Michael G 26th October 2013, 03:01 PM While wandering around a junk yard the other day I spotted what turned out to be an 18" square Wing granite surface plate. I have a soft spot for Wing - my 2 CI plates are Wing, so was interested to see it (what I will do with a 4th plate is a question that is in abeyance at the moment). Trouble is, it had a sticker on it saying out of cal. Not sure whether that means it does not meet grade or just was not checked when it should have been.
My cheese board came with a cal diagram that says it is 2.5 micron high in the middle, but I'm not sure it is reliable. All the calibration methods I've seen are for large plates. The cheese board is 9"x12", the small Wing is 4"x10" and the large Wing 16"x16". Apart from sending them on holidays to northern Victoria, does anyone have any ideas on how to check a small surface plate? At the moment the best I can think of is to make up something like a repeat-o-meter, but I am looking for measurements in the micron range and a standard sized repeat-o-meter could only make 1 or 2 measurements on the small Wing, so am wondering whether a DTI on a lightweight rigid arm would be better. A precision level seems to be out as it would need to be 4" long and have a sensitivity around 0.0125mm per metre (0.000125"/10"). The best I can see around is 0.02mm/m.
Of course, what I'd eventually like to do is map the surface and then refinish (diamond lap the granite, then use that to scrape the CI) but that is a pipe dream for another day...
Michael
Bryan 26th October 2013, 04:19 PM Michael you can easily split the lines on the level, which should get you into micron territory - in theory. If you don't have one you can borrow my cheapie. It's 200 x .02mm/m, so you would have to make a little doover for it to sit on with pins or something 100mm apart.
Ueee 26th October 2013, 04:28 PM Not really a help but is your "cheese board" a carbatec plate? The cert with mine says 2.2um high in the middle but after checking it off the headstone (that's what my wife calls the bog plate I got from Phil) it looks to be hollow.
Cheers,
Ew
Brobdingnagian 26th October 2013, 04:46 PM Depends on the grade. but yes you are looking to get into the micron - sub micro range. if you do use a DTI you need a very very flat surface as your reference.
Another option if you have a heaps of time and patience it is use the three plate method. A->B, A->C and Error from B<->C, all three need to be of the same size.
-J
Theberylbloke 26th October 2013, 05:06 PM Hi Michael,
My first thought was to slap it on a mill table, mount a sensitive DTI in the mill spindle and make passes to measure the non-flatness. You would need to use some sort of riser blocks under the surface plates to keep them clear of inaccuracy in the mills table.
Two problems to be resolved.
The first being the sensitivity of your DTI. I think 2um (Ok, so that's the best I own) is a pretty sensitive DTI and that may not be good enough for what you want to do. So some sort of lever arrangement might be required, which will bring it's own inaccuracies.
The second is wear in the mills slides. This could be got around, to a degree, by checking all of your plates and ensuring that they all sit with exactly the same X and Y origin point. Also mount the plates at one end of the table, that sees less use?
The result should be four sets of data points that you could say graph to find a common profile that was caused by the movement of the mill table on the worn slides. Subtract the common profile from your readings to give the true non-flatness.
I briefly considered the idea of just running the DTI on the mill table to get it's flatness. Most of us don't have mill tables that have recently been ground and hand scraped, followed by calibration, very small dents (sometimes called life) could well upset the accuracy of doing it this way.
It is possible that the plates vary slightly in thickness from one end to the other so they might sit slightly off flat in your set up. That could be got around by checking a plate then turning it 180 degrees on the mill and checking it again.
A third issue has occurred to me. It's going to take awhile to do all this!
There is probably a better way to do this, I'm just thinking about how I would go about it given, the metrology I own.
Cheers
The Beryl Bloke
Brobdingnagian 26th October 2013, 06:01 PM I'll note here that surface plate calibration is one of the most demanding/important calibrations.
Here are the methods I currently can think of
1. Laser interferometer (get its standard for form from the straightness of a laser beam, calibrated optics and electronics).
2. Planekator (get is standard of form from straightness of calibrated granite beam and indicators).
3. Autocollimator (get its standard for form from the straightness of a light and calibrated optics)
4. Differential Level (gets it standard from the parallelness of local gravity vectors (ie always points down) and calibrated levels)
5. 3 Plate method (Auto generation of standard of flatness via iteration of successive removal of errors) (EXTREMELY hard to do in practice).
-J
Making a planekator from cast iron would be an interesting exercise using a calibrated 00 surface plate.
6. Call it flat and not worry about it..... :P
Michael G 26th October 2013, 06:38 PM Yes, the cheese board is a carbatec plate. Until I know how flat it is, I probably shouldn't use it for anything else but it is portable and even with a 2.5 micron bump on it I think it still fits within the defined limits of a workshop grade plate.
Mounting a DTI on the mill is out because I doubt it's that good. A micron is not much and I'm prepared to bet that the table is not that uniform. I do have a DTI (or it may be a DI) that goes to microns I think
I had thought of using a 0.02mm level and as a happy coincidence when looking through ebay to see how good they got, I saw one that looked just like mine (0.04mm/m). Mine has not got any branding on it so apart from suspecting it was Swiss (metric fasteners), I didn't know who made it. It turns out I own a 12" Wyler! It's perching it on a couple of 4" centres that has me worried. Again, because of the magnitude of the measurement I suspect that the uncertainty of the set up will cause errors.
The plates are too heavy to do the A, B, C thing with them (and I haven't got another 2 CI plates to play with). The thought was if I made a truss up so it was very rigid and pivoted it off a pole (perhaps with a counterweight) it might be good enough to give me a datum plane. Two problems then become ensuring that the surface being measured is perpendicular to the axis of rotation and getting the plate to move so a map can be made without introducing error.
An interesting offshoot of the ABC idea is to make up some straight edges though - a bit of steel flat attached to a beam of decent depth may (after scraping) may be flat enough to use as a reference. Acetal pads as sliding bearings, small ball bearings for lateral guides, 3 point support...
Certainly possible so I might think about that one some more.
Michael
Stustoys 26th October 2013, 07:29 PM Hi Micheal,
Isnt the DTI is just going to be a pain as the ball is so small? I'd think you'd have to have a gauge block on the plate and measure off that.(you only want to measure the high spots....... right?)
I think what Brayn is talking about is to make a one piece cradle for your level to sit on that has the 4" measuring points on the bottom. At least that should get you repeatablity.
You'd need to have the plate mounted well so you were sure it wasnt moving or use another level as a ref.
Should keep you out of trouble for awhile.:D
Stuart
Michael G 26th October 2013, 08:33 PM Reading the standard for these things (GGG-P-463C), measurements have to be taken with a 3/8" pad on the indicator, so I'd either use a DI with a custom foot or or do as they do when using a plankator and mount the gauge on the plate and measure up to the reference beam.
One of the problems with doing this with a level is that these plates are darn small - if I take a measurement, move 4" and take another that's just about it - can't really see a trend, especially on the 4"x10" plate (which some kind previous owner has used as an anvil :C. I scraped it back to cheeseboard - interestingly the high spot was in the centre of the plate. I suspect that the hammer blows bent it, but another reason to work out some sort of checking device). I'm hoping that if I can get some sort of rail mounted device sorted out I can can take a "height" say every 25mm and map the hollows and high points. To lap a granite plate a CI lap and diamond grit is used, so provided I have a good idea where the lapping should take place there is no reason why the granite plates can't be resurfaced. CI is even easier as it is scraping and I have the gear for that.
This was a link I found yesterday which provides some background information.
Gauging Quality, Episode 1 - Surface Plates - YouTube (http://www.youtube.com/watch?v=NWObTpn6dTk)
Michael
Stustoys 26th October 2013, 09:19 PM But if you get the granite 18"sq one right first. Can't you then call that your ref. Then scrape the CI ones to that?
Michael G 26th October 2013, 09:49 PM But if you get the granite 18" sq one right first. Can't you then call that your ref. Then scrape the CI ones to that?
Well yes, but if I'm going to spend weeks making up some trick device to measure a surface plate I'm damn well going to measure them all! :~
Seriously I don't know anything about lapping granite so I may find that I can't produce a decent master plate this way and I may have to do them all individually (plus that 16" sq CI plate is bloody heavy - don't fancy flipping it over too often. I'd prefer to get an idea of the rough shape and take the bulk off before getting blue everywhere).
Michael
RayG 26th October 2013, 11:21 PM Hi Michael,
Maybe you could come for a visit and we can crank up the laser interferometer. The basic union jack pattern with a suitable step length.
The grading of flatness is a function of plate size, but for an 18x18 plate flatness
AAA 25 uin ( 0.64 um )
AA 50 uin ( 1.27 um )
A 100 uin ( 2.54 um )
B 200 uin ( 5.08 um )
There is a Rahn repeat-o-meter going at the moment Rahn Repeat O Meter FOR Checking Surface Plates IN Carrying Case | eBay (http://www.ebay.com.au/itm/Rahn-Repeat-O-Meter-for-Checking-Surface-Plates-in-Carrying-Case-/161133972903?pt=LH_DefaultDomain_0&hash=item2584554da7)
But my coffers are somewhat depleted just now...
Regards
Ray
Michael G 27th October 2013, 07:27 AM Maybe you could come for a visit and we can crank up the laser interferometer.
There is a Rahn repeat-o-meter going at the moment Rahn Repeat O Meter FOR Checking Surface Plates IN Carrying Case | eBay (http://www.ebay.com.au/itm/Rahn-Repeat-O-Meter-for-Checking-Surface-Plates-in-Carrying-Case-/161133972903?pt=LH_DefaultDomain_0&hash=item2584554da7)
But my coffers are somewhat depleted just now...
While I'd love to come for a visit I don't think it's going to happen any time soon - although I might send a surface plate via courier if I get keen.
Realistically for shed work a B grade plate is all that is strictly necessary. For bragging rights a AAA might top of the heap but given the environment that it will be used in it is for all practical purposes a waste.
As a side issue I still want to know who in Australia is calibrating all the surface plates around the place - I haven't heard of anyone and there are thousands of places with ISO systems who claim to use calibrated measuring equipment.
That's crazy money for the repeat-o-meter (I was looking at it earlier). When you look at what it is I would have thought about a quarter of that. As I've seen new advertised for a little over $900(US), I can't understand who would want to pay that much. Again, if I get keen I may have to make one - the trickiest part will be finding a precise enough gauge but even that is not an insurmountable problem.
Michael
RayG 27th October 2013, 11:44 AM While I'd love to come for a visit I don't think it's going to happen any time soon - although I might send a surface plate via courier if I get keen.
Realistically for shed work a B grade plate is all that is strictly necessary. For bragging rights a AAA might top of the heap but given the environment that it will be used in it is for all practical purposes a waste.
As a side issue I still want to know who in Australia is calibrating all the surface plates around the place - I haven't heard of anyone and there are thousands of places with ISO systems who claim to use calibrated measuring equipment.
That's crazy money for the repeat-o-meter (I was looking at it earlier). When you look at what it is I would have thought about a quarter of that. As I've seen new advertised for a little over $900(US), I can't understand who would want to pay that much. Again, if I get keen I may have to make one - the trickiest part will be finding a precise enough gauge but even that is not an insurmountable problem.
Michael
The guys we visited at the National Measurement Institute, would, I'm sure calibrate a surface plate if requested, but I suspect it might be a bit pricey. And they don't have a repeat-o-meter ( I asked ) :)
I think it's actually a bit worse than you think, not only are surface plates not generally checked, but I think a lot of the supplied calibration certificates are suspect, particularly the chinese cheapies... that said the ones we've checked are still 00 grade even if they don't match the supplied calibration chart.
I agree about a B grade being more than accurate enough for general work.. you'd need a temperature controlled, dust free environment to get much benefit from a better grade.
Regards
Ray
PS... The Rahn repeat-o-meter has a 20 millionth inch MAHR Supramess, so you'd need a 0.5 micron indicator as a starting point...
The Tru-Stone Rahn repeat-o-meter is $935 plus $640 for the Supramess plus $540 for the carry case, so that runs it up to $2115 for a new one to match the ebay listing.
Tru-Stone: The world's largest manufacturer of custom precision granite (http://www.tru-stone.com/pages/price_lists/price_list_popup.asp?id=8)
Stustoys 27th October 2013, 12:25 PM Hi Ray,
If you were "only" going to be checking to B grade, would you need the 0.5 micron indicator?
(Supramess is missing from my collection, though I have all the others lol)
The figures you give above are for local flatness right?
Stuart
RayG 27th October 2013, 12:45 PM Hi Ray,
If you were "only" going to be checking to B grade, would you need the 0.5 micron indicator?
(Supramess is missing from my collection, though I have all the others lol)
The figures you give above are for local flatness right?
Stuart
Hi Stuart,
Those numbers were for total flatness, not local flatness.
I found a formula for overall flatness specs for AA is 40 + (D^2)/25 where D is the diagonal, and the answer is in microinches, then double that for A and double again for B grades.
The definition of flatness, is the distance between two parallel planes that fully enclose the surface.
The basis of the local flatness ( repeat-o-meter ) specs are not so clear, I remember going down this rabbit hole once before and we found that it was based on a 0.375" probe for the US specs, but never found out what it was for the metric version.
Regards
Ray
Michael G 27th October 2013, 12:47 PM Attached are the tables from GGG-P-463C for flatness (two planes that sandwich the whole surface) and repeatability (deviation from local flatness). An 18" square plate has a diagonal of 25.4 inches.
291035291036
These numbers are in micro-inches. The numbers that Ray quoted are for flatness, and the repeatability (local flatness) requirement is tighter again. On the clip I linked to one of the statements made about the plate they were testing was that while it was flat to a particular grade, using a repeat-o-meter it failed the criteria and should be down graded.
Michael
Brobdingnagian 27th October 2013, 01:42 PM Attached are the tables from GGG-P-463C for flatness (two planes that sandwich the whole surface) and repeatability (deviation from local flatness). An 18" square plate has a diagonal of 25.4 inches.
291035291036
These numbers are in micro-inches. The numbers that Ray quoted are for flatness, and the repeatability (local flatness) requirement is tighter again. On the clip I linked to one of the statements made about the plate they were testing was that while it was flat to a particular grade, using a repeat-o-meter it failed the criteria and should be down graded.
Michael
All round that gauging quality episode was good.
As far as specs go, If anyone here is a student or an apprentice, I pretty sure you have access to all the australian standards the ones of particular interest are:
AS 1004.1-1998:Surface plates for metrology - Cast iron
AS 1004.2-1998:Surface plates for metrology - Granite
These are respectively equivalent to ISO
ISO 8512-1:1990
ISO 8512-2:1990
I can't remember now who has the 00 and 000 grades in their standard. AS and ISO grades are 3,2,1,0.
Also there is a newer US standard for plates ASME B89.3.7 – 2013 which supersedes the GGG-P-463C spec, is has a couple of new appendices on uncertainty and traceability.
-Josh
Michael G 28th October 2013, 09:48 PM I went back to day to have another look at that granite surface plate. I thought it was 3" thick and a B grade, but it turned out to be A grade and 4" thick. The next silly thing I did was bought it. Made in 1968, so around the same vintage as the rest of my gear
This of course means that I need to make up that repeat-o-meter to see how good it is. Once that is done I can check out the granite plate as well as the cast iron plate I have (although it will have to go as I don't need two - but I'd feel uncomfortable not knowing whether I was selling a plate on that was in reasonable condition)
One day I'll learn to leave the cards at home.
Michael
Brobdingnagian 29th October 2013, 12:42 PM : ) It's a good size...Maybe they will average out,
If you do make a repeatOmeter, I have some rough idea's on how they are put together.
-J
Michael G 29th October 2013, 08:53 PM All ideas are welcome Josh.
A friend emailed me through a page of the standard you mentioned with the testing requirements and they are pretty much the same. (I'm not sure why there is AS1004.1 and AS1004.2 as they say almost identical things). The test does not seem to have changed much at all.
I have a picture or two -
291349
and it looks straightforward although a horribly clunky design. You can almost hear the discussion -
"The prototype works fine boss, we just need to neaten it up a bit and then we can release them onto the market"
"No. We're too busy at the moment. Maybe later if we have time"
The one thing I haven't worked out is the twin beam arrangement. I'm guessing that the knob pulls the two beams together and so moves the indicator ever so slightly to adjust the zero (total travel for a supramess is around 10 thou I think) with the bolt the plunger is on for coarse adjustment.
Michael
markgray 29th October 2013, 09:11 PM Hi Michael
A repeat-o-meter is something I would love to make as I also have a few surface plates of unknown quality. I have a supramess and 2 mikrokator ( 0.00001 and 0.000005 inch scales) comparators that I could use. Does anyone have a basic drawing of the mechanics of the repeat-o-meter or a link to same and we can see what can be fabricated. Ray might even do some quality assurance and measure its accuracy against the laser interferometer?
I will go hunting on the net and see what I can find.
Ueee 29th October 2013, 09:24 PM They look pretty simple, but what do the 2 other handles do? (top an right end)
Presumably the actual contact area would have to be lapped super flat, as would the base?
Ew
Edit, Ahh, those handles....they are simple just handles aren't they?
Stustoys 29th October 2013, 10:11 PM But, at getting on to 12" long, isnt this going to be pretty useless for plates 18" and smaller?
Stuart
Michael G 29th October 2013, 10:24 PM Yeah, the handles are handles. Contact on the bottom is via 4 pads; 3 fixed on the main section and one fixed on the pivoting section. They would not have to be lapped flat but need to be "reasonably flat".
Mark, what I was going to suggest is that someone makes one first and then it can either go touring or be drawn up so anyone else who wants one can make one up. Scaling as I can from the photos I think that one is far heavier than it needs to be (I estimate around 10kg) and you could get the same accuracy from something at least half the weight. I'm even wondering about using Al rather than steel. The mechanics are very simple. Looking at the picture, the main block with the handles sits on 3 pads. There is a pivoting section (using a piece of spring steel) with a 4th pad and the indicator reads off the top of that. If the 4th pad is in a hollow or on a rise relative to the other 3 then the indicator will show that. The diagonal struts are to guard against over travel and as I've speculated above, I think the knurled knob and split beam are to provide some vertical adjustment to help zero the indicator. At the moment I'm trying to find out the lateral spacing of the 3 pads. GGG-P-463c states the pad diameter as 3/8" diameter and at 5" longitudinal spacing; AS1004 states the diameter as around 29mm with 100mm longitudinal spacing. At the end of the day provided that your surface table hasn't got pits in it (that is, any hollows or rises are gradual) it probably doesn't matter which set up you use.
I'm hoping to have a play this weekend
Remember though this is detecting local irregularities. The best analogy that I can think of is a road surface. A surveyor can use instruments to give you the total rise and fall of a road over a certain distance, in surface table terms that is the flatness. This device, while it could do that with a bit of thought is more the thing that measures the depth of the pot holes that the surveyor won't pick up unless his staff is exactly in the right spot.
The standards differentiate between the two by imposing different limits on what is allowed.
Michael
PS for Stuart - yes, but for plates under 30" in the diagonal, it is slid over the surface in a random manner. Apparently smaller versions are made but you are not necessarily conforming to the standard then (although it would give a good idea)
Brobdingnagian 30th October 2013, 01:04 AM Cool,
I'll draw it up to suit mahr.
-J
Stustoys 30th October 2013, 10:27 AM Not that I dont want to make a repeat-o-meter, but talking about Michael's problem. A repeat-o-meter isn't any real use until after you know the plate is flat(within spec)......... right?
A repeat-o-meter will pass this.(granted you'd need a small one ;) )
Brobdingnagian 30th October 2013, 01:46 PM any idea on where the flex point is? is it in the middle or on the bottom?
.RC. 30th October 2013, 04:11 PM You could always just blue a known good object on it, like a straight edge...
Michael G 30th October 2013, 05:41 PM Josh, the hinge is on the bottom (a strip of spring steel, which I just happen to have...)
Richard, I'll probably end up doing that but at the moment can't rely on that either. Classically to test flatness a straight edge is used with a dial indicator running on it (look up Planekator). These days of course you just send 60kg of granite on holidays to Victoria...
Stuart, you are right. A surface of constant curvature will pass a repeatometer test but if zeroed on a flat surface will work. However, another way would be to scrape something to the surface under consideration and then test both. The true (flat) surface will be the average reading. Once you have that well...
Michael
Brobdingnagian 30th October 2013, 09:15 PM cheers, I presume it is camped in place to give a knife edge flex point?
Michael G 30th October 2013, 09:43 PM Not as far as I can work out, no.
If we go back to the picture -
291512
The bottom hingy bit is there to provide something solid to measure against while having a pad of the right size on the bottom. Really you could make something up without it but you would have to put a large foot on the bottom of the indicator and you would then have to be very careful dragging it around so that you did not damage the indicator. As an added advantage the mass of the part makes it less likely that the measuring pad will sit up on a mote of dust or something. (A supramess is graduated in 0.00002 of an inch = 0.5 micron)
If a normal hinge (like a door hinge) was used, there would be enough clearance in the hinge that as the tester was slid around the table the indicator plunger would move relative to the setscrew that it sits on. Any surface imperfections, dust etc could potentially change the reading. So by having a solid hinge the indicator plunger is always in the same spot and (theoretically) not as prone to measurement error.
Just about any flexible item could be used but spring steel is durable, strong and is not affected as much by the environment as rubber or plastic might be. Incidentally, the only reason I can see for those diagonally shaped braces either side is just so that when the unit is picked up the 'hinge' is not over flexed. Total indicator travel is a thou I think, so they could almost be say M6 bolts with the braces having 6.5mm holes.
Michael
Michael G 30th October 2013, 10:07 PM Another variation (this is a Starrett device who now own Tru-stone and Rahn)
291523
Apart from the electronic do-da, it looks smaller and slimmer than the original version.
Michael
simonl 1st November 2013, 11:49 AM Hi Michael, hi all,
I've been lurking and reading this thread with interest. So, this Rahn meter is really just a glorified magnetic base and dial indicator slid around on top of the surface plate? By all means it's a little more refined with it's contact points but it basically uses the rear end of the contact on the plate as a reference to compare another point at the front?
Hence, the reason it measure local flatness and not overall? As stuart said, a perfect sphere would also show local flatness.
Have I got this correct? If so, this really would be a good project to make to test areas suspect of wear on an otherwise good surface plate.
Just getting my head around the concept….
Cheers,
Simon
Brobdingnagian 1st November 2013, 12:41 PM Here is a quick rough out of the shape and important features.
Based on feedback I'll do a more refined/detailed design.
based on the 5" centres of the US standard, easily changed to the 100mm centres of the AS Standards. I will also do a design suited to a 20mm probe (as that is what I plan to use).
-J
Edit: added indicator
Michael G 1st November 2013, 05:25 PM So, this Rahn meter is really just a glorified magnetic base and dial indicator slid around on top of the surface plate? By all means it's a little more refined with it's contact points but it basically uses the rear end of the contact on the plate as a reference to compare another point at the front?
Hence, the reason it measure local flatness and not overall? As Stuart said, a perfect sphere would also show local flatness.
Have I got this correct? If so, this really would be a good project to make to test areas suspect of wear on an otherwise good surface plate.
Yes, that's about right Simon. To test for flatness there are several ways - the most technical being a laser interferometer and the simplest probably being the afore mentioned Planekator -
291673 291672
Basically an upside down straight edge with the same sensitive indicator reading against it. Classically, the flatness is only measured along 8 lines where as for small plates the repeatometer is used all over. Once flatness is established then a repeatometer will show wear. Of course, it relies on the straight edge being really good too.
Yes Josh, that looks basically the item.
After realising that these were PDFs that could be rotated around, I looked underneath and think that the foot arrangement that you have should be reversed - that is, have the two feet at the hinge end. From memory that's the way that the AS has them
Michael
Brobdingnagian 1st November 2013, 06:31 PM Yes Josh, that looks basically the item.
After realising that these were PDFs that could be rotated around, I looked underneath and think that the foot arrangement that you have should be reversed - that is, have the two feet at the hinge end. From memory that's the way that the AS has them
Michael
I like the 3D pdf for many reasons, but you can also take measurements, isolated parts, do cross sections off them etc.
That's interesting. the GGG spec had them with the two feet at the back, hmmm which way to go? two at the front seem better to me as the center of mass is about 2" behind the centre foot.
-J
Michael G 1st November 2013, 07:59 PM That's interesting. the GGG spec had them with the two feet at the back, hmmm which way to go? two at the front seem better to me as the center of mass is about 2" behind the centre foot.
I think 2 by the hinge will be more stable myself. At the end of the day it probably doesn't matter as if the plate is flat then it's not going to change the reading wherever the feet are. Again it does not matter greatly, but it is annoying that GGG doesn't specify a distance between the 2 feet.
I'm a bit in two minds myself as the "new" plate was made in 1968 so is imperial and so would be to GGG-P-463 but the AS1004 version (below) seems more sensible. I think a greater influence on the accuracy of the gauge is going to be the foot size - although if you have a hollow that a 3/8" foot falls into but a 3/4" foot will bridge you could have trouble anyway.
What's your mass likely to be? I think the version I'm thinking of is going to be something like 3kg. Heavy enough to be stable but not too heavy to move around. I think both the nose and the hinging foot of both versions could be thinned down (which of course means the back can be lighter too.
Michael
291748
Brobdingnagian 1st November 2013, 08:11 PM What's your mass likely to be? I think the version I'm thinking of is going to be something like 3kg. Heavy enough to be stable but not too heavy to move around. I think both the nose and the hinging foot of both versions could be thinned down (which of course means the back can be lighter too.
Michael
The one in the pdf is ~5.5kg
Brobdingnagian 2nd November 2013, 07:21 PM More detailed Repeat Meter
-J
Please comment, question etc...
jhovel 2nd November 2013, 10:17 PM Looks really nice Josh.
I don't quite understand the purpose of the little block at the rear of the moving front section - given the shape of its bottom. I first thought it was just to clamp the (spring steel?) hinge, bit then saw the bump at the bottom. Incidentally, why not have the hinge on the bottom of the blocks, screwed on from underneath, since that wouldn't interfere with the feet. The gauge clamp could be just rectangular at the front of the top plate (like the Rahn), rather than the (nicer looking) somewhat difficult to machine round shape - my KIS principle... Also the slots in the retaining strips need to be just a few 10ths of mm long - or even just a tad oversized for the screws.
By the way, a very close look at the Rahn gives me the impression that its main body (the part with the handles attached) is made from a 4-layer sandwich of flat pieces - the top two about 1/2 the thickness of the bottom two. The fine adjustment screw behind the gauge seems to just spring the top plate with the gauge clamp against the 2nd one down. The gauge would therefore indicate just half the total deflection and therefore half the angular pitch of the adjustment screw. This is a clever way of making the adjustment very fine indeed.
Looks like a very 'doable' design now. Great work!
Brobdingnagian 3rd November 2013, 01:30 AM Looks really nice Josh.
I don't quite understand the purpose of the little block at the rear of the moving front section - given the shape of its bottom. I first thought it was just to clamp the (spring steel?) hinge, bit then saw the bump at the bottom. Incidentally, why not have the hinge on the bottom of the blocks, screwed on from underneath, since that wouldn't interfere with the feet. The gauge clamp could be just rectangular at the front of the top plate (like the Rahn), rather than the (nicer looking) somewhat difficult to machine round shape - my KIS principle... Also the slots in the retaining strips need to be just a few 10ths of mm long - or even just a tad oversized for the screws.
By the way, a very close look at the Rahn gives me the impression that its main body (the part with the handles attached) is made from a 4-layer sandwich of flat pieces - the top two about 1/2 the thickness of the bottom two. The fine adjustment screw behind the gauge seems to just spring the top plate with the gauge clamp against the 2nd one down. The gauge would therefore indicate just half the total deflection and therefore half the angular pitch of the adjustment screw. This is a clever way of making the adjustment very fine indeed.
Looks like a very 'doable' design now. Great work!
Thanks for the feedback Joe they are all very good points.
I'll see if my brain is still working at this hour and try to explain what I was thinking.
The Little block IS a clamp for the spring. I was thinking that a knife edge spring would be more consistent rather than one that was not supported all the way to the flexure point. I have included an alignment method for the spring as well, that should make it easy to make sure that the moving arm only up an down is not twisting as it flexes. you just loosen of the 6 screws on the clamping plate, insert a shim into the gap between the arm and the body and snug everything up again. One thing I'm not certain of yet is what length the flexure should be?
The other point about where the hinge is hopefully avoid mishaps to the spring if there is a lapse in concentration and one picks it up without having the side straps in place. having it at the top limits the travel of the arm, if it was at the bottom it could possibly kink the spring?
Good point on the side straps they only need holes that are oversized, definity a KISS there.
4 layers, I only counted 3, oh ok 3 layers plus the arm. I thought 2 would be simpler. But as it is I currently miss a fine adjustment for the indicator other than the indicator itself.
The difficult to machine indicator mount is only fancy to accommodate larger 20mm indicators. changing it to straight does not impact greatly, I was thinking of when I do the schematics I would throw in a variation or two that being one of them.
-J
Michael G 3rd November 2013, 06:46 AM The other point about where the hinge is hopefully avoid mishaps to the spring if there is a lapse in concentration and one picks it up without having the side straps in place. having it at the top limits the travel of the arm, if it was at the bottom it could possibly kink the spring?
From the videos I've seen, I don't think that the side straps are removed.
Michael
Brobdingnagian 3rd November 2013, 11:16 AM From the videos I've seen, I don't think that the side straps are removed.
Michael
Ok, more simplicity or more protection?
And do you think we need a fine adjustment mechanism?
-J
jhovel 3rd November 2013, 12:56 PM Josh, could you put up a dimensional sketch yet - or tell us how you can measure off the 3D pdf, please?
I think a fine adjustment would be convenient and easy enough to do.
I think I can see a faint line in the centre of the obvious upper layer which appears to line up with the split for the fine adjustment.
Brobdingnagian 3rd November 2013, 01:50 PM Josh, could you put up a dimensional sketch yet - or tell us how you can measure off the 3D pdf, please?
I think a fine adjustment would be convenient and easy enough to do.
I think I can see a faint line in the centre of the obvious upper layer which appears to line up with the split for the fine adjustment.
Sure thing Joe, the schematics are underway.
If you want to play around with the PDF if you right click you have all sorts of options, the one I always change it is the render mode to shaded illustration.
-J
Brobdingnagian 3rd November 2013, 03:44 PM Here are a couple of drafts, still working through the rest.
-Josh
Edit Added some more
Michael G 3rd November 2013, 04:27 PM Ok, more simplicity or more protection?
And do you think we need a fine adjustment mechanism?
I think they are just left on for ease of use (why take them off if you don't need to)
Given that the supramess range is not much at all, I'd say that fine adjustment is moved from the "nice to have" column to the "almost essential" column.
Michael
A thought - as repeatometer is probably some one's trademark and doesn't really describe the thing well at all, what are these going to be called? B&H indicator (bump and hollow)? Josh-o-meter? Oh bugger meter? (as that's what you say when you see the results)
Brobdingnagian 3rd November 2013, 04:33 PM I think they are just left on for ease of use (why take them off if you don't need to)
Given that the supramess range is not much at all, I'd say that fine adjustment is moved from the "nice to have" column to the "almost essential" column.
Michael
Ok Will do, It will just mean an extra layer, a measurement pad to be mounted on the top of the arm for the indicator, and a few more holes and screws.
Stay tuned for version 2.
-J
Brobdingnagian 3rd November 2013, 06:55 PM Revision 2.0 Draft: A simpler build.
RayG 3rd November 2013, 08:43 PM Looks good, when do we build it?
Regards
Ray
Brobdingnagian 3rd November 2013, 09:17 PM Looks good, when do we build it?
Regards
Ray
Soon...
I tweaked the design a little more, for simpler build and better use of steel. I also made the calculations for the deflection of the double beam thingy and for the springs free length.
Spring Length is 2.17 mm with 0.030" thick that give ~70µm deflection at the measuring point under gravity.
The double beams thingy gives an adjustment range of about 10µm.
-J
PS. Hope that covers every thing that people wanted?
PPS the stock material is 16mmx65mm, milled/ground to 15mm x 60mm, you would need about 850mm of 65mmx16mm flat bar.
PPPS the finished weight is ~4.6kg
PPPPS. what would be good for the beam adjustment, just a cap screw, or something more elegant?
Edit: added pdf
Stustoys 3rd November 2013, 10:09 PM Hi Josh,
Are you sure you dont need a plate under the cap screws on the hinge?(so its not hanging off the bolt heads)
As far as over travel protection. How about a bolt(couple of) through the hinged plate into the middle plate?
Stuart
Brobdingnagian 4th November 2013, 03:06 PM Hi Josh,
Are you sure you dont need a plate under the cap screws on the hinge?(so its not hanging off the bolt heads)
As far as over travel protection. How about a bolt(couple of) through the hinged plate into the middle plate?
Stuart
Both good points. I'm unclear if it is required or not, based on the FEA modeling it would suggest no it does not need an extra plate over the top. That being said there is 4mm clearance between the top of the spring and the bottom of the double beam so it would be a relative easy mod.
I like the screw idea, nice and simple.
So is everybody happy for me to do up the schematics for version 2.1? or are there more Great Ideas lurking about?
-Josh
Steamwhisperer 4th November 2013, 06:55 PM Hi Josh,
Whilst on holidays here in good ol' South Australia, I popped in to see Michael. I am now the proud owner of the 16" x 16" plate in question (well when I pay for it I will). It is now on tour with us and will be in Victoria soon maaaaaate. I may be popping over for a visit.
Phil
Michael G 4th November 2013, 07:33 PM Hi Josh,
Whilst on holidays here in good ol' South Australia, I popped in to see Michael. I am now the proud owner of the 16" x 16" plate in question (well when I pay for it I will). It is now on tour with us and will be in Victoria soon maaaaaate. I may be popping over for a visit.
Not sure if that plate had a date on it but if you are lucky you may be able to work out an approximate date from the exact name and address -
WING" Gauge & Instrument Co. (http://www.oldengine.org/members/kennedy/wing/gauge.htm)
As granite came in around WW2 due to CI shortages, there is a good chance that it is pre 1950's. I'm guessing that the small Wing I have is a similar age - Wing gauge and instrument was in existence from say 1947 to 1965. The granite plate that replaces it was produced in 1968 so is only 45 years old.
(Josh, I'm sure that was a AAAA grade plate when I had it - Phil's probably used it as caravan ballast and trashed it. My excuse anyway)
Michael
jhovel 4th November 2013, 07:39 PM Josh,
how much trouble and recalculation would it take to make the double/adjustable layers 10mm thick each and insert a 10mm rectangular layer between them and the base plate? I suspect we would get a fair bit more adjustment with a lot less force (= smaller knurled handle).
I would retain the 'coarse' adjustment of a bolt as the contact point with the gauge - and to suit different gauges.
What do you have in mind to use for the feet, given they will be sliding all over the surface plate? Should they be particularly hard and particularly flat and particularly smooth, or preferably self lubricating like cast iron or sintered bronze or just plain 'soft' like say brass?
Michael G 4th November 2013, 07:53 PM Looking at the Rahn version, I think the adjustment is simply a slab of 25mm thick material that has been shaped and then had a saw cut through the middle. Like you suggest Joe, i was thinking of a slab of 25mm material and then rebating in some 8 or 10mm material say 25mm wide both sides (welding or brazing in). Cutting from a piece of flat and doing the saw thing may work, but seems very wasteful to me, although probably quicker and easier than rebating in as I was suggesting I guess it is a balance between less adjustment force and more flexibility as the arms get thinner.
As for feet, I think it depends on what the device is running on. You don't want something that will ding up the surface so brass might be the go for a CI plate, perhaps CI or steel for granite plates.
Michael
Who's going to be making one of these (or a variant) and needs some spring steel shim?
Stustoys 4th November 2013, 10:04 PM A few more questions rather than great ideas.
The fine adjustment(other then on the indicator) is that really needed? shouldnt this be set once and forgotten.
To answer my own question "no some of us might what to use the indicator for something else between tests".
Why the angled cut at the hinge?
Re the weight. I wonder if its so heavy just so that moving it around with the handles lifting are less likely to effect it? If one was worried about that you could make handles with a pivot so they wouldn't let you lift it. (though that might be a little over the top)
Why spring steel? Just coz its handy and thin?
Stuart
Michael G 5th November 2013, 06:56 AM The fine adjustment(other then on the indicator) is that really needed? shouldnt this be set once and forgotten.
To answer my own question "no some of us might what to use the indicator for something else between tests".
Yes. Typically when being used you are meant to zero the indicator on the centre of the plate and then compare the rest to it. Zero adjustment may be handy for that too. Remember this is being used with an indicator that has sub micron divisions and the limits depending on the plate grade are between 1 and 5 microns. Some indicator movement is to be expected either due to wear or the initial grind/ lap.
Why the angled cut at the hinge?
No real reason. It just is and has been done that way. If I had to invent a reason I would suggest it is that way to prevent things being able to easily drop into the gap
Re the weight. I wonder if its so heavy just so that moving it around with the handles lifting are less likely to effect it? If one was worried about that you could make handles with a pivot so they wouldn't let you lift it. (though that might be a little over the top)
Weight is not the same as rigidity. Personally I think the Rahn is as heavy as it is because it was never refined much as part of the original design process. Some weight is desirable though so it sits well on the plate. Because of the gauge force and the cantilever arrangement my estimate is that 3kg is about the minimum that is practical
Why spring steel? Just coz its handy and thin?
More or less. A solid hinge is preferred to prevent movement between the main body and the hinged section in contact with the plate (could generate errors). After that it becomes a matter of finding something with dimensional stability that does not work harden and has a good fatigue life.
Michael
Michael G 16th November 2013, 01:40 PM Tester made to a level that it can give a number -
293804
Still needs a handle, feet to be "lapped", a strap to prevent over movement and some general clean up to make it look presentable. It's not quite a GGG-P-463 version or an ISO1004 as I have the centres from ISO and the foot pattern and diameter from GGG-P. I'm glad I got the supramess attached to the bench mic though. I was going to use one of the gauges that Bryan sent but as the movement is around +/- a tenth it would have been hard to see. Unfortunately for a class B (workshop) plate I'm allowed 110 microinches (1.1 tenths). This plate is labelled Class A (inspection) for which the limit is 60 micro inches and I had hoped that it had been sold off because it was out of class A but no such luck it seems. Having said that, this is the "rushed into use prototype" measuring device, so when properly finished I may get a measurement that is a bit closer to what I was hoping to see.
293805
Knurling of the fine adjust knob (which works better than I thought it would) was done using the method Rob spoke about and also worked well. I set the depth as half the pitch of the knurl and that seems to form nicely.
Michael
Stustoys 16th November 2013, 03:18 PM Hi Michael,
Looking good, now we just need to tweak it until your plate passes ;)
I've just been thinking(never a good thing). How much of an error will you get if the two single measuring points(for lack of a better name) arent dead center and square between the other two points?*
Stuart
*when the "two other points" roll
Michael G 16th November 2013, 05:46 PM One thing I did notice while playing was that in a traverse (either pulling towards me or pushing away) I'm "only" getting say 60 microinches of movement (or less in some positions). When I move to another location though the dial reading will change. The hinge is a bit of spring steel with some 3/16 holes in them and M4 button head capscrews to secure. Using a crappy bolt as a measuring pad relies on always measuring in the same place on the bolt head. I'm wondering whether the hinge can shift slightly when changing direction, disturbing the CBP alignment (CBP = Crappy Bolt Position).
I may need to counterbore the hinge bolts slightly and fit a close fitting sleeve to make up a mini shoulder bolt to prevent that movement.
Michael
Michael G 16th November 2013, 06:04 PM I've just been thinking(never a good thing). How much of an error will you get if the two single measuring points(for lack of a better name) arent dead center and square between the other two points?*
*when the "two other points" roll
Not sure what you mean. Neither of the standards give a tolerance on things, just nominals.
The three feel on the body are just to establish a plane, the 4th foot is there measuring a relative distance from that plane. (Strictly speaking I guess you are establishing a second plane with the 4th foot and the hinge axis). With a perfectly flat plate (or spherical one too) the distance from the surface to the plane is going to be constant.
Therefore, it shouldn't matter if the three points on the base are not geometrically symmetrical because their only function is to define the plane. Provided that the fourth foot is the right distance from the others, that precise geometry should not matter either. If one of the 3 feet defining the plane landed in a hollow in the plate, that might affect the reading (the roll?) but as the divisions are 20 micro inches over a 4 inch base, the error induced is going to be tiny.
(I think)
Michael
Stustoys 16th November 2013, 08:01 PM Hi Michael,
A picture should help me explain what I mean.
The first is the ideal pattern. Rolling the two bottom points changes the reading, as do the next two(by even more)
The last one is (to all intensive purposes) uneffected.
I think lol
Maybe I am thinking to much.:p Given the amount of "roll" there is likely to be wi;; it even be an issue? I have no idea, but 110 microinches doesnt give you much to play with lol
Stuart
p.s. How about 4 pins in the hinge?
Michael G 16th November 2013, 08:43 PM See attached.
293863
By my calculations (and by the way, I thought you and Ewan were the number jugglers around here?)
If a foot is in a maximum depth hole in the plate (110 micro inches) for the ideal configuration the dial will show a movement of half the depth of the hole. By comparison if the measuring foot is translated by 1/8" then the effect will be an additional (+ or -) 7 micro inches on the apparent movement shown on the dial caused by the hole. Roughly 12 1/2 %, or a 1/3 of a division on the supramess.
Doubt if I'm losing sleep over that one...
Michael
Ueee 16th November 2013, 08:54 PM By my calculations (and by the way, I thought you and Ewan were the number jugglers around here?)
I thought you let autoCAD do all the hard work.....:D:q
Nice work on the "MPT" Looks just like a Rahn, getting that Mahr was a really good score. I regard my 3 Mahr indicators as the best i have hands down......
Ew
Stustoys 16th November 2013, 09:28 PM (and by the way, I thought you and Ewan were the number jugglers around here?)
No No, I just come up with the ideas, I let others do the grunt work..... as I wouldnt really have a clue where to start.
I can follow your maths, but I'd be surprised if I came up with the same answer if I tried to muddle through on my own.
So as long as the marking out isnt done with a cold chisel.... it should be "close enough" :)
Thanks
Stuart
Oldneweng 17th November 2013, 04:59 PM I thought you let autoCAD do all the hard work.....:D:q
Nice work on the "MPT" Looks just like a Rahn, getting that Mahr was a really good score. I regard my 3 Mahr indicators as the best i have hands down......
Ew
Well, didn't you see that AutoCad drawing in Michael's post? :D
Dean
RayG 17th November 2013, 05:47 PM Looks good, I'm still waiting for Josh to make one :) Without a 20 uinch indicator, we will be using the Sony gauge, it's got 20 uinch resolution, (1/2 micron) plus stores max min readings.
How did your plate measure up, did it pass the 60 uinch test?
Regards
Ray
Michael G 17th November 2013, 08:46 PM How did your plate measure up, did it pass the 60 uinch test?
Hard to tell, but I don't think it will. As I said above I think I have some hinge movement which is causing me some grief. If I can get the movement out I think I should make the class B requirement. Looking at the plate under good light it is pretty sorry - some small chips, a deep scratch and several lighter ones. The other thing I want to do is talk to some of the monumental masons around here. I don't know how good their equipment is but they may be able either resurface it for me or give me some advice on how to do it myself.
Stuart suggested some pins to locate the shim but it is not all that thick (15 thou), so a pin may not do the job. I did think of cheating and using some bearing glue. The other option is to remake the hinge with 5/32" holes. As well I need to re-mount the gauge position. At the moment it is 3mm further out than it should be (adds roughly 3% error) and it irritates me!
Michael
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