Thread: Testing a second hand surface plate

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.

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

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

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

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

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

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.

Stuart

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 . 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

Michael

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?

Originally Posted by Stustoys

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

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

But my coffers are somewhat depleted just now...

Regards
Ray

Originally Posted by RayG

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

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

Originally Posted by Michael G

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

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