Thread: Testing a second hand surface plate

Not as far as I can work out, no.
If we go back to the picture -
rahn1e.JPG
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

Another variation (this is a Starrett device who now own Tru-stone and Rahn)
repeat2 (Medium).jpg

Apart from the electronic do-da, it looks smaller and slimmer than the original version.

Michael

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

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

Originally Posted by simonl

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 -
d_plan2 (Medium).jpg d_plan3a (Medium).jpg

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

Originally Posted by Michael G

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

Originally Posted by Brobdingnagian

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

ASmeter.jpg

Originally Posted by Michael G

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

More detailed Repeat Meter

-J

Please comment, question etc...

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!

Originally Posted by jhovel

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

Originally Posted by Brobdingnagian

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

Originally Posted by Michael G

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

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.