Repeat-O-Meter for Surface Plate Calibration

thingiverse

#INTRODUCTION Years ago when I first saw the Repeat-O-Meter, it seemed to me a very simple device that could be easily built. Why then did they retail for over fifteen hundred dollars, for such a simple device? So, based on photographs of the actual device, and Tom Lipton's improvement of various features, I've produced my own version of this device which anyone with a basic shop shoulld be able to build. I do not recommend 3D-printing this in plastic, I place this here anticipating a time when we will have cheap laser-sintering in various metal materials. It also requires a basic indicator, and the brass-bushing which holds the indicator can be modified for any indicator mount size. There is currently a mock-up indicator shown in the model, for demonstration purposes. PDF files for all screws required are included. --- # USAGE The basic way that this device is used is on a flat surface plate, to find local dips or high-spots. Using this can tell you if you need to have your surface plate reground and recalibrated or not. No more guessing. Zero out the RoM indicator on one spot, then run the device around on the surface plate and see if it records a positive or negative reading anywhere on the plate. Also, for this tool to work at all, you will need to press-fit the balls into the bottom of the device and then surface grind them co-planar *before* you cut the center-flexure. And ideally, they should be lapped flat as well. And you need a .0001" indicator. This device is fairly arcane, but I had a good time designing it. Enjoy, and let me know if you find any use for it. --- #Details of the Design There are a couple critical design aspects in this device. The key is the flexure mechanism in the bottom center. I've spaced this .050" from the bottom, which allows that part to bend ever so slightly with the surface plate. .050" may prove to be too much to get significant range of motion out of that flexure. In which case you have a couple options. Probably the easiest would be to simply surface grind the bottom of the part, taking off maybe .005" at a time until you get good motion. You would have to knock the hardened feet out to do that, and the part has holes to allow you to press them out from above without problem. If you worried then that the hole would now be too big to get the feet to press back in, the solution is simple. Take a ball-peen hammer and lightly Brinel the metal all the way around each hole after removing the balls (hit hard enough to dent the metal), and this will cause the metal to displace into the hole, making it ever so slightly smaller, giving you enough metal to press into again later. Another option would be to re-bore the flexure hole slightly larger, but this requires more tools and experience than just surface grinding the part. Another option would be to add weight to that part of the foot, as it is the weight of the foot that causes the flexure to operate correctly. Another piece of steel on top, or even lead, would add significant weight to the tongue and cause it to flex more. As for the angled support arms that seem to be constricting the tongue from moving, this is not the case. The screw is not tight on this part; there's a small amount of clearance both above and around it, using a bushing. This clearance allows the flexure to flex quite a bit, .025" in either direction, before the support-arm bottoms out and provides support. This feature only exists to keep the device from being destroyed by overextending the flexure accidentally. With the support arms, the device is robust in a way that it could not be without them. I have also thought about ways to create a part like this with other kinds of flexures. It might be possible to simplify the device quite a bit via other flexure designs. I will have to think about it. --- Video of the device in action by the original designer: https://www.youtube.com/watch?v=ZkCIVsP7-KY