Cross-roads
Added 2023-12-10 09:04:47 +0000 UTCI've been rubbing these things for days now, and I'm finding that when working with such teeny tiny numbers, it's easy to lose perspective/develop a certain level of insanity.
So I put it to the group: Is circa ±6 microns (best as I can measure) straight enough for a meter long straight edge?
I wanted to do a poll, but I'm not sure I can add images; these would be the options:
A) Of course it is you moron, hurry up and edit the video
2) No way lizard brain, get back in the shed and keep rubbing them out
Comments
At some point you have to trust your results and decide if that is an acceptable standard. A grade in the back shed is a seriously good result. Apply the Pareto rule of 80% of the result is from 20% of the effort, and you are now into expensively diminishing returns.
Geoff Williams
2023-12-22 03:52:49 +0000 UTCAnd yes, they're wrung together. I'm using a 10, and 8+2, and a 9+1.10
NotAnEngineer
2023-12-10 22:43:46 +0000 UTCYou could try using gauge PINS instead of blocks, since that gets you theoretical LINES of contact.
Jonathan L
2023-12-10 22:43:08 +0000 UTCTolerance is unknown (the "inspection sheet" probably shouldn't be trusted"), but I checked the stacks against each other on the surface plate and my (admittedly, very cheap) .001mm test indicator shows no variation.
NotAnEngineer
2023-12-10 22:42:54 +0000 UTC6 micron actually puts you within grade A, which is used in inspection labs. Congrats!
Jonathan L
2023-12-10 22:40:35 +0000 UTCAs for temp, the measurement standard is 20 +/- 1 C. Humidity is 50 +/- 2%.
Jonathan L
2023-12-10 22:39:12 +0000 UTCThe method is good. What's the tolerance on those gauge blocks? And you are wringing them, right? The fewer blocks in the stack, the better (although supposedly the wringing film is already accounted for http://starrett-webber.com/GB46.html).
Jonathan L
2023-12-10 22:35:36 +0000 UTCAlso I should say I have no idea about how twisted it might be
NotAnEngineer
2023-12-10 22:10:59 +0000 UTCMade a mistake there, I meant to say 6 micron deviation. The three surfaces are bluing in the same pattern, with a slight concavity in the centre. Two 10mm gauge block stacks placed at either end between any pair of faces allows a 10.01 gauge block stack to slide freely in the centre for about 1/3 the length of the straight edges, and I do get some bluing scattered around that cavity. It's crazy how much heat comes into it at this level, at the end I could only give it a dozen or so stroke before the surfaces started to become convex. I think this is why it's taking so long to get rid of the low spot in the centre; it warms up, becomes a banana, and then I'm just lapping at the belly. Once it gets back to equilibrium, the low spot appears. Let me know if you can see any flaws in how I'm measuring the flatness, it'll save me from getting roasted later.
NotAnEngineer
2023-12-10 22:09:56 +0000 UTC+/- 6 microns = 12 micron deviation. That is within the ASME spec for grade B surface plates. Just over half of that will be grade A. A third of that would be grade AA (used in calibration labs). https://www.mitutoyo.com/webfoo/wp-content/uploads/15004A.pdf My vote is option 3: start editing bc you reached a standard, but continue until you reach A or AA status.
Jonathan L
2023-12-10 21:57:48 +0000 UTC
