The Shop > Tools
Granite surface plate problem
PekkaNF:
--- Quote from: sparky961 on January 17, 2017, 06:05:39 AM ---
If the plate you have is in such horrible shape, is there any harm in trying? Mess it up and you still have plenty of material for professional resurfacing later on.
--- End quote ---
Well... the undulation somewhere 0,01 mm or something like it. Pitting normally does not matter, because any instrument will bridge over it. it Might be good for garage.
But it is not good enough for checking the straight edges, cast iron surface plates, laps and other references. Therefore I need one good one inside, where it is on pretty constant temperature, little drift, good light, less dust etc....
Very probably I will check it against next surface plate, see where it needs leveling, prepare as good laps I can make and try to lap it bit more true. After all I can do most of the stuff I need with it and if I have "non metrology" surface plate in garage it saves me from going back and forth.
i would very much like to hear more details how the lap was made straight, grit and measurement of the flat....that is hard to me because I have no coordinate measurement machine or planikator at disposal.
Pekka
sparky961:
You have presented a bit of a paradox here. In your first post you tell about using six different straight edges to check the surface plate. Now you're talking about needing the plate to check the straight edges - presumably the same ones.
The whole point behind the 3-plate method is that you can ORIGINATE a flat surface (THREE, for those who didn't pick up on that already). By definition, that means that by the very process you will end up with a flat surface (or three... ok, I'm done now). With this flat surface you can check how flat other surfaces are by comparison. The "lap" you speak of is each plate against the other two in turn.
Refer to "The Foundations of Mechanical Accuracy", Wayne R. Moore, Page 24.
They describe the process using cast iron surface plates and scraping but the procedure is identical for granite and abrasive. When rubbed together, high points/areas on "A" will be worn down by adjacent touching spots on "B". Likewise, those spots on "A" will remove material from "B". Do it enough, with sufficient rotations, the same amount with each pair, and using finer and finer abrasives (getting close to flat just the two together with some water is plenty abrasive enough) and you WILL end up with flat surfaces. You verify them against one another.
If you're a real masochist, you can do the whole process twice thus ending up with six plates. Compare between the two batches to remove all doubt.
I'd love to make a machine that does this automatically while I sleep, and sell the extremely flat plates for a hefty profit.
..... But more to the point, is this an intellectual exercise or do you really have a need for this level of accuracy? If you do, you can satisfy that need by spending A: a little money and a lot of time (originate a flat surface), or B: a lot of money and a little time (purchase new or resurface professionally). "A" is the typical hobbyist approach, while "B" is commercially favoured.
With the latter, you still can't verify the flatness without using a comparison surface of equally unknown flatness. You're just trusting that it is as good as they say it is. You can probably trust a reputable company, but when it comes to Chinese goods...... well, we've all been there, eh?
sparky961:
--- Quote from: PekkaNF on January 16, 2017, 08:00:03 AM ---...Then I got one almost free...
...edges chipped, dents all over, one end rough...
--- End quote ---
This should have been your first clue that it was not treated like the precision instrument it should be. Where I work, the surface plate might as well be just another table. I expect this is fairly common except in places where they're working with incredibly tight tolerances.
--- Quote from: PekkaNF on January 16, 2017, 08:00:03 AM ---Looks like the surface plate has low front, worn on the middle and rough spot on one end.
--- End quote ---
This is the typical wear pattern, except for the "rough" spot. Ideally a surface plate should be turned occasionally and the enlightened craftsperson will try to use different areas of the plate to spread out the wear. Instead, most shop monkeys will have their coffee sitting in one corner, a greasy rag smeared across the front, and their 2lb ball peen hammer sitting in the middle right next to that big missing chunk.
PekkaNF:
Yeah, it should have been a hint.....it still surprices me that such a precision instrument is treated as a kitchen table or worse.
There is no paradox....I haven't checked the straight edges conclusively yet....I have two each length camelbacks and one square flat and one trianangllasr one....easy to cross check that all but one are close to straight, but each two does not agree completely. Now if I had a absolutely good surface plate it would be easy to veryfy which way the slight error exists and what sort of action it warrants.
Originally I used all traight edges (some pretty good and the pretty good ones agreed close, but not exacly on each others) to see if they print consistely on the surface plate. If best traight edge prints the same on all sides, crossed and diagonals then i would have thought the surface plate is ok and probably is good enough accuracy.
But the prints did not stay constat on every straight edge.
Therefore my conclusion is that surface plate is not good enneoug to check those straight edges. One side of 800 mm straight edge printted fairly constantly union flag and poor on the better side of the plate, I deem that side sligtly convex, other side aproximed more straight.
I think.....
Pekka
PekkaNF:
Some pictures of the hangar queen.....
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version