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| Making a granite surface plate |
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| John Hill:
--- Quote from: Colh on November 18, 2010, 02:58:52 AM ---Hi Guy Checked the granite plates using the 300mm ruler from a Rabone CHesterman combination square set (set barely used, ruler still in orginal paper sleeve) as a straight edge and feeler gauges. Not looking good. The plates appear to be concave in one axis probably due to the polishing machine being out of tram. The worst had gap of around 4 thou in the middle of the 300mm rule with the best being about a thou. After reading what Franklin D Jones had to say, I think I might have a go at making 3 500mm straight edges (to reach across the diagonal of the plates)by fixing all three together, milling one side then scraping A to B & C, B to C, etc as per Mr Jones' writing. I will then use one of the new straight edges to test the plates again to confirm original checks. I think I will have to lap the plates against one another, again as per Mr Jones. Given the amount the plates are bowed I think I will start with the 240grit silicon carbide and then something finer to finish. Given that most things used on the plate will be in the order of 10s of mm long then the course finish provided by the course silicon carbide is negated in the first instance. A finer finish will be required if I want to use bearing blue to test flatness. May be a while before I get the this as I have to tram my mill and then make one of Bogs' tramming tools (two new DTIs should arrive next week while I am away for work). I am also half way through making a Harold Hall designed grinding rest. Will continue the thread when progress is made. Col --- End quote --- Colh, if I understand correctly, if you lap those three together in pairs you will eventually have three flats. If A fits B and B fits C and C fits A they must be flat. |
| RichardShute:
You are right about the A-B, B-C, C-A aspect for checking either a straight edge or surface plate, but I'm not sure about the grinding / lapping scheme. Grinding two surfaces together like that is _one_ way to make a lens and that's certainly not what you want here. If you use a seperate lapping block with suitable compound to do the lapping that would be fine. I should add I've never actually done the job myself so it's only a speculative comment.... so far. I have recently come by a battered camel back (for 99p! - it would have been rude to refuse) and fancy having a go at getting it back to spec. Richard |
| John Hill:
Richard, if you lap two together you will almost certainly end up with lens shapes, one concave, one convex. But if you cycle through three surfaces you will be lapping concave to concave and convex to convex as you go along and the end result will be flats. If you have three surfaces which can be shown to match each other, and they slide in all directions, then you have flats. They cannot be anything else. For two surfaces, if they match and slide over each other you may have sections of a sphere or they may be flats. But if you can do the same with three surfaces they must be flat! I dont know about straight edges such as your camelback but I assume the home shop way of recovering that would be to first grind/lap your flats. |
| RichardShute:
Hi John, yes, you are quite right, the typing got ahead of the thinking in my earlier comment, that and paranoia of possibly giving duff gen. I have a second 'known to be pretty reasonable' straight edge, longer than the camelback, but I would need a third for the traditional scheme. I'll have to see what turns out easiest. Cheers Richard |
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