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16th Jul 2020, 01:02 AM #1Golden Member
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Scraping a cast iron surface plate
I recently came into a 1245mm x 750mm cast iron surface plate. Unfortunately moisture had got under the cover and a lot of spots where heavily pitted the plate also had a number of scratches in it. It weighs 280kg so before I unloaded it at my place I took it to get surface ground it should be back within a week.
I would eventually like to scrape if flat enough to use to scrape straight edges and the like. I am investigating methods that allow scraping plates from smaller reference surfaces. I know this is possible and in fact a common on really large machine tools. I have found this video which shows the use of only a straight edge and an autocollimator to scrape a surface plate https://www.youtube.com/watch?time_c...ature=emb_logo unfortunately its not in English so the explanation of the technique is not clear.
Can anyone explain the method used here and are does anyone else know of other methods for scraping a larger plate from smaller reference?
Regards
Eric
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16th Jul 2020, 08:05 AM #2Senior Member
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Robin Renzetti just put up the first video in a series about using an auto collimator.
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16th Jul 2020, 01:56 PM #3Most Valued Member
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16th Jul 2020, 02:52 PM #4Golden Member
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Renzetti also did another vid re lapping in a large plate using a smaller plate: https://www.youtube.com/watch?v=Qm0QerRvtJQ. May not be useful for cast iron - dunno.
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16th Jul 2020, 05:35 PM #5
Hi Guys,
The three plate lapping method works well, but there are some gotchas.Best Regards:
Baron J.
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16th Jul 2020, 07:27 PM #6Golden Member
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16th Jul 2020, 07:32 PM #7Golden Member
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The plan would be to buy or acquire over time whatever was needed.
Those Japanese guys I linked to where clearly using a straightedge and a collimator to make that large plate flat. Having done some reading I think I understand how they went about it.
A 4ft or even 3ft straight edge should be quite possible to find in South Australia I would probably have to pay to have someone certify it as for collimator I am thinking US Ebay?
Of course it all depends on how flat it comes back from the surface grinder.
I have a Biax scraper so it isn't perhaps as brave as attempting it with a hand scraper.
If I mess it up I would just take it back to the grinder so the risk is kind of low.
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16th Jul 2020, 09:32 PM #8
Hi Guys,
Cast iron is perfect for lapping a cast iron plate. The fact that they are similar metals won't alter the lapping process.
Once the diamond grains have been pressed into the lap its the lapped surface that will have material removed from it. The lap of course will get clogged by the material removed and will have to be cleaned and re-charged with diamonds.Best Regards:
Baron J.
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18th Jul 2020, 12:17 AM #9Most Valued Member
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Eric, you'll need to live to 130+ to finish all those projects.
KrynTo grow old is mandatory, growing up is optional.
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18th Jul 2020, 01:05 AM #10Golden Member
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18th Jul 2020, 11:23 AM #11Most Valued Member
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18th Jul 2020, 12:27 PM #12Golden Member
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18th Jul 2020, 06:05 PM #13Diamond Member
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Must be a pretty dodgy grinding place, should be able to get it ground so you don't have to scrape it.
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19th Jul 2020, 12:45 AM #14Golden Member
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I would be surprised if the owner of a surface grinder big enough to grind that plate were dodgy.
Out of interest what sort of geometrical tolerances do you think a surface grinder should be able to hold over that size? (1245x750)
If it were flat such that it lay between two parallel planes 0.007mm apart then it would be a ISO grade 0 and between two parallel planes 0.014mm apart it would be a grade 1 (the other consideration being local flatness but I feel this is less of an issue with a surface grinder than with other methods of generation). If grade 1 was achieved I would be happy and the plate could be used as is for scraping the parts I actually want to scrape. Including a straight edge.
The guy driving the grinder thought he might achieve 0.01mm. However one of the grinding companies I visited with the plate indicated they would expect to achieve 0.025mm (albeit on a Blanchard grinder and it will in fact be ground on a peripheral wheel grinder).
I am not convinced that grinding will produce the geometrical flatness that you seem to think it should but maybe I will be surprised and delighted. Indeed I wonder why new granite plates are still hand lapped after grinding to achieve flatness?
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19th Jul 2020, 02:22 AM #15
Hi Guys,
Actually I would be very delighted to get 0.02 or 0.03. It would not only take a very good machine operator, but an extremely good grinding machine to achieve better !
I personally would be very interested to find out what is actually obtained !Best Regards:
Baron J.
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