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Thread: Calibrating a Surface Plate
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7th Apr 2013, 04:16 PM #1
Calibrating a Surface Plate
This might turn into a long running saga, or it might have a happy ending...
We (Josh & I) picked up a 72"x48"x10" surface plate at Auction just before Easter, and we got it into the workshop a few days ago, the question, now, is what exactly is it?.. It might be a complete dud.. I think I can remember Phil saying when we went to look at it before the Auction, something about being a good headstone.. could be more truth in that than I care to admit...
The surface is scratched and chipped in places, and the edges are chipped, the chipping on the edges is not such a problem, and we can live with a few scratches, but if it's badly worn it will be useless as a surface plate. If it's good enough we can use it for scraping larger parts. Depending on how it's worn will determine if it can be lapped back into service...
We got three lines done last night, but abandoned today's effort due to hassles with temperature fluctuations and draughts... we will try again later tonight.
One thing that was interesting, was a chip that had been repaired with some kind of epoxy crap, the area surrounding the repair measured high, probably due to a film of epoxy left after the repair, whereas the actual repair measured low.
The moral is don't try to fix chipped granite with epoxy, it would have been better just left alone.
There are no markings of any type on the plate, and only an engraved brass plate with the number 2607 rivetted to the steel frame, no idea what that means, maybe an asset number?
Just going by the plate thickness of 10" it's possible that it was a Laboratory grade AA at some point in it's life.
The numbers I have for the various grades of 72x48 plate I think it's Fed Spec GGG-P-463C
AA Grade (Laboratory) 48 x 72 x 10 Overall +-175 uinch Repeat +-30
A Grade (Inspection) 48 x 72 x 8 Overall +-350 uinch Repeat +-40
B Grade (Toolroom) 48 x 72 x 6 Overall +-700 uinch Repeat +-80
The repeat spec is for a 1" square, but the ISO spec for repeat is a 10mm ball?... that might mean we can meet Fed spec for repeat, but fail ISO if the ball rolls into a chip hole..
Final numbers in a day or so, then we will know if it's time to learn how to lap granite or not...
Regards
Ray
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8th Apr 2013, 09:32 AM #2Most Valued Member
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Hi Ray,
I wait in anticipation on the results. I hope you got a bargain rather than a boat anchor!
Cheers,
Simon
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8th Apr 2013, 09:59 AM #3
Ray as always ou start the ball rolling and have me doing searches to see what i find.
Heres one such very pro-type set up NT100 cutting centre with 100 disks for granite - YouTube
Then there is of course the Asian one Granite manufacturing in China 3-2011 - YouTube
I look forward to seeing what you can do I hope it comes out well.
Edited to add this very professional very long 30 mins presentation on surface plates Gauging Quality, Episode 1 - Surface Plates - YouTube
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8th Apr 2013, 12:02 PM #4
Hi Ray and Josh,
Looking forward to seeing how it goes. Can the interferometer do the ROM test too, or will you need to use something like the gauge Josh set up for testing square with?1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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8th Apr 2013, 02:06 PM #5
For some reason doing calibrations late at night seems to give more consistent results than daytime, everything is still and temperatures are more stable. We finished up doing the standard 8 line union jack pattern and repeated each line 3 times. consistency between successive runs was better than 10 uinches.
The way the laser interferometer works for measuring flatness is the same as the differential level method, the path length changes as the retroreflector tilts, so one of the parameters in the calibration calculation is the footprint size of the fixture, we've got the 2" 4" and 6" fixtures, and we went with the 4" fixture.
The calibration lines cross at various points like the center of the plate and middle of the edges, after applying the corrections you can tell if you have a good set of readings by the "closure" of the lines, this is a measure of the repeatability, and accumulation of measurement errors, and in the HP literature they recommend a closure of better than 100 uinches is a good calibration, we got a 3 line closure of less than 4 uinches ... ( ok, sorry that was bragging...), the bad news is that one of the other closure points was off by 160 uinches, so we'll have to repeat those measurements.. might have been some dust?
So now we have to repeat a few measurements and we can finalize the survey.... the initial numbers look encouraging...
Ewan, the repeatometer test is still the subject of on-going discussions... Thanks for the video's Ray, that Mahr video is excellent..
Regards
Ray
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8th Apr 2013, 03:04 PM #6Most Valued Member
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It's funny you mention this. It brings back memories (many moons ago) while studying at RMIT. Whenever we wanted extremely high magnification images with the SEM (Scanning electron microscope) it was best done at night when the trams and trains were less frequent and all was stable and still.
Such high resolution and accuracy requires such fussy conditions!
Can't wait for further info on your plate!
Simon
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8th Apr 2013, 09:23 PM #7
Calibration is complete.
The map of the surface plate looks like so...
All dimensions are in micro-inches ( millionths ) The purple areas are the least worn and are the highest regions of the plate, the reddish areas are the most worn areas, and it looks like the plate was against a wall and the majority of wear is along the edge facing into the room, except for a corner on the right?
Overall the plate meets the flatness spec for B grade, and if we were to lap the purple area down it would come into flatness spec for A grade, likely as not is was probably once AA grade, but not anymore
It's annoying, but not unexpected that we'd have to lap the least worn section to get to A grade. ( sigh )..
The repeatability spec for A grade is +-50 uinch, and for a B grade it's +-80 uinch with a 1" square footprint. At this stage we are wondering how to do the repeatability test...
Anyone got a spare repeatometer?
Regards
Ray
PS Interesting stuff Simon, we don't have problems with trams and trains, but night time is still better for quiet carefull measurements..
EDIT: Forgot to post a picture of the laser interferometer setup, the corner reflector in the foreground is not in use for this line..
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8th Apr 2013, 10:15 PM #8
Ray, I cant quite follow your colour coding. What's light blue and green identify? if it is a range, then I can see discrepancies, if green is flat spots (identical adjacent heights), then you missed a couple....
Might be good to put some more colour ranges in to get a 'false colour topography' picture of the whole thing (maybe conditional formatting in Excel).Cheers, Joe
retired - less energy, more time to contemplate projects and more shed time....
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8th Apr 2013, 10:41 PM #9
Hi Joe,
The green spots are intersections of measurement lines, the HP procedure deals with the calculation of offsets and those intersections will be the same elevation on both lines, or in the case of the central point where there are 4 intersecting lines the difference ( or closure) is a measure of how reliable the measurements are, they suggest if closure is greater than 100 uinches, you should repeat the calibration, better than 100 uinches is good.
The light blue border on the bottom and on the right is just the 4" grid layout.
Hope that makes it clearer... any more complicated I'll have to get Josh to explain..
Regards
Ray
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8th Apr 2013, 11:06 PM #10
Good work.
The blue had me confused too, clear now though.
Why not lap the purple areas and then everywhere else down to the red....Josh is the one that will be doing it right?
Not ever having seen a ROM, this maybe way off but isn't it just a large flat base, long enough to average out any errors and an indicator? You have the indicator and the means of making a base. Or are they far more complicated?1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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8th Apr 2013, 11:11 PM #11Banned
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I've seen one and one only Rahn repeatometers on Ebay. And that would have been a good 5- 6 years ago and they wanted $1200 for it.
I wrote to Forrest Addy at the time. Pointing out that one was actully on EBay. He wrote back with some thing like. "I dont need to buy one right now"
I never suggested that he should buy it. I was just giving him fodder to pontificate on.
Regards Phil.
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9th Apr 2013, 08:45 AM #12
Hi Ewan, don't know for sure, but I understand there is a special hinge that links the two blocks, I've not seen any details of the hinge design, there is a description on PM repeat-o-meter
And I see you can still buy them new.. Tru-Stone: The world's largest manufacturer of custom precision granite
I Always like reading Forrest in pontificate mode...
And yes, Josh is keen to do the lapping.
Regards
Ray
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9th Apr 2013, 09:10 AM #13Most Valued Member
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Ray I have no idea as to what's actually involved in lapping a surface plate. I would have thought it quite straight forward, at least in principle if not in practice, but have you read the information in the Machiner's Handbook regarding it?
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9th Apr 2013, 05:41 PM #14
Hi Pete, Thanks for that, (I haven't checked Machinerys Handbook yet) as far as I know, the lapping is done with a cast iron lapping plate, The ones I've seen usually have those cross-hatched grooves and the compound I imagine is diamond paste of different grit sizes
I understand that the guys who do the lapping in the factories where they make granite surface plates are all built like weightlifters..
Regards
Ray
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9th Apr 2013, 07:22 PM #15Philomath in training
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This isn't going to turn into "Honest Josh's reconditioned surface plate yard" is it? (One owner, only measured things on Sundays...)
Seriously though, I was looking for some one a while back who could check a surface plate and couldn't find anyone. If you had the laser calibrated you might find that there was a market for checking & recertifying plates.
I have a sniff on a real live repeatometer. PM me if you are interested and I can fill you in.
Michael
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