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- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] In-Shop Surface Plate Inspection
- Date: Thu, 4 Feb 2016 19:48:04 -0500
http://www.practicalmachinist.com/vb/general/shop-surface-plate-inspection-315725/
Thread: In-Shop Surface Plate Inspection
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02-04-2016, 12:10 AM #1
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In-Shop Surface Plate Inspection
I have a Metro-lab surface plate, black granite and about 12 x 18" in size. I believe rated to +- 75 millionths, but don't quote me on that. It's supported on three pads at the recommended centered 5/9ths plate size separation.
It's old and I'd like to verify decent flatness before I scrape a small lathe, it hasn't shown any obvious wear spots but I'd like to be sure.
I have most of the usual inspection kit (Surface gauge, height gauge, mag stands, a tenths indicator, parallel bars, V blocks, etc)so is there any way I can do a poor mans calibration? Don't need to read or certify any figures, but a basic idea of flatness would be very well appreciated. -
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02-04-2016, 12:50 AM #2
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We just discussed this and Charles closed the tread in the reconditioning forum. check that out. Do you have a larger graded plate?
Here is Dons method. https://www.youtube.com/watch?v=hui6pPZH36U
will get you close -
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02-04-2016, 06:15 AM #3
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Might seem a waste but a new import replacement plate may be the lowest cost solution. A 12" x 18" plate from Grizzly costs $41 plus shipping. An 18" x 24"- $56. That's about 1/3 a cal lab's minimum charge.
Demote the existing plate for abrasive sheet lapping and general crude work and save the new plate for work requiring special accuracy - like generating skinny straight edges, scraped prisms, and scraping the flats of female dovetails and gibs.
A second lower grade plate is a handy thing to have. It greatly reduces accuracy deterioration of the good plate. '
In the end demote your worn out plate of all trades for use as a pastry slab. A good baker would kill for a large cool surface on which she can work bread and pie dough. My sister used a 20" x 30" hunk of 2" aluminum plate I scrounged for her.
Cal you find a 24" x 36" plate in recent calibration? You could set yours up on jacks in the center, adjust it parallel to the reference plate and scan your plate with a 0.0001" resolution dial test indicator on a transfer stand. Draw an error map. -
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02-04-2016, 06:43 AM #4
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I honestly don't know why you guys want to screw around with this type of thing. Here in Oz surface plates are expensive and I can't even imagine how much it would cost to have one lapped and re-calibrated. In contrast, last week I looked up how much a specialist company was charging in the US (I think it was Arizona, don't quote me on that however), and it was around 200 hundred bucks for a guy(s) to come out to your shop, check your plate, lap it to Grade A, and re-certify it. You then KNOW it's good and don't need to guess what you may have. The surface plate is your master reference in your shop, so I think for that amount of money it's worth the investment. Incidentally if it doesn't require re-lapping, it's around a hundred bucks to have the plate checked. A lot of people will spend more than that on a night out on town to piss it against the wall, so I really don't see this amount of money as being a show stopper! Not to get a reference you know you can trust! Once you have that plate you can then make all manner of other test standards, by self-proving parallel and square, but without a reliable plate you're frankly screwed.
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02-04-2016, 07:11 AM #5
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You also need to take a look at this video. One of Tom Lipton's. Its worth the 30 minutes. I get the impression they are some how related to Standridge Granite
https://www.youtube.com/watch?v=EWqThb9Z1jk
You have to keep this scraping caper real.It's old and I'd like to verify decent flatness before I scrape a small lathe, it hasn't shown any obvious wear spots but I'd like to be sure.I believe rated to +- 75 millionths
It's a lathe, not a Jig borer. "B" grade plate will achieve anything you want to achieve with a lathe. -
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02-04-2016, 11:10 AM #6
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Starrett actually sells a 3 point jig that you put a sensitive indicator into the 3rd leg. The 3rd leg is hinged and will read out any tiny variations. Move it around randomly and flip it and do the same and you should have a pretty good idea of what you have.
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02-04-2016, 11:25 AM #7
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Don's method seems easy and reasonable with tool most of us have..but that just tells one the plate is poor. Don't think many can invest in the proper gauges if not in the plate business.
The bargain plates would make the stated grade and the name brands would exceed the stated grade IMHO.
Two hours shop rate (even 3)seems fair to get a name brand quality job..You cant compare a "just makes it" plate to a good one.
[a poor mans calibration?] if you had a 12" straight edge you trusted to 2 tenths or better you could do a blue-up test. It is not uncommon that you can find the good area of a poor plate.
A plate that has been turned often and so has an even bowl is hard to determine poor with simple methods..the blue test will show the bowl. -
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02-04-2016, 11:28 AM #8
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Do you mean the "Rahn Repeat-O-Meter?"
Originally Posted by yugami
Starrett actually sells a 3 point jig that you put a sensitive indicator into the 3rd leg. The 3rd leg is hinged and will read out any tiny variations. Move it around randomly and flip it and do the same and you should have a pretty good idea of what you have. -
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02-04-2016, 11:32 AM #9
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I'm subscribed to Tom Lipton on YouTube, so I saw it when it was first released. It is a good video. Originally Posted by machtool
You also need to take a look at this video. One of Tom Lipton's. Its worth the 30 minutes. I get the impression they are some how related to Standridge Granite
https://www.youtube.com/watch?v=EWqThb9Z1jk
You have to keep this scraping caper real.
It's a lathe, not a Jig borer. "B" grade plate will achieve anything you want to achieve with a lathe.
I know I don't need anything super precision to scrape this lathe in question, but if this plate has a hidden wear spot I want to find it and avoid that part. I also have a ground cast iron plate, from Xerox. That thing is out over ten thousandths, and it's not a lapping plate! I did some scraping on that before I realized it was a dish and it was impossible to produce accurate and repeatable prints. -
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02-04-2016, 05:49 PM #10
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One thought...
A 12"x10" surface plate isn't in the small fixed rate USPS shipping box size, but it certainly isn't expensive to ship.
What price can you get from a company to certify and re-lap, if needed, when you ship to them instead of them driving out to you? -
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02-04-2016, 07:50 PM #11
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a 18"x24" grade "B" plate is within 0.0003" an AA grade is 0.000075" overall, a calibration service can easily make your grade B into a grade AA Originally Posted by Forrest Addy
Might seem a waste but a new import replacement plate may be the lowest cost solution. A 12" x 18" plate from Grizzly costs $41 plus shipping. An 18" x 24"- $56. That's about 1/3 a cal lab's minimum charge.
Demote the existing plate for abrasive sheet lapping and general crude work and save the new plate for work requiring special accuracy - like generating skinny straight edges, scraped prisms, and scraping the flats of female dovetails and gibs.
A second lower grade plate is a handy thing to have. It greatly reduces accuracy deterioration of the good plate. '
In the end demote your worn out plate of all trades for use as a pastry slab. A good baker would kill for a large cool surface on which she can work bread and pie dough. My sister used a 20" x 30" hunk of 2" aluminum plate I scrounged for her.
Cal you find a 24" x 36" plate in recent calibration? You could set yours up on jacks in the center, adjust it parallel to the reference plate and scan your plate with a 0.0001" resolution dial test indicator on a transfer stand. Draw an error map.
. for about 200 bucks.
dee
;-D -
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02-04-2016, 07:51 PM #12
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a 18"x24" grade "B" plate is within 0.0003" an AA grade is 0.000075" overall, a calibration service can easily make your grade B into a grade AA Originally Posted by Forrest Addy
Might seem a waste but a new import replacement plate may be the lowest cost solution. A 12" x 18" plate from Grizzly costs $41 plus shipping. An 18" x 24"- $56. That's about 1/3 a cal lab's minimum charge.
Demote the existing plate for abrasive sheet lapping and general crude work and save the new plate for work requiring special accuracy - like generating skinny straight edges, scraped prisms, and scraping the flats of female dovetails and gibs.
A second lower grade plate is a handy thing to have. It greatly reduces accuracy deterioration of the good plate. '
In the end demote your worn out plate of all trades for use as a pastry slab. A good baker would kill for a large cool surface on which she can work bread and pie dough. My sister used a 20" x 30" hunk of 2" aluminum plate I scrounged for her.
Cal you find a 24" x 36" plate in recent calibration? You could set yours up on jacks in the center, adjust it parallel to the reference plate and scan your plate with a 0.0001" resolution dial test indicator on a transfer stand. Draw an error map.. for about 200 bucks.
dee
;-DLast edited by dcsipo; 02-04-2016 at 07:52 PM. Reason: duplicate
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02-04-2016, 08:06 PM #13
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I looked at my plate, it ACTUALLY reads .000035. Yes, +- 35 millionths. Or at least back in the mid 70's when it was made and stamped. Originally Posted by dcsipo
a 18"x24" grade "B" plate is within 0.0003" an AA grade is 0.000075" overall, a calibration service can easily make your grade B into a grade AA. for about 200 bucks.
dee
;-D -
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02-04-2016, 08:14 PM #14
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Surface Plate Tolerances Originally Posted by Spyderedge
I looked at my plate, it ACTUALLY reads .000035. Yes, +- 35 millionths. Or at least back in the mid 70's when it was made and stamped.
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- [machinist] In-Shop Surface Plate Inspection, Lawrence London, 02/04/2016
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