Surface Plate: Objective Standards?

[Uglydog]

[h=2]According to Connelly I will need a surface plate in order to scrape.
I have a 12x24 cast plate which has visible gouges secondary to presumed repetitive motion of an inspection device ($10 at a rummage sale).
I ordered one of the Busch 48inch straight edges. Raw cast, needs to be finished.
I will need a standard to calibrate both of these.
Found this granite surface plate 30x36 - $100 on CL. Price is right.[/h]http://minneapolis.craigslist.org/ank/tls/3782705105.html
[h=2]Looks like a garage item, vs industrial.
A 30x36 table gives me a hypotenuse of 46.86inches, not quite 48inches.
Not supposed to run over the edges. Thus, not good enough. Or, is it close enough? What does it do to my tolerances?[/h]Questions:
1) how do I identify if this granite plate is flat if I don't have a flat edge to assess it with?
2) is the "3 plate" method my best option?
3) other options?

Daryl
MN

 

[Richard King]

I know a guy who laps plates and tests them for accuracy who is in the Twin Cities. I will have to get it when I am back home this weekend. If the plate was in a shop it usually has a sticker on it show when it was tested and to what accuracy. We use a spec of .00005" per 12" when we scrape straight edges. But you never scraping anything you will need a friend to show you how. Or teach you. You can also get it ground. Call around up Ron at Precision Machine Rebuilders in Rogers and he can grind it fr you. Probably hold .0001" which should be close enough for anything you will need. I hope all you guys are telling Busch I told you to call them. Scratch my back so to speak.
Rich

 

[Ray C]

Unless you can transport it yourself, watch for shipping costs. I purchased a Grade B (12x18) for 50 bucks and spent another 60 on delivery fees...


Ray

 

[Tony Wells]

Daryl, for one thing, the average hobbyist, or even the average job shop and small production shops won't have the necessary equipment to evaluate a surface plate. It's a specialty. So it's really not practical to approach a used plate with that in mind. You have a choice of hiring it out, or trusting the sticker if there is one, as Ray mentioned. If I were going to use one to do a straight edge, I'd go ahead and either just buy a new one, factory certed, or if it were a bargain, buy it cheap and have it lapped to whatever standard I needed, and pay the piper. I know too many sticker-slappers. If the plate is granite, and black, it's relatively soft and the date should be closer to now than if it's a pink, which has more quartz content and hence harder. If you feel confident that it hasn't been used much, and trust the sticker....and want to take your chances.....go for it, but for making my own straight edge, I wouldn't want to. If I'm going to all that trouble, I want it to be the absolute best I can get it.

Richard's suggestion of simply having it ground certainly has merit. If you don't really need a surface plate other than for this, you could save a lot of work (and learning to scrape on an important piece) and just have it ground. Probably not all that costly. 0.0001 is nothing on a good grinder.

 

[komatias]

You need to ask yourself: WHY?!?!?!

Is it all an exercise to learn? If so, then the limit of length is ok and a good B grade will suffice. If you want to achieve perfection then you need good tools and only worth the effort if your bent on reconditioning a machine.

You could always remove a couple of inches off the casting could you not?

 

[Uglydog]

I know a guy who laps plates and tests them for accuracy who is in the Twin Cities. I will have to get it when I am back home this weekend. If the plate was in a shop it usually has a sticker on it show when it was tested and to what accuracy. We use a spec of .00005" per 12" when we scrape straight edges. But you never scraping anything you will need a friend to show you how. Or teach you. You can also get it ground. Call around up Ron at Precision Machine Rebuilders in Rogers and he can grind it fr you. Probably hold .0001" which should be close enough for anything you will need. I hope all you guys are telling Busch I told you to call them. Scratch my back so to speak.
Rich

Yep, Busch knows that I received the lead from you. I'll give Precision Machine Rebuilders in Rogers when the straight edge arrives (0.0001").
The CL ad that I posted the link to appears that it is in a garage. Certainly not committed to it. Merely trying to learn how to best proceed.

Thank you,
Daryl
MN

- - - Updated - - -

...average job shop and small production shops won't have the necessary equipment to evaluate a surface plate. It's a specialty. So it's really not practical to approach a used plate with that in mind.... .

New information.
Thank you,
Daryl
MN

- - - Updated - - -

You need to ask yourself: WHY?!?!?!

Is it all an exercise to learn? If so, then the limit of length is ok and a good B grade will suffice. If you want to achieve perfection then you need good tools and only worth the effort if your bent on reconditioning a machine.

You could always remove a couple of inches off the casting could you not?

Great question komatias,
Perfection?
I'm not sure what that is.
It seems that everything is about tolerances, with some work closer that others to perfection.

And yes, I've got several machines for which I'm hoping to improve the accuracy.

Daryl
MN

 

[DMS]

$100 seems like a pretty decent deal, even if that plate has seen better days. They can be re-finished. If the cost isn't too high, it may be the best route.

I purchased my surface plate (cheap, grade B import) for scraping. I got a cheap one because I wasn't sure how much scraping I was going to do, and it didn't seem that useful for anything else. Boy was I wrong. It's not something I use daily in the shop, but I do find it great for taking dimensions off of things, doing layout, etc.

 

[Richard King]

Daryl, for one thing, the average hobbyist, or even the average job shop and small production shops won't have the necessary equipment to evaluate a surface plate. It's a specialty. So it's really not practical to approach a used plate with that in mind. You have a choice of hiring it out, or trusting the sticker if there is one, as Ray mentioned. If I were going to use one to do a straight edge, I'd go ahead and either just buy a new one, factory certed, or if it were a bargain, buy it cheap and have it lapped to whatever standard I needed, and pay the piper. I know too many sticker-slappers. If the plate is granite, and black, it's relatively soft and the date should be closer to now than if it's a pink, which has more quartz content and hence harder. If you feel confident that it hasn't been used much, and trust the sticker....and want to take your chances.....go for it, but for making my own straight edge, I wouldn't want to. If I'm going to all that trouble, I want it to be the absolute best I can get it.

Richard's suggestion of simply having it ground certainly has merit. If you don't really need a surface plate other than for this, you could save a lot of work (and learning to scrape on an important piece) and just have it ground. Probably not all that costly. 0.0001 is nothing on a good grinder.

Daryl has an advantage, he lives 45 minutes from me. Around here there are several used machine dealers and several machine tool auction houses. It's pretty east to get a good plate. I can check a surface plate with a Starrett 198 level and a straight edge (Steel Rule type). Been doing it for years with lots of good results. Plus as I said I know a guy with an electronic level who laps plates in for a living. He demo'ed
how it is done at the MN scraping class we had last summer. I'll let you know....I'm home now. Rich

 

[loply]

Although it isn't going to give you a 'true' test of the whole plate, you can pop an indicator on a stand with the tip touching the plate several inches from the base and scan it around.

This won't test the plate to tenths but it may help you to identify if the plate is junk or not.

 

[Tony Wells]

I made my statement from a Metrologist viewpoint. It's probably far beyond the real requirements for machine scraping, but if you are buying a surface place for use as an inspection tool, you should know what you are getting. All I was really saying is that the average guy can't calibrate a surface plate. I can't, that's for sure. But I do know how it's done, and have had it done many, many times.

http://www.hudler.org/pub/HP/02-5952-0820 - App Note 156-2 - Calibration of a Surface Plate.pdf