Checking "long" straight edge from "short" surface plate

G'day guys, I wonder if you can tell me if it's still considered legitimate ie accurate, to spot and if necessary correct a straight edge which is longer than the diagonal of the master surface plate? In other words the straight edge would need to be done in stages. If so, how much "overhang" would be ok?

Thanks,
Pete

If you rig up a sensitive DTI with a long arm on a mag base clamped to a 'flat' (small surface plate) upside down on the big surface plate, you can extrapolate the flatness of the plate out to maybe 0.7 of the diagonal.
A lot then depends on the sag of the cantilevered overhanging straightedge, which varies with the cube of the depth of the straightedge.

You can evaluate that with the rig I described, by starting with the end of the straightedge just on the plate. Qualify the part of the straightedge that's on the plate with the DTI, then move the straightedge and the DTI progressively off the plate until the self-weight deflection starts to be a problem.

Obviously the DTI is essentially for picking trends rather than evaluating local flatness, but you can check the latter by spotting even if the straightedge is twice as long as the plate, provided you dream up a suitable way of supporting the far end of the straightedge. It's the straightness trends over a distance significantly longer than the diagonal which you can't check this way.

Having said all this, I'm a metrology munchkin as you can probably guess from my post - you'd probably get more sense over on the new metrology forum.

You can with great care work a straight edge about double the diagonal length of a surface plate. You need to take it in sections working careful overlaps so ensure the print on the lap matches center to end. Be careful how you handle the straight edge and limit the heat you put into it with your hands directly or by body heat radiation. Make sure any overhung part of the straight edge is lowered to contact the reference plate last.

Periodically, set the straightedge up on the surface plate supported on its insulators and indicate in its reference edge to the surface plate. This will double any error present. Use a "slip" to average out the scraped surface. Also by setting the straight edge under test on a solid surface and carefully leveling it a short 10 arc second level can be used to prove its linearity. By solid, I mean concrete or a large machine tooi bed or table. A wood bench or a castered tool chest will not do; the weight of the level will deflect the system and affect the readings.

There's a lot of quirks and tricks to calibrating surface plates and straightedges equipment from undersized references. Very credible work is possible but only by careful checking and cross checking can any work dome this way pass third party calibration.

Hi Pete,

It depends on what level of accuracy you wish to achieve.

If you're talking "real" accuracy, i.e. 50 microinches per foot or so, you need a surface plate longer than the straight edge. If you're interested in a half thou, perhaps you could do it.

One source of error is support of the straight edge. If it's not supported along its entire length, and accurately so, you introduce bending and bowing which translates to erroneous readings. So although you'll get readings, you can't believe them.

- Leigh

Ok well basically the story is that, as per previous posts, I'm VERY new to this so am setting myself up. If you recall a post about granite plates a week ago, I said that I need a surface plate so I'm trying to decide what size I will need. Unlike the USA, the surface plates are NOT cheap here in Australia, and if you start going big the pricing gets just stupid. One job I do want to do is restore my lathe. For that and pretty much anything else I can think I am likely to do, I figure a 1m straight edge should be fine. The trouble is it's likely to be used and then how to check that? I figure if I'm about the shell out a small fortune for a decent surface plate that's about to become my master reference I better be darn sure it's what I need!

Regarding the DTI, I have a Compac 215G coming which I'm led to believe is ok.

Hi Pete,

OK. Now that I understand the goal, perhaps I can make a couple of suggestions.

Hobbyists typically use small surface plates, with 12" x 18" perhaps the most common size. These are not terribly expensive, but that's obviously a relative term. Finding a used one in good condition (ebay or whatever) might be a reasonable option.

For the straight edge, I would suggest looking around for a machine rebuilder and see if you can use their plate. A friend of mine has a rebuilding business, and his surface plate is 72" x 144". Such businesses have large plates because that's what they need. An obvious advantage is that you know the plate is flat and in good condition.

Unless you're a real glutton for punishment, you'll probably find that your lathe will work just fine even if it doesn't quite meet original manufacturer's accuracy specs. Getting a used machine back to original condition can be a huge undertaking.

Good luck with the project.

- Leigh

It was pretty well hashed out in the previous thread on granite plates I mentioned, but to give you an idea, the cheapest surface plate I've been able to find here in Australia so far is about 500 bucks! Not quite the 28 dollar Enco specials you guys may be accustomed to

Yeah the attraction to restore old machinery probably makes as much sense as restoring old cars, you can buy a new one for half the cost and a quarter of the headaches Having said that, like everything else around my place, it will never really feel like it's truly MINE until it's been stripped down and rebuilt.
Pete

Getting a used machine back to original condition can be a huge undertaking.

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Understatement of the year!

Don't forget, other things you will need include angle block, reference piece for the way angle, a setup to bring the UNDERSIDE of the ways "in" with the topside, accurate level for 'survey" etc, a setup for the level to check "wind" in the ways (depending on way type), etc, etc. Preferably also a test bar for the spindle to check alinement, and lots of patience.

Oh, yeah, and another lathe and a mill to make 'stuff" you need in the process...... plus a very thorough knowledge of what you need to attain, and how to get there.

The last is only really obtained by some book learning plus experience, so that isn't necessarily a total stopper.

JST said:

The last is only really obtained by some book learning plus experience, so that isn't necessarily a total stopper.

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Well I bought Connelly's book a while back so that's a start

Yes I hear what you're saying and of course it will depend on just how far I want to go with all this. It's very unlikely to be bought back to factory spec. but it would be nice to be able to see where I'm at with things. Even if all I do is strip it, clean it and repaint it, I could probably do worse than checking where the worst wear is and just how difficult it will be to rectify while it's apart.

Did you ever check with a granite monument shop to see what they can do for you? Some of those guys are pretty sharp. I imagine if you explain what your goal is to enough of them you will likely get some very useful information and maybe a gift. Small cheap surface plates are not that great. Most buyer's of that kind don't have the means of checking the flatness and just conveniently assume the plate is flat. All the low priced Grade B plates are too thin. There is a federal standard for surface plates and how they are to be inspected that I have seen posted on the internet for downloading somewhere.

I agree with the comment about restoring a lathe. It is much better to have a working lathe than one sitting around in pieces. Better to spend your money buying another lathe and selling your "needs restoring" quality lathe. You can eventually upgrade to a good lathe that way and save all the worry and money for surface plates and straight edges. Depends on whether you just want a good lathe or if you want to play with scraping cast iron. Nothing wrong with either one. Eventually someone will buy a monument for you so use your time well.

Close Work said:

Did you ever check with a granite monument shop to see what they can do for you?

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No. It was a good idea, but in the end I decided there was no way I could guarantee any specific degree of flatness. If all I was doing was using it for layouts it would be just fine but IF I decided to start removing metal from this lathe (or any other machines) then I want to be pretty darn sure that the reference is good. Indeed the more I learnt about it, the more I realised that the surface plate was a rather important reference and pretty much every other standard would be traced back to that. Now I'm getting a much better idea of precisely what I need I'll probably be patient and wait for a good ex-government one or similar to come up at auction.

Pete

I think we have at least seven surface plates of different sizes with quality up to a couple labeled AAA and all were bought at auction or surplus. The thickness is important because granite is not very stiff, about a third of the stiffness of aluminum and 1/10th the stiffness of steel. The AAA grade plates are around 2ft. thick. I thought surface plates didn't get better than AA Grade until I stumbled on to these two Mojave and read the plates on them.

Close Work said:

The AAA grade plates are around 2ft. thick. I thought surface plates didn't get better than AA Grade until I stumbled on to these two Mojave and read the plates on them.

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Unfortunately, the accuracy represented by a particular "grade" designation varies with the size of the plate. A small A plate can be more accurate than a large AA plate.

All plates are available with 25 microinch precision if you specify same (and are willing to pay for it). These are frequently called 'AAA' grade, but that designation does not appear in the federal spec.

The accuracy of most granite products is given in "microinches per foot". The best I've seen is 25. I have a granite angle and a set of granite parallels which are rated 25.

- Leigh

Ref: Federal Specification GGG-P-463c page 6, section 3.3 Work Surfaces