Talk:Surface plate

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[edit] Shortages

The section about the use of glass seems to have some grammar problems, but I just want to check that I understand before editing. "when material and production dead lines were either in short supply or over stretched."? is this just a clumsy sentence or does "dead lines" have a special meaning here? Theoh 21:36, 17 January 2007 (UTC)

"Deadlines" is what the person meant. - Toastydeath 05:13, 18 January 2007 (UTC)

OK, fine, I have replaced that sentence with something less convoluted. Theoh 15:20, 18 January 2007 (UTC)

[edit] Granite Vs. Cast Iron

Wayne R. Moore, in Foundations of Mechanical Accuracy, details how the measured and verified long-term stability of properly poured cast iron is superior to the dimensional stability of granite and ceramics. Granite surface plates are fine for machines with lower accuracy requirements such as a CMM, but passing into millionths of an inch and beyond, cast iron is one of the few choices available for static, long term gage stability. Cast iron masters are used to produce laboratory-grade granite surface plates. The book was written in 1970; it is still in publication, and this section has not been revised by the Moore Special Tool Company. However, the age of the book has nothing to do with cast iron being a far more metrologically stable and suitable material for the extremes of accuracy.

I don't know how to link the citation I inserted to the section that needs it; my apologies. If someone would add it, I would be grateful. - Toastydeath 18:41, 26 July 2007 (UTC)

Lower accuracy measurement? Uh, I have to disagree with your recent addition to the article where I put the fact tag but my real world experience in high accuracy environments would be considered OR so I'll have to follow the rules and give in to your ref'ed material. --I already forgot 23:13, 26 July 2007 (UTC)

For the sake of clarity, I said "lower accuracy" as Moore Special Tool builds machines that have surfaces flat and parallel to a couple millionths of an inch over the entire machine envelope without any electronic compensation, and the machines still hold that accuracy today (arguably greater accuracy now than when the machines were first made, as they did not have the means to resolve the machine's full accuracy back in the 70's). The digital compensation maps that modern CMMs use to allow them to measure in the same realm that Moore was doing by purely mechanical means back in the 70's still have to be updated and re-certified as the granite shifts if the machine is to be used at its limit of accuracy. That being said, cast iron has a far greater thermal expansion coefficient and as the current article says, burrs, and is not useful in a laboratory environment not set up to create and handle master gages. In 98.5% of laboratory settings, a cast iron surface master is more detrimental to accuracy than a granite surface plate. I just wanted to add into the article that cast iron masters are still used, and where. - Toastydeath 05:24, 27 July 2007 (UTC)

Without addressing other issues, I added the fact tag to the statement that "cast iron remains most popular material for surface plates among machine builders, gage makers, and other high-accuracy industries". Having worked in the "high-accuracy industry", I personally know that the statement has a great potential to be false as I have never seen a cast iron surface plate used outside of being a lapping device to create more precise surfaces. If you have ever inspected and created objects to a .0001 of an inch or better, you now that thermal expansion and magnetism is a major concern during the inspection process and that cast iron could never be used for such inspection. Ever grind a semi-conductor die plate with gas channels that was held to .0001 or better that was .120 thick? That and numerous other high precision parts and gages could never be inspected on a cast iron surface plate because of the error created by the magnetism alone. The thermal expansion of a hand resting on a cast iron plate and oxidation would be enough to turn QC manager away from using cast iron for high precision parts so I find it interesting that the statement about cast iron remaining the most popular material would be added to the article as fact. I'm not saying it isn't fact, but its very suspect. --I already forgot 06:47, 27 July 2007 (UTC)

I can understand the skepticism, but without copying the first quarter of the book, I don't think I will be be able to convince you otherwise. I do recommend the book if you have interest, as it is still an industry reference. Perhaps you will still disagree. My personal experience has been doubts as to such claims in the beginning based on what I knew, but between actually getting the chance to read the book (it is difficult to find) and having the opportunity to speak with Jim Bryan, a pioneer in diamond turning and the gentleman who holds the patent on the telescoping ball bar, I became convinced that these principles are correct.

I do know that temperature is a huge concern. The book not only details temperature control and destratification, but also goes into the effects of gravity, sag, atmospheric pressure, indicator deflection, and others. It also details the control and correction of those factors as well. A granite surface master is not used as a surface plate - you do not inspect things on a surface master as you would a normal surface plate, and I am not suggesting anyone would ever want to use something that has the thermal expansion coefficient of cast iron for such tasks. The purpose of a cast iron master is to provide a flat surface; a surface, when flat, will be flat at 20 C and flat at 30 C, though dimensionally larger at 30 C than at 20 C. From this stable, accurate surface, you can then transfer the flatness or parallelism from the cast iron surface to the granite surface, for use in laboratory equipment. The cast iron plate can be used as an inspection surface, but this requires special training and consideration on the part of the inspector. When working down in the ten millionths and under range, any local heat change at all is going to invalidate your reading, no matter what material it has been made from.

I am not suggesting that cast iron is a superior material for inspection and I apologize if it sounded that way; trying to inspect a tolerance of .0001" or .00001" is well suited to a granite surface, and the disadvantages of working with a cast iron plate would completely outweigh the benefits. Cast iron surface plates are used as master references for flatness; I will change the article to better reflect that. - Toastydeath 10:07, 27 July 2007 (UTC)

"...without copying the first quarter of the book, I don't think I will be be able to convince you otherwise." Well, that sounds like WP:OR used to advance a position. The new change (the use of masters/standards) and a few other statements opens up a new can of worms but so it goes. --I already forgot 22:20, 27 July 2007 (UTC)