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Dueling gage blocks.

Dueling gage blocks

Just as we must evaluate the parade of new materials and processes that compete for a place on the shop floor, we must evaluate new materials and techniques that effect how we measure quality. And just as new ceramic materials are shaking up metalworking processes, they are also challenging the most basic of measuring tools, the gage block.

A big success in Japan, the ceramic gage block is attempting to make in-roads in the US and meeting stiff resistance. A gage block is a basic reference standard, something you measure other things against. To ask people to change their basic measuring standards is apparently akin to asking them to change their religion.

Thus, it take's a great "leap of faith" for users to believe that a "flimsy" ceramic could even survive in the metalworking environment, let alone claim to be a head-to-head competitor with time-honored metal gage blocks. But this perception of fragility is not based on fact or personal experience, just an inclination, an inhibition. After all, neither is malleable metal and ideal gage-block material, when a careless nick or scratch can introduce serious measurement error.

To help shed some light on this issue, here's a rendering of the key points of contention, distilled from a running debate between the two leading proponents: Webber Gage Div, L.S. Starrett Co (maker of chromium-carbide "Croblox") and Mitutoyo (maker of zirconia-based "Cera Blocks").


Up until the early 1950s, gage blocks were all made of ball-bearing-grade steel, and were vulnerable to wear and corrosion, and required frequently rechecking and replacing. Some of the challengers at that time for a more stable material included sapphire, titanium carbide, glass, ceramic, tungsten carbide, and quartz. All were rejected by Webber in favor of chromium carbide, produced by using a sintered power-metal approach. According to Webber, Croblox eliminated the problems of excessive wear, magnetism, instability, crazing, corrosion, and burrs - problem of not only ball-bearing tool steel, but also chrome-plated steel, stainless, and tungsten carbide. Their material can be polished to the mirror finish of a quartz optical flat, and has the optical reflectivity needed for interferometric calibration measurements.

With these advantages, Croblox enjoyed a long reign as a premium gage-block material, and there are more of them in use in the US today than any competitive carbide-based gage block (tungsten and titanium). However, traditional steel gage blocks (typically SAE 52100 with 1.4% chrome) still hold about 3/4ths of the total metal-gage-block market here, even though they wear out often and need replacing, on the average, every three years. Apparently, most users remain short-term thinkers, unwilling to pay twice as much for a premium gage block that lasts at least ten times longer.

Chromium carbide's case

In the late '80s, when Webber's metallic gage blocks were first challenged by ceramic alternatives, their reaction (not unexpectedly) was

"Why change? You already have the ideal gage block." To bolster their case, they cited these basic comparative advantages: * Faster stabilizing. Ceramics' thermal conductivity is three times slower than steel, while chromium carbide is twice as fast as ceramic (but less than steel). Thus, temperature stabilization takes much longer for ceramics - in one test, comparable stabilization times were 10 min for steel, 13 min for chromium carbide, and 20 min for ceramic. * Infinitesimal wear. While ceramic blocks will outwear steel by a factor of ten, chromium carbide outwears steel by a factor of at least 30 and as much as 50, and thus offers superior wear life to ceramic. (More on wear-life claims and counter-claims later.) * Greater availability. Available rectangular or square, chromium carbide blocks are presently offered in more sizes than ceramic, and in four grades versus two for ceramic. * Longer track record. With nearly a 40-yr lead over ceramics, Croblox have a far longer field history, while ceramic blocks have yet to prove they can withstand the "test of time." Webber Gage's own "Grandmaster" traceability standard - a 4" chromium-carbide block that has been shuttling back and forth to the National Bureau of Standards for the past 25 years - has never experienced a change in length greater than 0.000 000 5". Obviously, ceramic, the new kid on the block, cannot yet demonstrate that kind of long-term stability.

Ceramic's case

In rebuttal, Mitutoyo makes a case for Cera Blocks based on tests they ran comparing their zirconia against steel blocks, Starrett/Webber's Croblox, and also other ceramic blocks of silicon carbide, aluminum oxide, and silicon nitride. * Thermally matched to steel. In thermal characteristics, Mitutoyo points out, it is important to differentiate between coefficient of thermal expansion and thermal conductivity. If it were desirable, ceramic can have a very low coefficient of expansion (as low as 2.0 X [10.sup.6]/deg K for silicon nitride, used in some earlier gage blocks), but they chose zirconia because its expansion coefficient (10 X [10.sup.-6]/deg K) more closely matches that of steel (11.5), the material usually being measured. Croblox, on the other hand, has a coefficient of 8.4.

A Croblox, they admit, is considerably faster in getting rid of heat, more in line with steel, but once those temperatures stabilize at some temperature other than the standard 68 F, there's a slight difference in accuracy. A Croblox wil show a greater dimensional error than a Cera Block from that measured steel part at that new temperature, due to Croblox' greater difference in thermal-expansion coefficient. On the other hand, a ceramic such as silicon nitride would show an even greater thermal-offset error than Croblox because of its far greater difference in expansion coefficient.

Observes George Webber of Webber Gage: "That workpiece you want to measure is never a safe, stable lab temperature of 68 F. If you've just pulled a part off a grinder with a coolant temperature of 75 F, and gage-block temperature is 85 F, steel or chromium-carbide blocks will reach that 75 F much quicker than ceramic. I agree that there are applications for ceramic where you want the gage not to move thermally, but in my opinion, a gage block is supposed to move quickly to the same temperature as the workpiece. Our instructions tell people to use a heat sink, setting the gage block for four minutes on a large mass of metal that is at a stable temperature (not a granite surface plate, for example, which doesn't conduct heat well). Ceramic will not stabilize that quickly.

"This is necessary for measurements to 0.000 010", but not for the half-a-thousandth measurements where the average shop is now working. However, as these shops move closer to measuring millionths, it will become more apparent that their gages need the ability to stabilize quickly." * No rust. Ceramic's corrosion resistance is clearly superior to steel (but not to chromium carbide blocks), although Mitutoyo admits very minute dimensional change (0.1 micron or 0.000 004") can occur with exposure of ceramics to strong acids or bases. * Minimal wear. With its low friction coefficient and dense, homogeneous surface, zirconia's material loss due to abrasion beats both steel or chromium carbide, says Mitutoyo. Their abrasive wear test laid gage blocks on a cast iron surface and rubbed them back and forth thousands of times for a total distance of 10,000 meters. The results were 0.35 microns of wear for steel, 0.30 for Croblox, and 0.18 for Cera blocks. Photomicrographs and profilometer surface-measurement traces with a diamond probe confirm a significant difference in wear for this test.

Webber Gage's George Webber challenges this claim. "One key advantage we have over a testing lab is that we manufacture gage blocks by lapping them to final size. From this experience, we know very precisely the rate of cut of different gage-block materials using the same abrasive on the same lapping machine, and we feel this is identical to an abrasive wear test. Steel takes far less time to lap than any of the other materials, and ceramic cuts easier than either tungsten carbide or chromium carbide." * Long-term stability. Mitutoyo points out that dimensional stability after sintering of ceramic is much greater than the change steel blocks experience even months after heat treatment. But that's steel. Croblox, on the other hand, are made of sintered powder metal that is very stable immediately after manufacture and is not heat treated. * Permanently marked. Whereas identifying marks on steel blocks can corrode and become illegible, the laser-etched markings on Mitutoyo's ceramic blocks are more permanent over long periods of use. Croblox are similarly laser etched. * Better on impact. Fracture toughness is an obvious concern, but zirconia's toughness is superior to other ceramics, and it will not break or crack in ordinary use. Mitutoyo claims a blow strong enough to crack a Cera Block would render a steel block useless.

To demonstrate this in impact tests, they dropped sample blocks a distance of 70 cm to land on their edge on a cast iron surface plate. Photomicrographs of the ceramic block showed no effect, whereas a steel gage block picked up a burr, and the Croblox a 4-micron surface deformation.

Thus, they claim, their ceramic block does not burr and produce a false reading; at worst it will chip, but this has no effect on its measuring ability. In contrast, any dropped metal block will require polishing to remove edge burrs or deformations that distort its ability to lie flat and give a true reading.

George Webber admits chromium carbide is slightly malleable, and it will bubble when an edge gets hit, but disagrees strongly on the nature of a chipped ceramic-gage edge.

"Yes, ceramic will chip unlike a metal burr, but it is not a clean chipout - it still leaves a distorted edge that requires the same corrective treatment - a burnishing operation to recalibrate the block." * Better wringing. This, Mitutoyo feels, is a ceramic strong suit, thanks to its dense, polished surface. Ceramic blocks wring well with each other or when mixed with metal gage blocks. In tests to measure wringing force - the direct force required to break the bond - Mitutoyo found it took only 2.0 gm to separate Croblox, versus 3.8 for Cera Blocks, 2.9 for steel, and 4.2 for tungsten carbide. Thus, they conclude, the wringing force for ceramic is comparable to tungsten carbide, but nearly twice that for chromium carbide. Also, interferometer checks showed that wringing error, the accumulated error of stacked Cera Blocks, is negligible - 0.02 micron (0.000 000 8") for two 20.5-mm blocks.

Wringing out the old and new

"You're forgetting something here," cautions Webber, pointing out that wringing force is more of a function of the wringing film all users add to their brand-new set of gage blocks than block material. To create a sufficient force to wring blocks together, all gage blocks need this film. Veteran QC people wipe a gage block on their nose or the back of their hand to pick up body oil. The younger, more sanitary crowd apply a silicone fluid and similarly wipe the block clean to remove any excess.

Organic or otherwise, the resulting film thickness is less than 0.000 000 25", says Webber. Fortunately, if you leave too much film, the blocks will not wring, or if you have no film at all, the force is weak and unreliable. Thus, the process is self-correcting - a good wringing force is confirmation that you've done this properly.

"Wringing error is never significant," claims Webber, "and in fact is built into any calibrating interferometric measurement of gage blocks. It's always there - a half-millionth part of every block - every time it's used, even when a block is measured alone, wrung against an optical flat."

It has yet to be scientifically proven whether wringing force is due solely to molecular attraction, the vacuum created by the oil film applied to the two blocks, or more likely, a combination of both. Webber has measured a wringing force of 200 lb between two Croblox with a contact area of only 0.5". Since a pure vacuum would provide only 14.7 psi of locking force, he reasons that wringing force cannot be due to vacuum alone. Yet, the fact that a scratched block will not wring indicates that the vacuum is critical.

On the issue of whose blocks wring best, both sides agree that the Cera Block and Croblox surfaces are not identical under the microscope. Although surface finish is the same - equal to a quartz optical flat - grain structure is different because the ceramic powder grains are slightly smaller than the powder-metal chromium-carbide grains. Finer grain size implies fewer voids between grains, greater contact area, and greater wringing force if that force is molecular.

But if you can get plenty of pounds of wringing force with any properly-wrung block, this is hardly something worth quibbling over for long.


Thus, if you throw out the wringing controversy as minor nitpicking, the main points of contention remain thermal stability and toughness. Is it better to have a block that is slow to respond to minor temperature change or pick up heat when handled (ceramic)? Or is it more important for that block to stabilize quickly when moved from one environment to another (metal)? Do you really need to match the gage material characteristics to the measured part? And, finally, if your measurement people are going to handle gage blocks roughly, does it really matter which material you chose?

Used properly in a thermally stable QC-lab environment, either gage material will do very well. It's when you take them out into the shop that things get complicated. The key question is how important is the inevitable thermal error to the measurement you're making? Do you need to worry about millionths? You will need to know how fast your gage block will transfer heat and stabilize, and what dimensional effects that causes. Chromium carbide will stabilize much faster, but with a greater error once it stabilizes than zirconia, whereas zirconia if used too soon, before it has stabilized, will produce a temperature-difference error. Or in short, at temperatures other than 68 F, a Croblox produces a thermal-offset error and a Cera Block a time-lag error.

Although the initial price of Cera Blocks offered in this country is slightly less than Croblox, both will continue to suffer from a user reluctance to pay twice the price of conventional steel gage blocks for much longer-lasting premium blocks. With the conservative nature of US manufacturing and the natural tendency of individuals to resist change, it will be interesting to see how these two premium gaging alternatives fare in the years to come, particularly when so many metrologists seem stuck on steel.

PHOTO : Ceramic challenger: Mitutoyo Cera Blocks

PHOTO : Premium-metal defender: Starrett/Webber's Croblox
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:metal versus ceramic
Author:Sprow, Eugene
Publication:Tooling & Production
Date:Sep 1, 1990
Previous Article:Computerized tool management.
Next Article:Deep boring for happy landings.

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