Master Disc Measurement

Hello. I'm pretty new to the field (4 months) and had a question regarding proper measurement of a Master Disc. I'ts not a Go\NoGo, simply a reference standard for a snap gage.

The size of the disc is 4.690" and the class is XX.

Now the tolerance chart I have tells me that I have .000065" of tolerance...Does this mean I have +/-.000065"?

Thanx!

Bueller?......

4 months experience and you're making master gages?

I'm a little uncertain about modern gaging standards but in days of yore gages were made so part of the tolerance was the wear allowance. The few real gages I made I halved the tolerance and made the gage by that amount over the specified size.

In this level of precision choice of material, its heat treatment, and its post heat-treatment stabilization will determine the success or failure of the gage. Most steels as subgect to long term distortion and volume changes measured in the tens of micro-inches. You need a good gage steel, one that's through hardening.

If the material has not yet been selected I would reccommend Timken 52100 bearing steel or O5 tools steel. Follow the manufacurer's recommendations for heat treatment followed by two or more thermal cycles between draw temperature and a cryogenic soak avoiding rapid temperature changes.

Allow grind stock on all reference surfaces leaving lapping stock on the gaging fearures. Be sure to lap the gage centers before you grind. Roundness is tough to correct with a saddle lap. You have to build a series of ring laps.

So I suggest you make your gage to 4.690033" + 0.000010. These tolerance aren't hard to hit provided your gaging equipment is accurate, your setting masters or gage blocks are Grade AA, their calibration is current, you use the calibration sheet to determine the actual size of each block, and use this information to correct the block height. Temperature and cleanliness is excrutiatingly important.

This is a high precision gage after all. When it's done it will have to be certified. The guys in the measuring room will be pushed to reliably measure to +/- 0.000010. Make sure any error reported can be shown to be theirs.

I had a book once upon a time "Modern Gaging Practice" written in the 1930's. It's a prime source of the technology of mechanical gaging and includes much information on the practical side of gage manufacture, finishing, and calibration. If you're gonna make gages you need a text like this. No point in re-inventing the wheel.

i'm not making a gage, i'm trying to figure out the proper way to measure them...reason being, i'm questioning tolerance practice of the calibration house that is doing it for us.

my understanding is, that for a master disc of the aformentioned size and class the tolerance of .000065" was to be halved (.0000325") and your tolerance was +/- that.

the vendor doing it for us is allowing +/- .000065".

Sorry. I misread. I thought the problem was manufacture.

Measurement to millionths is readily done provided you have the basic equipment: a comparator equipped with a gage amplifier and LVDT measuring probe whose height is set with Grade A Jo Blocks. Set the probe withe the Jo blocks and zero the gage amp. As a check restack the blocks using a combination 0.000050" taller just to see if the gage amp picks up the difference.

Roll the disk under the LDVT and note the reading. Check repeat zero with the gage block stack. Check the diameter at at least 6 places and 2 (or 3) places along the length of the disk at each diameter site. This test will not determine roundness or other cylindrical errors; only diameter.

Note that dimesional readings to this accuracy are subject errors from thermal expansion that can swamp the tolerance you desire to hold. One degree F in 4.690" = 0.000032" expansion assuming the work is made of steel. That's half your tolerance. This is a thick cotton glove and constant temperature job where you allow an hour for the apparatus to equalize before you take readings. Pick a clean still space not subject to drafts and casual traffic. If your readings slowly change with your time near it, keep a thermal shield between you and the apparatus. A thermal shield can be anything from an aluminum foil bib and gauntlets to a piece of sheet metal you can reach around.

I'm assuming you don't have an in house measuring lab or enviromentaly controlled room. This is high precision metrology beyond the sensitivity of the best CMM's, dial indicating comparator, digital mikes whose resolution is claimed to be rated to 50 millionths, what have you. If you have to measure to -0/+0.000065 you have to get the same readings as a third party's measuring room and for that you need the best apparatus you can quickly obtain. There are several so-called "SuperMikes" resolving 10 millionths. You cannot take their direct reading accuracy for granted unless they have a current calibration sticker. Even then I strongly reccommend a comparason to a Jo block stack before and after each measurement.

It may not be worth it to do it in house. Most shops farm this class of work out to a calibration lab. Your task is probably a $60 job because the lab will have all the stuff right on hand, an environmentally controlled room, experiences personnel, and, being a third party, no stake in the outcome.

There are some ANSI standards or such which have definitions of various tolerance words. If the tolerance is stated "within 0.000065" or "0.000065" peak to valley" that means that the total peak to valley (+/-) fits between to lines spaced 0.00065" apart. "Tolerance of 0.000065" can be interpreted as 0.000065" plus or minus or as peak to valley meaning it is +/- 0.0000325". Careful reading of words required. Common argument. Best to get back to the ANSI, SAE, ASTM standard that applies and work on exact definitions from the right standard. Probably there is some equation thing giving the tolerance as a function of diameter since small diameter gage pins have much closer tolerances than large disks. Wherever that equation is given that same document will likely have an approved definition of "tolerance" as it applies to master gages it deals with.

I believe your cal. house is correct.
Master ring gauges are toleranced bilaterally (+/-.00065).

Go/NoGo plugs are toleranced unilaterally (+.000065 or -.000065 with the tolerance applied in opposite direction on the go and nogo.
Bob

Forrest Addy said:

I'm assuming you don't have an in house measuring lab or enviromentaly controlled room. This is high precision metrology beyond the sensitivity of the best CMM's, dial indicating comparator, digital mikes whose resolution is claimed to be rated to 50 millionths, what have you. If you have to measure to -0/+0.000065 you have to get the same readings as a third party's measuring room and for that you need the best apparatus you can quickly obtain. There are several so-called "SuperMikes" resolving 10 millionths. You cannot take their direct reading accuracy for granted unless they have a current calibration sticker. Even then I strongly reccommend a comparason to a Jo block stack before and after each measurement.

It may not be worth it to do it in house. Most shops farm this class of work out to a calibration lab. Your task is probably a $60 job because the lab will have all the stuff right on hand, an environmentally controlled room, experiences personnel, and, being a third party, no stake in the outcome.

Click to expand...

i am based in a temperature controlled room (68*) and have a Pratt Whitney Model C supermic with current calibration at my disposal...what i lack is the experience in proper disc measurement. in an effort to save my company some $$$ in outside vendor calibration costs, i'm tasking myself w/learning how to do this all in-house. i will be attending classes in metrology, but in the meantime, i like to soak up as much info as possible. i do appreciate all your input.

Metal working hobbiest

Sir,

High precision gage blocks and optical flats with a monochromatic light source can be used, in a temperature controlled environment to measure millionths of an inch by observing interference patterns. It is paramount that the item being measured is "super clean". All companies that manufacture optical flats are willing to explain the techniques. You will not have to depend on anyone else. You will not have to have your super mic calibrated and all doubt will be removed.

If calibration requires A2LA accreditation you would need to save the company about $100,000+ per year. A2LA audits cost about $50,000 plus recerts every few years. If your company produces parts for the automotive industry, then you most likely require A2LA certification.

There's alot more to the calibration of class XX master disc than you think. Using a PW supermic for class XX disc is marginally acceptable.

For in house calibration, the procedure outlined by Forrest Addy is the best way to go. As Mr Addy points out, temperature/humidity can eat away a serious amount of your tolerance. Also, gaging pressure eats away another 5 to 10 millionths (check Young's modulus for the materials).

BTW the calibration tol. for class XX disc size 4.510" to 6.510" is +/- .000065". Only MASTER class disc are to be WITHIN .000065" for your size range.

I suggest reading "Inspection and Gaging" published by Industrial Press. Check the recent post by Forestgnome with the NIST link for some great facts about metrology facts and history.

microinch said:

I suggest reading "Inspection and Gaging" published by Industrial Press.

Click to expand...

Excellent reference book. ISBN 0-8311-1149-6

Be aware that the standard (i.e. measuring instrument) you use for calibration must have a limit of error (i.e. accuracy) at least four times tighter than the item being measured. So you would need a tolerance of ±0.000016" or less for your cmm, per the manufacturer's specifications.

- Leigh

microinch said:

BTW the calibration tol. for class XX disc size 4.510" to 6.510" is +/- .000065". Only MASTER class disc are to be WITHIN .000065" for your size range.

Click to expand...

The .000065" limit of error applies to all ring gages of that size and class. However, its application differs depending on the gage type.

GO gages are strictly minus tolerance, so the true diameter can be anywhere from nominal to (nominal - 0.000065"); NO GO gages are plus; MASTER gages are split, so the true diameter can be anywhere from nominal +0.0000325" to nominal -0.0000325".

See the notes on pages 21-22 in the Meyer Gage Company Fixed Limit Gages catalog, available online from www.meyergage.com

- Leigh

The real Leigh said:

GO gages are strictly minus tolerance, so the true diameter can be anywhere from nominal to (nominal - 0.000065"); NO GO gages are plus; MASTER gages are split, so the true diameter can be anywhere from nominal +0.0000325" to nominal -0.0000325".
- Leigh

Click to expand...

this is exactly what i wanted confirmation on. the fact that the gage is a master and not a go-no/go told me that i was to split the tolerance...not allow .000065" both + and -.