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vernier scale depth micrometer .0001" resolution...


Jan 20, 2019
I'm working with two older machinists who insist on using depth micrometers WITHOUT a digital readout, and without a dial, so Vernier is all that's left as far as I know. The necessary measurement is part of the process to rebuild passenger rail car gear boxes on a commuter rail system, and must be measured in .0001" . One of the machinists has a depth micrometer with two scales, one indicates in .001" and then as you flip it over, it indicates the final portion of the measurement in .0001" Personally, I think the thing is a pain in the posterior. Here are my questions:

1. Can anyone provide the make and model number of a 0" - 6" depth micrometer with a RESOLUTION that measure to .0001" and has an accuracy of the same or greater ? Preferably with all of the increments simultaneously visible.

2. Is there any such thing as a dual thimble depth micrometer such as the dual thimble design on a Starrett No. 221 precision micrometer.
Measuring to .0001 with a conventional depth mic is a fools errand. You are going to have to step up your game to get your desired .0001 resolution and accuracy. Cost will probably be 2 orders of magnitude greater than a conventional depth mic. Oh yea, you will need a temperature controlled environment as well.
Just guess the .0001 measurements it will be just as accurate as trying to measure it with a depth mike in a non temperature controlled environment.
I hate to say this, but it is a management problem.

Set up an inspection station to check/verify their work with better instruments before it goes to the next stage. This can be as simple as a quick measurement made by yourself with gauges you trust.

If it passes, fine. Stop worrying. If it continues to pass with regularity, then you can shut that inspection down and just trust them.

If it fails, then have a "sit down" with them in the office and with witnesses. Tell them they are not performing their jobs properly with those instruments and they must work within the established parameters or they will be replaced with people who will. Be sure to document this. If they can not do the work properly with the gauges they want to use and refuse to use better ones and the work is sub-standard, then they can be fired for cause. It is their job to turn out accurate work and that should be in their job description.

Work with the personnel people in your company to be sure you are on sound legal ground. And if you have a union, work with them too.

And YES to that temperature controlled environment. The company must do it's part to make this possible in the first place. It probably needs to be a clean room as well. Not medically clean, but free of dust and other air-born contaminants.
Making a gauge is the only way to make .0001 in a bore if the part has a flat surface to gauge from with a surfacegauge and a dial indicator.
With not a good surface to gauge from thinking you can get .0001 is a fool's idea with most any device , even a CMM..
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Where is it stipulated that it must be measured to that tolerance. If you are rebuilding this “box” is that all measurements. Are you machining all new hard parts. There is a lot of things required if you need to maintain that level of accuracy. Your not only going to need a clean room to inspect, your going to need another to rebuild the box.
Thank you all for your responses, especially the information regarding a temperature controlled room. I really hadn't considered the temperature and dust issues but I was highly suspicious that a measurement of the radial runout of a shaft in a traincar's gearbox to the .0001" was necessary, or even meaningful. I have since obtained a copy of the manufacturer's rebuild procedures and will read it thoroughly to see what level of accuracy the procedure calls for. On a darker note, maybe some of the older machinists are just screwing with the new guys to see if they say anything.

The necessary measurement is part of the process to rebuild passenger rail car gear boxes on a commuter rail system, and must be measured in .0001"

How are THEY going to measure it?

My spidey sense tells me this either a typo, or Bullshit.

If this is measured after assembly, (as I suspect) then you need to have the assembly to measure the dimension in question.

We do this type of crap often, and we ALWAYS measure the dimension in question IN THE ASSEMBLY.

But, I highly doubt that a .0001 tolerance is required for a train part.

I was taught that to measure to 0.0001” you need a gage the is accurate to 0.00001” or 10x the accuracy. I’m thinking you are considering the wrong tools And worried about the wrong things. Measuring a depth to a 1/10 th sounds insane, dust will throw off your measurement. Not to mention the surfaces you measure to and from. Yes both surface reference features need to be considered. As been said “a fools errand”.
“On a darker note, maybe some of the older machinists are just screwing with the new guys to see if they say anything”.

Shops that work down to those tolerances are climate controlled, including the shop floor. Maybe the person that gave you the number is just having fun at your expense.