RE: [gingery_machines] new article: The Intersection of Art and Precision (Using a Caliper)

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Tuesday, February 1, 2011

 

Dave,

I most certainly welcome your comments! Please keep them coming.

You mention a "valid test" but I was writing about developing a feel for a caliper and having a test of that feel. I did not talk about how to use a caliper. I will add a note to the top of the article with this disclaimer.

My limited experience with the calipers tells me they are far better than within 0.015" but it does depend on how they are used.

Say I have a rod and want to bore out a matching hole using my mill. The rod is too long to clear the mill head and I certainly do not want to remove the block from the mill between cuts. The way I've solved this problem in the past has been to measure the diameter of the rod using my mic, and then us a telescoping gage to check the hole. After each cut I adjust the telescoping gage for a sliding fit, remove it and measure it with my mic. Since this is a differential measurement, the absolute accuracy of the mic is not important.

An alternate way would be to use an outside caliper to be a close sliding fit on the rod, transfer that reading to an inside caliper, and then test the bore with the inside caliper until it is a close sliding fit. Ideally, this is the same as using the rod as my gage. Of course the challenge here is to be able to tell how much to cut on the finish pass.

One error source I do see with the inside caliper is that the jaws have thickness and are square on the ends. If the thickness was zero, then the caliper would read the diameter, d. But with a thickness, w, and a measurement of m, I figure there is an error. If I have this right,

d = SQRT(m^2 + w^2).

For example, if I measure 1" with my caliper and the jaw is 0.1" thick, then d = SQRT(1^2 + 0.1^2) = SQRT(1.01) = 1.005" which is significant. If m = 0.5", then d = 0.510". If m = 2", then d = 2.002". As you might expect, the error decreases as the diameter increases.

If transferring dimensions, you would not be able to correct for this error. I'm not sure why they don't have rounded ends. I guess the work around would be to keep the OD of the rod on the outside caliper and then use a telescoping gage to pick up the ID.

The above problem would not exist for an outside caliper.

Rick

-----Original Message-----
From: gingery_machines@yahoogroups.com [mailto:gingery_machines@yahoogroups.com] On Behalf Of David Patterson
Sent: Monday, January 31, 2011 8:10 PM
To: gingery_machines@yahoogroups.com
Subject: Re: [gingery_machines] new article: The Intersection of Art and Precision (Using a Caliper)

Rick your probably getting tired of hearing from me. But your test is not a valid test, it not a real world application of this tool. I've been in pattermaking near all my life and in inspection using real world application of this and other measuring tools. Right now I program coordinante measuring machines. Basicly a 5 axis cnc that measures machined parts. The CMM is not as accurate as the calipers you have discribed in you article. Using those inside and outside calipers you will be lucky to get within .015, I have mititoyo vernier calipers that will read to 1/128" or .0075" you'll never get to that in a real world application with manual calipers. These tools are to be used with a rule, fractional or decimal, by compairing them to a known value and saying you can get that accurate in incorrect, although you may get lucky. a simple test would be to bore a 2" diameter using just the inside caliper to get to 2", the using just the outside caliper and
without checking the fit, until completed, turn a piston to fit leaving a .005" over all clearance. Don't check the measurment with anything but a rule. This will give you an idea of the accuracy of these tools. another test would be to turn/cut several parts to a given dimension the measure the repeatablity of your use of the tools. These are eyeball tools at best, using a rule and calipers.

Dave Patterson
odd_kins@yahoo.com
http://home.comcast.net/~oddkins/foundry_home.html

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