CNC Machinist Devices Help

MatthewVu_Pelco · #1 4/8/2009, 07:20 PM

There is a machine shop manager at my work place who is questioning the ten times rule of measurement.

We recently performed an inspection in his area and found an operator using a dial caliper to measure an ID dimension of 1.275 +/-.001. The manager says this is a perfectly fine practice. What standard exists - other than common sense or logic - that directly states the device must be accurate at least ten (or four) times the tolerance?

MatthewVu_Pelco · #2 4/8/2009, 07:41 PM

I performed a google search for "caliper vs micrometer" and all I got was one web page from Starrett stating this exact rule, but no explanation of where it came from. So how did this rule come to exist? Where is it? Who did it, and what authority do I have to apply this rule other than logic and common sense (which does not seem to hold much water in many American businesses - probably because it forces the one without common sense to look irrational, i.e. unintelligent).

machinist62 · #3 4/10/2009, 02:50 PM

the rule of thumb that I have been taught is that dial calipers are OK to be used when there is a tolerence of +-.005". Anything with closer tolerences is to be measured with a micrometer.

Dr Stan · #4 4/10/2009, 04:09 PM

Quote:

Originally Posted by MatthewVu_Pelco

There is a machine shop manager at my work place who is questioning the ten times rule of measurement.

We recently performed an inspection in his area and found an operator using a dial caliper to measure an ID dimension of 1.275 +/-.001. The manager says this is a perfectly fine practice. What standard exists - other than common sense or logic - that directly states the device must be accurate at least ten (or four) times the tolerance?

Let me guess, this "manager" has a liberal arts degree.

Try conducting a Google search using metrology as a key term.

MikeTF · #5 4/10/2009, 06:10 PM

Diagital, Dial or vernier calipers rely on too many variable to be accurate enough or repeatable to measurements an I/D. The contact area is extremely lone vs. a micrometer or dial bore gage. The calipers are not usually mastered to a standard that duplicates the part being checked. No friction thimble like a Mic., surface finish also causes problem etc... I measuring submicron tolerance for a living and also use a yard stick when it applies. I would not use a caliper on an I/D and expect +/-.005, they are more repeatable on and O/D beacuse the contact surfaces are flat and parelle, they I/D side is like a knife edge and the feel is less sensitive.

bradintx · #6 4/13/2009, 10:31 AM

How does the eventual customer measure the part? What does the feature you are measuring do (what is the function)? Have you done a gage R&R on the operation?

In your example - suppose a rod is run thru the ID and it should spin freely. Then I would think a plug gage would be the appropriate tool for measuring the hole. Of course you do not get variable data with this.

MatthewVu_Pelco · #7 4/13/2009, 06:47 PM

Thanks for asking Brad. The ID is used to press fit a bushing. It is some standard size, off-the-shelf pipe that we bore out from one end about .500 deep. The manager is stating that if the ID was too big, there would be no press fit (loose). But if that is the case, why bother even measuring?
And we have not collected data or studied gage R&R for this part.