Measuring hole size (diameter)

[JohnCTB]

I measure a lot of holes and core pins in molds. I read once (I believe in this forum) that a CMM is not the best tool for measuring diameters. I have recently had a coworker make the same statement. I use 5,7, or 9 points to measure a hole. I make sure I am measuring in the proper plane. I make sure my setup is correct. When I output the results I make sure to look at roundness error. If a hole or pin is not round then it will probably not check the same dia. with another method (Micrometer or pin gage) as the CMM. I believe PCDMIS uses a best fit method to construct the feature. As long as everyone involved is aware of the limitations of the inspection method I think it works fine. Any and all input or opinions on this are welcome. I often have to defend my machine and methods to the toolmakers that I work with.

Thank You,
JCTB

[Wes Cisco]

tolerance

I think what needs to be considered here is tolerance. If a hole is +/- .005" and form is not specified on the dwg. I will take the cmm dia as close enough. But everything that is .002" total tolerance or tighter gets backed up with bore gage or mics. The chance that there is a deviation in an area the cmm did not take a hit are too great with the tighter tolerances. You could probe the diameter to death, but do you have the time? A mic or bore gage is much faster. Remember the 10% rule, the uncertainty of your measurement method should be less than 10% of your tolerance. HTH

[tking]

I'm inclined to agree with your method of confidence in the diameter reading if the form is good. This is OK when working with molds and core pins where the form of the feature is usually VERY good. I don't think I would be as comfortable checking the holes in the parts made from the molds.

0.02 worth,

TK

[#2#]

I think what needs to be considered here is tolerance. If a hole is +/- .005" and form is not specified on the dwg. I will take the cmm dia as close enough. But everything that is .002" total tolerance or tighter gets backed up with bore gage or mics. The chance that there is a deviation in an area the cmm did not take a hit are too great with the tighter tolerances. You could probe the diameter to death, but do you have the time? A mic or bore gage is much faster. Remember the 10% rule, the uncertainty of your measurement method should be less than 10% of your tolerance. HTH

I agree with Wes that tolerance is a big part of this decision. If you deal with recurring parts you could always do a capability study. Start recording the cmm data and a data from a different method (bore mic.) and then when you have lots of data points decide which method best fits your needs. Then if you decide to go with the cmm's numbers you will have data to back up your argument.

[uber user]

its common knowlege that the best thing to check a hole with is a pin gage. bore gages are not too bad, those sunnen one's are very accurate. it depends on what type of hole you are measuring if its machined or ground then the cmm should give you pretty accurate results as long as you are square to the hole. if you are checking stampings, castings or moldings then i would rely more on a pin gage for hole size. i bet if you check a ring gage with your cmm it will give you a number pretty close to what is marked on the ring gage.

[Underspec]

Tell management to get a scanning head / probes. Otherwise take as many hits as you can. And/or provide max/min dimensions on the diameter.

[JohnCTB]

Thanks for the input. It's all good!
We build molds using part data + shrink. Usually we try to work right to the numbers. In reality, using CNC machines and EDM's we are +/- .003. Many of my inspections are based on the fact that the plastic parts don't always shrink the way the customer planned. They may have a tolerance of +/- .015 on a detail, but the part will not fit or function. Then I inspect the tool to prove we built it to data. It's all about who is going to pay to make it work. If it's not to data, we pay to fix. If it is to data they pay for an engineering change.

JCTB

[JamesMannes]

Keep in mind the different methodologies that PC-DMIS has to offer for algorithmic analysis...
(doesn't that sound fancy?)

LEAST_SQR
Least Squares ?This calculation type provides a method of fitting in which the average squared radial distance from the data points to the circle is minimized. The square root of this quantity is the Root Mean Square (RMS) distance. Since the RMS distance is based on an average, some points may be further than the RMS distance from the computed circle.

MIN_SEP
Minimum Separation ?This calculation type generates a circle that is halfway between two concentric circles containing the data points, with the difference of their radii as small as possible. The Min/Max math used by the MIN_SEP calculation minimizes the maximum error, or deviation, from the input data to the circle. The Min/Max error is one-half of the minimal separation. No input data points (or input features) lie farther than the Min/Max error from the Min/Max circle. This calculation determines whether or not all the input data (or input features) are within the given tolerances.

MAX_INSC
Maximum Inscribed ?This calculation type generates an empty circle with the largest possible diameter that lies within the data. PC-DMIS first computes a Minimum Circumscribed circle and requires that the center of the Maximum Inscribed circle lies within it. This option could be used for a circular feature that requires a mating stud. For example, if the input data represents a hole, then this calculation returns a circle with the diameter of the largest stud that will fit inside the hole.

MIN_CIRCSC
Minimum Circumscribed ?This calculation type generates a circle with the smallest possible diameter that encloses the input data (or input features). This option could be used when measuring a stud that would fit into a mating circular feature. The resulting feature would be the smallest hole into which the stud would fit.

FIXED_RAD
Fixed Radius ?This calculation type creates a circle of a given diameter, positioned so that the maximal radial distance from the data points to the circle is minimized. It is similar to the Max/Min math used in the MIN_SEP calculation except that,since the diameter is known in advance, the radius cannot vary. Only the circle's position is allowed to vary.

[Underspec]

Explain that in plain english Which is best used for diameters and studs?

[Bassbuff]

I certify Gages and fixtures alot. Some have pins set in them, some have Holes and then some have rails or guides. I can check them with the CMM just as accurate as using Mics or a Bore Gage. Most of the time, I do not use a tolerance, and I measure in .0001 of an inch with no problem.

[tking]

James Mannes
I am IRON MAN!!!

Thankyou, sir. Never found these terms defined so precisely and compact. Had to print it for the "treasures" binder I keep at the machine for reference.

Ya gotta love the wealth of knowledge available from the folks here on the forum!

TK

[JamesMannes]

That was copied out from the PC-DMIS help files fyi.
FYI

[tking]

That was copied out from the PC-DMIS help files fyi.
FYI

Thanks, never ran across them as I perused through the files.

TK

[dwade]

It's been mentioned here before but using a TP2 or TP20 is not a good tool for measuring diameters. Heard mention of bore gages and have used Sunnen bore gages with great success. I worked for a die shop that did a lot of jig boring and wire EDM holes and the only approved method for checking small holes was gage pins. Make sure there is no dirt or oil in the holes. I found a lite film of oil would kill me readings and my repeatability as the ruby would collect any grinding dust or minute traces of EDM scale left after the machining operation.

[kbotta]

Surprisingly, this has yet to be mentioned. Size of the actual mating envelope (AME). Did you know that, according to ASME Y14.5M-1994 that additional tolerance (also known as bonus) comes from the size of the AME? This might not seem a big deal to some, until you explore the meaning of AME. With any change in orientation of a feature (lets say a hole) the AME changes. If you were to simply check this dia with a gage pin, you would get the same results every time, right? As this negative feature's AME moves from MMC toward LMC you get additional tolerance, right?
For those that are feeling investigative, I invite you to take a look at 14.5 - pg. 85 Para. 5.3.2.1 (b). I think you will find this interesting.
Consider this: Take a negative cylinder (a part with one in it, or a coffee cup will do). Look straight down into it. That is how a gage pin will check it. Now, change it's attitude, or orientation just a little - and watch what happens to the size (hello, AME) of the cylinder. It 'winks' and grows smaller, right?
For feature's controlled by position symbol, this is where your 'bonus' comes from. The size of the AME...
Kev