ID's

[Ken379128]

I have heard alot about ID's not being able to measure them accurately I was running a program with and ID size of 2.246 to 2.250. The program is taking 6 hits in the ID. My question is this enough hits for this size ID. They were on Min and I rechecked them on a B & S validator that is still run by hand and they were 2.2469 2.2473 with 8 hits. What do you think? Any help is appriciated.

ps I hope spelling does not count

[bob mappes]

7 hits minimum is recommended

a size like that I would hit (if I read your tolerance at .004" total) 16 to 18 times

tighter tolerance than that I would go 36 or more hits

[Paul Barriage]

Much of the questionable issues with circles from here are:

1. Sensitivity of the probe.
Too much tension adjust deflection. - Late triggers.
2. Hole shape.
If it's not round, you get the average circle of the hits that you took. Not necessarily the proper hole size - (pin gauge)
3. As mentioned in #2, a pin gauge is used for more proper accuracy.



$.02

[John Kingston]

Paul and Bob both give good answers. I frequently compare my CMM measurements with other methods of measurement. I do this until I am comfortable with what my method of measurement/probing on the CMM gives. Remember that the CMM is one tool for measuring. It may be the most convenient and easiest, but it isn't always the best. It is there to enhance your measurement capabilities. This is why I preach understanding of other methods of measurement. You must be able to perform layouts on a surface plate prior to learning at the CMM. You also need to look at and understand the processes used in creating the feature. Is it drilled, milled, ground, honed, etc. ? Every process has advantages and disadvantages. What is best for this situation?
All that said, I would look at using 7 or more points. I would also do some type of comparison to a different method of measurement. Not another CMM, but another type of gage. (Bore Gage, tri-mic, etc.)

OK, done preaching, stepping down from soapbox!

[craiger_ny]

We have workholding here that I do bores that size in 6 hits because the material machine tool and workholding allows it. There are other jobs here I'd do it in 12 due to material machine tool and workholding. Tolerance also comes into play. Best bet is to ease yourself into it and err on the side of caution. As you gain experience with the processes at hand you will be able to tweek the program later. If this is a new or recently changed process program like you want to find something wrong with the parts. If it is a reliable long running process program for speed and efficiency. There are more factors involved than the size of the ID being measured.

Craig

[Wes Cisco]

WES (What everyone said.)

But most especially bore gage/tri-mic. The tolerance you describe is not extremely tight and I would not have a problem measuring this with my cmm, but as others have said, I would take into consideration method of manufacture, known abilitly, (or lack thereof), of the monkey pushing the button, etc , etc, etc. My main reason for posting is to say that if you are using a tp2 or a tp20 you need to be aware of the known lobing issue with those probes. Renishaw can provide you plenty of info about this. Even if you use a tp200 or other probe with no lobing, for tighter tolerance, (my limit is .002" total tolerance or less), the diameter should also be checked with a bore gage or micrometer by a competent inspector who will check the diameter thoroughly. The main reason I say this is that it is much faster to completely manually check the diameter for size and out of round than it is to program and run sufficent hits and dimensions to acheive the same with the cmm. My $.02 and you were overcharged. HTH

[JamesMannes]

Just an add to...


I don't think anyone mentioned the various algorithmic choices that are available to the PC-DMIS programmer for ID and OD circular features.

This is taken from the help files.



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.



I use the max_inscribe and min_circumscribe option quite a bit and find the resultant measurements to be very close to Pin/air gaging. As an example after measurement of an ID on the CMM and it measured small, I verified this by air gage and was off by .006 microns.

[John Kingston]

Good idea to add those Jimi, I just assumed those were taken into account. OOOOOPS! I ***umed!!

[Ken379128]

Running with a TP200 and a 2 by 20 mm tip. I will double check them with more hits and a mic if we have one that big, I may have to dig around in the dungeons of QA for one :-) seems like since we got the global the big shots thing manual inspection is a thing of the past.

[Wes Cisco]

Running with a TP200 and a 2 by 20 mm tip. I will double check them with more hits and a mic if we have one that big, I may have to dig around in the dungeons of QA for one :-) seems like since we got the global the big shots thing manual inspection is a thing of the past.

Yes, I am familar with that mentality. Since we got the plunge EDM no-one here wants to drill holes anymore. Why bother with the drill press when we can burn it!???!?!?!?!?!?!?!??!!? The CMM is a powerful tool, but there are some things it can not do well. The setup you have should be able to adequately verify the tolerance you need. I would take at least 7 hits. More if you seem to be seeing an out of round condition. HTH

[bob mappes]

This is why I preach understanding of other methods of measurement. You must be able to perform layouts on a surface plate prior to learning at the CMM.

It's just like the "machining" of the part itself.

The kids these days have NO hands on experience with cranking handles on mill and lathes...everything is done with a CNC now. No more "feel" of how the tools are cutting. No clue as to the Craftsmanship of Machining.

I find it amazing that when the guys in the shop can't figure out how to do a job...they come to me to tell them how!
Of course, having 10 years on manual and about 15 years on CNCs helps me to do just that.

Kids!!!