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Thread: how to measure circularity
04-22-2010, 02:05 PM #1
how to measure circularity
Help me out folks, I'm an engineer trying to get my set of numbers straight so I can sort out the issues with a manufacturing process. Long story short I was given a a table of numbers with circularity calculated but I don't think the inspectors methodology is right and I wasn't able to reproduce the results . Before I go throw sand in their face and tell them they did everything wrong and they are horrible people I want to make sure I understand this issue.
We have a rolled sheet metal cylinder called out at 15.000" on the I.D
The callout is for roundness within 0.030"
straightness within 0.015
The part was set on blocks and measurements taken in 6 places along the circumference. For straightness a straight edge was used and shim gauges used to find the area of lowest height along the longitudinal axis of the part. This was measured as straightness
Now for roundness the straightness measurements were used and the area of lowest height among the 6 areas it was clocked at was doubled and listed as roundness.
This feels wrong to me that would be taking measurements off the longitudinal axis of the part. My understanding is that circularity is the quality of every part on a circle to be equidistant to the center point. Therefore to measure roundness you would take Diameter readings at however many parts, and the Difference between the lowest and highest Diameter would be your circularity. we don't have anyway to spin this part right now precisely, so we're going with taking mic readings at 6 points (3 diameters)
We have gauges but apparently the parts are so far out the bore gauges and other tools for measuring inside diameters we had can't measure them (only go 0.010" or so)
so help out the clueless engineer.
Am I on the right track to calculating circularity?
04-22-2010, 04:05 PM #2
I think you lost me right around..."I'm an engineer"...
I think what you're actually looking for is "cylindricity", is it not?
If you have a lot of these cylinders to measure, you might want to have a hard gauge made up that has a thick-walled tube whose I.D. represents the part's maximum O.D., and slip that over a pilot diameter that represents the low limit of the I.D. of the workpiece.
A go/no-go gauge?
04-22-2010, 05:58 PM #3
circularity ??? do you mean ROUND ? like a circle is round??
I don't think the inspectors methodology is right and I wasn't able to reproduce the results .
I'm guessing your not old enough to know what this thing is let alone actually have one ( but I could be wrong ) so ask your mother if you can borrow her Lazy Susan.
Now set the sheetmetal thing on it and get it in the center and shimed so it's pointed straight up... Then use a dial indicator.
Rolled SHEETMETAL ?? round within 0.030 ?
How thick is this sheetmetal?
04-23-2010, 04:43 AM #4
04-23-2010, 10:08 AM #5
Okay I got this all sorted. Thanks for your confidence in my engineering ability. For the record I'm a welding engineer, but have a degree in manual and cnc machining. I don't pretend to be an expert but I'm familiar with gauging. with our current inspection setup we can't actually measure the circularity but we can take an approximation by clocking the part and measuring the diameter repeatedly.
The inspector said the part was okay, but when I went to input his numbers I couldn't reproduce his methodology when he described it to me, it didn't sound like circularity at all. I did my homework and then compiled my numbers. He was wrong.
and yes I know what a lazy susan is they were all around the place in michigan and Canada, but I feel like they are going out of style. Unfortunately I don't think one would make a good reference spindle.
and no it is not cylindricity. technically when you stack both straightness and circularity on the same surface you're calling out cylindricity BUT the tolerances were different and I don't have any measuring device that would let me tram the whole cylinder to do both in one shot. I've attached a picture from my very useful GD&T hierarchy pocket guide, that shows circularity aka roundness.
and those were hypothetical numbers. We are actually trying to hold LESS than 0.030" roundness on ~0.100" sheet stock rolled to roughly a 15" diameter 7 feet long.
Welcome to Aerospace manufacturing. and everyone wonders why it costs billions of dollars to launch rockets? it's cause some aeronautical engineer didn't realize it would cost a lot of money to make tubes at precise dimensions.... I forgot to mention these tubes are welded together that's why they called in me a welding engineer, because they are blaming the circularity on the welding.
04-23-2010, 10:42 AM #6
"Welcome to Aerospace manufacturing. and everyone wonders why it costs billions of dollars to launch rockets? it's cause some aeronautical engineer didn't realize it would cost a lot of money to make tubes at precise dimensions."
So I'm guessing from your statement above that you know the end use of the part and that it does not require this type of tolerance. Or are you just complaining?
I'd measure the diameter in a number of places around the circumference and if the variation exceeded the b/p callout I'd say that it is discrepant.
Your technique in measuring straightness is interesting. I'd worry that I couldn't verify that my straight edge was directly down the centerline of the part. Any variation off centerline will effect the measurement.
First thing I'd do is wander down the hall to QC, get together with their guy, figure out what'd make everyone happy and do that. The absolute last thing you want to do is end up with an adversarial relationship with QC.
04-28-2010, 12:57 PM #7
This is the kind of part handling/measuring issue that makes you tired just thinking about it. If I were going to need to do that kind of measurement, I'd maybe look for a decent size rotary table, or any rotary table with a faceplate on it large enough to handle the 15 inch diameter plus a combination square. Put the rotary table at floor level (maybe on a flat plate on 3 support points), set the tube on it, shim it to square checking all-around best case, then put a dial indicator on some kind of rigid stand and measure the runout down close to the rotary table. This would probably give you a way better indication of circular runout than you might currently have, although this method is just a better kluge than trying blocks and straightedges.
04-29-2010, 08:01 PM #8
I agree that rotating it is going to be the only way to be sure. The trouble with checking it using micrometers is twofold. Number one, you could possibly be squeezing it, trying to accurately use a 15 inch mic is hard enough on a rigid part, let alone a rolled sheet. This is not very likely with .100 thick stock unless you have gorillas working there, but i thought i would mention it. Secondly, If you check something which is tri-lobal with a mic, it is very possible to get 3 of the same readings, and be very out of round. To extremely exagerate my point, imagine mic'ing over a perfect triangle, you could mic over the 3 points, and say "this is a perfect circle". If this tube is being welded or formed in a 3 roll weld box you could easily have this knd of condition. Anyone who runs parts on a lathe in a 3 jaw chuck has run across this a time or two.
A Go/no go gage seems feasible as long as the OD tolerance is much much tighter than the circularity tolerance. If you have +- .03 on the OD, you would need a circularity gage for every diameter possible, not just .03 larger than the mean.
The best possible scenario would be mounting it horizontally using a bench center or engine lathe with a plate on both ends. Make a 16" dia. plate with a center in the middle, on the face opposite the center, cut a trepan which will locate the tube within reason. You should then be able to mount it, and use an indicator to check your circularity along the whole length.
04-29-2010, 10:16 PM #9
Seems to me that circularity and straightness are pretty forward. You support the tube at both ends, spin the tube and measure the greatest TIR, that is straightness. Don't put the tube on centers, put it on roller v-blocks (or similar). To measure circularity, you take TIR numbers at N different points, the range is max to min and should not fall outside your spec. As for the process, you probably should be welding it on a mandrel, then heat treating it on the mandrel, then hydraulically remove the mandrel. Sort of the same way they make carbon fiber tubing.
04-30-2010, 03:19 AM #10
You cannot use v-blocks as any out of round condition would let the true center rise and fall, not giving you what you need. Using v-blocks is essentially the same thing as using micrometers, the v-block is the stationary anvil and the indicator is the movable one.
05-19-2010, 12:01 AM #11
I agree with your first sentence, but the rest of your proposal doesn't live up to that aspiration...
measuring the diameter does not tell you where the centre of that diameter lies with respect to a reference axis.
As an example, think of the lobed shape you get from a badly set centreless grinding machine (like a very subtle tendency towards the form of a Wankel rotor). It will have an odd number of lobes, and it will measure the same diameter at all angles (which is how it 'fools' the centreless grinding machine)
Note, not just across the tops of the lobes, as an earlier post might lead you to assume
I'm only an engineer too, so I could be wrong (poking out tongue at prickly machinists!)
05-19-2010, 12:43 AM #12
05-19-2010, 04:44 AM #13
05-19-2010, 09:15 AM #14
Find someone who has a Tallyrand machine or purchase a used Tallyrand and you will find out more about cylindricity than you thought possible.
05-19-2010, 08:01 PM #15