Perpendicularity of cylinder

Hi everyone,

I am new to the forum and by searching the topics I couldn't find the answer to my question.
I need to measure the perpendicularity of a hollow cylinder according to its planar surface used as datum. I have used two different approaches in this regard, but I obtain very different results.
in the first approach, I measure 5 circles at different levels along the cylinder height [using auto circle feature] then I construct a cylinder from these 5 circles [constructed feature] and finally I chose Perpendicularity Dimension to measure the perpendicularity.
in the second approach, I use Auto Cylinder feature[using Adaptive Cylinder Concentric Circle Scan] to construct the cylinder geometry as showed in the attached image, where the scans are performed at the same positions along the axis compare to the previous approach. by measuring the perpendicularity according to the same datum as the first approach, this time I get perpendicularity values which are much worst than the previous case. [in first case I get 0.006 mm while in second case it is 0.098mm].
I would appreciate if you can help me find out which of these two approach is correct, or is there a better solution to measure the perpendicularity precisely?

Thank you in advance.

Attached Files

KIRBSTER269 over 7 years ago

How many alignments do you do, before scanning? Are your IJK's squared up? Why don't you post some of your code so we can pick at it a little bit.

Also what is your Newtons set at? I found 0.120 works pretty good
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louisd over 7 years ago in reply to Unitn

Technically, GD&T defines perpendicularity as the entire surface of the plane and the entire surface of the cylinder evaluated. Where the 'gray' comes into this definition is NIST's (unofficial) 10% rule, and how it applies to your drawing tolerance. You should be measuring everything with the ability to repeat within 10% (10 indications) of the allowable tolerance.

So, if your print says perpendicularity to .020" for a 1" long cylinder back to a 5" datum plane: you could get away with just two or three cuts, maybe just 4-6 probe hits each (scanning is not necessary with a tolerance that wide IMO).

But if your print calls out perpendicularity to .0003" for the same size part, you better be bringing the 'big guns' on sample points and cuts around the cylinder.

Number of points should always relative and complimentary to the tolerances on the print.
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AndersI over 7 years ago

I was wondering what could be the reason for this differences...

One factor may be the form of the cylinder - conical, banana curved, hourglass, non-circular etc. etc. - and depending on the form, it may be very important where you put the hits, and that you use enough hits to actually catch the form error. A rule of thumb is to increase the number of hits and cuts until the form error doesn't increase anymore, then you know how many you need for that particular specimen...
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