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Perpendicular Help

xbigfatgoat
Hello Everyone,

I'm having issues getting good results with the perpendicularity on this part that I'm checking. The drawing is posted.

Reading here on the forum I see that small cylinders could cause issues with perpendicularity. The parallel and concentricity come out great.

I've tried leveling to the datum B plane and the Datum A cylinder without any success.

Basically, if I report the perpendicularity of the cylinder with respect to Datum B I get extremely small numbers such as .00001.

If I report the perpendicularity of the plane with respect to the Datum A cylinder I'm getting measurements ranging from .003-.005.

The range comes from checking different parts but the measurements are consistent for each individual part.

I'm measuring cylinder A with two constructed circles using a 3mm dia stylus. Im using PCDMIS 2022.1.

Any help would be much appreciated. Thanks.

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  • Hi,

    this problem is quite common. This is neither a measurement error nor a machine error. The computer does everything right.
    The problem lies more with the engineer who thinks that the large plane has a secondary meaning.
    in reality, the cylinder A is much too short to represent the main axis (specify the vector)

    imagine you are trying to check the problem with a hand measuring device (e.g. a steel angle). I doubt you see a slit of light within the .25 height.
    (datum A is only to centering in 99% of these cases​)


    you can get around the problem by evaluating the concentricity to B|A instead of just to A

    I've already experienced that the "designer" can be very stubborn and doesn't see it, but it might be useful if you ask him if you're allowed to do it that way.​
  • I hate seeing this on a print. I know I'm gonna have to explain why the big variation every time. So imagine taking a gauge pin the size of your A datum and putting a ring gauge on that pin 0.25 down on to the pin, you will most likely be able to easily tilt that ring gauge from side to side. There is just not enough cylinder there to hold a ring gauge in place. Only the bottom surface would be keeping it from tilting, not the a datum cylinder. This is the same way a machine is going to react when measuring it. A 0.0001" out of roundness or size difference is going to affect it. If you feel you can't get through to engineering on the matter you could take a plane around the cylinder before measuring it. (kind of lying basically) It makes it more consistent.
  • in reply to markoor99
    Thanks for the reply, I see what you are saying regarding the cylinder size.
    When you say take a plane around the cylinder before measuring, what do you mean by that?

    I think I figured out what you mean, basically measure a plane that slices through the cylinder?
  • in reply to Henniger123
    Ok so it is because the cylinder is too short. Thanks for the reply.
  • Something else to consider is flatness of the face as this will have a direct bearing on the perpendicular result. I know your drawing does not show a flatness but it's always worth calling out.
  • in reply to markoor99
    something like a projection,
    the cylinder then has the same vector as your plane

    Construct Cylinder (Project Cylinder) first input a circle, second input a Plane
    (if you use only the circle then it will project the cylinder to the workplane.)
  • in reply to markoor99
    I created a projected cylinder but it doesn't seem to let me measure perpendicularity of a projected cylinder.
  • in reply to markoor99
    There is an option for sample points when measuring a cylinder. Thats what I meant about a plane before the cylinder. But that depends on if there is a plane perpendicular to the surface.
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