Creating an alignment on a cylinder

[[Je...]]

I am very new to Calypso, I used PC-DMIS for the past 5 years so taking me some time to adjust to this new programming. Every part I have is a cylinder so Im having an issue figuring out where to even begin for my alignment. Anyone have any ideas on how to tackle this? Thanks in advance

[[Cl...]]

IF the cylinder length is at least 3x it's diameter, you can use it for a spatial rotation. I use them all the time in Z+

[[Jo...]]

This has caused me fits too, especially without a model. It goes crazy unless you take 4 hits on lower level THEN take 3 or 4 hits on upper level. If you can use a start alignment to something easy. Then your cnc alignment to use the cylinder. I think a lot of the problem is with short cylinder height and manual hits it is having a hard time with vector.

[[Ow...]]

You can almost always get help here, although not always in a timely fashion.
However, for a beginning Calypso programmer, I’d recommend you purchase the Zeiss strategy cook-book. It explains a lot of strategies/evaluations for all kinds of parts and will most likely prevent some costly if not embarrassing mistakes.

https://shop.metrology.zeiss.com/INTERS ... gID=200000

General practices for spatial alignment:

The print datum's, tolerances and probe access may force otherwise but generally, for stability, you want to use the largest / longest surface for your spatial axis or you will end up causing projection errors.

Using a shaft as example, if you took a 1.0 inch dia rod that you knew was straight, and set the spatial axis off the end face of the rod, and the origins off the diameter by the face, when you measure the rod 2" or 3" or even more down the length of the rod, it will show it not being straight, even though you know it is.

Another example for planes/faces is if you used small planes for spatial then you may get location errors when projecting the features farther away than the size of the plane. i.e. if your spatial surface is 3x3" wide, then you wouldn't want to trust the location accuracy of features farther than 3" away from that surface

[[Ow...]]

Attached is a tech memo on alignments put out by several years ago but, still prevalent .

[[Ma...]]

Can cylinders not be used for rotational direction in Calypso? PC DMIS assigns a vector direction based on the direction of hits, so you can use them for rotational datums.

Is this not the case with Calypso?

[[SH...]]

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Yes, in calypso also change vector direction according to the direction of taking point, I think it common in all cmm.

Cylinder can be used for clocking , depend upon the orientation of cylinder and the drawing datum specification.

[[Ke...]]

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It's not just short cylinders. Poor manual probing on long cylinders gives the same issues. A simple Start Alignment works 100% of the time for me

[[Je...]]

So with this part what would form my alignment?

10732-XX revE.pdf

[[Me...]]

Plane on the end, circle on the .980 diameter and a line on the flat on the side for clocking. There is no GD&T so this is all linear and block tolerance dimensions. You could align the part numerous ways depending on how you place it on the CMM.

[[Me...]]

Out of curiosity, how would you align this in PC-DMIS? It should be no different with Calypso.

[[Je...]]

I went from measuring blocks that had a 0,0 point so it was very easy to follow a regular plane, line, point alignment. Now im measuring cylinders only. Its just an adjustment.

[[Cl...]]

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It's been a while, but yes, pretty much similar. If, for example the cylinder
was laying horizontally facing y+, you would rotate (level) to Y+ about Z.
Like I said, its been a while, I could be mistaken?

[[Ma...]]

I always shy away from using a cylinder for spatial. Instead I opt for a 3d Line with multiple circles making up the center points to construct the 3d Line. It has been my experience that the form of the cylinder can come into play when using the cylinder as a spatial orientation.

Mark

[[Ja...]]

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Mark,

I am curious about the influence of form on spatial orientation. I would assume that form would have an effect on spatial orientation only in the case of short cylindrical primary datum features.

Do you have a particular example that I could try to wrap my head around?

[[No...]]

I think most of the problems with cylinder vectors are due to possible discrepancies between the CAD display and the feature parameters. A manually probed cylinder can end up with a flipped vector, but you don't see it in the CAD window (or you do, but only if you have a good understanding of the local element coordinate system).

Basic rule: Never trust the CAD display! Always check the parameters in the feature dialog, especially the direction of the space axis, even if all the points look right.

[[Ma...]]

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There is an ISO spec that deals with this but I can't recall what that ISO number is right now. I'll look it up.

Just to be clear I'm not saying that you can't or shouldn't use a cylinder but for me the form measurement deviation affects the position and orientation of the cylinder. As Calypso creates a best fit cylinder, taking into account the evaluation method you have selected, things can change. Let's say you measured in LSQ but you have Form Datum turned on. Form Datum will definitely change the evaluation method when that cylinder is used as a datum.

I'd rather take multiple slices in LSQ and create a 3d Line when it comes to using the feature as a spatial orientation. This method eliminates most of the form deviation. Then go back and recall or remeasure the cylinder for form or other evaluations.

Just my two cents

Mark

[[Ma...]]

From The CMM Handbook

Datum Definition When the Datum Is a Cylinder


A datum is defined in ISO 5459:1981 [2] as:
.A theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which toleranced features are related. Datums may by based on one or more datum features of a part..

When measuring a part with a datum feature requirement that requires you use a cylinder as the spatial orientation datum (base plane datum) there are several considerations you must keep in mind. The surface of the datum cylinder will never be ‘true’. When measuring a part surface form, waviness, and roundness deviations will cause some variation in the calculation of the cylinder thus changing the orientation of the cylinder. The orientation (See Figure 1 a) is the angular deviation of 3d line through the center of the cylinder. It will be imperative that you take as many points when measuring a cylinder on a part that are equally distributed around each circle segment of the cylinder to create a more consistent best-fit cylinder.

A datum simulator is the best method to get consistent measurement values.

When creating a simulated datum the specs state:

ISO .A real surface of adequately precise form (such as a surface plate, a bearing, or a mandrel, etc.) contacting the datum feature(s) and used to establish the datum(s).
NOTE: Simulated datum features are used as the practical embodiment of the datums during
manufacture and inspection..

ANSI Y14.5: When a diameter is designated as a datum feature, the datum axis is derived from placing the part in a datum feature simulator. This simulator can be a pin gage (internal) or a ring gage (external). The datum feature is the surface of the part. The gage, the pin or ring gage is the simulated datum and is the datum axis.


Figure 1: Datum is the axis of a cylinder

If you measure the physical hole or the datum simulator an issue will arise. See Figure 1. This shows the largest inscribed cylinder and its axis for a hole. The orientation of the cylinder depends on the surface condition and form of the hole and might therefore not produce an unambiguous orientation. For that case the standard requires an equalization of the angles a in all directions of space. The same will be for the smallest circumscribed cylinder respectively and consequently its axis for a shaft. Due to the geometry of a cylinder it lies on two planes of orientation. These planes are the center axis of the cylinder and will be averaged during the spatial orientation process of establishing a datum structure on your CMM.

[[Ja...]]

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Thanks Mark, very interesting information. I will keep this in mind going forward.