Base Alignment best Practice

[[Ja...]]

I have a customer who inspected a 1st article we sent to them and rejected it for the TP being out of spec and the bolt circle diameter being out of spec. On a call with the customer they conveyed that they used Cylinder for the rotation in their base alignment.
The true position is .004 to Datum -A- which is the OD

I used a 4 point circle and then created a 3d line for my rotation in my base alignment.

My results and their results are drastically different.

Which way is the best way to achieve the best results?

Thanks

[[Jo...]]

I always use a projected line for rotation. Dunno if it matters, but a 3d line is likely a compound angle.

[[To...]]

Please add more details. Is it True Position to A only? Please attach drawing or sketch, or any part of.

In Calypso, if you use a plane for the Spatial and Z origin, and a cylinder for the X/Y origin, you can't use a cylinder parallel to the first cylinder for the Planar rotation. A circle will work all by itself for the Planar rotation. The center point of the circle feature will automatically project itself to the top plane. A 3d line from the circle to the cylinder will work too, but it is totally unnecessary. The fact that the 3d line might be angled has no impact either because it is only going to rotate about the Z axis until it is aligned with the closest primary axis. Remember the function of the planar rotation feature, in this case, is to stop rotation about the Z axis.

[[De...]]

I have found that the best (and most repeatable) results generally come from using a 2D feature for planar rotation.

A cylinder is a 3D feature can be misaligned in relation to the feature used for your spatial alignment and origin and therefore may cause problems with the repeatability of your measurements.

Having said that, the base alignment should not have an appreciable effect on true position if the true position characteristic is referencing the datum features correctly in the datum reference frame within the characteristic.

The base alignment should only be providing a repeatable means for 'finding' the position and orientation of the part. Then the features can be measured that matter to your final result.

If the results have major differences between them, I would look at the data for the features themselves. Is there a lot of form error? Is the probe shanking? Was the probe or the part dirty?

If that is not it, then the next step would be to verify that all the settings for algorithms, filters and outliers are correctly set in both programs.

[[Ja...]]

The true position is only to -A- and -A- is the OD

The picture shows what I did for alignment

and what the print callout shows as well

Alignment question 2.PNGAlignment question.PNG

[[Cl...]]

In this scenario, I would project the circles onto the
(same) top surface, so they share a common plane
(Z=0) and the construct the 3d line.

[[Jo...]]

screenshot of your alignment ?

[[Me...]]

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Are you and the customer both doing a best fit alignment and allowing rotation around Datum A for the hole pattern? Maybe that's the difference in results?

[[Ow...]]

If you're recalling all the bolt holes into a circle for the 8.268 bolt circle, make sure you're using just recall, not recall feature points.

[[De...]]

If they are only called out to A, then they can freely rotate around the Axis of A, however unless you did somethign to allow them to freely rotate then Calypso will automatically default to the rotation from the base alignment.

This is likely why your results do not match your customer's.

There are a couple ways to allow the pattern to freely rotate about A, the way I get around this is to do the following.

Step 1, Create an alignment to Datum A
Spatial : Datum A
Planar: N/A leave blank
X Origin: Datum A
Y Origin: Datum A
Z Origin: this will only determine your Z origin - I generally leave it blank so the zero point is the same as the base alignment.

This creates an coordinate system that is centered on the Datum A Axis.

Step 2, Create a Geometry Best Fit Alignment

From the Resources Menu, choose Utilities, then Geometry Best Fit

Open the Geometry Best Fit in the characteristics screen.

Where it says alignment, choose the alignment you created in step 1.

Click the Select Elements button, and choose all your cylinders for your holes.

Click the Evaluation Constraints button, and make sure that rotation around Z is checked and all others are not.

This creates a new coordinate system which is determined by using the Alignment to Datum A as a starting point and rotating the data for all the cylinders around it's Z axis until the rotation is found that gives the best results for all of the features chosen in the select elements step. This rotates all of the features as a group but allows the pattern to freely rotate around Z so the base alignment is not included.


Step 3, Create your True Position characteristics.
Inside each True Position Characteristic, you can now choose the Geometry Best Fit alignment from the dropdown menu in the Datum Reference frame area.

The other method for doing this to click the Bore Pattern button inside your True Position characteristic, and then clicking the Best Fit Icon to setup your pattern. In the window that comes up you will want to choose select elements and then choose all your cylinders. then make sure rotation is checked and translation is not.

Then hit OK and back in your characteristic setup your Datum Reference Frame as you normally would.

This will essentially do the same thing as the steps I outlined above, personally I like the first method because I control the entire process based on what I want it to do, and because it gives more flexibility in reporting the results. That is only my opinion though, either method should allow you to unlock the rotation of the pattern from your base alignment.

[[Ja...]]

Owen Long, I did do the bolt circle as just recall.

[[Jo...]]

what do your actuals and nominals look like in the constructed bhc ?

[[Me...]]

I don't understand the need for a constructed bolt circle. It's a basic dimension and the position of the bolt circle isn't the same thing as the position of the individual bores. Maybe I'm missing something.

[[Br...]]

Derek laid it out for you step by step. Datum A is your primary Datum feature. You need to create the cylinder in order to simulate the datum axis that will control the orientation of the datum reference frame. I like to use the second method Derek brought up though..

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To me, this is a lot cleaner and allows you to use the bore pattern plot which is an extremely helpful visual aid. I'm not sure why you have less freedom with reporting here though? I feel as though I have the same as the first option. Either way do what Derek said and it will be right to your drawing. No need to make a circle out of all the circles though. Are you trying to report basic dimensions or something?

[[Ma...]]

Hi

I have a question, are you making Datum A as a circle? because you should do it as a cylinder. It is also not necessary to make the 3D line, with the circle it is enough to rotate the alignment.

I think you can use the bore pattern option within the position, this calculates all the positions of your holes.

Regards

[[De...]]

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It is nothing major, its just when I report a pattern using bore pattern it reports featurename^1, featurename^2, etc.

One of our customers requires us to have holes called out as featurename A, featurename B, etc for patterns of holes.

[[Br...]]

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Oh yeah I see what you mean. You can probably customize something like that with Piweb.

[[Ri...]]

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Quite similar to what I practice with one addition. Since 2001, my first time with CMM's, I was told the Base Alignment should only be used to establish the location of the part plus a Clearance Plane (What exists in the volumetric envelope of the CMM), My Base Alignments are always simple 3,2,1 alignments or as simple as can be regardless of datum's. I keep it simple to align.
After that, I create a group called "CMM Alignment Features", this is what will control Datum's, features and additional alignments thru out the program.
This also stems from old school information, that Base Alignment features do not utilize Filters and Outlier elements (I'm not sure if that is still true today, haven't been to a seminar in quite a while), about 5 years ago I was at a local "Lunch and Learn" and recall the instructor saying the later is still true.