Alignment for very long part

[QCinTN]

I have a long part with a Datum slot on one end and a Datum hole on the other. Due to machine size limits, I can't get to both of them in one set up. My idea was to measure all of one end using some alignment holes other than the datums and then move the part and check the other end using those same alignment holes. What I don't understand is how I could use a Datum hole in the first set up and a Datum slot from the second set-up for the final dimensioning. Any ideas on how to do this? Thanks.

[bw_bob]

The datums are on the print for a reason. You have to set your part up using them. It sounds to me like your SOL....Sorry

[#2#]

It depends on what other datums you have. If the slot is a referenced datum for different features than the hole then you might be able to do some measuring. If you need both the slot and the hole to do an alignment you are most likely SOL.

You could measure it by using a created feature at a location relative to your "alignment holes" to simulate the hole datum but I wouldn't.

[Wes Cisco]

I have a long part with a Datum slot on one end and a Datum hole on the other. Due to machine size limits, I can't get to both of them in one set up. My idea was to measure all of one end using some alignment holes other than the datums and then move the part and check the other end using those same alignment holes. What I don't understand is how I could use a Datum hole in the first set up and a Datum slot from the second set-up for the final dimensioning. Any ideas on how to do this? Thanks.

A sketch would help. What does the Datum slot constrain. Can that be constrained by the alignement holes you mention, also how precise are these alignement holes? You might be better with clamping a dog to the part and using it instead of the holes. Make sure you put in your report the method you use whether it be hole or dog. Strictly speaking Bw_Bob is right, but sometimes you gotta do what you gotta do, just make sure you let the customer know what you are doing. CYA HTH

[QCinTN]

It's a basic flat sheet metal part (couple of flanges). Datum A is the main surface, B is the hole & C is the slot. Everything on the print is called out as TP 1mmc A|B|C. I was hoping, with that kind of tolerance, that I could come up with something that could merge the set-ups. Oh well. Thanks again for the responses.

[bw_bob]

Go to your customer and tell him your CMM envelope is not large enough. Maybe he can help you out. ???

[cmmguy]

Measure the first datum that you can "see". Measure the two holes that will be visible from both ends. Setup an alignment on the two holes. Report the datum feature from those interim alignment features.

Now move the part, remeasure the interim alignment features and set alignment on them like you did before moving. Recreate the first datum using the reported information from the first position. This virtual feature should be off the machine somewhere.

Now measure the second datum and establish your datum between this second measured feature and the virtual datm that you created.

If you have features on both sides of the move then you will need to be careful on how you get their information. I thought there was a "move part" feature in PCDMIS.

(or)
Buy a bigger CMM

[Don Ruggieri]

Equate Alignment

You should look into the Equate alignment and/or the LeapFrog functions of PC-DMIS. I believe you can accomplish your goals with these. They do add some risk/variation to the whole process, but sometimes there is no alternative. . .

[Wes Cisco]

You should look into the Equate alignment and/or the LeapFrog functions of PC-DMIS. I believe you can accomplish your goals with these. They do add some risk/variation to the whole process, but sometimes there is no alternative. . .

Thanks Don, I am going to add that to my collection of euphemisms.


Manager : "The customer rejected these parts you checked. What happened?"

Employee : "Gee, I don't know for certain, but I would suspect the abscence of an aggressive metrological equipment acquisition policy has necessitated the use of a procress that incorporates an unacceptabel level of risk/variation that was not noticed because of the continous throughput encouragement environment.

[BOB PARSONS]

Big Part

WE have done this many times pretty much how cmmguy said in his above
post, use common sense when picking your virtual alignment, with the
tolerance you have, you should be fine. Also, will the part fit if you put it on a 45
degree angle, the part doesn't need to be straight on a cmm, if all the features are
on the top.

[dccfreak]

The equate alignment has worked well for me many times in the past. There is a potential risk/variation factor involved, but, if done correctly this should be a minimum. First, try to locate some good well defined features somewhere in the center of the part while a fair distance apart. This way, you can establish a solid temporary datum structure while measuring your part. The key is to locate features for this alignment that can be utilized in both part positions.

Basically, use these features, align measure all of the features within the envelope of the machine, move the part forward or back (depending on whichever position you choose to begin with) and then repeat the previous alignment used in the new position. You can then measure the remaining features. With all features measured, you will have a pretty screwy looking graphics window, but now, you should be able to go to the alignment menu and choose the equate function. The key here is to equate the most recent alignment to the first. This will "marry" both halves and give you something that will represent the entire part and if done correctly should appear as such in the graphic display. Now you can use your print datums to perform a final alignment which should allow you to dimension the part with a minimal amount of error. Be aware that the point data taken for the temporary alignment features must be taken very critically to reduce the margin for error.

When you have no other alternative, this is very useful, but it should only be used as a last resort due to the risk of error involved. HTH

[Matthew D. Hoedeman]

Can you reach both holes if you use a star probe and a 82.5,0 and 82.5,-180 angle on your head? I have had to do this type of thing. Depending on the amount 'too long' the part is, you might be able to get them both. I have effectivly increased the 'length' of my CMM by 10 inches using this method. Sure, it sucks, but it will work.

[Paul Barriage]

I would consider making a fixture, at least for the datum holes. The level points can be balanced.

Make the 2 pins for the datum holes, then holes or pins closer, so you can reach them both directions. Check the locators within tolerance to these alternate fixture holes.