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# CMM fixture design help needed

## CMM fixture design help needed

(OP)
Hello,
I am a mechanical designer used to design welding fixtures and visual inspection fixtures but for the first time I am asked to design a fixture for a subframe part to be measured in a Coordinate Measurement Machine(CMM).The CMM is Wenzel brand bridge-type granite CMM using Metrosoft software. I am new in CMM subject and if someone experienced can answer my questions I will be glad.

Here are my questions:
-How can I fix my fixture to the CMM?
-Does it matter which features I use to align the part in the fixture? If I use some holes where machine needs to measure, how can it measure them?
-If I use two holes to align the part(one round pin, one diamond pin) must the pins pass the holes tight?
-Does the machine use its own coordinate system or does it zero itself by using RPS holes of the part which are mentioned in the part drawing? Or If something else, how?
-If the machine zeroes itself by using RPS points how we can measure if RPS points are at the correct place?

these are the questions in my mind now. Thanks in advance.

Regards,
Taner

### RE: CMM fixture design help needed

Do you know and understand GD&T? The granite surface should be your primary datum plane. Features to use to align the part are dependent on the requirements of the GD&T defined on the drawing.

Can you attach your fixture to the table frame holding the granite block? I've also seen some metrology blocks with threaded inserts, flush with the top surface. These are used for bolting fixtures to the granite block.

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### RE: CMM fixture design help needed

As MadMango says, you need to look at the drawing of the part to be measured and make sure your fixture simulates the explicit or if not such a great drawing, implicit datums.

If memory serves there may be some slight differences based on if drawing is to ASME or ISO stds on how you need to simulate the datums etc.

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### RE: CMM fixture design help needed

(OP)
Thanks a lot for your answers. Yes, I understand GD&T. So, if the primary datum plane is granite surface, the fixture must be very accurate so that the parts Z direction and the CMMs Z direction will be the same.
The part to be measured is similar to this one: The A reference is bottom surface of 4 bushings at the edges(A1 A2 A3 A4) in which number 2 and number 3 elements go. B reference is a hole next to one bushing on the left and C reference is a slot hole next to the other bushing on the right. So do I have to use these features to align the part in the fixture? If yes, how will CMM measure those holes if there are pins inside? Do I have to remove the pins once the part is fixed? Or does my fixture have to be enough accurate that if part goes in, I assume those holes are in tolerances?
About the granite block I am not sure yet if I can attach the fixture since the CMM is not in our plant but in customers. I am waiting answer from customer about it.

### RE: CMM fixture design help needed

A fixture for CMM is not normally needed.
If this part is "stiff" or "strong" enough to stand on it's own(no deflection)then it would simply be placed on CMM table, and aligned using datum features.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

### RE: CMM fixture design help needed

Based on ASME Y14.5M-1994

"So do I have to use these features to align the part in the fixture?"

Yes I think so in order to inspect per the drawings intent (unless there is some way to fudge it with the CNC as HGMorgan says, but the fact you're developing a fixture implies it cant' be faked adequately) - though having four datum points defining datum A is a little unusual if it is a rigid part.

Potentially measurement of the size of the holes could be done as a separate step - I think. Sorry I'm not an inspection guy just moderately GD&T competent.

You say you're new to CMM, are you used to inpection gauging and knowing how much of the tolerance you can use up on your tooling etc.?

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### RE: CMM fixture design help needed

(OP)
OK, so I think I should use one pin in the B datum hole, another pin in the C datum slot hole and 4 Z supports under 4 A datums and clamp the part only on 3 of them, otherwise the part might bend. Maybe I should remove the pins after clamping, I will discuss this with customer.

KENAT, I am new to CMM and also not experienced in gauging. But we are not the one who is producing the part. it is some other manufacturer. We will only design the holding fixture for CMM.

Another question: where is CMMs 0,0,0 point? how does it zero itself regarding to the part?

### RE: CMM fixture design help needed

You don't "clamp" the part down to Datum A, as that would imply you are purposefully deflecting the part to make contact with the fixture. The part should lay on Datum A in the "free state", located by your a pin (diameter sized for allowable tolerance) for Datum B, and a possibly a rectangular block (sized for allowable tolerance) to register the slot for Datum C. If Free State is not called out on the drawing, then you might want to ask the manufacturer if clamping is allowed.

The CMM operator will define the zero point by touching a probe to the defined Datums. Software will then calculate deviations from the datum planes when the part is probed by the CMM. You just need to know the operating envelope for the CMM probe, and ensure the part and fixture are within that operating space.

"Art without engineering is dreaming; Engineering without art is calculating."

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### RE: CMM fixture design help needed

A CMM is like all CNC machines, it has it's own Machine Axis (0,0,0 origin point and XYZ vectors).
When you place a part on CMM and align thru datum features, you create a new "Local Part Axis" per Eng drawing/model.
The part can be sitting anywhere on the CMM table at any kind of angle.
The alignment datum features are checked to show good alignment, and the rest of the features are checked from there.
The alignment to Eng Datums (per drawing/model) makes it easy to output a CMM report that relates to the part dimensions & tolerances.
No fudging or faking.
CMM's eliminate most fixtures. A flimsy part might need a holding fixture (we have used shims at times), but most products don't need a fixture.
We will sometimes use clamps (very light pressure), tape, clay or hot glue to hold a part from moving, but the holding method can't deflect the part in any way.
Also note that eng will sometimes call out allowable restraint, I.E.: 5 lbs of pressure every 12 inches, etc.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

### RE: CMM fixture design help needed

HGMorgan, my understanding is there is some difference between use of CMM to measure to ASME or ISO stds based on how the dataum planes are defined.

For instance measuring a number of points on a surface and coming up with an average surface plane to simulate a datum would not really comply with ASME datum plane definition.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

### RE: CMM fixture design help needed

Hi Kenat,
We are getting off topic here.
Unless that part is so flimsy that it can't hold it's shape, we do not need a holding fixture.
I can place the -A- datum of that part on the CMM granite surface table and check it with feeler.
Then I take 3 points on CMM surface table at edge of part envelope and create CMM -A-.
A CMM only needs 3 points to create a plane, it is not required to take a bunch of points and derive an average plane.
There are times that multiple points and an average plane is useful, however, not usually for alignment (unless call-out on ENG).
A discussion about datum simulation can get long and involved, however, all nominal theoretical datums have to be simulated in our real world, with the tools and processes we have at hand.
A surface table and the high points is just one way to do it.

Thanks!

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

### RE: CMM fixture design help needed

(OP)
Really thanks all for helpful comments. I didnt know that it is written to drawing how the part must be measured. After I read MadMangos comment about "Free State" I checked the drawing and It is written on the bottom edge that at "measurement fixture in Z is clamped on features RPS3,4,5 and RPS6 and max entanglement from RPS6 in Z +-2mm"(I translated from German). Now it is clear that I have to clamp the part on those points. Does the +-2mm tolerance mean that after RPS 3,4,5 are clamped, when RPS6 is clamped it can deflect part max 2mm in Z? HGMorgan; is this the similar thing you said about "allowable restraint"?

HGMorgan; you say that "The part can be sitting anywhere on the CMM table at any kind of angle.". Does that mean that my fixture does not have to be so accurate if it doesnt matter where the part is? So no machining needed at the fixture frame. Am I right?

### RE: CMM fixture design help needed

The only benefit to a fixture on a CMM is to eliminate the first-run manual setup to 'teach' the machine where the part is. There are two alignments. The initial alignment are the features you pick to touch that tell the machine where the part is on the table. These have nothing to do with datums or measuring. Think of it as just teaching a "blind man where his coffee cup is" so he can then infer the datums (knowing where the features are because obviously he knows what a coffee cup looks like) and then being able to touch off on the features (before picking it up).

So you put your part on the granite. You either put some pins or stops or something to mark where it is, so you can put each and every part in the same place and run it on 'auto pilot' so to speak. You then manually drive the probe to touch the surfaces you've indicated in your software to teach the machine where the part is. Then your program begins - it seeks and touches the datum features to create the alignment - then it goes and touches the features to inspect. The datums are independent of that initial manually-recorded alignment that just says "Here's my part" before you tell it to "go measure stuff".

When we have to run inspection on 100% of parts or if we have to run a large sample, we just set up some pins (there are threaded inserts in the granite, and we have grid plates we made to go onto those if needed) to use as locating stops. When it's not feasible to have pins in the way, we'll sometimes just have a 1, 2, or 3 posts and hot-glue the part to it, when practical. It varies on geometry obviously.

Forgive me, my terminology is crap. I hope I was clear enough, though. We use Calypso with our Zeiss CMMs but I expect it's similar.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

### RE: CMM fixture design help needed

(OP)
No worry, JNieman. you gave me very good information, thanks!

One question: if I use pin in a hole, does it need to be a hole which doesnt need to be measured? How otherwise can it be measured if there is pin inside?

### RE: CMM fixture design help needed

The minimum requirements to establish a reference coordinate system with your CMM is 3 points for the primary datum plane, 2 points for the secondary datum plane, and a single point for the tertiary datum plane. Using an inspection fixture can make things easier when checking large numbers of parts. But it is no problem to establish a dimensional coordinate system simply by probing a few surfaces of the part. And the engineering drawing should provide all of the information needed to establish these datums.

### RE: CMM fixture design help needed

Hi Taner41,
Sorry I didn't reply sooner, got slammed with some hot projects.
If a fixture is used, it will have to be accurate.
What we are saying is "you don't need a fixture".
I guess my 1st question should have been, what is driving the need for a CMM fixture?
A CMM fixture with the accuracy and features needed would not be far removed from being a Check Fixture, and if you have a CF, there's no need to go to CMM.

As far as holes vs pins, the CMM can survey either one: 4 pt's on outside of a pin or 4 pt's inside an ID.
We don't normally use pins because of pin to hole fit deviations.

As far as the RPS6 +/- 2mm, that sounds like "designated restraint"(ENG is telling use how to restrain the product),
I.E.; clamp RPS3,4,5 on 2mm block(those 3 points should clamp flat) and RPS6 can range from touching surface table, to up to a 4mm gap from table.
Impossible to say for sure without review of drawing.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

### RE: CMM fixture design help needed

(OP)
I am not sure but I think these can be two of the reasons why they need a fixture: 1. Because there are also features to be measured under the part, if you put the part directly on the granite, you cant measure them. 2. (even though I find it weird) Some customers want to measure the part as it is assembled to the car. So when it is clamped.

#### Quote (HGMorgan)

As far as holes vs pins, the CMM can survey either one: 4 pt's on outside of a pin or 4 pt's inside an ID.
We don't normally use pins because of pin to hole fit deviations.
I was thinking about this one but I thought they wouldn`t use this method because of the reason you said. You lose accuracy because of the fit clearance. Could it be possible to remove the pins with a sliding mechanism after the part is clamped?

#### Quote (HGMorgan)

As far as the RPS6 +/- 2mm, that sounds like "designated restraint"(ENG is telling use how to restrain the product),
I.E.; clamp RPS3,4,5 on 2mm block(those 3 points should clamp flat) and RPS6 can range from touching surface table, to up to a 4mm gap from table.
I think this is good solution.

#### Quote (HGMorgan)

Impossible to say for sure without review of drawing.
Unfortunately I am not allowed to share the drawing.

### RE: CMM fixture design help needed

Okay making a couple of assumptions, this part is for automotive and needs a fixture to support it.

Pretty much all automotive work will be measured in carline but the fixture does not need to be. The normal case is to create a plain from the top of the plate and have four ground bushes that when they intersect this plain will have X,Y&Z dimensions referring back to carline, see examples on website.

From the base plate you build up support, usually the exact points are denoted on the drawing and will often he held down with toggle clamps or similar directly above these. The location points will again usually be denoted on the drawing and will generally be a pin and diamond pin, better still sprung tapers so diameter is not a factor.

Not sure if that makes any sense, but I know what I mean.

### RE: CMM fixture design help needed

(OP)
Thanks ajack1. I understand what you mean. But wouldnt it make more sense to create CMM coordinates directly from the part, not from the fixture? Then we would eliminate the initial fixture manufacturing tolerances, from the bushings till the locating pins.

### RE: CMM fixture design help needed

You can do that but once you go away from carline it does not relate to how it interacts with other parts. The part doesn't have to be in carline it can be whatever suits best for measuring or manufacturing.

Working from the fixture gives greater control than just working from the part, especially on parts that are pressed or moulded with lots of 3D form.

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