Building modular CMM part holding fixtures

Written by Dan Smith

So – engineering just brought you a new part that is going into production, and you need to measure it with your CMM. Sometimes it’s a breeze, but other times the part shape and/or the datums and features of interest on the part make it a very challenging problem. Some of you already have this down smooth, maybe because all the parts you get are similar, and you figured all this out a long time ago. For the rest of us, here are some of the things in my checklist when I am asked to set up a holding fixture.

Before we go any further, this discussion is directed at parts that are rigid enough such they can maintain their shape in an open setup. It also does not address the situation where a part is required to be held in “body position”. Those conditions require a different class of fixture.

It may be the case that this is a long run production part, and management is thinking that they want a dedicated fixture. Maybe you are one of the folks in the camp that believes there should always be a custom fixture, or maybe you don’t like to deal with fixtures and would rather delegate all that out. Well keep in mind that if you delegate it, you lose control of what they give you. A modular fixture system is a good way to develop and test concepts and the proper approach to holding that part while it is still in the prototype stage, and can save a lot of potential aggravation that occurs when you are given a CMM holding fixture that was developed without any input from you.

1) Review the features to be measured: The first obvious starting point is that it’s about the part, and what needs to be measured on the part. Just like writing the actual CMM inspection program, the first task is to review all the parameters of the part inspection requirements. Ideally you can develop one fixture setup that will allow you to measure everything in one position. Job one is to consider what position allows the part to be fully interrogated. Unfortunately, that is not always the simplest task from a fixture standpoint. If possible let gravity be your friend when establishing the part position for the fixture.

2) Part Position: given the features that need to be measured, try to visualize a part position that gives the CMM access to all the datums and features of interest. If multiple setups are required to get to all the part features, it is likely that multiple setups were also required to machine a part completely, and your multiple setups can simulate the part position for specific operations, and your CMM routines can be developed to measure the features produced in those specific operations.

3) Positioning the locators: once you have a position in mind, try visualizing a 6 point nest, so you can immobilize that part in space. Sometimes you need burn a few brain cells to figure it out, but it is almost always possible to get there. Think 3-2-1. Just like the datum setups for a CNC part program, most parts can be staged with a plane-line-point scheme. Build the fixture the same way, going from plane, to line, to point.

Now you are looking at that part, and your eyes are drawn to all those nice prismatic machined surfaces… sure would be nice to locate on those… but if you do – are you are still able to measure that surface or an adjacent feature of interest? Oops we have a problem, can’t locate on that. Not to worry (yet), remember it’s usually better to give the CMM full access to those anyway. Keep in mind – fixture builds tend to be much more stable when the locators are positioned near the outer periphery of the part.

4) Test the fixture stability: OK, so you have a plan – go ahead and try to build it up. Don’t get all happy yet, now do a little test, either light tapping or a bit of a wiggle test. Is the part truly immobilized? Sometimes it seems like it should work, but for some reason it doesn’t. If this is the case now, consider:

– is the part truly seated in the fixture, as in do all the part surfaces touch the fixture locators?

– is there a clamping force that is defeating a locator, or pulling it off the locator? Keep in mind that best practice is to clamp directly against a locator, and/or clamp “inside” the locators, for example if you are setting A datum plane with (3) locators, make sure your clamps are inside the triangle formed by the locators.

If you are still having trouble stop and think about it a bit before you start tearing it all down. If the magical aha moment doesn’t come to you, well sometimes you just need to start over. It may be that there are other surfaces you can locate on, or at least space the locators out differently on the surfaces till you get a secure fixture.

A solid percentage of the time, I end up modifying the fixture from the original concept. That is the beauty of a modular system; you can play around before you commit (something your mom would frown on). Very often I don’t get called in to a company until someone has one of those #@##! parts that is driving them nuts trying to find a way to hold it on their CMM. Since this is usually a product demonstration, I have to develop a fixture on the spot and make it all look easy (it’s my job). Sometimes it just takes a couple tries to get it just right. I can say the infinite adjustability of a t-slot style fixture is very helpful – rarely is a part geometry suited to match up perfectly to the specific increments of a grid plate system. It also seems like there is always some little obstruction that required a slight adjustment of a locator position, or the locator needs to be moved just a bit to give the CMM better access to a feature of interest.

If you are inspecting a part that is flexible, like many plastic or thin walled parts, it is very easy to distort the part surface with clamping forces. Ideally in this case you clamp directly over the locators, but this is not always an option. Given the low force that a CMM touch probe generates, a low force clamp or one that traps the part in position by touching the part surface in the direction opposite the locators can solve for this. Here is a photo of a self-wedging clamp we use that addresses this issue:

This clamp assembly can be used with a swivel head allowing the clamp to exert force normal to the part surface.
About this article:
AA Jansson is a distributor for FixLogix modular fixture systems. This is a t-slot style system that is very flexible, often greatly simplifying fixture setups. The FixLogix system is also less than ½ the price of the old grid plate style systems, while offering superior functionality.
Dan Smith is a sales engineer at AA Jansson, with 32 years of experience in the metrology field. He is also the inventor of the FixLogix system, and has written many articles about fixtures and other metrology topics for CMM Quarterly and other trade journals.

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