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by Bill Reilly

How many times have you spent hours struggling with vices, blocks, clamps, and even glue, in an attempt to create some sort of rudimentary CMM fixture to hold the part you need to inspect? If you just rolled your eyes, then read on.

Having spent the past 20 years programming CMM's and inspecting an untold number of parts, I've got loads of experience with part fixturing applications and have often been frustrated trying to find a suitable method of restraining a part for inspection utilizing "bits and pieces" from the tool shop. More often than not, a custom built CMM holding fixture is either not in the budget or is an afterthought, having been ignored during the planning stages. Now the CMM programmer has to figure out some way of holding the part, which is not always simple, and they're usually under the gun to get the part inspected after every other stage in the process has eaten up all the time.

I hate to paint a picture with one broad stroke but it's been the truth in the vast majority of cases over the years. The inspection process is typically an afterthought and once parts are available, and all the time has been used up, the CMM programmers/inspectors are expected to pull results out of their...uh...hats, let's say hats. And, of course, it needs to be done today.

The CMM programmers reading this will relate. The inspection process is an experiment of sorts, and, as with any experiment, there are variables which must be controlled in order to obtain accurate and reliable results. Perhaps the most critical of these variables is the method you'll use to restrain the part, and the fixture is the key to replicating your experiment. This is your starting point and it needs to be right.

Proper part fixturing requires the part to be restrained securely, while still leaving all features to be inspected accessible to the CMM probe. This requires that the six degrees of freedom be controlled but a CMM fixture only needs to hold the part securely to accomplish this, it does not need to hold the part on the datums in most cases. If you decide to use the fixture to locate the datums, it's no longer just a holding fixture, now it's a gauge, and needs to be very accurate to replicate the datum points. In fact, the preferred method is to leave the datums accessible so you can create the datum alignment using the CMM. It's especially useful if your fixture locates the part in a repeatable manner so that a manual alignment is not required for each part.

Now, at one end of the spectrum, you've got a custom built, dedicated CMM holding fixture that will do the job perfectly. On the opposite end of the spectrum, you've got your vices, blocks, clamps, and of course, the glue brothers....crazy and hot (you just rolled your eyes again). In the middle somewhere, lie the modular CMM fixture kits, which have always been intriguing to me but seemed a bit pricey at several thousand dollars for a starter kit, and so I've always gone to either end of the spectrum. Having co-owned a contract inspection lab and gauge/fixture shop in the past, I'm quite familiar with how much it costs to build CMM holding fixtures and it's usually been cheaper to build a CMM fixture than to have a modular CMM fixture system at my disposal...until now.

I was recently introduced to the FixLogix modular CMM fixturing system and it impressed me with both its versatility and price point. Fixlogix has taken what already existed on the market, improved upon it, and offers it a fraction of the cost. Now there's a kit that would normally run you well over $4000, available for less than $1200 in many cases. The justification just got much easier. A gauge/fixture shop won't get far with your $1200 building a dedicated custom fixture, and it's just that, dedicated. If you have any engineering changes to the part, you may need to fork over even more money to modify the fixture, not to mention lost time. Don't get me wrong, dedicated fixtures absolutely have their place, I'm merely pointing out their limitations when compared to a versatile modular system.

The Fixlogix system utilizes a T-Slot system for positioning and fastening components to the plate, allowing for infinite adjustability in multiple axes, not restricting you to a grid of tapped holes. The plates are constructed from anodized aluminum extrusions, which are just as rigid as a solid gauge plate, but at a fraction of the cost. Another innovation I was thoroughly impressed with is the FixLogix spring clamp, which due to its unique design, allows for near zero clamping force while still locking the part in place. This is extremely useful with flexible and delicate parts and cannot be accomplished using the typical spring clamps available on the market, which require you to push the clamp onto the part to achieve the desired clamping pressure, resulting in a significant force on the part surface. In addition to these innovations, FixLogix also utilizes stand-offs as a base for many different components such as rest buttons, conical pins, hard stop pins, jack screws and magnets, reducing the number of dedicated components required.

If you've read this far, you're clear I'm biased towards FixLogix. Of course, I could have written extensively about all of the other systems on the market, but having done my own research and spent the past 20 years programming CMM's, I figured I would save you from tedium and focus on what I believe to be the best all around modular CMM fixturing system available on the market today. In fact, I was so impressed with the system I now offer the FixLogix system as part of my turn-key CMM program packages.

Bill Reilly Metrology offers expert CMM programming in Mitutoyo MCOSMOS, Zeiss Calypso and PCDMIS as well as complete turn-key CMM program packages. Visit Bill Reilly Metrology online for more information. This email address is being protected from spambots. You need JavaScript enabled to view it.

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