Custom fixture design

My friends and I have a project we wish to showcase during the next upcoming Maker Faire Rome 2018. We believe it’s an amazing idea, which involves engraving a massive amount of aluminium dog tags, like the one below, and for massive I mean a couple hundred pieces.

aluminium tag

If this is not the first post you read on my blog, you should have already guessed I enthusiastically accepted the task as an opportunity to use my amazing Othermill Pro (a.k.a. Bantam Tools Desktop Milling Machine).

My first approach was to use a piece of double-sided tape to fix the aluminium tag to the work surface, but that was not only time consuming and tedious, but also very inaccurate. I decided to take advantage of the threaded holes available on the sacrificial surface of my machine, but I needed to avoid scratching the tags while tightening the M3 screw, so I milled a plastic washer out of a bottle cap knowing those are made of HDPE.

At this stage, I had obtained an acceptable degree of precision and repeatability, but fixing and unfixing the tag was still requiring almost one-third of the work-time: too much for my tastes.

It’s here where the story of designing a custom fixture starts. I started listing my constraints and requirements:

  • reduce the time consumed by fixturing as many tags as possible at once
  • must be compatible with the fixturing tools my machine provides (existing holes, precision fixturing bracket, etc…)
  • one fixture applicable for both sides of the tag
  • the fixture must not scratch the tag surface
  • the tags must be held tightly, strongest forces will be horizontal
  • have the tags precisely located on all the three axes (engraving depth is only 0.1mm)

I then moved onto Fusion 360 to draw a few ideas and soon a fixturing for four tags emerged: made out of plastic should satisfy all the points above.

As you can see, the horizontal cutouts provide the flexibility necessary to grab the tags by their longest sides while the bolt and nut pair should be able to exercise enough force to squeeze the rig. If you have a look at the third image, you’ll see this rig will have one side accessible when mounted: I can reach the rig from the back to do/undo the bolt for tightening/releasing.

I had a piece of black Delrin (a.k.a. acetal resin) laying around which was a piece of cake to shape into its final form. I then used a hand drill attached to a vertical guide to drill the hole for the bolt: accuracy is not of the greatest importance for that, I just had to be sure to go straight. Add an M5 x 70mm bolt (not very easy to find, I must admit) and an M5 nut and this is what I ended up with.

Fixture Animated

The result is amazing in terms of usability, precision and, above all, time: I managed to shelve out one-third of the total work-time with great benefits for my family-time. Apologies for masking the engraved tags: I don’t want to spoil the surprise for our visitors at MRF18.

The entire Fusion 360 project is published in the cloud but also available for download: I’m eager for comments and suggestions.

The component assembly including the machine bed, spoil board, alignment bracket and tall alignment bracket is also available in the cloud.

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