Thank you for voting / Ponoko test fit pieces!

First – a big thank you to everyone who voted for me in the sparkfun / ponoko / Geekdad contestI won!

From here on out I will be documenting my process as I go from rough prototype to finished laser cut beer vending machine!

There will be an upcoming longer piece on how to design for laser cutting. I have been amazed as to how many variables you need to keep in your head, and I haven’t even gotten the pieces yet! It seems like once you get it all figured out it is very systematic, but my design wasn’t nearly as tight as it needed to be in order to be laser cut.

This post is all about prep work. I have been talking with Josh at ponoko, and he suggested that I cut a few test pieces to figure some things out. I am planning on using t-bolt assembly. The good thing about that is that it ought to withstand both the thermal rigors of being refrigerated and the physical stress of having 12 oz cans banging around in the machine. The bad thing is that it adds another unknown to the design. I decided to test a bunch of different slot and tab widths to find one that fits as close to perfect as possible.

One interesting thing about designing for laser cutting is that you can easily control the cut dimensions within 0.1mm, but you don’t have any control over the thickness of the acrylic. Due to variances in manufacturing, the 3mm sheet could be +_ 15%! On a 3mm sheet your thickness could be off by up to 0.4mm. This means that for a tab and a slot, you have to make the short sides precise, and the wide sides a bit sloppy. Definitely an interesting twist.

The other thing that I needed to check out with my test pieces is the friction of acrylic-on-acrylic. My dispensing unit currently slides up and down in order to release a can. I cut a few different width slots to see how the plastic behaves. That’s the kind of thing you can only guess and test.

Here is a pic of the test sheets as they came in from ponoko. Here is the bottom of the sheet with the backing on it.

Here is the top of the sheet with the backing on it.

Here I have stripped the backing partially off.

Here I have all of the backing off.

This is just the parts with the rest of the plastic removed.

Here are a few shots of the fittings.
This shows the slot test rig. I don’t have the proper metric button head bolts, but #4-40 1/2″ bolts worked out good enough for the test fitting.

This shot is just from a different angle so you can see how the whole thing fits together.

In the end it seemed like the -.2mm sizing worked best for the slots and tabs. That means that the slot is .2mm smaller on both axis. If you make the slot and the tab the same size, then they end up respectively bigger and smaller due to the laser burning a small amount of material away. That is called kerf. Ponoko recommends .4mm for a friction fit if your piece is laser cut on all 4 sides, but I actually had a fair amount of trouble getting the .4 tabs into the slots. In an interesting twist, they would fit together on one side of the slot test piece, but if I flipped it over they wouldn’t! I think that may have something to do with the fact that the laser cuts at a tiny angle, but that is just a guess. Either way, my plan is to keep it on the loose side to allow for a bit more material variation. I started everything off with the t-bolt plans from oomlout. I bet they have done some pretty extensive testing, but I wanted to make sure I didn’t end up shipping some laser cut garbage.

If you want to check out the design for my test rig, you can download it here. If you just want the tab and slot design, then here is the eps for you!

Now that I know what works and doesn’t work, I will have to refine my design a little bit and then get to lasering! Don’t worry, I’ll keep you updated along the way.

2 replies on “Thank you for voting / Ponoko test fit pieces!”

  1. Thanks for going into so much detail about this Ross – it’s a great example of how to cost-effectively test the parts of a design most likely to not work out, without having to make the rest of it first.

    We look forward to following your findings and advice on the rest of the project!

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