Sunday, 7 August 2011

Dome Goal

I got my homemade Milestag UMT board working this week. This has forced me to think about acrylic domes again. I bought 4 pairs of acrylic domes from LasertagParts whilst in the USA last year, but now I've used them all.

As mentioned previously my neighbour made me some dome molds from a weird slate/resin composite. These are just right but I needed to create a hole in a sheet of wood to allow me to force the soft acrylic sheet around the mold.

The problem I had was that the outer diameter of the mold is 36mm and the nearest hole saw I have is 44mm. A 44mm hole is too big to force the soft acrylic sheet around the bottom of the mold.

To solve this I drilled a 44mm hole in a sheet of 6mm MDF and then printed a bush to reduce the internal diameter of this hole. Here is the CAD:

And the printed part:

It has a flange at the bottom to prevent it from being pushed out of the MDF:

Top down you can see the diameter reduction:

I cut a second sheet of MDF to provide a flat surface to place the mold on. Here it is forcing 2mm acrylic, heated at gas mark 9 for 4 minutes, over the mold:

And with the forcing plate removed:

I'd previously sprayed the mold with PTFE to aid removal:

The molds have a threaded hole underneath. This allowed me to hold the mold in the vice whilst I gently prised the acrylic free. As this mold has a draft angle it came off quite easily:

The remaining problem was now the removal of the dome from the sheet. To do this I used my 44mm hole cutter on two 15cm square pieces of wood which I then placed on top of each other with the holes aligned. With the hole cutter in my drill press this prevents any downward force being applied to the dome, which might crack it. I applied very little force whilst cutting and I let the acrylic soften from the tool friction as this seemed to prevent cracking. I destroyed one dome by pressing too hard:

Here are sheet and dome after separation:

I cleaned the rough edges on my belt sander:

Here it is after grinding:

Total time is about 15 minutes, 4 of which is waiting for the acrylic to soften.

Can I repeat it? Of course!

These are now available at Tagbits.

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