Monday, 30 May 2011

Fully Focused

Nophead kindly helped me to get my RepRap commissioned last week. I've been building it now for about 5 months and I was pleasantly surprised when it sprang into life. Thanks to Nophead for all the help!

Anyway, I've been looking into designing an IR focusing tube to fit in with the modular gun design that I am working on. I wanted a unit that would bolt together with a few screws and allow accurate focusing of the IR beam. I also need the design to be flexible enough to fit on different gun designs.

Here is my first attempt:

It is a 6 part assembly comprising of a lens retaining ring (red), the lens (blue),
focusing tube (grey), IR led mount and slider (green), focus screw mounting plate (purple) and focus screw retaining plate (yellow).

The plates are fastened with 2 self tapping screws which are not on the model. Also, the focusing screw is not shown.

Here is a picture of it assembled:

There is a 6mm 'bar' at the top of the tube allowing the tube to be fitted inbetween 6mm MDF sheets when using an odd number of sheets in a gun design. This could easily be increased to 12mm for even numbers of sheets, or a multiple of 9mm if sheets of that width were used. Two holes in this bar allow an M4 bolt to hold the tube in place.

I printed the whole thing this weekend, and after an aborted print caused by the plastic getting too hot due to bad thermistor connections I got this:

A view from the top of the lens tube with the LED mount above:

Here it is assembled:

From the back you can see the self tapping screws holding the focusing screw plates in place and the encapsulated head of the focusing screw:

And from the front showing the LED mount in place through the lens:

I'm more than pleased about the way it has turned out. Just need to modify my gun design to use these then I can start knocking them out!

Saturday, 7 May 2011

Dome Again!

Today I had another go at making acrylic sensor domes. I've given up on vacforming as acrylic is not malleable enough to form properly this way with the equipment I have.

Instead, I decided to try press forming. This process involves heating the acrylic to make it soft, then pushing a tool into the sheet to make the desired shape.

As a test I made a simple cylinder out of two layers of 12mm MDF that I cut out with a holesaw. I fastened them both with a woodscrew to a base:

I fastened an a6 sheet of 2mm acrylic in the clamping plate I made a few weeks ago, and heated it under the grill. This was my first error, the grill is too fierce and the acrylic started to bubble. I tried it anyway as it had gone soft.

The plan was to use a piece of MDF left over from cutting the cylinder. This is about 2mm larger diameter than the cylinder and I used it to force the soft acrylic sheet over the tool.

Here's a picture after the operation. The MDF sheet used to force the acrylic is on the right.

It worked quite well, but as I had not put a draft taper on the tool, I broke the acrylic trying to remove it:

It occurred to me that the metal frame was not really necessary for this method of molding, so I made tried again with a new sheet of acrylic. This time heating it in the oven (gas mark 9, 5 minutes at the top) placed on the MDF sheet used to force it over the tool. I also smeared some vaseline as a release agent onto the cylinder.

Here's me waiting for the acrylic to harden:

With the forcing plate removed:

It was still difficult to remove the formed piece afterwards, but it did come off:

I then used the same sized holesaw to cut the piece out from underneath:

Much better than my vacforming attempt. Here's what I learned:

1. Draft angle is essential.
2. Use oven at high heat and time the heating process.
3. Don't use a grill!
4. Forcing plate makes all the difference.

I'm going to try again soon with a better tool. I think that will result in a dome that I can use.

Thursday, 5 May 2011


My Milestag UMT PCBs arrived from China today. Just under 14 days since I placed the order with them. Not bad. The quality is good with holes centered well on pads, and for about 25UKP for 10 boards, inclusive, they are a bargain:

I just need to populate them. Bit busy with RepRap at the moment though...

A pre-built version of this board is now available at Tagbits