Portable Workbench Build Process
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Posted 15th March 2021
I realised I didn't post a photo of the slot bushes in the state they were in after turning:
I'll start with a small update on the material. With the parts a fairly simple shape (i.e. before the slot was milled in them), I measured the OD, ID and length carefully and put them together on the kitchen scales. A simple calculation gave the density as 7.45 kg/m³. That's much more in the bronze range than the brass one, so I suspect these are some sort of bronze. One reference I used quoted aluminium bronze as 7.7 kg/m³ and generic tin-based "bronze" as 7.4 to 8.9 kg/m³; another quoted generic tin-based bronze the same but aluminium bronze as 7.8 to 8.6 kg/m³. Who knows, but it's certainly not heavy enough to be brass.
To make the slots, I started by mounting a bush in the milling vice. I'd intended to do this with a v-block between the moving jaw and the bush, but none of my v-blocks are big enough, so I just clamped it as if it were a bit of flat bar and hoped for the best! I used a DTI to find the exact centre of the bush:
Then put a big milling cutter in an ER40 collet. This cutter is somewhere in the region of 24 mm, but it's not an exact number: I think it was a 24 mm or 25 mm cutter that had been resharpened a few times when I bought it second hand.
I started opening out the slot by repeatedly moving the table over (in X) by a millimetre and then gradually plunging the cutter down. I did this until I'd reached ±8 mm.
I then moved the cutter over in tiny amounts in the Y direction and took passes all the way around the slot. This was intended to open the slot out and also smooth it up at the same time. Smoothing it up was a bit optimistic: I ended up with lots of chatter at the ends of the slot:
When I did the second bush, I used a slightly different technique - doing more of the plunge cuts and then only using the X feed to clean up the material between the circles made by the plunge cuts. I thought this would leave a much worse finish but it actually looked really good.
After cutting the slot, I spotted and drilled the 5 mm hole that is used with a dowel pin to make sure the slot is horizontal when it's inserted in the moving jaw.
I deburred both bushes and then for the bush that had lots of chatter, I then wrapped a bit of wet & dry paper round a 22 mm bar of steel and cleaned it up a bit:
The other one didn't need any clean up at all. The end result of that was that the slots are slightly different sizes (about 0.1 mm), so I'll probably make the tubes that run inside the bushes to match each individual bush (and make sure I put it all together the right way round!)
The finished bushes - I'm really glad these are complete as I was dreading that milling operation.
They even fit!
Page 20 of 67
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