Thursday, March 19, 2009

The Turner's Cube

So last night, being a bit stumped on other projects due to lack of parts, but also bored, I decided to start on a Turner's Cube, also sometimes called a Machinist's Cube. It's just a curiosity or conversation-starter, tho shop lore has it that apprentices and vocational school machinist students were once given this project (without instructions on how to accomplish it!) as a rite of passage in order to graduate.

Since this was going to be a show-off piece, I wanted it to be precise. I started small partly to keep the material free and partly because I wanted to do this on the mill instead of the lathe. I might do one on the lathe another day, but to do so will require me to either A) buy a four-jaw chuck - which I can't afford; or B) make a custom fixture.

So I cut out a little piece of aluminum from some 5/8 sheet, and started milling it into a "perfect" 15mm cube. 15mm was the largest dimension I could fit into the material, although I could have gone a little smaller with .5"

Since the dials on the mill's cranks are all graduated in thousandths, and the DRO only covers the table axes and not the knee height, and I needed to use the knee height instead of the quill because of its greater precision, I worked in inches, converting my target dimension from 15mm to 0.591", more or less.

I managed the first two dimensions just fine, controlling their thickness to ± .0005", but when I got to the very last pass, to take off the remaining .006", I somehow managed to miscount or make a data entry error on my calculator (which I was using because I was a little tired and didn't trust myself to add and subtract numbers in my head), because I somehow managed to go from .602 to .587 (a .011 cut) instead of .591 (the .006 cut I thought I had dialed in). I thought about it a while last night and have thought about it again this morning, and I don't know how I did it.

So now I'm quite frustrated because I wasted several hours machining an imperfect cube. (clearly the absolute suggested by the word "perfect" is an unattainable goal, so I was shooting for ±.001 which is well within the capabilities of a big mill.

So I can either re-machine the cube I have down to some other arbitrary dimension, like 10mm / .3937", 0.5" / 12.7mm, or whatever... or I can start over. I'll probably just machine it down to 1 cm.

I am NOT ready to try making the "true" Turner's Cube, where the cubes inside are cut free and remain trapped inside the outer cubes. That requires either a fly cutter (which I don't have) with a specially ground tool, or a 4-jaw chuck and a specially ground tool on the lathe. Bah, humbug!

Also, doing this the way I plan to do it on the mill means a lot fewer steps, but also restricts the diameter of the circular features to the sizes of end mill I have in my set.

Oh well, I'll try again tonight.

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