Pop Quiz 2 Update 5

Sunday, June 1st, 2008 @ 3:42 | Bots, Pop Quiz 2, Project Build Reports, Stuff

But before we start, here’s the toolgasm of the day.

I suddenly feel very insignificant.

Anyways, onto Pop Quiz.

Hey, drivetrain parts! These things are tiny. Very, very tiny. Like, holy crap that’s tiny. The gears are metric 0.4 module, which is roughly 64 pitch. The total reduction is 3:1, from an 8 tooth pinion to a 24 tooth main gear. Fortunately, the pinions have 1.5mm bores, and the drive motors have 1.5mm shafts. There is no way I can bore one of those out.

The drive gears are a little better – 3/8″ in diameter, so somewhat reasonable. They will be bored to 3/16″ and mounted on a custom hub-shaft thing. I’m not sure yet how I’ll manage mounting them on the lathe. And I’m wondering if sub-thousandths precision is required for these things…

The bearings and shoulder screws are for the weapon motor.

Onto the frame. This is the fixture for the UHMW, a Convenient 5″ Square Aluminum Plate. Eight holes are drilled into the plate corresponding to the locations of the drive pod retaining slots seen in the first rendering. I decided against locating them along the perimeter, corresponding to the top and bottom cover holes, because they would be too close to the edge of the aluminum.

5 inches also happens to be the maximum opening width of the vise. Whew.

I’ve known that Bridgeport mills could wiggle their heads in a spherical envelope, but didn’t figure out that it was controllable (i.e. I don’t have to hold a 100+ pound head assembly and jiggle bolts at the same time) until tonight, when I discovered that the head is mounted on worm gear drives which allow for easy, controllable tilting with a wrench.

Milling the front slope on PQ wasn’t going to be painful after all.

If the head joints are servo-controlled, does this constitute a 5-axis mill?

If the rotary table base and tilting table of a conventional 5-axis mill are then added, is it a 7 axis mill?

What kind of creepy things can you machine with that?

Mounted, zeroed, squared up, and ready to go. This is a 6″ square slab of UHMW that will be reduced to a 5.25″ square slab. Then it will be milled down to .375″ thick, from half an inch (Actually .485″ as measured – UHMW manufacturing tolerances are an absurd +/- 0.025 on the thickness)

Uh, oops. I seem to have milled my fixture screws. There goes the hex drive.

Now a 5.25″ square piece of aluminum with a 45 degree slope front.

And now the planing down begins. For this, I removed each fixture screw as the cutter got close to it, then reinstalled it after I cut past the screw hole. This didn’t seem to affect the finish, but is definitely not the right way to do things.

UHMW forms very strange looking, continuously spiraling curls because it is such a soft and malleable material that the last few thousandths just deform away from the cutter instead of being diced.

And the entire slab has been reduced to 3/8″ thickness.

Next was hollowing the slab out. With a 1/4″ endmill and an obscene spindle speed, I blazed a square around the internals, leaving a 3/8″ thick wall. However, I had to route around the fixture screws. This is no problem, since once I get an 1/8″ endmill the area will be detailed and recut anyway.

After cleaning and cutting away the flashing (UHMW also loves to leave lots of edge flashing), here’s the result. The sides are straight. The illusion of waviness comes from the edges, which have been massacred with an Xacto blade.

I haven’t finished out the corners, nor the slots of drive pod holding, since I need to scrounge an 1/8″ endmill or something similar (maybe a spiral bit).

Ahh, the poor fixture screws, milled from all directions. I think I’ll have to buy a stack of 4-40 screws in order to finish this.

After the frame is done, it will be time to proceed onto the drive pods! I really need to see if this crazy semi-direct-drive idea with frankenmotors will work. There’s still time to redesign the whole bot.




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