I bored a couple of timing sprockets today, these are for the turret drive and live tooling drive for my CNC lathe project. Normally something like this would not be worthy of posting, but in this case they required a bit of thought to hang onto for boring. There wasn't really enough hub to hang onto and they are a bit of a PITA to dial in under the best of conditions, you have to have the bore concentric with the OD of the teeth, and you can't chuck on the flanges. So I tried something a little different this time.
First some pie jaws for my 3 jaw. Didn't really need full pie jaws to hang onto the sprocket since i'm working in the center of the chuck but I might need them for a large hollow part someday
So start with a piece of 1/2'' aluminum plate. This is bolted down to two thickness of 3/4'' with 3/8-16 x 1 1/2 cap screws. Drilled & tapped the MDF. Normal drill for 3/8-16 is 5/16'' but in MDF I go one size smaller to get 100% thread depth. You can tighten the cap screws pretty tight without stripping if you have about 3 diameters or more of thread depth. The MDF is bolted down to the table and the top piece is screwed to the lower piece with drywall screws. The two Phillips head deck screws held the plate in position while drilling the holes. The center bolt goes down to a T-nut, adds a bit of security for the outside roughing operation.
And this is what happens when an end mill gums up with aluminum, it just kind of melts through the plate.
I should have been watching a bit closer, but I had my back to it working on the lathe. I basically did everything wrong here
Using a 4 flute rougher rather than a 2 or 3 flute. Not enough coolant flow, let the chips build up, in addition to pushing it kind of hard. It actually overloaded the motor and tripped out the VFD, first time that has ever happened.
But wait, there's more...
The VFD fault output WAS NOT (it is now) tied into the E-stop circuit so the servos kept happily running along until I hit the E-stop. Of course moving the parts around and knocking the head out of tram.
So tram the head, find 0 again and finish the cut. Surprisingly, the endmill did not break, nor was it damaged by the aluminum welding to it, cleaned right up and I finished the job with it.
Finishing the profile. I could have built the pie jaws with a drill press and a band saw, but it's just too easy when you have a CNC mill sitting there. I'm getting lazy in my old age.
And the mill work is done.
So next over to the lathe and prep them for use. First something in the center to you can load the jaws outward when tightening the cap screws. This is the ring I turned to preload the chuck jaws for boring the center to size. I should note here that my chuck jaws are removable 2 piece jaws, with single piece jaws you would not be able to do this. Interesting camera perspective, that's an 8 inch chuck, makes my hand and 5/16'' allen wrench look huge.
Then using the same ring in this case, tighten the jaws down on the ring to load them for boring to size. In this case the bore is a bit smaller than the ring because I needed clearance for the flange on the sprocket. So make the bore exactly the same size as the OD of the sprocket teeth. That way the sprocket is captured all the way around and held concentric in both axes, just like a collet.
So now ready to bore the sprockets to 22mm. I made them on size because I want them to shrink on. I'll heat them up pretty hot to install. Did pretty good, hit the target dimension dead on for one of them, and 0.0001'' under on the other. They started out at ~15mm bore.
And done with this part of the operation
So next is the keyways. the keys are 6mm and I don't have a 6mm broach, so time to make a quick keyway cutter. I grabbed a trashed brazed carbide bit, drilled a 0.358 hole (T drill) in the shank to clear the edges of a 1/4'' HSS lathe bit, and put a set screw in the end to hold the bit in place.
Then I ground the 1/4'' bit into a 6mm keyway cutter. I just did this by hand, grind a little bit, measure, rinse, repeat until it's correct. I managed to hit 6.01mm and decided that's close enough.
Cut the tool bit to length. Then just set up in the lathe and pretend that I'm a shaper.
So tomorrow back to the rest of the lathe project.