Friday, April 4, 2008

Just Repeat After Me: "More Power"

And say it like Tim Taylor in Home Improvement.

I just couldn't get the Sherline to machine 6061 aluminum. Even spraying cutting oil as I was working just wasn't doing the job. Fortunately, most of the semiprecise work was done. The only real issue left was taking .025" off the thickness of this piece of aluminum, so that it would fit in between the "ears" of the mount base.

I discovered that I could sand it thinner with a belt sander--although it wasn't blindingly fast. Still, I wanted a faster solution.

As an experiment, I chucked up the end mill in the drill press, set the speed to 200 rpm, and dropped the workpiece into the drill press vise. Perhaps because the drill press vise was able to hold it down more tightly, when the end mill hit a patch that might have caused the Sherline to stop, or to jerk the workpiece out of the vise, it just kept going. The X and Y controls on the drill press vise aren't as precise as the Sherline vertical mill, and there simply isn't the flatness with the drill press table, but I was about to rough cut (and let me emphasize rough) enough material that it is now thin enough to slide into position. I also ran the rough surface over the belt sander a bit. It still looks bad, but not quite so bad.

I do think a better mill vise would help a lot. At least one reader suggested that since the mill vise is holding the workpiece in one direction (in this case, the Z-axis), I should use a clamp to hold the workpiece in the X-axis as well. This seems like a good idea--although I didn't have a clamp small enough to do this without interfering with other parts of the mechanism.

Another reader suggested that because of the relatively light frame of the Sherline tool, the vibration from the milling (especially when doing "climb" milling) might be jostling everything loose. There's some merit to that claim as well, I suspect.

It does seem as though the Sherline vertical mill, as cute as it is, may simply not be up to tasks like this. I'm sure that there's a vertical mill that is 50% larger and more powerful than the Sherline--still something that you can mount on a bench--but I fear that it isn't just 50% more expensive!

Oh yes: I mentioned that I had ordered a .600" reamer and a .59375" drill bit. I wasn't reading the list at McMaster-Carr carefully enough. The drill bit arrived as expected; it was actually a .060" reamer--a bit small for this application!

One of my readers suggested a technique that was very clever, and actually worked well. Put the .59375" drill in the chuck slightly offset by a shim. The drill will now turn around a slightly different center, making a larger hole. The shim that I used was several layers of sandpaper. This enabled me to enlarge the hole to .598"--almost big enough. But I couldn't find a shim that was just a little thicker, without being way too big.

So I was reduced to a technique that sounds more dangerous than it is (I think). I set the drill press to 200 rpm, and pulling the workpiece slightly to one side, rotating the workpiece in a circle. Remember that nearly all the cutting power of the drill is near the bottom, so you are getting nearly all the benefit there. It did enlarge the hole, and kept it quite round, to .602". A little polishing with sandpaper and I now a press fit for the cross piece.

UPDATE: Over in newsgroup rec.crafts.metalworking, I poured out my woes, and received several very useful responses--one of which was that if the workpiece was substantially taller than the jaws of the mill vise, this wasn't going to work because leverage would be against me.

This seems to be part of the problem--maybe most of it. I put in a piece of 6061 scrap that I cut off the workpiece with a bandsaw, one small enough that it was just barely above the jaws--and I was able to run an end mill across the surface just fine. At .010" slices, it worked just great--not a single snag, stoppage, or loosening. I tried one pass at .015" and while it didn't stop, I could see that this was the upper limit of what I could do.

When I turned that scrap on end, so that there was about 3" sticking above the jaws, the old problems started to appear.

I think this means that if you have a workpiece that is fairly big, you should get yourself a mill vise with very tall jaws. Does someone make a mill vise for micromills with varying height jaws?

Obviously, you should turn a workpiece the direction required to get maximum grip from the jaws, and mill vertically rather than horizontally--but sometimes that's just not possible.

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