Saturday, June 12, 2010

I'm Glad That I Made These Expensive

Generally, the caster assemblies that I make for telescopes are in one of three categories: round ones that go inside the tripod leg, round ones that lock onto the outside of the tripod leg, and square ones that go inside the tripod leg.  One of the weird exceptions are the casters for the Vixen HAL-110 and HAL-130 tripods

These tripods have such oddly shaped legs that none of those strategies seemed to make sense, so I decided to make a replacement for the "foot" that would replace it.  The "feet" on these tripods are held in by four M4 screws, so I machine an insert that replaces the foot and its insert--and the standard screws go into holes that I tap in the insert.  It looks a bit odd once installed, but it works well.

The most annoying part of making this rather complex part has been my continuing learning experience with the vertical mill.  Previous postings have discussed the problem of not using the right type of end mill, and trying to work around insufficient capacity in the standard mill vise that comes with the Sherline vertical mill.

Today was one of those days where I put quite a few hours into getting a set made up for a customer--and I'm glad that I charge a bit of a premium for these, because of it.  Past difficulties have made me very wary of trying to use the Sherline mill vise for this--but today I managed to make everything work.  Today's lessons learned:

1. There is a socket head 10-32 screw that does the clamping on the Sherline mill vise.  As the socket head wears out, it gets harder and harder to really lock down the workpiece.  Fortunately, I keep a spare or two lying around.  It's time to replenish my spare collection.

2. I now use the roughing end mill for all operations on this part.  Yes, this one is only 3/8" diameter, so it takes two passes to cut the 0.50" slot through the insert that I make.  But it does not catch the workpiece and throw it, as happens with some of the two and four flute end mills.

3. I noticed that some people make what are called vise stops for the Sherline mill vise.  A vise stop is something that attaches to the mill vise and provides a place for a workpiece to stop (as you might guess from the name).  The advantage of this is that if you are making a number of pieces that are identical, you can get away with using an edge finder only on the first piece, which is located against the vise stop.  Then, your handwheels are correctly zeroed for all subsequent workpieces that go up against the vise stop. 

I haven't made a vise stop--but because all of these workpieces start out as a rectangular solid, and I machine them within a few thousandths of inch of each other, I can use the edge of the mill vise as a rudimentary mill stop.  I can rub my finger along the transition between the workpiece and the mill vise and feel a discrepancy of .005" pretty effectively.  For these parts, that's quite sufficient accuracy.

4. One of the slowest parts of machining is cutting away excess material.  These workpieces needed to be machined down to blocks that are 3.42" x 2.62" x 0.75".  I asked Interstate Plastics to cut me a piece that was 2 5/8" wide by 3/4" by 11".  The 2 5/8" actually came out about 2.68", so there was only about .060" to remove--and just squaring that dimension was .020" of the cutting.  The sheet they cut this from was actually 0.752", and was close enough that it was a waste of time trying to improve on that.

The first piece that I cut from the 11" long stock was 3.50"--and while .080" doesn't sound like much, trust me, my fly cutter really doesn't like taking off more than .010" at a time--maybe .020" if I move the fly cutter slowly across the end.  The closer to you get to the desired length, the better.  After I had trimmed that first one down to 3.42", I used it on the chop saw to set the length for the next two workpieces.  Both came out at about 3.45".  The act of squaring the ends took off most of the .030" that needed to go away.  In machining, time is money, and material that you throw away in the trash can is time and money wasted.

5. I have been making the 0.5" slot through the middle of this piece by using the bandsaw to make two 1 1/2" long cuts, then using a .4375" drill bill on the drill press to knock out holes between the cuts.  This means that I am only having to clean up the slot--not cut it from scratch, which is very slow.  The bandsaw, unfortunately, doesn't really cut acetal all that well, because it is so much harder than wood.  As a result, unless I go very slowly, the blade starts to bend to the side.  I have concluded that it is simpler and faster to just skip the bandsaw part of the operation.  I can use the .4375" drill bit to make several holes in a row, and the results are just about the same.

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