Thursday, November 3, 2016

There's A Lot More AR-15s Out There

If you look at the sales numbers of AR-15s, it's huge.  But I suspect that is an understatement of the number out there.  For my current machining project, a new type of handtool, I need to cut two 5/16" slots on both sides of a 1" OD aluminum tube.  They should be as straight a line through the slots as possible.  Most 5/16" end mills are short enough that I have to cut one slot, rotate the tube 180 degrees and cut the other side.  This is not easy to do with high accuracy.  (Cutting across the top solves this, but this is a 5" tube, so not easy to clamp adequately: I tried.)  So I went to look for a long reach 5/16" end mill:

Interesting how many of the comments were from people using it to complete an 80% AR-15 lower.


Will said...

My question is whether your mill system is capable of keeping that long of a bit stable enough to hold any sort of tolerance. Plus, is the collet setup sufficiently true to keep the tip wobble within reason. Don't forget that that bit makes a very long lever arm, so cutting travel will have to be very slow. Are you expecting to cut both sides of the slot at once, or making a second pass to cut a wider slot than 5/16"?
Make some practice cuts first!

Frankly, I would be considering getting an indexing head to mount the part in, and cut the slots with a short bit. You may be unpleasantly surprised at how much deflection you can have in that sort of milling bit.

Michael The Magnificent said...

At least as evident from your typing skills, it appears as if you've recovered from your stroke. And thank God for that.

Clayton Cramer said...

I really don't need high precision on this slot. The big issue is that the mill tends to catch in the slot and stop turning.

Will said...

More hp needed. You have too much surface engagement (180 degrees), and the resultant chatter drives the cutting edges into the material and locks it up. This is partly due to all the components not being ridged enough to keep that cutting end from wobbling. It doesn't take much flex at the tip to cause problems.
You really need to upgrade to a bigger mill/drill, at least, to minimize these sorts of problems. Of course, it never goes away completely. You can always create difficulties, but it's much nicer to only run into them when pushing the boundaries of your equipment on an infrequent schedule. Yours are an everyday occurrence, it seems. Can you add a room off of your garage to fit a real mill and lathe? Maybe a very well insulated shed that is sort of/temporarily connected to the garage for heat and winter access? You have a paying hobby. Why make life difficult to deal with it? Spend time and effort on more important aspects of it, and stop slogging through a swamp. Drain it and move on.