Monday, October 31, 2016

Heresy: End Mill in a Drill Press

I am developing a new hand tool.  I have made one out of acetal that works well.  Obviously, as a commercial product, it needs to be aluminum or steel.  I need to dig a 3.5" deep 3/4" diameter hole in the workpiece, centered.  To get it really centered, the obvious choice is the lathe.  The Sherline lathe isn't spectacularly powerful. but putting a 3/4" 2 flute end mill in a chuck in the tailstock at least gives a centered start of a hole.

Obviously, you don't try to mill by moving X or Y.  Been there watched the chuck drop out. This is strictly plunge milling. I put the end mill  in the drill press, and it roughly follows the starter hole down, at least good enough for my purposes. Th At 3600 rpm, the end mill does a lot of ugly bouncing.  At 2500 rpm, it is better.  I suppose the 4 flute end mill would be a better choice for aluminum.  Would going to a really low speed work better?  This chart suggests 4482 rpm, 43 inches/min.

I thought about the mill but again, it has low power.  Would just dropping the feed rate to .1 or even .01 inches per minute do a better job?  I don't care if it takes hours to complete.

UPDATE: The 3/4" mill cuts aluminum in the CNC mill at .001 inches/minute and low motor speed.  A while back I claimed that at a low enough feed rate, this mill would cut through a battleship.  (1x10-15 ipm.) Would starting with a 1/4", then 1/2" mill speed this up?  Less material to excavate?

2 comments:

Jay Kominek said...

as i understand it, drill presses aren't rigid enough to keep an end mill from grabbing at the side walls of the hole.

i haven't tried what you're doing, but just some off the top of the head ideas: 1) drill to nearly 3/4" and then use a reamer; 2) use a variable helix end mill (should reduce any sort of chatter)

Will said...

I suspect that most of the problem with the drill press is the typically low quality bearings in the head. I've encountered this myself, but haven't yet bothered changing them. I expect that they won't be cheap, as grade 7 is probably needed.

The other part of the problem is that without a high quality knee action, you have to extend the ram to drill/plunge cut, and that exacerbates the loose parts tolerance situation.

Use as short a mill bit as possible when trying this, as the longer the lever arm, the more side movement you get. Also, check the runout of the chuck, as some of them can be really bad in this regard, which further aggravates the problem.

Perhaps you can acquire an actual mill/drill that would fit in your space requirements, in place of the drill press.