Drilled A Tapered Hole. But Did Not Mean Too.

M

MAlcocer

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I was drilling some aluminum today in the mill (g0704) and spotted all my holes with a #3 center drill. Drilled 10 at 3/8 and one at 1/2 inch. All through a piece of .600 inch stock. The half inch hole is .505 at the bottom and .520 at the top. All of the 3/8 holes were tapered as well. I did not run the bit much past the stock. Ran at 1500 rpm and had good ribbons coming off.

What went wrong? I don't recall ever having a hole come out tapered like this. Any advice to prevent this from happening. Yes I should ream to get an accurate hole but I was not expecting a taper. Also used a brand new set of hss bits. Any advice is welcome.
 
Is your hole tapered or is the profile more like a counter bore? A drill bit with unequal lips will drill a larger than nominal hole until the tip breaks through when the remainder of the hole will be closer to the nominal size.
 
I'm going to go out on a limb and suggest the problem in chatter.
Try drilling the holes with out the center drilling. I find center drills produce a lot of chatter, so I quit using them in preference to spotting drills.
Either way, the spigot on the center drill should be larger than the web on your drill.
 
Both of those sound reasonable. I used a smaller center drill and had some chatter. It appears to be more of a taper than a counter bore. Brand new ground drill bits i would hope they were cut equally. Would i get better results progressively drilling the holes larger sizes?
 
RJSakowski said:
Is your hole tapered or is the profile more like a counter bore? A drill bit with unequal lips will drill a larger than nominal hole until the tip breaks through when the remainder of the hole will be closer to the nominal size.
Click to expand...
Wow, never thought of that. That is actually a pretty good test for unequal cutting edge lengths. Thanks!
 
another consideration is if the table is truly square to the spindle, any deviation from square will show a taper.


the drill will try to glance, ever so slightly and the end result is an egg shaped hole.
 
So to test what caused it. If I drill a hole with out a spot or center drill and get the same results then its most likely unevenly ground bits (they look visually fine). I will also try and drill a spot/center drill that is larger than my drill bit and see if that works. Any other ideas?
 
What's the run-out of your mill spindle?

If it is turning a little out of centre then the hole will be larger than expected. Add in the flex of the drill bit and it could end up tapered.

-brino
 
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