Keep breaking drill bits

[Chris64]

I have to drill a whole series of tiny holes through 5/16 thick 6061. The catch is that the holes are only .04"

I can't seem to do it. I keep breaking bits. I've tried various RPM's and feed rates. If I feed to slow, the bit seems to pack up and break. If I feed too fast it breaks faster. Slow RPM doesn't pack up as bad, but breaks...High RPM seems to pack-up. Really High RPM seems to fling the material really good...and then breaks.

So, if you haven't noticed...all my variations result in breaking. I'm running out of drill bits. The best combination so far seems to be around 2500 RPM and 2ipm. I've been pecking about .15-.15-.1., I've also tried to straight drill it in a single motion...results have been about equal.

Any idea's? I've also tried spraying cutting fluid on the Drill, but this actually seems to make things worse.

[HuFlungDung]

One factor to keep in mind are the proportions of drill diameter to hole depth. You'd never think of drilling a 1/2" dia hole 3" deep in one peck but that is essentially what you are doing with your .040" drill @ .150" peck.

So, peck about .025" with a complete retraction from the hole each peck to allow the fluid to get down the hole. WD40 would be good for this one. Run as high rpm as you have, at maybe .0002 to .0004"/rev feed.

[Geof]

Reduce your peck dramatically; go to .05 or even down to .02 and keep coolant flooding the drill and hole. You want the retracting drill to pull coolant down into the hole.

Two minutes slower than Hu. I am getting faster.

[HuFlungDung]

That's ok, Geof, I cheat because I'm a touch typist.

[Chris64]

Originally Posted by HuFlungDung You'd never think of drilling a 1/2" dia hole 3" deep in one peck

You wouldn't?!?!?!

OK...when you put it that way...

It just seems like that would take forever. Plus I'm not really running any coolant...my spray mist sprung a leak so I've just kept it off. I guess I need to fix that now.

Thanks for the suggestions

[Geof]

Originally Posted by Chris64 You wouldn't?!?!?! OK...when you put it that way... It just seems like that would take forever. Plus I'm not really running any coolant...my spray mist sprung a leak so I've just kept it off. I guess I need to fix that now. Thanks for the suggestions

Replacing drills can take forever also .

If you don't have coolant flow can you use something like putty or plasticine to build a little collar around the hole and fill it with coolant. Or build a tray to hold everything and fill that; it works.

[lerman]

Pecking does two things: it breaks the chips and it clears the chips. On occasion, I've used a cycle where I break the chips (by pausing for a few revs of the drill) and one time in five or ten, retract the drill to clear the chips.

That can save a lot of time compared with retracting each time. If I were you, though, first I would get it to work; then get it to be faster if necessary.

Ken

[Willbird]

Also how long of drills are you using ?? and what are you holding them with ?? A long drill can bow from the force of pushing it into the metal. As the other poster said the hole is 8 diameters deep.

I did find this fact that I did not know, maybe I can remember it :-)

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Jobber length drill

Jobber-length drills are most common type of drill. The length of the flutes is ten times the diameter of the drill, that is they will drill a hole that is 10D deep.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


also if your drill does not start properly, that is if it walks a bit to the side...as you drill deeper it bends the drill.



The deepest holes I ever drilled were in H13 steel (gummy nasty stuff), they were .125" dia, and upwards to 4" deep, I used Titex or Guhring drills(professionally re-sharped using a sharp drill each 8-10 holes), ran them 50 surface feet per minute, fed them .001" per rev, and pecked every .025"...and flooded the cavity they were drilled from with water soluble oil coolant...the only time I ever broke one is when I tried blowing the holes out with air between pecks :-)...the peck cycle stopped .01" away from where it was cutting before it pecked when it returned. I think the .01" is important because the drill can bow as I said, and if you do not leave some cushion on the return it can hit bottom while still in rapid. (you may be hand feeding ?) One fella I worked with got that deal to walk a drill over 1" when it exited the other side of the part, his spot drill was running out so he didnt get a good dead center spot.(probably left a little conehead POINT I bet)...this 1/8" hole was down in the bottom of a 1/2" dia spring pocket with a 118 drill point down in there 2" deep.



most drilling I have done a GOOD dead center spot drill seemed to make things go much nicer, .001-.003 per rev feed, with at least a 1 second dwell. either .03" deep with a #3 center drill (still a 118 point at that depth) or about the same with a 120 degree Garr carbide 1/4" spot drill. Any walk at all before the drill starts is gonna ruin your day in my experience.



Bill

[Chris64]

The center issue is an interesting point. I'm holding the drill with a basic Jacobs Taper drill chuck (#2 I think). It closes down to nothing very precise so it works good for this. I tried only leaving .4" exposed (the depth I needed) and it didn't help...possibly was worse. I thought leaving the extra length worked better because it gave a cushion for any small errors/deflections. If that's the case, those errors we're likely caused by a wandering drill bit.

What I'm thinking might be a good idea is to only expose a very small amount of drill bit (.1") and run around and pre-peck all the holes. This would prevent wandering and ensure that the holes are pretty darn close. I'll try that when I get home.

[Willbird]

Also depending on what the hole is for, does the .04" dia have to be the whole depth ?? or can it be started a larger size ??

A ball endmill MIGHT leave a nice start too, they do really really well for that on angled surfaces for some reason, IE they do not walk away from the angle.


Bill

[cobburn]

spot the hole and peck no more than .020"...it may take long but will take longer if you break all your bits.

[keen]

Yeh - spotting the hole with something like a short centre drill or spotting drill is essential. CNC "pecking" is new to me but I have done a lot of manual small hole drilling. Drilling in a "well' of thin cutting fluid is good, and the drill needs to be set to lift right out of the hole after a small time cutting. This is most important once say the depth is beyond 4X diameter.