When peck drilling, say a 3/8" hole, in 6061, what feed rate, peck depth and retract depth would be appropriate?  I've been using about 5IPM, 1/8" peck, 0.05" retract.  That still results in some pretty good "strings", and drilling holes takes quite a while.
What would you guys use?

Regards,
Ray L.

Depends on the rigidity of your machine, quality of your drill, (I would use non-coated bits on alu. regardless of the bit quality) coolant used, among other things. If It were me, I would start about 1200-1800 rpm, 15-20 IPM, and use an oil based coolant. Usually when I start with something, I make changes during or after the first hole. If your hole is deep, it would be a good idea to set a retract depth. Say every half inch or so. The oil based coolant is of great importance to avoid the aluminum from sticking to the bit. Once it sticks, it's done for---end of story. WD40 works, but not really optimal. If you DO happen to have aluminum brazed into the bit, drilling some steel with a good coolant flow will usually clean it off pretty well. The chip break doesn't really matter all that much, as long as its breaking the chip, so .03-.05 should be fine. With aluminum, I would rather have a bit longer strings than very short ones that would cause the soft chips to seize up in the bit. Not long enough to accumulate around the bit, ending up like a ball of yarn, though. If the strings are to long, turn the feedrate up, or turn the spindle speed down. These figures would be a "in the ballpark" starting point for the machine I use.

Sam,

    Thanks for the quick response.  Not sure I fully understand all of it though.  The machine is, for all practical purposes, a Bridgeport (it's a very literal Taiwanese clone - if I painted it gray, nobody would know it wasn't a real BP).  I guess I did OK on RPM - around 1500.

    "If your hole is deep, it would be a good idea to set a retract depth" - I thought the idea of peck drilling (I'm probably wrong....) was plunge, pull back, plunge, pull back, etc.  Should I instead plunge, stop, plunge, stop, etc. and only on deep holes actually retract periodically?

    Sounds like I need to crank the feed up a lot higher....

Regards,
Ray L.

It sounds like your using the same machine as the one I use. A simple peck drilling cycle is just as you stated. Plunge, chip break, plunge, chip break, etc. However, if your hole is deep enough to cause a problem of evacuation of chips, a retract would be advisable. Plunge, chip break, plunge, chip break, plunge, retract.... or some form thereof. Yeah, 5IPM is kinda (allot) slow.

If the strings are to long, turn the feedrate up, or turn the spindle speed down.

OR lower the peck.

In soft materials it is sometimes better to peck in .020" pecks with a high feed rate of say 3 to 4 times normal and 1.5 times spindle speed with a retract of .010" and flood coolant.

This way low power spindles get a chance to recover before the next cut and also the chips are small and clog free.

Using this method you can drill holes much bigger than normal and much faster than normal. Give it a try but don't tell anybody, its a trade secret.

Graham

It sounds like your using the same machine as the one I use. A simple peck drilling cycle is just as you stated. Plunge, chip break, plunge, chip break, etc. However, if your hole is deep enough to cause a problem of evacuation of chips, a retract would be advisable. Plunge, chip break, plunge, chip break, plunge, retract.... or some form thereof. Yeah, 5IPM is kinda (allot) slow.

Quote

If the strings are to long, turn the feedrate up, or turn the spindle speed down.

OR lower the peck.

Sam,

    I modified my post to do 5 pecks with a 50mSec dwell between, then do a retract.  Worked much better, and I upped the feed to 15IPM - no more strings, and no clogging!
   Thanks!

Regards,
Ray L.

Graham,
    Interesting approach!  I'd like to try it, but part of my reason for wanting to minimize retracts is I've CNC'd my knee, and for now I'm going through the bevel gears.  All those short pecks and retracts woudl really hammer the poor gears.  But, when I get the ballscrew in , I'll give that a shot!

Regards,
Ray L.

Ray,

  do you have some sort of assist on your knee ? Air? Curios.

Ed