Gundrilling plastic

Having issues gun drilling in plastics, .159 hole up to 13" deep. 33" drill length w/ two moveable supports about 9" apart plus support at part face.Hole is not going straight, seems to wonder to skinny side of part. No burning/melting of part and good finish. 3000 rpm, .400 ipm, new .159 drill. New at this. Any information appreciated.

As you mentioned a ”skinny side”, I assume the part isn’t round or you might counter-rotate the drill and part. Are the “chips” evacuating down the chip gullet? A build up could push the drill off center. Can you predrill a hole to insure the gundrill starts straight? Is the machine properly aligned? Worked at a shop with a huge trepanning machine that would drill straight in steel but would drift as you described in aluminum. The headstock was misaligned with bed due to more than a few mishaps.
Is the drift always in the same direction? Are you using new gun drills or reprints?
Sorry , so many questions come to mind.

E189552 said:

As you mentioned a ”skinny side”, I assume the part isn’t round or you might counter-rotate the drill and part. Are the “chips” evacuating down the chip gullet? A build up could push the drill off center. Can you predrill a hole to insure the gundrill starts straight? Is the machine properly aligned? Worked at a shop with a huge trepanning machine that would drill straight in steel but would drift as you described in aluminum. The headstock was misaligned with bed due to more than a few mishaps.
Is the drift always in the same direction? Are you using new gun drills or reprints?
Sorry , so many questions come to mind.

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All good questions and might add single flute or double flute. Axial relief (clearance) might be increased. choose a gun drill that has more flute opening (if such exists.) increase coolant pressure or a more slippery coolant(if such exists). Is it really drift or could it machine or part tram. Does the start axial cutting forces pull the part/drill off desired start position?

QT:two moveable supports about 9"... is that/there a bushing for starting the spot on?


I don't have much experience in gun drilling plastic but much in gun drilling iron and steel.
do you sharpen in house or send sharpening out? Primary should be ground with a 320 or 500gt wheel (or finer). primary should look mirror like IMHO.

I did a job many years ago like that. Six deep holes spaced out around the edge of a round bar. Think of a revolver cylinder. All the holes curved towards the OD. Less mass on the skinny side material heats up quicker and softens the material, the drill tip will drift in the soft direction. Customer changed the spec and we milled slots around the edge and he covered the OD with Teflon tape. The material may have been slightly less dense towards the OD, whatever the cause the holes were going to all drift towards the outside of the bar. Drilling slow and using chilled coolant could help a little but it may just be the nature of the material with no cure.

Which plastic? …...it can make a big difference.

Sounds like non-uniform material properties...Molded part? Maybe consider a special drill with much longer-than-usual cylindrical guide section? Can you mold the hole, perhaps with a taper, so you only have to ream it straight

magneticanomaly's molding the part with a hole already in it is sort of an outside the box solution for a machinist.
A couple of other outside the box ideas.
#1 ball mill channels in to pieces and weld/glue them together

#2If it is temperature causing the problem, clamp the piece (skinny side) to a chilled plate while drilling maybe a vortex tube or 2 could maintain the chill. Thermal grease could help with temperature transfer.

#3 start with a larger piece of plastic, drill though the middle then mill or grind the skinny side to the desired thickness. If the problem is material density then the material may really warp as you mill away one side.

#4 If the internal hole does not really need to be straight drill from opposite sides, hopefully the 2 holes will meet in the middle.

Is the hole still on size but off angle , straight? might you put an indicator to the part side and so see how/where it swells, perhaps give more support their. does it go the same way off on a number of parts?

likely a time/job killer but might you drill smaller then line bore/ream with a steady at other end.

.159 drill that might be tough. 13" deep

If the direction repeats can you tram off the part so the outcome will be in spec.

Yes a .159 tool can bend easy, single flute gun drill that log will try to wonder. two flute perhaps need be center cutting/ both flute ends the same. Don't think I ever saw a center cutting gun drill. Never did any gun drilling plastic.

Solid carbide two flute might be better..I did make some plastic-use special cutters for a fiborus rubber on a military part, can't tell much about that because friend had edge on the market for that part.. but it was only for 1.5" thick rubber part with having string like fibers (perhaps kevelar) in the mix.

Lured2Fish said:

Having issues gun drilling in plastics, .159 hole up to 13" deep. 33" drill length w/ two moveable supports about 9" apart plus support at part face.Hole is not going straight, seems to wonder to skinny side of part. No burning/melting of part and good finish. 3000 rpm, .400 ipm, new .159 drill. New at this. Any information appreciated.

Click to expand...

What kind of plastic???
What type of machine are you doing this on?

....

In my experience, I need to know the answer after I ask the question. But I'm a maniac, sooo different strokes I guess.

R

Thanks for advice everyone, very helpful, turns out larger kidney shaped oil hole (opposed to round) in drill bit and using a different cutter grind along with greatly increased coolant pressure (1300 psi) has it on point. Tips really got me on right track.