Do I need a J drill or what?

Hello all you plastic machinists

I am drilling 7mm holes in Delrin, 50mm deep, to take HT leads which are a nominal 7mm and rubber cased. I started with a long 3mm drill, and then a new 7mm twist drill, in stages, and at a modest speed. The bigger drill came out hot to the touch - no more than 60 or 70 C. I then used a new 7mm machine reamer and it produced a very small amount of very fine shavings. But the HT leads will not fit. The first 3mm goes in, but as soon as resistance is felt, no furtherr movement is possible. My guess is that the give of the plastic under the drilling loads is springing back too much.

Is it likely that a new J size drill (0.277 inches = 7.036mm) would shave off enough? Or if not, what size?

Or should I try putting the delrin block in the deep freeze overnight? Or CO2, or liquid nitrogen?

I did think of drilling right through and using twine to pull the leads through under tension - then blanking the exits with plastic plugs. Might that be a solution - but the HT leads have a wire core, not carbonised string, so may not stretch much ?

I await your comments with extreme interest ...

Regards, Tim

Far more likely that the HT isn't exactly 7mm and/or that as the lead is pushed in the rubber coating bunches up and expands slightly. I'd go up in increments of 0.1mm until the lead fitted.

Andrew

Vaseline? (or its more sophisticated cousin silicone grease?)

So you are pushing the HT lead into a blind hole? If so, could it be air pressure preventing it travelling any further, if a tight fit?

Lubricate

You will probably find the HT lead needs to be pulled through rather than pushed. Some silicone grease would also prove helpful.

Regards

Gray,

Getting back to the machining side of things Delrin is generally very stable when machined but it is a plastic and so thermal conductivity is inherently low.

Standard machining rules apply. Razor sharp tools, low speed, heavy cut, decent feed and keep it cool. Heat input is fundamentally related to length of cut so you want as much material out as possible per unit length.

Never had much luck with stepping out, shaving cuts or reamers. Drilling straight to size, unless teh hole is really big, always worked for me. But I do have the means to make any drill up to 3/4" very sharp indeed.

Clive

The hole is not a dead end, as I am aware that the air has to get out. There is a side hole where a contact will be made. I am also clear of the advantages of lubrication with silicone grease (to avoid rubber attack) and both these dodges were applied to my problem.

But thanks anyway - I should perhaps have explained further, but long questions don't get read.

The reason I used a 3mm pilot drill was to avoid the 7mm drill wandering.

Cheers, Tim

Plastics tend to push away from drills so using a 7mm drill will produce a hole smaller than 7mm. Also agree with AJ about using larger drills in .1mm increments 

Tony

Edited By Tony Pratt 1 on 24/06/2023 14:22:54

IN that case, all you can do is bigger drill I suppose. Can you measure how big the existing hole is? And what is the exact size of the nominal 7mm HT cable? Might give some idea of how much bigger drill you might need.

Maybe test drill in some scrap to establish exactly what size you need.

Tony

I've always found Delrin drills true and to size given a razor sharp drill with undamaged lands. Dormer factory grind is barely sharp enough.

Fortunately my Clarkson attachment will put that extra edge on .

Hafta say that if I needed job to come dead right first time I'd factor anew drill into the budget. If I did lots of plastic work there would be dedicated drills, just as there would be if I did lots of brass.

Nylon of course is different story. Given a sniff of a chance that runs away rather than cut.

Clive

It depends a lot on other details. If the hole can go right through at 7mm, consider making thin barbed semi circular (or less) splints that grip the insulation. Feed these through the hole and pull. You need to put the insulation in stretch mode. This supposes it is rubber like, not semi-rigid. The splints should act to compress the insulation, forced down by the sides of the hole. You need to go too far to release the splints, then draw back wire to final position and bung hole.

I have had poor success pushing wire into tight holes, the insulation merely expands to block progress.

Leave the delrin in the freezer overnight. Drill through it with the 7mm bit tomorrow morning.

If you are trying to push it in I doubt you will succeed unless the hole is oversize, if you want it a tight fit I would drill all the way through the delrin then plug after.

Pull the HT leads through (taper the end as a lead in), if you have some braided sleeving slip it over the end (lubricate with silicone) and use that to pull it. Cut the end off and then pull it back to allow plug fitment.

Edited By Richard Millington on 24/06/2023 19:56:46

It rather reminds me of this joke:

**LINK**

But I digress, as others have said you need to pull it in.

I use delrin / acetal for making small gears 6/8mm dia and have never had a problem with drilling and reaming holes to size as well as bobbing teeth, the whole idea of it is that it is an engineering plastic known for its ability to machine to reliable dimensions. Are you sure that's what you have ?

The easiest option would be to drill for a sliding fit and assemble with some RTV silicone sealant. That should take care of any sealing against moisture.

I think I'd try using a wood drill bit - the twist drill type with a centre point.

Martin.

For what it is worth, I did a quick calculation:-

Heating Acetal to 120C:

Thermal expansion coiff of 140ppm x 100C >> 1.4%

for 7mm diameter, hole size increase is roughly 0.1mm

Cooling elastomer to -20C

Thermal expansion coiff of 140ppm x 40C >> 0.64%

for 7mm diameter, insulation size decrease is roughly 0.04mm

So circa 0.14mm increased clearance ( about 6 thou)

Is this enough for feasible assembly? Probably not.

Edited By Macolm on 25/06/2023 10:11:40

The OP mentioned he used a machined reamer am I correct in thinking these are the same size right to the tip except for a very small amount to aid starting in a hole.

Sorry for not being able to help with the original problem