Drilling for delrin problems

[Charl]

Been having problems with my delrins lately. The delrin I have is 10.7mm and the closest bit I can get 10.5mm. Up to a couple of weeks ago, I have been having no problems at all; the fit (both in stem and mortis) was nice and tight after drilling.
But then my bit became blunt. And the problems started!
I re-sharpened, but then it drilled a hole of 10.3mm. Re-sharpened again and then it drilled 10.4mm. Bought a new bit of (supposedly) 10.5mm, which now drilled a hole of 10.4mm!
So all in all: cracked stems and shanks all over the bench, because Charl try to force it!
Anybody else that had similar problems? Maybe I'm just sharpening the S&D bit wrong?
Any help will be appreciated!

[JHowell]

Been having problems with my delrins lately. The delrin I have is 10.7mm and the closest bit I can get 10.5mm. Up to a couple of weeks ago, I have been having no problems at all; the fit (both in stem and mortis) was nice and tight after drilling.
But then my bit became blunt. And the problems started!
I re-sharpened, but then it drilled a hole of 10.3mm. Re-sharpened again and then it drilled 10.4mm. Bought a new bit of (supposedly) 10.5mm, which now drilled a hole of 10.4mm!
So all in all: cracked stems and shanks all over the bench, because Charl try to force it!
Anybody else that had similar problems? Maybe I'm just sharpening the S&D bit wrong?
Any help will be appreciated!

I know how this is, I have a "magic" drill bit that is exactly the right size for my Delrin stock. If I were you, I would make a finish reamer exactly the size you want. I seem to recall seeing drill rod here in very precise increments, but if you can't find drill rod that is 10.5 or 10.6 mm you can turn a section down to just the diameter you want. Then grind away almost half of it. You now have a bit similar to the spoon bits for chambers. Your grinding line will have to be straight and free of burrs or nicks; you may want to dress the flat part with a stone for the last percent or so. The bit may need to be hardened -- I usually use A2 because it doesn't warp too badly and holds a nice edge even if it's not handled optimally -- but tool steel will last a while in briar and rubber even without hardening. Long enough to see if it works, anyway. Follow your drill with this reamer and you should be able to control the size perfectly.

The other option is to drill the holes undersize and then bore them to final diameter, but I'm not sure what sort of lathe you have.

Edit: come to think of it, you may be able to buy a commercially made reamer that is the perfect size. I know reamers come in very close increments. McMaster has them every .005", and offers a 10.5mm reamer that probably really is 10.5mm. 10.7mm comes out to .4212", and McMaster offers a .4210" reamer that would probably do nicely.

My way is thriftier, though.

[pennsyscot]

I glue the tenon into the stem. Then, I spin the stem in the lathe and sand the tenon until I have a nice fit. I make the final fit with 1000 grit. I also use a delrin mortise sleeve as recommended on Downie's web site. It makes for a super smooth fit. I'm just a rook, but it works for me. I use delrin that is sold in english dimensions. My 5/16" delrin rod measures .312" and drill bit measures .309". I think that you probably need to find a better drill bit.

[KurtHuhn]

One thing I noticed is that you're using delrin that's fairly large - 10.7mm is kind of big (that's about 7/16" for the USians in the audience). One thing I've found is that the probability of getting a nice snug fit of delrin into a mortis rises considerably when you use a smaller hole. I don't know why, but it seems I can use any damn 1/4" drillbit I have in my drawer to drill a mortis, and the delrin will slip in and give a nice snug, but not too snug, fit. For 5/16", I need to be a bit more careful and keep tabs on the two magic drill bits - one for briar, and one for ebonite, since one won't work on the other material. I tried to fit 3/8" once, and gave up after going through about five or six bits with no luck.

Also, it's worth mentioning that delrin tenons will be much more compliant if you fit them after you drill them for the airway. You don't say if you do this or not, but I figure it couldn't hurt to toss it out there.

[Charl]

Thanks for all the replies, guys!

Unfortunately I don't have a lathe yet (I'm not prepared to pay $500 for a 9x20 here in SA), but will try your idea Jack.

Kurt, I'm using 10.7mm because (for some strange and wonderful reason), 8mm is near impossible to get over here. IF you are lucky, salesmen will tell you that it MIGHT be ordered and MIGHT be here in about a month's time!

Oh by the way: found out early on that it's easier to fit the delrin after drilling the airhole

Thanks again!

Mmm! I can get hold of 6mm, maybe I should try that!