Drill digging in Delrin

Howdy, gang.
For the MOEPED 6 (remember the MOEPED?) I need some control cables routed over 1-1/8 inch tubing so I bought an aluminum two-piece shaft
collar and placed some 3/32 holes in it. As expected, they wandered from the face drilled, coming out the back a bit off location. Then I got a couple of same-size same-type shaft collars in lightweight Delrin and used a 1-1/8 hole saw as a guide, clamping the aluminum jig collar over the Delrin work collar. I learned two things for my next try:
1) The holes are placed right on the drilled side of the jig collar, so THAT side should face the drilled side of the workpiece. Also, the second feature should go on that side of the work.
2) Drills dig in Delrin. I am nursing a tiny thumb cut proving this. Next time, I'll clamp the collars and holesaw up in a six-inch three- jaw chuck that will resist lifting. I'd thought of a hold down the Super Shop has on it, but now that I've botched a part, the heavy chuck is clearly the way to go. It'll let the part float to follow the small hole; a hold-down wouldn't allow that. Yes, I know about blunting the edge. I think I should do that, too, next time
You know, it's a bitch to get this kind of shit right the first time. There goes about $38 worth of Delrin, but then again, here comes $38 plus worth of knowledge. If I didn't believe that, there'd be no freakin' way I could soldier on through this kind of disappointment. As is, I've pretty much seen it all before and am largely immune to the upset..
When finished, the collars will be around 9 inches apart on the tubing and the control cables will enter one collar, the sheaths will stop there, the cables will continue, and the sheaths will pick up on the other side. So the feature I am trying to make (eight times correctly the first time through) is a 3/32 through hole 1/2 inch long and a 6 mm stopped hole around 3/8 deep. It's not that hard. I could serial- ream the features and there'd be no digging. When finished, some cables will trip switches or measure control pressure with a strain gage and others will interact with each other to lock out overshifting. The OEM cable routing sucks; it's a bracket applied with fiberglass tape.
I sure am enjoying the Super Shop! It does everything well.
It's hard translating what you know into what you do the first time through.
Why is that?
Doug Goncz Replikon Research Seven Corners, VA 22044-0394
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Hi Doug,
Not sure why you are having so much trouble drilling Delrin. We use a lot of it in some of our products and it behaves very well. We drill holes for M4 to M10 and never have any digging in. I am not sure of the exact rpm but when I am in the shop later today I will put a tach on the drill-press we use. The work must be held firmly and rpm (guess) is low in the 1200-1500 range and feed is fast. We also machine and drill it in the CNC machines and I just looked at one GCode file and it calls for 1600-rpm and 20-ipm feed.
We do not square the edge of the drills as for brass, they are normal screw length 135deg with relief.
It sounds like your collars maybe Nylon and not Delrin as I don't recall every seeing Delrin dig in when drilling UNLESS being held by hand or not clamped properly.
Happy to help more if you need it.
Dave
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Gunner Asch writes:

I have an ongoing need for *black* Delrin rounds in roughly 2.5 inch diameter by 5.25 inches long or more, either solid or up to 1.375 core. I currently pay $28 delivered from Enco for a 12-inch length of 2.5 dia black Delrin that makes two of these blanks.
[I tried to email you but earthlink.net sez, "550 snipped-for-privacy@earthlink.net...User account is overquota"]
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On Mon, 18 Feb 2008 15:18:22 -0600, Richard J Kinch

Ill see what I can do. Straight delrin or graphite impregnated?
Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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Gunner Asch writes:

Straight. Gets made into an optical assembly for medical and scientific instrumentation.
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On Mon, 18 Feb 2008 17:21:27 -0600, Richard J Kinch

Ok, Ill check with my sources. It comes feast or famine.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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On Mon, 18 Feb 2008 10:16:55 -0500, Brian Lawson

Nooooo! Slow speed and aggressive feed is what works with delrin. You want long clean curly chips climbing the flutes of your drill. Try it! You could use a bit of ethylene glycol (AKA antifreeze) as coolant, but I never do. It's not necessary if you get your speed and feed right.
Try it!
Even spit-sizzle warm is too hot for delrin.
As Brian notes, a center drill is your friend with delrin. Use it. Every time! If the follow drill starts straight, it'll run straight. The converse is also true.
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Keywords:

I'm sure this is old news to many folks, but one of the biggest problems with drilling in plastics is their high coefficient of thermal expansion, combined with poor thermal conductivity.
When the plastic around a hole being drilled heats up, it wants to expand. But because the plastic around it hasn't bothered getting warm yet, the only place it can go is to squeeze into the hole you are making. The exciting part is when this causes more friction against the drill, which causes more heat, which causes more expansion which means BANG! Suddenly the part siezes onto the drill bit & something (usually expensive and/or painful) gets trashed.
Depending on what you are doing, a little cooling can go a long ways. I've used soapy water, soluble oil & almost any other liquid I have handy. Just be sure to check that any solvents won't do nasty things to the plastic short or long term. I have cans of compressed freon gas (electronics "dusters") that make good spot coolers when inverted. If I have to drill a hole and things start getting a little warm, I'll back out the drill & frost it up before continuing.
Doug White
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