Accurate hole in Delrin?

I am making a bearing for a 1/2 inch polished shaft in Delrin (black nylon) and find it is a bit tight and despite use of a 1/2 inch reamer (a bit old though) remains tight. The shaft measures about 2 thou under 1/2 inch.

I have tried freezing it an re-reaming (no good) and heating up the shaft (a little better), but short of a new reamer - any ideas?

I have had trouble with Delrin before - it seems to 'relax' after machining, is this just me or a general thing?

It's not entirely you! Many plastics, and gunmetal/bronze, have a tendency to close up slightly after reaming. However, if the shaft is 2 thou under I'd be slightly surprised if Delrin closed up that much. So I suspect the reamer. Try drilling it rather than reaming, or bore it.

Andrew

Do you mean Delrin or Nylon ?

It sometimes does move when large amounts of material are removed. Best to get it within .020 over finished size and let it sit a few days somewhere warm (not hot), then finish to size.

Same thing with many plastics.

JD

If you bore the Delrin with a sharp cutter or CCGT insert you can achieve a good running fit, fine finishing cuts. As Jeff says, large cuts cause trouble.

Delrin is a bugger to machine grabs drills and even when bored to size the size can change overnight ! Razor sharp tool and bore to size, good luck.

Roger,

As Bob suggests: Delrin is not Nylon

... useful comparison here

That said; they can all be troublesome.

.

My Dad taught me a trick which often helps on difficult materials:

• Either make a 'toolmaker's reamer' or use your slightly blunt one
• Use a burnisher [which might be a carbide insert] to raise a small burr on one cutting edge. ... This needs to act rather like a cabinet-maker's scraper.
• Make your finishing cut with this; without pausing or backing-off.

It may, or may not, work for you; but it's worth a try.

MichaelG.

Plastic tends to have a high coefficient of thermal expansion, so it expands when it gets warm, much more than metals. This can make the part shrink an hour after cutting to be tight on a bearing from a reamed hole size.

but it's also very difficult to take a small cut in plastic with anything other than a razor sharp tool. And when a tool doesn't cut it rubs and produces heat.

Perhaps bore it with a nice sharp HSS tool until you're happy with the fit ? Or as Andrew said a sharp drill's worth a try.

Cheers - Will

Quite agree you wont ream it with a sized reamer it will be well under size.

Better way as Will stated bore it but keep It cool.

Hi Roger

Rather than making the bearing from a block of plastic, would it be possible to make the block from Aluminium or stainless then fit a say 2mm plastic sleeve. Companies like IGUS actually sell precision plastic dry sliding bearings with an aluminium body. There is a company in the US that makes them too, I cant remember their name! fairly inexpensive. You can also but ready made plastic Teflon or other bearing bushings from most bearing suppliers, even cheaper.

I suspect a large block of plastic will always be inclined to move a bit rather than a thin metal supported insert.

Regards
John

I have made a few cleaning rod guides from Delrin and Nylon 66 (they fit into the breech of rifles to keep things concentric). They are usually about 6" long.

The rifle cleaning rods have been ground to precisely 0.200". I found that it required a 0.202" very sharp HSS (Sutton) drill bit at low rpm to make a hole for a sliding fit using sprayed coolant for the drill bit.

I used a spray can of Inox MX-3 as it was handy and had a spray tube. Lanolin based, it's good for the lathe.

Check the swarf, if it starts to clarify it is getting too hot. The deeper the hole, the greater the friction so keep withdrawing the bit, feel it, if it's too hot to touch, slow down.

"Softly, softly catchee monkey"

* Danny M *

Thanks everyone. I checked the invoice - Acetal Delrin. The job is not critical, made mostly from the junk box and a bit of Delrin rod left over from a previous job. I had some holes tighten up on the previous job - so tricky stuff. Actually the polished rod is only 1 thou under size (checked with Moore & Wright) - so less surprising it was tight. Thanks for the tip about raising a burr on the reamer, think I will try that. Next time - more thought first!

An adjustable reamer may be of use in this material. You only want it slightly over size to make sure it does not cut a lot off, just a very fine swarf that is more like dust. Keep it cool and repeat a few times at the same setting until it stops cutting.

Martin

For a tool makers reamer use a bit of the same size as the shaft that is going to fit the hole. After you have cut the angle with a hacksaw, file the angle flat, and leave a wee burr. Don't harden, this will do aluminium, and brass, I'v used it on my hot air engines. Ian S C