Since it is a good insulator Delrin gets hot when drilled and reamed and seems to like closing down on the tools so unlikely to be an accurate size when reamed. I would bore it to size with a sharp single point boring tool. Stop to let it cool before the final sizing cuts. Once the bore is to size plug it with a suitable piece of metal and turn the outside. You may be lucky and not get this but I would expect the swarf to come off like a piece of silly string (I wish I remembered to video this when it happens) and tangle up on every available surface.

Martin C

Another one in favour of boring. I use a dead-sharp bit with a few degrees of back-rake and a fine feed.

For turing the push-fit diameter, I can't see you'd get away without internal support from a mandrel, at least if you bore first. How much grip you get depends on several factors, but light cuts and fine feeds – again with sharp tools – would be what I'd try.

I've made quite few bits from acetal, but yours certainly presents a couple of challenges.

Edited By Mick B1 on 08/07/2021 10:01:05

I have reamed Delrin with a mild steel D bit reamer and it cut to size. I turned the reamer to size and put about a 1mm chamfer on the front end before milling it to a D form. I'd obviously ream the bore before turning the OD.

Things would be so ,much easier if the 30 mm Delrin rod could pass into the mandrel of the lathe; although this implies a larger machine. If it would this would be less wasteful of material.

If not, you will have to cut off just enough to hold in the chuck to machine the top hat bush and part off.

You will then need to repeat for the second bush. Wasteful of material, unless you use a steady while each bore is made, so that enough material can be cut to make two bushes, part off, and apiece in the chuck..

So you would be thinking in terms of about 90 mm long, minimum.

To compensate for the compression when the interference fit bush is pressed into the tube. (Don't know how well Loctite works with Delrin, and with a wall thickness of less than 1.5 mm (28 – 25.4 = 2.6 / 2 = 1.2 mm allowing for running clearance, AND the Loctite needing a clearance ).

May I suggest finish bore to VERY slightly oversize (The bore will close in when the interference fit bush is fitted to the tube ).

Howard

I think this task is being seriously overthunk!

Depending on the intended purpose, if this was a 4ft length ground bar between two 3" long journals (created by line-boring) then one might need be able to get a very close running fit. For Sam's application (bearing in mind we dont know how true or straight the shaft or the tube is, nor do we know the surface finish of the shaft.

For a a shaft running in a plastic bearing to have a long life, the shaft surface needs to be very smooth and preferably polished. There are plastics specially formulated to run without lubrication and ready made bushes are an online purchase away (Google IGUS).

The longer the bushes are (and 3" is unusually long for a 1" shaft!) the more the straightness of the shaft becomes important. I know nothing about boats and rudders or what forces are involved but even a 1" shaft bends when it has overhanging loads at its ends so aiming for a close engineering fit seems a bit OTT for this application.

If you do want to machine your own bushes from Acetal then rough drilling/turning the OD and ID oversize with the stock projecting from the chuck followed by turning and boring to size with very sharp cutting tools is the way I would recommend. My only other recommendation would be not to make the bush a press fit in the tube as its likely to buckle before its pressed all the way home.

Ian P

Glad that it came to a satisfactory conclusion.

Win / win for all concerned.

Thanks for the update.

Howard