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How to turn this bush

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Sam Longley 108/07/2021 08:19:58
857 forum posts
30 photos

I have to make 2 bushes from Delrin. I have a piece of delrin tube 30 diam with 20 hole & 250 long supplied by client.

I have to make 2 top hat bushes to fit inside a rudder tube internal diam 28mm. Inside this goes a 1 inch rudder shaft. The bushes are to be 80mm long overall with the top hat part about 10mm long & 30mm diam The bush goes 70mm into the tube.

The tube is 4 ft long & I can put this & the shaft on my bench for fitting purposes.There is a bush at each end & the reason for the bush is that wear over 45 years has caused excessive play.

One can see that the bushes will have a wall thickness of 1.5mm so be very delicate.

I do have an expanding reamer that will ream the 1 inch Ok ( assuming I can ream Delrin) I have drills that are 7/8 diameter to bore the tube prior to turning out with a boring bar to a size that can be reamed

I did think of making an under sized mandrel. Boring the centre, mounting on the mandrel, turning the outside to a press fit in the tube then reaming to size to take the shaft.

Is this the way to go about it? If I get it wrong I do not have any spare delrin as I need the offcut of delrin for another bush

Can one ream delrin with an adjustable hand reamer? Any suggestions please

Martin Connelly08/07/2021 09:18:57
avatar
1844 forum posts
195 photos

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

Emgee08/07/2021 09:44:24
2146 forum posts
265 photos

I have found the same as Martin, whatever wall thickness when reaming using a machine reamer, after first drilling and boring, the reamed hole comes out really under reamer size, so boring to finish size is appropriate.

I have no experience of adjustable reamers.

The top hats shown are delrin with a 14mm bore, the SS316 stud is M8&M5

Emgee

dscf0004.jpg

Edited By Emgee on 08/07/2021 09:48:34

Mick B108/07/2021 10:00:03
1996 forum posts
114 photos

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

Bo'sun08/07/2021 10:17:06
492 forum posts

Hi Sam,

Have a look at Joe Pieczynski's video on turning thin walled tube. I know it's not quite the same, but it may yield some suggestions.

https://www.youtube.com/results?search_query=Joe+Pieczynski+thin+wall

Glyn Davies08/07/2021 10:46:44
131 forum posts
43 photos

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.

John Haine08/07/2021 10:54:34
4086 forum posts
241 photos

I doubt that a reamed fit is needed for a rudder, in fact a bit of play may help when salt gets in. I'd just bore it out.

Sam Longley 108/07/2021 11:26:41
857 forum posts
30 photos
Posted by Bo'sun on 08/07/2021 10:17:06:

Hi Sam,

Have a look at Joe Pieczynski's video on turning thin walled tube. I know it's not quite the same, but it may yield some suggestions.

https://www.youtube.com/results?search_query=Joe+Pieczynski+thin+wall

Brilliant link. Thanks for that . I had no idea how it could be done so easily ( for him anyway)

Much apreciated. Now to have a go wink

duncan webster08/07/2021 12:27:16
3445 forum posts
63 photos

I think delrin swells when subject to prolonged immersion, so don't make it too good a fit

Edited By duncan webster on 08/07/2021 12:27:40

Sam Longley 108/07/2021 12:43:18
857 forum posts
30 photos
Posted by duncan webster on 08/07/2021 12:27:16:

I think delrin swells when subject to prolonged immersion, so don't make it too good a fit

Edited By duncan webster on 08/07/2021 12:27:40

Lots of boats have delrin bushes.However, Nylon is a big No No.

here is an extract from Direct plastics on line data sheet

So I expect it to work Ok. But thanks for the comment

Good abrasion resistance
Good mechanical strength
Low coefficient of friction
Pretty good heat resistance
Good electrical and dielectric properties
Low water absorption
Mick B108/07/2021 13:46:08
1996 forum posts
114 photos

Not AFAIK through water absorption, but I have noticed some dimensional instability on parts I've made from acetal, leading to stiffer rotation in moving parts than when just finished.

Not branded Delrin though, just black acetal from the Bay - maybe it's the cheap stuff I buy...

laugh

Howard Lewis08/07/2021 14:08:09
5224 forum posts
13 photos

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

not done it yet08/07/2021 15:43:21
6251 forum posts
20 photos

Firstly I would not even consider drilling ~two mm out of the pipe with a 7/8” drill.

I would start by cutting slightly surplus lengths of the delrin.

Affix (by a couple of screws?), to a piece of drill rod and turn to outside dimension.

Then bore a holder, on the lathe, to accept that outer dimension. Affix each tube, in turn, to be bored to the desired internal dimension.

Tidy up the ends and job done.

 

Edited By not done it yet on 08/07/2021 15:47:25

Ian P08/07/2021 18:01:27
avatar
2512 forum posts
102 photos

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

Sam Longley 108/07/2021 18:06:27
857 forum posts
30 photos

Job has been finished. Thanks to Bo'sons post above I followed the video by advanced inovations. The supplied tube distorted in the chuck so I started by turning a nut to the internal bore & forcing into the tube with a piece of studding. . This meant that I could clamp the work in the jaws 25 mm securely.

I turned a short piece of oilon as a test 25mm long to get the sizes correct for the shaft & the press fit into the tube.

I drilled the tube with a 1 inch drill bit,(this was still tight on the shaft as it was over 1 inch diam) then in one pass bored it to a clearance fit on the shaft that I required. I (26.2) then turned the outside right up to the chuck jaws in one single cut ( 28mm). So the bush was very thin.( walls .9mm) Overall length 80mm

I then advanced the work in the jaws of the chuck & because I had the piece of steel in the middle I was able to tighten down on a 12mm length. I then deep scored the work at the length for the top hat & finally cut the score with a hacksaw.

I used a carbide tipped boring bar as it was the only one long enough & a disposable tipped cutter for the outed diam. The big problem was the stringy swarf requiring regular stops to remove with scissors.

It all went well & a delighted customer left with 3 bushes fitted

Thanks everyone for their input

Howard Lewis08/07/2021 18:20:25
5224 forum posts
13 photos

Glad that it came to a satisfactory conclusion.

Win / win for all concerned.

Thanks for the update.

Howard

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