Will I have a fight trying to cut a thin wall 3.5" Delrin bushing?

Trboatworks · Jul 26, 2015

I have to make a 5" long bushing with ~3.5" bore and ~.1875 wall thickness.

Before I start cutting- am I facing a nightmare to spin this down on the lathe?
I have a length of 4.5" solid rod and am figuring to cut the bore in first than skim down the shell to size.

This is a rudder bushing which is going to be set with epoxy into a corrosion trashed aluminum rudder tube so I suppose the bore has to be good but the outer cut can be a bit wonky.

Any tips on setting up to support the work as I cut?
I am figuring on just chucking up a length, facing and boring to size then cutting the OD/parting off all with no tail stock.
I wonder if that damn slippery stuff will hold in the chuck for that much messing around..

Is stock such as this ever tapped and bolted to face plate for this sort of operation?

I think the job is a good candidate for a cast in place epoxy/graphite bush but the maker has grease grooves chopped all around the stock so that route is a no go.

Thanks all

Milland · Jul 26, 2015

If you need a "slippery" surface for the bushing you might be better off with a Teflon-filled Delrin, rather than straight acetal. I'd guess that you would like a round, straight bore when you're done, as the OD isn't critical, so beware of distortion from clamping in a three jaw if you're working from a hollowed slug. If you're leaving a plug of material at the end for support you may have residual stress that messes up the bore when you part off. If you have a good six jaw, I'd turn a thick tube 1" longer than the part, flip it around in the chuck for the finish turning and part.

Leave a course finish on the OD for epoxy "tooth", unless you're going to etch the OD prior to epoxying.

Trboatworks · Jul 26, 2015

The material is supplied by shop I am doing the work for so- no say com-padre, I don't know what they purchased.

I was thinking if cutting a ~7" length of the rod and essentially making a cup with solid left in chuck end till parting off tube section length I need for bushing..
Four jaw chuck.

This job is a good example of boat work.

The rudder tube is aluminum bolted to hull skin with welded flange leaving a spigot through hull to exposure in sea water.
The tube is turned inside to carry length of thin bushing with stainless solid rudder stock within.

The sea water exposure and proximity of stainless caused the aluminum tube to blow up with corrosion compressing the bushing till the rudder stock is clamped unmovable.
Sooooo- scrape out oxides, place new bushing and wait for it to happen again.
Older boat so it all took awhile but still- lousy build for long term and no change with fix.
Greased fitting so the whole mess is lubed but lousy design- job security I suppose...

Bobw · Jul 26, 2015

Of any plastic on the planet, delrin/ass-a-tell is probably the easiest to do this with...

I've done similar, you're on the right track. Make a cup, part it off. Distortion from
the chuck should be minimal, delrin isn't squishy like teflon or UHMW. I'm not saying
you can't distort it, but its won't come squirting out of the chuck on you.

alonzo83 · Jul 26, 2015

When it comes to rudders. you will have to measure the bore port to starboard top mid and bottom.

measure the bore bow to stern in the same manner.

You will find that flanking and steering rudders wear considerably different so you need to make sure that each one is measured and each bushing is cut and labeled for that bore, marked for the areas filler needs to be applied. (EDIT) and marked for what direction the bushing gets installed.

If the wear is as bad as I have to deal with I would recommend line boring. (EDIT) the bores I deal with vary as much as .040.

The id of the bushing can be a fairly loose tolerance. usually +0.010. The od should be an interference fit with the smallest portion of the bore measured.

I am not sure why a plastic was selected for this application. We only use 954 alumabronze.

Just my 2cents from a riverboat guy.

Trboatworks · Jul 26, 2015

alonzo83 said:
When it comes to rudders. you will have to measure the bore port to starboard top mid and bottom.

measure the bore bow to stern in the same manner.

You will find that flanking and steering rudders wear considerably different so you need to make sure that each one is measured and each bushing is cut and labeled for that bore, marked for the areas filler needs to be applied.

If the wear is as bad as I have to deal with I would recommend line boring.

The id of the bushing can be a fairly loose tolerance. usually +0.010. The od should be an interference fit with the smallest portion of the bore measured.

I am not sure why a plastic was selected for this application. We only use 954 alumabronze.

Hmmmmmm

40' sail- old off shore race boat.
Aluminum rudder tube with bore totally trashed- epoxy fill to makeup on all sides.

Would the 954 take the sea water immersion with interference fit in aluminum rudder tube if such fit were possible?

My experience is any of the bronzes take out aluminum if contact allowed..

If I were driving this ship- the whole tube/gland would be replaced with composite tube and acetal bushings.

In any case- I was just asked to cut bushing- the crew doing this job will just butter everything up with epoxy and call it good with alignment established by top bushing and gland..

alonzo83 · Jul 26, 2015

Trboatworks said:
Hmmmmmm

40' sail- old off shore race boat.
Aluminum rudder tube with bore totally trashed- epoxy fill to makeup on all sides.

Would the 954 take the sea water immersion with interference fit in aluminum rudder tube if such fit were possible?

My experience is any of the bronzes take out aluminum if contact allowed..

If I were driving this ship- the whole tube/gland would be replaced with composite tube and acetal bushings.

In any case- I was just asked to cut bushing- the crew doing this job will just butter everything up with epoxy and call it good with alignment established by top bushing and gland..

I am not even going to pretend I know anything about aluminium hulls.

954 bronze is highly corrosion/wear resistant chemically stable material.

I think you mean any bronze not in an interferance will chew away at aluminium yes it will in fact it will wear out damn near any common metal before it gives away.

If the crew doing the install does not know what they are doing you should be standing by with an install tool, some all thread, an impact gun and a big hammer to get the busing jacked into place. If they think the epoxy alone will be a long lasting fix they will be in for a surprise 6 months from now, make a spare to throw on the shelf.

Also I made a few edits to my original posting.

Regards,
Alonzo

DavidScott · Jul 26, 2015

Delrin/ acetal is a poor bearing material since it has very poor wear properties. Another problem is epoxy does not stick to it. I use acetal to make molds for epoxy parts, works great since I don't need to apply anything to keep the epoxy from sticking to the molds. I can't think of a plastic that would be a good bearing material and be something that epoxy would stick to. If it has to be plastic then burn the surface you will be epoxying to with a torch first. I have been told that you can epoxy to polyethylene with this method.

John Welden · Jul 26, 2015

DavidScott said:
Delrin/ acetal is a poor bearing material since it has very poor wear properties.

Really?

……..

alonzo83 · Jul 26, 2015

DavidScott said:
Delrin/ acetal is a poor bearing material since it has very poor wear properties.

http://www.sdplastics.com/delrin/delrin[1].pdf

Philabuster · Jul 26, 2015

There are far better materials to use nowdays, but regular white Delrin is not bad.

There are many different flavors of Delrin out there now. For example, we use the blue Delrin for steel shot peen masks on our production parts whereas the white Delrin would not last a single shift before getting eaten up.

specfab · Jul 26, 2015

To OP -- your plan sounds like it will get the part made, just don't try to feed too heavily into the end of the part. There's a good bit of leverage presented by a 7" long rod if you only have about an inch of chuck jaw height, although 4.5" diameter helps. If your chuck isn't badly worn, you should be OK.

I would agree with the comments about epoxy bonding the material, although it sounds like you don't need to worry about that;-)

DavidScott · Jul 26, 2015

Ok, so maybe "poor" is the wrong word. Delrin/acetal is best for snap fits because of it's creep resistance and low moisture absorption. For a wear bushing I would pick a Nylon over Delrin, but you will still have the problem of bonding to it, and in this application Nylon's moisture absorption may be an issue. If you go with a Nylon choose a grade that has lower moisture absorption and adjust your clearance to suit. What about one of the composite bushings to start with? Does this bushing get greased once installed?

Edit: I have some plastic bushings where they rub against a steel surface with a water/stone slurry lubricant. I use delrin, nylon, and polyethylene for the different bushings. The different plastics wear at a very different rate, with delrin easily dead last. Nylon lasts about 2 to 3 times better and polyethylene lasts several times longer than nylon. Granted delrin will work for your bushing but there are far better materials to make it out of.

I need to hold a thou on my nylon bushings so to do this I leave the plastic rod in a bucket of water for a few days before machining it. Most of the dimensional change in nylon is right after molding or extruding since it was just dryed very thoroughly and needs time for the moisture content to stabilize. When I worked for a molder some of our nylon parts would get boiled in water before we would ship them to stabilize the size.

Trboatworks · Jul 26, 2015

Ok- thanks all.

Re- Delrin.

This is a common rudder bushing material in sailboats as installed OEM.

The diameter is often large and loads/speed relatively low.
In this case the bushing was pressed in.
The corrosion in tube crushed the bushing and siezed gear- wear was not excessive for age.
The material must have good wear, low friction, compatible with aluminum and stainless, and low water adsorption - a better choice out there than Delrin?

This is a bandaid fix- not my call but I am betting the epoxy will hold enough to not spin or shift...

Heavey Metal · Jul 26, 2015

do the id

stuff a mandrell in it with a center drill hole in the end

do the od

DavidScott · Jul 26, 2015

Do some grease grooves in the o.d. to give the epoxy something to hold on to. With sharp tools with small nose radiuses you shouldn't have any problems machining to size holding onto the end of the rod. Rough i.d., rough o.d., finish i.d., finish o.d., and part off. This would be what I would try first. Delrin is one of the easiest machining materials there is. What are the tolerances?

adama · Jul 26, 2015

My all time favorite job was a rework on the rudder bearings of a large fishing boat, they at least were a stainless rod in a nylon bush in a stainless flanged fitting. Boat was brand spanking, they ploped - launched it in the river the day before i got the call.

Turns out the original machine shop held real good tolerance on that nylon - press fit and a real close tolerance on the sharft too, practicaly no woblle at all at assembly.

Problem was 24 hours in the drink and the nylon swells, locking the stearing solid, they ripped the hydraulic steering mechanism out the boat hull try to force them to shift. We got to renginer them once the boat was back on the shore and the rudder sharfts with attached bushings were delivered to the shop. Took about 40 tons to slide them of the sharft on the large press :-) Opened em out about 15 thou and they fall on and most importantly after spending the night in a bucket of water they still had some play :-)

Motto is, plastic swells when wet, you need to allow for it! Rudders are not the place for precision fits, yeah they dont want to be lose lose fits, but a bit of clearance is no bad thing.

spaeth · Jul 26, 2015

Trboat,
I would also follow Heavey Metals advice. Hang it out there and
face, drill and bore the hole to size. Make a mandrel of steel or
aluminum for a good fit the entire finish length with a center
hole in one end. Turn the OD using a sharp pointed threading type
tool and a slow feed which will give you a grooved finish for the
adhesive to stick too. Go for it.
SPAETH

gnorbury · Jul 26, 2015

Epoxy is not going to work to hold the bushing in the old rudder tube. 3M5200 might, but even then it's not optimal.

Replacing the whole darned bearing assembly & rudder tube is the correct way to go. Luckily it won't be your fault if this "repair" fails.

Limy Sami · Jul 26, 2015

Trboatworks said:
The material is supplied by shop I am doing the work for so- no say com-padre, I don't know what they purchased.

I was thinking if cutting a ~7" length of the rod and essentially making a cup with solid left in chuck end till parting off tube section length I need for bushing..
...

I do this sort of thing all the time, and that's the way to do them.

Tips

1. Use dovetail chuck jaws and machine matching DT on end

2. Turn up a flanged plug the bore size to use with TS for OD turning,- it makes life SOOOOOO much easier.

3. Speed down feed up, sharp tooling.

4. If od finish critical, you might need to play the speeds and feeds game to get rid of resonance.

5. Have fun, oh and make money

Will I have a fight trying to cut a thin wall 3.5" Delrin bushing?

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