View Full Version : Lathe Turning of Rubber Seals ????

09-16-2010, 06:37 PM
I'm in a bit of a bind here, I have some old hydraulic rams that are in need of new coupler seals. You cant buy the seals individually any more, you have to buy the whole coupler, too expensive. I know that there are places that custom make seals which are turned on a lathe, If your turning rubber round stock, say Buna-N or EPDM rubber I would imagine that the cord stock is held in the lathe by a collet for minumum deformaty. What I'm wondering is if anyone knows if there is any special tooling used to cut, turn face or drill soft material such as what I mentioned. They would have to be very sharp tools. These guys that do this stuff seem to keep it a secret. I think any rubber around 80 durometer should be machineable with proper tooling.


09-16-2010, 06:48 PM
I think the big secret is to put it in the freezer over night. On the coldest setting you have. Plan your work and work quickly.

09-16-2010, 06:49 PM
Freeze it just before use and if needs be at various stages of machining.

Lathe tools need to have a cross-section like a very fine, small - and very well polished/honed finish and very sharp. Use plenty of coolant (water only is preferred). Take is slowly - very. For roughing, a cutting edge like a parting tool with a "chip breaker" works well too (tool is like the old-fashioned HSS parting-off tool).

Make sure the tool actually "cuts" and does not "rub".

Try a series of "test runs" first to get the "feel" of/for it.

Machine rubber the same way.

If it softens or gets any more than "cold" it will "grab".

Coarse high-speed "burrs" work well when the job is cold as well.


09-16-2010, 07:15 PM
The biggest factor determining machineablility will be the durometer of the material.

I've done a fair bit of urethane(probably about the same as rubber) in the 90A and harder durometer. That's a bit harder than the sole of a sports shoe. Softer than that and you won't have much luck, in my experience.

We use high positive carbide insert tooling because that's the sharpest thing we could find.

Freezing overnight does help, but you have to work fast.

Grinding can sometimes be a better method.

Dr Stan
09-16-2010, 07:22 PM
Would it be possible to make a mold and cast them from Chicago latex?

09-16-2010, 07:42 PM
I don't think freezing the piece will do much good since the flexability of the material won't change much at 25 degrees. Maybe at -300 degrees if you can get some liquid nitrogen but by the time you ran from the refrige to the machine the part would be warm.


09-16-2010, 07:57 PM
You can stink the shop up grinding it with a Dremel.

Liger Zero
09-16-2010, 08:20 PM
+1 for freezing and +1 for grinding. Grinding with a stone works so much better than cutting it with a tool.

09-16-2010, 08:25 PM
Freezing it will work - to a degree (sorry) - and its better than shop or machine temperature.

I forgot to add/say that the water "coolant" should be "lubricant" - with just a small dash/drops of kitchen-sink concentrated detergent. Too little and it loses its lubricity and too much may cause the cutter to "slip/rub".

It does not need to be in the main coolant tank/system - just apply with a brush from a tin which is in cold/chilled water.

These burrs work well too - but will heat the job if you are not careful to keep them and the job cold:

With a 1/4" shank, they require a die-grinder or similar with a 1/4" collet and lots of speed and "grunt" - like this:






09-16-2010, 08:37 PM
90 durometer rubber packer elements, as used in BP's infamous GOM well are often reduced in OD, so's to pass through out of gage wellbores. ie 9 inch reduced to 8 inch OD, etc.

Elements are fitted snug/solid onto a mandrel, turned slowly and shaved using a tool which looks for all the world like a woodturner's half moon tool, a razor sharp, curved cutting edge with all kinds of clearance. 2 inch dia. tubing was often 'sharpened' and held in a fabrication at such an angle to produce the shaving/shearing action.


09-16-2010, 08:41 PM
Could you punch them out of sheet stock with a hollow punch? If the OD was a little too big or they needed to be an exact OD, you could sandwich them between 2 pieces of steel of a slightly smaller OD and turn or grind to size.

09-16-2010, 09:39 PM
Rubber grinds quite well, but it does stink up the place and makes a mess. Other than that, you can use a fine stone and get a pretty decent finish. I think the trick with rubber is to have the cutting edges skimming it at a very high speed, so the inertia of the rubber molecules keeps it in place as each cutting edge removes the smallest bit of material. That's why grinding works so well, and no freezing required. By the way, urethane isn't likely to get much harder unless it's very cold- and how are you going to keep it that cold over the time it takes to mount it and cut it- . Better I think to grind, have the stone fresh (not loaded up with anything) and use a high SFM.

Most any time I grind on something like an O-ring, I'll turn a fixture to mount it on. Sometimes it's a friction fit, other times I'll fit it onto a boss then add a disc to clamp against the side of it. I keep a stone sharpening tool handy to touch up the stone, and that makes grinding easier and less likely to distort the rubber during the process.

If you do want to freeze it before turning, consider making some kind of mount that you can quickly attach to the spindle somehow, then freeze the mount with the rubber piece on it. The cool will last longer between turnings, but of course it will take longer to re-cool.

09-16-2010, 10:25 PM
Would it be possible to use a razor blade to do the cutting in the lathe? A hobby razor would fit nicelly in a boring bar holder.


09-16-2010, 10:38 PM
I once tried turning rubber from a dog chew toy on my lathe. When I left, I was still trying to clean the "GUM Powder" off my lathe...
Does not turn very well. If it is soft, it will tear more than anything.

09-16-2010, 10:39 PM
Do you have the size of these?

09-16-2010, 11:37 PM
The size of these seals are .437 od x .220 id and the height is .188. I found some Buna tube, 7/16" od x .250 id, I'll cut some 1/4" segments and put one in a 5C collet and mount it vertical on the surface grinder and square up the end, flip it over and grind down the other end to the .188. I think that will work. I did try to punch these out of a piece of Buna sheet. I have a Mayhew punch set that will do various sizes of id and od combinations all in one shot but what happens when you punch them out is they tend to skew to one side or end up hour glass shaped. In case your wondering what these seals are for ...... I have an old Blackhawk 4 ton port a power, the seal is in the ram half of the coupler.


09-17-2010, 12:16 AM
I have turned alot of polyurethane for suspension bushings on cars. I start with a solid rod. Cut it down to four inch lengths, freeze them and drill and bore them one at a time out of the freezer. Then I mount them on a mandrel, they are at room temp now. And use a belt sander to take down the OD. Works for me. Oh, I cut to length with a bandsaw, my tolerances are not nearly close to yours. Thank God. If they made choppers like I make cars they would be outta pilots :) JR

09-17-2010, 12:29 AM
I don't think the tolerances are that critical on something that is going to be compressed as most seals are. I think if I'm with in + - .010 of an acceptable fit it will work just fine. Have you ever checked the tolerances of O-rings, thier very gracious.
I'll post some pics if I get the time.


09-17-2010, 05:16 AM
Make yourself punches, and punch the seals.

09-17-2010, 08:24 AM
I had to make a rubber bush for a gearbox out of urathene (the part cost $500 to buy as you had to buy a complete gearstick assembly)

I used a shackle bush and ground it with the dremel (used the drum sanding attachment), with huge success.... Turning it was a complete failure... You can freeze it but it will thaw in seconds...

09-17-2010, 09:19 AM
I had to make a rubber bush for a gearbox out of urathene (the part cost $500 to buy as you had to buy a complete gearstick assembly)

I used a shackle bush and ground it with the dremel (used the drum sanding attachment), with huge success.... Turning it was a complete failure... You can freeze it but it will thaw in seconds...

You are spot on Ringer, freezing sounds good in theory does not work in practice, unless you have liquid nitrogen maybe. I have had good results grinding rubber and silicone rubber on a cylindrical grinder.

Dr. Rob
09-17-2010, 09:32 AM
As others may have said, I use an Xacto knife in the toolpost, or a sharpened tube for holes.

Soapy water, oil, WD40, anything for lube.


09-17-2010, 10:08 AM
You could keep it cold while turning by directing a stream of compressed "greenhouse gas" at it. or wear gloves and use a stick of dry ice to get it cold again between cuts.

09-17-2010, 09:31 PM
I finally had some success in making the seals. I ended up grinding the ends flat on the surface grinder after I cut the segments out of some buna tube. The first picture is my first attempt to punch out the seals from buna sheet. As you can see I can't control the shape. They end up with an hour glass shape or a slant. Hard to punce compressable material.
The second picture is the finished product which was ground.
The third picture is the ram and hose coupler halves and a look down inside the female coupler of the ram. You can see the thin washer that sits on top of the seal. The way this deal works is much like the way a garden hose washer seals againt it's mateing parts. The male end of the coupler compresses the seal when the tightened and a small portion of the seal kind of blurps up through the center of the washer making flat contact with the face of the male coupler. The washer allows the ram to piviot with little resistance on the hose coupler. This is really a low grade or poor design for a hydraulic coupler, it would have been much better to go with a quick detach type like on a snow plow, but they are big and expensive. So anyway..... it seems to be holding. The problem with the original seal was that it leaked when not preassurized, when preassurized it was OK.



Don Young
09-17-2010, 10:23 PM
This thread is very informative to me. I have several connectors of that type and have a feeling some are going to leak when I try to use them.