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machining rubber

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I have to machine some pcs out of nitrile rubber, it has a 75 shore a hardness any
sugestions on how to tackel this stuff.
 
I used to have a rubber moulding company in a previous life and we used to machine some some bits out of rubber and a lot of bits out of polyurethane.Rubber grinds good but PU doesn`t and PU is easier to turn than rubber.For drilling we got good results by grinding the drill like a Forstner woodbit,looks "W" shaped when viewed from the side,for turning we would grind HSS up with very steep top rake.We made a lot of special fixtures and used a lot of double sided tape to hold pieces for machining.We used to stick 150lb PU mouldings 48" diameter onto a faceplate for turning.These were pigging discs for cleaning the inside of subsea pipelines.Interesting times they were.
Regards,Mark.
 
I have never cut anything out of the material you are speaking of, but I once had a job that was made from cotton impregnated rubber (kind of like a car tire) and I could not machine it at all. I do not know if it was because of the cotton our just no experience so I ended up making a steel rule die to cut the profile in an arbor press and a fixture to cut an angle on the face of the parts using a buck knife that was very sharp. For the holes I made punches and located them on an old mill and punched them through the rubber. I made one depth cut using an exacto knife mounted in a collet on the mill and finished its bottom in a fixture using the buck knife. I think I used a 15-degree relief on all of the knife-edges and made them from stainless. They needed to be very sharp to go thru easy. Made about 100 parts that were vibration dampeners for aircraft avionic equipment. Worked real good and held the ridiculous +/-.005 tol. Good luck
 
Try to get acces to liquid N2.
(maybe a university or a hospital near you ?)
Drop the part in a simple bowl of lN2 , wait till it stops boiling and machine it .
HSS and toolbit worked well for me.
Be sure to have every tool you`re going to need at hand and everything set up in advance tough.
Including pliers (or isolated gloves) for handling it , anything frozen with lN2 will cause SERIUOS blistering when handled the wrong way.
Good luck.

Lambert
 
I think Mark has the best suggestions. Rubber doesn't want to cut. It deflects into itself under tool force thus it overcuts or undercuts unpredictably unless the tool us just perfect.

The best tool I've used for rubber was ground up from tool steel. It was shaped a bit like a cape point chisel sticking up with mirror bright edges. It's a trick to get the end clearance just right. Use soapy water (Ivory Flakes if they still sell it or dish soap) for a coolant.

Grinding works best for maachining rubber like it was raw stock. I used to make all kinds of wierd nitrile rubber plugs and fittings on a tool and cutter grinder using common white wheels.

Rubber exapands like crazy when heated. If you have to hold accurate sizes be sure to keep the work cool. I used a spray mist.
 
How long are the parts?I have done my share of turning rubber but the method that I use will not work well for parts over 2" long.If your parts are shorter than that I would suggest fabricating a thin "knife" tool out of an old HSS parting tool that resembles the blade of a dagger then setting it up so it is parallel with the lathe bed and approximetly on center with the center of the material then use it to remove the material by plunging it into the face of the material.The "sleeve" you just cut away can be removed by either setting up the same tool perpendicular to the bed and parting it off or with a utility knife with the spinde running in reverse(VERY important).This method works well but it will not work well for removing a tiny amount off the parts OD.Try to select material that is oversize enough to take a good size roughing cut and a decent size finish cut as well at least for the first part and once you have it zeroed you can switch to a material that is only slightly oversize if you wish.Also,if the parts you are machining will have a hole through the middle,drill the hole first(drills ground with a brad point like they use for woodworking work well for this)and then set up the part on a mandrel supported by your tailstock and this is even better yet.
Good luck,
Dave
 
I agree with Forest on rubber machining. What you want is as close to a razor blade sharpness as possible and an extreme positive rake on a rounded nose tool. If you are taking a finish cut you may find it useful to coat the part with something like Duco cement or other glue that hardens to a fairly had surface. Paint it on with the part left in the lathe (I am assuming you are turning the rubber part) and leave your cutting tool set up so you can go in for a finish pass. The harder and stronger coating of glue will hold the surface of the rubber and make it effectlvey look like a harder rubber than the Shore 75A when you cut the last amount of rubber and the glue layer away with the last pass. If it doesn't work for you then you may have a mess to cut off so I would try it out when the part is oversize. I have heard of spraying electronic circuit freezing stuff on rubber but the shrinking that causes will make it difficult to hold dimension. Rubber rolls are ground as for as I know.
 
Thanks for the great ideas. I have to turn the pcs from a 3.250 od to .500 od x1.500 long shaped like a funnel. DO you think dry
ice would help,i tryed puting them in the frezer to make them harder,helped a little.
 
I don't know much about machining but have been a rubber chemist for 20+ years, so know a little about rubber. Freezing rubber is the way to go when machining but you will need to be around 40 to 60 below, so a frezzer won't work. 75 duro is fairly hard and that will help also. Dry ice should be cold enough, your problem will be keeping it frozen as you work it. If you use liquid N2 and get it too cold, it will break like glass. ALSO!!! It will shrink when you freeze it, so if you measure it cold, it will be too big when it warms up.

hope this helps

dan
 
When we machined rubber rolls (Duro 75),
we mostly ground them with a open sharp course wheel.
As Forrest said, any tools (HS) should be sharp. Ours were like Kitchen Knives .

Also, we used Talcum powder like a coolant !
It keeps the rubber from sticking to the tool and itself when it heats up during the cut....a bit messy but good results when trying to hold .005 tolerances

Tried freezing, but changes in size as it warmed up drove us nuts for tight tolerances
 
Lambert and Pitkin1 have pointed you in the right direction. If you can't access liquid nitrogen, the alternative is a mix of dry ice and acetone. Around here, you can buy dry ice at the grocery store and acetone at the hardware/paint store. If tolerances are important, just adjust your dimensions to account for thermal expansion/contraction. You can cool your tooling if needed and you can also pour the cooled acetone over the part being machined. It doesn't take a lot to keep a part cooled. Just remember that acetone vapors are flamable. Beyond that, think about liquid CO2.
 
When turning rubber, take a lesson from swiss type truning. When turning the OD, only expose 1/2 or less of the material protruding from the collet (or chuck). This will have minimal deflection when turning such a short length. Then open the collet and advance out some more stock. Turn some more of the length and repeat this until the entire length needed has been turned. --Doozer in Buffalo
 
From taking a lesson from swiss type turning, I mean keep the work as rigid as possible. A swiss type lathe has a guide bushing that supports the material/part that moves with the toolbit. Sort of like a follower rest on an engine lathe. A true swiss machine has a stationary toolbit and the headstock and tailstock both grip the part, and they move on the bed. When turning a material like rubber that has loads of deflection, only turn a little sticking out of the collet or chuck as possible. If you do it in steps, the overhang you are turning is only a small amount, not the whole length of the part. --Doozer
 
I have to machine some pcs out of nitrile rubber, it has a 75 shore a hardness any
sugestions on how to tackel this stuff.

I was wondering if you figured out how to machine your part? I have some parts that are 75 Shore A and trying to figure the best way to machine it
 
I had to make plugs to fit and expand with a through bolt to seal a bore. I was able to hold size in a cylindrical grinder. Used water based coolant and changed it everyday. Made a big mess but I was able to get the job done. Tried turning with a razor sharp tool in a lathe but found grinding far superior.
 








 
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