8 Finish on Oring Grooves?

[crazy^millman]

Okay have a project where the customer is asking for a 8 finish on a oring groove. Normally process I have used is to jig grind. I have been asked to do this more and more in one operation on a CNC Mill. I have really thought that maybe a burnishing tool might accomplish this task. They have an 8 Finish on the bore and we will be able to achieve that with burnishing no problem, but never tried it on oring groove. Anyone else done this in 15-5PH?

[Newbeeee™]

Ron, is it a face groove and a circular groove?

If so, what about milling it U/S to rough, and having a one piece ground (carbide) hollow form tool and plunge it to finish?

Slow revs and dwell - it would give you circular lay???

[RaiderX]

prob not what is needed but have achieved this by milling best finish poss. and polish with cratex. def. not a production aproach..

[crazy^millman]

Yes this is intended for lights out and doing as much to the part as possible. In the past I got the engineers to go up to a 32 finish, but this group is not going to budge on that. If it needs to stay an outside process it can. I have thought it might be possible to do with a custom made burnishing tool. I am going to get with the manufactures of these types of tools and see what we can accomplish and keep everyone posted.

[YoDoug®]

How deep in the bore is the groove? What about a coolant driven high speed spindle with a small grinding wheel?

[Guest]

The best finish I've ever been able to obtain on a machining center was an 8... in Aluminum. It was with a 5 flute tool, high RPM, low chip load. It was basically polishing the part. So it's doable.

 

If it were my project, I'd get aa custom ground CBN grinding wheel  and finish grind it (on the Machining Center of course). The tool may cost aa little bit, but do it once and do it right I like to say. You can have it right, or you can have it cheap, pick one. :

[jeff]

customer is asking for a 8 finish on a oring groove. 

Sounds like an engineer that's fresh out of college. 

[Jim at Gentex]

Sounds like an engineer that's fresh out of college. 

 

That's what I was thinking.

 

Just because you CAN call for something like that doesn't mean you NEED to do it.

Frickin kids, huh?   

 

But IF you must do it, as James said, you gotta grind it, AND, it's gonna be 'spensive. 

[Newbeeee™]

Depending upon the pressure application, as long as you have circular lay a 63 finish can be more than adequate.

[crazy^millman]

Well I can share this much. It helps things go into space.

 

I mentioned that they up the tolerance and it was well we already get it that way so support it that way. :scooter:

[YoDoug®]

I know from experience that those sometimes crazy tolerances and finishes on a print actually matter. When I worked for the fishing reel company we did a lot of testing of components like o rings and quad seals. The difference between a 8-16 finish in a groove vs a 32-64 finish may double the life of the seal or increase the pressure rating immensely. When your designing something that goes in severe environments, salt/corrosive, extreme temperature/pressure, etc you sometimes have to push the limits on tolerances to make sure the service life is acceptable. 

[Guest]

I know from experience that those sometimes crazy tolerances and finishes on a print actually matter. When I worked for the fishing reel company we did a lot of testing of components like o rings and quad seals. The difference between a 8-16 finish in a groove vs a 32-64 finish may double the life of the seal or increase the pressure rating immensely. When your designing something that goes in severe environments, salt/corrosive, extreme temperature/pressure, etc you sometimes have to push the limits on tolerances to make sure the service life is acceptable. 

WHen something is going to a place where servicing the item is nearly or completely impossible, the only option is to go crazy on the tolerances. I worked in a shop where almost everything we made went either in space or in a nuclear environment, the materials were nasty, the tolerances were crazy, and the deadlines were insane. I learned a LOT there. :

[Powertrain604]

Hey guys, very interested in bringing this old topic back up.

We have been fighting this issue for years and have yet to find a good solution.

A little explanation;

Our application is for seal rings in multi-speed transmissions. Grooves are similar to an o-ring groove, but in some cases very narrow (i.e. the one we're working on right now is only 0.08" wide x0.12" deep on a 1.50" dia shaft). 

These are special seal rings made out of high end polymers (i.e. vespel, etc) which run at very high PV factors and having grooves cut very square to the shaft axis and a target finish of 8 is required or the life of the ring suffers. Parts are not going into space, but we have test data substantiating the requirement for these fine finishes.

Not a high volume production, single digit to 10's of parts in an order.

We've been trying to assist our machining vendors with developing processes and tooling to reliably cut these grooves without much success. 

Any guidance would be appreciated.

Thanks

 

[Kyle F]

@Powertrain604 Hmm I've milled a decent amount of Vespel but have never had to deal with any tough surface finish requirements so I may not be able to be of much help,.. but easiest option would be to get a tool that you can run something like a reaming drill cycle on.

https://www.thinbit.com/products/mill-a-groove/index.php

check these out.

you can get custom ones as well here is a screen shot from a recent quote of mine:

theoretically you could get 2 or even 3 of the same tools, but maybe have the first tool a thou smaller and it cuts .001 off of finish depth, then tool #2 can come in and take the last .001" off the wall and floor, and then tool 3 could spring pass tool #2 and remove any deflection (which would be super overkill on something like vespel haha I would probably go this route on inconel though)

[#Rekd™]

Try High Speed Steel vs Carbide with Vespel.