Burnishing Counterbore face

[A1BERZ]

Burnishing is HIGHLY dependent on the pre-finish.
If the finish is not consistent you will be chasing your tail forever

I've burnished thousands of tubes made from aluminum, stainless & titanium.
Each one had different running parameters, but all needed to have a specific pre finish roughness so the burnishing process would work.
If the pre-finish is too smooth, its just as bad as being too rough.

do you have any good resources on how we would determine a range for our pre finish, or is it just trial and error?

 

[A1BERZ]

Two flute end mills have relief called 'dish angle' (taper up towards the center) that is going to be heck on your finish.

Use the 6mm endmill to interpolate the bottom of the counterbore ~flat~ and not plunge to the bottom with a 2fl end mill.
This will guarantee it is flat prior to burnishing.
I can imagine the inconsistencies with roughly +0.002" more material at the Ø.281" diameter from the Ø.437" diameter
(caveat: assuming 1.5° dish angle & I did my math correctly. Carbide Bob will know the exact amount)

We initially tried to get away from using the custom ground 2 flute and attempted to use a burnisher endmill from Fraisa but unfortunately the test part was rejected becasue the customer could see tool marks after the burnish caused by interpolating the endmill.

 

[A1BERZ]

I would look into getting a stub 3-flute carbide endmill ground flat as a finishing tool, it would be much more stable than the 2-flute you're using now. I'd keep the dwell as short as possible, but still ensure at least a full revolution occurred during it. Mount it in a short, rigid holder for best effect.

I think the point about the burnisher being used for too long is a good one, but it's worth some careful experimenting on some scrap 316L before you commit. You can work harden 316 very quickly, and going beyond that surface hardening "ideal time" may lead to fatigue failure and spalling of the upper surface.

We were provided a 4 flute tool by our customer to try after we struggled with the 2 flute and we still got about the same results for finish and consistencies of tool life.
To your second point we do have it mounted in the shortest holder available and the tool is sucked up in the collet as much as it can be. Also out of curiosity when you say to long are you saying to many attempts or just the total length of time the burinsher is engaged in the material is too much?

 

[Mtndew]

do you have any good resources on how we would determine a range for our pre finish, or is it just trial and error?

If you're using a Cogsdill tool, it should have come with instructions.
Still you can find them on their website.
When we were diamond burnishing the O.D. of some tubes to get <10 micro, the pre-finish was supposed to be in the range of like 40-60 micro.

 

[MCritchley]

I live in the high vacuum world so I understand what you are after.

Start with the best finish you can then burnish. I would finish that face with an endmill that is not touching the sidewall and run it slow so your finish is around a ten.

Only burnish once, you mentioned three passes, that’s bad.

Try using oil or just some straight coolant concentrate dripped onto the face before burnishing.

Also as others have mentioned talk with the vendor.

 

[Milland]

Also out of curiosity when you say to long are you saying to many attempts or just the total length of time the burinsher is engaged in the material is too much?

As others are saying, essentially both. It's a matter of upsetting the material too long, taking you out of "burnishing" and into fatigue failure risks. And ditto for the endmill, the ideal dwell would be just enough to complete the cut, but not having the edge rub.

One thing I'm curious of - when I did high-vac work, we tried to vent everything that wasn't part of the actual flow path or chamber volume. So either screws would be vented, or grooves would be cut to prevent captured volumes. It sounds like your customer wants a screw head to act as a seal, and unless the screws are designed to do that, it's (IMO) not ideal.

 

[jccaclimber]

do you have any good resources on how we would determine a range for our pre finish, or is it just trial and error?

The times I've used Cogsdill the local sales guy or home office (depending on region) has always given me a speed/feed/roughness/stock remaining recipe recommendation. I've never needed to follow up because it didn't work, but they've always emphasized that I should if I have any issues. Have you tried them?