Ball Burnishing of a 6 Lobe profile using CNC

[formcast01]

Hi,

Here is the problem: we manufacture fasteners and among those there are some Self Clinching Fasteners. They have a special feature under the head that helps "clinch" them to the sheet metal. The die that makes that feature breaks very fast. Currently we use EDM to burn the profile of the feature. The surface roughness is not that good and is very difficult to polish that kind of profile (6 lobes). I was reading about the burnishing and I wonder if we can use it on our dies using a CNC machine. The profile is like a flower with 6 lobes. The radius on the bottom of the profile cavity is the starting point of the fracture (most of the times). I was thinking of improving the surface finish and work harden that area in order to improve the fatigue life of the die and for that I was looking at the Ball Burnishing process. Any thoughts on this subject? Would it be feasible?

Thanks,

Formcast

 

[btrueblood]

Burnishing works by creating compressive stresses in the area of the part under the tool. Not clear to me how you would get a ball to "crunch" the metal inside a tiny cavity, with a likely tinier radius (I am imagining the clinching fasteners I've seen used before, in 1/4" size and smaller...)

 

[CoryPad]

Roller burnishing is an excellent method to produce compressive residual stresses and improve fatigue life. As btrueblood mentioned, small features are difficult (but not impossible) to work on. Another potential problem is the die material and its surface hardness. Shot peening is another option.

For assistance with roller burnishing, try Ecoroll:

http://www.ecoroll.com/

 

[MikeHalloran]

I'm thinking maybe you could try burning the hole a little small and getting it electro-polished to size. That should give you a better surface finish. Electropolishing does _not_ induce compressive stresses, however.



Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

There are lots of things you could try, as a rule of thumb the smother the finish the longer the life of the part. You say the EDM finish is not that good, that would be where I would start, can it be improved using different settings or electrode materials?

If not look into micro polishing, this can be done mechanically, electrically or chemically, speak to someone who specialises in this field, or wait for a post on here from someone who does.

Look at the tool steel you use and hardness, many people stick with the more generic steels they know but there are some amazing “new” ones out there, I have always found Bohler Uddeholm a very good company with excellent technical support but you may have a preferred supplier.

Finally look at coating the part, this can offer huge improvements and don’t just go for the more generic coatings. I have always found Oerlikon Balzers to be an excellent company that offer very good technical support and a first class service.

 

[TVP]

Roller burnishing may be somewhat beneficial, but there are other things that would have more impact. Make sure that the EDM layer is completely removed. Microblasting is a good method for this. Polishing the surface to a roughness of Rz 1 maximum will certainly improve tool life. But it sounds like the biggest improvement would come from making the die a multi-segmented design, out of a high quality tool steel (not D2 or M2, try a PM grade from Bohler Uddeholm), with a large amount of prestress (shrink fit). Coatings can improve wear resistance for tool steels that are strong, but not wear particularly wear resistant.

 

[formcast01]

Thanks, guys, for the promptness! Yes, I agree, it is a small feature, but we do sizes up to M12.
The material is M2, Rc 58-60. I'll consider all suggestions.
We did order it outside and we asked for a better finish on the feature, but we didn't get quite the finish we were looking for. Burn it in-house, polish it a bit on the surface and breaking the top edge gave us the same tool life, so it's not practical to pay extra money for something you can do. Still, the tool life is poor and this is what I was trying to change. Thanks again for the help.