Internal edge radius on machined spring

[prdave00]

I have a compression spring machined from cannulated MP35N bar stock. The resulting "coil" cross section is rectangular with square edges. I'm looking for a way to radius the internal edges so the part that slides within does not grate against the edges. Radii ing the external edges is OK, but not required. So far I've considered Electropolishing, electrochemical machining, abrasive flow machining like Kennametal's Extrude Hone, and mechanical processes such as Flex Honing.

Any other thoughts or comments on my considerations this far?

 

[israelkk]

Tumbling

 

[TVP]

Shot peening + some type of secondary process like conventional tumbling or isotropic surface finishing (REM superfinishing) or one of the ones you mentioned. I think electropolishing would be difficult for this kind of part/application, whereas one of the mass finishing options would likely produce an outstanding combination of shape and roughness.

 

[prdave00]

israelkk: I'll look into tumbling. Sounds like a low cost solution and maybe something the machine shop I get the parts from can do in house.

TVP: Conventional shot peening has minimal effect on breaking the internal edges. I imagine lance or deflector pin peening may be better, but I think it would be marginally so. I'll investigate isotropic surface finishing further. Can you supply me with any leads?

I have parts out for electropolishing so I should have some idea how well it works by end of week. The unfortunate thing about EP is that I'd need to account for the material allowance in the as machined part - something I can't do with already fabricated parts. There's also the whole question of when to shot peen. EP may effectively remove the entire cold worked surface of a shot peened part, while the other mass finishing methods may not require as much material removal and thus a material allowance may not be required. The flip side is to peen afterwards, but the surface finish quality, regardless of how it was achieved, may take a hit (no pun intended).

 

[israelkk]

Beware of any chemical or electrochemical machining that may cause hydrogen embrittlement if the parts are already heat treated. Any chemical or electrochemical machining must be performed before heat treatment. Before passivation, heat tint removal needs to be mechanical, such as: fine mesh aluminum oxide blast cleaning.

 

[TVP]

Here are a couple licensees of the REM isotropic superfinish process as well as Metal Improvement (CASE):

http://www.cryogenicsuperfinish.com/rem-isotropic-finish.html

http://www.nwsuperfinish.com/

http://www.metalimprovement.com/case.php

 

[prdave00]

Thanks gents and thank you TVP for the leads.

 

[MikeHalloran]

Electropolishing may not be effective unless you insert an electrode down the center of the part, and you may still have difficulties getting the field where you really need it. I'm also not real thrilled about using any electrochemical process on a highly stressed part at any stage of manufacture.

If the part is big enough, you may be able to run a Flex-Hone through it to knock off the internal corners.



Mike Halloran
Pembroke Pines, FL, USA

 

[prdave00]

Mike: Thanks for the input. I've never considered (or maybe should have but didn't) hydrogen embrittlement (HE) in my past experience with electro-polishing (EP). israelkk was very specific about his concern and I'm assuming your aversion to EP is for the same reason. If not, please enlighten me.

There seems to be a lot of chatter out there about the effects of EP increases on HE but no consensus that it is adverse in this regards. Two sources, found using Google, both express differing opinions on whether EP results in increased risk of HE (e.g.

http://www.bssa.org.uk/topics.php?article=131,

http://www.finishing.com/475/98.shtml).

Furthermore companies that offer EP and ECM/ECD services taut that there is negligible risk of HE, but then again their job is to sell their process.

Both Latrobe Specialty Steel and Carpenter (soon to be one in the same) note the high resistance to HE of MP35N, so I'm wondering if I have a little more leeway than if my design was made of a different alloy.

Regardless both electrochemical/electrolytic machining (as you described having an electrode in proximity to the surface) and Flex Honing are on my list. I'm actively pursuing the latter with Brush Research Manufacturing. I should have an idea over the next couple weeks. EP definitely has its drawbacks and I have my fingers crossed I don't have to continue down that path.

 

[israelkk]

I was referring to chemical cleaning or process like EDM or wire EDM that produce hydrogen or any other process that produces hydrogen during the process.

 

[TVP]

Dave,

MP35N is very resistant to HE, so you should not have any problems with electropolishing from that standpoint. I agree with MikeHalloran that there may be little electropolishing on the ID unless a center electrode is used.

 

[MikeHalloran]

My other concern with electrochemical processes on cyclically stressed parts comes from a story that was related to me some years ago by my then mentor.

A particular auto manufacturer had invested some money in electrochemical grinding of axle shafts. Think EP plus abrasive grinding sort of combined. Production rates were astonishing. Finish was great. Dimensional control was just fine, as good as any other technology.

Just one problem; essentially zero fatigue life.

They found out at the proving ground. Luckily nobody got hurt when the axle flanges cracked at the bearing shoulder and fell off.

Program and machinery scrapped.


Mike Halloran
Pembroke Pines, FL, USA