Electropolishing ID of 316LSS blind-hole cylinder

[BlueOvalBruin]

Hello,

I have a small 316L SS machined cylinder and I need to polish the 5mm ID for functional purposes. Only 1 end of the 24mm long cylinder is open. My company tried electropolishing the cylinder before I joined them but couldn't make it work. The polished area didn't make it halfway down the can. I think they gave up on electropolishing too early and maybe we need a different vendor that can do the small blind-hole cylinders. Any recommendations on vendors, or if you believe this type of part cannot be electropolished effectively, any recommendations on alternatives? Thanks.

 

[CoryPad]

How polished does it need to be? You could press a smooth die/punch down the hole to make it smoother. You could produce the part via punch forming to avoid the rough surface altogether.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BlueOvalBruin]

Cory (nice name BTW),

It needs to have a surface finish or 0.3 Ra (micrometers) [12 Ra in microinches] or finer. We tried deep drawing the cylinders but the constant wall thickness caused problems at high pressure. We needed some extra thickness on the base of the can and the OD is constrained by other parts. We are also considering roller burnishing, but it might work harden the surface and perhaps harm our seal.

 

[Compositepro]

To electropolish the inside of the cylinder all you should need to do is use a wire electrode that extends into the cylinder. A tube brush with plastic bristles mightwork as the electrode and fixture.

 

[unclesyd]

How is your hole made?
What is the finish now?
How good is the hole as is as drilled?
Aside from the fine finish what else is require?

We polish a similar hole in our manufacturing of polymer spinnerets. We use one of three methods to polish what we call a counterbore, essentially a blind hole. Depending on the requirement we use a needle lap and if a little better finish is needed we go to Helical Lap. If someone wanted a ultra smooth bore we would first lap it to help true it up and using our in house electropolishing process, similar to STEM drilling for the electropolishing. After you read the description of our process you might check and see if there is anything in polishing business.

Look under internal bore and needle eye laps for mechanical laps. I would call them i you need any assistance.

http://www.helicallap.com/index-2.h

Here is another supplier of needle eye laps.

http://www.penntoolco.com/catalog/products/products.cfm?categoryID=5643

A little rant>

This may be self serving but I will have to take credit for the whole process even though I didn't get a patent and GE did. This still rubs me the wrong way as I have a widely distributed memo about my process dated 4 years before GE's patent application for STEM drilling.

The process.

I was asked to help in developing a process to deburr the drilled holes at the bottom of a counterbore. At the time I had been doing some ECM of flat plates. Realizing that the ECM process could do the job I made an electrode from a hypodermic needle where I squared the point and insulated the stem leaving only the annulus open. Using this apparatus with flowing electrolyte and current through the needle I was able to successfully deburr the drilled holes. My first apparatus had to traverse the counterbore very slowly to keep from hitting the burrs at the bottom. As this machine would only move at one slow speed and the fact the I left the power on while withdrawing the electrode and as I was checking the deburring job I noticed the walls of the counterbore had a beautiful polish.
This apparatus was used for thirty+ years.

 

[BlueOvalBruin]

unclesyd,

Thank you for your reply, here are some answers to your questions:

Q: How is your hole made?
A: The part is made on a screw machine and the final ID cut is made with a single-point cutter.

Q: What is the finish now?
A: We've been hand polishing the bores to the desired surface finish for R&D testing, but it is inconsistent and definitely not a production process.

Q: How good is the hole as is as drilled?
A: The surface finish callout is a 16 (microinches). I can't say whether or not we are getting that fine a finish since I have no way of inspecting it. The machinist said they would try to do it, they wouldn't say they can't do it but no guarantee they can.

Q: Aside from the fine finish what else is required?
A: We have 2 small side ports (one near the open end and one near the closed end) which need to be deburred. Electropolishing could potentially kill 2 birds with one stone.


What surface finishes are you getting on these various methods (needle lap, helical lap, etc.)?

 

[unclesyd]

Cylindrical geometries that have been achieved with helical laps:
.000005" (5 millionths) roundness
.000010" (10 millionths) straightness
Surface finish better than one microinch

Trying not assume anything but I think the one microinch = 1 RMS as Ra is normally expressed in microns. If my recollection is functioning correctly we were consistently below 2 RMS with mechanical finishing.
I would contact Helical Lap personally with your problem. If they can't help they can check with their parent company, Engis. Engis has diamond pins in your diameters.

Information about other processes.

While looking through my notes I came across some work done by Extrude Hone for us on a different process. They have two systems that be able to finish you bore, the Micro Flow AFM and the ECD. We were using them mainly for the Surftran thermal deburring system. We did us their AFM process for some polishing using the one way system.
There website isn't specific about their capabilities so another call is warranted. Aside from them doing work there a lot of shops that have this equipment that would probably be better for short runs.

http://www.extrudehone.com/afmpro.html

 

[TVP]

Brian,

I would definitely investigate roller burnishing. Yes, it will work harden the surface somewhat, but this should not have any negative effect for a typical elastomeric seal. Is the seal lubricated? Lightly lubricated seals are used everyday on hardened (> 50 HRC) piston rods that are subsequently hard (> 1000 HV) chrome plated. Some seals have are integrally lubricated meaning that there is no external lubricant, just a small amount compounded into the rubber.

 

[BlueOvalBruin]

TVP,

The seal (dynamic) is not elastomeric, but a spring-energized PTFE seal to reduce friction. Will a work hardened polished surface have any different effect on the teflon seal than a non-work hardened polished surface?

 

[unclesyd]

CoryPad,
I was thinking along the same line as your post in using what we call a broach. We used a 0.009" broach to smooth and size the capillary on the other end of some of the counterbores that we were polishing. You can best describe our broaches as rain drop (point) on the end of a pin. The rain drop shape was used to provide clearance while traversing the capillary. This is what you would call ballizing on a stick. Without taking a slice off the end of the point you are limited to the radius of the point in how close you can get to the bottom. We did shorted the point by about 0.003" for the flat bottom holes mentioned above.
This worked in our case as the bottom few thousandths of the capillary didn't play into the flow of our polymer.
I check this morning with spinnerette manufacturing people about grinding BlueOvalBruin a couple of this type broach. I was about a year too late as the machine that could grind this size has been scrapped. The remaining machines the limited to a 2mm dia pin.
When the need to deburr some holes along with finishing the bore came into play I stopped looking at this approach.

 

[TVP]

BOB,

In my opinion, no there will not be any difference between a work hardened vs. a non-work hardened surface. The important aspect is that the surface is polished (or burnished).