Finishing Suggestion for AISI 1144 Turned Part

[AllAngles]

I have a CNC turned AISI 1144 shaft that is about 140mm long and 10mm OD. There is another (aluminum) piece that fits over the shaft that must slide smoothly along the axis and I am using composite bearings pressed into the outer sleeve for the interface which works well. The issue I'm having is getting the "feel" of the turning marks out of the sliding mechanism. The Ra of the OD on the shafts right now is probably 200-250 uIn. The bearing manufacturer recommends a max Ra of 90 uIn.

While I think the shop can improve the finish somewhat from where they're at, I (and they) do not believe they can get consistently and cost-effectively down anywhere like 90 Ra. So, I'm looking for some thoughts on the most cost-effective way (in volumes of about 500 pieces per batch) to get from say a 250 uIn Ra to near 100 uIn. I also need some kind of finish coating on the part for basic corrosion protection - enviroment is automotive interior so nothing too remarkable required there.

I was considering electropolishing/passivation but I don't know anything about it other than what I've read and I see it much more commonly used on Stainless than not. Using a vibratory tumbler on the parts is an option but not an attractive one as it's time-consuming in these volumes.

I would appreciate any thoughts/ideas.

Mikah B.

http://www.allanglesdesign.com

 

[btrueblood]

"it's time-consuming in these volumes."

What kind of volumes are we talking, exactly?

It ought to be dead simple to get even a 32 rms finish on a turned part, even on 1144. Sharp tools with generous nose radius, and a slow feed rate. It does take a little more time, but that's the price you pay for finish. Maybe a burnish tool, or just a little time on the lathe with an abrasive pad. Or shoot the parts through a centerless grinder for the finish step. All of which take time too, but maybe less than the fine pass on the lathe, you'd have to experiment a bit.

If the problem is that the 1144 is tearing (leaving little pockets/craters where chips have torn out), try sourcing a finer-grained material. Stressproof from Niagara is one such, and it can be polished to near mirror finish, from experience.

 

[AllAngles]

Quantities are 500 pieces at a time about every 3-4 weeks. The parts are being fed through-collet on a CNC lathe so any kind of manual operation like scotchbrite or equivalent would add significant cost.

I'll talk with the shop and see if they've tried your suggestions on getting a better finish. Your thought about tearing may be what's going on as they've struggled to get a nice finish on these parts from day one. I'm familiar with Stressproof and maybe we'll give that a shot.

The shaft does have a feature at one end that is more than the 10mm OD so centerless grinding would be fairly manual (though it might be workable).

I appreciate the suggestions!

Mikah B.

http://www.allanglesdesign.com

 

[dgallup]

The sulfur content in AISI 1144 makes it tough to get a mirror finish but you are not asking for that, normal turning process can easily produce an Ra of 63 or less. Are you single point turning? A shave tool might be better. I would think quantities of 500 in this size would be simple to clean up in a vibratory finishing step but I always prefer to avoid secondary ops if possible.

You might want to look at the condition of the material too. If you are using annealed bars you might want to try cold drawn bars instead.

As for coating, electroless nickel should work, there are many other possibilities.

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[mfgenggear]

allangles

first of all to add to the above great comments here are my suggestions.

#1) if the bars can be purchased heat treated, this will improve on machining.
because when the parts are dead soft a good finish is hard to obtain,it will tear & smear.
dgallup is correct.

#2) a good lathe or screw machine with collets & bar loaders (for better production)
is a necessary. 63 micro even 32 micro can be obtain. someone is flocking the dog.
but see #1, soft material will tear & smear. heat treated bars will enhance the finish during machining.
also if the added cost for heat treated bars is not possible very high RPM with very light
depth of cut & feed rate with with 80 deg & geneous radii (R .06 or bigger) carbide tools should do the trick.
it will not be as shiny but will have excellent finish. as btrueblood suggested.

#3) a simple polish with emery during turn will greatly improve finish but will slow down the parts per min.

#4) I have being working with & have machined parts, for 37+ years and your guy's are
not doing it right.

HTH
Mfgenggear