Machining Delrin Round Stock?

[RVJimD]

I am turning some 1/4" delrin round stock and find it gta smaller toward the headstock. I don't notice this problem with metals. Any hints or method I can use to reduce the problem? I am loosing almost .001" per inch. I am trying to make little axels for my idler tumblers on my crawler undercarriage project.

Thanks,

Jim

 

[aeroHAWK]

You haven't noticed with metals because they don't deflect as much, since they are stiffer. It still happens but it is so small, you haven't seen it.

The solution is not easily defined given the lack of information about what the part looks like. If you can use a center in the tailstock, it would help. But if the part is too long and skinny, it will bow in the middle. You can also look at the tool geometry. High speed steel sharpened and stoned razor sharp is important. Carbide inserts are not a good idea here.

 

[Wreck™Wreck]

Buy a Swiss type lathe this will solve all of your flexy stock problems.

 

[brino]

I believe one problem could be tool bit shape. Plastic can be sticky and the bit digs in if there's and backlash in the lead screw.

I think that a tool more like a brass one, larger rounded tip and little or no top rake.
Some say really sharp, but always flat topped.

-brino

 

[aeroHAWK]

brino,

I have to disagree here. I machine delrin all the time - today as a matter of fact. Delrin won't do what you suggest, especially small diameter like here. But I do agree the tool geometry can have an affect.

 

[RVJimD]

I tried a different tool without a radius and it was a bit better. I will have to keep working at this...

If I didn't say, I'm trying to make a few shafts from 1/4". They need to be 3/16" in the center 75% and each end a bit smaller to fit into wheel collars.

Thanks for the ideas...

Jim

 

[Bill C.]

You haven't noticed with metals because they don't deflect as much, since they are stiffer. It still happens but it is so small, you haven't seen it.

The solution is not easily defined given the lack of information about what the part looks like. If you can use a center in the tailstock, it would help. But if the part is too long and skinny, it will bow in the middle. You can also look at the tool geometry. High speed steel sharpened and stoned razor sharp is important. Carbide inserts are not a good idea here.

Plastic material cuts better with HHS bits. I know Nylon shavings will either melt or ball up on the part if it gets to hot. The shavings will not break so no chip breaker is needed. Good luck

 

[RJSakowski]

One way to get around the flexing issue is to start with larger diameter stock. If you use 1/2" diameter and finish as you go, the flexing will be minimal. Delrin cuts easily so you can cut in a single pass if you have the right tool. Use sharp tools and +1 on the HSS. The cost differential between 1/4" and 1/2" is minimal.

 

[mikey]

Delrin likes very sharp edges with large relief angles and large side and back rake on the HSS turning tool. This reduces cutting forces and cutting temperatures so less melting occurs. Finishes get better with a larger nose radius, too; somewhere around 1/32" nose radius works well. Delrin also likes slower speeds and faster feeds than metals. Watch your depth of cut. Delrin cuts easily but the lubricity can cause the part to slip in the chuck if it doesn't have tailstock support. For trimming the ends of the shaft a knife tool works well; just put a radius on the tool and it will cut fine.

I am curious if this isn't a lathe alignment or leveling issue, though. I cut Delrin all the time and do not have taper issues so you may want to check these possibilities out.

 

[aeroHAWK]

Mikey has good suggestions. With the small diameter you are working with, I would keep the nose radius pretty small. That will keep the cutting forces more axial than radial, therefore less likely to push the material away from the tool. Another thing you may try is to raise the tool slightly above the centerline. This will help in case the part is raising up due to cutting forces.