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Nylon Tube Melting Where Riding on Steady Rest

wmtmachining

Plastic
Joined
Sep 20, 2023
We can't figure out what is going on here - we are boring a PA6G Nylon tube (24" OD, 20" ID finish size), but the material eventually starts to melt only where it is riding on the steady rest rollers.
None of the rollers are seized; they spin freely and we are running well under their rated limit (tube is spinning at 300RPM).
Any idea why this material is melting?
Any ideas on a fix? Lead time for a new casting is 6 months.....


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I suggest flood coolant or air blast on all three bearings. My guess is the bearings slip rather then roll smoothly.
You could make nylon tires for the steady bearings.
Bill D
 
Rollers are spinning at 3000RPM, not 6000.
My guess - based on the quality of the CNC Lathe - is that the rollers aren't quite aligned with one another, so they are probably slipping, even if just slightly.
We'll try the coolant and see if that takes care of the issue.
 
Make up a sleeve to protect the work, Well oiled leather would do it.

ditch the rollers.
 
You say the nylon is spinning at 300 rpm and then you say the rollers are spinning at 3000 rpm. The only way that would be possible is if the rollers were 2.4" in diameter. I'm betting Garwood is more right about them trying to spin at 6000 rpm (rollers ~1.2" in diameter).
 
Aligned or not, those slick rollers are slipping. Can you grit blast them or add a knurled sleeve? Either way you'll probably have to slow the workpiece rpm.
 
You say the nylon is spinning at 300 rpm and then you say the rollers are spinning at 3000 rpm. The only way that would be possible is if the rollers were 2.4" in diameter. I'm betting Garwood is more right about them trying to spin at 6000 rpm (rollers ~1.2" in diameter).
The rollers are 62mm in diameter....the math is right.
 
I was going to ask about roller diameter but I see that you're on top of it. Are the roller bearings tight or loose? IIRC the premium quality for steady rests is stiffness. So if the bearings are tapered roller or needle, and they're new then I think that the resistance keeps em from spinning fast enough and you end up with friction.
1) lubricate surface (reduce friction)
2) flood coolant (reduce friction, remove heat)
3) ensure that rollers are aligned and rotate really freely (reduce friction)
3) reduce speed. The recommended cutting surface speed for nylon is 200-300 sfm (600-900sfm for cast nylon - still way below 2000sfm). Any reason for the super-fast speed? May have to engage the bull gear to get to 30 rpm!
Good luck!
 
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Have you touched the rollers? Are they hot? Is there some reason you need to spin that fast? You can cut the speed way down on nylon and just kick up the feed. There's very little tool pressure on nylon with a razor sharp tool...

Remove the bulk of the stock at low speed, and kick it up only for the finish if you have to. I'd not even do it then - just grind up a piece of HSS with a broadnose and run high feed, low RPM for that too.
 
Need rollers with a wide face on them, like around 3" and coated with polyurethane and sleeved with a heat resistance thermoplastic like PEEK. Time to redesign the steady rest fingers.
 
So, this is a recurring but, I guess, low volume job? I'll say it out loud, but it is probably impractical. Wouldn't an air bearing be sweet? I am thinking not a fully circumferential bearing but three pivoting pads maybe each an inch wide and 4 to 6 inches in length. For those with experience making them, is that totally crazy? Be gentle in your responses, please.

Denis
 
So, this is a recurring but, I guess, low volume job? I'll say it out loud, but it is probably impractical. Wouldn't an air bearing be sweet? I am thinking not a fully circumferential bearing but three pivoting pads maybe each an inch wide and 4 to 6 inches in length. For those with experience making them, is that totally crazy? Be gentle in your responses, please.

Denis
for a start I don't think the outside of the material would be round enough for an air bearing.
 
Might you try to run one of your scrap parts with a wrap, or double wrap of SS tape?

I would make your roller wheels a lot larger, could make them larger and wrap them with some kind of tape that would get more traction to the plastic part.

What is your cutting tool device? could multiple cutters cut down the time that part is in the turning?
Could the cutting tool be turning rather than that part?
 
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Back in time we had a similar problem with the rollers on a hydraulic steady on a CNC tearing up the surface on a piece of cast iron bar but not that big. Shop made a tapered bushing and split sleeve that slipped over the OD of the finished surface of the bar. It had a take-up nut that tighten the bushing against the sleeve which shrunk against the part. Worked like a charm.
See no reason why you couldn't do something similar for this job.
 








 
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