Turning PTFE?

I need to make some parts out of PTFE. As it is expensive stuff, any advice anyone can offer so I don't waste the material? I have in the past made the same part out of bronze and other plastics with no problems but have been asked to copy it in PTFE which I have never worked with.

Will be done on a manual lathe which only goes to 1800 rpm. Part is 3" in diameter and after turning will have a small amount of milling done to create a hex on one end.

So, I was thinking HSS would be the best choice? I have all the grinding equipment needed to make a razor sharp tool but what geometry on the tool?

I don't want to waste $2000 worth of material because I have no idea what I am doing!

Laverda said:

I need to make some parts out of PTFE. As it is expensive stuff, any advice anyone can offer so I don't waste the material? I have in the past made the same part out of bronze and other plastics with no problems but have been asked to copy it in PTFE which I have never worked with.

Will be done on a manual lathe which only goes to 1800 rpm. Part is 3" in diameter and after turning will have a small amount of milling done to create a hex on one end.

So, I was thinking HSS would be the best choice? I have all the grinding equipment needed to make a razor sharp tool but what geometry on the tool?

I don't want to waste $2000 worth of material because I have no idea what I am doing!

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I suppose you don't want to hear this but I once worked at a place that had decided to diversify and one of the new projects was the machining of PTFE components. A pal of mine worked there briefly. They built, fully equipped and staffed a workshop with about 30 skilled machinists, day shift and a night shift. After about 12 months they closed the department down because they couldn't make it pay. One of the reasons was they had a very high wastage rate, components that came off the lathe within tolerance at night were scrap in the morning once they'd cooled down and vice versa. It appears that the PTFE they were using was very prone to temperature fluctuations.

Regards Tyrone.

Just to add some detail to the above:



The thing to watch out for is that big spike in the middle, which happens somewhere around the mid-point between 0°C and 50°C aka room temperature. it's about .003 mm/mm of expansion just in a few degrees difference.

My granddad once bullied a customer into accepting some PTFE parts which were "over spec" by throwing them in the freezer for 5 minutes, and then having him measure again, and would have needed to reject them for being under spec in that short of a time. That was the 1970s, your results may vary.

I have only used PTFE once or twice but neither experience was good. I tried making some swing arm bushings out of it and I second what the others have said about expansion and contraction issues. I ended up using PEEK and all was good.

To add to the others - very prone to cold flow, so be careful how you hold it. What seems secure one moment will be loose the next. If it's 3" OD, I'd want to use pie jaws to hold it to spread the load.

I like using sharp cutters, but not very aggressive on clearance angles, which can pull the part from the fixturing. So new drills with edges "dubbed", etc.

If it's a filled material (glass fiber) it'll be more stable at the cost of being somewhat abrasive.

From what I've learned over the years - your mileage may vary

1 Heat is the enemy, razor sharp high clearance tools will reduce friction there fore heat.

2 Don't run too fast 200 - 300 FPM is adequate

3 Keep the feed up as high as possible, less cuts / inch of travel = less heat, and you'll find the heat comes away with the swarf.

4 Get the material to the room temperature it's going to be measured and machined at, ............taking a bar off an overnight truck in the winter and straight in to a warm shop is asking for trouble I LEAVE MATERIAL TO TEMP STABILISE AT LEAST 12 HOURS

4A Pay much attention to Boxcar Petes Temperature chart !!!

5 Whatever the drawing states, you WILL be pushed to hold much better than +/- 0.002'' and the bigger the part with higher mass = bigger tolerances.

6 Mic and caliper readings require a very light touch, (easy to squeeze a part) often less than a ratchet or friction mic thimble - practice with a gauge /Jo block to get an accurate pre ratchet reading.

7 Coolant, .....beware of warm coolant, at least one PM er contacted me with tolerance problems, which (after a lot of head scratching )were down to the coolant actually heating the part !!!


8 Chuck pressure, keep that to a bare minimum, dovetailed jaws with a matching dovetail on the part are the way to go for turning slugs

YMMV

Actually it's not as bad as it sounds, ....more a case of getting your head round the material.

Oh yes, ................if you think PTFE's $$$$ ..................look up the price of the same sized piece of PEEK!

P.S. If anything else comes to mind I'll post it

Not sure why anybody would design or attempt to make close tolerance parts out of PTFE. I've machined the stuff with no problems but would never expect it to be stable better than .010" or so.

You're not alone Conrad ..............I've seen some right Lulus, and declined to quote many of them.

What is this part normally PTFE is used for seals an gaskets. It's very week and rubbery.

I have made lots of different types of seals from HD-PTFE don't be afraid of it. Grind your tools with a lot of backrake 15* or so. Put a 1/32" rad on the point and home it feed .005in/rev 5-8 hundred rpm and take deep cuts.

Keep in mind it's super soft and it's slippery you can really smash it bad when clamping on it. Did I mention it's slippery? As far as holding size, pay attention to the tolerance on the print if they know what they are doing it will have a wide tolerance. It's usually smashed into final shape when the parts are put together.

A few years ago, I got a print to quote. Fricken teflon. It was some sort of washer.

There was a table for the ID and OD dimensions. It was broken down into 2 degree F
increments. And the size differences weren't trivial.

I don't save my rejection quotes, but the e-mail went something like this.

HAHAHAHAHAHAHAAAAAA NO!!!!!!!!!!!!!!

The shit sucks.

While back, had a part spec'd out of fricken "teflon". Told them that was a bad idea,
and making the parts was a nightmare. Told them what you need is some delrin. Teflon
may be slippery, but it wears away as quick as a crack addicts savings account. I
happened to have some delrin drops kicking around, and made a few. A few weeks later..
"Can you make us some more out of delrin?". $$$$$$$$$$$$$$

Teflon blows, there are better coatings and there are better solids. Its slippery,
that's it. Its not dimensionaly stable. Its not wear resistant. Its hard as hell
to hold because it has the consistency of warm freshly chewed bubble gum.

I think Teflon is the only plastic that makes UHMW look good. (from a machinist perspective).

Bobw said:

Teflon blows, there are better coatings and there are better solids. Its slippery,
that's it. Its not dimensionaly stable. Its not wear resistant. Its hard as hell
to hold because it has the consistency of warm freshly chewed bubble gum.

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I will correct you slightly. It is slippery and it doesn't care about exposure to damn near any chemical known to mankind. Once you lengthen that list a bit, I agree there isn't too much more good to say about it. Good heat resistance too, for a sealing material. But the immunity to almost every kind of chemical is quite a good reason to use it.

BoxcarPete said:

I will correct you slightly. It is slippery and it doesn't care about exposure to damn near any chemical known to mankind. Once you lengthen that list a bit, I agree there isn't too much more good to say about it. Good heat resistance too, for a sealing material. But the immunity to almost every kind of chemical is quite a good reason to use it.

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Concur. It's benefit is its almost perfectly inert, impervious to anything but some seriously nasty shit like liquid rocket fuels.

I turned some semi-regularly in the shop in college. It cut readily with sharp tools (duh) but I couldn't tell you anything else about it.

It does feel funny to handle though.

Go figure. My first machining job was at Greene Tweed making all types of seals out of this lovely stuff. I didn't know any better and had a great supervisor that gladly showed me many tips and tricks. The pure Teflon we machined with hss. We used to sharpen the turning tools on a surface grinder, then we would burnish around the nose radius with something like drill rod, rolling the burr upward. Very sharp and great finishes. We had the temperature sensitivity down and knew basically where to run on the floor so the parts would pass final qc at a specific temp. The glass filled stuff we ran pcd tipped tools and it would still eat them up pretty good.

Another charming aspect of PTFE/Teflon is the noxious fumes it puts out if you do manage to light it on fire:

Lung Effects. Teflon (PTFE: polytetrafluoroethylene). CAS No. 9002-84-0. Fluoride Action Network Pesticide Project.

"Overheating of PTFE generates fumes of highly toxic PFIB and poses a serious health hazard to the human respiratory tract. PFIB is approximately ten times as toxic as phosgene [2]. Inhalation of this gas can cause pulmonary edema, which can lead to death. PFIB is included in Schedule 2 of the Chemical Weapons Convention (CWC), as a result of the prompting by one delegation to the Conference on Disarmament"

Of course, when you look at polycarbonate, Delrin, and a bunch of other polymers you quickly come to the conclusion that burning plastics is a Really Bad Idea.

i wonder how it works in a frying pan.

dian said:

i wonder how it works in a frying pan.

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With an ignition temp north of 500 degrees C, if your pan lights on fire you won't be eating that meal anyhow.

I'm an avowed user of SS cookware. It can be more difficult to clean but if push comes to shove I can use scotchbrite on a die grinder with no ill effects.

Cole2534 said:

With an ignition temp north of 500 degrees C, if your pan lights on fire you won't be eating that meal anyhow.

I'm an avowed user of SS cookware. It can be more difficult to clean but if push comes to shove I can use scotchbrite on a die grinder with no ill effects.

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Stainless works OK if you are cooking something really saucy with acidic ingredients. Otherwise, come to the dark side, we have surface-embedded oils!

A bit more on-topic for PTFE, one thing that's fairly unique about it is that it has almost no elastic memory - you can push it around by hand and it'll resist you a bit but then just kinda stay there. Probably not that useful for you to know, but maybe if some features are out of size you can just push them around a bit and have them go in/out of spec and stay there.

Fun part about being in the sealing business is that 9 times out of 10 there is nothing wrong with the seals and you need to convince your customer that the problem lies elsewhere without pissing them off.

BoxcarPete said:

Stainless works OK if you are cooking something really saucy with acidic ingredients. Otherwise, come to the dark side, we have surface-embedded oils!

A bit more on-topic for PTFE, one thing that's fairly unique about it is that it has almost no elastic memory - you can push it around by hand and it'll resist you a bit but then just kinda stay there. Probably not that useful for you to know, but maybe if some features are out of size you can just push them around a bit and have them go in/out of spec and stay there.

Fun part about being in the sealing business is that 9 times out of 10 there is nothing wrong with the seals and you need to convince your customer that the problem lies elsewhere without pissing them off.

Click to expand...

Ha, been there! Stuff like- 'this gasket won't seal for shit' well, the pipefitter was stoned, the welder is drunk and the flange is cocked about 5deg, think that may have some affect?