Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

On Sep 16, 4:37*pm, Scott wrote:
Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

Would help to know what the fluid is and it's highest operating
temperature! Lots of fluids soften many types of plastics and
temperature certainly does.

Paul

Scott,
I have some plastic rod, it is supposed to be UHMW.
UHMW is supposed to be good for bearings.

I have good luck using:
Sfpm, can be just about anything, up to 500.
Tools, SHARP with high back and side rake, in excess of 15 degrees,
diamond honed sharp, moderate radius.
Feed, high feed rates.
Coolant, none.
Drilling, high feed rate.
Accuracy, should be able to hold a few mills, but some have reported
changes after machining.
Chips, lathe chips will be stringy and stretchy, tangling in the
chuck, and grabbing. Thin strings will wrap around the part,
sometimes requiring removal.
Heat, not a problem, as long as high feed rates are used.

Dave J.



On Sep 16, 4:37*pm, Scott wrote:
Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

Chips, lathe chips will be stringy and stretchy, tangling in the
chuck, and grabbing. Thin strings will wrap around the part,
sometimes requiring removal.
Heat, not a problem, as long as high feed rates are used.


The trick here is to use a shop vac. Aim it right at the cutting tool.

Karl

"Mechanical Magic" wrote in message
...
Scott,
I have some plastic rod, it is supposed to be UHMW.
UHMW is supposed to be good for bearings.

I have good luck using:
Sfpm, can be just about anything, up to 500.
Tools, SHARP with high back and side rake, in excess of 15 degrees,
diamond honed sharp, moderate radius.
Feed, high feed rates.
Coolant, none.
Drilling, high feed rate.
Accuracy, should be able to hold a few mills, but some have reported
changes after machining.
Chips, lathe chips will be stringy and stretchy, tangling in the
chuck, and grabbing. Thin strings will wrap around the part,
sometimes requiring removal.
Heat, not a problem, as long as high feed rates are used.

Dave J.

Use high speed steel, Dave didn't say it, only implyed.

On Tue, 16 Sep 2008 16:37:05 -0700 (PDT), Scott
wrote:

Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

What Dave said, but you might want to try acetal (Delrin). Holds
dimension, and is more rigid than UHMW and nylon.

Bob

Bob wrote:
On Tue, 16 Sep 2008 16:37:05 -0700 (PDT), Scott
wrote:


Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott


What Dave said, but you might want to try acetal (Delrin). Holds
dimension, and is more rigid than UHMW and nylon.

Bob

Also IIRC acetal has very low water absorption whereas for nylon it can
be significant and causing dimensional swelling. I have been told of one
case where this caused seizure of a bearing when the nylon bearing
equipped machine was left in a damp atmosphere. The OP mentioned water
immersion so it needs to be considered.

David Billington fired this volley
in :

Also IIRC acetal has very low water absorption whereas for nylon it
can
be significant and causing dimensional swelling. I have been told of
one
case where this caused seizure of a bearing when the nylon bearing
equipped machine was left in a damp atmosphere. The OP mentioned
water
immersion so it needs to be considered.


About 75% of all the machining I do is plastics, just by the nature of
the tools we need.

I'd recommend not using UHMW polyethylene for bearings/bushings unless
they are only lightly loaded. Polyethylene compresses easily, and
"crawls" or "creeps" under even moderate loads.

As said here, nylon has its own problems, but is probably the most
stable and strongest plastic you can use, presuming you allow for
moisture expansion.

For moderate loads, I'd recommend Acetal, perhaps filled with PTFE or
Molybdenum disulphide. Even unfilled acetal is a good choice except
for high loads or high speeds. And you'll really like the way acetal
machines -- it has none of the bad habits of "fuzzing" like nylon and
polyethylene do. Clean, attractive finishes can be had in a single
pass with a sharp tool. Fine feed rates don't punish you, either. So
taking down that last half-thou is easy.

The belt tensioner on my old F.E. Reed lathe has an acetal bearing cut
with enough clearance to slide easily across the face of the jackshaft
arm. It's got probably 1000 hours of actual run time on it, and I can
discern no significant wear to it or the shaft on which it runs (dry).

LLoyd

On Tue, 16 Sep 2008 16:37:05 -0700 (PDT), the renowned Scott
wrote:

Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

Suggest you avoid nylon- it's quite hygroscopic and changes dimension
significantly with moisture changes.

Second the suggestion for Acetal (Delrin). 3" diameter rod is $36/ft
at mcmaster, which should be at the high end of the price range (try a
plastics distributor).



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Scott,
All plastics are somewhat hygroscopic, some are worse than others. For that
reason alone, never use a coolant, as the dimension you cut to will not
remain in time. Delrin is better than nylon, but there are plastics that
will swell only slightly, check with the manufacturer for swelling specs
when submerged. Marine bearings are typically clearanced .002 to .003" per
inch of diameter to compensate for absorbtion.
Steve

"Scott" wrote in message
...
Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me). UHMW is much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? Does UHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead of UHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

On Sep 16, 7:37*pm, Scott wrote:
Forgive me for asking a plastic question in a metal newsgroup, but it
seemed to be the most applicable place.

I have to machine some round bushing-like parts. The inner diameter of
the bushing is going to hold a rubber packing seal (similar to a
chevron or V seal). Fluid inside the mechanism will be low pressure
water. One option I have is to machine these seals out of a metal like
aluminum or brass. Another option is to machine them out of plastic.
I'm thinking that for prototyping plastic is the way to go (presumably
faster to work with than aluminum or brass). However, I've never
worked with plastic before.

So I'm looking to buy some plastic rod stock, in 3" diameter.

Checking a few online merchants like OnlineMetals.Com, I see that
Nylon is pretty pricey, about 25% more expensive than the same
dimension of aluminum (this surprises me).UHMWis much cheaper, about
70% less than the same dimension of aluminum.

Can someone give me some info on machining these plastics? DoesUHMW
machine fairly easily? Is there any reason why I should buy the
significantly more expensive Nylon instead ofUHMW? What about
durability? I'd like the prototype to hold up to some hours of use.

Thanks,
Scott

Hi Scott,

I sell various plastics and have put a "Help center" on Google groups
for UHMW;

http://groups.google.com/group/uhmwtips

UHMW is a pretty easy plastic to machine, but you won't get a high
level of accuracy from machining because it moves. It's also only
suitable for low-speed bushings - it melts at even moderate speeds.

Delrin is usually preferred for its ability to be machined to exact
specifications and it's a lot slipprier than UHMW.

Nylon is also a good alternative, but it's very brittle - you might
look into an oil-filled Nylon like Nylon GS or GSM.

Eric