Any advice for milling polypropylene?

Discussion:

(too old to reply)

Dragonhunter

2003-12-08 00:59:48 UTC

Heya,

I'm milling a circle in polypropylene about 1" diameter with a prototrak
using a 1/8" endmill, dont know the speed... maybe 5k. The finish is
kinda crappy- there are little indents around the circle. Feed rate is
10 in/min I think. I'm using coolant. Any advice how to get a nice cut
in the stuff?

Thanks,
DH

Joe Osborn

2003-12-08 01:08:43 UTC

Permalink

Post by Dragonhunter
Heya,
I'm milling a circle in polypropylene about 1" diameter with a prototrak
using a 1/8" endmill, dont know the speed... maybe 5k. The finish is
kinda crappy- there are little indents around the circle. Feed rate is
10 in/min I think. I'm using coolant. Any advice how to get a nice cut
in the stuff?
Thanks,
DH

Little indents are probably from recutting chips. First off, use a sharp (ie,
brand new) 2 flute .25 or .375 end mill. A .125 is pretty small for a 1"
circle. Use a heavy chip load (say .005 inches per tooth for a .25 EM) and a
relatively slow speed (say 3000 rpm). Peck into the material when plunging to
prevent the polypro from wrapping around the mill. You can use air instead of
coolant if you want, but coolant (water based) is OK too. Make sure it is aimed
to flush the chips out. Do a .010 finish pass. Should work fine.

Hope this helps,

Joe O.
Joe Osborn

OMW Metalcrafts
"Custom Precision Machining
in all Metals and Plastics"
23 Pamaron Way, Ste. E.
Novato, CA 94949
WWW.OMWMETAL.COM
EMail: ***@AOL.COM

Dragonhunter

2003-12-08 02:51:42 UTC

Permalink

Thanks for the great info!! One question, what does this mean:

"Use a heavy chip load (say .005 inches per tooth for a .25 EM)"

???

Thanks,
DH

Post by Joe Osborn

Little indents are probably from recutting chips. First off, use a sharp (ie,
brand new) 2 flute .25 or .375 end mill. A .125 is pretty small for a 1"
circle. Use a heavy chip load (say .005 inches per tooth for a .25 EM) and a
relatively slow speed (say 3000 rpm). Peck into the material when plunging to
prevent the polypro from wrapping around the mill. You can use air instead of
coolant if you want, but coolant (water based) is OK too. Make sure it is aimed
to flush the chips out. Do a .010 finish pass. Should work fine.
Hope this helps,
Joe O.
Joe Osborn
OMW Metalcrafts
"Custom Precision Machining
in all Metals and Plastics"
23 Pamaron Way, Ste. E.
Novato, CA 94949
WWW.OMWMETAL.COM

Joe Osborn

2003-12-08 05:00:32 UTC

Permalink

Post by Dragonhunter
"Use a heavy chip load (say .005 inches per tooth for a .25 EM)"

This means as the cutter turns, it is taking a .005" cut per tooth per
revolution. This means if you are running at 3000 rpm, the cutter should be
moving at 30 ipm. If you drop the rpm to 1500, it means you will now need to
move at 15 ipm to maintain the .005 chipload. Chipload is a good way to think
about how you are cutting things. A relatively soft, easily burred and
easy-to-melt plastic like polypro likes a big chip, sharply cut, so that the
minimal heat generated is carried away in the chip and you don't have the
cutter spending a lot of time in one place heating up the material. A big chip
means you want a 2 flute end mill (and not too small a one) so the endmill
doesn't clog up (ie, it has lots of space between the flutes). Chip load, rpm,
feed rate and surface speed are all interrelated. Take a look at Machinery
Handbook, they have a good section on this.

Good luck!

Joe O.

Joe Osborn

OMW Metalcrafts
"Custom Precision Machining
in all Metals and Plastics"
23 Pamaron Way, Ste. E.
Novato, CA 94949
WWW.OMWMETAL.COM
EMail: ***@AOL.COM

Gary H. Lucas

2003-12-08 15:26:32 UTC

Permalink

Post by Dragonhunter
"Use a heavy chip load (say .005 inches per tooth for a .25 EM)"
???
Thanks,
DH

Post by Joe Osborn

Little indents are probably from recutting chips. First off, use a sharp (ie,
brand new) 2 flute .25 or .375 end mill. A .125 is pretty small for a 1"
circle. Use a heavy chip load (say .005 inches per tooth for a .25 EM) and a
relatively slow speed (say 3000 rpm). Peck into the material when plunging to
prevent the polypro from wrapping around the mill. You can use air instead of
coolant if you want, but coolant (water based) is OK too. Make sure it is aimed
to flush the chips out. Do a .010 finish pass. Should work fine.
Hope this helps,
Joe O.
Joe Osborn
OMW Metalcrafts
"Custom Precision Machining
in all Metals and Plastics"
23 Pamaron Way, Ste. E.
Novato, CA 94949
WWW.OMWMETAL.COM

Since it is a semi-auto machine anyway, I'd use a shop vac while cutting to
yank the chips right out. The blast of air rushing into the vac nozzle also
provides quite a bit of cooling too.

Gary H. Lucas

Proctologically Violated©®

2003-12-08 19:25:43 UTC

Permalink

We make bracelets for somebody--not sure what the material is, just
know it's clear. We have used roughing ems, followed by a finish cutter.
I'm not sure what the limiting factor is with plastics: chip load
or melting onto the em, as with aluminum. Or if chip load for plastics is
the same as chip load for metals. But basically we push it to the melting
point, and back off. For cooling as well as chip removal, as much pressure
as you can get w/ coolant--w/ typical centrifugal/low pressure pump deals,
choose your nozzle diameter "optimally" for best flushing effect.
Gary's notion of a vacuum is intriguing and neat, but I think the
air flow generated by vacuum is just not strong enough, due to poor
directionality inherent in suction. But coolant AND a vacuum removal system
would be pretty neat.
With some plastics I think you can recycle--ie, SELL--the chips.
We woulda been able to sell ours, but for some reason they didn't like
shavings and coolant contamination. Bags and bags and bags of the stuff.
Shop looked like a winter Xmas scene.
----------------------------
Mr. P.V.'d
formerly Droll Troll

Gary H. Lucas

2003-12-08 20:37:10 UTC

Permalink

Post by Proctologically ViolatedÂ©Â®
We make bracelets for somebody--not sure what the material is, just
know it's clear. We have used roughing ems, followed by a finish cutter.
I'm not sure what the limiting factor is with plastics: chip load
or melting onto the em, as with aluminum. Or if chip load for plastics is
the same as chip load for metals. But basically we push it to the melting
point, and back off. For cooling as well as chip removal, as much pressure
as you can get w/ coolant--w/ typical centrifugal/low pressure pump deals,
choose your nozzle diameter "optimally" for best flushing effect.
Gary's notion of a vacuum is intriguing and neat, but I think the
air flow generated by vacuum is just not strong enough, due to poor
directionality inherent in suction. But coolant AND a vacuum removal system
would be pretty neat.
With some plastics I think you can recycle--ie, SELL--the chips.
We woulda been able to sell ours, but for some reason they didn't like
shavings and coolant contamination. Bags and bags and bags of the stuff.
Shop looked like a winter Xmas scene.
----------------------------
Mr. P.V.'d
formerly Droll Troll

The vacuum I used was an old Milwaukee that cost about $700. If you stuck
the hose down to the table then slide it over a 1/2-13 tapped hole 1-1/2"
deep the jet of air created by the vacuum going into the hole shoots all the
steel and aluminum chips right into the vacuum nozzle. The shrieking sound
hurts your ears too. So plastic chips are really easy. With polypropylene
on a lathe we could grab the end of the chip and continuously pull it into
the tank the whole time we were cutting. Man, what a shock you get from the
static when you reach into the tank to pull out the ball of chips!

Gary H. Lucas

PrecisionMachinisT

2003-12-08 20:39:31 UTC

Permalink

Post by Gary H. Lucas
The vacuum I used was an old Milwaukee that cost about $700. If you stuck
the hose down to the table then slide it over a 1/2-13 tapped hole 1-1/2"
deep the jet of air created by the vacuum going into the hole shoots all the
steel and aluminum chips right into the vacuum nozzle. The shrieking sound
hurts your ears too. So plastic chips are really easy. With

polypropylene

Post by Gary H. Lucas
on a lathe we could grab the end of the chip and continuously pull it into
the tank the whole time we were cutting. Man, what a shock you get from the
static when you reach into the tank to pull out the ball of chips!

I hear spray fabric softener spray helps eliminate static.

Also sometimes used when mixing small batches of explosive chemical
formulations.

--
SVL

Cliff Huprich

2003-12-08 23:34:40 UTC

Permalink

Post by PrecisionMachinisT
I hear spray fabric softener spray helps eliminate static.

You can get anti-static spray in good computer supply shops IIRC.
Used to use it on carpets in computer areas.

--
Cliff