I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.

"Andrew VK3BFA" wrote in message
oups.com...
I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.

I would think a hole saw or a fly cutter would do a much better job. A
lathe would be last on my mind and only after a few single malts.

"Andrew VK3BFA" wrote in message
oups.com...
I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

When I needed some 1/4" thick x 3" OD acrylic disks recently, I bought them
from McMaster-Carr. It looks like they may have been cut with a hole saw,
though without using the center drill as the disk is solid throughout.

For a one-off larger disk that has a center hole, I used a shop-made stepped
arbor, with aluminum disks on either side of a square acrylic plate and
turned it down on the lathe.

Mike


--
NewsGuy.Com 30Gb $9.95 Carry Forward and On Demand Bandwidth

I think I'd band saw it to rough size first, then gently go at it on the
lathe to finish. You can put the part on a piece of wood while band
sawing so it doesn't scratch.

Assuming that Perspex is a clear plastic, like plexiglas, I'd use a
sharp tool and a very slow speed. The stuff heats up and becomes sticky.

Pete Stanaitis
--------------------

Andrew VK3BFA wrote:
I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.

Andrew VK3BFA wrote:

I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.


Since it's got a hole in the center, how about chucking a near square
piece of wood in a 4 jaw in your lathe, drilling a small pilot hole in
its center, putting a couple of pieces of double sided tape on the
perspex (I believe we call it Plexiglass here.) and using a wood screw
and washer to secure it through the pilot hole. (Maybe a cardboard
washer too, to protect the area under the washer.) The double sided tape
will give it plenty of drive without slipping.

Then, slow speed and you can run the tool bit into the wood a bit to get
all the way across the width of the Perspex.

HTH,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

Andrew VK3BFA wrote:
I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

I did this to make some inspection hole covers for my neighbour's
homebuilt aircraft. BTW, Perspex = Lucite = Acrylic
Cut out approximately square piece of Perspex a little larger than the
required disk from sheet. Drill 4 holes near the corners.
Secure this to a square piece of wood (e.g. 3/4" ply) with four small
wood screws.
Mount in four jaw chuck.
Press firmly on the center with a live center. A bit of old inner tube
will prevent scratching.
Use a pointy, sharp bit to cut your circle at SLOW speed.

A few strokes with a file or abrasive paper will clean up the sharp edges.

Ted

On 16 Apr 2006 06:41:10 -0700, "Andrew VK3BFA"
wrote:

I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.

I rough-cut them with bandsaw or sabersaw, so there's only 1/8" or so
of rough periphery to turn smooth to size on the lathe.

I take a scrap of 3/4" or 1" bar (CRS, brass, aluminum, delryn,
whatever's handy), chuck it, in the lathe, face it, drill it, tap
it. Cut (bandsaw or sabersaw) and center drill a slightly undersized
plywood backing disc to keep the disc from flexing. Fix the plastic
and wooden discs to the mandrel (still in the chuck) with a bolt
and large washer or another plywood disc. Might want a rubber
washer under the large washer to prevent scratching. Turning is
then easy peasy with a sharp but radiussed tool at low speed. You
don't want much rake on the tool, take light cuts. Tool should be
at center height or even just a skeense below center, definitely not
above center. You don't want it to grab, or to lift the disc away
from the backing disc.

"Andrew VK3BFA" wrote in
oups.com:

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

As others have said, I would sandwich the acrylic (perspex) between some
wood or aluminum or what have you. That will help with break out at the end
of the cut.

Acrylic has a very high thermal coefficient of expansion. It's better than
six times that of steel and three time that of aluminum. It's also a poor
conductor of heat. To get the best results you need a sharp tool, high
speed and low feed rates. For a HSS tool you should run around 400 SFM and
..001-.005" IPR feed rate.

The tool should have zero top rake and zero back rake. Positive rake tools
will only increase the chipping problem and will try to make the work and
tool suck into each other. However, the tool needs massive amounts of end
and side relief to to the thermal expansion. Around 30 degrees end relief
and around 20 degrees side relief. Hone the cutting edge so that it's very
sharp. Lighter depth of cuts are better than deep cuts. I would take no
more than .04" per pass.

D Murphy wrote:

"Andrew VK3BFA" wrote in
oups.com:


Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?


As others have said, I would sandwich the acrylic (perspex) between some
wood or aluminum or what have you. That will help with break out at the end
of the cut.

Acrylic has a very high thermal coefficient of expansion. It's better than
six times that of steel and three time that of aluminum. It's also a poor
conductor of heat. To get the best results you need a sharp tool, high
speed and low feed rates. For a HSS tool you should run around 400 SFM and
.001-.005" IPR feed rate.

The tool should have zero top rake and zero back rake. Positive rake tools
will only increase the chipping problem and will try to make the work and
tool suck into each other. However, the tool needs massive amounts of end
and side relief to to the thermal expansion. Around 30 degrees end relief
and around 20 degrees side relief. Hone the cutting edge so that it's very
sharp. Lighter depth of cuts are better than deep cuts. I would take no
more than .04" per pass.

I've heard of guys rigging up shop air (a gentle stream) to help with cooling
when turning plastics.

GWE

Grant Erwin wrote in
:

I've heard of guys rigging up shop air (a gentle stream) to help with
cooling when turning plastics.

For production jobs I've used an Exair cold gun (vortex tube).

http://www.exair.com/vortextube/vt_page.htm

If you can rig up a shop vac to collect chips as they come off the tool it
will help prevent the work or chuck from grabbing them and wrapping them up
into a big fur ball.

In article ,
D Murphy wrote:

If you can rig up a shop vac to collect chips as they come off the tool it
will help prevent the work or chuck from grabbing them and wrapping them up
into a big fur ball.

The best position (IME) for chip removal is to strap/duct tape the
vacuum nozzle right on top of the toolholder, or as close to that as can
be managed, so chips come off and go straight into the vacuum.

--
Cats, coffee, chocolate...vices to live by

On 16 Apr 2006 06:41:10 -0700, "Andrew VK3BFA"
wrote:

I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?

Words of competence?

Andrew VK3BFA.

I had to make a hole in my replacement aircraft windscreen for the
fuel filler which was an inch off the side. it was easy in the end.

I would drill the centre hole undersize you you can clean it up after
the process.
use a 1/2" by 1" length of wood just a little longer than the radius.
drill a hole in one end and reverse the drill bit to use the drill
stub as a pivot pin.
cut a slot at the other end to take your favourite perspex lathe bit.
I used a piece of 1/8th square tool steel sharpened at one end.
put a bolt through the end to clamp up the slot and hold the tool.

put the drill stub into the hole into the perspex and pull the tool
around by hand. adjust the angle of cut so that it cuts well. some
negative rake on the tool bit works well. cut in half way.
reverse the perspex and cut in the other half. when you have cut
through and pushed out the centre clean up the last little dags with a
fine file. then drill the centre hole to final size.

when I did my fuel hole in the windscreen it was with some
trepidation. turned out to be really easy and took 5 minutes.
half the trick is to do it by hand so that there is no heat buildup in
the cutting area.

Stealth Pilot

Flycutter would be the way to do this if you don't have a router.
Drill your hole in the blank, then replace the drill bit in the
flycutter with a piece of steel rod so the hole doesn't get wallowed
out. I'd sandwich the blank between two scraps of wood for holding it.
Do it in a hefty drill press or a mill. One sort of off-the-shelf
flycutter looks like this:
http://www.generaltools.com/product....52&sectionid=2

More like a trepanning tool. Run very slowly. They've got these
fairly cheap at the local hardware store, I don't know what you'd have
available where you are. I've got a couple of these in different
sizes, they work well if you've got a heavy enough machine.

A small router in a router table would be the way to go, since you've
got the hole. Use that as a pivot and just rotate it against the
cutter until done.

I've also done small plastic disks by roughly cutting them into a
circular shape, sandwiching them between two chunks of door skin on a
bolt, then putting that in an electric drill and using a wood rasp to
smooth things up and reduce it to final diameter. Polish with sand
paper. Could do the same on a drill press, just make sure the bolt
doesn't walk and the chuck doesn't drop.

Another method would be to do as above, stick a sanding drum in the
drill press, clamp something for a pivot pin in place on the table and
then spin the blank on the pivot against the sanding drum.

I wouldn't try to lathe turn it at all.

Just number of different options.

Stan

Andrew VK3BFA wrote:

I want to turn up a round perspex piece to fit in a circular hole in
the front panel of a radio I am building. I start with a piece of
perspex 4 inches square, and turn in down to the required fit.

Had one go at it today - interesting. Generates enormous amounts of
swarf - ok, can live with that. Started by using a pointed tool to cut
it to roughly the right diameter - when it broke right through the
piece, it went "whang" and threw a big chunk across the room.
Mmm....not good.

Whats the proper way of doing this without generating missiles of
plastic (or steel, come to think of it.) - what sort of cutting tool
should I be using, and what speed should the lathe be running at, ie
fast or slow (yes, very technical..) Any lubrication required - it was
cool to the touch.

Its got a quarter inch hole in the middle to allow the vernier drive
shaft to pass through to the tuning knob - I cut the head off a bolt
and used this to chuck it to the lathe. Its got a bit of wobble,
probably need to get a straighter bolt - was grabbed out of the junk
box.. Is there a better way of mounting the project?


I've turned some "watch crystals" for dial indicators. I never did one 4"
diameter, but I'm sure I could. I faced a mandrel of steel scrap very
smooth
and super glued the acrylic to it. It could take some pretty heavy cutting
without the super glue debonding. (I cleaned both parts with alcohol before
sticking them together.) With acrylic, you want to take light cuts and
keep the
cutter moving back and forth across the work as you come down to final
diameter. Cutting to approximate shape with other tools first, rather
than trying
to turn the square blank is a good idea. Debonding it is pretty easy,
you can run
hot water over it for a few minutes and then just pull it off the
mandrel. Some
superglue will be left on the part, you can remove it with a brand-new
single
edge razor blade, and then polish if needed. I think acetone will harm the
acrylic, so don't use that to clean up.

Speeds can be fairly fast, like 1000 FPM (roughly 1000 RPM) as long as
you take light cuts. The one foot circumference of the part will
prevent heat
from building up much in the workpiece.

If you want to make an arbor for turning parts with holes in the center,
just take
a piece of scrap and turn to say 1/4" dia, and thread with a die. Leave
a shoulder
on the arbor, and a nut will clamp the piece against the shoulder, so it
should
turn true.

Jon

Jon
I have a nice DI sitting on my desk with no crystal. Sure, I can
buy one from LongIslandIndicators, but I've been trying to figure out
how to make one. The shaft protrudes beyond the height of the trim
ring, so the original bezel was convex. I'm not picky about it being an
exact duplicate. I'm curious if you had that issue and how did you
handle it?
- -
Rex Burkheimer

Jon Elson wrote:
I've turned some "watch crystals" for dial indicators. I never did one 4"
diameter, but I'm sure I could. I faced a mandrel of steel scrap very
smooth
and super glued the acrylic to it. It could take some pretty heavy cutting
without the super glue debonding. (I cleaned both parts with alcohol
before
sticking them together.) With acrylic, you want to take light cuts and
keep the
cutter moving back and forth across the work as you come down to final
diameter. Cutting to approximate shape with other tools first, rather
than trying
to turn the square blank is a good idea. Debonding it is pretty easy,
you can run
hot water over it for a few minutes and then just pull it off the
mandrel. Some
superglue will be left on the part, you can remove it with a brand-new
single
edge razor blade, and then polish if needed. I think acetone will harm the
acrylic, so don't use that to clean up.

Speeds can be fairly fast, like 1000 FPM (roughly 1000 RPM) as long as
you take light cuts. The one foot circumference of the part will
prevent heat
from building up much in the workpiece.

If you want to make an arbor for turning parts with holes in the center,
just take
a piece of scrap and turn to say 1/4" dia, and thread with a die. Leave
a shoulder
on the arbor, and a nut will clamp the piece against the shoulder, so it
should
turn true.

Jon