How do I turn a disc?

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.

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

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

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Andrew VK3BFA wrote:

To hold and turn a thin piece of stock in a lathe, you can make a sandwich:

1.Scrap aluminum against the face plate or three jaw chuck with jaws removed, 2. plastic piece, 3. scrap aluminum. Use the tailstack with a live center to tighten against the faceplate, friction will hold everything. far easier if you don't have a lot of material to remove.

Karl

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

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

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 news: snipped-for-privacy@g10g2000cwb.googlegroups.com:

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 news: snipped-for-privacy@corp.supernews.com:

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

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.

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:

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

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

J> I've turned some "watch crystals" for dial indicators. I never did one 4"