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Tony
09-29-2003, 05:18 PM
the need has arisen to turn two nylong "discs" ... about 8" diameter x 1/4" thick.

once i face the first side, how do i turn it around and hold it to face the otherside without it buckling?

i recall a few times some time ago.. about crazy glue or vacuumcleaners or something.. but havent been able to find it via Search.

thanks,
-knucklehead

Evan
09-29-2003, 05:39 PM
You could make a vacuum faceplate. With that size of disk it should hold well. Need a rotating coupling on the end of the spindle for the shop vac. The faceplate could be a temporary one like I used to make the rings on my telescope. In this case I would use a stub of 4"x4" wood, bore a center hole the same size as the through hole in the spindle and use some plastic pipe that fits inside both. Chuck this in the four jaw. Cut a 9" disk of 3/4" high density particle board and screw it to the 4"x4". Bore center hole for vacuum into it and true the face. Cut another 9" ring of particle board with 7" inside hole. Screw this to the faceplate. True it up, use the shop vac to collect the sawdust as you work. Cut another disk 9" particle board and inside a 7" circle drill a bunch of 1/4" holes. Screw this to the ring and true it up. Sand the surface well and smooth. Coat the suface with candle wax, melt a little and brush it on, wipe it off. If your vacuum can pull 4" of vacuum the holding force will be about 200 lbs. Should work.

Joel
09-29-2003, 05:57 PM
Do both of the faces have to be perfect? I would try chucking it up by a relatively small diameter, but not too short stub that has been center drilled. Face the one side, flip the disc and press it against a large diameter flat-faced arbor held in the chuck using your live center. Cut the stub away until the part breaks free. Easier still would be to cut the stub off and sand smooth if tolerances permit. Double sided tape might be enough to hold it against the arbor for a light finish facing cut.
That nylong can be a bear to machine. http://bbs.homeshopmachinist.net//smile.gif

Joel
09-29-2003, 06:09 PM
If you are cutting the disks out of sheet stock, rough cut the O.D. oversized, then retain the part with screws to a wood disc beyond the 8". Chuck up the disc, face, then cut the diameter with a homemade trepanning cutter. Of course, if you were using 1/4" thick stock, you could simply cut disks out on your drill press with the center drill removed.

Tony
09-29-2003, 06:11 PM
evan: i really was only joking about the vacuumcleaner.. (!) .. machinists really scare me sometimes.

joel: if you think NYLONG is difficult to work with, you should try touch-typing with 3 bandaids on the tip of an index finger

what you *shouldn't* try is cutting towards yourself, deburring nylong parts, with a sharp razor.

would double-sided tape really do the trick?

double and triple-checking my spelling,
-knucklheb

Tony
09-29-2003, 06:16 PM
i have one piece of (expensive) 8" diam x 20" round stock. planned on bandsawing the discs out. woodscrew and a backer sound like a good idea.

thanks,
-tony

Forrest Addy
09-29-2003, 06:17 PM
Machine the bored undersized. Build spud. Turn the faces with a double tool set-up taht faces both sides at once.

Alternatively, make some wedge shaped aluminum soft jaws that give the part almost full back-up. Grab the part by the OD and face each side. If you lightly tap in the part against the jaws on the second side you're sure to get good parallelism.

Evan
09-29-2003, 06:26 PM
Facing the nylon won't be a problem once you have it held. It turns very nicely. Drilling is a different matter. I use lots of plastics in my projects. I'm glad I don't have to cut a 8" nylon bar stock on a bandsaw. That is going to be tough. It will want to overheat badly. The blade will wander as well. Use lots of beeswax on the blade and take it in small stages. Rotate the work as you cut and leave the thickness at least 1/8" oversize.



[This message has been edited by Evan (edited 09-29-2003).]

Joel
09-29-2003, 06:27 PM
BTW, use particleboard. You could press the part against the oversized board with the tailstock center against a center-drilled arbor, and face to the arbor. Then clamp the perimeter with scrap and screws, and face the center section. The good double-sided tape sticks like hell, never used it on nylon though (clean it good with alcohol). A quick test will tell you all you need to know. Use a slower speed and light cuts if you try. Probably lots of other ways to do this.

[This message has been edited by Joel (edited 09-29-2003).]

gunsmith
09-29-2003, 06:42 PM
You turn bothe sides against a disc of similar size to your nylon plates. I use a commercialy available double sided tape. This tape has no problem holding even under heavy cuts in both plastic and aluminum. Biggest problem I have is pulling it free of the tape after the turn. I usually make the disc I am turning against a bit larger and this will compensate for miss alignment when you make the switch to the opposite side.

darryl
09-30-2003, 03:49 AM
I really like the vacuum idea, however, have never used it. I doubt that a vacuum cleaner would 'suck' enough to give sufficient holding power, even a cheap vacuum pump should do better. I find that full contact of the disc to the faceplate is usually needed to be able to face a part accurately. That's not easy to get by pressing on the disc with a live center, or double-sided tape mounting. Depends on the stiffness of the material. If I had any quantity of discs to be made, without center holes, I'd be rigging up a vacuum holding faceplate. I do have a vacuum pump and tank, so maybe it's feasable for me, where it may not be for you.

Evan
09-30-2003, 02:07 PM
Darryl,

Any old shop vac can suck at least 4" of mercury (about 45 inches water). Some of the really good ones approach 100 inches of water. That would provide over 400 lbs of holding force. Should be enough. Should probably put in some internal re-enforcement pieces to avoid collapsing the faceplate though. Also, the shop vac has the volume capacity to deal with some leakage.

Come to think of it the design could be much simplified. Just use a single piece of HD particle board and router some radial groves out from center.

Heck, I think I'm gonna make one. I'll let you know how it works.

Slight math hiccup here, not 400lbs, it's 200lbs for an 8" disc. Previous post is wrong too, should be 100lbs, still probably good enough.

Incidentally, if you could actually suck close to a real vacuum then it gets silly. 8" disk would be held by over 700 lbs of force.

[This message has been edited by Evan (edited 09-30-2003).]

debequem
09-30-2003, 04:50 PM
I am a newbe at this, but if I had to do it I would face one side in a lathe and then cut it off a little thicker.

Place faced off side down on a mill table and clamp it enough to hold the part. Face rough side with mill and move clamps to finish it off where the clamps were first placed.

You could make a vacuum plate to hold it on the mill table, too, but I'd try clamping it first because it is faster if it is only one peice. Make a fixture if you need to do several.

Alternatively, if you have sheet stock (and I think you said you do not) the right thickness, I would rough cut it into a disk and outfit the tailstock with a live center and make a 7" diameter plate to fit into the nose of the live center. Make another 7" plate that fits in the lathe chuck and clamp the work between the two plates using the tailstock.

Once clamped you can turn the flat stock to the exact diameter.

Marv

mpbush
09-30-2003, 11:06 PM
I second what Gunsmith said, us woodworkers couldn't survive without our miraculous double sided carpet tape. It has adhesive on both sides, you lay one side against the faceplate, rub it down, peel away the paper backing, stick your plastic to it and itsn't going anywhere. It is so strong that on thin items I have to make sure not to use too much tape or I will break the item before it comes free.

The amount of adhesion this stuff has is amazing. If you turn one side and then flip it over and attach it to a stiffener board, the tape will keep it from rotating.

Michael

[This message has been edited by mpbush (edited 09-30-2003).]

darryl
10-01-2003, 02:33 AM
Evan, if you make the vacuum faceplate, I'd be interested in how it performs. I can see how a vacuum could easily be drawn through the spinlde hole, with a rotating coupler with seals. Wouldn't need a large passageway for the air, unless there's too much leakage around the part being held. Should be easy to rig the (wooden?) face plate with a seal to the spindle bore as well. I do wonder how the vacuum cleaner would like it, as it's relying on air flow for cooling, and in this app, there shouldn't be much airflow, or the holding power wouldn't be there.

Evan
10-01-2003, 02:42 AM
I'm willing to take a chance on the vacuum. It's been clogged with swarf enough times and hasn't smoked yet. Trickiest part (not very tricky) is the rotating coupler. A little leakage here and there shouldn't matter at all.

One more cool thing is all that force holding the part is not exerted on any part of the lathe or bearings.



[This message has been edited by Evan (edited 10-01-2003).]

Cass
10-01-2003, 03:01 AM
We use vacuum chucks all the time. Be sure to use safety clamps on any vacuum chuck in case power fails to the vacuum pump during the cut. A vacuum cleaner will probably not work very well and neither will a high vacuum pump. You need both high volume and high vacuum in my experience because of the need to overcome leaks which may happen at any time. Lower volume and pressure start to work when the area of the part is very large due to the reserve capacity under a large area an the large force even a small amount of vacuum causes. You can make things work a good bit better by putting a storage tank in the vacuum circuit to give some reserve capacity in case of a leak. One bit of fluid mechanics to be aware of which is the reason vacuum chucks don't work as well as you might think is that flow through a slot or orifice increases as the third power of the width of the opening. This means that if an acceptably small leak (for example 0.002")happens to open enough to double the width of the crack to 0.004" the flow of air goes up by a factor of 2 cubed or 8 times; thus a small leak that opens due to machining forces such as chatter can rapidly overcome the vacuum flow rate and allow the leak to double in size again and again and there goes the part. Safety dogs may prevent you from getting killed. We use big high volume vacuum pumps that give us about 28 inches Hg vacuum and there is an audible loss of vacuum alarm on the machine and a vacuum guage and we have a large tank in the system for a reserve of high vacuum. Vacuum fixturing works great if you do it right. Even with really high flow the part and fixture must fit each other very accurately or you will have to use a sealant or an O ring(s). Just holding the part down or using vacuum to pick up parts is easy compared to holding the part down against machining forces.

gunsmith
10-01-2003, 07:57 PM
Knucklehead
I don't want to discount the vacuume cleaner method but I can tell you from experience that most shops that turn plastic use tape. You can get it in variouse strengths as well. Try CANTECH (Canadian Technical Tape) Montreal Canada or CANTECH in Johnson City TN. U.S.A. This is not carpet tape either, although on some occasions it will work also. It may be hard to believe but sometimes if it doesn't have a thousand moving parts it can still work and better. Remember " KISS" (keep it simple stupid), said to me many times.

Alistair Hosie
10-01-2003, 08:32 PM
Use double sided tape its perfectly strong, enough woodturners like myself use it all the time for just such a process, or alternatively use a large cup chuck from MDF make a disk slightly larger than the size you want then cut a recesss so that the disk is a tight fit them fit it with a blow from your fist lightly all round. When you face it off it will be a nice snug fit and will be easily removed to do the other side.That's how woodturners sometimes finish off the bottoms of bowls etc and make a nice finished bottom without any chuck marks or spigot buttons. Alistair

Evan
10-02-2003, 12:49 PM
Cass, Darryl,

I went looking through one of my storage sheds last evening and found something I dimly remembered having. A brand new small Gast oilless rotary vane vacuum pump. It will do 20"hg at 3450 rpm on a 1/4 hp motor, 2 scfm (free). I also happen to have an empty helium tank that can't be refilled which should make a decent small reservoir tank. I do belive I will build a nice little low vacuum system. It might be a while before I try the vacuum chuck but I do intend to. I even have filters, just need some fittings.

darryl
10-02-2003, 04:14 PM
In case anyone's interested, I built my own vacuum pump using the crank and lower end of an outboard motor. All other parts including cylinders, are homemade. My design philosophy centered around having zero space left at the top of the stroke of the pistons. The valve surfaces actually sit flush with the cylinder head, and the stroke can be adjusted so the piston actually contacts the head on the upstroke. Works great, and I use an old 100 gal propane tank as a vacuum resevoir. One drawback is that the room smells of mercapton, the odorant that's added to propane to enable leak detection. Another drawback, and this might affect anyone's pump use, is that extended running heats it up too much, and it takes far longer to 'fill' the tank than it does to deplete it.

gross9
10-02-2003, 04:47 PM
can the material be obtained in flat stock, the right thickness ?
if it can, sandwich it between 2 thicker disks and turn it. or use the double faced tape.

beckley23
10-02-2003, 10:13 PM
Years ago I did a job surfacing 6061-T6 alum 10" X 20" X 1/4" thick.Had to take thickness down to 3/16". Mounted the piece on #2 K&T table covered with dbl back tape, made sure the piece was down and faced with 12" dia. 24 tooth carbide tipped face mill direct mounted to the spindle. Slow feed rate so that the cutter wouldn't throw the work piece. Unfortunately the table travel wasn't enough for the cutter to clear the piece, this was R&D for surfacing method, but the tape held.