My research: Cutting Plastic With Rek-O-Kut

[mischmerz]

Fellow trolls:

Over the last two months or so I have been experimenting with cutting plastic - myshank stylus and blanks. The relatively small prices for 10" blanks allowed me many, many tests. Especially to get suction working reliably. Here's a short overview what I did and what I found out:

Background: Rek-O-Kut / Grampian / Long shank diamond / No heat

My suction system worked just great with lacquer - but not at all using plastics. Well, I tried the "American" way: Gimme more suction. So I got myself a real nice pump that pulls about 100 quarts per minute. This thing was too loud for my studio so I ran a garden hose and an extension cord across the attic into the garage (just about 15 yards or so total length). I now had all the suction one can ask for. I could here it sucking air though the 1/4" pipe while wearing headphones. Guess what: Didn't work. Or it did - sometimes .. for a few minutes. So - I upgraded to a 3/8" tube - I had enough suction to play with. The tube ended behind the stylus and that worked better, but still not reliable enough.

I had to go back to the basics.

And to the known culprit of all the problems known to mankind while cutting plastics: Static. Whatever I did, I just couldn't avoid it. Those swarf threads were clinging to everything - it started to really p** me off. So I fought the static. First: Everything on my lathe is now grounded. That helped a little. But what really makes a big difference: Propylene Glycol. That stuff is almost magic. Just one or two drops and the static is gone. The tread is being pulled into the suction tube and everything is just fine.

But - of course there's a but: The suction system - even with reduced force, introduces a crackle and some squeaking noise. It was quite audible in a cut - and I was able to trace it to my - let's say "interesting" way of routing and warping the suction tube. I constructed a new tube (I guess version 25 or so) taking good care to avoid all unnecessary constrictions and turns. This reduced the crackle and squeaking to maybe -40 db or so .. only audible in empty grooves while cranking the amp up all the way. I will go back to the drawing board to re-route the suction more to the position were the swarf actually originates - which should allow me to reduce the force of the suction even more.

So - in summary:

-Even powerful suction systems wont be able to work against static;
-Static can be reduced by grounding and polishing the plastic with a few drops PG, making the cutting thread behave almost like lacquer thread;
-Suction tubing needs to be routed carefully to avoid restrictions to the air flow;

I will report back when I have my new suction tubing in place. Thankfully I ordered two 25 ft rolls 1/4" and 3/8" aluminum tubing - so I still have plenty to play with.

Michaela

[mischmerz]

Ok - Here's part II.

I had a few another unsuccessful or .. well less successful tries but the current version is actually working pretty well. I designed a "plug-able" suction with a 3/8th tube mounted to the bridge at the right side of the cutter head allowing a 1/4" tube to be plugged in. That allowed a number of easy test and modifications to find the best position of the swarf pickup without the need to waste too much aluminum tubing. I cut quite a few PVC's by now and had no problem with the chip being sucked away. However - that only works well with the PG treatment as described in the previous post. Without it, I need much more suction power and it usually balls up on me after a few minutes.

suction_new.jpg

I still have the crackling and occasional squeaks but that might just be the unheated stylus. Now that I have my suction up and running, I am now able to proceed to a heated stylus. One baby step after another.

Stay safe

Michaela

[grooveguy]

Good progress, Michaela,

And thanks for sharing your experience with the group. Have you ever tried a negative ion generator for dissipating static? Back in the 1960s, I earned quite a bit of experience with high speed tape duplication. A one-inch master, recorded at 7.5ips, would run as a continuous loop in a vertical bin at 240ips, or 32:1 relative to real time. That master would build up a tremendous static charge that would ultimately lift the tape out of the bin and cause it to spew all around the room.

We turned to 3M for help. They brought a block of aluminum about 2 inches square and maybe a foot long. One long side was covered with wire mesh, and underneath was a very powerful source of atomic radiation... so strong that we would have had to get a license from the government to use it. The ladies who ran the machine wanted nothing to do with that.

Next we tried a negative ion generator, something that was popular with the 'earth people' back then. Anyway, it worked like a champ we rarely had a master spill after that.

Here's a picture of a negative ion generator I bought as a kit from Ramsey Electronics years ago.

ions.jpg

It isn't made anymore, but Amazon offers a few, and eBay sells the high voltage part for ten bucks or less. Note how just the high voltage moves enough air to almost blow out that flame. You'd need to get the HV generator part off eBay and build something like this with those tubes and sharp nails inside to generate the negative ion airflow. If this is of interest I could take more pictures, or maybe Ramsey has documentation on their Website for the stuff they don't make anymore.

Good luck!

[mischmerz]

Good progress, Michaela,
And thanks for sharing your experience with the group. Have you ever tried a negative ion generator for dissipating static?

Thanks and - now .. that's funny. I just bought a Simco Aerostat Ionizing Air Blower. This unit is supposed to be a sharp sword in the fight against static. Small enough to be installed on the lathe. I am exited about that and will, of course, report back.

One more piece of information in regard to my plastic cutting and suction experiences: I understand it to be crucially important that the endings of the suction tubes are very smooth inside and out so that the thread wont find anything to get stuck at. But I don't have the tools to bring the ends to a smooth finish - I simply used clear nail polish to "paint" the openings of my suction tube. Works really nice.

Michaela

[Skint]

Excuse my naivety, but what is it that limits what materials can be cut on any given lathe? I'm thinking the Rek-o-Kut was not designed to cut PVC, so are there issues with the cutter head, or is it all down to the shank used? i.e. does fitting a diamond shank/stylus and vacuum to "any ol' cutter" theoretically enable it to cut PVC?

[mischmerz]

Excuse my naivety, but what is it that limits what materials can be cut on any given lathe? I'm thinking the Rek-o-Kut was not designed to cut PVC, so are there issues with the cutter head, or is it all down to the shank used? i.e. does fitting a diamond shank/stylus and vacuum to "any ol' cutter" theoretically enable it to cut PVC?

Well - its a matter of stylus and pressure. In order to cut PVC, you need a diamond stylus and a counter weight - because the stylus pressure should be below 20 grams. I had to add to the counter weight of my system to reduce the weight of the head (and the stylus). I guess I couldn't cut into concrete because the Rek-O-Kut tunable doesn't have enough torque and I wouldn't know how to get the sledge hammer attached to my bridge

But yes - any lathe with the right stylus should be able to cut plastics.

Michaela

[grooveguy]

The stylus and media are, I think, by far the most important aspects of this situation. Damping of the cutterhead is certainly a factor; damping, that is, by the medium as its cut. But with sufficient horsepower 'headroom' in a decent head, and driving it harder, it should be possible to cut almost anything, as members on this group have proved. Compatible styli and media are the most important thing.

I read recently that diamonds, as hard as they are, may not be as good as synthetic sapphire (aluminum oxide) for record cutting... something about the grain of the material. I wish I'd tagged that article. Also, some materials just don't like to be 'cut.' Ask a machinist who is called upon to turn an object out of some of the esoteric plastics like Delrin and other acetal homopolymers. A real challenge compared with turning aluminum and brass. Some plastics want to tear or rip rather than cut nicely. Again, finding the right material and the proper cutting tool are paramount.

But the lathe mechanism, itself, should have only minor bearing on the issue. My first recorder, when I was a kid in the '50s, was the ubiquitous General Industries deck with an Astatic crystal cutterhead. At 78 r.p.m. the turntable had only marginal horsepower to overcome the cutting 'drag.' I remember copying Rossini's William Tell Overture for a friend with classical music tastes. When the loud parts came in, evidently the turntable slowed down as the steel stylus cut deeper into the groove walls, creating a noticeable speeding-up of those parts on playback. The effect was greater on a lacquer blank than on the little coated cardboard discs that were still available at the time, and which evidently posed less of a load on the cutter. But any decent turntable today should be capable of constant speed, even as challenging materials are cut by whatever stylus is required to create a quiet groove.

[markrob]

Hi,

I think many of those older rim drive turntables had way more torque than modern servo controlled hi-fi turntables. The servo's don't seem respond well to the varying cutting load near the torque limits. They really were not designed to see much more that the light load of a modern playback stylus at low tracking forces. That they work at all for this is a plus.

Mark

[sakuszi]

Hi,

I think many of those older rim drive turntables had way more torque than modern servo controlled hi-fi turntables. The servo's don't seem respond well to the varying cutting load near the torque limits. They really were not designed to see much more that the light load of a modern playback stylus at low tracking forces. That they work at all for this is a plus.

Mark

Mark, I did drive direct pitch, it works perfectly, soon video, 150mm in 4 seconds back. This is repaired fast ride, of course, FB encoder analog!