Machning of Plastic pipe to create a helical opening.

[Parusky]

Does anyone know of an efficient procedure to machine a helical opening in a plastic pipe?

The purpose of the design is to convey small dense particles at a low production rate from the bottom of a water bath. It encompasses a helical pipe rotating within pipe in a close tolerance.

I can use a rotating flat spring but would prefer to use plastic pipe for corrosion and temperature reasons.

Any suggestions or advice greatly appreciated.

Thanks.

DJT

 

[CSLufkin]

I assume you don't have access to a 4th (rotary) axis CNC? If so it is a snap.
If you are trying to do this on a manual milling machine it will be a bit of a challenge, but not impossible. How long & what diameter is the pipe and the helical cut area? How accurate do you need the Helix to be? What machines do you have access to (lathes, mills drill press's) and how big are they?

 

[griffengm]

Parusky,
Why do you want to machine pipe into a helix when you may be able to wrap plastic flat stock to form your helix? If you require an edge wound helix I believe that could be done using plastic strip as well unless you need an uncommon polymer.
Sounds to me like you are wanting to move shot from the chilling pool of a shot tower.
Any particular reason you do not want to use some of the plastic chain components presently available?

Griffy

 

[Parusky]

Dear CS,

I have the resources of a fairly large machine shop that is a little bit in the dark ages. The longest bed lathe is an 8 footer. Although things have been retrofitted in most cases with NC contols it is mostly verniers. I must admit that GriffenGM's solution is a lot more appealing than attempting to do this on a lathe. The rigidity of the platic pipe would require some sort of mandrel.
I would need at least 8 feet of length and an opening of about 1/4".

Dear GriffenGM,
You are correct about the lead shot except that it is not from an open pool. The shot has to be removed from the bottom of a water filled tank. It is not formed with a tower drop but a little bernoulli principle stuff and gravity. The temperature of the water is 200 degrees F and I would like to bring the shot out continuously. The tank bottom is an sharply angled inverted pyramid terminating with a piece of open topped steel pipe. The pipe is horizontal for about 20" and then angled up at a 70 degree slope to a height of 6 feet. I considered full flexible augers like Flexicon makes but discarded them due to temperature limits and lifting restrictions. The density of lead shot is .45 #'s per cu inch. Their design requires the tubing be filled to function correctly. The machine is producing only 1.7#'s per minute so why not remove it as it is produced?

I looked at other conveyors and all had their limitations. I am attempting to keep the water level static and undisturbed. I can visualize a plastic chain running in a close fit channel also working. Problems with the submerged sprockets and the idler sprockets for the top loop at the angle transitions? Please elaborate. This system is self draining.

As to the flat plastic idea it sounds loke it would be heated and wound around a mandrel similar to spring production? I anticipate having to place braces to maintain the diameter of the plastic spring under the minor torque?

Any other ideas,suggestions,comments or techniques would be greatly appreciated.

 

[CSLufkin]

The 8' bed lath should be able to do the job if they can adapt a spindle (Air. or Electric), or even a die grinder to clamp into the tool post. This way they could put an endmill in this spindle and cut a helix thru a wall of the pipe. This would be like cutting a real coarse thread with the split nut on the lathe. It could be a little scary, but with a descent setup, perhaps a mandrel, or a follower rest, you could git-er-done.

 

[griffengm]

Parusky,
Lufkin is correct about being able to turn the helix on a lathe. I have cut long lead helical slots on a vertical mill but like the lathe it would need multiple setups to treat 8'.
Are you wishing to turn a helical groove in the o.d. of the pipe or to cut through the wall? If through the wall, then you are removing much of the rigidity anyway.
Maybe this comment will jog someones memory. Sometime in my misspent life I saw an apparatus that used a router to generate the spiral flutes in wood for furiture legs. IIRC, the operator turned a crank which rotated the blank and moved the router along a linear rail. It was not real complicated and used a cable or rope to slide the router.
Since this is plastic, maybe it could be done in sections and glued together.

Griffy

 

[Parusky]

Dear Guys,

Thanks for the advice. Another way would be similar to a rifling button cutter inside the pipe. I have seen the machinery to do button cutting and it used a twisted set of four square guides anchored in a circular end plate to drive the single bladed tool mounted on a a round mandrel. The pipe doesn't rotate; the tool head does.

The plastic pipe does not have enough strength to survive the process with the lathe.

The idea was to cut a spiral through but keep tabs to hold the whole thing together and hand saw out the tabs to gain a spiral.

I like the idea of a piece of flat bar plastic and will play with that idea. I can run it through a plastic pipe heating unit to gain enough pliability to run it over a mandrel at slow speed. I can put locating pins on the steel mandrel to maintain the correct helix and spacing. Got to figure out what the springback is though. I haven't found anyone who sells a flat plastic spring that would work.

I have though about a piece of plastic chain running in side a plastic channel mounted in a steel rectangular section tube with enough room for a tension cog on the bottom end. The shot would fall into the captured chain and be run up the channel to the dump opening. The return loop would be loose. It would not interfere with the shot dropping through the water. Machining a T-shaped slot in the channel is not a problem as much as it would be getting the correct bend after machining.

Still at the playing stage.

Any other ideas for accomplishing the task.