How to Make/Machine a Twisted Tube?

[mrelet]

I have a plastic tube design with either a hexagonal or octagonal cross section. Wall thickness is constant. Overall, part is 3" long, 0.48" major diameter, 0.02" wall. The twist is that the cross section forms a helical path with a desired constant pitch of between 0.5 to 3.0. The most important aspect to keep is the internal helix.

I can get a decent prototype with SLA or FD ABS plastic. I wanted a smooth metal working model, but machine shops tell me it is not within their capabilities [ including swiss screw machines]. Assuming any plastic, anybody know how this can be mass produced ? I have an Injection Molding quote, but tooling costs are steep. I'm leaning towards extrusions. But, so far Extruders have nixed it ["haven't done an internal helix"] Any ideas?
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If you've got some decent volume, injection molding will certainly be the most cost effective. Tooling cost is almost always reduced to pennies per part, IF the volume is there. You could try a broaching shop to see if any are willing to give you a fair quote on broaching an internal helical profile. It's a tricky process and you'll probably get a lot of blank stares, but you might run across some who are up to the challenge. Mandrel forming or swaging of a tube is another option. Good luck.

 

[Adrian2]

Dear mrelet;

Could this plastic tube not be extruded in long straight pieces and then heated and hand formed around a suitable mandrel and allowed to cool. Then cut to length ?

Plastic Compressed air hoses are made that are helically wound around a form in this manner.

There must be a plastic tubing supplier to the Pneumatic/Hydraulic industry that does this.

Regards Adrian

 

[jfc]

Use your sla part as a pattern and have it cast in metal.

 

[xanvil]

Have you looked at the extrusion blow molding process? An extruded tube of molten plastic is grasped between two mold halves, then injected with a shot of high pressure air to conform to the mold shape. The cycling time is seconds. Search the web for extrusion blow molding to see examples.

 

You mentioned that you were looking for a smooth metal prototype, but all discussions have been about plastic.

HK small arms (guns) makers use polygonal barrels. I think they're swedged around a mandrel. They have about the same pitch.

You could use low temperature melting metal - cerro alloys melt as low as 140F, and use silicone molds. This is not as easy as it sounds, but it can work.

Seems that injection molding would work fine, but be expensive to tool.

Extrusion die would "simply" have to be made in a helix with maybe some extra length in the extrusion direction. This extra length could be cooled too stabilize the plastic.

This would likely be a puzzle piece die, not something easily done with wire edm - the helical surfaces, if you examine them with a machinist's eye, I don't think will be "ruled surfaces" and therefore will not be machinable with wire edm. A sinker could do it from a machined electrode though!

Lots of fun!

 

[Viktor]

The easies way for mass production is to use a special broach. For plastic, it should have a very long tool life and the tolerances can be kept as high as you need. Use the same approach as the gun producers do.

Regards
Viktor

http://viktorastakhov.tripod.com

 

First I need to understand what the tube looks like.
You describe a tube with multi faces of constant thickness meaning that the inside of the tube is also hexagonal or octagonal. Than you mentioned a helical shape that I believe it is only possible if the pipe is twisted.
The wall thickness of the tube is 0.020" and the major diameter of the pipe, I assume to be inside diameter, is .48".
The lenght is 3".
Not an easy part to form from metal and the tooling for a metal syntering process will be high. However cold drawn over a mandrel will also be expensive.

Antonio Reis

http://www.vitrom.com

 

[RobWard]

Do yuo really have to use metal?
I don't know the application, but we have had great success in replacing many metal parts with plastic. (We cast high performance polyurethane). The trick is getting the right material and grade for the application.
If you can find a plastic with the qualities needed for the application, you might find the manufacture isn't such a problem.
Remember: there are many, many different materials out there and getting the right one can save a good deal of time, money and effort in the long term.
(But if you must have metal, you might want to look into hydroforming) "I love deadlines. Ilove the whooshing noise they make as they go past." Douglas Adams