looking to cut a mandrel to do a little tubing bending - coil

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Good day :
I am looking to make a plastic or brass mandrel for 1/2" tubing. To that end, I need to get my feed to be in the range of 1.25 Threads per inch. Current machine can do 1/8" per Rev. so I need a 6x improvement on this.

Option 1.) making / or buying some spur gears to get the gearing up to the 0.75 inch / rev., But there is a bit of cost and time to do this, as well as additional load on the gears. Currently my leading option but it is not a good one.

Option 2.) use a motor connected to the apron feed wheel and a encoder connected to the spindle. Even more $$ and time, and not to short of a full cnc.

Option 3.) convert to full CNC big $$$'s and ? is it the right thing to do. (My lathe is a 9x20 type. lite home stuff +/-)

Option 4.) ???? Any great Ideas would be very welcome! ( Note: I am not looking to have the lathe motor bend the tube! = not near enough torque.)

I am looking to do something like
 
Are you planning on producing the mandrels in large quantities or make one mandrel and produce coils in numbers?

If the later have a machine shop make the mandrel, this will be far less expensive then modifying a 9X20 hobby machine to produce one part.
Roughly like so.
i-2N9wSVn.jpg
 
Have you actually checked that you cannot turn 1.25 pitch threads? Remember the threads listed on the plate are usually just the common ones. My lathe does not list 1 1/4 T.P.I. but by using the program 'LatheGears',(I'm lazy),it comes up with 25 combinations using the stock gears, add in the extra change gears I have and the number is in the hundreds. Might pay to check out what you have first.
Roughing out in a couple of passes then swapping to a form tool and taking it easy should do the trick.
 
You will be hard pressed to find a hobby level machine that will thread at a .800 lead.
 
Thanks for the comments, and yes I am still thinking on this one :

This project:
It is, for only one for now. ( if it works out; then, many coils to be made from the one mandrel ) so I may need to have a mandrel made some where? (and make a manual follower to work with it, like a std. floor mount tubing bender).

Other options that I thought about, but maybe not the right way to go.
1.) 3d print the outside of the mandrel and slide it over a pipe. (issue = may not hold up, even with polycarb)
2.) cut a temp. lead screw and follower on the lathe (current is 16TPI so cutting 8TPI only doubles the speed = 4tpi even with some gear sub =not 0.8"/rev)
3.) making a set of cog pulleys and belt for the ratio about 12 to 1 (not going through the change gears at all) this could work but ? but would this be less $$'s and effort than having a mandrel made? Yes I know 0.75/in but this is not an issue as the coil can (will have to stretch and compress when assembled)

Option # 3 was the only one I think would have a chance of working, and I could back feed the spindle from the lead screw the 12/1 thing)

Wreck (any chance you have done this already (your drawing looks about correct for 1/2" tube inside 4" 0.065 for the exchanger))

In any case still thinking (it is not really hurting my head yet but getting there)
 
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Have never made a coil mandrel but have some experience with single tube bending dies and followers.
Large helix conveying screws are common in labeling and filling applications when the containers must be closely positioned for filling or labeling and are most often made from low friction plastics that will not mark the container, this will not work in your case.

I know little or nothing about how they are made but having closely examined several which I modified for particular applications they appear to have been milled. And yes, I work for a company that makes parts for the machines that glue the annoying folded information sheets to drug bottles (these are called top serts).
Not this one however, this is the first search result that turned up after a "screw conveyor" search.
 
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