Cutting a 45deg helix on a Myford lathe

[Stewart HartParticipant @stewarthart90345]

Ive been asked a ? by a reader about cutting a 45Deg helix on a myford lathe using the change gears, he is aware that the myford change gears won't cater for this but was wondering if it could be archived with a special gear ratio, I was unable to answer his ?.

The only method I know for this type of job would be to mill it using a dividing head geared to the mills feed screw, I did discuss the possibility with him for fabricating the part. Its fore compressor vane on an experimental turbine he's working on.

If any one could help I'll pass the information on to him.

Cheers

Stew

[Stewart HartParticipant @stewarthart90345]

[John HaineParticipant @johnhaine32865]

A method was described in the book on the Quorn build. This uses a milling spindle mounted on the cross slide to cut the thread, with gearing down from the leadscrew to the spindle. Feed was by turning the leadscrew handwheel. In effect it's what you describe but using the lathe as a mill.

[Neil WyattModerator @neilwyatt]

I think he may have to do it the hard way:

Make an extended gear banjo so he can gear the leadscrew done sufficiently. The exact ratio to get 45 degrees will depend on the diameter.

Mill the slot with a toolpost attachment rather than trying to cut it.

Drive the lathe by hand from the leadscrew not the other way around.

Neil

[BazyleParticipant @bazyle]

Depending on size it should be possible to rough it with a Dremel or battery drill for the powered spindle then clean up with very fine conventional tool cuts.

[Roderick JenkinsParticipant @roderickjenkins93242]

The longest lead I can get on my S7 is about 5 inches which equates to a diameter of 1.6" for a 45 degree helix. That's using a 6 gear train. If you can contrive an extra gear pair then that would help enormously. What we really need is a worm and wheel somewhere in the system – I don't know if anybody has ever done that, I guess you would need to put bevel gears or crossed helicals in their as well.

Rod

[John PParticipant @johnp77052]

Hi Stewart

As some of the other contributors have said a useful starting reference
would be the Quorn book.This 45 deg angle would be more about the
lead required to obtain this angle at a given diameter .Some photo's
posted in the album "Helix" will show in photo 1 some guide hobs
made by these methods ,the sizes indicate the cutter diameter ,the cutters
all have a nominal 30 deg helix angle. As can be seen when all viewed as a
common diameter they all look very different ,the 1/16 inch one is only
two start the other two are four start.

In the second photo is the gear change quadrant for the Myford made to cut
these guide hobs,since there is no indication on the diameter turbine
blade /wheel some indication of the ratio needed between the leadscrew to
headstock to get 45 deg at a typical model size turbine of 60 mm nominal
diameter would be about 60 to 1 .
There are other complications when making these sort of forms in this way,
Using a toolpost milling atachment and just offsetting will result in producing
a tombstone effect on the blade ,the blade will be undercut at the root and
thick at the tip to avoid this some adjustment to offset and also rotation of the job is
needed to correct.Sometimes these sort of jobs look at first quite easy to
do and can be much more difficult to end up with the desired result.

The third photo shows this 718 inconel turbine wheel milled from a
solid blank,to do this required a considerable number of fixtures and a
rotary table .

The fourth photo shows a model compressor wheel made from HE 15
aluminium milled on the Dore Westbury using 4 axis cnc ,usually these
are done on 5 axis type machines ,probably beyond the pockets of
most model engineers.

I find it easier to use this prototyping wax for test cuts it can be saved and
remelted for re-use saving metal and tooling as can be seen here with
this nozzle guide vane part in photo 5.

Interested to see how this works out.

John

[MuzzerParticipant @muzzer]

Posted by John Pace on 20/02/2016 11:14:41:

…The fourth photo shows a model compressor wheel made from HE 15 aluminium milled on the Dore Westbury using 4 axis cnc…

Sounds as if you have a CNC-controlled Dore Westbury with a 4th axis? That must be a unique machine! Certainly seems to have cut the mustard on the compressor and turbine wheels – incredible to manage this in the home workshop.

Couldn't see any pictures in your albums – any chance of a photo or two?

Murray

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