Beam coupling spiral - how do you cut it?

hello friends,

I need to machine a 3 start 0.012" wide spiral on a ss beam coupling.
(OD of coupling is 5/8" and ID is roughly 3/16")
I am wondering how are these spirals on couplings machined?
they are usually very thin, and using a slitting saw may present problems as the spiral will
force the saw to bend around it thus breaking the saw or widening the spiral slot.

Is wire edm better suited for this operation?

If so - is it possible to cut such a spiral without supporting the wire on
the opposite end?
what I mean is piercing the coupling mid way and starting a ( 3 start) spiral? basically using the wire to erode material as if it was a "0.015" end mill? simultaneously turning the rotary axis around the axis if the coupler and advancing longitudinally to achieve the spiral?

Howdy, MULTUSB,

Please go to EDM Today magazine's archives to read about "Turn & Burn" and see examples of your parts on page 11.

You will need a servo-controlled rotary axis and good tooling (both are covered). Your programming will be quite simple cutting in the X-A axes or Y-B axes, depending which way the rotary device is orientated.

Good luck!

Bud

(Don't forget to include start holes in your estimate.)

You might mention that you asked this question on 2 January and got 12 replies, so you are interested specifically in the EDM aspect of the question. (Frankly, I think you should be using a slitting saw and accept that the slot width will be marginally wider than the actual saw width. If it's critical, just buy a custom saw that produces the desired result.)

Hi MULTUSB200:
As Bud has commented, a rotary axis that can couple to the control and respond to feedback from the control (so NOT just an indexer) is needed to burn spirals on the wire BUT:

With a 3 start spiral you can not cut it in the conventional way.
Cutting spirals conventionally means making start holes aligned with the centerline of the coupling, threading the wire through the holes and then advancing the wire down the length of the part while turning it slowly so the pitch is correct.
You do this for half the number of starts, because each pass cuts a start on each side of the part, one on the top and one on the bottom.

You have to think of the wire as a glorified electronic bandsaw so the wire must be able to pass in a straight line right through your part.
If that condition is not met, you cannot make the cut.

Since it's a 3 start spiral each slot is oriented at 120 degrees from the next one and there are no slots that are at 180 degrees from each other, so no joy.

A very tedious workaround is to bring the cuts in from the side and in this instance, you can fake it to some degree by tilting the wire at the helix angle of the spiral and burning in from the side while rotating and advancing the workpiece going back and forth along its length in increments no deeper than one wire diameter.
Once you get from outside to inside of the tube that will become your coupling, you break the wire, do an index, pick up your new start point, and do it all again, going around the part with as many indexes as needed to complete all the starts.
On a 3 start coupling such as yours, you do the whole messy affair 3 times.

On thin walled couplings with a fine pitch it can be made to work OK if the slots are not fussy...on thick walled couplings with steeper pitches it will not work because obviously there is not a straight line path for the wire in the same way there is not a straight line path for your slitting saw.

So it will always be a bodge unless the coupling has an even number of starts, or is so huge that the entire lower head of the wire can be positioned inside the bore of the coupling and one side at a time can be cut.

It is possible to make a contraption that will bolt to the lower head to dog-leg the wire so it can be introduced into the bore of the coupling from one side only, just as my previous example with a huge coupling, but the customer will have to be prepared to spend enthusiastically to custom build the gadget in order to re-route the wire in this way.

So it's a nightmare part...some bloody "engineer" must have designed it !

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

The coupler in my surface grinder is the spiral type, but the width of the slot is zero. I mean zero, and the thing is under significant tension like an extension spring with pretension. Not a clue how they did it.