Cutting a trapezoidal tap from lead screw

mcdanlj

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I would like to cut a T8*2 2mm lead (single-start) ladder tap from a piece of matching lead screw. It only needs to cut plastic, so cutting it from lead screw should be fine. (That would work to cut aluminum or brass too if I want, but it's not what I need to do for now; in any case, I wouldn't be cutting steel with it.) I want to make something with very little play, because I want to use it for the Z block for a CNC router.

I tried making a 2-flute tap with a 2⁰ angle on the compound setting the taper, but it was too aggressive and took a lot of pressure, and it had small but perceptible play. I milled out the flutes by holding the tap in a v-block on the mill, and put the edge of the mill slightly past the center, to add some rake to the cutting angle. I turned a section at the tip to the minor diameter to align the initial cut.

The T8 lands are 1mm deep, so naively if I set the compound to 1⁰ I'll have to make the tapered section of the tap almost 60mm long. That seems excessive.

Questions:
  1. Cut the taper first or the flutes first?
  2. How many flutes would you use for cutting plastic with a ladder tap like this?
  3. Thoughts on taper angle on the compound for the ladder step?
thanks ☺
 

aliva

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I needed to replace the compound nut on my lathe. But I had to buy the nut and screw, so I had the old lead screw. So I used it to make a tap. It was an Acme 5/8-8 ( I think). I cut the Acme screw to an appropriate length. put it in lathe and tapered the end about 5 degrees, Then in the mill to mill 4 flutes with a 1/4" ball mill. I then hardened it with oil. I used it to tap a spare compound nut made of bronze. Worked like a charm. If you cut 4 flutes you should be good for plastic, brass, aluminum. I never tried mine with steel as It is only needed for the lead screw nut which is bronze only.
 

mcdanlj

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Thanks! 5/8" is very close to 8mm. It sounds like probably my feeling that 2° was too aggressive was actually poor flute formation rather than excessive step size.

Mine is 304 stainless so the only hardening I'll get is work hardening... :)

I've ordered carbide ball nose mill for cutting the flutes.
 
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