# Cutting a trapezoidal tap from lead screw



## mcdanlj (Sep 29, 2019)

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:

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


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## aliva (Sep 30, 2019)

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.


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## mcdanlj (Sep 30, 2019)

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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## Bob Korves (Sep 30, 2019)

mcdanlj said:


> Thanks! 5/8" is very close to 8mm.


5/16" is close to 8mm...


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## mcdanlj (Sep 30, 2019)

@Bob Korves  Urk. Indeed. I know better.


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