Die Threading Perpendicular to a Shoulder

ACHiPo

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I need suggestions on how to use a die to cut perpendicular threads.

I'm making a stud for my size E Multifix tool post. I had a choice between 1/2"-13 and M20-1.5 threads into the t-nut because those were the biggest tap/die pairs I had. I chose M20-1.5 as that was the closest to what Adam Booth used when he made a stud for a customer's size E Multifix. Selecting the M20-1.5 means single point threading of the stud is out as I only have imperial threads on my Logan.

My first attempt ended up being off by at least a degree--not good when I want a nice precise seating against the t-nut. My second attempt I turned a shoulder equal to the root diameter. I pressed the tailstock chuck against the die and started tapping. After about 3 threads, it appears better, but it is still not perfectly perpendicular.

I have plenty of length on which to experiment, but I'd like to get this done so I can mount the tool post.

What's the best way to proceed? As a contingency I popped a 3/4-16 tap and die into my Drills and Cutter order, which gives me the option of single-point threading, but that will require me re-making the t-nut.
 
I would single point thread it. Cutting big theads with dies requires a lot of torque and getting really close to a shoulder is difficult. Much easier and cleaner to screw cut it.
 
Single-pointing is the only way to get the precision threads you want. Pick an imperial thread if that's what it takes to get good threads off of your lathe, but I wouldn't count on nailing it with a die unless you have a tailstock die holder with a deep recess.

Or you can do what I do when I need a part like that: Go to McMaster and buy a stud of the length and diameter you need with rolled threads.
 
Single-pointing is the only way to get the precision threads you want.
It's somewhat reassuring that I didn't have my head totally up a dark stinky place!
Or you can do what I do when I need a part like that: Go to McMaster and buy a stud of the length and diameter you need with rolled threads.
I like the idea of a stud with integral spacer to hold the toolpost, but it would be an option (and much simpler) to use all-thread. I looked at McMaster Carr for M20-1.5 all thread is very limited and REALLY expensive--100 mm (4") of all thread is $41 and that's only for grade B-7 which seems kinda wimpy. Their low-strength version is only $26, but I think I need/want something stronger? I also looked for 4" bolts, but again the selection is really limited for the 1.5 pitch and nothing close to 100mm long. Belmetric has a 10.9 grade 100mm bolt for $11 that is also an option.
 
Since you are using the stud with a t-nut (vs. threading it into the machine), then you can choose any thread you want. If your multifix will support a 20mm stud, then it's much beefier than I imagined. I think a similar stud in a 3/4" thread would be well under $10. You can also get some nice flange nuts or spherical seating nuts to top it off. I wouldn't dream of using all-thread for a machine part, that's the stuff commercial plumbers use to hang sprinkler pipes from the ceiling in warehouses. Not a strong alloy (fact), not a good material for machinery (in my opinion).
 
Since you are using the stud with a t-nut (vs. threading it into the machine), then you can choose any thread you want. If your multifix will support a 20mm stud, then it's much beefier than I imagined. I think a similar stud in a 3/4" thread would be well under $10. You can also get some nice flange nuts or spherical seating nuts to top it off. I wouldn't dream of using all-thread for a machine part, that's the stuff commercial plumbers use to hang sprinkler pipes from the ceiling in warehouses. Not a strong alloy (fact), not a good material for machinery (in my opinion).
Just looked at 3/4-16 stud: high strength 4" stud is $12 at McMaster. Much more reasonable.

I'm going to try again using the tailstock die holder, beveling the first 0.100" or so up from the root-diameter starting point. If I'm not still able to get it perpendicular, I'll lop it off, bore it to just be a spacer, and find a stud and flange nut.
 
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