sharpen carbide or make a hss holder?

[Zamfir]

I need to make some special knife pivot screws that are 64 TPI. I successfully ground a threading tool in HSS for the mini lathe. Now I have a larger lathe with 1" square tool holders. The external and internal threading insert tools only go down to 48 TPI. I am sure that has to do with the radius ground at the tip.

My question is, can I take one of these cutters made for 48 TPI and sharpen it more so I can make the 64 TPI threads? This would be my preferred solution so I do not have to mess with shims and such each time changing out a tip.

Or, would it be better to make a 1" shank tool holder to hold the hand ground HSS 1/4" tools (made from 1/4" blanks) I know I can make?
Or, maybe someone already makes something like this?

Thanks!
Eric

 

[Zamfir]

Here is the cutter I made.
It worked real well!

But inserts are easier. I guess this would end up being an insert of sorts if there was a holder. And I do have a lot of blanks that could be used. Hmm.

 

[mikey]

You can use a P-type parting blade and grind a 60 degree tip on that. It works well. I used one to cut a 00-90 thread once but I have no idea where that tool is now.

 

[joshua43214]

MSC lists lots of very fine thread inserts, though I didn't see any lay down style in these.
Iscar makes laydown inserts in metric that will cut .35mm pitch (64TPI = ~.39mm pitch).
You would need to reform the tip if you are trying to match an existing imperial thread.

It will take you a bit more time to make an insert holder, than it will to grind and hone a big old 1/2" HSS bit down to 64TPI. The insert holders are super easy to make (other than TopNotch style), and don't need to be made from anything special.

Honing a 48TPI insert down on a diamond lap would not be a big deal for you since you already know you can do it in HSS. Doing the inner edge on a lay down insert would be tricky though. Easier to hone down an upright insert since you can get at both edges easily on those.
The inserts are pricey though. I would just try it with whatever you have laying around first.

As for making a holder, I use a shop made holder for my lay down inserts (11ER?), took about an hour on the mill and works perfectly for fine threads. The handful of 8TPI threads went with no issues, though I didn't make cuts deeper than 0.015" so no idea how it would do on heavy thread cutting. I made it from scrap mystery metal. I drilled the hole ~0.001" closer to the sides of the pocket and rounded the face of a random socket head screw so it drives the insert tightly into the sides of the pocket.
For cutting fine threads, you don't need anything fancy. There is very little force on the insert compared to regular turning. I would bet that for this application, you could make the holder from 7075 and it would perform just as well as a pricey holder on a manual lathe unless you will use it enough to deform the seat.

I also suspect you can use the same insert for both internal and external in your application, so prolly best to make a LH internal tool, and use it as a RH external tool.

A word of caution honing inserts. Diamond maybe be harder than carbide, but the base material of the lap is not. You can very easily gouge the lap while forming the tip. That said, I think it is better to do this by hand since it would take a very rigid set up on the grinder to cut a tool this fine in carbide.

-Josh

 

[Zamfir]

Thanks for all the input! inserts are great but there are so many of them and searching for them evidently is not as straight forward as I would have thought..
Finally I stumbled across this insert that fits the dang tool holder perfectly.
https://www.mscdirect.com/product/details/73243552

google eventually got me there...it just took a couple weeks of poking around. I thought it was just silly someone would not make these.
Carmex - 16ER External Thread, Right Hand, 64 TPI, UN Laydown Threading Insert
P25C Grade, TiN Coated, 3/8 Inch Inscribed Circle

glad I did not need to make these parts right off the bat and am just thinking ahead! now there is plenty of time to make the collet holder for the chuck to hold the wee little rod that needs the threads..