Tinkering over my pay grade in the shop this week. Get a cup of fresh coffee.
I use a LeBlonde Regal lathe with a 14" swing. The Aloris quick change tool post default uses a 3/4" square shank lathe bit. Some small gunsmith projects have small dimensioned places or metal stock where a smaller lathe bit tool would be handy. I wanted to fabricate a 3/4" to 1/4" tool bit adapter sounds simple enough. The project requires milling a 1/4" mortise to hold 2-1/2" tool bit, which would be secured within the mortise with 8-32 set screws; make a 1/4" slot, drill and tap holes, what could be easier ? The mfg. 3/4" tool bit shanks are really stout so I wanted to replicate that rigidity for my adapter.
I started with a 6" x 1" x 1" 4140 alloy chrome moly stock that was the hardest material I have attempting to rough it on the mill to 5"x 3/4"x 3/4". I was using TIN-coated high speed steel bits, not carbide end mill bits. I couldn't mill it or accurately band saw it; was able to use the big chop saw to cut it. Enough of that program. Learned 4140 was not a good starting point and I should have used some stock closer to my desired final dimensions.
Sourced a 5"x 3/4" x 3/4" lathe bit blank made of M-2 high speed steel; said it was easy to grind, not. Got some 1/4" carbide end mills and was
able to make the 1/4" slot for smaller tool bits. Next to drill 8 holes for set screws. Started with #29 ga. Monster brand cobalt bits, pecked it almost all the way thru the first 1/4" hole, bit broke as it was just breaching the far side of the hole (where they often do). Was able to take punch and knock the residual bit out the bottom of the hole. Next up full carbide bit with lots of AnchorLube took 25 minutes to peck the next hole. Switched to a International Carbide Corp square
die bit and it worked really well (highly recommend it), finishing the next 6 holes in about 20-30 min. I then used my 3-flute (HSS) Supertap in the last hole for my first threading; tip snapped off at ~1/8" in the hole; an opportunity to use broken tap extractor! So I have my project 2/3 done, just need to tap 8-32 holes.
What would be the carbide brand and style to use for the threading? I am leaning towards a 2-flute spiral tip style.
Plan B is to give it to some one to thread it that knows what they are doing!