Tapping a large hole

[spike7638]

1. I'm a complete amateur machinist
2. I've got access to basic machine tools ca 1980: no CNC or anything.

Back here https://www.hobby-machinist.com/threads/brazing-together-bronze-nuts.67149/page-2#post-562325 I was describing a piece I want to make. The first step is taking some 1 3/8 silicon bronze rod that I've got, facing and center-drilling one end, drilling a 21/32 hole about 1/2" deep or maybe a little deeper, and then facing, center drilling, and drilling a 3/4" hole from the other end to meet that one, leaving about 1/2" of 21/32. Then I'll tap the 21/32 to 3/4-10. Now I'll be able to run 3/4-10 threaded rod in from that end, and have it run through the larger hole in the remainder.

When I'm done, the piece will be about 2" long: the first half-inch threaded, about 1.5" of clearance hole. (The exact amount that's threaded doesn't matter much, as long as it's at least 0.4".)

I think I can do all that with some confidence on the lathe, although being sure not to make the clearance hole too deep will require a little care.

My problem is with tapping the 3/4-10 threads. I bought a nice fresh taper tap. I have some Tap Magic. I'd really like the tap to go in square to the hole. I see two choices:

1. Grip the piece between two vee-blocks in a vise on the mill; make sure the faced bottom with the larger hole is sitting square on the bottom of the vise. Set up the tap in a really big tap-wrench. Center the table so that the quill is exactly above the center of the hole. Put something like a live-center into the quill, and use this to press gently against the tap as I take the first turn or two with the tap wrench; with each 1/4-turn or so, advance the quill a little to maintain a little pressure and be sure that I'm tapping square. Remember to ease the quill when I need to unscrew the tap wrench to break the chip. Once it's in a turn or two, forget about the quill and just keep tapping.
2. Do something clever with the tailstock on the lathe, which should make the quill go exactly into the center of the piece without any further alignment of the work, etc. Turn the lathe headstock by hand to do the actual tapping (?)

Obviously, I don't actually know how to do #2, and I don't know whether it's even possibly a good idea. And it's possible that both strategies are wrong, but something else is right. I'd appreciate any suggestions. I've tapped a good deal of stuff up to about 1/4-20, with moderate success (esp. after I learned about the existence of cutting oil about 30 years ago!). But 3/4-10 just seems really intimidating, and I don't want to mess it up more than 2 or 3 times.

I'd welcome any advice before I start making chips...
--John

 

[T Bredehoft]

Instead of drilling your two diameters, bore them, both size and concentricity will thank you. (Drill just undersize, bore to size. ) Holding a 3/4 tap in your tailstock may be a problem, so use your live center to center the back end of the tap. the cutting edge will center the tap in the hole (bored concentrically, remember?) Turn the chuck by hand, with a crescent wrench on the square shank of the tap to keep it from turning. With your third hand, advance the live center to keep up with the tap.
Hey presto, its done.

 

[Ferrous Turner]

You can get a tapping guide to fit into your tail stock. They look like this:

It aligns with the notch in the end of the tap (if it has one).
Otherwise look for tap guides in Google

 

[Robert LaLonde]

If the tap is floating and not gripped in the tail stock, put the lathe in low gear and just turn the tap. Then you only need two hands. One to turn the tap handle, and one to advance the tail stock. If you have a small or mini lathe where low gear can still be easily back turned by hand an adjustable wrench on one the chuck jaws and a block of wood or aluminum (so you don't ding the ways) works well enough as a chuck brake. A block of aluminum that lays flat on the ways with a stop block of same bolted to the face at one end means you still only need two hands.

____________________
| __________________
| |

 

[spike7638]

Instead of drilling your two diameters, bore them, both size and concentricity will thank you. (Drill just undersize, bore to size. )

OK. I have to admit right here (remember how I said I was an amateur?) that boring scares me, and I don't really know how to do it.

The notion of having a cutting point way deep in a hole where I can't see it, on the end of a bar that might be a bit bendy, working into a metal that folks sometimes describe as "grabby" ... well, all that makes me want to put on a set of hockey-goalie pads before I even get near the machine. Besides, we're talking about a hole that's 2/3 of an inch in diameter. Do I use a half-inch boring bar? And if I do, how is there room for a tool?

Perhaps what I need is a basic boring tutorial. I've bored something on a lathe perhaps twice in my life...

At any rate, the spring-loaded tapping guide seems like a good idea. In fact, I think I have something that'll do the trick already in my toolbox. And the hints from others about putting some aluminum down so that the tap-handle or crescent wrench doesn't ruin the ways...that seems like good stuff too.

Just how much "oomph" is going to be required to turn this tap, though? Will my arms be sore when I'm done? Or could any 10-year-old do the job just fine.

 

[Z2V]

I made this tap holder a few months ago. I saw it here and made it but I honestly don’t remember who gets the credit, sorry. I used 1/4” drill rod for the center and the rest was cut from some 12L I had on the shelf. The actual chuck is from Irwin. It came in a two pack, up to 1/4” and 1/4”- 1/2” I believe. The body slides on the center so the tap feeds naturally as it cuts.

 

[Ferrous Turner]

Boring is not too bad. I would get a piece of scrap to practice on until you get comfortable with the process. I figure I'm always going to screw up on the first try with something new so it doesn't surprise me. I watch a lot of Abom79 videos on YouTube. He explains things well and does a variety of jobs. He has a number of videos where he is boring stuff.
As for the big tap, bronze isn't very tough to machine so you shouldn't have much trouble with it. Do you have a big old tap wrench for that?

 

[P. Waller]

You will already have the part in a lathe, machine tap it from the tail stock.

 

[RJSakowski]

Depending on the finish you require in the 3/4" hole and the length of your tap, you may want to run the tap in from the 3/4" side. The tap will lightly score the 3/4" bore as the o.d. on the tap is slightly larger than 3/4" but the tapped hole will be true to the 3/4" bore. It will take some "oomph" to cut that thread. You will most likely want to lock the lathe spindle and use a good sized tap wrench.

 

[spike7638]

Depending on the finish you require in the 3/4" hole and the length of your tap, you may want to run the tap in from the 3/4" side. The tap will lightly score the 3/4" bore as the o.d. on the tap is slightly larger than 3/4" but the tapped hole will be true to the 3/4" bore. It will take some "oomph" to cut that thread. You will most likely want to lock the lathe spindle and use a good sized tap wrench.

Good point. I'll probably want that to be a bit bigger than 3/4, so that there'll be no chance of binding. (THe ID isn't critical for this part).