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Tapping a large hole

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spike7638

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#1
  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

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#2
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

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#3
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

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#4
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.

____________________
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spike7638

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#5
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

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#6
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. F7ACD66E-B0C5-4233-A38B-EB60E194756E.jpeg 991485AF-8DF8-433C-B719-078BE209549D.jpeg C2A5F553-DBDA-4E7B-BF8E-E0609FFD11F8.jpeg
 

Ferrous Turner

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#7
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

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#8
You will already have the part in a lathe, machine tap it from the tail stock.
 

RJSakowski

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#9
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

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#10
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).
 

Silverbullet

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#11
First I suggest you watch some boring being done on YouTube. Many good videos Ck abom79 , Keith Rucker , the lazy machinist .
Others there also but these do a bit better explaining.
And hi and welcome to the site. Your machining operations are not hard once you watch you'll understand better. Bronze is grabby but not hard to machine one suggestion I do know start with your largest drill bit over the usual small drill first. Brass and bronze will really grab when drilling a larger hole in a small starter hole. Try a scrap piece and see drill a 1/8" inch hole in a 1/2" deep then try a 1/2" drill bit and see what happens.
 

brino

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#12
Two thoughts:
1) Practice both the boring and tapping on a scrap piece, that way you make all your mistakes there.
2) If the application allows, drop down to 75% threads to make them much easier to cut.

-brino
 

GoceKU

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#13
I've made myself an MT5 holder that i use for power threading, for smaller taps i often use drill chuck to hold them.Cutting a small 60 degree taper on the entry hole increases your chance to start the tap square to the hole, you can use an live centre to follow the tap in the hole, if you don't have big enough tap wrench you can always use two adjustable wrenches 180 degrees apart, good luck be careful.
DSC_0104.JPG DSC_0107.JPG
 

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BtoVin83

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#14
Sure you can us a tap wrench on a 3/4- 10 tap if you like getting blisters and s***. Here's how we roll in oil country. after preparing the hole to be tapped insert the tap in the hole and run the tailstock up to the tap with center in the tailstock (most large taps have a center hole in the butt end), lock the tailstock down. Take a suitable sized crescent wrench and fit it on the square drive. By hand rotate the chuck until the wrench makes contact with some part of the carriage. In a low gear start the late and KEEP UP WITH THE TAILSTOCK HANDWHEEL (i.e. keep a gentle pressure on the tap). We have used a boring bar, tool post or the compound to block the wrench from rotating. I don't think I would start with a bottom tap but plug and taper tap work fine. Through holes are naturally better than blind hols as you don't have to guess how far to go in. Ran an old Monarch with a clutch and you could drift the clutch on really large taps. Oh and don't try to reverse the tap out this way do it by hand.
 

mikey

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#15
John, I know you said you aren't comfortable boring so this might sound even worse but when I have large threads like that, I bore the hole and screw cut the threads. That way I get the fit I need with very little chance of scarring up the outside of the part when it slips in the chuck like it often does when using a big tap.

Unless you need the part done ASAP, it might be a good opportunity to learn to bore accurately and also learn to internally thread on your lathe. Practice on scrap and then go make your part. It would be time well spent.
 

spike7638

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#16
Sure you can us a tap wrench on a 3/4- 10 tap if you like getting blisters and s***. Here's how we roll in oil country. after preparing the hole to be tapped insert the tap in the hole and run the tailstock up to the tap with center in the tailstock (most large taps have a center hole in the butt end), lock the tailstock down. Take a suitable sized crescent wrench and fit it on the square drive. By hand rotate the chuck until the wrench makes contact with some part of the carriage. In a low gear start the late and KEEP UP WITH THE TAILSTOCK HANDWHEEL (i.e. keep a gentle pressure on the tap). We have used a boring bar, tool post or the compound to block the wrench from rotating. I don't think I would start with a bottom tap but plug and taper tap work fine. Through holes are naturally better than blind hols as you don't have to guess how far to go in. Ran an old Monarch with a clutch and you could drift the clutch on really large taps. Oh and don't try to reverse the tap out this way do it by hand.
I figured someone was going to say something like this...but that just takes more courage (and coordination) than I've got. :) Besides, if I manage to do this at all, we're talking about 4-5 threads total, so I'm hoping that blisters won't be a big issue.
 

spike7638

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#17
John, I know you said you aren't comfortable boring so this might sound even worse but when I have large threads like that, I bore the hole and screw cut the threads. That way I get the fit I need with very little chance of scarring up the outside of the part when it slips in the chuck like it often does when using a big tap.

Unless you need the part done ASAP, it might be a good opportunity to learn to bore accurately and also learn to internally thread on your lathe. Practice on scrap and then go make your part. It would be time well spent.
I DO kind of need the part soon, so I'll probably go the "tap it" route. But some rainy day this summer, when it's too rotten to be sailing, I'll head into the shop and try to practice this whole "boring and cutting internal threads" thing, even if it is something I'll probably never do again in my life. :)
 

mikey

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#18
... some rainy day this summer, when it's too rotten to be sailing, I'll head into the shop and try to practice this whole "boring and cutting internal threads" thing, even if it is something I'll probably never do again in my life. :)
If you stay in this hobby, I can almost guarantee this is something you'll do again, and probably a lot more than you think. Besides, developing skills is never wasted. However, I understand that time is a factor so have at it with the tap. For big threads like this, I start the tap on the lathe and transfer the part to a vise to finish. A lathe chuck often slips with a lot of torque and you'll ruin your part; a vise will hold it more solidly.
 

homebrewed

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#19
Maybe you've already done this job, but if not, I have a comment about boring bars and machining grabby materials. If you have a brazed carbide boring bar, they usually have very little top rake, which reduces the tendency of the material to grab. Just make sure to install the boring bar so the top surface is perpendicular to the work and you should be good to go.

BTW, if you are concerned about going too deep with your boring bar, get yourself a carriage stop. One of the handier lathe attachments for the $ IMHO. Run the bit up to the work, set the carriage stop, zero the compound dial. Back the carriage off, advance the compound to the depth you want to cut. Done, no more worries about going too deep. Don't use power feed unless you want to crash the carriage into the stop!

Also, standard twist drills can be very problematic with grabby material. I have a mini lathe and have had the drill chuck pulled right out of the TS when I was enlarging a large hole in brass. And forget about drilling to a specific depth! It drove me crazy until I learned about the practice of "dubbing" drill bits for stuff like brass. I modified several (using bits from a cheap HF drill set) and the difference was amazing. Dubbing basically modifies the cutting edges of the drill so they have a neutral rake. I use a 600-grit diamond stone for this. It only takes a few passes per edge, just keep the stone in a plane parallel to the body of the drill bit while you are grinding the edges.

Drill bits that have been hand-modified like this probably won't drill real straight. So you still will need to get to your final ID with a boring bar. That's what I did.
 

British Steel

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#20
First I suggest you watch some boring being done on YouTube. Many good videos Ck abom79 , Keith Rucker , the lazy machinist .
Others there also but these do a bit better explaining.
And hi and welcome to the site. Your machining operations are not hard once you watch you'll understand better. Bronze is grabby but not hard to machine one suggestion I do know start with your largest drill bit over the usual small drill first. Brass and bronze will really grab when drilling a larger hole in a small starter hole. Try a scrap piece and see drill a 1/8" inch hole in a 1/2" deep then try a 1/2" drill bit and see what happens.
About the drill grabbing, I keep a set of inexpensive HSS drills for brittle plastics and brass/bronze with "dubbed" edges: the cutting edges are ground back in the flutes so that they have a 10 to 100 thou" face (from small to large) square to the material, this means they cut by scraping and won't dig in and grab/snatch/self-feed (The flat just needs to be wider than the feed per edge-tooth).
It takes about 5 minutes, starting with a sharp and symmetrical drill lay it parallel to the top of an oilstone with the cutting edge on the corner, a few strokes at a time and check the flat, repeat for the other side.
A side effect of dubbing the edge is that usually the drill web thins on the side of the stone, which helps reduce the pressure necessary to drill the hole, helpful in the harder bronzes☺

Dave H. (the other one)
 

spike7638

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#21
I managed to get the whole job done, largely due to the encouragement from folks here. Here's a picture of the old piece (a but with a piece of copper pipe brazed to it, which was great for a zinc with a 3/4 ID, but not for a larger one with a 1" ID, which is what I wanted to use -- it's thicker, hence lasts longer...yay!) To the left of that is the new piece. The material looks a little spotty, because it's made from a piece of an old 1.25" propeller shaft whose zinc got knocked off; the bronze then do-zincified in spots, leaving pink copper-y areas ... but only about 1/8" deep, so mostly of no concern to me, although the final piece does look as if it has some mange.

The tapping, which had worried me, turned out to be pretty easy --- there's nothing like having a completely new tap, and only needing to cut about 5 threads!

IMG_20180426_221827.jpg

Thanks again to everyone for the encouragement.
 
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