Tapping a large hole

[Silverbullet]

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]

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]

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.

 

[BtoVin83]

Sure you can us a tap wrench on a 3/4- 10 tap if you like getting blisters and ****. 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]

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]

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]

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]

... 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]

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]

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)