Lathe thread cutting

[BFBOB]

I'm no novice at running a lathe, but thread cutting kind of eludes me - at least doing it well.
Can anyone point me at some basic BASIC videos on the subject? Practical Machinist - at least what I've seen there - is too advanced.

I need to thread some rod to repair a small vise, and it needs to look good as well as work well. So far my efforts look like they were hand-hewn with a broadaxe.

Thanks in advance

 

[daveroy]

Google tubal Cain, he has some great machinist videos. Besides he reminds me of my grandpa!

Inside:

outside:

 

[lathedog]

Tom's Techniques is another good machining channel. here's the first of a 4 part series on threading

 

[Beachside Hank]

Single- point threading is better done on larger thread diameters; use of a threading die is better for smaller. The smallest I've ever done is 3/8"- 16, and even then I had to chase it with a die to clean it up. I think Tubal Cain also advises this.

 

[remileblanc]

What size/pitch of thread are you cutting? If its for a vise I would assume either square thread or acme.

 

[Ign]

I single pointed 6-32 in school, turned out pretty good. A thread file is nice to have for most threads

Make sure your tool is square to your work and the key to good threading is LOTS of little passes with plenty of "free" or "air" passes in between where you don't advance the tool at all. Just hit the same number on your lead screw dial every time if the lathe is older.

 

[Steinmetz]

Single- point threading is better done on larger thread diameters; use of a threading die is better for smaller. The smallest I've ever done is 3/8"- 16, and even then I had to chase it with a die to clean it up. I think Tubal Cain also advises this.

I agree with this.

 

[Steinmetz]

What kind of material are you using?

 

[Shadowdog500]

Single point is fun to do once you learn. The mrpete222 videos are hard to beat for learning. I mostly single point if it is and oddball size and I don't have a die to cut the threads. I've also had really hard stuff like drill rod that I had to single point because it was a PITA to use a die on.

If I do have the die. I still do it in the late while turning the chuck by hand and using a drill chuck in the tailstock behind the die holder to guide it on straight. They also make tap and die holders for the tailstock.

I only have a mini lathe with change gears so just setting up to single point threads takes at least twice as much time as it would take to use a die. If I had quick change gears and was all set up, maybe my attitude would be different.


Broke it up into three paragraphs, Teken would be proud!!!

What kind of lathe do you have? Any photos?

Chris

 

[Kevin54]

Material makes a huge difference when threading. Some threads will be as furry as a bear and others will be as pretty as can be. I seem to always have a problem with tool steel when single pointing.

Tubalcain has a lot of good videos on it. One thing to remember....if you don't single point on a regular basis, don't have someone standing around asking questions while you're trying to concentrate on it. And make a couple trial runs with the compound back a ways or no stock in the chuck. All it takes it one lever grabbed wrong and you will have a wreck. I've not had a real wreck, but I have busted a few cutters. I did have a coworker though that was threading and shooting the **** at the same time, and didn't get the half nut disengaged. The compound into the chuck makes a hell of a racket before it stalls out.

 

[A_Pmech]

Hewn with a broad axe huh? I've had a few turn out like that!

I'm going to assume you're trying to cut a thread with the standard flat-topped tool. They have a habit of causing tear out issues in mild and low alloy steels because the cutting geometry is all wrong. For some reason they insist on teaching this tool geometry, yet any old-line manufacturer of turned lead screws would laugh at such a tool.

On the further assumption that you're making a 60˚ thread profile:

Swivel the compound to 30˚ and grind about 31˚ of relief on the back side (non cutting side) of the tool. Now, grind about 15˚ to 25˚ of side rake into the tool, being careful to grind right up to, but not over the cutting edge of the tool to sharpen it. The back rake will be ground in 30˚ from the centerline to stay parallel with the cutting edge. Hone the cutting edge face to deburr and refine it.

The resulting tool will cut freely with a very good surface finish. When you set up the tool, you'll have to hold the thread fish against the workpiece and align the front cutting edge.

 

[Shadowdog500]

Hewn with a broad axe huh? I've had a few turn out like that!

I'm going to assume you're trying to cut a thread with the standard flat-topped tool. They have a habit of causing tear out issues in mild and low alloy steels because the cutting geometry is all wrong. For some reason they insist on teaching this tool geometry, yet any old-line manufacturer of turned lead screws would laugh at such a tool.

On the further assumption that you're making a 60˚ thread profile:

Swivel the compound to 30˚ and grind about 31˚ of relief on the back side (non cutting side) of the tool. Now, grind about 15˚ to 25˚ of side rake into the tool, being careful to grind right up to, but not over the cutting edge of the tool to sharpen it. The back rake will be ground in 30˚ from the centerline to stay parallel with the cutting edge. Hone the cutting edge face to deburr and refine it.

The resulting tool will cut freely with a very good surface finish. When you set up the tool, you'll have to hold the thread fish against the workpiece and align the front cutting edge.

Any way you could do a step by step photo essay of this method or make a video.

If not, let me know, I'll go try it tonight and can make a video
(if you can stand to watch threads being cut on a mini lathe)


I usually set my compound to 29.5°, why do you do 30°?

Thanks,
Chris

 

[gtermini]

I started using these insert tools a few years ago: http://www.mscdirect.com/product/88228762

They leave a nice finish in most mat'ls and have a decent pitch range. One trick to a nice clean thread is on your last pass, feed in 1 or 2 thou with the cross slide so that both sides of the tool take a fine cut. Mild steel is a ***** to get a nice finish on. I use Rapid Tap with trichlor to help with finish, but when I end up with a ragged thread, I just wire wheel the hell out of it. The wire wheel deburrs the thread peaks and burnishes the faces.

Greyson

 

[Steinmetz]

"...I usually set my compound to 29.5°, why do you do 30°?…"

It really doesn't make an appreciable difference.

 

[Steinmetz]

I single pointed 6-32 in school, turned out pretty good. A thread file is nice to have for most threads

Make sure your tool is square to your work and the key to good threading is LOTS of little passes with plenty of "free" or "air" passes in between where you don't advance the tool at all. Just hit the same number on your lead screw dial every time if the lathe is older.

Using a follower rest?

 

[justanengineer]

Single- point threading is better done on larger thread diameters; use of a threading die is better for smaller. The smallest I've ever done is 3/8"- 16, and even then I had to chase it with a die to clean it up. I think Tubal Cain also advises this.

JMHO, but single point threading is for times when you either dont have a tap/die, cant use one, or it doesnt make sense to use one, nothing to do with size of thread. Ive done some ungodly tiny ultra-fine threads on a few occasions, the worst part is inspection after.

As to the OP, having a good toolbit thread gage for the specific thread type and using it is important IMHO. I prefer the square or arrow shaped ones as you can grind your bit to fit the notch in the gage, then square the toolbit and use the gage between the toolbit and chuck to ensure the bit is ground equally on both sides.

 

[A_Pmech]

Any way you could do a step by step photo essay of this method or make a video.

Chris,

29.5° allows the tool to shave the "back" thread flank. In theory, at least. In reality, it just tends to gall the "back" thread flank with a flat-topped tool. When using a tool sharpened as I discussed, the geometry of the back side of the tool is totally unsuited to cutting, possessing a high negative rake. It's best to not force that into the work.

If I remember, the next time I thread something with HSS I'll take a video of the tool

 

[Shadowdog500]

Chris,

29.5° allows the tool to shave the "back" thread flank. In theory, at least. In reality, it just tends to gall the "back" thread flank with a flat-topped tool. When using a tool sharpened as I discussed, the geometry of the back side of the tool is totally unsuited to cutting, possessing a high negative rake. It's best to not force that into the work.

If I remember, the next time I thread something with HSS I'll take a video of the tool

I gave it a try. That bit is a little bit of a ***** to do freehand. My thumbs are still burning.

Chinese Blank

Grinding the bit(note the guide lines, table always set to a 7° rake)

The finished bit

Getting a stainless rod ready.

Ready

Oh boy, set change gears for 14TPI (yeah they are plastic, but i can get a whole set for around $30)

Line up the threading bit.

Finished thread (came out OK, but not great)

I've always set my compound to 29.5° From the southbend book, so the compound isn't much different. I did notice that I could take a much deeper cut than when I was using a forming tool.

The side of the thread that was being face cut was smooth, but I noticed that the other side of the thread was passable, but wasn't quite as nice. Normally I make the last pass with a forming tool a 0.001-0.002" plunge cut to face both sides and I feel I get a nicer thread.


I'll keep playing with this bit to see how it works on different metal.

Do you form this bit by hand or do you use a guide?

Chris

 

[Provincial]

I have found that there are some handy sources for quality, free-machining, strong steel. One of the best is Caterpillar bolts. I scrounge all the Cat head bolts I can find (the 3400-series uses 3/4" bolts) and they are long enough to make most smaller parts. The price is usually free!

 

[Kevin54]

One other thing also for ones that are using a 60 degree carbide threading cutter.....take a diamond hone and stone the tip of the cutter to five it a few thousandths radius. It doesn't have to be anything big, but you want to round off the sharp point. Even a radius as small as .002-.005 will make a world of difference in your cut

 

[Steinmetz]

If you're threading stainless, it's bound to be difficult. Higher Cobalt content in the blank, or use carbide tooling. Make sure the speed is correct.

 

[Shadowdog500]

If you're threading stainless, it's bound to be difficult. Higher Cobalt content in the blank, or use carbide tooling. Make sure the speed is correct.

Good point, I'll try a few different metals today.

I hand form all of my tools, I may make a tool grinding guide.

Chris

 

[Provincial]

The reason they teach using a flat top tool is because it is easier to see the angle of the "v" when testing with the gauge. Once you angle the top of the tool by adding back rake, it gets harder to see the angle properly. Making a guide or fixture will address this problem.

Stoning a HSS or Cobalt tool cutting edge will remove the small grooves left from grinding and in many cases helps get a better surface finish. This helps if the more drastic measures, like feed, speed, and tool angles haven't fixed the problems completely.

 

[BFBOB]

Thanks for all the replies. I'm learning more about how little I know! To answer some questions:
My lathe is a 25 year old 9x20 Buffalo (the Taiwan variety) with change gears.
For tools I use indexable carbide almost exclusively. I only use HSS when doing small inside turning. The insert is Fansteel 09906181. Triangular, TPG 221. It has quite sharp points, but with a definite very small radius. Maybe 1/64? I got this batch some years ago and really don't remember what it was supposed to be for - but I generally try to get something like this in a kind of middle-of-the-road variety rather than have a dozen types.
For stock, I've tried using a plain hardware store bolt , and a piece of 3/8 rod out of a lawn chair. I have some 3/4" shaft stock from Enco that I got for an axle where strength was an issue, but it's been years and I don't remember the alloy. I hate to turn down 3/4 to get 3/8, but maybe that's next.
The carbide insert holder holds it level- zero rake. The sides look to have plenty of relief- looks like about 20*.
Technique - I thought I knew how to use the thread indicator dial on the lathe, but when I opened the half nuts, backed out and reset the cross carriage, waited until the dial came around to the SAME number as when I started, the second pass was way off. Like halfway between the first threads! And it only got worse.
I finally resorted to the European way, leave the half nuts engaged, stop, back out cutter, reverse lathe. That at least got the second pass in line with the first.
I am using the compound to feed the tool, and I have it set at 30*.
I do have diamond stones, but have not done anything to the carbide inserts. That's the whole point isnt't it?!
If there's a special insert for threading I expect that's what I need. I'll check the Enco catalog.

Good thing this isn't a time-critical project!

 

[Shadowdog500]

Thanks for all the replies. I'm learning more about how little I know! To answer some questions:
My lathe is a 25 year old 9x20 Buffalo (the Taiwan variety) with change gears.
For tools I use indexable carbide almost exclusively. I only use HSS when doing small inside turning. The insert is Fansteel 09906181. Triangular, TPG 221. It has quite sharp points, but with a definite very small radius. Maybe 1/64? I got this batch some years ago and really don't remember what it was supposed to be for - but I generally try to get something like this in a kind of middle-of-the-road variety rather than have a dozen types.
For stock, I've tried using a plain hardware store bolt , and a piece of 3/8 rod out of a lawn chair. I have some 3/4" shaft stock from Enco that I got for an axle where strength was an issue, but it's been years and I don't remember the alloy. I hate to turn down 3/4 to get 3/8, but maybe that's next.
The carbide insert holder holds it level- zero rake. The sides look to have plenty of relief- looks like about 20*.
Technique - I thought I knew how to use the thread indicator dial on the lathe, but when I opened the half nuts, backed out and reset the cross carriage, waited until the dial came around to the SAME number as when I started, the second pass was way off. Like halfway between the first threads! And it only got worse.
I finally resorted to the European way, leave the half nuts engaged, stop, back out cutter, reverse lathe. That at least got the second pass in line with the first.
I am using the compound to feed the tool, and I have it set at 30*.
I do have diamond stones, but have not done anything to the carbide inserts. That's the whole point isnt't it?!
If there's a special insert for threading I expect that's what I need. I'll check the Enco catalog.

Good thing this isn't a time-critical project!

Any way you can post a video of what is going on.

When I first started cutting threads stuff like this would happen to me too. I thought I was doing everything right but I really wasn't. If you post a video perhaps some here can figure out what it going on.

There are rules on what number you can catch (ie even odd etc.)based on the tpi you are cutting. I usually just use 1 for all of my cuts so I have one less thing to remember.

Any chance you have a metric lead screw installed. I understand from reading about my mini lathe that the Asian lathes usually come with either an imperial or metric leadscrew based on where you live. If yours has a metric leadscrew installed that may be your problem. Try a gear combo for a metric thread and see if you can hit the same thread every time while engauging the half nuts.

Chris