Threading Project Causing Brain Damage

Hi All,

I am seeking some help with a small threading job on a manual lathe. I am trying to externally thread a piece of 2" OD DOM with 20 TPI and I an having no success which could be as simple as the compound set angle.

I should mention that I am a hobby machinist, although I have been playing around for 20+ years.

I have turned a recess down to the minor diameter of 1.9387 per the 3A fitment shown in the Machinery's Handbook with the intention of cutting the threads to the depth of the minor diameter (tool bit just touching the previously cut recess). Everything on the lathe is set up as usual with the compound being set at 30 degrees, however this is a new 14" x 40" South Bend made in Taiwan and I have not yet cut threads so I am not certain what angle it should be set at such as 30 or 60 degrees. Now the problem. When the depth of cut gets close to the minor diameter of the recess, I have already lost the of major diameter of 2" and the threads are not formed correctly.

I thought to remove the tool bit and align one of the cutting edges with the edge of the compound to see if it was pointing parallel to the faced edge of the part. To my surprise it was not and in order to achieve this, the compound has to be set to 45 degrees. I have not attempted to re-cut the part yet and wonder if anyone has experienced this or has a suggestion.

Thank you,

Steve

The compound angle is not critical. In fact, you can thread using the cross-feed straight in and ignoring the compound altogether. It is important that the axis for the thread pass feed fall within the flank angles of your thread. So, +-30 degrees off perpendicular to your work centerline, for a standard 60 degree thread form.

It is critical to align the tool properly. In practically every situation, that means the centerline of the threading tool profile is perpendicular to the centerline of the work. It sounds like you are rotating the compound slide axis to align your tool. Don't do that, rotate the toolpost instead! If you have a non-rotatable toolpost, then, under the assumptions that (1) the toolpost slots are parallel with the compound slide axis and (2) you are using a "straight" toolholder not an angled one, set your compound perpendicular to the work centerline.

"The threads are not formed correctly". If what way are they not correctly formed? Asymmetric thread profile? Jagged or stair-stepped thread flanks? A closeup photo would be worth 1000 words here, and would help us a lot to diagnose this.

Are you using a full profile insert? If you used an insert for a finer therad that could cause loss of the OD before the tip of your tool reaches the minor diameter.

If you use the compound, point the handle at your bellybutton and call that zero, whatever it may read. Now turn it counterclockwise by 29 or 29.5 degrees. Done. Now, set the threading tool perpendicular to the part.

Depth is a problem because you don't know the size of the flat or radius on the tool tip. Ideally you should do a 3-wire measurement or fit to a nut. Touching the land may or may not get you what you need.

Be sure the tool has sufficient clearance angle on the leading side and is sharp. Use oil.

Pictures would help, otherwise it's just guessing.

R

sfriedberg said:

The compound angle is not critical. In fact, you can thread using the cross-feed straight in and ignoring the compound altogether. It is important that the axis for the thread pass feed fall within the flank angles of your thread. So, +-30 degrees off perpendicular to your work centerline, for a standard 60 degree thread form.

It is critical to align the tool properly. In practically every situation, that means the centerline of the threading tool profile is perpendicular to the centerline of the work. It sounds like you are rotating the compound slide axis to align your tool. Don't do that, rotate the toolpost instead! If you have a non-rotatable toolpost, then, under the assumptions that (1) the toolpost slots are parallel with the compound slide axis and (2) you are using a "straight" toolholder not an angled one, set your compound perpendicular to the work centerline.

"The threads are not formed correctly". If what way are they not correctly formed? Asymmetric thread profile? Jagged or stair-stepped thread flanks? A closeup photo would be worth 1000 words here, and would help us a lot to diagnose this.

Click to expand...

I never had a problem threading previously, but as I said, new lathe so things may be a bit different. I started by setting the compound to 30 degrees and then aligning the tool to be square with the face as usual using the QC tool post. The thread form is stair stepped when completed. I do realize that I can use the cross feed for threading which uses both sides of 30 degree form tool and I can disregard the angle of the compound, but I was trying to use the compound. I am not sure what is happening here, but I will continue to work on it.

Thank you for your response.

Steve

Illinoyance said:

Are you using a full profile insert? If you used an insert for a finer therad that could cause loss of the OD before the tip of your tool reaches the minor diameter.

Click to expand...

I am using a 60 degree carbide insert. I don't recall the radii at the end, but it is really pointy and I have used the same one previously on mu last lathe without issue.

Thank you,

Steve

Conrad Hoffman said:

If you use the compound, point the handle at your bellybutton and call that zero, whatever it may read. Now turn it counterclockwise by 29 or 29.5 degrees. Done. Now, set the threading tool perpendicular to the part.

Depth is a problem because you don't know the size of the flat or radius on the tool tip. Ideally you should do a 3-wire measurement or fit to a nut. Touching the land may or may not get you what you need.

Be sure the tool has sufficient clearance angle on the leading side and is sharp. Use oil.

Click to expand...

I think that my bellybutton is off a few degrees too. but I do get your point. I do think that it is time for some pee dee wires.

Thank you,

Steve

If the threads are stepped on back side then you are feeding at an angle greater than 30deg.Any angle less than 30 deg will tend to cut on both sides of the tool.when you feed straight in then it will tend to cut equally on both sides, however the front will be positive and the back will be negative which will tend to push the tool.
That is the reason for feeding slightly less than 30 deg ,to give a shaving cut on the backside and not leave steps;greater than 30 deg steps the tool away from the backside of the thread.

Sounds like you have the compound at the wrong angle. Conrad summed it up, compound straight in, parallel to the cross slide is zero, no matter what the numbers say. Rotate 29* from there.
If you have it 29* off parallel to the bed, then it is advancing the tool too much in the Z axis as you feed in, causing the steps you are seeing in the trailing side of the thread.

Mr. Fixit said:

I am using a 60 degree carbide insert. I don't recall the radii at the end, but it is really pointy and I have used the same one previously on mu last lathe without issue.

Thank you,

Steve

Click to expand...

But is it for threading or just a 60 degree triangle insert? The later has no helix angle and if not positive rake with high relief it could be rubbing. Use a threading insert or or simply grind a suitable HSS tool.

14x40 and 20tpi? I'd just feed it "straight in". i.e. use your cross-feed and ignore the compound.

I believe that the compound on your lathe is indexed the same way my 13X30 South Bend 0* is parallel to the bed and 60* is 30* from perpendicular, in other words set the compound to 60*

I totally agree. I hate it when these questions come up and we can not be sure just what is being done or just what the results are.

"I am not certain what angle it should be set at such as 30 or 60 degrees."
Some lathes are made with the scale printed one way and others are made the other way. The key idea is that the angle of the compound is 30 or 29.5 degrees from the angle of the cross feed. The cross feed goes straight in or at 90 degrees to the main axis of the lathe. The compound would be set 30 degrees from that line regardless of the markings on any scale.

"When the depth of cut gets close to the minor diameter of the recess, I have already lost the of major diameter of 2" and the threads are not formed correctly."
Just what is lost? Do you mean you have cut below the major diameter, perhaps leaving a burr? Was the tube only 2" in diameter to begin. Or was it somewhat oversize? Did you cut it down to the proper 2" or actually just a few thousandths less (2.0000" - 1.9904" according to my copy of MH)? If you have the proper minor diameter and if your tool has the correct geometry (60 degrees and the proper flat at the tip) than you should not cut below the major diameter when the minor diameter is reached by the tip of the tool.

"I thought to remove the tool bit and align one of the cutting edges with the edge of the compound..."
Align which cutting edge? A cutting edge of that tool but that you removed? And align it with what edge of the compound? I find this hard to envision. A photo would really help here. Or a sketch.

I mentioned a possible burr above. Almost every thread that I have cut with a single point tool has had a burr and I had to remove that burr before I could judge or measure that thread. I usually use a brass brush for that. You can not judge a thread with a burr. You must deburr it after EVERY pass of the tool if you are going to measure or judge anything.

The other thing that I can think of is that you have the compound set to the wrong angle. Read my description of the proper angle above.

litlerob1 said:

Pictures would help, otherwise it's just guessing.

R

Click to expand...

I seriously doubt that he has a rubbing problem with a 2" - 20 TPI thread. Even if it is just a 60 degree insert, it would need to have almost zero clearance angle to rub. The rake angle has almost nothing to do with it. I would not worry about rubbing.

Lakeside53 said:

But is it for threading or just a 60 degree triangle insert? The later has no helix angle and if not positive rake with high relief it could be rubbing. Use a threading insert or or simply grind a suitable HSS tool.

14x40 and 20tpi? I'd just feed it "straight in". i.e. use your cross-feed and ignore the compound.

Click to expand...

If OP is using a triangle turning insert the radius is WAY too large for that fine a thread. He needs the experience of grinding an HSS
threading tool.


As to setting the angle of the compound with the degree marks stamped into the lathe consider this: Some lathes read zero when the compouns is parallel with the bed;others will read 90.The simplest way to remember how to set the compound is to set it parallel with the trailing flank of the thread.

EPAIII said:

I seriously doubt that he has a rubbing problem with a 2" - 20 TPI thread. Even if it is just a 60 degree insert, it would need to have almost zero clearance angle to rub. The rake angle has almost nothing to do with it. I would not worry about rubbing.

Click to expand...

0.463 degree lead angle.. Definitely not a problem.

No one has told him to get a thread gauge, the little piece of sheet metal with 60 degree notches that you use to align the bit.

Feeding straight in with the cross slide can work but it produces two chips that try to occupy the same space at the same time with the obvious results.

It sounds like he is cutting a butress thread because the tool is at the wrong angle. All this is lathe operation 101, not some deep mystery.

Bill

A friend just starting out called me over to look at why his thread was not turning out right. His compound was set at 30 and it looked like a butress thread. Turned out it needed to be set at 0. I think it was one of those unmentionable machines. Hope this helps.

OP, lets keep this simple, you need one of these;



Though it isn''t the first process, you just need one. (I'm not referring to the Threading tool)

Then put the Compound wherever you want it, put it backwards if you want,(2"-20 is about .03" deep. no big deal) but align the Tool the way it shows in the picture.

If your still getting a shitty Thread set-up the compound to 30º or less. This should be very easy regardless of what the Scale on the Machine says. It's going to say 0º (which would asinine) 30º or 60º, it should be very obvious which is correct, it'll be the same Angle as the Tool. Then re align the actual Threading tip like in the picture.

In the OP it says it's from DOM, you did turn the pipe to the correct OD right? 1.9985-1.9904"

R

Thank you all for your responses, very helpful in getting me to think a bit more about it. I am using a threading insert, not just a triangle insert and I do use the thread gauge when setting up. What I believe the issue is (although I have not yet had the time to re-cut) is the fact that the lathe strike mark on the top of the carriage is facing dead forward. When you swing the compound to what is really 30 degrees from the handle facing forward, the strike mark is covered completely. Soooo, I think that I was setting it at 60 degrees previously when the mark was exposed and I had it set to 30 on the scale. The way the scale reads around the bottom of the compound, the 0 mark puts the compound either forward and aft, or side to side. In essence, the mark that is struck on top of the carriage is useless for threading (I think) Maybe the attached photo will help. Unfortunately I won't be able to run the machine until Friday due to work. Thank you all again..

Steve