Gunsmithing Need help with threading on lathe (UPDATE)

[cattleman99]

I’ve been working on threading some muzzles, however my threads come out with a forward angle towards the muzzle, rather than straight up and down. Any machinists know what I can try differently?

29.5 degrees on the compound, tool squared to the work piece via fish gauge.

Threading 1/2-28

 

[Mordamer]

I'm guessing your compound is set at 60 degrees instead of 30. Some lathe compounds reference the angle with 0 degrees being with the compound feeding straight into the work. Other lathes are referenced 90 degrees from that where 0 on the compound feeds straight along the bed.

 

[Mordamer]

Is your compound pointed more at the Chuck or more across the bed? It should be pointed more across the bed.

 

[Alabusa]

Looks like you are feeding in with the compound???

 

[Danny1788]

Pictures are worth 1000 words.

 

[Supersubes]

Certainly looks like the tool angle is wrong. Shoot a pic of your setup.

 

[cattleman99]

It’s one of those compounds where 0 degrees is feeding straight into the work. I have it set just a hair past 60 degrees on the compound. Yes, I’m feeding in on the compound.

 

[cattleman99]

 

[Supersubes]

It’s one of those compounds where 0 degrees is feeding straight into the work. I have it set just a hair past 60 degrees on the compound. Yes, I’m feeding in on the compound.

What cutting tool are you using, inserts or ground HSS?

 

[cattleman99]

This picture is just showing the compound angle. The tool post and tool are both perpendicular to the work piece when it’s used.

 

[cattleman99]

What cutting tool are you using, inserts or ground HSS?

Carbide inserts.

 

[Supersubes]

I stick a magnetic base on the back foot of the carriage and run a test indicator on the tool itself(not the holder it’s clamped in). When you run the cross feed back and forth, the tool should be perfectly perpendicular. More accurate than a fish gauge. If that’s correct, your insert holder may be bad.

 

[cattleman99]

I stick a magnetic base on the back foot of the carriage and run a test indicator on the tool itself(not the holder it’s clamped in). When you run the cross feed back and forth, the tool should be perfectly perpendicular. More accurate than a fish gauge. If that’s correct, your insert holder may be bad.

i'll do that shortly. Even if the insert holder is bad, if the insert is square to the work, then there shouldn't be an issue should there?

 

[Supersubes]

i'll do that shortly. Even if the insert holder is bad, if the insert is square to the work, then there shouldn't be an issue should there?

By bad I mean not holding the insert correctly. So ya, if the insert is square, and the compound is set properly, you shouldn’t have an issue.

Are you just starting, or did you change something to cause the skew?

 

[1moaoff]

Tool post or tool holder angled?

 

[cattleman99]

By bad I mean not holding the insert correctly. So ya, if the insert is square, and the compound is set properly, you shouldn’t have an issue.

Are you just starting, or did you change something to cause the skew?

Just checked runout on the tool while it was in the holder. I pushed it clear to the back of the holder so it was square against it, as I tighten the set screws down it tilted a bit. 17 thou taper across the length of the tool itself.

 

[cattleman99]

Tool post or tool holder angled?

Tool post and tool holder were both squared against the chuck using 1-2-3 block, then the insert was double checked against work piece with fish gauge.

 

[cattleman99]

By bad I mean not holding the insert correctly. So ya, if the insert is square, and the compound is set properly, you shouldn’t have an issue.

Are you just starting, or did you change something to cause the skew?

Just got this lathe recently and just now found the time to do some work on it

 

[Danny1788]

What tool holder is it? Are you saying the tool is rocking to one side and now it’s not flat?

 

[cattleman99]

What tool holder is it? Are you saying the tool is rocking to one side and now it’s not flat?

It is a tool holder from Grizzly. I can feel and see the tool rock when I tighten the set screws, as if its not sitting level on the bottom of the tool holder. This only happens when it is pushed all the way into the tool holder as it should be.

 

[Supersubes]

It is a tool holder from Grizzly. I can feel and see the tool rock when I tighten the set screws, as if its not sitting level on the bottom of the tool holder. This only happens when it is pushed all the way into the tool holder as it should be.

Can you take a couple of pics of the tool holder and tool?

 

[cattleman99]

Compound was at 60.5, I just loosened the bolts temporarily*

 

[J McGriddle]

Inserts usually have a range or are pitch specific. Double check that you have the right insert for that pitch.

 

[cattleman99]

Inserts usually have a range or are pitch specific. Double check that you have the right insert for that pitch.

Range of these inserts is 16-48 I believe. Quite a large range

 

[Alabusa]

Yes, I’m feeding in on the compound.

There is your problem. With the compound on an angle and feeding with it, you are moving the insert laterally as well as into the part. Feeding with the cross slide moves the tool in a straight line perpendicular to the part. Feed with the cross slide instead of the compound and your threads will be the same on both sides.

Makes sense?

 

[cattleman99]

There is your problem. With the compound on an angle and feeding with it, you are moving the insert laterally as well as into the part. Feeding with the cross slide moves the tool in a straight line perpendicular to the part. Feed with the cross slide instead of the compound and your threads will be the same on both sides.

Makes sense?

ill give that a go tomorrow and we'll see how that works.

 

[J McGriddle]

Unless I'm missing something, shouldn't your compound be set on 29.5 degrees? Your lead angle looks correct because it is formed by the insert, your trailing angle looks off because you are moving in at 60 instead of 30.

 

[Supersubes]

There is your problem. With the compound on an angle and feeding with it, you are moving the insert laterally as well as into the part. Feeding with the cross slide moves the tool in a straight line perpendicular to the part. Feed with the cross slide instead of the compound and your threads will be the same on both sides.

Makes sense?

Many many people thread this way. If the setup is correct, youre feeding along the right flank, and only cutting along the left flank and tool tip. This significantly reduces cutting loads.

You can feed straight in if you like, but the compound infeed as a technique is not his problem.

 

[cattleman99]

Unless I'm missing something, shouldn't your compound be set on 29.5 degrees?

Its labelled differently than others

 

[cattleman99]

There is your problem. With the compound on an angle and feeding with it, you are moving the insert laterally as well as into the part. Feeding with the cross slide moves the tool in a straight line perpendicular to the part. Feed with the cross slide instead of the compound and your threads will be the same on both sides.

Makes sense?

If I feed in with the cross slide, then what is the purpose of setting the compound to 29.5 degrees?

 

[Supersubes]

Unless I'm missing something, shouldn't your compound be set on 29.5 degrees?

Look at the way his machine is marked.

 

[Supersubes]

If I feed in with the cross slide, then what is the purpose of setting the compound to 29.5 degrees?

It wouldn’t matter, it can be at any angle you like because you would never touch it while threading.

The rest of the setup is the same.

 

[Mordamer]

There is your problem. With the compound on an angle and feeding with it, you are moving the insert laterally as well as into the part. Feeding with the cross slide moves the tool in a straight line perpendicular to the part. Feed with the cross slide instead of the compound and your threads will be the same on both sides.

Makes sense?

He should absolutely be able to feed in with the compound without a problem. That is the whole point of setting it to 29.5 degrees. Feeding with the compound is considered the ideal method and results in less tool pressure. Feeding with the cross slide works, but is using the threading insert as a form tool and will result in higher tool pressure, most likely worse surface finish, and is more likely to break the carbide insert. Feeding with the compound is thread cutting 101.

 

[FCrifles]

Your thread picture sure look like your compound is off. Have you checked it with a machinist protractor?

 

[Driftr76]

^^^^
Agreed

 

[cattleman99]

Your thread picture sure look like your compound is off. Have you checked it with a machinist protractor?

I have not. As of now I’m going off my compound, which seems to be at 29.5 (60.5)

 

[J McGriddle]

Calculating your rise over run and indicating the tool is a surefire way to know you are setup on the exact angle.

 

[J McGriddle]

This could be the problem. Your toolpost should be perpendicular to the bore axis.

 

[cattleman99]

This could be the problem. Your toolpost should be perpendicular to the bore axis.View attachment 7115614

It is, as mentioned above that was taken at a different time, just to show compound angle. It is perpendicular to bore axis via 1-2-3 blocks on the between the chuck and tool post

 

[Praeger]

View attachment 7115540
Compound was at 60.5, I just loosened the bolts temporarily*

Your tool holder looks like it is sitting directly on top of the compound slide. Did you center the insert to your spindle bore? If not, a quick check is to insert a live/dead center on your tailstock and slide it over to your cutter and compare height. The point of the center and the top of your insert should be the same height.

If the cutter is too low, it may affect the thread form because the thread is a helix shape.

Also, using a three jaw chuck to practice is fine. Just make sure to use a four jaw chuck and indicate the barrel by the bore when you are actually threading a barrel.

 

[cattleman99]

Your tool holder looks like it is sitting directly on top of the compound slide. Did you center the insert to your spindle bore? If not, a quick check is to insert a live/dead center on your tailstock and slide it over to your cutter and compare height. The point of the center and the top of your insert should be the same height.

If the cutter is too low, it may affect the thread form because the thread is a helix shape.

Also, using a three jaw chuck to practice is fine. Just make sure to use a four jaw chuck and indicate the barrel by the bore when you are actually threading a barrel.

It’s not bottomed out on the tool post. There’s a good gap, it’s centered. I use a 4 jaw chuck for threading muzzles, 3 jaw for practice because it’s fast.

 

[Mordamer]

Couple more questions:

Is there any chance you accidentally got carbide inserts for a 55 degree whitworth thread or some other thread besides a 60 degree profile?

When feeding in on the compound each pass should primarily produce a chip only on the left cutting face of the insert. Is this what you are observing?

 

[cattleman99]

Couple more questions:

Is there any chance you accidentally got carbide inserts for a 55 degree whitworth thread or some other thread besides a 60 degree profile?

When feeding in on the compound each pass should primarily produce a chip only on the left cutting face of the insert. Is this what you are observing?

The inserts say they’re for 60 degree threads. I’d have to check on chip load this afternoon if I make it to the shop

 

[Rubicon Precision]

Every time I have seen this, was because people were set up 60* from perpendicular, rather than 30*, due to the way the compound is labeled.

It’s the only way I can think of that would cause this.

 

[smoooth1]

Could be wear / play in the lead screw, cross-slide, or in the screw on the compound?

Push and pull on the toolpost to see if you can move anything. May be slight movement, maybe significant?
Something is definitely changing your pitch along that short distance!

Be sure the screws and slides are all clean and free of any chips or build up that could cause deflection. Same goes for the gear box.

Best of luck!!

 

[smoooth1]

You did mention that you could see the insert move as it was tightened.
Wondering if the INSERT is moving under load pressure?? That could be throwing the pitch off too.

 

[RonA]

Looks like something is moving.

 

[cattleman99]

You did mention that you could see the insert move as it was tightened.
Wondering if the INSERT is moving under load pressure?? That could be throwing the pitch off too.

The tool bar moved, not the insert. It couldn’t be moving under load but I’m not sure how I’d check that

 

[JBoomhauer]

How many passes? Should have pretty light tool pressure, certainty not enough to move anything.

 

[cattleman99]

How many passes? Should have pretty light tool pressure, certainty not enough to move anything.

5 thou cuts, at any number of passes