Internal Threading Problems: Carriage Moves

[Matt in TN]

I've done internal threads once before with no issues. I went to do it again last week and it didn't go so well, and I can't figure out what I'm doing differently. Material is 4130 prehard steel, thread is 1"-16. I turned the ID to the listed minor diameter in the Machinery's Handbook, and then started threading. I set the compound to the same angle I use for external threading and crank the compound OUT 0.005" each pass instead of cranking it IN (for external threading). I was running 50 rpm. I set the tool tip height so a 6" scale was exactly vertical when pinched between the tool tip and the OD of the part.

I noticed the carriage "jumps" when the first thread engages, and then the tool just follows the existing thread instead of cutting a new path. In later investigation, I found when I pull up on the front of the carriage there is significant vertical free play between the carriage and the ways. There is no such free play when I pull up on the back of the carriage.

Is this a machine problem (South Bend 9A), or a technique problem? What am I not doing right?

 

[bill stupak]

do you have enough clearance on the cutting tool, maybe it's rubbing

bill

 

[dbassing]

You said that you set the compound to the same angle as when you thread externally. When threading internally the compound is set so that the handle is on left side. External the handle is to the right. Same angle different direction of feed.

 

[TomS]

I've done internal threads once before with no issues. I went to do it again last week and it didn't go so well, and I can't figure out what I'm doing differently. Material is 4130 prehard steel, thread is 1"-16. I turned the ID to the listed minor diameter in the Machinery's Handbook, and then started threading. I set the compound to the same angle I use for external threading and crank the compound OUT 0.005" each pass instead of cranking it IN (for external threading). I was running 50 rpm. I set the tool tip height so a 6" scale was exactly vertical when pinched between the tool tip and the OD of the part.

I noticed the carriage "jumps" when the first thread engages, and then the tool just follows the existing thread instead of cutting a new path. In later investigation, I found when I pull up on the front of the carriage there is significant vertical free play between the carriage and the ways. There is no such free play when I pull up on the back of the carriage.

Is this a machine problem (South Bend 9A), or a technique problem? What am I not doing right?

Are you cutting a right hand or left hand thread? If you are cutting a right hand thread and the compound is set the same as an external thread your tool is cutting, or trying to cut, on the trailing edge. What's happening is the tool pressure is taking up the slack in your half nut and pushing your carriage forward.

Tom S

 

[Matt in TN]

Are you cutting a right hand or left hand thread? If you are cutting a right hand thread and the compound is set the same as an external thread your tool is cutting, or trying to cut, on the trailing edge. What's happening is the tool pressure is taking up the slack in your half nut and pushing your carriage forward.

Tom S

This must be it - I'll try again with the compound handle on the left. I guess I just got lucky with my first attempt. Thanks!

So the vertical play in the front of the carriage is normal and nothing to worry about?

 

[TomS]

This must be it - I'll try again with the compound handle on the left. I guess I just got lucky with my first attempt. Thanks!

So the vertical play in the front of the carriage is normal and nothing to worry about?

Check to see if you have a sliding gib on the front. If you do there may be shims between the gib and the underside of the bed. Taking shims out will snug it up a bit. If you don't have any shims then you will need to machine the gib to reduce the clearance. I'd shoot for .003" to .005".

Hope this helps.

Tom S

 

[Jeff M]

A couple of things:
1) Try thighening the brake on the carriage to make sure it is pulling and not floating in the slack when you start.
2) I find it much easier to cut an internal thread with the chuck running backwards and feeding from the inside out. You can cut some thread relief in the bottom of the hole, with the tool retracted move to a position that will let you start in the area of the relief, move in to your "zero/starting point" with the spindle stopped. Put the lathe in neutral, turn the chuck to engage the half nut in the proper orientation, then start the spindle. You can take a finish cut at pretty high rpm's to get a good finish.
One other thing to note: I think .005" infeed for each pass is a bit much. I usually only infeed .002-.003" and I do an air pass after every few passes. Your cutting oil can also be a factor. Make sure you are using something made for carbon steel.

 

[Jeff M]

I forgot to mention that (and it is probably obvious) the tool will be facing the back of the part if you thread in reverse from the inside coming out. the compound should be 29º to your left. 1/2 degree into the trailing edge.

 

[davidh]

i just turned some 15/16 - 16 tpi parts. I left the 29 Degree compound angle the same as for an outside thread and turned the tool upside down and cut from right to left on the far side of the bore. that worked fine for me.
also, possibly you could compensate your tool height for the "up-lift" your experiencing. . if your cutting on the near side, lower the tool bit a bunch, maybe start at .010 below center and keep dropping it until it either works or not. . .
if your cutting the far side, upside down, raise the tool bit the same .010.
I'm sure your not the first one that needed to compensate for a bunch of wear. . . . . . .

 

[Bill C.]

For curiosity sake with the lathe power off engage the half-nuts and see how much travel there is when you grab the handwheel and move it forward and backwards. If there is a lot might want to add spacers to reduce the slack, just a thought.

Good luck