Threads cocked?

[rock_breaker]

I am trying to cut 7/8" X 14 TPI in a blind hole in a 1 1/2" aluminum rod. I have cut the external threads on a piece of mystery metal that fit a factory nut closely-can be run on easily. I am using a 4 way tool holder to hold the boring bar that has the cutting tool in the 45` end. I ran the carriage up against a stop on the way and used # 1 only on the thread dial. I did not cut a relief groove as I thought the bit would do that with each pass. The work piece was turned in forward direction with the cutter upside down on the far side of the hole. The carriage ran left to right. When I finally got to size the "bolt" was at an angle. The threads were oversize and when I got the bolt in more squarely it was still out of line.
The things I will do differently on the next try are: 1.Cut a relief groove 2.Chamfer the bolt and the hole 3.Blow the cuttings out after each pass 4. Try putting some lubricant in the hole. Perhaps this post belongs in the goof and blunders dept. but I still don,t understand why the "bolt" would not go in square. Any suggestions will be appreciated.
Ray

 

[Dave Paine]

If I am understanding what you wrote, if the lathe was rotating in the normal "Forward" rotation, moving the carriage starting at the end of the hole and moving it to the right you may have cut a left hand thread. The cutter being mounted upside down on the far side just all better visibility of the cut.

I think if you want to start at the end of the hole and feed to the right you would need to run the lathe in "Reverse" rotaion.

 

[rock_breaker]

If I am understanding what you wrote, if the lathe was rotating in the normal "Forward" rotation, moving the carriage starting at the end of the hole and moving it to the right you may have cut a left hand thread. The cutter being mounted upside down on the far side just all better visibility of the cut.

I think if you want to start at the end of the hole and feed to the right you would need to run the lathe in "Reverse" rotaion.

I thought about this several times and will recheck the threads. Looking at the bolt and the internal threads it seemed the bolt matched the nut. Thanks for this tip, am on my way to the shop to give it a go.
Ray

 

[brino]

I am using a 4 way tool holder to hold the boring bar that has the cutting tool in the 45` end.

Wait.....do you mean a 60 degree threading tool held at 45 degrees?
If so, that will NOT present the proper cutting tool angle to the work.
Your internal threads will not be symmetric, but will be a non-standard buttress thread.
https://en.wikipedia.org/wiki/Buttress_thread

-brino

 

[Technical Ted]

I also think you need to run the lathe in the reverse direction. I think you cut a left hand thread. Joe Pie has a couple of YouTube videos showing how he does this. Here's one:

Ted

 

[Cadillac]

Yes you probably did a left hand thread. You can run the tool upside down on the operator side but will need to reverse spindle direction. You can also use a left hand tool cutting on the far side of id with spindle in reverse.
You want to make sure you angles are flipped so your not rubbing the tail end of cut.
The joe pie video is very informative too.

 

[rock_breaker]

Wait.....do you mean a 60 degree threading tool held at 45 degrees?
If so, that will NOT present the proper cutting tool angle to the work.
Your internal threads will not be symmetric, but will be a non-standard buttress thread.
https://en.wikipedia.org/wiki/Buttress_thread

-brino

Hi brino,

Some time back I cut some internal threads using the 45` end of my boring bar so that I could get close to end of the hole. Prior to grinding the 1/4" HSS cutting bit I drew a schematic of the boring bar then the two faces of the proposed 60` cutting shape long enough that I could measure with a protractor.
At the bench grinder I approximated the angles required; the right side is not to difficult 45` off the end I believe, the left side requires grinding into the side of the cutter bit. 15`, don't hold me to these angles as I am redoing this from memory. With the two sides ground close I then slide the cutting tool in the boring bar. Using the thread gage against the boring bar and the new point in one of the "Vs" on the side of the gage I could see what corrections had to be made. This is much like aligning the cutting tool on an external thread, it is a crude approximation but I have done this on several occasions without failure of the threads. I do calculate the depth of cut and test the cuts with a "bolt" when nearing the full depth of the thread. Of course relief has to be ground as with any HSS cutting tool.
Have a good day
Ray

 

[rock_breaker]

If I am understanding what you wrote, if the lathe was rotating in the normal "Forward" rotation, moving the carriage starting at the end of the hole and moving it to the right you may have cut a left hand thread. The cutter being mounted upside down on the far side just all better visibility of the cut.

I think if you want to start at the end of the hole and feed to the right you would need to run the lathe in "Reverse" rotation.

Yep! I have oversize left hand threads. That's what I get for thinking.
Ray

 

[RJSakowski]

You cannot cut a left hand thread, either external or internal, without adding an idler gear to the gear train to reverse the direction of rotation of the lead screw relative to the spindle.

 

[kd4gij]

You cannot cut a left hand thread, either external or internal, without adding an idler gear to the gear train to reverse the direction of rotation of the lead screw relative to the spindle.

His MK3 like my craftsman has a reverse tumbler gear. You can run the lead screw in forward or reverse just by moving a lever.