Thread cutting on the lathe ??

[welder]

I have several projects that require thread cutting but I cannot seem to grasp how to cut a thread on the lathe. I have watched all the videos I could find and talked to the guys at at work about it but I still cannot grasp it the threads always end up cross threaded. I will practice on the weekend when I'm in the shed it's driving me crazy as everyone I've talked to said it's easy

[Ueee]

Hi Andre,
Firstly, is your leadscrew metric or imp, and what sort of thread are you cutting? Are you using the threading dial or reversing the machine? Sounds to me like your not using the thread counter properly if you are using it, or if it is a metric leadscrew you have the wrong gear on the counter. I have a thread counter in a box somewhere. That is where it stays....i just reverse the lathe (i have a metric screw)

[Anorak Bob]

Andre,

I taught myself by reading Peter Hercus' Text Book of Turning which is similar to South Bend's instruction manual. If you are interested I'll post you my South Bend manual. Let me know.

Once you work out the basic procedure it's easy.

BT

[morrisman]

I have several projects that require thread cutting but I cannot seem to grasp how to cut a thread on the lathe. I have watched all the videos I could find and talked to the guys at at work about it but I still cannot grasp it the threads always end up cross threaded. I will practice on the weekend when I'm in the shed it's driving me crazy as everyone I've talked to said it's easy

It's not easy for a learner . I'm still not that confident about doing it.

Above everything else You need to concentrate and re check everything before you go ahead . It only takes one little slip up and it wrecks everything .

The HSS tool needs to be as sharp as you can get it with a nice edge .

I use neatsfoot oil on thread cutting , horse shops sell it for applying onto saddles .

Sounds like your not picking up the thread if its cross threading on you .

The tool needs to be very rigid in the post , you don't want any deflection or movement of the top slide or the post . Are you plunge cutting or doing the 29 degree compound way ?

MIKE

[welder]

The thread I am cutting is a 3/4" 10 tpi my lathe has an imperial lead screw and the thread chasing dial has no witness mark so I just left the lead screw engaged. I am plunging the tool not feeding with the compound.

[Ueee]

The thread I am cutting is a 3/4" 10 tpi my lathe has an imperial lead screw and the thread chasing dial has no witness mark so I just left the lead screw engaged.

Well FM......i don't know where your problem is, unless you have some slip in the geartrain. Do you have the compound set to 30deg, and are you using the cross slide to back the tool out to reverse and then back in, advance the cut with the compound and cut?
BTW, your not cross threading the screw, you making multi lead threads.....
Did you add the compound remark or did i miss it. You are far better off IMO using the compound.....but that leaves even less to screw up so i really have no idea......

[morrisman]

The thread I am cutting is a 3/4" 10 tpi my lathe has an imperial lead screw and the thread chasing dial has no witness mark so I just left the lead screw engaged.

The normal method is to engage the half nuts for a pass with the dial mark lined up on the edge mark. . Then after a cutting pass you disengage the half nuts and return the saddle to the dial chaser mark ... If its an even TPI you want to cut ( 2,4,6,8. tpi ) , you can engage the half nuts on any mark on the dial . MIKE

[Jekyll and Hyde]

The thread I am cutting is a 3/4" 10 tpi my lathe has an imperial lead screw and the thread chasing dial has no witness mark so I just left the lead screw engaged. I am plunging the tool not feeding with the compound.

If you're leaving the halfnuts engaged, are you withdrawing the tool far enough (to clear the part) on the return journey? And on the return journey, are you going far enough past the start of the thread that when you start your next cutting pass all the backlash is taken up before the tool engages the cut?

If the answer to both of those is yes, then my guess is that the part is moving in the chuck, or the tool/holder is moving....

*EDIT* whoops, one more - you're not, by any chance, changing spindle speeds around whilst threading? Like maybe using a higher speed to reverse, or selecting a higher speed part way through cutting the thread? That will also cause you to lose the thread, don't change the speed once started.

[Ueee]

Is you lathe reversible or are you turning it by hand? (often i turn the chuck (well drive belts) by hand for short threads) And, are you using a back gear?

[Stustoys]

I dont understand the "cross threaded". No matter where the leadscrew is picked up the thread pitch should still be the same. maybe I am taking "cross threaded" to literally?

*EDIT* whoops, one more - you're not, by any chance, changing spindle speeds around whilst threading? Like maybe using a higher speed to reverse, or selecting a higher speed part way through cutting the thread? That will also cause you to lose the thread, don't change the speed once started.

Shouldnt do. The leadscrew gear train comes off the spindle.

Stuart

[welder]

By "cross threaded I mean the tool cuts off the crest of the thread I will get some photos of my set up on Friday which will hopefully explain my problem.

[shedhappens]

welder how tight is the gib on the compound ?

Maybe the compound screw is turning slightly as you are cutting the thread, I spoze you would be looking at the thread and not notice it if that is the problem ?

[Jekyll and Hyde]

I dont understand the "cross threaded". No matter where the leadscrew is picked up the thread pitch should still be the same. maybe I am taking "cross threaded" to literally?

Stuart

I took it to mean, as someone mentioned earlier, more like a multiple start thread, which I think is what Andre just confirmed...

Shouldnt do. The leadscrew gear train comes off the spindle.

Stuart

You are correct, of course. Now where the hell did I get the idea that changing spindle speeds caused a problem - I was sure I'd done it at some stage and had issues. Maybe I just read something somewhere and misunderstood it... Think I might do an experiment when I get some time.

[Ueee]

You are correct, of course. Now where the hell did I get the idea that changing spindle speeds caused a problem - I was sure I'd done it at some stage and had issues. Maybe I just read something somewhere and misunderstood it... Think I might do an experiment when I get some time.

Its O.K J and H, thats where my question about the back gear was heading.......

[JohnQ]

Andre

I have a PDF copies of numerous South Bend publication, if you PM me with your email address, I'll send you this Bulletin ( 3.5 meg ).

1936 - How to Cut Screw Threads in the Lathe - Bulletin 36A


Regards
JohnQ