Screw cutting mistake

[Arnak]

Note to self :-

When screw cutting with the half nut permanently engaged, remember that the forward and reverse switch handle is the long chrome one with the red knob.... not the short black one with the black handle that engages / disengages the half nuts! :fan:

NB:

It is difficult to get the lead screw re-engaged in the correct place to pick up the thread to be cut in the right place!

Try to pay more attention next time.... :wall:

I did get it cut eventually...

Arnak

 

[ShedBoy]

Did you do much damage to the machine or the part? When I first had a go at screwcutting I used that method, I then watch a Tubal Cain video on youtube and learned how to use the thread dial. It was much less stressful and one les thing to worry about. I found once I got a system going it is quite enjoyable even reverse threads are fun. Do you not have a thread dial?
Yes getting it lined up again is not easy but it can be done, I managed it on a reverse 1in 9tpi for a lathe spindle.
Brock

 

[mu38&Bg#]

No problem, just engage the half nut anywhere and adjust the cross slide and compound until the tool fits the thread again. You'll have to have a nut to check the thread depth or measure it. I've done it more than once.

Greg

 

[Ken I]

I know how to use the dial, but I have had just too many bad experiences where either I got it wrong or the half nuts didn't fully close and the job was ruined.

My personal preference is to screw cut in reverse - away from the chuck or out of the blind hole is particularly useful - with the tool upside down.

That way I need never disengage the half nuts, there is no risk of crashing into the job, chuck, bottom (whatever).

You eliminate the "terror" of screwing toward a shoulder or into a blind hole having to rely on luck and skill to disengage at exactly the right spot - consequently it allows you to get away with short undercuts.

It also allows you to cut at much higher rpm's than you would dare use going towards the chuck.

(Obviously if you have auto-disengage this is all somewhat moot.)

Drive the machine forwards slowly and finally by hand - position the tool in the undercut - set to next depth - double check you are in reverse and go.

It takes a little longer but never screws up.

(Except if you forget to change back to reverse - you can guess how I know this.)

Curiously whenever I suggest this to a machinist they give me that look reserved for dangerously unstable mental patients - but if they see me actually doing it will normally comment "that's an interesting way of doing it." but that might just be patronising the lunatic.

2c
Ken

 

[ShedBoy]

Curiously whenever I suggest this to a machinist they give me that look reserved for dangerously unstable mental patients -

I have a sign that says "CAUTION STABLE AREA" on the wall in my shed so I can only be classed as a dangerous mental patient :big: :big:. Whatever works is good. I think we have all forgot to reverse at some stage.

Brock

 

[steamer]

Thread cutting is a tense thing regardless...even if you ARE a lunitic... :big:

Being a lunitic, I think, actually improves your chances of doing it right!

Dave


+1 for Gregs description of picking the thread back up....it works fine

 

[Arnak]

Hi Folks,

I don't use the indicator dial as I never seem to get it to engage in quite the correct place. ???

So I use the permanently engaged method with the lifting on the return adaptor, that works fine.

I just got a little distracted and grabbed the wrong lever... more than once. *club*

I did get it done in the end though. ;D

Arnak

 

[mu38&Bg#]

Gosh I must be crazy. Thread cutting doesn't raise my pulse one bit. I don't even set the back gear anymore, and just thread at ~20Hz on the VFD so that must be 120RPM considering the belt settings I normally run. I have run my fair share of threads by hand cranking the spindle as Ken describes. The bottom up threading he talks about is something I know about but never tried, with the reverse switch in place on the VFD finally, it's on the to do list.

There's lots of things going on during threading, but once you get procedure sorted to a checklist, it's really no more complicated than talking on your cell, eating lunch, on the expressway, in traffic, with a manual transmission in the car...... *club*

 

[rake60]

I've cut a lot of threads in the past 30 years.
Everything from a 10-32 to a 2 inch lead buttress thread.

I'd much rather thread than part off a finished piece.

I can't say my threading experience was all smooth sailing.
I've lost leads and chipped tools just before finish.

One of my worst threading errors didn't really involve threading........
I was running a lathe at work that I had never used before.
About half way through the shift I was getting pretty comfortable with
the controls. I dialed in a cut, reached down and gave a firm jerk up on
the on the half nut lever, instead of the feed lever.
That got exciting real quick!

Rick

 

[user 4756]

My first attempt at thread cutting was fabulous...got the changewheels set up, was frightened of turning into a shoulder so cut backwards...a beautiful left hand thread...it was supposed to be right hand :big: :big: :big: :big:

Andy

 

[fcheslop]

Hi Andy,I did the same funny now :big: :big:but not at the time.We live and learn well some times.
best wishes Frazer

 

[rleonard]

Similar story here. A guy gave me a nice machine vice but the lead screw was broken. An easy fix to make a new one but it would not screw in.

I had made it right hand thread, not left hand like it was supposed to be.

Bob

 

[Hopefuldave]

Note to self :-

When screw cutting with the half nut permanently engaged, remember that the forward and reverse switch handle is the long chrome one with the red knob.... not the short black one with the black handle that engages / disengages the half nuts!

NB:

It is difficult to get the lead screw re-engaged in the correct place to pick up the thread to be cut in the right place!

Try to pay more attention next time....

I did get it cut eventually...

Arnak

Using Imperial-leadscrew lathes at home and work and cutting a lot of metric threads, I make a point of *not* letting go of the fwd/stop/reverse lever - at home the VFD brakes the spindle pretty quickly (brake resistor fitted and 3 seconds of DC braking for faster stopping when it gets to very low RPM), at work I stomp the brake bar, same result! I just wish the works lathe had a 2-speed clutch like at home, so I would wind back 5 times as fast as the cutting RPM - worrying when you have better machinery at home...

Coarse internal threads in blind bores still increase the pucker-factor somewhat, though... I've found a nylon cable tie on the threading tool set to the depth of the run-out groove helps a lot, and I cut *wide* run-outs when I can get away with it

I find the worst bit is having to realign the tool with the thread if I can't avoid taking the work out to check I'm to size (e.g. fitting to threads still attached to a machine or a motorbike - I don't want to pull the forks off a bike to make a custom replacement nut for the steering stem! TOO MUCH WORK!), but running forward with the tool backed off and then juggling cross and top slides works well (especially with some blue on the work and my Squinting glasses on), need to be careful when it's the finishing 1 thou" pass, I find!

Dave H. (the other one)

 

[warf]

I never cut metric thread on my grizzly till recently and found that with the american lead screw and the change gears for metric I had to leave the half nut engauged. Didn't dawn on me, but, with the simple ratios in the quick change gearbox, a metric lead screw is the only way to make the half nut work for metric.

 

[rkepler]

I never cut metric thread on my grizzly till recently and found that with the american lead screw and the change gears for metric I had to leave the half nut engauged. Didn't dawn on me, but, with the simple ratios in the quick change gearbox, a metric lead screw is the only way to make the half nut work for metric.

And with the odd pitches you need 3 different metric lead screw pitches for the thread dial to have any meaning and the half nuts to be used. There are some geared threading dials including this one designed to let you use an imperial leadscrew while threading metric pitch:

http://www.dgrdesigns.co.uk/metradial.html

 

[dman]

I never cut metric thread on my grizzly till recently and found that with the american lead screw and the change gears for metric I had to leave the half nut engauged. Didn't dawn on me, but, with the simple ratios in the quick change gearbox, a metric lead screw is the only way to make the half nut work for metric.

depends on the lathe. and if you do the math there is eventually a point you can re-engage the lead screw. if the metric thread is a multiple of 1 you can re-engage in multiples of 5 inches on the dial but since the dial ususally reads in 4 inches you have to make sure you keep track really well. not ideal for sure. if the pitch is .75 or 1.25 or god forbid .8 mm (it exists) then you probably don't have the bed travel (or time to wait for the dial to turn that many times) to do this.

i''ve actually messed this up on a left handed internal metric thread (first metric thread) on a tractor turnbuckle it went really well till the last cut and i ended up double cutting the thread at full deapth. :wall:

then i looked at everything and realized that turning the spindle backwards is the best way to do it but without a brake on the spindle i didn't trust it.

then the shop i was working in got a hardinge lathe and threading suddenly became easy. the quick change gear box on a hardinge changes the lead screw direction and a stop can be setup to throw it in neutral at the end of a cut. it indexes the gearbox to the spindle in a way you can restart on any thread no matter what the pitch without ever stopping the spindle. i'm guessing that it works on the input of the gearbox and only engages in one spindle position.