Reverse thread cutting

You would need to be threading very near to the chuck to be in any danger of running into it,no thread goes right up to the chuck.

I leave the leadscrew/halfnuts engaged all the time,while threading,using the angled top slide to set the dept and the cross slide to put on the cut. When you reach the end of your cut,wind out the cross slide and reverse the lathe back to the starting point,while bringing in the top slide a bit deeper. You get into the habit of using both of them together,and it works a dream.

Sparey was using this approach back in his day,and it still works.

The only time I have to watch it,is when I,m cutting large threads like 21/4"x 8 as on my lathe spindle,since the carrage is really moving and while I might not hit the chuck,I could run out of road,so to speak. Result is much the same. Chips might not be blue but the air sure is !

Lathes do not like things like that.

I do not have half nuts to disengage and I cant disengage the feed as I will not be able to pick the thread up again, I dont thread close to the chuck but I've made adjusting screws with large heads and have run into them because backing the tool off then stopping the lathe and putting it in reverse and starting it again can get very frenetic. I find the slowest speed on my lathe is a bit quick for thread cutting also and my next project was a three phase motor and an inverter but the budget has just been blown on sorting my car out.

Martin P

Another option, if you are afraid of running into the chuck (or shoulder on shaft) while screw cutting is to fit a crank handle to the spindle and turn it by hand.

Paul.

I didnt realise that there are lathes where you cant disengage the lead screw, how do they work..?

Ron,

If you look at the picture of the lathe you will see the feed screw for the saddle is permanently engaged and the manual handle is far right of the bed, at the other end the lower lever in the headstock engages the feed with the chuck so when I'm thread cutting I have to keep it all engaged otherwise I will loose position.

Martin P

Thanks Martin, I just imagined that all lathes with a lead screw had half nuts but obviously not.

Ron

Martin,

One thing that you may find useful when screwcutting is a 'flip-up' tool holder.

... I think it was Martin Cleeve that designed a rather good one.

MichaelG.

MichaelG,

I've found the workshop practice book on thread cutting by Martn Cleeve and youtube video's on flip up tool holders.

Martin P

Of course you must keep the leadscrew engaged while threading,otherwise you will lose position,but that does not prevent you from stopping the machine and winding out the cross slide when you reach the end of your thread.

Then you just reverse,back towards the tailstock,set the cross slide back to zero and bring in the topslide to increase the dept of thread. A low speed is a great help,dont know what your,s is but when I,m cutting course threads,I find that even 50 RPM is a bit fast ! It,s great for most other threads.

Never seen a lathe like that,with only shaft,must take some getting used to !

Perhaps it would help if you cut a groove just short of the bolt head and get ready to reverse as you approach it.

Larry,

50 rpm would be nice, my slowest is 160 rpm, its surprising how far the saddle will move at that speed.

Just received a blank message from you.

Martin P

"Of course you must" - broad and sweeping generalisation! Only if you don't have a thread dial on the lathe or are cutting pitches not compatible with the dial (some metric pitches on an imperial lathe as an example). When you cut a thread it is common to make an undercut at the shoulder as a run out for the tool, depends how quick your reactions are and the speed of the job as to how wide that needs to be. If reverse cutting ideally you still need an undercut to position the tool in before starting the cut. On 'proper' lathes there are various creative solutions to assist with screw cutting, Holbrook have a stop system that will disengage the saddle / lead screw on a dog clutch for you at a predetermined point, you can then retract the tool and return to the start and use the dial to re-engage. Colchester had a quick retracting cross slide, small lever under the normal hand wheel which when tugged snatches the slide back to clear the thread. Again you can then disengage the half nuts return to the start put the slide back, wind on the next cut and re-engage. Both methods make thread cutting quite quick as you don't have to reverse the lathe. On some of the big traditional lathes I have worked on like Denham or DSG if you had to reverse the motor after each cut you would be there all day with the inertia in the system!

Paul.

.

Here is the design for the one I like: **LINK**

https://archive.org/stream/PopularMechanics1976/Popular%20Mechanics-04-1976#page/n105/mode/2up

MichaelG.

I had one of theses CL500 as my first machine, IIRC the lever at the bottom left on the headstock, at the end of the leadscrew is a dog clutch & can engage / disengage the lead screw. As shown the leadscrew is engaged, you simply flip the lever over to the left to disengage. It is the arrangement of the gearing on the leadscrew that gives you your threading set up. The handle shown under the saddle is the saddle lock.

George.

Edited By mechman48 on 24/12/2018 13:38:27

Unless I have misunderstood the recent posts, I think that the handle on the Leadscrew, shown on the CLM500 is intended to allow accurate positioning of the saddle. Trying to rotate the Chuck by that would involve a lot of effort, since you are geared UP, so the for a given torque at the Chuck, you would have apply that figure multiplied by the gear ratio between Spindle and Leadscrew.

A Mandrel Handle, as it's name suggests, fits into the back of the Mandrel and turns it for slow cutting of threads, or to provide extra torque at low speeds. (As when working close to the Chuck, or cutting threads upto a shoulder)

Howard

The Emco Compact 5 can't be disengaged from the leadscrew as that's how the saddle is traversed along the bed. There's no rack.

My first lathe , a 2 !/2 inch EW way back in 1960 had a fixed leadscrew nut,though a previous owner had cut the leadscrew near the headstock and fitted a dog clutch. Quite a nice small lathe .Some years later I bought ML7 ,vast improvement,since then I have read of some ingenious improvements to an EW particularly those by Martin Cleeve and more recently a method ( found on the net) of using one half of the leadscrew nut ,split horizontally and a mechanism to lift the nut into engagement with the leadscrew. I have thought about reverse screwcutting but never tried it as I cut a lot of threads using HSS thread chasers, and that would not work too well if reverse cutting. one thought if an old lathe had a worn leadscrew ,some are/were quite bad at the headstock end of the screw,if the cutting was in reverse the right hand flank of the threads would not have had much use and the pitch of the screw being cut would be far more accurate, unless of course the lathe had been used for a lot of left hand threads.

MichaelG,

Again thank you, I shall enjoy reading the whole magazine. Mechman48 the lever under the saddle is only for clamping the saddle, if you attempt to engage the feed when clamped it will break the shear pin, the feed shaft can also be set to cut thread via the gear solection at the back of the headstock, I dont have a thread dial so have to stay engaged regardless of what thread I'm cutting.

Martin P

I have a very early Warco WM250 (circa 2005) which has a forward / reverse switch. However, in reverse it runs much slower. I have never had cause to use it and don't know if the torque is also much reduced. Has anyone any experience of this?

…' Mechman48 the lever under the saddle is only for clamping the saddle'...

That's what I meant by saddle lock...

George.