Running a Myford S7 in Reverse

I want to have a go at screw cutting on my Super 7 and it seems that by running the lathe in reverse I can cut away from the chuck - hopefully less stressful! However, I have read that the chuck can unscrew with disasterous consequences! Is some kind of draw bar arrangement the only answer, or is there another solution. Stupid question: is there some way to fit a camlock type chuck - would this help. Any ideas most welcome. Beginner so please bear with me.

Mitch

I know it feels safer to cut away from the chuck but it is probably worth practicing disengaging the half nuts which can be done very precisely with a bit of practice, as you have a super 7 you have a clutch to stop the spindle and as screw cutting is usually done at slow spindle speed to give you a decent reaction time it is not as hard as you think to stop the spindle as well if cutting metric threads. I think someone has linked to the Oxtool demo on YouTube of how the threading indicator can be useful even when cutting metric threads.

Mike
Oxtool video

Edited By Mike Poole on 07/02/2022 19:17:07

Been screwcutting I’m my S7 for more than 30 years and have never had a problem. The great thing with the S7 is the mainshaft locking pin enabling the chuck to be well tightened. Also when you consider that the forces for screwcutting are much lighter than for normal turning operatios

You will be screwcutting in back gear so it is pretty slow anyway. However it is also a good idea anyway to make a mandrel handle so you can go even slower and provide rapid braking if necessary. Also reverse can then be applied quite quickly but remember to withdraw the tool first.

You ask is there another solution, well you could always convert to left hand threads if you are screw cutting internal and external mating halves.

regards Martin

Screw cutting is not a race. The tool removes exactly the same amount of material whether the work is revolving a 1 r.p.m. or 1000 r.p.m. so you should have plenty of time to release the half-nuts and withdraw the tool and even if you don't withdraw the tool at the same time it will only cut round groove at the end of the thread. In fact if you were to be using a lathe with an Ainjest rapid threader it does this anyway as there is no automatic withdrawal at the end of the cut when the device trips, so it is accepted practice.

You will be surprised just how easy screw-cutting is when you have done it a couple of times and you can get quite adept at releasing the half-nuts and withdrawing the tool in no time at all. BTW, when you come to cut an internal thread I recommend inverting the tool and cutting the thread on the rear face of the bore. This means you still withdraw the tool, rather than advancing it, at the end of the cut and thus alleviates any risk of forgetting to reverse the withdrawing action and causing a disaster.

You could place the tool upside down at the backside of the lathe. Then you don't need to change the rotation of the spindle.

Doesn't make any difference, feed is still towards chuck with lathe running forwards. Bottom speed back gear is really slow, just practice dropping the half nuts out with the tool clear of the job, it's not difficult. When you finally get to cutting metal machine a groove for the tool to run out. I learned doing 1" whit studs,i wouldn't recommend that as a starting point on a S7

my WM250 doesn't have back gear, and so far hasn't needed it.

I tend to screwcut larger threads that aren't worth buying tap and dies for, or that would be too much work. This

is a new headstock bolt for a 200kg bell, that needed M20 threads on each end. The end you can see there needs about 100mm of thread, so doing it efficiently was important: I cut it at about 120rpm and 0.2 DOC for all but the last spring passes. The test piece I did was more like 200rpm, but I didn't have enough material to remake the job if I screwed up.

Here's the rest of the parts

that have been in use for over two years; my bolt has a relief at the end of each thread unlike the original die cut one that failed noisily...

If you don't have the hand-eye co-ordination to disengage the halfnuts before hitting the chuck at low rpm such as backgear speeds etc, you probably are not safe to be using a lathe.

Take a bit of scrap bar and practice cutting a thread on it a few inches away from the chuck so there is room for error. Put a felt mark on the bar and practice disengaging the halfnuts as you get to the mark. You will soon get the hang of it and find it no big trick to disengage within a quarter or even eighth of a turn of the same spot every time without fail. And practice retracting the tool at the same time so you dont need to turn a finishing groove in the job.

With a bit of practice you will be able to screwcut comfortably at the highest backgear speed of about 90rpm or whatever.

Thanks very much guys for the advice and encouragement. I have now managed to cut a few quite passable BSF threads - cutting in the conventional direction! My desire to investigate cutting the threads with the lathe in reverse stemmed from watching YouTube videos from 3 of the well known machining gurus, who recommended the reverse method. Anyway it has been very satisfying to have accomplished it.

For the project I am doing I will now need to delve into the mysteries of trying to cut some metric threads with my imperial Super7B. Even though my lathe has a gearbox, I do have the full set of change gears, but I don’t suppose any of these will help me though. I am sure there must be a ‘how to do it’ somewhere on the forum, or in back issues. I would be grateful for a pointer.

thanks again

mitch

Well done. You are better than those YouTube self-appointed gurus!

Yes I am sure the metric on imperial with QC gearbox has been discussed on here before. If you do a search of the site using the Google search facility on the homepage, not the one at the top of the forum page, you should find them. Depends on what change gears you have in your "full set" as to how you go.

Forum member Brian Wood has written the definitive book on the subject: "Gearing of Lathes for Screwcutting" if you you really want to get into it.

Edited By Hopper on 10/02/2022 14:12:49

It's been mentioned in the thread "four index thread indicator", that an electronic lead screw obviates the need for all the rigmarole of change gears, chuck direction and clasp nut engagement problems. That is certainly the case.

I consider it to be the most useful addition that I've made to any of my workshop machines. I used the design developed by James Clough on youtube, buying-in the pcb that he sells. It is configurable for any lead screw pitch (imperial or metric) and a number of screw pitches as well as controlling the direction of rotation of the lead screw for right- and left-hand threads. An added bonus is a spindle rev counter. It wasn't cheap, I'll admit, but I consider it money well spent.

John

Mitch, have a look at this thread for metric threads on a Myford with a gearbox **LINK**

As far as cutting in reverse unless it's a very large course thread its unlikely to cause you a problem, I do it regularly when cutting internal threads in a blind bore.

John

Forward or backwards, I never understood why we choose the correct cutting speed for ordinary turning, but for screw cutting we use very slow speeds. Obviously there is a problem of engaging / disengaging the half nuts or even dog clutches at speed and the reason why slow cutting speeds are used. So how come we end up with acceptable thread finish at slow speed.

There are auto retract tool holders that help us to increase cutting speed, but these are not used by many of us hobby engineers.

Bob

Hopper, thanks, I will order the book today.

johnF, thanks, really good thread. I was going to try and buy the metric conversion set, but really did not fancy dismantling the whole thing every time I wanted to go to lmperial threads. The 2 new gear solution looks good. As I suspected they are not in the set of change gears I have.

John Hinckley, thanks, I like watching James. He is in a totally different league, but as I am more comfortable with cnc routing set-ups than I am with thread cutting, I will probably be able to follow him. I will watch with interest.

mitch

You can use the clutch to stop the thread cutting very effectively. I don't disengage the half nut when screw cutting. I create a thread runout area, either for the external or internal threads. If the thread pitch is fairly course, ie over 20tpi or over 1.25mm pitch, I just used the back gear to slow it down, to give more time. Then turn off the motor, reverse motor direction, wind away from the cut,(retract the tool) and use the clutch to engage and wind back to the beginning. Now that I have a VFD(variable frequency drive) I use a micro switch to stop the lathe when it gets to the run out area. Seldom do I cut threads without a thread relief area, as it prevents chipping of the threading tool. In this video you can see it stopping in the thread run out area. It generally will stop within 0.1mm every time. By hand , you would run in the lower rpm range and just disengage the clutch.

https://youtu.be/LW1-UfYKGCE

I find screw cutting by manual turning the lathe spindle with with an insert crank handle to work very well. You have total control of everything and no chance of a crash or anything disastrous like that.

When returning for another cut, just withdraw the tool and crank in the opposite direction. No need for a thread indicator dial.

When using this method, I disengage the bull gear from the spindle so the only load is on the gearing, lead screw, and carriage. For most usual threads this takes surprisingly little effort.

Before anyone dismisses this idea, I suggest you give it a try first. I think you will be pleasantly surprised.

My method for internal threading for a M71x1 telescope to camera adapter.

**LINK**

Emgee

Yes, I recently made a crank handle for my ML7 and it's one of those "why did I not make one of these years ago" things. Super useful.