Running a Myford S7 in Reverse

There's other reasons for running the Myford backwards as well as screwcutting.

When I picked up a part built Quorn, I needed to re-cut the taper in the main spindle.
I set the whole spindle tube up in a couple of fixes steadies, so the spindle ran in its own bearings.
The taper was re-cut to get it central again, correcting the original builder's error; The top slide was in the normal position and used conventionally with a boring tool.

Whilst the top slide was set over, it made sense to turn up more arbors for the grinding wheels.
Yes I could have cut the tapers conventionally, with the tool at the front of the work, but that would mean the thin end would be at the chuck, and I couldn't easily turn the taper, parallel portion and the thread on the thin end, all at the same setting.

By running the lathe in reverse, I could cut on the back of the workpiece, have the thick end at the chuck, turn the parallel section(s) and also add the thread at the end with a Herbert die head, and everything was guaranteed concentric.

A drawbar in the 4 jaw is easy, as the throat through the chuck is larger than the hole through the backplate, so a simple washer, threaded rod and a nut will suffice.
The 3 jaw (Griptru) is a bit more awkward, as the holes are the same size, however there is a gap between the front of the backplate register and the recess in the back of the chuck.
Again a large washer is used, but this time with a taper on the edge, and two large flats, such that it will fit sideways up the bore, and then cockle over into position.

It does obviously restrict the length of a workpiece, but no danger of the chuck coming loose whatever work you are doing.

Bill

Peak4,

thanks for that insight. My Pratt Burnard Chuck has a bore the same as the spindle and so I cannot fit a washer for a draw bar. Consequently, when I was going to cut threads in reverse, I had decided to buy a new, larger chuck with a correspondingly larger bore so allowing the draw bar washer to bear on the backplate. This seemed like an expensive solution! Also, the 125mm chucks ( which is the min size I would have needed to allow the washer) looked very big on the Myford, and I was concerned that the Chuck my have been too heavy (c 5kg) and damaged the spindle bearings??

I am now in the process of making a spindle handle, but I want to incorporate a through bore so that if I do decide to change chucks and have a draw bar, I can use it with the handle.

thanks for the help

mitch

You still need the tool to be close to the chuck, either internal or external threading. While reverse cutting seems easier because the power feed is away from the chuck, the initial part of getting the tool to the shoulder and then reversing is still there. One advantage of threading in normal rotation towards the chuck, is when you have an older lather or one that has slop in the whole assembly. Starting early takes away that slop and you have a great thread right up until the end. Either option will require a thread relief area. Sometimes it is easier to just turn the machine over manually, with either a hand crank or just turning the chuck with your hands. There is no wrong way to do something. Just that some ways pose more risk or danger than other ways. With internal threading and boring, I often either use a marker pen or paper tape, to visually show the max depth of the part to the cut as a guide. In regards to reducing the chuck from coming loose on the Myford S7 , I have often thought of making a ring collar, with a set screw, and then drilling a matching hole through the back of the chuck body, and use a brass or copper plug to make it lock onto the spindle, like some of the Hardinge chucks have with their screw on fixtures etc. The 4jaw for a Myford S7 would need to be drilled right through for this to be effective, something I am reluctant to do myself. If I find that I need a largish work piece that needs to be cut in reverse and would be putting enough load onto the chuck that a risk of it coming loose could happen, then I would drill a hole through it to make a locking pad. The only reason for making an outer ring to carry the set screw, is to not be distorting the chuck body in some some way with the small diameter flange that is on the chuck backplate.

Most of all, enjoy the model engineering and have fun doing it. It is a great hobby, and many people who are not professional machinist often have great ideas and ways of doing things .

Neil, thanks. It’s great that so many of you have responded so quickly, and with really positive and helpful advice, I cannot quite visualise what the collar you suggest would bear on, I will try and find a picture of a Hardinge lathe chuck arrangement.
my P and B 4 jaw does have a larger bore than the lathe spindle, but when gripping work smaller than the chuck bore the back of the jaws are very close to the spindle and so yet again there is no clearance for a draw bar washer.

mitch

Mitch, re the handle/drawbar thing, I'm not sure you would really need to use both at the same time, as, by design, everything will be happening slowly.

Is your PB chuck on of the Myford threaded ones, or is it on a backplate.
If the latter, there's nothing to stop you temporarily removing the chuck, turning a recess on the front of the backplate, and then reassembling.
A washer with trimmed opposite sides, will then slide up the bore, and drop into the recess you've just made.

Bill

Bill,

you are right, and it will be easier to make the handle without bothering with the through bore. My 4 jaw is the threaded version without back plate, but the 3 jaw does have a back plate. I had not considered boring a recess in the back plate because I did not think I could get the washer in without separating the assembly each time. Although I will probably not bother with the draw bar for now, I may machine the recess anyway and experiment with filing flats on the washer and inserting it as you say. I suppose there are always solutions! A lot cheaper than buying a new Chuck. Than you, Mitch.

Could you not have cut on the back of the job, but turned the toolbit upside down and run the lathe in the forward direction? IE like a rear parting tool.

Power threading away from the chuck is wonderful because you can cut at top-speed without any risk of smashing into the headstock or the spinning chuck. Unfortunately the technique is risky on lathes fitted with a simple screw-on chuck because the forces tend to undo it, which spoils the thread, and might even cause the chuck to fall off.

All machine tools have limitations, and screw-on chucks are one of Myford's! Although they work well in most circumstances:

1. Chucks can jamb so tightly that owners often damage the lathe trying to get them off. Avoid over-tightening by regularly loosening the chuck and don't use the machine heavily for years without checking the chuck, or allow rust, or the oil to gum up in the threads!
2. The lathe can't be used willy-nilly in reverse! Has to be driven with due care and attention. But reverse cutting can be done provided the operator resists the urge to hack metal at full throttle, and checks the chuck is still secure every so often.

I'd only consider modifying a Myford for guaranteed safe reverse threading if a lot of threading had to be done in a hurry. For occasional work, it's easier to slow down and thread carefully. In the forward direction avoid crashing the carriage, in reverse don't cut so hard that the chuck unscrews.

Modifying the chuck and spindle mechanically is one option, an Electronic Leadscrew is another. Or admit the Myford is a poor choice for this particular application and replace it with a lathe fitted with a bolt-on or cam-lock chuck.

Dave

Certainly yes, and from what I can recall, I did look at that, but because of tool and toolholder sizes, I would have had to raise the QD holder too far up the Dickson style toolpost, such that I couldn't clamp it properly.

I'm sure I could have found a much smaller bit of HSS, rather than the carbide insert I used, but just thought running backwards was easier overall.

Horses for courses I guess, since I prefer not to skin cats.

Bill