Back gear is a different train of gears to drive the spindle more slowly. Likely it has locked the spindle because you are still trying to drive the spindle at a higher speed – ie two gears selected at the same time.

Edited By not done it yet on 15/05/2022 18:46:50

The Flexispeed is the same as the Cowell lathe, the grub screw in the centre of the three step pulley has to be released so that the pulley turns freely on the mandrel. Then the back gear cluster can be rotated into engagement. You may find that you get increased end float on the mandrel, I never really solved that problem on mine.

The mandrel on the Flexispeed has a reduced diameter rather than a flat where the grub screw tightens onto.

Neil

I hav sent you a message.

Edited By Neil A on 15/05/2022 20:44:42

I have a very large single-step pulley on the "motor" side of the counter shaft of my Flexispeed and a three-step pulley on the other end, driving the lathe itself. This gears it right down, giving a top speed of just under 500rpm and low speed of 190rpm without the back-gear. With the back-gear engaged, it goes down to 50rpm. I find these to be a useful range of speeds which seem not to stress the machine.

James.

Interesting debate to be had about the merits of classic back-gear versus modern electronic speed control.

Some jobs, like threading and turning castings, benefit from spinning the chuck slowly – 50rpm or less. Back gear is the traditional solution. Big advantage is gearing down increases torque and keeps the motor operating inside its comfort zone. This is particularly important if the machine has a single-phase motor.

The disadvantage of back-gear is the fiddle-faddle of putting it in and out of action. Not a problem if a lot of slow speed cutting is on the agenda, but becomes a nuisance when the work in hand involves many fast speed changes. I think fast speed changes are the most useful pattern in a general purpose workshop.

Electronic Speed Control is the modern alternative. It allows the lathe to be spun at any rate at between minimum and maximum at the turn of a knob. Very handy whenever a job hops between cutting small and large diameters, whilst switching between fast Aluminium, medium speed steel, and slow cast-iron. Unfortunately running electric motors slowly reduces their power output and they get hot: the operator has to make sure he doesn't cook the motor or overheat the electronics! The power loss at slow speed may not be serious because, depending on the type of motor, the electronics can compensate. Older lathes keep the motor cool with a cheapo impeller that doesn't work at low RPM: newer ones often have a separate cooling fan.

Basically a trade-off between stamina and convenience. I prefer the convenience of electronic speed control, but on my hobby lathe its still necessary to change a belt to get low speed. (25 to 450rpm). Speed on the high belt is about 150 to 2500rpm, which is good for most of my needs. (Including threading, done in reverse away from the headstock at 150rpm or faster…)

Given speed control, I've never felt the need for back gear, but it depends on what the lathe is used for.

Dave

To be pedantic (who me) reducing speed doesn't necessarily reduce torque, it does reduce power. The normal reason to reduce speed is bigger workpiece, so the cutting load is at a larger radius, so for constant torque you need smaller depth of cut. I have a vfd, but still use backgear as running motor too slowly means the cooling fan is ineffective. I have seen industrial variable speed motors with separate cooling fans, a bit ott for a little lathe

Interesting to see the different views on using a variable-speed motor apparently to replace the use of back-gear.

I see it as a co-operation, gaining the advantages of both, not merely replacing one with the other.

Although I have fitted a VFD and 3ph motor to my Myford 7 and Harrison L5 lathes (and Myford VMC mill and BCA jig-borer) I do use the cone-pulley and back-gear ranges to their full. The Harrison has an all-geared headstock anyway, so engaging back-gear is merely by moving a lever.

Even with the L5 revolving at only about 60 rpm, estimated, the motor is still at about 900-1000 rpm, happily in the controller's green sector. The motor – machine pulley ratio is about 2.5:1, I think.

One unexpected bonus of converting the ML7 is that the 3ph motor runs very quietly. The original motor did as well of course, but it made the cabinet resonate loudly.

I am restoring a Denbigh H4 horizontal mill but will give it only a conventional single-phase motor and high-ratio belt and/or gear drive. Similarly my little EW 2.5" BGSC lathe is perfectly happy with a modern 1/4HP 1ph motor linked at about belt 3:1 ratio to the original countershaft.

Horses(power) for courses perhaps, but my feeling is that variable-speed 3ph motors can enhance, not merely replace 'cos we can/ it's modern / thought "better" /…, the machine-tool's existing transmission system.

Duncan –

Much as my approach although I don't know the actual figures for my VFD, motor and lathe. My way is, if it's cutting nicely, the control position is in the green sector and the machine doesn't seem to be struggling, I must have it about right; and I judge it by hearing as well as sight!

Much of the time my ML7 runs with the belt on its lowest setting, and I use the VFD for a sort of "fine-tuning".

The electronics on that lathe don't show the frequency. Those on the other machines do, but I don't know how to relate them numerically to motor speed, though there is an approximate scale printed on the controller.

I use both HSS and carbide tooling. One thing this has shown me is that carbide insert tooling – which is really made for high-rate industrial production machine-tools – does not actually have to be driven at crazy speeds to be effective. "Because it can" does not axiomatically mean "So it must". It is though often easier, depending on the metal grade and quality being machined, to obtain a decent turned finish with HSS.

As Nigel, here, too.

My lathe and one mill have infinite mechanical variable speed control but they are both powered by variable three phase motors (as was my previous lathe). I have never yet needed to select back gear on my current lathe.

My other mill is now powered by a three phase motor with VFD – even though it also has a six speed gearbox. The table power feed is also speed controlled by a VFD, even though there is a choice of 4 (or more) mechanical geared alternatives, for the table feed, with only a 50 Hz supply.

My surface grinder is fitted with a VFD also. Partly because the motor was the original 3 phase item, partly because the speed can be ramped up over several seconds and partly so I can adjust the wheel speed should I ever need to (like if I fit a smaller grinding wheel, requiring a higher optimum rotational speed, or wish to ‘soften’ an abrasive wheel by slowing it).

The stand, for my Centec, was often criticised due to vibrational noise, when fitted with single phase induction drives – so there is yet another advantage (like Nigel’s) of fitting a 3 phase motor on that machine. Likewise my previous lathe was sometimes reported as noisy at slow speeds (only when fitted with a single phase motor).

Everything leads me towards avoiding single phase motors, wherever possible/practical. But this thread was not really about this aspect of our machines? Specifically, it was about the Flexispeed back gear, I think.

NDIY

As I said… yes… to a point.

I also said I use the pulley steps and back-gear, not only for torque but also to keep the motor running fast. The speed controls have amber and red sectors warning you the setting is too slow there. All right for a few moments perhaps but not long duty.

I did and still do not advocate never using the mechanical reductions, but instead using them in conjunction with the electronics!

The cabinet was noisy at all spindle speeds, because the motor was still running at its single speed.

You are right though. The question was about the back-gear on a Flexispeed lathe – but – not because the lathe is a Flexispeed. It was about the purpose of the gear.