internal screw cutting

Nigel,

Your comments ring true, I tried out thread cutting with a chaser for a brief review in the current MEW. I found it was easiest for me to use my e-stop (coast to stop on my inverter) which stopped the work within less than a turn under load, then retract, disengage and wind the saddle back..

Yes, half way through the first cut I thought "what happens next?" and resorted to disengaging the half nuts. Fortunately it was just a 'scraping' cut to check I had the right gear setup.

Neil

Neil

Have to wonder just how repeatable the "stops in less than a turn is". If it really is very repeatable maybe a bed stop incorporating a microswitch for automatic stop at repeatable position is a practical proposition. In principle a system with some sort of spring loaded actuator device able to move past the switch without damage if things don't work could be quite simple. A plausible concept would be a double diameter rod made so that one diameter safely passes the switch when the button is up and the other when it is down with a short cone between the diameters to operate the switch. I have a salvaged device of this style, but with two switches, in one of the it will be useful one day boxes.

Something of this ilk, but incorporating a 6 position turret with individual micrometer stop adjustment, is on the to-do list for my Smart & Brown 1024 VSL. (Unusually for a high end industrial toolroom lathe the the 1024 VSL has no spindle drive clutch relying on simply switching the motor on and off.)

A further thought for those without VFD drive is to use said switch to control an electromagnetic clutch off a car air-conditioning compressor interposed between to the motor shaft and drive pulley. I have a feeling such use of an air-conditioner clutch has already been suggested in MEW some time ago.

Clive

It is a clutch that grips the modified large wheel, with it you can run the slide against a fixed point without cousing damage. With screw cutting it is of no use and you change it for a normal large wheel. You can find a few more pictures in my album. Niko.

Martin,

I would also check the threading dial for engagement with the lead screw and wear as it may be jumping a tooth as you wind the saddle back

if you are not using the compound slide set at half the included angle method for threading (ie it is set parallel to the spindle axis or similar )then also check your compound slide for end float

Ian

> Have to wonder just how repeatable the "stops in less than a turn is".

That's a good question clive, All I can say at the moment is it's probably as consistent as being "very quick with both hands"!

It will be easy and safe for me to wire a microswitch in the circuit (a fully isolated 10V), I suspect for real repeatability I would have to fit a dynamic braking resistor (100R 200W is specified) and set the deceleration time to be very short. This would then mean I don't have the 'gentle wind-down' stop I enjoy now.

Neil

Neil

Very short deceleration times always seem unkind to the machine and probably need proper design taking into account the momentum of various components with proper consideration to the way drive line is made. Industrial machines with built in electromagnetic brakes usually seem to have significant coast down before the brake is applied. Presumably for good reason.

For a screw cutting stop repeatability only needs to be good enough to ensure that the machine always stops with the tool within a sensibly narrow run-out groove. I guess something around ± 1/4 of the thread pitch variation in tool stop position would be more than acceptable which doesn't seem wildly out of line with your less than a turn estimate on current VFD settings.

Clive

Just cottened on to this thread (no pun intended), and this may be off the wall, but why not turn the threading tool upside down and cut the thread from in to out? Even if your'e cutting metric on imperial you can wind the saddle in to a stop, fidgett in the back-gear, put on the cut, and cut from left to right.Just remember old GHT's advice about holding the saddle back as the tool pops out of cut.

Remember I'm a bit of a luddite, if it's not mechanical---I struggle.

Regards,

Alan.

I resorted to cutting the thread in reverse like that. although with the tool at the back of the work and not upside down. Mainly due to the long time it takes my Bantam to coast to a stop. My VFD can't be configured to stop any quicker, so threading can be rather fraught. If cutting metric on an imperial machine you still need to get the tool back to the starting position without disengaging the half nuts (ie under power) but you can do this with fewer distractions going on, so slightly reduced chances of screwing it up. That's the theory at any rate.

I have almost completed a variant of Graham Meeks' clutch I designed for for the Bantam (see my albums), which should help considerably. This operates on the spindle, not the leadscrew and gets around the problem. I'll finish it once I am no longer preoccupied by the latest distraction...

Merry

Muzzer,

I'm curious about your reverse screwcutting. If, as you say, you haven't inverted the tool but cut outwards from the far side of the bore by my reckoning you cut a LH thread. I have used this technique on occasion to produce cap ends and the like but the provisos are

1 You need a full undercut at the blind end of the bore to accommodate the tool whilst the work is stationary
and
2 Beware of unwinding the chuck off its spindle as the cutting torque in reverse tends to undo screw on chucks, predominantly found on modelling lathes. Moderate cuts and a good slam as you screw on the chuck are generally sufficient to keep it intact.

Bob D