Mini Lathe back-gear

[redvan22]

Hi,
Hope everyone is well and safe.

I watch KA videos where the he does threading using a tail stock die holder and always says he puts the lathe into back-gear. I have concluded that back-gear is a high-torque, yet very low speed setting apparently very useful for threading using a die. His standard size lathe offers this setting of course.

As an owner of a mini lathe, this setting is not available (presently) and it got me thinking; is there a modification that could be made to obtain high-torque and very low speed for threading in the aforementioned manner?

I have made several attempts at trying to thread a long part using a 10-32 die only to find later on that the thread resembles a belly dancer, swinging from one side to the other of the shaft rather than be a nice straight threaded shaft. I know what causes this; uneven pressure on the die holder while I manually cut the thread.

Sure, I can slow down my lathe to a similar speed but as soon as any cutting force is applied, it stops and the fault light comes on. Ugh!

Being able to create the part on the lathe using a tail-stock die holder would end my frustrations.

So, what say you all, is there anything that can be done to a mini lathe to obtain this?

Mike

 

[benmychree]

Yes, there is, buy a bigger lathe!

 

[redvan22]

benmychree,
My wife sitting behind me now, because we were looking at online furniture when the email notification came through, insists I do not answer this post in my usual sarcastic manner to match the comment posted so I will be nice and only say....
I knew some one was going to say that.

 

[LEEQ]

It's not elegant, but you can turn the chuck by hand with the die in the tailstock. Some folks make up a handle for the spindle.One other thing to note is that there are chasing dies, and there are thread cutting dies.

 

[RJSakowski]

Back gears on a lathe exchange reduced speed for torque multiplication. A VFD can reduce speed but ir can't increase torque. If your lathe is powered by an external motor, it is possible to increase torque with a set of pulleys. If your motor is built into the housing, most likely there is no solution. In eother case though, you may be limited by the strength of your machine. Too much torque can do serious damage.

 

[RJSakowski]

Threading a long shaft with a die will seldom provide a straight thread. There are too many factors that influence the cutting operation. One way is to start the thread by single point threading and finish the thread with a die. The partially formed threads will act as a guide.

 

[benmychree]

Dies do not really work well when used in the lathe, I've often seen that wobbling problem, getting a die started straight is not so easy, even when using a tailstock die holder. The hand crank idea posted above may be a good cure.

 

[Aaron_W]

I think the Atlas 6" lathes are the only common small lathes with back gears, or at least I understand they have them anyway. Most of the current mini-lathes use a DC motor with rheostat like speed control, but some like my Sherline combine a DC motor with a pulley drive, so by using different size pulleys you can trade speed for torque or torque for speed.

The standard 2 speed pulley can be set for higher speed (70-2800 rpm), or higher torque (45-1400 RPM) and they offer an optional high speed 2 position pulley allowing a range of 150-2200 RPM and 1500-10,000 RPM. I suppose if someone wanted more torque it would be possible to make a lower speed higher torque pulley. I believe Taig lathes use a similar set up.

Sherline does threading with a hand wheel and gear train, which takes the motor out of the equation, however I have seen mods where people have powered the hand wheel.

If you have one of the gear drive Chinese mini-lathes I'm not sure of your options, as I'm not that familiar with them but maybe a gear change is possible? I have seen mods for the 9x20 lathes to add a back gear, I think somebody on this site has made that mod to their 9x20 basically adding a transmission to the lathe. The 9x20s are different in operation from the 7x and 8x variable speed lathes.

I guess this is just a long winded way of saying your options depend on your specific lathe, except to say there are small lathes with back gears or alternatives to offer very low speeds.

 

[LEEQ]

When I'm doing some Design On the Go type engineering I tend to think in parts my machines can readily make. Perhaps a design modification could get you into using a storebought screw in conjunction with a something you made. If you were to spin the chuck by hand with the die holder in the tailstock, you might start with just a bit of screw sticking out of the chuck and pulling more out once the die reaches the chuck. Cut the threads in increments reducing the amount of tiny shaft sticking out unsupported.

 

[hman]

A possible alternative for running the lathe slowly is to build a crank for the ouser end of the spindle. There are lots of examples out on the interwebs.

I built this one for my Grizzly 9x20. I'm a bit paranoid about using a simple crank ... accidentally turning the lathe on would be entirely too interesting! So I cut out a plywood disk. The washers opposite the crank handle cover a hole (~1 ½") thru the disk, filled with lead to act as a counterweight. Note that the shaft is sawed at a shallow angle. Tightening the red knob pulls the far end toward the disk, jamming the two halves of the shaft inside the spindle bore.