Feedscrew on Compound Slide

[Antman]

I needed to go to a real machine shop in the next little town and while there I was taking a look at their machines. Their smallest lathe is a 1 metre BC. I noticed all the lathes have coarse leadscrews, mine has only a 2mm pitch. I forgot about what I should have really taken note of, the pitch of the compound slide screw. I have made some progress on my toolpost spindle, and have sorted out a mounting system for the ¼ hp motor, borrowed from my little drill press. Now what I want is a ball turning tool to make the pulleys’ U groove to use round 6mm polycord. I want to make a ball turner something along the lines of a standard compound slide. The compound slide on my lathe has a 1mm pitch 60 degree leadscrew, which seems very fine to me. I was wondering, do serious lathes have a much coarser screw on the compound, and are they usually Acme threads?
Thanks again,
Ant

 

[Tin Falcon]

I was wondering, do serious lathes have a much coarser screw on the compound, and are they usually Acme threads?
Thanks again,

looking at my south bend vs my 7x yes 1 turn = .100 inches vs .050 on the 7x.
And yes the south bend has an acme thread. on the cross slide.

You also need to keep in mind that lathe cross slides are sometimes calibrated for diameter so the tool moves only 1/2 what you dial in .

10 pitch 1 turn = .100 , 20 pitch 1 turn = .050, are probably the most common used threads for machine tools. 40 pitch 1 turn = .025 is standard for inch micrometers. I would be guessing at mm threads . but I would guess mics have a .5 mm thread small lathes a 1mm and a large lathe 2mm the metric machine experts can correct or confirm this info. Hope this helps.
Tin

 

[Antman]

Thanks Tin, but I am asking about the threads on the compound top slide not the cross slide.

 

[Tin Falcon]

since the dials are the same on a given lathe I would expect the threads on the compound to be the same as on the cross slide. I can see the cross slide screws without taking things apart. So even though I uses the cross slide as an example I would expect the compound to be the same scenario.
Tin

 

[Hopefuldave]

Larger lathes will use either Acme or square threads on the feedscrews, mine has 8tpi on both cross and top-slides (marked 0-125 thou"), and a 6tpi threading leadscrew - Acme for the leadscrew, as a square thread is a lot harder to engage. Metric lathes will have similar pitches (e.g. 2.5mm to 5mm pitch feedscrews, a 5mm pitch leadscrew) on the bigger (well, medium sized) machines.

The smaller lathes have much finer pitches, as threading gets a lot more difficult as the pitch-to-diameter ratio increases, as the helix angle increases with the ratio and square threads can be almost impossible to cut - I guess they could increase the leadscrew diameter, but it would look a bit odd having a 2" diameter leadscrew on a minilathe - the narrow fine-pitch plastic geartrain might object to driving a 4tpi screw, too

One thing that often bothers me is the step-up ratio from spindle to leadscrew on the small machines - ideally a lathe shouldn't be made to cut a pitch coarser than its leadscrew in order to keep the required leadscrew torque for a reasonable depth of cut within sensible limits, but a lot of the hobby lathes with 1mm pitch leadscrews have charts for 3 or 4 mm pitches so the leadscrew will be running at 3 or 4 times the spindle speed?

Dave H.

 

[Maryak]

Ant,

My lathe has a cross feed of 4mm per turn and the compound is 2mm per turn or half the pitch of the cross feed.

IMHO this is fairly normal on a lathe with powered cross feed. Feeding with the compound is normally a manual job and the finer thread helps give a more consistent feed rate.

Best Regards
Bob

 

[metaltrades01]

Now what I want is a ball turning tool to make the pulleys’ U groove to use round 6mm polycord.
Thanks again,
Ant

The easiest way to cut the grooves might be to grind a piece of High Speed Steel to 3mm radius (and turn the job slowly)

 

[prof65]

You can use one of these tools for turning your pulley, insert is 6 mm dia:

http://glanze.com/indexable-tool-holders/mc-tc-srdcn.html

Roberto

 

[Antman]

Thanks Bob and Dave for your useful and interesting replies. I think I will have to go for a 1mm leadscrew then it will have to be a V-thread, I can't imagine cutting such an Acme. If I could fit 125 divisions on a hand wheel 1.25mm pitch might be easy to work with, or 75 divisions and 1.5mm. Roberto and metaltrades I'm determined to make my ball turner. I have already made the dovetail ways (quite nicely, I think) on my Atlas7, and I don't want to fight with formtools or inserts.