Wrong Ballscrew Size?

[ozzie34231]

I'm converting my lathe and I'm having a hard time deciding on a Ballscrew size for the Z axis.
I've read and considered the thread about roller nuts for the original Acme screw, but since it's not a new idea, I've decided that the ballscrew is still the way to go for me and a metal cutting lathe.
I think my choice is between 5/8" and 3/4" rolled screws. I used double nut rolled screws on my Shoptask conversion with very good results.
The new lathe is a 14-40, (bigger than I need), and while I'd like the 3/4" screw, the price is a lot higher; I'm on a retirement budget.
The applications are strictly hobbiest, no heavy cuts, speed not needed.
So my question is:
Is a 40" axis too long for a 5/8" screw and what are the consequences of using a screw that might be too small?

Thanks,
Ozzie

[mxtras]

The consequence of too small of a diameter would be whip at high RPM. Without looking at numbers, I think you will be fine with a 40" long 5/8" screw within reasonable RPM but it would be best to do a little research just to be sure.

There is info on the net about RPM, diameter and whip. I will dig around and report back in a bit.

What is the pitch of the ballscrew you are considering? Any idea on max RPM or desired traverse speed?

Scott

[LeeWay]

I am using a 5/8" Thompson screw and single nut on my router. The X screw is about 46" long. This is pushing about 150 pound gantry on roller skate bearings. I can and do mill some aluminum on this machine, so it is pretty rigid. I can rapid this @ 300 IPM with 495 oz motors and gecko 201's on 62 VDC PS. I typically cut @ 150 IPM though or a little less for tight curves. CV wants to overshoot sometimes with this size gantry. I slow to 80 IPM on tight curves with prefect results.

There is no whip at all in these screws. They should well be capable of driving what you want on a lathe. I have a small lathe project coming up and intend to use these on it as well. Smaller than what you have. No need for that kind of length for my use. Mine will likely be about 14 or 15 inches of travel for the carriage.

I really think the bigger screws will be way overkill for less than a production machine.

[ozzie34231]

Thanks for the info guys.
The servo I'll use tops out at 3000 rpm, but I'll probably use a 2 to 1 reduction. The .2" pitch would indicate a possible 300 ipm but on my Shoptask I've never gone beyond about 60ipm. With this longer bed maybe I'll want more speed, but the fact is that most hobby stuff is small and done in a confined area.
I do some woodwork also and I might try using the lathe on wood, assuming I can control the dust. If I really do that I might want more speed. Multi passes on a long spindle might prove slow with 60ipm rapids.
So, how fast is fast, Scott?
Regards,
Ozzie

[mxtras]

I was thinking 1500+RPM! That would send your carriage down the bed of your lathe as quick as an aircraft catapult!

I think you are good with the 5/8".

Scott

[ozzie34231]

That's cool Scott.
I'll go with the 5/8". I have the installation pretty much mentally designed. I'll use the original leadscrew gearbox bearing bosses as well as the bearings on the ass end, (engineering term), for guides to mount the new screw and angular bearings. The nuts will be threaded into a common carrier and backed out to obtain near zero backlash; no springs. I've had better results from that type installation on the other machine.

Some other thoughts I've had on the screw size is that most work will be done on the angular bearing end of the screw, (90%), within 6", and that any strain on the screw will be pulling rather than pushing as that is how lathe work is usually done, (right to left). My wild ass guess, confirmed by you, is that'll work fine!

Ozzie

[mxtras]

Looking at some generic numbers, 45" of 5/8" screw supported on both ends (one end fixed, one end "floating") is perfectly acceptable up to 950RPM with a max load of 400 pounds. These are perfectly safe operating conditions and could likely be exceeded by some margin. I would fix the chuck end and "float" the tailstock end since most work will be done near the headstock. Lock one end up in thrust and radial and lock the other down in radial only. 42" of steel will grow around .0025" for every 10 degrees F.

950RPM = 190IPM with a .2 pitch screw. Your motors spin at 3K, so you could easily reduce by 3:1 or more to improve your potential resolution and increase torque and acceleration. Just a though.

The critical speed for a 42" long, 5/8" diameter rod is around 2500RPM with one end fixed (angular thrust) and the other simply supported (radial bearing). Personally, I would not spin a 42" long, 5/8" rod that fast. Things go from fine to 'bang' in about a split second.

Scott

[ozzie34231]

Thank you Scott,
Good information. I understand it all. Regarding the pulley ratio and resolution, I must take into account the computer I'll use and the speed it can handle along with Mach 3.
With my Shoptask the Mach 3 25mhz choice has been plenty so I haven't experimented with higher frequncies, but I can probably go to 45 or maybe 60mhz. The computer is a 1.6 mhz.
I also have to consider how much speed I really want. A code error while running a lathe can be much more damaging than with a mill or router. Crashing into a spinning chuck is not a pretty sight. I'm careful with the code, but I've had crashes, mostly at tool changes.
You seem to have some engineering smarts. Can you guess at the torque of my servos. I have a small collection of them, all with nearly the same nameplate info. 75V 5.5A 3000 RPM. I actually run them on 70V in defferance to Gecko life.

Thanks,
Ozzie

[mxtras]

One watt equals about 0.001341 horsepower (hp)

Your motors = 412.5 Watts, so that equates to just over 1/2 a horse (.553HP).

I am not too swift with motors but I believe this is running power, not peak. Hopefully someone will chime in and offer up some better knowledge!

Scott