Old South Bend conversion...

[courtiron]

Regardless as to whether or not this may be practical, I would like some opinions regarding the size of a servo drive to power the lead screw, which would likely be converted to a ball screw.
I started out looking for a gear box for this old lathe, but the cost projections started looking like a conversion might at least be more interesting, if not practical.
While it may be unusual this could be just a single axis system, keeping the cross-feed and compound intact, and adding the other axis later.

thanks
paul

[mike_Kilroy]

did i miss another post with more info? if so, sorry. if not, then not much to go on to give opinion. but i can say it is typical in machine tools to have servo motor continuous torque rating equal to at least 5x friction torque level....10x is better.... that said, why not stick a torque wrench on the screw shaft and see what friction torque is? list here for comments. put in more detail if you want too also - like screw dia, length, pitch, speeds in IPM u want, how much force in pounds you wanna push with while drilling or cutting. THEN an educated guess on motor size will result

[courtiron]

I guess that I did kind of start in the middle of my thought process.
Let me restate the question and provide some background.
I have an old lathe with ways that are in excellent shape.
It is not much to look at beyond that.
I have 45 years of electronics as a hobby, and have repaired and trouble shot equipment at component level.
I have NO experience directly with CNC.
I have been operating manual machine tools as a hobby since 1963.
I still have my first TRS 80 and it still works.
I can cast aluminum and bronze in my small foundry, I have a blacksmith shop equipped with an 18 KW induction heater and have been welding longer than I have operated machinery. I am retired and have the time to pursue foolish ideas.

All that being said, I should have asked for more basic information relative to converting a lathe to cnc operation. Having no knowledge of what exactly to do, I thought that perhaps someone who has converted a lathe might pass on the motor information that they used in their conversion. Given that, I could select a servo driver and power supply, I usually go big anyway. I would likely pull off the banjo and fit a sprocket to the lead screw and start there. I don't know how fast the carriage will need to travel, but only that it would have to be fast enough to thread with. I know for certain that it will be faster than I can crank it now.

My methodology might be flawed, but I was looking for a starting point, since I have little else.

The drive is to be converted to 3 phase VFD with a 2 speed toothed belt reduction... (1000 rpm is about as fast as I want to spin anything here) likely a headstock speeds of 1000 rpm and 400 rpm for the two top speeds.

At 8TPI, given a headstock speed of 100 rpm, the lead screw would then need to turn 100 rpm for 8 TPI. Say 200 RPM for 4 TPI. Any finer thread would be slower. 7.5 FtLbs to move carriage (breakaway torque as measured.... running will be less) times 15 = 113 FtLbs.

200 RPM would give me about 25 IPM, most likely about as fast as the screw could be driven given the bearings that it now has.

Pick 2000 rpm for the servo on the lead screw and you got 10:1 reduction for the servo drive to the lead screw, yields 11.25 FtLbs required from the motor. HP = Torque x RPM ÷ 5252, gives me about 4.28 HP min.

I appreciate the fact that the lead screw should be replaced with a ball screw, but I will likely start with that... (to paraphrase John Wayne) 'cause it's not all I got, but it's what I got.

Thanks for looking

paul

[mike_Kilroy]

Dang Paul! With your background and experience, you are absolutely ready to go for a cnc retrofit! I think you would do good running servo/stepper sizing program to see what-ifs on how to proceed. I'd suggest Kollmorgen Motioneering:

MOTIONEERING® | Kollmorgen

since servos easily go 2000+rpm and you think 200 is enough, and since servos get sorta expensive ($2500each ballpark) at 10#-ft size, gearing seems to be in your future; that motioneering program can help you see what size motor you would need with and without various gear ratios....

check out mach3 and the other free? computer based cnc programs - sounds perfect for what you describe.... some folks take that base and build complete cnc around it - such as machmotion.com - i think some of their complete units are as low as $ 2k

list screw dia and length too; that tells what it is rated and how fast it might go before whipping.... just more data points to help do it right.