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View Full Version : Infinite gear ratio for leadscrew on lathe



Rotate
02-24-2003, 04:44 PM
On the weekend, I was reading a book on cutting screw on lathe, and it was describing in detail the mathematics of gear ratio for inch/metric screw cutting, and the potentially inaccuracy introduced by the gear ratio not being precisely correct.

Are there lathes, where the leadscrew is driven by a separate servo motor? If you had a high resolution shaft encoder on the spindle of the lathe, you should be able to digitally create any ratio you needed with extremely high accuracy. Better yet, the servo should have the feedback coming from the DRO so that any inaccuracy in the leadscrew pitch can also be compensated.

More day dreaming about machining utopia.

Albert

John Foster
02-24-2003, 05:12 PM
I think you just described a CNC lathe. John

wierdscience
02-24-2003, 08:38 PM
Some lathes claim to do both inch and metric threads buy if they have either an inch or metric lead screw they can do one or the other accurately but not both.Now the shop I work in has an Italian made lathe that has a leadsrew with what I believe is called a module threaded lead screw this machine does inch,metric,whitworth and module threads and it will do them all without changing gears,studs etc.Before we had that machine we had made a set of stud gears for ower old Hendey so it would do metric in a pinch,it does a pretty good job but on coarser threads it had a lot of error if you used the thred dial instesd of reversing the motor and backing out with the half nut closed.The amount of error in the lead of the thread amounted to I think about 1 thread in 550 basicly it was close enough for most work anyway.

Uncle Dunc
02-25-2003, 12:53 AM
If a 127 tooth gear will fit in your gear train, you can cut exact metric threads with an inch leadscrew. 25.4 mm/inch x 5 = 127. If you have 37 and 47 tooth gears, you can cut metric threads very, very close to exact. With a 16 TPI leadscrew and a collection of 12 change gears, I worked up a thread table for my lathe that will cut most standard metric threads to within microinches of pitch error per thread

Dr. Rob
02-25-2003, 04:03 AM
YES! Coincidentally, I was just about to start a new thread about this- I bought a machine a while back that has exactly this feature. A stepless variable drive longitudinal feed mechanism that seems to drive three ball bearing races against a round axle. Simple, cheap and effective. Why don't all machines have this?

More later, I'm pressed for time now.

GJLawlor
02-25-2003, 09:36 AM
Hi Dr. Rob:
Any chance there's a photo of this very interesting sounding drive?

Don Warner
02-25-2003, 11:17 AM
<font face="Verdana, Arial" size="2">Originally posted by GJLawlor:
Hi Dr. Rob:
Any chance there's a photo of this very interesting sounding drive?</font>

As I remember this infinite linear drive was mfg. by Amacoil ( not sure of the spelling).
However, to me the adjustment would be tedious, like trying to set a taper attachment on a lathe. A lot to cut and try.
Where as a precise digital ratio could be set with a servo/stepping motor drive.


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Don Warner
02-25-2003, 11:17 AM
<font face="Verdana, Arial" size="2">Originally posted by GJLawlor:
Hi Dr. Rob:
Any chance there's a photo of this very interesting sounding drive?</font>

As I remember this infinite linear drive was mfg. by Amacoil ( not sure of the spelling).
However, to me the adjustment would be tedious, like trying to set a taper attachment on a lathe. A lot to cut and try.
Where as a precise digital ratio could be set with a servo/stepping motor drive.


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Axel
02-25-2003, 11:38 AM
What book whas it that you read?

Dr. Rob
02-25-2003, 12:43 PM
No, man- very simple. No cones, no difficult adjust. Just a big knob. Turn left, go left. Turn right, go right. Turn lots, go fast. Turn a little go slow. To the left is dial-in preferred rate. It's on my Koepfer Gear hobber. Maybe more info later. Very rushed, also very bad mood as befitting troublesome day in shop. Why are all the machines disagreeing with me?

gvasale
02-25-2003, 09:32 PM
The good old Atlas folks used 52 and 44 count change gears for their metric threading on suitable lathes and claim an accuracy of 1/3000.

Thrud
02-26-2003, 01:29 AM
Albert
TRAK makes a lathe that is driven by servos. The handwheels are damped to simulate regular handscrew wheels, but in reality they drive the semi-cnc lathe. I like it, but it is not cheap ($36K US here)

The Harris Alpha is also a CNC lathe that can be used like a manual with added features like constant linear velocity for turning and automatic caculation for SFM by material type (all preprogrammed into the lathe at teh factory). The Harris is one hot machine, but it is also $80K CDN (manual/semi-cnc/full cnc).