Ballscrew advise...

[Dave's_Not_Here]

Two questions:

1.) Is a 12mm ground ballscrew with 3mm lead acceptable dimensions for a Z axis ballscrew on a router table setup?

I was trying to keep my specs in the 16mm to 25mm by 4mm to 6mm range but was wondering if my range is too narrow?

2.) How "course" would you recommend going on a ballscrew for a Y axis... I see some decent ground ballscrews in the 12mm to 20mm leads (sometimes a little bigger) and since they are not in a "vertical" orientation" holding any static loads without power, (like the Z axis when the power is off) I was wondering if they would be too course.

Seems they would move considerably faster than a 5mm lead, which could be a benefit... but then what impact/sacrifices do those mm ranges have on resolution and other factors...

Thanks!

[ViperTX]

12 mm for a short leadscrew is acceptable....short being 18 inches (45cm) or less.....3mm lead is rather slow.....you should try for 6mm....

[Dave's_Not_Here]

Viper...
Thanks just what I was looking for... thanks for the length info. as well. Most of what I am looking at are in the 12" to 18" range for Z axis....

Still curious about the Y axs... hope someone has the same insights about them...

Dave

[ger21]

Imo, 12mm or (1/2") is a very good lead for a router. You really want to be able to move as fast as possible, and the higher lead will keep your steppers spinning slower, where they have more torque.

If you look in the K2 CNC thread in the commercial router forum, you'll see where Taus was trying to gear his 5 tpi ballscrews 2:1, but wasn't happy with his perforance. I suggested he gear them 1:2, which gave him about a 10mm lead, and he got much better performance.

[Dave's_Not_Here]

Gerry...

Following that same thought, what do you think about a 16mm +/- diameter X/Y axis with a lead in the range of 10mm to 20+ mm?

At what point do you feel the lateral or horizontal movement (speed) from a high lead screw begins to compromise the accuracy/resolution? Can the lead be "too fast" assuming feed rates, depth of cut, etc. are adjusted properly for the lead?

Seems there should be a margin where the benefits of a high lead screw's speed and the torque from slower motor speeds will intersect with the accuracy/resolution... (given a tight zero backlash ballnut, etc...)

Dave...

Added info:

Here is an example...

[ger21]

Originally Posted by Dave's_Not_Here At what point do you feel the lateral or horizontal movement (speed) from a high lead screw begins to compromise the accuracy/resolution? Can the lead be "too fast" assuming feed rates, depth of cut, etc. are adjusted properly for the lead? Seems there should be a margin where the benefits of a high lead screw's speed and the torque from slower motor speeds will intersect with the accuracy/resolution... (given a tight zero backlash ballnut, etc...)

There are a lot of factors involved. What are you cutting and how fast do you want to cut it? Does you're spindle have enough power to keep up with high feed speeds? How much torque do your steppers have? If you can get higher lead screws cheaply, you can always use belts and pulleys to tune them to best work with your motors. Without additional gearing, I'd try to stay around 1/2" lead. That gives you full steps of .0025. I wouldn't want to use a higher lead unless you go with servos. The machine I'm building now will use acme screws with a 1/4" lead (1/2-8 2 start). I'm hoping to be able to cut at 100 ipm. The next machine I buils, I'd like to go twice as fast, with maybe 400ip, rapids. Using a large and fast router at work makes slower speeds really seem slow.

[Dave's_Not_Here]

Originally Posted by ger21 There are a lot of factors involved. What are you cutting and how fast do you want to cut it? ...I am cutting everything from Hardwoods to Aluminum... for furniture, signs, jigs, etc... Does you're spindle have enough power to keep up with high feed speeds? Will be running 3.5 HP router and eventually a 5-10 HP high frequency spindle... How much torque do your steppers have? I am aiming for a minimum of 640 oz/in and most likely in the 1,000+ range... also, servos are not out of the question If you can get higher lead screws cheaply, you can always use belts and pulleys to tune them to best work with your motors. Without additional gearing, I'd try to stay around 1/2" lead. ...I want to minimize the number of "components" that have to "work together" to accomplish the task... more parts mean more potential problem areas and adjustment issues... at least in my mind... so, what combination of motor specs and ballscrew specs will allow me to eliminate "additional gearing" and other components... That gives you full steps of .0025. I wouldn't want to use a higher lead unless you go with servos. ... what is the calculation for determining the steps? Also, since servos are not out of the question, why do higher lead screws ( which means faster transitions... right?) work better with Servos (higher speed motors... right?) Seems to me, higher lead screws would benefit a slower running Stepper that is trying to maintain optimum torque curves at slower speeds, where they function best... What am I missing? Do I have my assumptions confused? The machine I'm building now will use acme screws with a 1/4" lead (1/2-8 2 start). I'm hoping to be able to cut at 100 ipm. ...I would like to hit the 200-250 IPM range with room to move if needed... as for rapids, 400-600 would be a nice place to start... The next machine I buils, I'd like to go twice as fast, with maybe 400ip, rapids. Using a large and fast router at work makes slower speeds really seem slow.

... more input/thoughts?

[ger21]

With steppers, there are 200 steps per revolution. A screw with 1/2" lead needs to turn twice for 1", so 400 steps per inch. 1/400 = .0025

With higher lead screws, you start to lose resolution. A servo's resolution is dependant on the encoder, and can be changed to work with specific components. Most of the time, the servos will need to be geared to utilize their full speed range.

There are formulas avialable for determining the required motor size for a given acceleration and speed.

http://www.pacsci.com/support/downloads/index.html
http://www.orientalmotor.com/support..._recommend.htm

If I was designing a router to use a 10HP spindle, I'd want to be able to cut at at least 400ipm. We cut at 400ipm at work with a 10HP spindle, and that's the limit of our machine. I'd go faster if I could. At those speeds, you almost have to use servos, though, or you're resoution would really suffer.

[Dave's_Not_Here]

Okay... so assuming I have a "goal" ralative to spindle HP, Feed Rate's, etc. Is it correct to assume that I could build the router (ball screws, servos/steppers, encoders, etc) with that target in mind and use the 3.5HP router... even though I might not get the maximum potential from the router until I upgrade the spindle... keeping in mind that I would have to adjust the servos accordingly? Guess what I am thinking, is all I want to do is upgrade the spindle... not rebuild the entire mill...

... Another off-topic question... I see a lot of pneumatic actuators and guide systems... but no one seems to be using them for CNC axis control... What are the issues, drawbacks or restrictions with these...

[mxtras]

Pneumatics and closed loop are compatible - there are some AWESOME positioning systems out there from Bimba, Festo etc.

BUT - I would not use them for machining/routing. Air has tendency to be a little spongy.

Scott

[ger21]

You can always use a slower feedrate with the 3.5HP router, or take lighter passes at higher speeds. You're speed is controlled in the g-code. BUild you're machine to be able to cut as fast as possible, and set up you're g-code to accomodate the spindle you're using. Depending on tooling and what you're cutting, you may be able to cut faster than you'd think.