[mechhamo]

Hi to all members

i have recently removed x and y axis lead screws to be replaced by ball screws but there is a couple of thing i don't understand if someone could help it would be very appreciated


i weighed up the pros and cons for z drive and decided i would drive the quill and not the knee so i would like to ask if a 16mm ball screw would be larger enough for the z axis , it is a fair size machine it has 32mm lead screws on x&y axis.

what lead would be best for the new ball screws or does the motor to ball screw ratio change things as well as changing speed with the software, it appears to me that the more lead there is the more load would be put on the servo motor at first movement is this correct and have any impact .

what size servo motors would be suitable, and does the lead on all 3 ball screws need to be the same or can this be overcome with gearing or software programing.

can the z axis servo motor be of a smaller size than x&y axis motors because it is moving less weight ,is this correct

kind regards wayne

[Jim Dawson]

A 16mm ball screw should be more than enough on your machine. A 750W servo would be more than enough to drive it, that size is common on machines like yours. I think a 1605 would be a good choice. The lead does not need to be the same as the X & Y screws, you set the encoder pulses / inch in the software when you are setting up the motors.

A larger lead does require more motor torque to produce the same thrust. Double the lead, double the torque requirement. With a 750W motor, you will have more available torque than you can possibly use. Your machine will only handle about 300 lbs or so of quill down pressure. That's enough to quickly drill through steel plate with a 1/2 inch drill and no pilot hole. I actually turned the torque on my motor down because I didn't want to break something, the torque is normally adjustable in the drive software.

[mechhamo]

thanks so much jim for your response i am learning slow but i am learning

jim you mentioned that a 750w motor would be fine for the z axis so i assume that a 750w size would also be able to be used on x&y is this correct if so this is good news as 750w motors ain't that dear.

i have a couple of other servo motor questions if possible

1) As far as the servo motors are concerned do they need to have brakes for this application

2) would a nema34 servo motor be suitable as far as encoder resolution and are they closed loop

3) how long would a set of 32mm c7 rolled ball screws on x and y axis last if the machine was not used day in and day out

4) are c7 rolled ball screws able to produce quality accurate end products.

thank you for your help

[Jim Dawson]

Jim, I'm not sure the relationship between lead and torque is that simple.

Rotational inertia is an important factor, particularly for bigger diameter screws. The higher the lead, the lower the rpm needed which reduces the impact of rotational inertia.

You are absolutely correct. And in the case of a large VMC this would absolutely be a factor. But in this case we are working with a BP size knee mill for home shop use, so rotational inertia really can be ignored in this application. In most cases rapids would be limited to about 100 IPM and cutting speeds in the 30 IPM or less range, along with mild accelerations.

thanks so much jim for your response i am learning slow but i am learning

jim you mentioned that a 750w motor would be fine for the z axis so i assume that a 750w size would also be able to be used on x&y is this correct if so this is good news as 750w motors ain't that dear.

i have a couple of other servo motor questions if possible

Based on my experience, a 750W servo is adequate for all axes for a BP style machine.

1) As far as the servo motors are concerned do they need to have brakes for this application

No, brakes are not needed

2) would a nema34 servo motor be suitable as far as encoder resolution and are they closed loop

NEMA 34 servo motors are available with high resolution encoders.

A servo is not a motor type, but rather a system. By definition, the term servo implies a closed loop. You driving your car is a servo system, with the loop closed at the controller level. You, the controller, takes in various inputs from what is going on around you, and then output control signals to the steering, throttle, and brakes. Those output signals are amplified by the drives (power steering, engine, and power brakes) so that a small input signal to the drive can command a lot of power where needed. Works exactly the same way in a machine tool with the exception that the feedback is from encoders rather than sight, sound, and feel in the case of a car. In the case of a machine tool, you set the end goal by providing the G code to the computer. In the case of your car, the end goal might be to go to the store to pick up a quart of milk. In either case, you program in the end goal and press go.

The loop can be closed at the drive or at the controller level. A stepper motor with encoder feedback that closes the loop would be a servo system, adequate for a light duty system.

Most motors today that are called servo motors are 3 phase Brushless DC motors (BLDC, also called AC servos) as opposed to the older brushed DC motors. A BLDC motor without encoder feedback is just a BLDC motor, not a servo.

NEMA 34 refers to a frame size, not a motor type. And you have to be careful when buying a NEMA 34 frame motor, not all of them follow the exact specs for the NEMA 34 standards, most notably the shaft size, the mounting flange is usually correct. The various marketing departments have really confused the newer end users by miss-naming their products. It's a jungle out there.

3) how long would a set of 32mm c7 rolled ball screws on x and y axis last if the machine was not used day in and day out

As NIC 77 says, ya get what ya pay for.

4) are c7 rolled ball screws able to produce quality accurate end products.

This is where I get to jump up on my soapbox. Accuracy is a function of the job setup, you need to make the adjustments to the cutting parameters to achieve the accuracy that the job requires during the job setup. Repeatability is what is really required. With a proper job setup, I would expect a properly tuned machine to make the same part on each run. But there is no substitute for a tight machine with no backlash.

[NIC 77]

A larger lead does require more motor torque to produce the same thrust. Double the lead, double the torque requirement.

Yes and No. If you're talking about something that is providing a force while not moving, that's true.

When a machine is accelerating into a cut, it's different. The available torque from the motor to provide "thrust" or what I would call "available cutting force" is only what is left over after accelerating the axis.

For stepper driven systems you'll hit a speed where a higher lead ballscrew can actually provide more cutting force. This is in part because the torque of a stepper falls off with increased RPM. You might be limited to 200 IPM or less with a 5mm lead ballscrew, but be able to attain 350 IPM with a 10mm lead, and have more cutting force than the 5mm from let's say 150IPM and onwards in the speed range. Whereas the cutting force available with the 5mm lead will be a ridiculous amount, that you would never use, at lower speeds. It's just a generalization, because the diameter of the screw and the acceleration settings play a huge part in it.

Servos, it's a different story obviously as the rated torque will be continuous for most, if not all, of the RPM range. Really, I'd have to put the numbers in my spreadsheets to make a comparison between different leads and diameters for a given acceleration and motor. 32mm diameter ballscrews have substantially more rotational inertia than 25mm diameter ones.

My questions for the OP would be what kind of acceleration and top speed are you going for? And what are the lengths of your ballscrews? What 750W servos would you use, link to a datasheet? Do you want to turn this thing into a speed beast with 0.5G or more acceleration? Is using a belt driven gear reduction out of the question? For example, 10mm lead with 3:1 gear reduction from the servo.

The fact that it's a mill conversion with Ways instead of linear rails (is that true?), also makes it harder because the friction of these components...I don't know how to deal with them.

3) how long would a set of 32mm c7 rolled ball screws on x and y axis last if the machine was not used day in and day out

The higher end manufacturers will supply all kinds of calculations to figure that out. The lower end ones don't. 32mm diameter ballscrews are overkill. 25mm diameter with 750W servos is fairly common I think.

Actually, 20-25mm diameter, 750W servos, and 5mm lead are fairly common things for a mill conversion. If you were going with steppers I would say to definitely use a 10mm lead. If you are going to use 32mm diameter I would say to use a 10mm lead also. Just my opinion.

Have a read through this thread:

https://www.cnczone.com/forums/verti...rum-posts.html

And the one below from post #71 onwards (10mm vs 5mm lead and 32mm dia vs 25mm dia, same thing as you, actually look at the graphs in post #73 I made for someone else, there's my opinion, it will be about the same for you):

https://www.cnczone.com/forums/servo...-servos-4.html

4) are c7 rolled ball screws able to produce quality accurate end products.

It's not the C7 rating per se that is your biggest concern. It would be the backlash in the ballscrews. Many different manufacturers of C7 ballscrews. Some have backlash, some don't. Buying stuff from China can be a bit like rolling the dice. I'm not an expert on it really.

I don't think I'd pair a 16mm diameter ballscrew with a 750W servo, I'd be too concerned about damaging it, but like Jim said, you could tone down the servo output. I don't know anything about driving quills, I'll say that right now.

[mechhamo]

Yes and No. If you're talking about something that is providing a force while not moving, that's true.

When a machine is accelerating into a cut, it's different. The available torque from the motor to provide "thrust" or what I would call "available cutting force" is only what is left over after accelerating the axis.

For stepper driven systems you'll hit a speed where a higher lead ballscrew can actually provide more cutting force. This is in part because the torque of a stepper falls off with increased RPM. You might be limited to 200 IPM or less with a 5mm lead ballscrew, but be able to attain 350 IPM with a 10mm lead, and have more cutting force than the 5mm from let's say 150IPM and onwards in the speed range. Whereas the cutting force available with the 5mm lead will be a ridiculous amount, that you would never use, at lower speeds. It's just a generalization, because the diameter of the screw and the acceleration settings play a huge part in it.

Servos, it's a different story obviously as the rated torque will be continuous for most, if not all, of the RPM range. Really, I'd have to put the numbers in my spreadsheets to make a comparison between different leads and diameters for a given acceleration and motor. 32mm diameter ballscrews have substantially more rotational inertia than 25mm diameter ones.

My questions for the OP would be what kind of acceleration and top speed are you going for? And what are the lengths of your ballscrews? What 750W servos would you use, link to a datasheet? Do you want to turn this thing into a speed beast with 0.5G or more acceleration? Is using a belt driven gear reduction out of the question? For example, 10mm lead with 3:1 gear reduction from the servo.

The fact that it's a mill conversion with Ways instead of linear rails (is that true?), also makes it harder because the friction of these components...I don't know how to deal with them.



The higher end manufacturers will supply all kinds of calculations to figure that out. The lower end ones don't. 32mm diameter ballscrews are overkill. 25mm diameter with 750W servos is fairly common I think.

Actually, 20-25mm diameter, 750W servos, and 5mm lead are fairly common things for a mill conversion. If you were going with steppers I would say to definitely use a 10mm lead. If you are going to use 32mm diameter I would say to use a 10mm lead also. Just my opinion.

Have a read through this thread:

https://www.cnczone.com/forums/verti...rum-posts.html

And the one below from post #71 onwards (10mm vs 5mm lead and 32mm dia vs 25mm dia, same thing as you, actually look at the graphs in post #73 I made for someone else, there's my opinion, it will be about the same for you):


https://www.cnczone.com/forums/servo...-servos-4.html


Hi jim very interesting read i was not aware of the significant impact of rotational inertia has on different size ball screws
and i will factor this in when i select motors, i think there might be a problem using 25mm screws because the x axis screw is 1500 long and i want the machine to move fairly quick this might cause shaft whip not to mention 32mm shafts are less inclined to bend if a limit switch failed but i can also now see by the math if correct the advantage of a smaller shaft but i think for me based all factors i am better of with the 32mm screw with 10mm pitch and use 3 phase a/c servos.

also i will not be using machine for mass production so if it takes a bit longer it don't matter but i still want the machine fairly fast

It's not the C7 rating per se that is your biggest concern. It would be the backlash in the ballscrews. Many different manufacturers of C7 ballscrews. Some have backlash, some don't. Buying stuff from China can be a bit like rolling the dice. I'm not an expert on it really.

so your advice would be make sure its a double nut.

I don't think I'd pair a 16mm diameter ballscrew with a 750W servo, I'd be too concerned about damaging it, but like Jim said, you could tone down the servo output. I don't know anything about driving quills, I'll say that right now.

kind regards wayne

[NIC 77]

I wrote you a very long reply but the forum crapped out on me when I went to "preview post" and I lost it all. I hate when that happens.

If it were me, I'd go with 2510 ballscrews using either direct drive 750W or even 400W with 3:1 belt driven gear reduction, whichever is easiest to do and is within budget. That would give a top speed of around 400 IPM at 1000 RPM (assuming a critical speed on the ballscrew of around 1500 RPM) and you could run it at 0.5 G, so for a mill, it would be fast, faster than you need.

You could also use 2505 using a direct drive 750W but you're maximum speed (due to whip) and the maximum acceleration you could reliably get would be much less. I'm not seeing an advantage to doing this, but it would work. How fast do you really need to go? The 1500mm ballscrew length is the reason why there are disadvantages to using a 2505 vs a 2510 ballscrew, even though it is servo driven, but if the ballscrew was 400mm long, for example, the 2505 direct drive with a 750W would out perform the 2510.

If you insist on the 3210 ballscrews, the ideal might be a 750W with 3:1 belt driven gear reduction, but you could direct drive it also at a reduced acceleration, perhaps 0.2G, which would work fine also.

That's just a preliminary look based on continuous torque and doesn't include alot of factors. Also it's just my opinion.

so your advice would be make sure its a double nut.

No, it's not.

I think that some of the people who do frequent mill conversions will actually repack the inexpensive Chinese ballnuts with slightly oversized balls and things like that if they find backlash instead of using double nuts. Someone who has done it can correct me.

I'm not certain that if you buy the inexpensive double nut ballscrews on eBay that they're really guaranteed to be better. Also, do they even make those in the size you want?

You might want to look at something like a TBI based in Taiwan ballscrew. They come in different preload varients on a single ballnut. There are resellers on Aliexpress and Alibaba who can get you the custom end machining and lengths that you want. Just do some searches on those sites. You just send them a message and ask for a quote based on your lengths and end machining. If you went with a "P2" preload on a TBI ballscrew, my guess is that you'd be just fine. Might cost twice as much as the standard made in China ballscrews, but way less than something made in Germany or Japan.

I'd much rather prefer a 25mm diameter TBI ballscrew over a generic 32mm diameter variety, but I haven't used them myself.

https://www.tbimotion.com.tw

The inexpensive fixed end bearing supports can have backlash in them also. Something like "SYK" or "TBI" fixed end supports are a good idea, also available on Aliexpress or Alibaba.

If you decide to go for the more inexpensive Chinese rolled ballscrews that most of us use, BST Automation on Aliexpress has good customer service, and it's a better option than buying from eBay. I received a long ballscrew from them that was bent, and they replaced it free of charge. That's one problem with the Chinese ballscrews, is that sometimes they don't arrive straight. I've heard so many stories of this happening, and many eBay sellers will just say too bad.

I'm not saying that the inexpensive Chinese ballscrews are bad, there are so many different suppliers and grades of products made there, simply that based on the number of reviews and comments I've seen, it's a bit of a roll of the dice.

[mechhamo]

I wrote you a very long reply but the forum crapped out on me when I went to "preview post" and I lost it all. I hate when that happens.

yes nothing worse

If it were me, I'd go with 2510 ballscrews using either direct drive 750W or even 400W with 3:1 belt driven gear reduction, whichever is easiest to do and is within budget. That would give a top speed of around 400 IPM at 1000 RPM (assuming a critical speed on the ballscrew of around 1500 RPM) and you could run it at 0.5 G, so for a mill, it would be fast, faster than you need.

so its alright to go down from 32mm to 25mm without any whipping with such a long screw at 400 IPM.
also i would like to keep machine as a manual mill as well so i guess i have to have a gear reduction to achieve this if so would a 2.1 ratio be ok with 25 mm shaft and 750w 3 phase servo motors, i don't need it to be super fast but i don't want it to be painfully slow either, i plan on using it to make help with cylinder head porting because i am a mechanic, and also after i get it up and running i would like to convert a rotating table into two other axis to make it a 5 axis but at this stage i am not convinced if this is possible but you might be able to set me straight about that.
there would be one advantage of going to a 25mm shaft is i might be able to use the existing yoke as it has a 40mm bore where as a 32mm ballnut have outside diameters of around 50mm this would help me greatly because i have no mill to make it now.

kind regards wayne



Wayne, I edited your post for clarity. I apologize if I didn't get it all put together as you intended. Jim

Hint: Take a look at how the [<start>QUOTE] and [<end>/QUOTE] brackets the quoted text. Remove the <start> and <end> for actual use. You can put these anywhere you want in the text. But it must have a start and an end.

[pippin88]

Jim, I'm not sure the relationship between lead and torque is that simple.

Rotational inertia is an important factor, particularly for bigger diameter screws. The higher the lead, the lower the rpm needed which reduces the impact of rotational inertia.

[mechhamo]

Hi everyone

i have recently posted questions about what size ball screws i should use on my knee mill cnc conversion, jim advised me to go down from 32mm ballscrew to a 25mm ballscrew to improve the performance of the conversion, upon measuring up the old shaft it steps down from 32mm to 25mm where the end support bearings go so if i used a 25mm ballscrew there would be no edge that the torrington bearings can fit against leaving the ballscrew floating along the x axis has anyone come across this and how can this be solved or am i forced to use a 32mm ballscrew any help would be very appreciated.


kind regards wayne