First Build 5 x 12 Steel Frame CNC router

[Ger21]

One question, (of many im sure) do you guys find you need to grease the ballnut more since it is sometimes spinning rather fast?

On our Morbidelli, it's recommended to grease the spinning nut every 40 hours of run time. linear bearings and non spinning nuts get greased every 3 hours of run time with an auto lube system.

But a lot more grease goes into the spinning nut, as it's much larger.

[Scott Damman]

So Im looking into and learning more about long ballscrews and how to run them. Obviously spinning the ball nut is a way to go but each unit gets to be pretty costly by the time you are said and done.

Another thing I found was supports along the ballscrew which effectively push the critical speed to a higher rpm. Has anyone ever looked into this type of design?
http://www.linearmotiontips.com/achi...d-ball-screws/

Thermwood does this on the long axis of some of their CNC machines. It looks like the support flips as the ballnut passes and then supports the ballscrew again on the other side of the ballnut.


I assume these are just nylon but cant find any further details on this type of support. I wonder if it could be considerably less expensive to incorporate this, provided the same performance is achieved in the end. Thoughts?

So still learning about the CSLabs IP-A and trying to decide if the extra money is worth it. With all ballscrews, I dont know that there will ever be a time that the system could move while the drives are disabled? I.e. I would have to manually turn the ballscrew or the motor to put it out of position. However, if Mach knows absolute position does that mean that the system needs to be "homed" less often? Between cuts etc. Are there other benefits to having Mach know the absolute position from the resolver? I hope to make a decision here soon on a motion control card... If anyone has any further thoughts I greatly appreciate it.

Thanks to Fred over at BST for working with me, I have ordered motors, 2 for X, 1 for Y, and 1 for Z. I decided to go with servo on Z as it seemed silly for the price difference to have that axis be a stepper.

Next up is HTD timing gears/belts just need to figure out speeds and gear ratio. Thanks Boyan for posting the 20t:30t Starting point and after putting it on paper sure makes sense to me.

So some math just to make sure I have things right
Lets say 20000mm/min travel
Ballnut is 5mm per revolution

(20000mm/min ) / (5mm/rev) = Ballscrew rpm => 4000rpm

Gear ratio 20t:30t = 0.666

4000rpm * gear ratio => Motor Speed => 2666rpm.

The motors I purchased from Fred are 3000rpm so this looks reasonable to me.

[Ger21]

Figure out what you want your max velocity to be, and gear it so it reaches that at 80% of the servos rated speed.

[Boyan Silyavski]

ball nut is 10mm per revolution. Only z is 5mm

[Scott Damman]

ball nut is 10mm per revolution. Only z is 5mm

Figure out what you want your max velocity to be, and gear it so it reaches that at 80% of the servos rated speed.

Gerry thats just the problem, dont know what I want or need my max velocity to be...

Boyan, so you are taking the gear ratio the other way then? i.e. screw speed (or ballnut speed) * 1.5 to give motor speed. If I take the same equation from before your 20,000 mm/min with a 10mm/rev screw gives a motor speed of 3000rpm?

What would be the disadvantage to doing a 5mm screw? I know the torque issue with gearing the motors the other way. However I ended up ordering 1.8kw 6Nm 3000rpm motors since they ended up being cheaper than the 1kw motors so I think I can sacrifice some torque to a gear ratio. Are there other issues with 5mm? I havent ordered the long screws so I can go either way just asking the question.

I am thinking about implementing brake resistors, I do this on my VFD's (to run 3 phase woodworking equipment) for quick motor braking. Im just trying to decide if the inertia of the gantry being so high warrants braking resistors or not. Has anyone needed to add them to their setup?

My motor order was as follows (in case anyone is interested)
Qty: 3 - 6Nm 1.8kW 3000 rpm servo motors - these will be for 2 for X axis and 1 for Y axis
http://www.aliexpress.com/store/prod...761095604.html

Qty:1 - 2.4Nm 0.75kW 3000 rpm servo motor - this one is for the Z axis
http://www.aliexpress.com/store/prod...760999511.html

[Boyan Silyavski]

i was trying to say / but as i was busy at workshop/ people typically use for woodworking machines xx10 screws on all axis and xx05 on Z, as this more or less gives compromise between speed, precision and step count/pulse train needed to generate movement


So say you have a servo motor that will spin to 3000rpm. From look of the servo it says 2500PPR/ pulses per revolution/ if i am not wrong. And ball screw moves 10mm per revolution.
Then ball screw needs 1/10 rev to move 1mm. If no geared then 3000/max rpm/x10=30 000mm/min max speed , like Gerry says thats your max velocity in Mach3. So from above it seems then you will need 250pulses per mm , so 1mm/250pulses=0.004mm resolution.

There is something important that Gerry told you but you missed his point. The servos should not be greatly OTT as you may think, they must be closed mutch to what effort they will do, otherwise youwill run in other problems. read literature about servo sizing. Its not a good thing to be much bigger than job they are meant to do.

Then read carefully page 14 from my build thread where Johnatan explained me very well how to calculate details and especially about snappiness.

And by the way i was suggesting 20t at the motor and 30t at the rotating ball nut, not vice versa. So you finish with slightly geared up machine, 0.7-1kw servos, and very fast acceleration. But just do your self the calc. My gantry is moved by 2x 400w servos, not 2x1.5kw ha ha

[Scott Damman]

i was trying to say / but as i was busy at workshop/ people typically use for woodworking machines xx10 screws on all axis and xx05 on Z, as this more or less gives compromise between speed, precision and step count/pulse train needed to generate movement


So say you have a servo motor that will spin to 3000rpm. From look of the servo it says 2500PPR/ pulses per revolution/ if i am not wrong. And ball screw moves 10mm per revolution.
Then ball screw needs 1/10 rev to move 1mm. If no geared then 3000/max rpm/x10=30 000mm/min max speed , like Gerry says thats your max velocity in Mach3. So from above it seems then you will need 250pulses per mm , so 1mm/250pulses=0.004mm resolution.

There is something important that Gerry told you but you missed his point. The servos should not be greatly OTT as you may think, they must be closed mutch to what effort they will do, otherwise youwill run in other problems. read literature about servo sizing. Its not a good thing to be much bigger than job they are meant to do.

Then read carefully page 14 from my build thread where Johnatan explained me very well how to calculate details and especially about snappiness.

And by the way i was suggesting 20t at the motor and 30t at the rotating ball nut, not vice versa. So you finish with slightly geared up machine, 0.7-1kw servos, and very fast acceleration. But just do your self the calc. My gantry is moved by 2x 400w servos, not 2x1.5kw ha ha

Yeah I think I did miss understand what Ger meant by OTT. Anyway Fred is holding the order for me so I can make some changes if I want.

Alright doing the math for the inertias of the system I did it based on a rotating ballscrew at the moment.

Some assumptions:
with 2 motors each motor on the X will handle 1/2 of the gantry inertia
Steel pulleys
2/3 ratio on the pulleys

Ballscrew inertia = 7.4E-4 kgm2
Gantry equivalent inertia (1/2 of total) = 3.44E-4
Pulley inertia Driven = 9.86E-5
Pulley inertia Drive = 1.48E-4
Total Inertia = 1.33E-3

Motor inertia for 1.8kw from BST page = 7.6E-4kgm2

So Inertia ratio with the 1.8kw motor = 1.75

I also ran Jonathans Matlab Script and got
Feedrate in m/min: [x y z]=[20.0 10.0 7.50]
Inertia in g-m^2: [x y z]=[1.56 0.96 0.80]
Torque in Nm: [x y z]=[4.29 2.63 2.27]
Inertia ratios: [x y z]=[2.06 1.27 1.06]

It looks like my X axis needs 4.29 Nm, how much above this number generally do I go?
These numbers show a slightly different inertia ratio for X but pretty close.

I know the general rule of thumb here is the keep the ratio under 10 but is there a lower limit as well? Ive searched to see if there is any information about that particular motor for inertia ratio but I havent found anything. Boyan I see your motors show an allowable of 12 but I dont see that for the motors BST sells (searched the model number online as well) so not sure how high I can go with them.

I guess based on these numbers the motors are probably oversized however by how much? These motors show 6Nm continuous, how much is that number trusted for the chinese motors? If I need 4.29Nm per the torque calculation?
Should I be closer to an intertia ratio of 5,6,7,etc?

Dont know if I know more now than I did or less...

Thanks for the heads up

[JAZZCNC]

Then ball screw needs 1/10 rev to move 1mm. If no geared then 3000/max rpm/x10=30 000mm/min max speed , like Gerry says thats your max velocity in Mach3. So from above it seems then you will need 250pulses per mm , so 1mm/250pulses=0.004mm resolution.

Boyan Your forgetting the Encoders are Quadrature so that's 2500 x 4 =10,000ppr.!


Scott welcome to the reality's of BIG heavy Machines. Need to Slow down with the Ordering and rushing in. The fact you even considered 5mm pitch screws and Steppers on such large machine shows you need to do much more research.

You enter another world which becomes expensive very quickly if you go rushing in without doing home work.!!

[Boyan Silyavski]

Boyan Your forgetting the Encoders are Quadrature so that's 2500 x 4 =10,000ppr.!

I am always confused about that info. Why are they never more clear. When i bought my Samsung servos, it stated 2500ppr, then i thought like you say- that they are quadrature, but it went they really meant 2500 pulses per revolution. So i actually for my machine have them programmed in Mach3 at 2500/10=250x(30t:20t)=375steps

Thats why i said he must check in servo manual and every time i think about servos ppr i check in manual.



Scott,
they are talking about controlling the resonance frequency, so if you are near it will be ok.

The bst motor inertia could not be different than typical servo motor inertia. Just make sure you know the correct length of motor and shaft diameter and you could use data from comparable motor

[Scott Damman]

The fact you even considered 5mm pitch screws and Steppers on such large machine shows you need to do much more research.

Sometimes I forget about the speed of the screw also being an important factor. I put together a little table for resolution based on ballscrew, based on my calculations I need to go with a higher pitch or up the gear ratio to keep the speed ~80% max. Upping the ratio of course hurts resolution so Im looking for a 20 pitch screw. Higher pitch also helps keep the ballscrew speed down, but of course hurts resolution per mm.



Thoughts on this?