[embraced]

Hello,

I'd like to know what Servos & gearing I should use on my system. I am fairly certain that I will use the 400W 4-axis system they sell, but I may sway to another setup if the community insists on an alternative.

Firstly, the machine is a CNC router, with a moving gantry design. The gantry moves along THK 20mm Linear rails, with a total weight of approximately 80kg. Module 1 gear rack is used as the drive mechanism for the X and Y, with the gear PCD being 30mm. 1 rotation of the output drive corresponds to 94mm of linear movement.
The Z-axis of the machine has a travel of 200mm, and the moving portion (for which the servo will have to move) is approximately 20kg. This is mounted on THK 25mm Linear rails, with a 16mm diameter ballscrew of pitch 5mm.

My question is the following: What gear reduction would be required of the DMM-Tech motors to A) move the Z axis at an acceptable acceleration/speed without an additional lift assist mechanism, and B) Rapids of ~20,000mm/min (800IPM in imperial), on the X and Y axis.

I am reasonably certain that steppers would be adequate for what I require of the machine - but with some of the jobs that are required of it, missing any steps would be a catastrophe. The noise level is also likely to be unacceptable for me.

I have asked the same question of DMM-tech, but I have not received a response as of yet.



Many regards from Australia,



Lindsay

[mactec54]

embraced

Are you using ( 2 ) 400W motors on the X axes, if so 4.5:1 will give you a high rapid speed, This also depends how well everything moves on your machine, 5.5:1 is more were it should be, but you won't have the same rapid speed, On the Z axes you could use 1:1


I have asked the same question of DMM-tech, but I have not received a response as of yet.

They are doing a engineering/automation show, so they may take longer to respond to your request

[embraced]

Thanks for the reply mactec.

Yes, two motors are used on the X axis. the design is similar to the CNC router parts design.

How do you suggest I should achieve that gear ratio? Currently I'm using a 60 tooth T5 pulley (the largest that is available, to my knowledge) mounted on the same shaft as the 30T mod 1.0 pinion.
To achieve a 4.5:1 (or 5.5:1) ratio I would have to mount an 11 tooth or 13 tooth pulley on the motor shaft - something that would be difficult, or impossible due to the size of the motor shaft (14mm diameter).

Do you have any ideas how I would achieve that ratio with my current setup, or an alternative?



Regards,


Lindsay

[mactec54]

embraced

You could do it by using a double reduction, (2) 60 teeth & (2) 24 teeth pulleys, this means a extra shaft & bearings, it would give you 5:1, you should not go below a 20 tooth pulley though, as there is not enough belt engagement with the smaller pulleys, I have used a 10 tooth before with the 14mm shaft before plus using the full key with no problems

[embraced]

Great idea.

Thanks again, mactec.


Regards,



Lindsay

[MetalStorm]

This might seem like a stupid question, but considering it was originally stated that missing steps via the use of steppers is not an option, why are pulleys being considered, I would have thought the backlash element of this would cause some problems? Especially as tuning of the servos require them to be rigidly attached to the axis, any looseness will result in overshoot or oscillation.

[mactec54]

MetalStorm

No questions are stupid questions

If you are using the Gates GT series belts/pulleys, you will have no looseness/backlash if installed correctly, even if you did have a loose belt, this would not cause overshoot or oscillation,

Oscillation is mostly a tuning problem, overshoot, can be a tuning problem, but most times when this happens the motor is to small for the job it has to do,or the inertia of the machine is to great for the motor to over come

The servo motor will always go to its commanded position, what ever it is driving, if there is slop/or backlash in how the motor is coupled to the machine, then table/gantry Etc will not reach the same commanded position

It is always important to size the motor for the job it has to do

[embraced]

To update, I ended up getting 400w AC servos with 8:1 planetary gearboxes on them. The gearbox shaft is coupled to a 50mm diameter pinion, and engaged upon the rack with a spring and pivot mechanism (a la CNCRP design).

The only thing I dislike about this setup is the length of the motor sticking out from the side of the gantry, and the moment this causes around the pivot.

[mactec54]

To update, I ended up getting 400w AC servos with 8:1 planetary gearboxes on them. The gearbox shaft is coupled to a 50mm diameter pinion, and engaged upon the rack with a spring and pivot mechanism (a la CNCRP design).

The only thing I dislike about this setup is the length of the motor sticking out from the side of the gantry, and the moment this causes around the pivot.

Yes you would need to have the 90deg gearhead to tuck them in