Rotating Axis Controlled by a Ballscrew

Hi Everyone. 

I am new to CNC and This is my first post here. I am trying to build a 4th Axis that is controlled via a ballscrew. In the image attached you can see the motor and bearing for this setup. You can also see the linkages to control the rotation of the axis with the turning of the ballscrew. I wanted to know how I can setup my LinuxCNC to control the axis via the stepper and compensate for the change in angle based on turning the stepper. Thank you for your help in advance, and I apologise if this topic is not covered here. I'm not sure what to search for to find my answer.


 

Don't do this design! Do it worm gearbox . Just to make the project a success. There should be no play in the spindle.

Hi, thanks for the reply. I don't understand how this would create play. I designed it this way specifically to reduce backlash. I only need a travel of 45 degrees both ways and it needs a lot of torque to the weight of the spindle and motor being offset from center. The linkages are designed to have press-fitted bearings with dowel pins inside so I feel there should be no play. Also, the position of the ball-screw and the length of the linkage were designed to make sure that within the desired travel there are no dead spots. Is there something I am missing which you feel could result in backlash? Thanks again

Also something like this was used by Centroid for their cylinder head porting and polishing machines. Except they used to turn the work piece rather than the spindle. In their newer design they claim that the b-axis is controlled by a ballscrew but it is not visible

A simple (simplistic?) way would be to use the lincurve component to compensate for the non-linear rotational distance.  Your model also needs to incorporate a pivot at the ballscrew mount since the screw will swing as the B-axis travels.

I would also install a high resolution encoder directly on the B-axis so LCNC can use a PID to adjust for screw mapping and lincurve errors.

But yea, I'd also abandon this idea and use a direct drive motor or a roller cam gear - or even a harmonic with a brake.  Even more simple - a big gear with two pinions and two servos.  One of the servos is in position mode and the other is programmed in torque mode to oppose the position servo to take up backlash.

Based on the given information about the requirements and constraints I cannot advise anything and cannot tell wether it will be successful, clever or foolish.

My search terms would be: (without deeper knowlede)

lookup table
kinematics

Digging through the hal component list and look if I could convert the linear movement to an angle in the hal.
linuxcnc.org/docs/stable/html/hal/components.html

Greetings.

high resolution rotary encoders tend to be more expensive than a linear scale, so for a sine bar like arrangement I was thinking if a linear to angle system would benefit from. High torque, and high accuracy. At the cost of a very limited angular range.
 

Hi,

Thank you all for your contribution. I apologise for not giving enough information here so let me correct that by sharing this video of the mechanism in motion.



Secondly, Timo you are absolutely correct about the pros and cons you listed. The cost is lower than a harmonic drive, the torque is greater due to the huge ratio. In this example over the range of travel, the average ratio of the stepper motor rotation to the b-axis is 85.2 and at a speed of 500RPM it takes 2.55s to do a full 90-degree sweep which is fine for me. If I need it to be faster I can change the ration by changes the ballscrew lead.

The thing I wanted to mention here is that the relationship between the ballscrew travel and the angle of the b-axis can be defined by a function rather than a mapping in order to get an exact and accurate relationship. It is similar to the relationship between the travel of a piston and the angle of the crankshaft, as explained in the video below.



Is there anyway I can insert this function into LinuxCNC?

Alright, I will list some disadvantages that I see. There are more moving parts and more expensive bearings. The ball screw needs to have a large diameter and be of a high grade. The biggest drawback of this drive system is that it will create not only axial loading on the screw but also radial loading. In other words, the play will depend significantly on the size of the screw.
If you decide to go another route I can give some ideas

Yes I agree that this will put radial load on the ballscrew, I didn't think of that. Ok, I'm open to other ideas please do let me know.

The best option is a worm gear like 4 axes do. The other option is something similar
www.haascnc.com/service/troubleshooting-...set---ngc-lathe.html