30degree clearance/deadzone when changing direction of axis

Hi, 
I am quite new to CNC and I am currently making a CNC from a manual mill while doing my masters degree.  I noticed that when I start spinning the ball bearing screw in the other direction ( change direction) there is a 30 degree deadzone  so basically my motor would spin thinking that the Mill head is changing direction and location but it would create an error. 

ON the Z axis this problem is eliminated when I attach the mill head (100kgs) which pulls all the bolts and plates holding it  down thus eliminating the clearances. BUT my X and Y are on the plate underneath and there is no way to use the same trick there as there is no weight to be added which would tighten the connection between the plate and motor( all the bolts plates metal parts holding it)

My first ideas was - okay can I add a laser distance sensor to measure the X Y Z location at all times disregarding the need to track the motor position with its sensors. I still don't know if this is possible.

What do you think of this problem. Can I fix it in the software? Can I fix it with distance sensors? Can I fix it mechanically(probably can if everything can be made much tighter with no "give"/"clearance", but I doubt I can make it better)? (I have not yet started using the linux cnc software hence am still unaware of all the magic is capable of)

Thanks

 

What you're describing is called backlash. You want to minimize it mechanically as much as possible through preloading.

If you cannot do that, it is possible to use an absolute sensor of some kind (usually a linear encoder) to help compensate the issue. You still need (velocity) feedback from the servos to be able to control them, and the servo tuning gets tricky.

LinuxCNC can also "blindly" compensate for backlash through software only. Again, tuning the motors gets harder (you need to tune them so they can handle both the loaded and the unloaded motion).

If you have mechanical backlash you also cannot do climb milling.

So. have you used distance sensors for x and y as an addition similar to the limit switches? Is that possible, does linux cnc support that you have feedback with extra sensors such as distance sensors. As currently I only have the encoder as feedback which is as you said for absolute position, which won't help when there is backlash as the encoder would sense movement while there would be none.

Could you propose fixes for preloading? The Z axis is easy. onces I install the head of the mill it introduces enough weight to preload it, but I am not sure how to accomplish that on x and y

Very insightful. We are almost done with the CNC I will try to incorporate these ideas. Thanks

I am using Steppers. I am not sure if this post applies to me. Does it?

I see. Great I will read more about it. Thanks :3 .