Ball screw question.

[Nth]

In the ends of the rod of my ball screws are little holes. Are these holes important or just a byproduct of manufacturing? I was planing on drilling them out a little, tapping them, then using them to attach encoders. Does this sound like a bad idea?

[gio666]

The holes on the end of the ballscrew are for live center for lathe before machined , live center keeps ballscrew straight when machined , you can drill it no problem

[rowbare]

Originally Posted by Nth In the ends of the rod of my ball screws are little holes. Are these holes important or just a byproduct of manufacturing? I was planing on drilling them out a little, tapping them, then using them to attach encoders. Does this sound like a bad idea?

Is there a specific reason why you want to attach encoders to the ballscrew? They should go on the motor. There is a sound technical reason for it though I can't tell you what it is off the top of my head (hysteresis perhaps?). You should research it before taking this approach.

There is double feedback approach where there is the encoder on the motor as well as an encoder that measures the position of the axis. However this takes a special controller. The CNC Brain was going to do this but who knows what is going on with that...

bob

[Nth]

Thanks for the replies. The motor is connected to the ballscrew via belt. My thinking is by putting the encoder on the screw it'll keep track of the actual table position even if there is lost steps or belt slippage under load. I'm new so maybe I'm wrong. Anyone see a problem with this?

[hanermo]

Well, it depends.
The first answer is don´t do it.
The second answer is yes it can work.

For an encoder to work at the screw, you must use and have built an industrial level backlash free mechanism.
This means position control belts (HTD etc), not movement belts (XL etc).
High tension (but your motor axels probably wont support this).
Adjustable mount for the motor to tension the belts correctly.
Very good alignment, so back and forth movement wont change the initial movement characteristics of the motion control system.

Servos dont have lost steps. Is this for steppers ?
If you are using steppers, and you only want to see the encoder positions in mach3, then by all means put them at the screw.
If you want to try and control a servo system pid loop, the mechanics must be tightly coupled.
This means HTD belts, 15 mm wide, or better, and above mentioned mounts etc.
High res encoders are better, less chatter.

But you cant run hi-res encoders without a SS or other hardware motion control device.
If it´s for EMC, you cant run hi-res encoders as the max speed in sw is around 45 kHz. Again, you need a hw control board, and it gets more complicated or expensive.

I built a hi-res servo belt transport using 10k encoders, and 3:1 transmission to screw.
Works excellent, better than 1 micron resolution.
But it is neither the easiest nor the cheapest solution.

Power ? Motors ? Resolution (microns desired, screw used, etc.) Sw ? usage (mill, lathe, grinder ?)
Everything affects it.
Building a PID transport is not hard. Building a poor one is easier. Flexible mounts will spoil it.
A servo PID is a function of the (number of components x max error of worst) squared.

Yes, you can drill (tap) the ends if you want. Easy. Wont hurt anything.

As I said, belts are more accurate than you think. Better than 1 micron for sure.
Unless you have a Hardinge HLV or similar, all other mechacanical components will be weaker than HTD belt resolution.

Originally Posted by Nth Thanks for the replies. The motor is connected to the ballscrew via belt. My thinking is by putting the encoder on the screw it'll keep track of the actual table position even if there is lost steps or belt slippage under load. I'm new so maybe I'm wrong. Anyone see a problem with this?

[RICHARD ZASTROW]

The encoder on the shaft is by itself no guarantee of table positioning. It will give you an accurate position of where the screw is.

Any error in the lead screw or backlash will have to be compensated for elsewhere. A linear feed back system will give you correct position of the table, but our electronics contributors can give you better info about that set up.

Dick Z

[Nth]

This is for a stepper motor system with a toothed belt going between two toothed gears. Here is a pic of what I'm working with.



This is a top down shot. The stepper and belt is normally enclosed in a box the size of the black bottom panel you see in the picture. There is no room for an encoder on the motor's shaft itself. All I'm trying to do is add the ability to counter lost steps and give something solid (but not absolute) in the way of motion detection. Right now the software sends a pulse and assumes something happened. I figure it is better to add some actual feedback system then take things on faith and the end of the shaft seemed like a perfect place to put an encoder wheel.