Can linear scales be used in conjunction with mach 3 software?

[komatias]

Thanks for the info on the motion control and KFlop boards I will do some research on that tonight. I guess the big professional gear runs off closed loop then?

Some do. The thing is that most professional high end machines will also run servo motors so you need the encoder feedback anyway. The feedback is mainly through the encoder on the motor with the linear scales in parallel.

Looked at length at the KFlop and it is a very nice piece of kit. Like you I wanted to tie in the linear scales with the stepper motors to have absolute accuracy.

Unfortunately for the cost and the faffing with coding lead me to use a different solution.

Considering that stepper motors are extremely accurate in their stepping, it is not usually worth the extra expense on the servo based system for hobby work.

There are also steppers that have feedback on them and know if the steppers have lost steps due to mechanical issues. See the "Leadshine hybrid servo"

[Ger21]

To avoid lost steps from the stepper motors, you will need to consider a motion control board and not use the parallel port.

This really has nothing to do with lost steps. Thousands of people use Mach3 with the parallel port without losing steps.

Whatever you do, you will need to use an external motion controller because there is no way to get the encoder signals into mach3 without one.

You can get encoder signals into mach3 through the parallel port. You'd probably need a second port, though, as one probably won't have enough inputs.

But regardless of whether you use a parallel port or motion controller to get the signals into Mach3, Mach3 can't do anything with them, except display the values in DRO's.

The motion controllers that can close the loop with Mach3, do it on their own, and Mach3 never sees the signals.

[m_c]

The others have covered the practicalities of Mach3 and closed loop, however closed loop using linear scales will struggle to deal with any excessive amounts of backlash.
.
Lets say for example you have the equivalent of 20 steps backlash between the stepper and scale. You command a move of say 30 steps, and want to try holding within +/- 5 steps. The controller commands a move of 30 steps, but due to the backlash, the table only moves 10steps, still 20 steps short, so the closed loop control commands another 20 steps and you theoretically stop at your requested place. But then the cutter pulls the table, and you've suddenly overshot the target point, at which points the controller commands a move back within range, and the process continues.
Another example is the table moves overshooting the target due to deceleration, and you end up with a continual oscillation.
.
Using the PID loop typically found within closed loop controllers, you can tune to handle each of these situations, but never really in a satisfactory way. Increase values to enable quicker corrections, you'll likely end up with a continual oscillation (which in the case of servos can be so severe you cause them to overheat in a matter of seconds), reduce the values to reduce how quickly errors are corrected, and you end up with an over-damped or very sluggish to respond system.
.
And that's before you consider the mechanical problems from excess backlash. With excessive backlash, climb milling will likely be out of the equation, and overall accuracy will be reduced.
.
Personally, I'd put the additional cost of adding scales/closed loop into removing the backlash in the first place. A on open loop machine with minimal backlash will always be better than a similar but closed loop machine with excess backlash.