Sliding across an axis?

[gremlin101]

I was wondering if someone could tell me how machines like a laser engraver or any cam machine is capable of moving across a bar (axis anchor) so smoothly and how does it know where it is on the bar?

 

[IRstuff]

One configuration might be worm screws with optical encoders or interferometers.

TTFN

Eng-Tips Policies FAQ731-376

 

[mappel]

Pre-loaded Ball Bearing Slides for smooth accurate movement and a linear scale with encoder to measure movement.

 

[cbrn]

Hi,
generally it works like that:

1) How does the machine "know" where it is?

- the machine has an "absolute-zero" ("machine-zero") which is a series of mechanical stops placed on each axis
- at startup, the machine axes are moved arbitrarily towards the mechanical stops until they reach them. Then the machine "knows" it has reached its "zero" and the coordinates' counts are reset to zero.
- some kind of locator is provided (mechanical "touch probe" or optical device) in order to exactly determine where the piece to be machined is placed with respect to the "machine zero". A "theoretical" value is included in the CNC program of the machined part: from the "machine-zero" to near the programmed position, the axes move at full-speed, then the machine slows down to an appropriate "approach speed" until the exact position is reached (the piece must be "touched" or "seen"): this will be the "part zero".
- the difference between the programmed position of the "part zero" and its real value is recorded as a fixed compensation
- All the subsequent machining is made using the CNC program corrected by the compensation, and relative to the "machine zero".

That's why a complete reset procedure must be done each time a machining cycle is interrupted for some reason (interrupted with EXIT from the CNC program, not a simple pause).

2) what is used to track the position?

The most precise CNC machines nowadays use a combination of techniques:
- mechanical drive: still now, nothing beats the worm screws with automatic backlash compensation. Correctly executed devices can achieve linear motion with location precision down to 0.1 micron and speed uniformity in a range of +/- 0.01 mm/s. They are driven by step motors, generally
- motion detection / measuring: the best is a parallel between optical scales and high-precision encoders (by high-precision, I mean an encoder which has a resolution at least twice the locating precision of the mechanical actuator). The results of each detector is combined by the driver in order to mutually compensate the errors. Of course, there is a control feedback so the driver auto-compensates in function of the effective response of the axis it drives. This kind of system is used for example in 6- or 7- axes mills-grinding machines (e.g. Rollomatic, ANCA, etc...)

Regards