CNC Stepper Motor Controller with CNC Software Program SuperCam

CNC Mechanism Design and Construction Notes

Currently this page is in draft form. rambling notes....

Once you have decided to your own machine.  You will find yourself faced with a long list of questions.  Making a computer controlled XYZ mechanism can be a daunting task, but with the proper attitude it can also be a very interesting endeavor.

For this discussion the assumption has been made that a XYZ or XY mechanism is going to be driven by stepper motors with no feed back.   This is the most economical path found so far.  

Linear feed back systems are a thing of beautiful in operation.  The additional programming and required electronics tends to detract from the objective.

A prominent question is, what kind of linear drive is going to be used.  
    Lead screw 
    Rack and Gear
    Timing Belt
    Cable Drive
    Radial Worm Drive
Each has advantages and disadvantages.  

The most powerful is the lead screw and nut, speed can be a factor to get power speed is often lost.  Ball nuts can easily get foreign material in the internal raceways, requires extensive cleaning.  The replaceable nuts have a lot to offer and has become our choice for driving wood routers. 

Probably the fastest is the Cable drive.  This is where a thin aircraft type cable is rapped around a grooved pulley.  The grooves are helical to make sure the cable does not wind over itself.  Power is limited to the strength of the  cable and that cable has to be wound around the smallest step distances.  The possibility of the cable twanging limits the length to probably less than a meter.

The Rack and Gear could also be described as a linear gear and a spur gear.  This has the problem of keeping the gear in good mesh along the motion path.  The gear that engages the rack should be kept as small as possible when driving directly from the stepper motor.  A very innovative designer has used a timing belt held firmly to a surface and then used a timing belt pulley as the spur gear, the application was to control a laser.

Experience has shown that the major cause of a mechanisms in accuracy is not the step distance.  Having a very small step distance does not guarantee a machine that can accurately produce parts.   This leads to linear bearings and the choices to be made there.  The problem is to contrive a mechanism that can freely move in one axis and not in any other.  

Motor sizes
Thinking that more power is always better can get you in trouble.  Watching a machine destroy itself because you left a wrench on the table which just happened to work itself into a mechanism jamming location can happen.  Rule of thumb you do not what motors stronger than the structure of the mechanism.  

Stepper motors also have the characteristic of having less speed as the size goes up.  

The NEMA 23 motors can work with loads up to 25 lbs.

The NEMA 34 motors can work with loads in excess of 100 lbs

The lead screws that have one turn per inch and eight threads have matching nuts that produce one linear pound of force for each 3.53 oz/in of rotational torque.  The nuts are rated at 70lbs maximum force.   Using a motor with a torque of 470 oz/in you can expect 133 lbs of linear force.  Using two motors synchronized you could expect to be able to move 266 lbs with two motors.

Nothing is perfect.

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Super Tech & Associates
3313 East Hillery Drive
Phoenix, Arizona  85032  USA
Voice 602-867-1755 
FAX 602-867-1426
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Last revised 06/06/01 14:37
Copyright 2000 Dennis L. Bohlke