Plasma design questions

[HMD]

First off,I have a Hypertherm powermax 600 that im wanting to use for my 5x10 table that is still in the design mode.Can this be wired up and run with a plazma pak 4 from CandC?My other question is if I use rack-n-pinion does it have to be a preloaded r&p.I will have drives on both sides.Or do you use anti-backlash rack-n pinion which is two gears on one track if I am seeing this right.Can or do you have the pinion gear directly on the servo motor.If done this way do I need a gear reducer?Sorry if some of this sounds stuppid.Can pinion racks be bought in 12' lenths and if so where?Thanks,Brad.

[jcc3inc]

Brad,

Use 20 degree pressure angle R & P. You can get 2oDP, 20 degree PA from: www.stdsteel.com/gear.htm

It costs about $23 for a 6 foot length of 1/2 x 1/2" in quantities of 2. Get pinions to match from Boston Gear or Martin Gear and Sprocket. Be sure the pinions have 18 teeth or more to avoid undercutting on the pinion teeth.

Use spring loading to engage the pinion into the rack. The spring force needed will be:
(.37 x the Max drive force anticipated). If you need 50# drive force, this means 18# of spring force on the pinion.

Have the motor/gearbox pivot line set so that it falls on the rack pitch line.

Regards,
Jack C.

[Torchhead]

First off the pinion gear is a ratio INCREASE. Take a standard 1" dia pinion gear (doesn't matter the pitch) and it move s the load a little over 3 inches for each rotation. That means you get three times the speed but lose three times the torque and resolution. You end up with a design that will reach astounding rapids (i.e. 2400 IPM) but give you rounded corners (bad acceleration) and a best case accuracy of less than .015. Any speed over 500 IPM is just wasted except for bragging rights! It's like building a go cart that will go 300 MPH but stalls on a 5% grade!

While direct driving a pinion is attractive from a construction standpoint it has too many cons to make it a good idea. There are some table vendors that actually tout the direct drive as being a "feature" (no belts to break or replace). I leave it to you to consider why it is not the best engineering design.

The MP1000 with the Plazpak will run a Hypertherm 600 just fine. While it (600) does not have the built in connections that the 1000 series has for machine interface, you can still pick up what you need and use the CT transformer we supply to sense for Arc Good. We have several shops using the 600.

Tom Caudle
www.CandCNC.com

[jcc3inc]

Sirs:

While I have used direct drive with good results, and my measured accuracies are .001" or better, proper gearing will generate the best results. Timing belt drives for gear reduction are favored by many homebuilders because of their simplicity, but do suffer some friction loss in operating with good belt tension. A geared reduction is superior, but harder to achieve for most do-it-yourselfers.

The ideal solution should provide the needed max travel speed with more available torque than normally is used. You can optimize your performance by using the motor specs, showing torque vs stepping rate for a given driver voltage. Also remember that with microstepping, the process simulates sine/cosine energizing of the coils, so that when one coil is as max current, the other is at zero. However, the stepper motor torques are specified with BOTH coils energized; therefore microstepping will result in only .707 x the specified (two coils energized) torque. Some improvement might be had by overdriving the stepper coil current. Some manufacturers do show results from overdriving.

Regards
Jack C.

Merry Christmas To All