Rack and pinion with Hobbycnc

[Sieg]

I posted this on the Hobbycnc support group, but Dave is not responding.

Maybe someone here can help me out. This is the post I made:

I am designing a new router driven by rack n pinion. It is inspired by the Mechmate, which appearently uses 20 pitch racks, and 20 teeth on the pinion. Pinion diameter = 1". What amazes me is that they dont use any gear reduction. This means if I wanted to run the machine at 100 IPM, the motor speed would have to be: 3.14159 x 1" = 3.14159 inch per revolution. 100 / 3.14159 = 31.83 RPM This seems rather low to me! My current acmerod driven machine uses 846 RPM to reach 100IPM. So my question is, can your steppers handle this? What is their ideal RPM? Are my calculations correct? Can I do this without gearreduction? cause that would sure make things easier. If not, what is the ideal gearratio? Thanks

Btw this is about the 305oz-in Hobbycnc package.

[pointcloud]

Unless you are buying heavy motors make sure and gear down... I had bought a router that was like that.. Freaking waste.... I now have it geared 4 to 1, it is much stonger and much more stable....

btw I have 1200 oz servos, on a 4'x8' router.... Max rpm on the motor is 2000.....



Edit... The 4 to 1 is befor the rack, which is the same pitch as you have....

[Sieg]

Ok, I also received a PM saying the Mechmate nowadays uses Oriental motors (what are those?) with 7.2 to 1 reduction.

Is there a relation between oz-in and ideal rpm? So that I can get the most out of my 305 steppers?

[pointcloud]

Oriental, I am guessing that this is a glamorus word for chinease?

Sounds like it has a 7.2:1 gear head on it?

I am sure that there is a happy median for oz-in/speed...

[jcc3inc]

Sir,

On my machine (which was made primarily as a CNC test bed) I used direct drive off the steppers. The steppers are Pacific Scientific 200 oz-in and the rack and pinion are 1/10 inch circular pitch. My 20 tooth pinion results in a resolution of .001" using 10X microstepping. If I were more serious about routing on a regular basis I would either opt for beefier steppers of utilize some sort of gear reduction.

Using 20 DP rack and pinion is quite practical; if you use 20 degree pressure angle, you should have a pinion with 18 theeth minimum in order to avoid undercutting.

Attached are some pix of my drive arrangement.

Regards,
Jack C.

[ger21]

Originally Posted by Sieg Ok, I also received a PM saying the Mechmate nowadays uses Oriental motors (what are those?) with 7.2 to 1 reduction.

Oriental Motor makes Vexta motors, which are probably a bit better quality than the standard cheap chinese imports. They've certainly been around much longer. They make the closed loop system that the ShopBots use as well.

www.orientalmotor.com

[Mike Nash]

The MechMate design also uses Gecko drives. There is a pretty substantial torque difference between the Gecko and the HCNC. The Gecko driven stepper motor can realize usable torque at the higher speed required by the gearing. But it's also going to have more torque at low speeds. Both substantially due to the higher voltage power supply that can be used with the Gecko, but the anti-resonance "circuit" of the Gecko may also contribute. The original MechMate design did not use gearhead steppers.

[Gerald_D]

The MechMate story on motors is posted here. You can pretty much slap any motor onto the MechMate's mounting plate, even a belt drive if you want to. There is no such thing as the "right" motor for a MechMate - depends on your budget and desired resolution/speed/backlash.