acme 2 start rod vrs. gear up box on z axis

[drummer 1]

Hi, as a newbie to both cnc and this site I have a question regarding bumping up the speed and torque of z axis motor.
first some info; On my z axis I am using a unipolar stepper, 12 volts , 2 amps/phase, driven by belt with equal size pulleys on both the motor and the allthread 1/2''-13 tpi thread rod. I am using mach 2 . right now in this configuration I am getting 12 inches per minute without stalling the motor. I would like to use this motor as opposed to buying more gear right now. I am running the unipolar off a bipolar drive I built using cmos translators and a l298 ic driver.the wires are hooked up for full coil, as I have found this to run faster and stronger than half coil.I wanted to use acme 1/2-10 2 start thread but nearly immposible to get hear in canada without paying crazy $$. My question is this. Instead of using acme 2 start rod, can I get faster motion without losing torque if I simply keep my 1/2-13 allthread rod and use a smaller (1/2 size pulley on the driven(threaded rod) end? will I lose torque? any ideas on this subject would be greatly appreciated

[ahren]

Drummer,
If you go to a half sized pulley on the driven end, you are putting your motor at a 2:1 mechanical disadvantage in terms of torque -- that's just how pulleys work. That being said, you will be able to run your motor slower, and steppers produce more torque at lower RPM, which is good. Unfortunately, with your current setup, you're only going about 156 RPM to get your 12 IPM. Without knowing the torque curve of your motor, it's hard to say, but I doubt you'll get double the torque by changing from 156 RPM to 78 RPM.

If anything, you might try going the other direction, and give your motor more voltage, which will keep the torque from falling off as fast as you increase RPM's. 12 IPM is pretty slow -- it sounds like your motor is having trouble just overcoming friction on the all thread. If you can get your motor to a place where it isn't struggling so much, you might be able to run it 5-6 times as fast, overcoming the reduction you're getting from the pulleys.

And if you really want to improve your z, upgrade to ACME (even regular single start). All thread is extremely inefficient as a drive mechanism. Fastenal carries ACME, and they have a Canadian storefront, I believe. Alternatively, I offer to ship other parts to Canada with orders from my store, so if you wanted to buy an ACME nut from me and some rod from somewhere else, I'd be happy to ship them both to you. Good luck with whatever approach you take!

Best regards,

Ahren
www.cncrouterparts.com

[drummer 1]

Thank you for the suggestions offered. I tried fastenal but I felt robbed at the crazy markup in $. What you said about increasing the voltage sounded good I saw another thread about daisychaining power supplys to get higher voltage taps.A quick look at the neibourhood curbside road trash netted me 3 extra power supply's. worked like a charm and very easy to do. Now I had 24 volts to run the motors. Next a very simple ohms law calculator from the internet gave me the proper current limiting resistor value., which for my motors ( 24 volts and 2 amp) turned out to be 12 ohms at 50 watts. The local electronic store sells them for $4. each.I then realized I could wire my motors in full step, which simply means not using the common wires on my unipolars and recieved more power. 1/2 hours work and a fan from the gleaned garbage computer towers gave me what I was hoping for power wise. Then a $5 piece of 1"diam. acetal X 12" lenghth from the local plastis place and a 1/2" tap from canadian tire produced many very nice and smooth antibacklash nuts.I am now running at 30 inches per minute which for my small engraving cnc is plenty, and the homemade driver runs cool and clean.I can even daisy chain more power suppl'y to produce 42 volts ( the max volt input for my motors) but the wattage requirments make this unfeasible.The outcome for someone new like myself is satisfaction at knowing that a little bit of thought and a great community such as cnczone is, can produce acurate machines for low cost.
Thanks again for your ideas.