Steps for HOWTO Build a home made CNC machine (JGRO router)

Intro Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Step 11 Step 12

Step 6: Choose a stepper motor and lead screw

This router uses 3 stepper motors directly attached to 3 long threaded rods (lead screws) to move the platforms and gantry. I have not bought any stepper motors or drivers for this project yet. The materials so far have been very cheap, but adding stepper motors, drivers and their power supply will cost around £200. I need to see whether I can make the rest of the machine, before I spend a lot money.

The are a number of options for the lead screw. The plans I am using state that 1/4-20 allthread should be used for the lead screws, but this is not available in the UK. Alternatives are M6 and M8 allthread. It is clear from the cnczone forums that using ACME or trapezoidal thread with a larger pitch, increases the speed of the machine greatly. Finally there is the option of using a ballscrew, the most efficient and expensive lead screws. I have used the motor, lead screw and gearing guide by Arvid Brodin (available for free download at www.cnczone.com) to estimate the stepper motor and lead screw requirements for this machine. See the attached table below for the results.

There is a good Open Office spreadsheet, at the EMC2 wiki, that you can use to calculate the size of stepper motor (or servo torque) that you need, Link: mechanical spreadsheet .

I have calculated the cutting speeds and forces for the lead screw options, assuming a stepper motor that provides 1.8 Nm of torque and revolves at 300 RPM (or 5 revolutions per second). To know whether a given motor could provide 1.8 Nm torque at 300 RPM you need to consult the manufacturers speed torque curve for that motor. As RPM increases torque decreases, and I could have to settle for a much lower cutting speed in practice.

M8 thread has the most similar mechanical properties to the 1/4-20 allthread. Interestingly using M6 allthread you would get 172 N of cutting force, but I am going to use M8 allthread for convenience and economy, in the knowledge that it will be very slow (0.00625 m/s or 15 inches per minute), although it should win in a race with a common snail (average speed estimated at 0.001 m/s). I have the option to upgrade the lead screws to trapezoidal thread later.

Suppliers of small quantites of steppers etc. (UK or Euroland)

• www.motioncontrolproducts.com (stepper motors, drivers and power supplies)
• www.damencnc.com (lead screws, nuts, motor couplers, stepper motors, drivers and power supplies etc.)
• www.cnc-plus.de (stepper motors, drivers and power supplies)
• rswww.com (expensive but stocks most things)
• www.fish4parts.co.uk (cheap motor couplers)
• www.worldofcnc.com (Marchant Dice - all things CNC: linear motion, couplers, spindles, stepper motors etc.)

Images for this step (click to open full sized image)