Greetings from Colombia

[coinhunter]

Hello everybody, I'm a newcomer in the world of CNC, but in some days I've learnt a lot of things, Unfortunately in my country CNC is not a common science and to get the parts to assemble a machine is quite difficult, so i decided to import some elements from USA, it's expensive because of taxes and shipment but this is the only solution I've have found. I need your help to choose a correct kit for my project, I´m planning to build a 40" x 32" x 10" table and I don´t know what is the correct size of the stepper motors, at the beginning I thought that a kit including 470 oz/ inch 23 nema motors was right, but I was really dissapointed when I saw the shafts, they are really thin, am I wrong? So i decided to get the nema 34 625 oz/ inch stepper motors, the kit I mention is from Kelings, tell me you ideas and thanks.

[Richards]

My preference is to use nema 34 size motors. The ones that I own have a 1/2-inch shaft. In comparison, the nema 23 size motors that I own have a 1/4-inch shaft.

The 1/2-inch shaft motors allow the use of an 18-tooth or a 20-tooth pulley (XL size), which gives at least 3:1 reduction when used with a 60-tooth or 72-tooth pulley on the shaft. Using a 20-tooth and a 60-tooth pulley allows the use of a 150-tooth belt because the two pulleys are only separated by about 3.25-inches.

A 3:1 belt drive, with a 1.25-inch spur gear gives a per-step resolution of 0.00065-inch, when driven with a Geckodrive stepper driver. That resolution should be small enough to give excellent results.

The 34-size stepper motors that I have, have inductance ratings of 3.5mH or less when wired half-coil, which means that they can be used with power supplies of 60VDC or less.

A size 34-stepper has much greater surface area when compared to a size 23-stepper motor, so the heat is dissipated better.

Unless you are going to use the motors direct drive (without a belt drive), a 300 oz-in motor or 450 oz-in motor is adequate. With 3:1 reduction, that 300 oz-in motor develops 900 oz-in of holding torque. Without a gearbox or belt-drive, I would suggest a motor that gives 600 oz-in of torque. That is almost exactly what my Shopbot PRT-Alpha motors developed.

A small table has less mass to push, but you'll still want to have motors that can drive the cutter deep and fast.

You choice of 34-size 625 oz-in motors would be fine. They would be bigger than you need if you use a belt drive, but just about perfect if you don't use a belt-drive. Just match the power supply to the steppers so that you have optimum torque without excessive heat and you should have a very nice drive package.