Greetings All,
I was inspired by this website this past May to build rather than buy my CNC plasma table. I went about gathering all the parts through surplus sites and the ubiquitous eBAy. I spent a good couple of months researching various design approaches and drew up my complete table in Solidworks to share on the site here as well as document the buildup. In the beginning I fell into the "more is always better" approach and over engineered a good deal of it based on the choices I had for components. I spent a bit more many times as I figured if 320 oz. motors are good 1300 oz. motors would be great. I wanted the performance of servos and the quiet backlash free advantages of belt drive. So I worked toward those ends.
Which brings me to my quandry. For the X and Y I have 3 1300 oz. servos rated at 3000 rpm at 90v. I have a complete robot module for the Z. I have geckos to drive them and intended to use a 75v power supply, which should give me 2500 rpm, correct? ( please correct me if I am wrong about any of this!?) Well, only after buying new brecoflex belt and pulleys (one of the costliest parts of the build) did I start to do the math for reduction to give me 200ipm to 400 ipm at 2500rpm with the 4 inch diameter pulleys driving the X and Y. With 12.567 inches travel per revolution, my intial idea that I only needed 3:1 reduction was a joke, a holdover from discussions of using rack and pinion. I only need 32 rpm to achieve 400 ipm! reducing 2500 rpm to 32 rpm requires 78:1. Hitting the internet for something in this range I only encountered a few 100:1 gearheads (I felt 314ipm a decent compromise) and a few 50:1, but almost nothing in my Nema 34 size. I finally settled on a couple of used 100:1 Bayside right angle gearheads I will have to adapt slightly. As well as a 50:1 Bayside that is freaking huge, I didn't pay much for it, so I probably wont use it as it would add 50 lbs to my gantry, which looks like it will weigh between 150 and 170 as it is, due mostly to the weight of the capistan robot module.
And now my question, I had purchased enough servos and geckos to drive both sides of the gantry, but with only two reducers in hand should I give up and use one for both the X and the Y and just run a jackshaft? I believe I have enough torque available, and that would leave me a spare gecko and servo for the future. I liked the idea of squaring the gantry with each home position, but I guess if both sides can be adjusted with the belt positioner / tensioner that wouldn't be an issue. I was also worried that the cumulative error that even the minimal backlash of the two gearheads might rack the gantry.
Any advice would be appreciated, as I am more mechanical in nature and this is all new to me. Thanks in advance!
Cheers, Erick Davidson
Zweihammer Armoury