I want to DIY.. some questions please!

[Tenson]

Thanks for the detailed reply.

So the problem with a single central ballscrew is that the gantry can twist a little back and forth? While the problem with using dual ballscrews + motors is that it can twist should one motor stall? I don't think my budget with allow the custom machining of ballscrew to fit a timing belt. Where did you get yours from JazzCNC?

The X axis will be 80cm wide. How about running a single 2505 ballscrew on the Y axis with a bigger motor? There is a nema 34 12Nm motor on Zapp Automation. However it needs 6.2A per phase, so what is a suitable driver for it? Is it best to use 2x 6A drivers, one per phase?


What is the problem with 5mm pitch on a 150cm axis? I'd have thought it best to use the same screw pitch on both X and Y axis so they have the same speed, provided the motors have enough torque.

Oh, and finally how can I build an auto tool change? I'd really like to have that at least to choose between 2 bits. Hmm.. I just searched that and it looks like I'd need a special chuck with gearing to transfer power from the motor, plus a system to open and close the chuck. PITA.

[JAZZCNC]

Ok the screws come from Chai at linearmotionbearings which you'll find on ebay. He's a great guy and very trust worthy. It's best to let Chia do the machining as he doesn't charge very much and will machine to your dimensions if asked. I do this with him all the time.

Yes single screw can twist the gantry when cutting at the outer edges, the wider the gantry the worse it will be. At 80cm your on the edge IMO.
If you do proceed with single then make the distance between the bearings as wide as possible to help combat this and have a strong wide cross brace undernieth.

The 25mm screw would work but you don't need 12nm motor 6nm would be ok.
I think you miss understand the phase/drive relationship you only need a drive which can handle the motors rated amp's, so in the case you gave 6.5A. you don't need 1 drive per phase.
The thing you have to be aware of is the differnce between nema 23/4 and 34 motors. Larger 34 motors tend to spin at lower speeds than 23/4 motors. typicly a 34 will runout of puff around 1000-1100rpm where as 23/4's will spin 1500rpm and above. The difference comes from how they produce and carry the torque up the rev range, 34's tend to hold higher torque further up the rev range then drop off really quick as it peaks. 23/4's will carry torque further up the rev range all be it a lower rate and gradually drop off to nothing.
Also 34's need really high voltage to get best performance. Voltage is key to speed and torque in both 23/4's and 34's but 34's require much higher to get best performance.

Unless you have a very well equiped workshop or lots of money then forget autotool changer far too expensive.!! . . . . better alternative is to buy Gerry's Mach screen it's about as close to autotool changing as you'll get without the expense, find it here and it will be the best $20 you have ever spent. http://home.comcast.net/~cncwoodworker/2010.html