Hello everyone, been a part of this wonderful forum for some time, but I've never really utilized it to its full potional since I've been a manual machinist for the last 6 years.
I have an opportunity to run a HAAS vf7 at work this upcoming summer, so I have been slowely trying to teach my self gcode and haas control.
This of course finally sparked the cnc curiousity.
So I have a craftex brand ct129n, aka bf20, that I will be converting. I purchased a stepper kit from motiontek.ca, 438oz/in motors, 50v 4.2amp microstep drivers, 48v 400w power supply, 6axis break out boar, limit switchs etc...
Origionaly my research led me to believe this size motor would be alright for this mill, but I am finding they're on the lower end now that I have them in my hands. Mainly for the z...
Now I'm not looking for 150ipm rapid speeds, I just want to build something to learn on and cut aluminum with. So I'm just looking for a bit of feed back on my current plan since my heads a little overloaded and I'm starting to lose focus I think heh.
For starters I was going to go the acme route. I ditched the idea of using stock lead screws since 10tpi sounds like it would be on the very slow side, plus my stock lead screws had a slight lead error.
Next idea was to use off the shelf 8tpi acme all thread. Unfortunately over 10" I discovered a massive lead error and found I was missing a thread on all three bars. So I decided to machine my own lead screws.
8tpi, and I went for a 6tpi acme thread profile for a thicker root and thinner tooth, this way on the flip side I had more meat in my nylatron nut's teeth.
Now I was going to only support one end with thrust bearings for the x axis. Total thread length is 15". That shouldnt whip or cause any problems will it? I kind of went ahead and machined it for a single mount set up for the sake of machining but never stopped to think about it too thoroughly.
For my nuts, like I mentioned I was going to use blue nylatron. Material I had on hand is .750" thick, which will give me 6 threads. For the X & Y axis, my origional thought was to mimic the stock nuts, but make .060" metal plates for either side of the nut to aid rigidity since I have nearly 7/8" to work with in the slots. With the metal plates, then I also dont have set screws digging into plastic.
For the z I was going to ditch the stock design. I was thinking of mounting my stepper on the top offeset at an angle towards the head and using an 8tpi leadscrew directly driven by my stepper just like the x and y.
My nylatron nut was going to get mounted to the heads dovetail slide. This one I was thinking of using thicker material and getting 12 threads of engagement at least. With weight in mind, my crude calculations lead me to believe my motors through this drive train will give me about 95lbs/in of holding power. So I was also going to mount a 70lb gas strut on the same lead screw assembly to help offset the heads weight.
Any concerns? I've been filling my head with other peoples ideas for the last few weeks and confusing my self, so I just wanted to print my origional plan and see if anyone had some warnings or tips. Thank you for your patience and advice.
I also did a CT129N conversion and was a complete CNC noob when I started. So I'm no expert, but I'm not sure the torque would be enough for the z-axis. Should work fine if you use a belt drive with reduction I would think. Since you're not concerned about speed, you'll at least be able to increase the torque and won't need the gas strut. Myself, I use a Nema 34, 906 oz-in on the z-axis in direct drive setup without any problems.
Besides lead errors, you also need to think about the resulting backlash from using nylon struts. So before deciding to machine your own lead screws, you might want to take a look at anti backlash ballscrews. I got mine from linearmotionbearings2008 on ebay. Look for the RM1605 for x & y axis and RM2505 for the z. He will machine them according to your drawings. Might save you the time and trouble of making your own.
If you haven't already, check out this thread of another CT129N conversion for some ideas. http://www.cnczone.com/forums/bencht..._0704_cnc.html
Good luck with your build.