5-axis project questions

[georgebarr]

I am planning to build a 5-axis CNC machine. I already have a 3-axis CNC mill and just need to build the 4 & 5th axis. The main requirement I have is that it be backlash free which general means it should be a direct drive system rather than use worm gears or belts. Please correct me here if I am wrong. Although there are backlash free worm gears, eventually they do require re-adjusting which is a disadvantage.

My main design concept is to take a manual horizontal/vertical rotary table which uses a worm gear and convert it to a direct drive rotary table. I have one main question for anybody out there. How can I do this direct drive conversion? Any kits, plans, blog sites, and etc? If I have to do it on my own, does anyone have suggestions or comments before I begin on this project?

Thanks,

[Steelpolish]

I have recently converted my rotary table to cnc power. The worm gear can be adjusted to no backlash, but after a period of use, it would have to be re-adjusted. Our application uses the rotary table spinning only in one direction, so backlash wasnt a big issue. I also thought of possibly making it a direct drive, but I was worried at how much torque would be necessary to spin it. My idea on doing it was to machine a back plate that bolted to the back of the rotary table with a center bore to mount the stepper motor shaft. Then machine a housing that bolted to the outside of the rotary table, and allowed for the stepper motor to be bolted to that. Just my 2 cents...

[will gilmore]

I hear 5 axis CAM is big $. I'd figure that part out first unless you are using one axis for simple positioning.

[rab]

What sort of parts are you trying to produce ?

[georgebarr]

rab, I plan on milling some M4/M16 rifle lower/upper parts and anything that my hobby interests can think of. There is some difficulty on doing it via 3-axis in that you have to re-adjust the stock aluminum material in various ways and the machining is broken down in several steps. Also, one must build a couple of fixtures to insure the stock is aligned and etc. With a 5-axis, I do not have to worry about most of these details. I can mill one side and then rotate the part to mill the other side. There is less operator setup. Also, I am not very experienced in machining and have limited time to devote to this hobby.

The steppers I looked at has a minimum of 1.8 degrees per step. With a 10x microstepper, the minimum movement is 0.18 degrees per step if stepper is mounted using direct drive method. This is not accurate enough. Servos I hear has an accuracy dependent on the encoder. So if I use a high resolution encoder like a 40K line per rotation encoder, I can get a 360/40K = 0.009 degrees minimum movement or I can go with a higher resolution encoder like a 60K which will give a 0.006 minimum movement. I am not sure if Mach3 can handle very high resolution encoders. I can also consider mounting this servo to the rotary table using belts with say a 1:10 ratio to get better accuracy, but have to be careful with the backlash. Also, I am considering NOT using the rotary table and just build things from scratch. Ultimately, I should end up with a trunion type of 5-axis machine.

The main design requirement is no backlash which can ruin expensive stock if things are worn out. I fear of making many identical parts to only find out that some backlash introduced some milling defects. If I use timing belts, I got to make sure they have no stretch to them, if one exists.

Thanks,

[DRD]

If you plan on going 5 axis, run the servos for sure. There is no room for any error on 5 axis work. Also plan on programmig in with g93, inverse time also.
I own a Haas 5 axsis machine, and have programmed for awhile. When you go to 5 axis, it's a whole new world for sure.
Good luck.

[georgebarr]

I read on another thread that Bob Warfield recommended using Harmonic Drives/Gears which offers zero backlash. I am indeed looking at this possibility and finding out the cost to get one of these. They are just like gearheads where a servo/stepper can be mounted on to them. They offer gear ratios from 50:1 to 160:1 so the accuracy and high torque should not be a problem.

My friend works as a machinists and will convert my CAD to Gcode 5-Axis program. Therefore, the 5-axis CAM is not a problem at this time.

[veteq]

Originally Posted by georgebarr My friend works as a machinists and will convert my CAD to Gcode 5-Axis program. Therefore, the 5-axis CAM is not a problem at this time.

Does he has a 5 axis postprocessor that can talk to MACH, dont get me wrong, but i think you are thinking to easy about just making a quality 5 axis machine.

Harmonic drives start at 1000 dollar a piece for a nice one for making athe products you want.

Kind regards,

Roy B

[BobWarfield]

Originally Posted by veteq Harmonic drives start at 1000 dollar a piece for a nice one for making athe products you want.

Yeah, but they're cheaper in the used/surplus market and the high ratio models seem less in demand, though they come up less often too.

Cheers,

BW

[veteq]

Almost forgot, had contact with this company in China.
They make nice units ant pretty precise, also have models with zero backlash.
Around 250 dollar for a new nice unit, worth a try, for real!!!.

They have good communication and are making these drives for years now, they specialize in it.
http://www.hanzh.com/English/gongsi.htm

Kind regards,

Roy B.

[georgebarr]

Originally Posted by veteq http://www.hanzh.com/English/gongsi.htm

I requested a price quote from www.hanzh.com for some Harmonic Drive Sets and Gears. This is what they sent me:

XB1-100-100-C : USD 600 (component set)
XB1-100-100-G : USD 730 (gear set)
XB3-A-100-100-C : USD 610 (component set)
XB3-A-100-100-G : Sorry , For XB3-A, only gear component set is available.
XB3-B-100-100-C : USD 710 (component set)
XB3-B-100-100-G : USD 925 (gear set)

The Gear Set has the Component Set inside it's housing. These prices are way too high for me. Instead, I bought a new harmonic component set for $141 total and may buy a second one. Ebay is much cheaper than a company from China.

Now, I have to select the bearings to use and the trunion table design. I decided to use Taper Roller Bearings since they provide good radial and axial loads and the prices are cheaper than the other bearing designs. I plan to buy seals for these bearings.

Thanks,

[veteq]

Hey George,

Those prices are high, what precision class and preload code did you asked for? My quotes to Europe where more then half as cheap.

Stays strange, our Chinese friends...

regards