Dual Drive on Long Axis?

[fyffe555]

Mike,

Looks good, couple of comments. increasing L2 does a lot for stability and effectively reduces F2. Spreading L2 means the forces at the bearings will distort the pipe less due to load of the gantry.

Moving the cutter side bearing past the cutter will reduce the same moment diagram for forces generated by the cutter in use.

Increased L2 will reduce the racking moment.

It's worth doing the same analysis for the gantry in the other plane, Most home built gantry's are sufficiently braced and so can go out of square in both the horizontal and vertical.

Andrew

[spalm]

I just got my dual X leadscrew machine going. I went with two motors instead of a belt. It seemed easier to me and about the same cost with more power. I bumped up my HobbyCNC to control 4 motors and I then told mach2 to slave them and it works like a charm. With hand pressure, I can not rack it or stop it. Either way, it seems like a good solution to reduce gantry racking.

Steve

[joecnc2006]

Originally Posted by spalm I just got my dual X leadscrew machine going. I went with two motors instead of a belt. It seemed easier to me and about the same cost with more power. I bumped up my HobbyCNC to control 4 motors and I then told mach2 to slave them and it works like a charm. With hand pressure, I can not rack it or stop it. Either way, it seems like a good solution to reduce gantry racking. Steve

I imagine you created an identical setup on the rt. side of your machine.
Also You may want to place a simple box enclosure maybe with a hinging top around the entire length of the leadscrews to avoid dirt and material from gettings all over the leadscrews and also would help protect the rails from falling debre.

Joe

[spalm]

Joe, point well taken. Leadscrews on the sides of a machine need to be protected from swarf and other junk no matter the drive type.

Steve

[ynneb]

The first machine I built, had a 1.5 m gantry driven from one side. The second machine I built is driven by one servo with a drive shaft the goes along the gantry and drives the gantry from both sides. You could do dual motor drive, but for some reason it doesnt feel as right in my mind, as a drive shaft connecting to both sides. Many have sworn that dual drives are good though.

Having had both machine to compare to, the dual side driven gantry is rock solid and makes a world of difference to the machine. Bracing the gantry etc will not help as much and will just put bad loads on your linear drives. A linear drive is not meant to stop racking, its sole purpose is to guide the gantry in a straight line.

The very first animation I ever posted on this forum was the one below. While it was done out of naievity by a wet behind the ears CNCer, many years ago. It may give you some ideas. (Possibly not, I just had to blow the dust off it.) (I should do an updated one)

[harryn]

Hi, as you can probably tell by the dates of the prior posts on this thread, I have been doing a lot of searching around for information.

BTW - this is for a home build router setup. It is not intended to be commercial duty, but I like fast cutting and want decent results on a 4 x 8 sheet - something like +/-0.010 in or less if possible / affordable.

The animation from ynneb above is exactly what I am thinking of doing, but of course with a few twists:

a) Long Axis rotation
- Imagine that the X axis shown in the animation is my Y axis
- Instead of being horizontal, the motion is vertical
- Yes, there really is a method / need to the madness
- This axis movement is approx 6 ft / 2 meters long

b) Y axis rotation / change
- Imagine that the Y motion in this animation is now the Z axis instead
- The direction is the same, just the router is facing sideways instead of the normal "down"
- This axis movement is approx 18 inches / 1/2 of a meter

c) The X axis is a moving gantry
- Details still in progress
- Setup sort of like a plywood cutting setup at a lumber yard.

Perhaps more simple - just rotate the picture 90 degrees clockwise and imagine the router going into the material when the motion goes left and right.

Some design assumptions

1) Belt selection
- Brecoflex AT 5 by 50mm belt (2 each as shown)
- Zero clearance timing pulleys

2) Gantry mass - approx 30 Kg / 70 Lbs
- The force due to gravity will be largely reduced by using constant force spring coils.

3) Desired speed
- Cutting - approx 12 Inches per second (300mm / sec)

4) Acceleration - I don't know - perhaps 1 G ?

5) Timing pulleys - approx 100mm dia / 4 in dia.

Questions:

1) What shaft should I use to mount the pulleys ?
- I am assuming at least 1/2 in / 12 mm for the short axis, and perhaps 1 in / 25mm for the longer one
- I have seen shaft in McMaster which has keyways cut all the way through it - good idea ?
- How do you keep the shaft from moving around laterally ?

2) What bearings should I use to mount the shaft ?
- Is a bronze bearing ok ?
- What about the plastic bearings from Igus ? http://www.igus.com/igcat/ig_cat1.asp?p=ig_23_4.gif
- I have mostly hand tools, so life needs to be simple here on bearing mounting.


3) How do you mount the bearings in a way to achieve reasonable alignment ?
- It seems like this will require adjustment capability to really work. How is this done professionally, and perhaps more importantly, by DIY'rs ?

Thanks

Harry