Frankenbot CNC Router

[dpgoldberg]

Greetings. I had previously posted the development of this design in the main forum but as I am ready to start construction, I thought it would be more appropriate to move the build thread over here. If you want to see the evolution of the design, check out the design thread here:

Design Advice Needed

This is my first CNC router project, though I have built quite a few stepper motor driven motion control rigs over the years for movie and TV work and the principles and mechanics are very similar. Still, I am very much a Newbie when it comes to CNC machines so any and all input is welcome. Especially if I'm doing something stupid!

The router is going to be built using bits and pieces and linear movers from old projects that I have lying around. Hence the name Frankenbot. I thought it appropriate! The Y Axis mover is made from Dual-Vee components attached to a 2" x 4" aluminum box tube. The Z Axis is a mover using skateboard bearings. The gantry will move on the Y Axis on a set of 20mm supported rails and pillow blocks I found on Ebay. Cutting range is about 44"X by 22"Y by 5"Z. I'm planning to use either a Porter Cable 690 or 892 router for the spindle.

I'm planning to use 1/2" Acme precision threaded rod and anti-backlash nuts to drive each axis, though I am not sure if I should go with 2-start or 5-start threads. The Y axis will be driven by rods on both sides, tied together by timing belt chain and pulleys.

[Phife]

Congrats on your start..

Ive been using the 890 (891,2,3,4,5, same router different accessories) for about a year, long jobs (5+ Hours) and I really like it, its never giving me trouble until about a week ago.

Its got plenty of power and the speed control is very nice and smooth, its a feedback type and has a magnetic disc "encoder" so it can keep constant speed even under variable load. the 690 has no speed control and IMO spins too fast for our machines, 23000rpm.

I usually keep my router in the 10,000-16,000 rpm range for most things. I cut at around 120IPM max, my machine starts to flex and bounce too much for my liking at fasters speeds.

[dpgoldberg]

Phife,

Yes, I'm leaning towards the Porter Cable 892. PC now has a variable speed version of the 690 but I figure the extra horse power of the 892 will be worth it in the long run.

Dave

[dpgoldberg]

So I'm considering two different methods of driving the dual X Axis lead screws. The first is to use a single stepper and connect the two screws via a timing belt and pulleys. The second is to use dual steppers. The dual stepper configuration would be cheaper, at least in regards to mechanical components.

Are there inherent advantages or disadvantages with each method?

How do I slave the two steppers to each other? Are they cabled to the same driver or does each go to a separate driver and the software links them? Does a program like Mach 3 allow for this sort of configuration?

[CarveOne]

So I'm considering two different methods of driving the dual X Axis lead screws. The first is to use a single stepper and connect the two screws via a timing belt and pulleys. The second is to use dual steppers. The dual stepper configuration would be cheaper, at least in regards to mechanical components.

Are there inherent advantages or disadvantages with each method?

How do I slave the two steppers to each other? Are they cabled to the same driver or does each go to a separate driver and the software links them? Does a program like Mach 3 allow for this sort of configuration?

A timing pulley and belt can slip when a 90 degree turn is done. The belt needs to wrap around the pulley more if there is a heavy cutting load on it. Your belt layout looks ok to me except that the motor is driving the belt at a 90 degree turn. The belt would have to be pretty tight to not slip. A wider belt would help, but it adds more expense.

The two motor direct drive is better. You just wire the motors to turn in the same direction, and set the slaving option in Mach3 Configuration menu to slave the X and A axis.

It is possible to run both motors off of one driver if the driver can handle the current load of both motors. I don't recommend trying it on a driver that doesn't have suitable I/O protections. It seems to be rarely done though, and most of us just buy a 4 axis driver (G540) or four separate drivers (G203V) or similar.

CarveOne

[dpgoldberg]

most of us just buy a 4 axis driver (G540) or four separate drivers (G203V) or similar.

CarveOne

Thanks Gerry. If I use a 4 Axis driver to run this, and later want to add a true 4th axis (rotator) can I add a single driver to the system and will the software allow it?

Dave

[CarveOne]

Thanks Gerry. If I use a 4 Axis driver to run this, and later want to add a true 4th axis (rotator) can I add a single driver to the system and will the software allow it?

Dave

Yes. Mach3 allows a couple of other slaving combinations that will let you free up A axis for the standard rotary axis. As long as you write/generate your own gcode you don't necessarily have to follow the standard. You just need to test or simulate any new gcode to be sure it does what you expect it to.

CarveOne

[dpgoldberg]

I'm narrowing down my components list. I will probably go with a Gecko G540 4 axis driver and something like Keling's KL23H284-35-4B 387 oz. in. stepper motors. Now the choice of Acme threaded lead screws. It looks like the main choices are:

1/2-10 2-start (.200 lead)
1/2-8 2-start (.250 lead)
1/2-8 4-start (.500 lead)
1/2-10 5-start (.500 lead)

It seems like a reasonable speed goal would be 200 ipm. The X axis screws are 55" long and the Y axis screw is 33" long. I understand whip can be a problem with longer screws at higher speeds and can limit top speed. However, I would gladly trade precision for speed.

Any advice on these options would be greatly appreciated.

[ger21]

I think you'll need to use .5 lead for the 55in screw, if you want 200ipm without whipping.

My .25 lead screws whip at 150ipm.

[dpgoldberg]

I think you'll need to use .5 lead for the 55in screw, if you want 200ipm without whipping.

Okay, is there any advantage between 1/2"-8 4-start and 1/2"-10 5-start?

[dpgoldberg]

I'm considering two different ways of doing the bearing blocks for the Acme leads screws. The first is to leave the ends of the 1/2" screws unturned and use 1/2" bore bearings, couplers and shaft collars, as shown in this cut away diagram:


The second option is to turn down the ends of the Acme screws and use smaller bearings, couplers and shaft collars, as shown in this diagram:


Can anyone see and advantage of one over the other? Other than not having to spend the time turning down the lead screws. Oh, I'm going to use helical couplers rather than the solid ones shown.

[dpgoldberg]

And here's a third bearing block option using a captured 1/2" bore radial bearing with 1/2" thrust bearing on either side...