3 Axis CNC router

[JoeHarris]

Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

I intend to build the timber table next so I have a base to work up from - any comment would be greatly appreciated before it is too late!!

[irving2008]

Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?

No, even digital drivers can miss steps. A stepper only misses steps if you're asking it to perform outside its torque curve. As long as the torque available at the speed/acceleration you are operating it at is sufficient to move the motor onto the next step, or prevent it moving to the next step when inertia comes into play on deceleration then it'll be fine. Microstepping makes it much harder and the risk much higher esp if you go over 1/4step.

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

Shouldnt make any difference as long as belt tension is sufficient to maintain meshing with the pulleys, tho avoiding long unsupported runs of belt reduces the possibility of skipping a tooth due to belt 'flap'

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

If you mean diagonal bracing, then yes it wouldnt hurt

I intend to build the timber table next so I have a base to work up from - any comment would be greatly appreciated before it is too late!!

Good luck!

[Jonathan]

Some of the digital drivers have a stall-detect feature which changes an output on the driver if the motor stalls. You can connect this to the e-stop so that the machine stops moving to prevent any damage should only one of two motors stall. However I don't think this feature alone is that big an advantage since if you're stepper motors are stalling, even just occasionally, then there is something wrong with the system - either you are trying to get too high acceleration/speed from the motors or there's a mechanical fault. If there's something causing the motors to stall then one should find the problem, not compensate with a safety feature.

Once you have tuned the motors properly they will not miss steps or stall. Plenty of people, myself included, use two stepper motors on their X-axis with standard drivers and do not have problems.

[JAZZCNC]

Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?

No it doesn't go away but if the digital drive has a feature to monitor stall detection and send an alarm signal like the AM882 drive do then it's possible to use this to informthe control software to stop the machine and therefore the other motor before any damage is done.!! . . . . You'll still have lost steps and run out of position slightly but no damage is done.

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

No don't make much difference other than you have 2 long lengths of belt to handle flap so in the middle limits this slightly.

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

Absolutely bracing never hurts.!! Even a little is Certainly better than none and the moving mass of the gantry decelerating from speed will easily twist an unbraced frame.

[m.marino]

I know others have chimed in and given very good information, my two cents is as follows:

Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?

No digital driver do not remove the risk of missed steps. I am running twin screws with twin drives (one slaved to the other via Mach) and using digital drives (the very AM882's that Jazz refers to). I had been running them at 8.5m/min velocity and 1.75m/sec^2 acceleration for a good while, without any stalling what so ever at all. That was for the X axis as each one I tune a bit different. I am using 1605 ballscrews and direct drive. Currently I am running them at 8m/min V and 1.5m/sec^2 A and the little bit of jerkiness that was present has gone. Running a good solid voltage for the motors helps a lot. Currently running 60V and will be running 68V when I finish building the new power supply (almost have al the parts and slowly getting comfortable with how to d o it correctly).

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

I will leave this to those who have experience with belt drive systems.

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

Yes, rigidity is your friend as long as it does NOT cause binding or increase enertia unduly.

I intend to build the timber table next so I have a base to work up from - any comment would be greatly appreciated before it is too late!!

Yes when building your table do the best you can to make sure the surface is truly flat and set up points with which to bolt the machine to it so that once built the timber frame can be secured and act as additional part of the rigid structure to help absorb vibration from the machine.

Good luck and keep asking, will help as able.

Michael