Stepcraft 840 won't move

I managed to unblock the three axes but now each one of them gets stuck when it reaches the end of the axis.

Hi Carlokko.

Please see my recent posts on this subject which are summarised below.

http://www.stepcraft-systems.com/en/forum/assembly-and-maintenance/3777-stepcraft-is-an-unbaked-product-and-i-hate-it#39996

http://www.stepcraft-systems.com/en/forum/operation/3176-clicking-noise-x-y-axes#40100

I believe that the majority of Y axis 'mechanical tuning' issues that people are experiencing, particularly with the larger footprint machines are due to two factors (assuming that your track roller assemblies are correctly set and run freely in the extrusions without excessive play):

(a) Screw mounting hole size and/or position constraining the end plates from aligning correctly.

(b) the lack of some free play in the bearing holes in the end plates.

The instruction manual does not stipulate how to address these issues because they involve modifying the machine.

1. It seems counter-intuitive but I understand that there needs to be some free play in the lead screw mechanism, especially at the non driven end of the axis. Largely speaking, the track roller and extrusion assembly provide the alignment while the lead screw & lead nut assembly provide the linear drive. The two functions are not fully interdependent. This must be a characteristic of the lead nut/lead screw linear drive type and will be more of an issue the larger the machine.

Note that the aluminum T slot bed is machined to a precise tolerance and will therefore constrain the available extrusion movement.

You will almost certainly need to remove the orange powder coating from the bearing seats. You might also need to remove the powder coating and file away some of the metal in the end plate screw mounting holes to provide sufficient screw clearance to avoid them misaligning the extrusions or applying stress to the lead screws when the mounting screws are tightened. When the end plates are tightened up to the extrusions, both lead screws should turn easily by hand/fingertips.

Take care not to drive the Y axis too quickly by hand operation of the drive belt or the generation of back EMF will risk burning out the Y axis chip in the control PCB. A sensible precaution is to disconnect the four wires from the control PCB.

2. Once there is sufficient free play in the lead screw(s) and the mounting screw holes are big enough to stop the end plate misaligning the extrusions when the screws are tightened, align them by loosening the end plate or gantry, drive the track rollers/gantry to one end, align each (in the case of Y axis) then tighten up the end plate screws. This process acts to self-align the assembly.

3. Resistance in the stepper motor (when it is de-energised of holding torque, obviously) might be due to the superglue used to fix the bearing in place bleeding onto the shaft and causing fouling. Exercising it by hand might free it off. If not, remove and clean.

4. As an additional precautionary measure if things are still not going well is to reduce the full travel speed (G00) from 3,000 to maybe 2,250 to 2,500 mm/min in UCCNC or Mach 3 or whichever control software you are using.

I hope the above insights help you resolve your issues.

Regards.

Doug

Hi Doug

Just to add my "two pennyworth" to this topic - I believe that many of the problems builders are having in "tuning" their machines are due to powder coating over critical areas.

Examples are the bores and seating for the track rollers and the bearing housings for the ballscrews. If I were assembling my machine now, I would certainly carefully remove all traces of orange in those critical areas.

The basic parts are certainly machined to a high degree of accuracy by a CNC machine, and that basic accuracy is clearly severely compromised by a variable thickness of powder coating. Perhaps Stepcraft should consider masking critical areas before coating.

Another potential trap for the unwary is the alignment of the 4 Y-axis fixed rollers so that the two sides are perpendicular and parallel. A simple jig consisting of two suitably sized rods clamped to align all 4 rollers before they are finally tightened would greatly improve the overall accuracy of the Y axis and hence the chances of jamming.

On the larger machines particularly, it is most important that the final tightening of screws is done with the machine on a dead flat surface. If the basic assembly is twisted, the chances of free movement over the whole range of Y movement are small to say the least. Fitting the T-slot aluminium base is probably well worth the extra cost so as to minimise this source of frustration.

One final point - if it is found necessary to slow the maximum slew rate of any axis to get full range travel, then something is certainly misaligned or over tightened!! The motors specified have more than enough torque available at rated full speed to easily move the axes if mechanical adjustments are correct - I've checked this out and measured it on the Y axis of my own machine !!

Regards

Peter

Hi Peter.

I agree with everything you have said there.

I have also experienced random binding of motors at specific sections of the travel at 3,000 mm/min which I can't explain.

As I've said in other threads, I've got my track rollers aligned and lead screws moving freely with light finger pressure so I've decided to reduce rapid speed as an additional conservative measure due to random odd issues.

I just got a new Y motor because I found some odd binding when turning the original one by hand off the machine... Inexplicable.

I disassembled it, removed the stator then screwed the rest back together, fitted the pulley wheel and used it as a dummy low resistance unit for manual jog testing with the drive belt fully coupled.

Doug

Hi Doug,

For your interest, I've attached a picture of my crude setup to measure the torque required to drive the Y-axis. The cord goes to an offscreen spring scale which reads the force required.

This measurement was repeated at either end of the axis, and the results were both around 10% of the rated holding torque of the motor alone.

Sound like you had a foreign body inside your old "Y" motor - either that or a very defective ball bearing, most unusual.

Regards,

Peter