Wobblycogs CNC Mk2

[Jonathan]

hi Jonathan, when you say 4 seems excessive to you what exactly are you referring to? if it's the drivers then i'm at a loss because i was under the impression that each motor needed a dedicated driver, or have i the wrong end of the stick? (again !!!)

I was referring to stepper drivers. You do need one per motor, but my point was you don't have to use the same driver for every motor. There are arguments for getting different drivers for slaved motors which don't apply to the other motors.

[Clive S]

Paulro Have you posted in the correct place as this post is 9 months old!!! >Clive

edit Obviously not !!

[Jonathan]

Paulro Have you posted in the correct place as this post is 9 months old!!! >Clive

edit Obviously not !!

He was referring to post #64 ... had to Google search the forum to find out!

[PAULRO]

i'm still getting to grips with the forum!!! i'll have to get one of my young fellas to bring me up to date with all the new forms of communicating just when i think , yeah, that's makes sense ! that's the way to go ... i read on and find one of you bright sparks has another alternative and that really gets the hamsters running in different directions

[Wobblycogs]

Hello again . Seems a bit quite around these parts, where's everyone gone?

Anyway, it's been a while (cough, 10 years) but I've finally got around to starting to build this CNC. I started by building a sturdy bench for it to sit on. I was on safe ground here. The cross members are 2x9's and it's covered with a sheet of 18mm ply.



I made my first mistake while making one of the support brackets for the x-axis box section. I was rushing and I managed to break a drill bit. In the replacement hole I then managed to break a tap.


Moving swiftly on... Here's a shot of all the parts I've put together so far carefully arranged to make it look like I've done more than I have. I was going to mount the linear rail to the box section when it occurred to me that might not be a good idea. Obviously, I've got to get the x-axis exactly perpendicular to the y-axis. I feels like I should build the y-axis and mount it before drilling the mounting holes for the x-axis. Does that sound about right or should I mount the x-axis rails now and then try to adjust them square to the y-axis later? To adjustment them I'll have a tiny bit of play in the rail mounting screws or I can get more by opening up the holes that mount the x-axis rail assembly to the frame. It feels like I'll only get one shot at this so I don't want to screw it up.

[Neale]

Not quite sure what the problem is about squaring - it might not be as bad as you think!

On my own machine, I bolted down one rail to the frame as straight as I could. I then used the gantry to align the second rail to it, to guarantee parallelism - just enough slack in the mounting holes using M5 (from memory) bolts to allow the second rail to move slightly, then slide the gantry back and forth to get it parallel and tighten bolts. I built the gantry as accurately square as I could, and once the machine was almost finished I could take test cuts and then adjust the gantry to be as square as I could measure from the test pieces. How to square it? Again, in my case, I was driving the gantry at both ends so I could adjust the home position at each end of the gantry so that each time it homed, it automatically re-squared itself. I had arranged the gantry itself to be bolted to "feet" at each end which carried the ball slides and during the initial squaring, I could slacken the bolts slightly to allow the gantry to swivel slightly, then tightening once I was happy. The process does rely, obviously, on having a master and slave drive to the gantry, one at each side, and the motion controller needs to support separate homing of master and slave axes.

There is an underlying design principle in all this. For a home-built machine, where you do not have the machining and assembly facilities to build accuracy immediately into the machine, you need to build in wiggle room so that once built, you can adjust everything. "Adjust on assembly" is an expensive nightmare in a commercial setting so you pay a fortune for manufacturing capability to build in accuracy. As amateurs we don't have that luxury but our time costs a lot less so we do it the hard way! But it works - witness the number of successful machines described on this forum. Good luck!

[Wobblycogs]

Thanks, Neale. I'll have another think about it. What you describe is pretty much what I'd planned on doing and then I was standing there yesterday looking at the parts and I started wondering if it was going to work. My concern in terrible ascii art is something like this:

  y axis (in home position)
        V
    ________
    /      /
   /      /  <-- The x rails are parallel but not perpendicular to the y axis
  /      /

Ah, now I've drawn it out I can see the method you describe will work. You just use the second x-axis stepper to pull the gantry into alignment.

Thanks again. I'm sure I'll have plenty of other stupid questions :-)

[Neale]

I had a quick look back through the thread after I wrote my last post (thread started a long time ago!) and it seems that you are driving both gantry ballscrews by belt from a single stepper. In this case you could adjust gantry squaring by tweaking the belt position on one or other of the ballscrew pulleys a tooth or two at a time - I guess that that is what you were planning? I was able to just move a home switch trigger point slightly but at the cost of doubling up on steppers, drivers, switches and wiring, and general configuration complication!

[Wobblycogs]

Cheers, I've just looked through the thread and noticed there's no good images of the back of the machine. There are two steppers on the x-axis, they are set down a little and then connected by short belts to the screws. It allowed the machine to be a little more compact.

This build has, indeed, taken a really long time for a variety of reasons, just over ten years at this point I think. I'll get there eventually.

I don't suppose you know a good source for ball screws do you? The ones I bought originally have seized. Somehow condensation must have formwed in the ball nut and they have rusted solid to the screw. I thought my workshop was dry and warm enough for that to not be a problem.

[Neale]

No personal experience of buying ballscrews from anyone still in business, I'm afraid. BST Automation via Aliexpress is often recommended in this forum but I have never dealt with them myself.