Computer-Controlled Machining

aka ShopBot Assignment

Task is to make something big using CNC Router, if the lab has a ShopBot then use that, else use some other large CNC machine to make some relativity large object. Plywood or similar material could be used.

A Gaming Chair for Work.

I'm planning to make a chair, like,

'Rapid Prototype'

Redesign

Our instructor Fransico and my brother in law who is an orthopediatrician had a few suggestions. Also I myself thought this chair is not 'lazy' enough for me!
So I redesigned.

module copy_tran(vec=[5,5,0])
{
  children();
  translate(vec) children();
}


sd = 1;          //diameter and the width of the slots in flexure
sl = 10;         //length of the flexure slots

fw = 50;         //total width of the flexure
fl = 200;        //total length of the flexure

sepslotx = 6;     //spacing between slots along x
sepsloty = 3;     //spacing between slot along y

$fn = 64;        //smoother render, this number gives how many frangments are used for a circle

difference ()
{
  translate ( [ 0, -sepsloty] )
  square ( size = [ fw, fl], center = false);

  for ( i = [0 : 3])
  {
    for ( j = [0 : 20])
    {
      copy_tran(vec=[i*(sl + sepslotx), 2*j*sepsloty, 0])
      copy_tran(vec=[(sl + sepslotx)/2, sepsloty, 0])                 //duplicate and translate the copy
      union()
      {
        square ( size = [ sl-sd, sd ], center = true );

        translate ( [(sl-sd)/2, 0, 0])          //right circle
        circle ( d = sd );
        translate ( [-(sl-sd)/2, 0, 0])         //left circle
        circle ( d = sd );
      }
    }
  }
}
      

More R&D

I was skeptical about my design, not the shape but the dimensions. Also I had to choose a good flexure pattern, and we are running out of plywood for any some major testing. So I started to search internet for similar designs, or designs that use the 'living hinge' design.
I found a few interesting resources.

• 50 DIGITAL WOOD JOINTS
• Curved laser bent wood by Aaron Porterfield at instructables

Re-Designing again.

A few more tweaks were made and the i also included the flexure.

I decided to reduce the length of the the teeth (the bend part that holds on to the 'T') because the original tooth is very hard to handle, it takes a lot of effort to assemble and disassemble.
s

Download the original Rhino file here

Routing

I used Vcarve-ShopBot Edition to make the tool paths from the DXF. I'm not talking about it much, just the following

• Used 1/4" 2 flute, downcut.
• Feedrate of 45, each pass cutting 3.5mm.
• Spindle speed of 12000rpm.
• Using 18 mm plywood, the slot is 9 mm deep, so is the notches for the T on the flexure.

I also saved the individual tool paths, separately, just in case.

As you know, the maximum bed area of the ShopBot is 8ftx4ft, and my layout takes only about 75% of it. So I loaded the plywood to the machine, then clamped them at the far end(where no cut is done).
After this I set the origin, then made a few marks where the tool will never go while cutting the parts. This is where I'm going to use screws to secure the plywood further.

As I said before, the job takes about 12 hours, maybe less. And shopbot is a machine that deserves some respect and attention, else it will 'teach you a lesson of your life'.

So, let's make some noise

The shopbot is painfully noisy machine, so without the hearing protection, be prepared to have bad sleeps and strange sounds.

Now everything is ready, don't forget to switch on the vacuum and don't start routing without starting the spindle (happened with me once!). Also keep the chuck as tight as possible(tighten is a human, don't break it). For lengthy jobs like this, it's better to have the entire tol paths in multiple files, and in between(or at regular intervals) the cuts you should check for any loose bolts, loose chucks, clogged vacuum etc.

'Disasters and Disaster Management'

During the routing, Once the bit came loose and started to make some marks where it's not supposed to, I detected this immidiately form the change int he noise and killed it, had to tighten the chuck/collet again and re-do the Z-zeroing step.

Another issue came when I stopped the machine for a break and switched off the power. during this time while inspecting the job, I accidentally moved the YZ-carriage, I lost the origin, and I haven't noted the relative position of the set origin compared to the machine's absolute origin. If you don't want to face this awesome issue, please 'home' the machine to absolute origin and then move to your required origin(origin of the job), before keeping it as the new origin, note down the positions with respect to the absolute origin.

But in case this happens to you,
Choose a known position like a drill hole or two different positions like a 'wall'/cut along X and a 'walls' along Y to recever the origin. The 'walls' work the best(as long as they are parallel to the axes). I used the walls/cuts along X and y to recover my origin.

The procedure is as follows, from the G-Code or a the V-Carve 3D view, choose a know position, then the move the bit to that positions at small incremental steps, smaller and smaller as you get closer to the target. Once near the target lower the Z below the material(I presume that the material has been removed at previous job), and then move the axis you want to set with increments like .1 or .05 mm. Keep the spindle rotating WITH YOUR HAND, DO NOT POWER THE SPINDLE DURING THIS STEP. Once the toll barely touches the material, set the coordinate of the positions the one from the G-Code/V-Carve, if you are setting one axis at a time, then set only one at a time even in this step.

Before proceeding to assembly, mind the tabs we used to secure the job, if you lift pieces blindly, you will damage the job. So patiently cut every tab and use a file to even out the surface.

Assembly

The overhanging flexure to the left will not be there when I finally assemble it all together. Also, you might have noticed that my head is just outside the boundaries, but his too will not be there when the top is mounted to the frame, as I'll be sitting bit lower. But I agree that it could have been a little longer or curved a little bit; next time...

Other References

• Living Hinge Swatches