Week 7

Computer Aided Machining

About the Assignment

I am doing a backup tape shelf for the University IT department, their requirement was that they had a system of daily, fortnightly and monthly bakcups and wanted something which could fit into a shelf and keep the tapes neatly organised in the same manner. I got the dimensions of the tapes here. For our lab and personally, I wanted a camera stand, which I have designed as well and fit into the same 8x4 feet MDF 11mm sheet.

Materials needed

  • CNC machine
  • Drawing software - Rhino in this case. This covers the steps taken to draw the tape shelf.
  • Machine control software - PartWorks in this case

How it was done - Drawing

  1. Fire up Rhino, draw the plan for the shelf (300x100 mm) and divide it into 12 equal parts to accomodate 12 sony magnetic backup kits in a row.
  2. Since 3 such rows were required for storing daily, fortnightly and monthly backups, the next step was to draft the height (aka elevation) (330x300mm) to understand the height and form of the vertical components required.
  3. To connect the vertical and the horizontal components, I chose to go ahead with the 'COMB' joints or 'Waffle' structure by using a modular design of the components to allow flexibility and modification in the assembly of the shelves.
  4. After making the horizontal and the vertical components, the next step was to make the base for all the components, on to which all these would fit on
  5. To understand the assembly better, I made a 3D model of shelf assembly. This was done with the "Extrude" command
  6. To avoid the rounded edges formed by the movement of the milling bit, I added the provision of 'DOG-BONE JOINTS' to all the notches.
  7. Since I was planning to use the 8'x4' MDF board sheet for cutting, I nested the components for two of my projects (camera stand and the shelves) on one sheet.
  8. The final file is linked here as a 3dm and here as a dxf

How it was done - Cutting

  1. AS the shopbot is parked at a safe height and distance from point 0 or origin, take the spanner and chuck nut and change bits as needed.
    Types of bits we use, not shown in a ball nose for finer cuts.
  2. Power up the Shopbot - for us at CEPT Fablab, this is by switch, and turn on the vaccum
  3. Turn on the Shopbot motor with this knob - Rotate to the right to turn it on. The key controls a lock mechanism, and this is engaged when turned right to the "engaged" position.Note how we have tied the spanner and chuck nut to this key to ensure they are not seperated or lost.
  4. Power up the laptop with the shopbot controller installed, and if its the first run of the day, Start the Shopbot controller software. In the remote , hit the reset button to power up the contoller box. In the software, Under the menu item "Cuts", select the job to warm up the spindle . The software runs through a sequence of commands and prompts you to start the motor, so it can run at a safe height at about 9,000 rpm for about 9 minutes. This warms up the machinery and ensures good lubrication of the shaft and gears.
  5. Open up Partworks (2D if you have simple cuts - pockets or through cuts and 3D if its a complex shape). Plan the type of cut you want
  6. In the first screen, got when you start a new file, the left side menu-window set the size of the job, define height of the material your cutting, and select the Origin. Disable "use Origin Offset" as this option allows you to set a point as origin, but cut a predefined distance away from it. Last important option to select in this screen is the Units of measurement and to centre the job.
  7. Once it loads, on the right side of the screen, pin the toolpath menu which is usually hidden. This screen allows different types of toolpaths
    Profile cut is a through cut
    Pocket cut is a shallow cut, not going throught the material
    Drill cut is a through cut, vertically
  8. Select Profile Cut
    Profile cut is a through cut
    Start Depth defines the height at which the cut is started, and Cut Depth is the depth upto which the cut will be done. Ususally, it is best to add an 0.25-0.5mm to the cut depth so that the material is cut through completely.
  9. Tool selection
    Allows you to select different types of bits, and defines the RPM (revolutions of the bit per minute) and the Feed Rate of the material and Plunge rate of the material in inches / second
  10. Passes option allows you set the number of times the bit will move through the entire length of the job, cutting away a smalled depth of material to reduce the chances of failure and breakage of the bit
  11. Adding Tabs helps keep the cut in place, the Tabs option allows you to do this.
    Selecting number of Tabs
    Positions of the Tabs in the parts being cut
  12. Selecting the next option at the end of the toolpath menu gives this popup if the cut depth is greater than the material thickness as a warning
  13. The preview toolpath option simulates the toolpath
  14. TO save the toolpath for a cut, select the profile you just created Then click the save toolpath button In the resultant screen, leave te default option of Inches as shown and click save
  15. In the ShopBot Controller, select the Movetoolpath button A screen pops up, allows you to control the location of the the drill This menu also lets you set the location as Zero
  16. Once the file is open, this screen pops-up Here you select "Start". The next 4 screens follow in sequence
  17. From this point, the process is automatic
I started the cutting with 11mm plywood. As the wood started to split due its low quality, I switched to MDF. The Casette rack is completely modular, and looks like this when assembled with 2 racks The Camera stand came out like this


I used this week to make 2 large objects - a camera stand to document my work at home and a casette stand for the College IT department.