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Computer controlled machining - personal cheat sheet

Based on the midterm review I've written this page. It's like my personal cheat sheet/ check list related to VCarve Pro and the ShopBot workflow

For the complete manual of the ShopBot use this link

For the complete manual of VCARVE Pro use this link

For the feed and speed calculator of MIT use this link

Note

Before using VCarve Pro I have already created a 2D design of what I want cut-out using the ShopBot. I have used SolveSpace sofar but it can be done with OpenScad / FreeCad as well (and many others that are able to export the design as PDF).

Refresher course

Feeds, Speeds, Flutes & Chip Load

Feeds and speeds are the two main numbers that decide how the tool and the material behave together. If they are wrong you get burns, tear-out, broken bits, or ugly edges — a bit like the “monster chewing” effect we saw on our OSB during the group assignment.

In short:

  • Spindle speed (RPM): how fast the bit is spinning (our ShopBot max is 18 000 RPM).
  • Feed rate: how fast the bit moves through the material (usually in mm/s or mm/min on our machine).

Chip load is the thickness of material each cutting edge (= sharp edges = teeth that do the actual cutting) removes in one revolution. Too high, then the bit deflects or breaks. Too low, then you are not really cutting; and it could generate more heat which could burn the wood.

And the channels ('groeven' / 'kanalen' in Dutch) between the cutting edges of the bit are called flutes. The flutes evacuate the chips (= the removed material).

At the Waag, our standard 5 mm bit has 2 flutes (so also 2 cutting edges). More flutes give a smoother finish but need slower feed because there is less space for chips to escape. Fewer flutes let you go faster but leave a rougher surface.

MIT's calculator is primarily based on the formula:

***Feed rate (mm/min) = RPM × chip load (mm/tooth) × number of flutes ***

VCarve Pro

Why use VCarve Pro? Well without a proper toolpath the ShopBot has no idea how deep, how fast, or in which direction to cut my design.

So a toolpath in VCarve Pro is the exact route the milling bit will follow, together with all the cutting parameters. It turns my design = 2D vectors (from SolveSpace/OpenSCAD/FreeCad) into G-code (.sbp files) that the ShopBot can execute.

There are several options here of which I used the most: - Profile toolpath which follows the outline of a vector (inside / outside / on the line). I used this for the outer rectangle and all my kerf lines. - Pocket toolpath which clears material inside a closed vector, leaving a flat bottom at the chosen depth (e.g. the floor inlay or the eyes/mouth on the top plate).

And, for each toolpath I have/can set extra parameters: pass depth ≤ 2.5 mm, feed rate, spindle RPM, tabs (3–4 mm long and thick), climb milling for cleaner edges on plywood, and allowance offset when I need a tighter or looser fit.

To cut the ShopBot needs a milling bit and speed.

As talked about already above, I worked with a 5 mm diameter, 2-flute straight end mill (this hadd a right-hand spiral which up-cut behaviour helps for good chip evacuation upward). In short:

  • Cutting diameter: 5 mm
  • Number of flutes/teeth: 2
  • Shank: fits the ER25 collet
  • No special coating observed
  • Helix angle: standard for wood
  • Protruding length: always reset via Z-touch plate after every bit change or machine restart.

Rotation speed of the spindle is set manual on the ShopBot. I kept the spindle at 14000–18000 RPM depending on the test (see group assignment).

Note

During my assignment I assumed that the bit was aligned correctly all the time and it just worked and that it had a diameter of 5mm. As Henk mentioned "never again assume the bit is aligned correctly just because “it worked last time”. Also do not assume that's still a 5mm bit just because we always use it.

After any bit change, machine restart, or even a long pause I do the following:

  • re-zero Z with the metal touch plate directly on the sacrificial layer,
  • do a quick visual check (white-paper shadow test at low RPM) for runout,
  • and run the first drilling holes as a check to see if the ShopBot is in the same universe as I am.

When in doubt about speed/ feed use the MIT Feeds & Speeds Calculator. During my week I used as a practical rule for 17–18 mm birch plywood on this ShopBot:

  • Kerf cuts / dense areas → 40 mm/s at 14000 RPM
  • Normal profile cuts → 60 mm/s at 18000 RPM

ShopBot

The ShopBot is de Waag's large-format CN; the machine that precisely cuts, drills, pockets, and carves big sheets of material (mainly wood like birch plywood or OSB, but also foam and sometimes soft metals). It is a 2.5D / 3-axis machine: it moves freely in X (left-right) and Y (front-back) across the bed, and the spindle moves up and down in Z.

Specifications:

  • Bed size: 250 cm × 130 cm
  • Spindle: CAT40 tool holder with ER25 collets, max speed 18 000 RPM. We turn it on manually with the key attached to one of the hook wrenches.
  • Controller: Old Windows 7 PC running ShopBot3 software (SB3). Files are transferred via USB stick.
  • Sacrificial layer: A sheet of plywood on the metal bed that we screw our stock to. It gets holes over time and is occasionally levelled with the service bit.
  • Typical bit: 5 mm diameter, 2-flute straight end mill (right-hand/up-cut spiral for good chip evacuation in wood).

Note

The machine is deterministic — it does exactly what the G-code tells it, even if that means crashing into Mars. Most problems I had (wrong Jog Home, Z not zeroed correctly, warped stock) came from setup mistakes, not the machine itself.

Checklist – from design to finished cut

Design & Export

  • Make sure the 2D design has closed vectors only
  • Export it as PDF (this is the most reliable in VCarve Pro)
  • Optional: rotate 90° in Inkscape if needed.

VCarve Pro – Job Setup

  • Start new project and enter exact material size in X/Y and measured thickness (e.g. 17.8 mm).
  • Import the PDF and set the material origin (lower-left).
  • Create the separate toolpaths:
    • Drilling holes first (sanity check).
    • Kerf/profile cuts (on line, 40 mm/s, multiple passes).
    • Outer profiles (outside, with tabs).
    • Pockets where needed.
  • Always preview in 3D first then save it (separate .sbp files).

ShopBot Physical Setup
- Power on the machine and ventilation (check if suction is ok otherwise fix with duck tape 😉).
- Check whether the a bit is installed and that it meets your requirements. - If not install the bit you need: collet into nut first, then bit (never the other way round).
- Home machine (absolute X-Y zero button).
- Make sure you have room to secure the material onto the sacrificial layer by moving the spindle away using the arrow keys. - Secure stock with lots of screws (especially warped plywood) - Then set jog home: arrows / page down to where the material starts = corner. Then choose option Menu - Zero XY. - Take photo of coordinates!!!! - Z-zero with touch plate on sacrificial layer (this layer is fixed on top of the real bed and is sacrified all the time) - Manually turn spindle on with the key, let it warm up. Set speed manually - Do a quick visual check (white-paper shadow test at low RPM) for runout,

Execution
- Load first .sbp (drilling holes) as universe check.
- Be ZEN, stay present, never leave the machine running alone - Press START. Pause only during air/travel moves if needed.
- After the job: remove tabs carefully, sand the edges.

Post-processing & Learning
- Measure actual radii / fit and feed the numbers back into the next design (meten is weten!).
- Document everything while it’s fresh.

Safety reminders
- No loose clothing, protective shoes - Again ZEN mindset.
- Listen for strange noise/vibration.
- Ventilation working? - If you smell burning or see sparks then stop immediately, remove dust sack, throw out window if on fire.