Week 7 : Computer-controlled machining - REVIEW

Week 7 assignment could be categorized as follows:

• Group assignment

  • Lab safety training

• Individual assignment

  • Design, milling, and assembly (stackable organizer)

Basics of CNC milling

Common wood sheet materials

Material	Description	Pros	Cons
Plywood	Thin sheets of wood stacked with alternating grain (e.g. horizontal vs vertical) and glued together.	Strong, dimensionally stable, good structural strength.	Surface not as smooth as MDF.
MDF (Medium-Density Fiberboard)	Wood fibers and resin pressed into dense sheets with a very smooth surface.	Very smooth finish, great for painting and CNC.	Heavy, produces fine dust when cut.
MDO (Medium-Density Overlay)	Plywood with a resin-treated fiber overlay for a smooth, durable face.	Strong like plywood, smoother surface, good for paint.	More expensive than plywood, harder to find.
OSB (Oriented Strand Board)	Compressed wood strands (the sausage i.e. a mix of leftover woods) bonded with adhesives.	Very inexpensive, structurally strong.	Rough appearance and definitely cannot do flexures (i.e. not flexible)
Lexan (Polycarbonate)/bulletproof glass	Extremely impact-resistant transparent plastic.	Nearly unbreakable, high impact resistance, clear.	Expensive, scratches easier than glass.

Stock means raw piece of material/workpiece a user starts with before cutting or shaping it.

Tooling

Tool	Description	Pros	Cons	Typical Uses
Drill Bit	Cutting tool designed to make round holes by cutting straight down into material.	Fast and efficient for holes, simple to use, widely available.	Cannot cut sideways well, limited to hole-making.	Drilling holes for screws, bolts, dowels.
End Mill	Milling cutter with cutting edges on the tip and sides that can remove material in multiple directions.	Can cut sideways, make slots, pockets, and contours; versatile in CNC and milling machines.	More complex to use, slower for simple holes, can break if plunged incorrectly.	CNC routing, slot cutting, pocketing, profiling, shaping parts.

End mills cutting

• The most common end mills have 2 to 4 flutes (cutting edges).
• Fewer flutes are used to remove material quickly. More flutes are used for finishing passes to provide smoother surface.
• Center-cutting vs non-center-cutting
  • Center cutting : Cutting edges meet at the middle, allowing the tool to plunge straight down into the material.
  • Non-center-cutting : Evacuates chips more efficiently and are commonly used for side cutting.
• Up-cut vs down-cut
  • Up-cut : Pull material upward during cut. This produces a clean finish on the bottom of the workpiece but a rough surface at the top.
  • Down-cut : Push material downward during cut. This produces a clean finish on the top of the workpiece but a rough surface at the bottom. A common strategy is to start with a down-cut pass to clean the top, then follow with an up-cut pass for the bottom finish.
• When an end mill or drill removes material, it doesn’t just disappear — it produces chips, which are tiny shavings, curls, or fragments. If these chips aren’t cleared away efficiently (e.g. moves it away out of the route of end mill), it could lead to overheating to the wood and the machine.

Types of milling cuts

Cut Type	Description	CNC example	Laser cutter analogy
Slot	A long, narrow channel cut into material, often the width of the tool.	Cutting a track for a bolt or sliding part.	A through cut
Pocket	A recessed area cut into the surface that does not go all the way through.	Carving a cavity for electronics or magnets.	Laser engraving or rastering to remove material depth without cutting through.
Contour	Cutting along the outline of a shape (internal or external path).	Following a curved path around a design feature.	Vector cutting along a drawn path.
Profile Cut	Cutting the final outer shape of a part to separate it from the stock material.	Cutting the outside of a plate.	The final vector cut that releases the part from the sheet.

Sacrificial layer : A stock/material placed under the workpiece so the cutter can go all the way through without hitting the real table.

To check again about vises, bar clamps,vises, sacrificial nails - REVIEW

Lab safety training - REVIEW

Safety training on 2nd Week of April

runout, alignment, fixturing, speeds, feeds, materials, and toolpaths check

Most important characteristics values and parameters in CNC milling

• Milling diameter (mm) : The diameter of the milling tool. Determines how wide a cut is and affects material removal rate.
• Speed (RPM) : The rotational speed of the spindle (the part of machine that holds and spins the cutting tool)
• Feed rate (mm/min) : The speed at which the tool moves horizontally across the material.
• Depth of cut (mm) : Depth of the plunge of the milling tool into the material per pass.

Toolpath

• Kerf
• Runout : A slight deviation in the tool’s rotation caused by tool not being perfectly fixed or vibrations. Runout can make the effective cutting diameter slightly larger than expected and affect surface finish.
• Conventional vs climb milling -
• Tabs vs onion skinning (for stability) :
  • Tabs : Small uncut sections left intentionally so the part stays attached to the stock after cutting. Like small uncut bridges.
  • A thin layer of material left along the bottom of a cut part, so it remains attached but can be easily separated later. Similar to brim in 3d printing.

Source : cncyangsen

Chip Load in Woodworking

• The recommended chip load for woodworking is approximately 0.0025 to 0.025 cm.
• Chip load per tooth is calculated as:

\[ \text{Chip Load (per tooth)} = \frac{\text{Feed Rate}}{\text{RPM} \times \text{Number of Flutes}} \]

• If the chip load is too high, cutting becomes noisy and rough.
• If the chip load is too low, the tool rubs the wood instead of cutting, which can burn the material and reduce tool life.

Depth of cut should not exceed the milling tool's diameter. Cutting deeper than the tool diameter increases the material removed per pass (one complete movement of the tool along the material for a cut) which puts excessive stress on the tool and spindle**

Design, milling, and assembly

The advantage of CNC milling over laser cutter is that thick wood boards could be machined and also more complex three-dimensional shapes could be made.

The design is inspired by users that live in short-term accommodation with the need of stackable organizer that can be easily assembled and disassembled - so joints are interlocked with each other without permanent fasteners or glue. This modular concept then also means users can add or remove layers depending on their storage needs.

stack-organizer-v1 by ichbinea on Sketchfab

Several types of joinery methods are also explored, namely the wedge, through, and the blind joint.

Dogbone and t bone

find buy tool joints