Types of Sink Cut-Out
- TOP-MOUNT (Drop-In) — the sink sits on top of the countertop, with a projecting rim
- UNDERMOUNT — bonded underneath the countertop, invisible
- FLUSH MOUNT (Integrated) — level with the countertop, the most premium option
- DOUBLE BOWL — two separate cut-outs with a supporting bridge between them
- OVAL / CURVED — a special shape, with detailed CNC programming
The Standard Process — 8 Steps
- Obtain the cut-out template for the sink model (from the manufacturer)
- Draw the countertop and the sink cut-out in CAD
- Set the pierce (entry) point in CAM (in a corner that will not be seen)
- Check the internal corner radii in CAM (min R12-15)
- Make sure the slab is properly held on the table
- Pierce — start at a slow speed (1,000-1,500 mm/min)
- Internal contour cutting — standard speed (2,000-2,500 mm/min)
- Lower edge finishing — the internal profile with five axes (where available)
The Sink Manufacturer's Cut-Out Template
Every sink model has its own cut-out dimensions, supplied as a PDF in the installation guide. Those dimensions have to be transferred into CAD:
- The internal dimension (the sink's inner opening) is NOT the cut-out dimension
- Top-mount — the cut-out dimension is 5-10 mm larger than the internal dimension
- Undermount — the cut-out dimension is 2-5 mm larger than the internal dimension (an allowance for the rim)
- Internal corner radius — follow the manufacturer's recommendation
- Waste hole position — it varies from sink to sink
- If the manufacturer has a DXF file, use it directly (the safest route)
The Pierce Point — A Critical Decision
The pierce is the point where the cut begins — where the blade or nozzle first enters the cut-out. The wrong pierce position leaves a visible defect.
- It must be IN A CORNER THAT WILL NOT BE SEEN — the rear corner of the sink (hidden once installed)
- Not close to the centre of the slab — it should be close to the outside of the contour
- Do not put it at the start of a corner radius — you want a smooth transition
- The cutting speed during the pierce is LOW (1,000-1,500 mm/min)
- On a CNC bridge saw the blade runs slowly for the first few seconds of the pierce, then speeds up
- On a waterjet there is a "low pressure pierce" mode (the pressure is reduced, which prevents cracking)
A Common Mistake
Putting the pierce point at the front edge of the sink — it stays visible after installation. Always place it at the rear edge, and if possible on a corner radius.
Internal Corner Radius — Minimum Values
| Material | Min Internal Corner Radius | Reason |
|---|---|---|
| Marble | R12-15 mm | Brittleness + stress concentration |
| Granite | R10 mm | Harder, but still sensitive to stress |
| Engineered stone | R20-25 mm | The resin matrix is stress-sensitive (a manufacturer warranty condition) |
| Quartzite | R15 mm | Hard and brittle, so a balanced figure |
A sharp corner (R below 10 mm) concentrates stress over time and micro-cracking starts — particularly under heat, damp and the stresses of use.
Cutting Order — Why It Matters
When you machine a sink cut-out, the order of cutting is critical:
- Do NOT cut the OUTER CONTOUR first — the slab is then free and chatters while the sink cut-out is being made
- Cut the INTERNAL CONTOURS first (sink + hob) — while the slab is still firmly held on the table
- Then the OUTER CONTOUR — the "countertop, finished product" shape of the slab
- Last of all the fine detail (edge profile, V-cuts and so on)
- This order maximises structural stability and accuracy
Lower Edge Finishing (The 5-Axis Advantage)
The lower edge of the sink cut-out is where the user's hand touches. A straight cut leaves a sharp edge there:
- FOUR AXES — the lower edge is polished by hand (5-10 min of extra labour)
- FIVE AXES — the lower edge profile is milled automatically (1-2 min)
- The result: on five axes the sink cut-out comes off the machine completely finished
- Lower edge profiles — eased, half-bullnose, 45° chamfer
- On a top-mount sink the lower edge is not visible — so it is optional
- On an undermount sink the lower edge IS visible → so it is essential
The Waste Hole
- The standard diameter is 90 mm (the common waste size)
- If it has to be cut on the CNC — an additional pierce plus an internal contour
- Some sinks come with the waste hole already formed (so the CNC does not cut it)
- An extra tap hole (35 mm, if required) — at the edge of the sink
- A built-in soap dispenser hole (32 mm) — optional
- All the holes in the same CAM file — a single setup
Quality Control
- Verify against the template — the cut-out dimension and the sink must match
- Internal corner radius — check with a radius gauge
- Any witness mark at the pierce point — has it been cleaned up properly?
- The lower edge easing (five axes) — no roughness
- Surface quality — no cracks under a magnifier
- A trial fit of the actual sink (a temporary test before installation) — does it fit?
Common Problems and Their Remedies
| Problem | Cause | Remedy |
|---|---|---|
| The sink does not fit | The cut-out dimension is wrong (internal and cut-out confused) | Use the manufacturer's DXF |
| A crack at an internal corner | The radius is too small | Min R12 (marble) / R20 (engineered stone) |
| The lower edge is sharp | 4-axis machine + the hand work skipped | Finish on five axes, or polish |
| The pierce mark is visible | The pierce position is wrong | Move it to the rear corner |
| Surface quality is poor | The speed is too high | Reduce the feed rate by 20% |
| The slab broke after cutting | Inadequate workholding | More clamps plus support underneath |