General Cutting Overview
Before initiating a CNC job, it's essential to configure your CAD/CAM toolpath with more than just a feed rate. Choosing the correct cutting strategy—whether roughing or finishing—ensures efficiency, precision, and prolonged tool life.
Preparing the Toolpath
Depending on the material and project complexity, it's often advantageous to perform two distinct passes:
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Roughing Pass: Removes the bulk of the material quickly.
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Finishing Pass: Refines the surface with higher precision.
Roughing Pass Strategy
The goal here is speed and efficiency. You can achieve this by:
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Using a larger bit (e.g., straight flute) to cover more surface area.
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Increasing tool load, which removes more material per pass.
Techniques to Increase Load
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Increase Z-axis depth per pass.
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Increase X/Y-axis step-over.
For example:
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A 35–40% step-over of the bit diameter is a common roughing setting.
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For MDF with a 6mm carbide cutter, a 2mm depth per pass at high feed rates is effective. The chip should resemble wood shavings, not fine dust.
Note: If your cutter produces a high-pitched or “unhappy” noise, the RPM or feed rate is incorrect. This can lead to burning, wear, and even bit breakage.
Finishing Pass
The finishing pass enhances surface quality and detail:
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Use a smaller bit.
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Reduce cutting depth.
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Decrease step-over to 3–5% of the bit width.
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Increase feed rate while keeping the tool load light.
This results in smoother surfaces and finer detail, ideal for decorative or high-precision work.
Understanding Step-Over
Step-over defines how much the tool moves laterally between each pass.
Example:
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Cutting a 12mm pocket with a 6mm bit:
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100% step-over: Two passes (6mm each). Fast but rough finish.
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50% step-over: Three passes (6mm, 3mm, 3mm). Slower, but smoother finish.
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Reducing step-over improves surface quality by minimizing scallops between passes. The trade-off is longer machining time.
Router / Spindle Speeds
Spindle speed is critical to match material and bit type. If you're using a hand trimmer like the Katsu 101748 (mechanically similar to the Makita RT0700), ensure you're aware of the tool's fixed or variable RPM settings.General Tips:
Too slow results in burning, tool wear, and gumming (especially in materials like MDF).
Too fast without matching feed rate causes chatter or material tearing.
Listen to the machine. A smooth, consistent cutting sound indicates a correct speed/feed combo.
The speeds of the router is defined in the user manual. I have checked both Makita and Katsu and they both seem to have the same RPM for each of the numbered speed settings on the dial. I have included the table below as reference.
| Setting | mm-1 |
| 1 | 10,000 |
| 2 | 12,000 |
| 3 | 17,000 |
| 4 | 22,000 |
| 5 | 27,000 |
| 6 | 30,000 |
Calculating Feed Rate
To calculate feed rate and tool speed for cutting the optimum chipload is one of the most important parameters of all to get right. Chipload is the size of the material that is removed by each cutting edge (flute) in a single revolution of the tool bit. If the chips are too small i.e. we create dust then heat is generated, blunting and burning out the tool.The formula to calculate the feed rate is:
f = n x cpt x rpm
| f | feed |
| n | number of flutes, the cutting edges |
| cpt | chip per tooth, the chipload or material removed by each revolution of the tool bit measured in mm per tooth |
| rpm | revolutions per minute of the tool bit |
‘Ball Park’ values for chip load
| Bit Diameter | Hard Woods | Softwood / Plywood | MDF / Particle Board | Soft Plastics | Hard Plastics | Aluminum |
| 3mm | 0.08 – 0.13 | 0.1 – 0.15 | 0.1 – 0.18 | 0.1 – 0.15 | 0.15 – 0.2 | 0.05 – 0.1 |
| 6mm | 0.23 – 0.28 | 0.28 – 0.33 | 0.33 – 0.41 | 0.2 – 0.3 | 0.25 – 0.3 | 0.08 – 0.15 |
| 10mm | 0.38 – 0.46 | 0.43 – 0.51 | 0.51 – 0.58 | 0.2 – 0.3 | 0.25 – 0.3 | 0.1 – 0.2 |
| 12mm and over | 0.48 – 0.53 | 0.53 – 0.58 | 0.64 0.69 | 0.25 – 0.36 | 0.3 – 0.41 | 0.2 – 0.25 |
Final Notes
Proper feed rates, step-over, and RPM settings are crucial for quality CNC machining. Understanding and applying these fundamentals saves time, reduces wear, and improves your final product.
Next Steps: Limit Switches and Safety
While this guide focuses on cutting dynamics, setting up limit switches is equally important for machine safety and job repeatability. Proper switch configuration prevents tool crashes, over-travel, and damage to your stock or CNC machine.
A follow-up guide dedicated to switch calibration and best practices for homing your CNC axis will be provided.
Limit Switches: Overview
Limit switches have two main purposes. Firstly and most importantly they tell the controller board When one of the axes has moved to its limit stopping the machine from crashing into its support potential causing damage. This is done by using some kind of switch. There are normally 6 switches, two per axis. When any of these are activated it cause the machine to stop.And secondly the switches are used to inform the controller of the home position when the machine is setup ready for a cut.
Switch
In the most simple cases the switch is normally a Snap Action NO / NC switch. Any style of switch can be used but it’s best to keep things simple.The snap action means that pressure is applied to activate the switch and when pressure is released the switch is deactivated.
The NO / NC means Normally Closed / Normally Open and refer to the mode that the terminals of the switch allow, one terminal per mode with a third terminal that is connected to ground.
Normally Closed means that the contacts of the switch are normally closed which results in the circuit being fully energised. Activating the switch opens the contacts and cuts the power to the circuit, a voltage drop.
Normally Open switches have, you guessed it, the contacts of the switch normally open stopping the current flow until the contacts of the switch are closed completing the circuit, a voltage rise.
Normally closed switch circuit are most common (your home lighting is an example). Normally closed are normally used for safety systems because of a distinct character, if your switches lose power it would be the same as the switch being activated, a voltage drop. This allows the circuit to be instantly activated in other situations i.e. if the wire connecting the switches or the switch itself fails. With a normally open circuit it could be days even months before you know you had a problem by then it’s too late. (normally at the point when you take that deep inhalation of breath as you see your gantry smash into the side of your machine).
To get started a NO circuit is sufficient but you should always look towards moving to NC for safety. The complexity of the circuits increase when we start adding noise filtering, more on that subject later
Arduino CNC Shield Limit Switch Pins
Please note when using either NC or NO you mush tell the Arduino that you are using limit stops and whether it is NC or NO. This way the Arduino knows to look for a high (voltage rise) or a low (voltage drop) when detecting stop signals.“To use hard limits with Grbl, the limit pins are held high with an internal pull-up resistor, so all you have to do is wire in a normally-open switch with the pin and ground and enable hard limits with $21=1. (Disable with $21=0.) “ … From GRBL Wiki
Use of NC instead of NO is enabled by configuring $5=1 in grbl:
“$5 – Limit pins invert, boolean
By default, the limit pins are held normally-high with the Arduino’s internal pull-up resistor. When a limit pin is low, Grbl interprets this as triggered. For the opposite behavior, just invert the limit pins by typing $5=1. Disable with $5=0. You may need a power cycle to load the change.” … From GRBL Wiki
The pin for a limit switch is marked on the shield with the name of the axis followed by either a + or a – (one for one end and one for the other). There are two terminals, one connected to ground (easily distinguishable by looking at the circuit as all the grounds of the limit switch will flow to the ground point of the power jack) and one that is used as sense.
Normally Open
With a simple NO circuit it is a mater of placing a switch (using the NO and Ground terminals) in-between these pins which causes a voltage to flow through the switch from the sense to ground when the switch is activated. This is a basic switch circuit.There are a number of ways to implement this; duplicate this circuit for all the axes + and – pins meaning that you would have 6 circuits coming off the board,
Use one single circuit with 6 switches coming off one connection (a stop is a stop no matter where it comes from!),
Or the preferred root 2 switches on one circuit with a circuit per axis. This keeps the wiring down but allows each axis to be isolated allowing you to home a axis and know exactly which axis has tripped.
If more than one switch per circuit is used then they have to be wired in series as one switch must be closed to activate a stop. If this was done in parallel then the both switches would need to be activated. Note: The above wiring instructions can’t be used for NC circuits which must be connected in parallel as the circuit is always on.
Normally Closed
The circuitry for a Normally Closed circuit differs slightly as we have to take into action continuous current flow. Rigging up a single switch is just a case of using the NC terminal.But when we add more switches to a circuit we have to wire them in parallel rather than series so when one is broken the current stops flowing
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