CNC: Feed Rates, Limit Switches, Passes and Speeds
This post is for a reference for calculating tool speeds and feed rates for different materials and setting up limit switches.
Depending on the project it may be beneficial to machine the project with two passes. The first pass is called a roughing pass which aims to remove as much unwanted material as quickly as possible. The second pass is called a detailing pass, smoothing the edges and adding the intricate detail.
A larger bit such as a large straight flute to create a large pocket can be used in the first pass to remove large amounts of material. Additional passes can be made with progressively smaller bits progressing us to a point where we can swap to a smaller bit for the more intricate parts of the project.
Increasing the load on the tool means removing more material by cutting further and deeper into the stock. This can be done by increasing the depth on each decent on the Z axis and / or increasing the step-over value for removing material on the X and Y axis.
For roughing a common step-over setting is around 35 – 40 % of the bits diameter being used.
The cutting depth on the other hand will depend on the ability of the CNC machine and the material being used. In the case of using the MDF with a 6mm solid carbide cutter, a common approach seems to be to use 2mm steps and a very fast feed rate. You shouldn’t get fine powder dust from the MDF but more like wood shavings. If you go too slowly the bit will gum up, burn out and blunt quickly due to the build up of friction. When a tool makes an unhappy noise it means that either the RPM or feed rate are wrong.
What is step over?
A tool can be configured to make several adjacent cuts rather than passing the full diameter of the bit for the next cut. For example, to cut a 12 mm width pocket with a 6mm bit we can make 2 passes to remove the material. Though the fastest option this has two disadvantages. Firstly a high load on will be applied to the bit so a slower speed rate will be needed and / or a decrease in the depth and, secondly a ridge of unwanted material, called a scallop, will remain between these two cuts leaving a rough texture. By using step-over we increase the amount of passes but still maintain or increase the feed rate. In our example if we change the step over from 100% to 50% then we will cut the diameter of the bit (6mm) on the first pass then 50% (3mm) of new material on the second and finally another 50%, completing our 12mm width pocket in 3 passes rather than two. This results in a smaller scallop left. The less material removed in each step the better the finish (but the longer it takes to complete).
The formula to calculate the feed rate is:
And secondly the switches are used to inform the controller of the home position when the machine is setup ready for a cut.
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
“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.
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.
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
General Cutting
preparing for the cut
Before making the cut we have to first prepare the path within our CAD/CAM package with a few additional settings other than our usual feed rate. To do this we have to decide on what type of pass we are going to make.Depending on the project it may be beneficial to machine the project with two passes. The first pass is called a roughing pass which aims to remove as much unwanted material as quickly as possible. The second pass is called a detailing pass, smoothing the edges and adding the intricate detail.
Roughing Pass
To remove as much material in the shortest time possible one or a combination of the following may be used:- A larger bit to move as much of the material as possible.
- Increasing the load on the tool to remove more of the material at any one time.
A larger bit such as a large straight flute to create a large pocket can be used in the first pass to remove large amounts of material. Additional passes can be made with progressively smaller bits progressing us to a point where we can swap to a smaller bit for the more intricate parts of the project.
Increasing the load on the tool means removing more material by cutting further and deeper into the stock. This can be done by increasing the depth on each decent on the Z axis and / or increasing the step-over value for removing material on the X and Y axis.
For roughing a common step-over setting is around 35 – 40 % of the bits diameter being used.
The cutting depth on the other hand will depend on the ability of the CNC machine and the material being used. In the case of using the MDF with a 6mm solid carbide cutter, a common approach seems to be to use 2mm steps and a very fast feed rate. You shouldn’t get fine powder dust from the MDF but more like wood shavings. If you go too slowly the bit will gum up, burn out and blunt quickly due to the build up of friction. When a tool makes an unhappy noise it means that either the RPM or feed rate are wrong.
Finishing Pass
A finishing pass is the reverse, removing smaller amounts of material adding detail and smoothing the surface. To do this we can use a combination of a smaller bit, reduce the depth of cutting and decrease the step over by about 3-5% width of the bit. We will be running the tool faster with less load rather than slower with more.What is step over?
A tool can be configured to make several adjacent cuts rather than passing the full diameter of the bit for the next cut. For example, to cut a 12 mm width pocket with a 6mm bit we can make 2 passes to remove the material. Though the fastest option this has two disadvantages. Firstly a high load on will be applied to the bit so a slower speed rate will be needed and / or a decrease in the depth and, secondly a ridge of unwanted material, called a scallop, will remain between these two cuts leaving a rough texture. By using step-over we increase the amount of passes but still maintain or increase the feed rate. In our example if we change the step over from 100% to 50% then we will cut the diameter of the bit (6mm) on the first pass then 50% (3mm) of new material on the second and finally another 50%, completing our 12mm width pocket in 3 passes rather than two. This results in a smaller scallop left. The less material removed in each step the better the finish (but the longer it takes to complete).
Router / Spindal Speeds
To calculate the correct speed rate we need to know the speed of the tool being used. Rather than a spindle I use the cheaper alternative, a hand trimmer (also known as a mini router) the model of which is a Katsu 101748 which is apparently mechanical similar if not identical to the more expensive Makita RT0700.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 |
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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