User's Guide

Simple Speeds calculates the spindle speed and feed rate for a CNC router operation. It is designed specifically for CNC routers, focusing on those that are non-industrial machines, as commonly used by makers, hobbyists, and small businesses.

This guide covers the topics listed here, in order of how the fields appear in the calculator. Use the links to jump down to a section.




Selecting “Imperial” will change all input fields and results to imperial values:

  • Inches (in)
  • Horsepower (hp)

Selecting “Metric” will change all input fields and results to metric values:

  • Millimeters (mm)
  • Kilowatts (kW)

The units can be toggled at any time. Any existing inputs and results will be converted to the new units selection.

Select your machine

The calculator optimizes the operation for you based on the specific machine you are operating.

Machine Make

Use the dropdown selection list to choose the manufacturer of your CNC router.

Machine Model

Once a manufacturer is selected, use the Model dropdown selection list to choose the specific model of your CNC router.

If your make or model is not included in the dropdown list, you can request it to be added. Send an email to [email protected] and we’ll work with you to add your machine to the calculator. Even if it’s a DIY machine of your own design, we will add it for you.


Select the router or spindle that you are using on your machine. If yours isn’t listed, choose “Other” and please send a message so we can add yours to the list.


The Power field will be populated automatically. When “Other” is selected, manually enter the Power rating of the router or spindle.

Material you are machining

Material Type

First select the category of material to narrow down the material selection list. Note that Hardwood and softwood does not refer to the hardness of the wood, but rather the type of tree as defined by the forestry industry. For example, Balsa is considered a hardwood even though it is very soft.


Select the material you will be machining/cutting/carving. The material is used to determine the appropriate surface speed for the operation.

Tool Material

Select the material that the cutting tool or bit is made of. Carbide tools are most common and recommended for CNC routers. Select Carbide for solid carbide, carbide tipped, and any coated bits. High Speed Steel (HSS) tools are more commonly used on milling machines at lower surface speeds (less than 5000 RPM).

Tool Diameter

Enter the diameter of the cutting tool that you are using. This should be the diameter of the cutting edges that are in contact with the work piece, rather than the diameter of the tools shank (aka shaft) that mounts in the collet.

As an example, for a chamfer operation, the bit changes in diameter from almost zero at the tip up to some maximum. Enter the approximate diameter where the tool will be in contact with the stock you are machining.

Cutting Edges

Enter the number of flutes, inserts, or cutting edges on the end mill, router bit, or cutting tool. This must be at least 1 and is typically not more than 4, but could be up to 10 on some specialty bits. 1 or 2 is most common on the CNC routers that this calculator is designed for.

Chip Load

A default chip load is provided, based on the prior inputs from the user. This default can be used by a beginner or if there is no better information available to the user.

However, it is recommended to determine an ideal chip load based on manufacturer’s recommendations for the specific cutting tool being used. Most manufacturers provide this data on their websites for each cutting tool, or for broader categories of tools.

If you have a generic, unbranded tool you can look for a similar tool on another manufacturer’s website and use those recommendations. For example, an unbranded (or non-major brand) 0.125”, two flute, upcut, uncoated, flat end mill was bought on eBay or Amazon. You can go to a site like Amana Tools and find a similar tool with the same dimensions, number of flutes, flute geometry, uncoated and see the recommended chip load.

Radial Width of Cut

Enter the width of the cut (WOC). Also known as stepover or radial engagement.

For a slot cut (full-width), or to cut a shape out of a piece of sheet stock (aka through-cut) this will be equal to the diameter of the cutting tool.

If not doing a full-width cut, it is generally better to keep the WOC to a low value, and increase DOC as much as possible.

Axial Depth of Cut

Enter the axial depth of cut for your operation. This is also known as stepdown, depth of cut (DOC), or axial engagement.

For a full-width cut, adjust the DOC as necessary to keep the cutting forces low enough to not overcome the stiffness of your CNC router.

If not doing a full-width cut, set the DOC as deep as possible. Then adjust the WOC as necessary to keep the cutting forces low enough to not overcome the stiffness of your CNC router.



Spindle Speed

This is the recommended speed to run your router or spindle. It is based on the mechanical properties of the material you are cutting and the diameter of the cutting tool you are using.

Feed Rate

This is the appropriate feed rate for the CNC router to achieve the user entered chip load while running at the recommended spindle speed. Chip thinning is already accounted for when appropriate.

Material Removal Rate (MRR)

Material Removal Rate describes how efficient an operation is. The more material you can remove per a unit of time, the faster the job will be completed.

It is generally desirable to maximize MRR. At some point, based on the limitations of your machine, surface finish quality will suffer at higher MRRs due to deflection and chatter.

Using a roughing and finishing strategy can optimize the operation by removing the majority of material at high MRRs, then light passes that remove the final amount of material at a lower MRR. This provides a nice great surface finish and more accurate final dimensions.

Cutting Force

Cutting force for the operation is helpful in knowing if the operation will be too aggressive for the CNC router’s limited stiffness. This is commonly the limiting factor for how hard a CNC router can be run.

Required Power

Required Power helps to understand if the router or spindle has enough power for the operation. Be aware that as RPMs are decreased on a router or spindle, the available power will be less than the rated power. Power is usually not a limiting factor for most operations on CNC routers.


For further assistance: [email protected]