Developing Custom Speeds and Feeds Recipes
With ProvenCut machinist Vince Ramirez
Speeds and feeds recipes are essential for CNC machining operations because they are what we use to define our cutting parameters for the toolpaths we program. For a 3-axis CNC like the Bantam Tools Desktop CNC Milling Machine, there are four key cutting parameters you’ll need to define when programming your CAM:
Spindle speed (RPM, revolutions per minute) relates to how quickly the cutting edge moves along the material
Feed rate (IPM, inches per minute) controls how quickly the tool moves through the material
Axial depth of cut (ADOC, aka stepdown) is how deep your tool cuts into the material
Radial depth of cut (RDOC, aka stepover) is how much space there is between tooling passes when cutting a material
SFM to RPM
When it comes to spindle speed, two values to keep in mind are surface feet per minute (SFM) and revolutions per minute (RPM). SFM is how quickly an end mill moves across a given distance, while the RPM is how many revolutions an end mill makes across a given distance. To better illustrate this, take a look at this crazy bicycle.
Notice how there are big wheels and small wheels.
If you ride around for a minute on this bike, all the wheels are going to travel the same distance. However, the smaller wheels will need to rotate faster than the larger wheels. Bringing this back to CNC machining, this means that tooling with larger cutting diameters will have higher SFM and, in turn, you’ll be able to program a higher RPM.
In order to arrive at the appropriate spindle speed for your tool, you’ll need to calculate your SFM and then your RPM using the following formulas:
To save you time, many manufacturers publish SFM tables to help you calculate your RPM. And as you gain more experience with machining and creating your own recipes, you’ll get a better idea of the SFM (and in turn RPM) ranges you want to stay within when using a specific tool on a certain CNC machine.
Sweet Spot
Once you have a better handle on the range you want to stay within, you can begin to further hone your recipe. Finding the sweet spot for your speeds and feeds recipe is often a balancing act between material removal rate (MRR), tool life, and surface finish.
Material removal rate is how quickly you can machine a toolpath or part
Tool life is the duration of time you’re able to machine with a tool until it’s no longer usable
Surface finish is what the part looks after it’s been machined.
The image on the right is from Bob Warfield’s CNC Cookbook. This site is a great resource for CNC machinists!
Chip Load
One of the keys to unlocking that sweet spot is chip load. When a tool is cutting a material it takes a bite out of it. Chip load can be thought of as how big or small of a bite your tool takes. But a bigger chip load isn’t always better. While taking bigger bites may result in a higher MRR, it may shorten the lifespan of your tool and/or impact the quality of your surface finish. As a result, a good strategy to use is to start with a smaller chip load and progressively increase as you get a better idea of your tool and CNC machine’s capabilities.
Live Q&A Recap
To help you gain more insight into how to think about speeds and feeds, we asked machinist Vince Ramirez to join us for a live Q&A. Previously, Vince was part of our Desktop PCB Milling Machine remote residency and now he is working with Saunders Machine Works to develop ProvenCut recipes for the Bantam Tools Desktop CNC Milling Machine.
Vince has been deemed the Maverick of desktop CNC machining and for good reason: His surfaces are top notch. And in this Q&A, Vince shares variables he takes into consideration and strategies he uses to arrive at his speeds and feeds recipes. Let’s dive in!
Q&A Overview
About Vince and his experience with desktop CNC machining (1:54)
Defining speeds and feeds (5:10)
The balancing act between material removal rate (MRR), surface finish, and tool life (7:55)
The speeds and feeds sweet spot (9:22)
What is chip load? (11:20)
What variables impact speeds and feeds recipes (12:20)
Not all CNC machines are created equal (15:15:13)
Why Vince always considers the K factor (17:40)
Vince’s workbook (18:39)
Listening to your cuts (25:30)
Managing chatter with desktop CNC machines (33:20)
Show and tell (34:40)
Tips for avoiding breaking your tool and welding a tool (42:45)
Thread milling (45:30)
Machining steel with the Desktop CNC Milling Machine *Caution* (46:40)
Last minute questions and closing remarks (51:10)
Additional Links & Resources
Saunders Machine Works thread milling calculator
Fusion 360 Workflows using the Desktop CNC Milling Machine