Manufacturing: Feeds and Speeds
lets talk about the feeds and the speeds, and the birds and the bees.
First I want to say hello to everyone, this is my first post on this forum. I wanted to write a topic that I hope will contribute to helping people who are new to machining by providing a better understanding of how a tool works in relation to the material.
Disclaimer: Please understand that my experiences and theories are based on personal experience with large commercial CNC routers and not with the X-Carve; however, tooling interacts with the material being cut the same regardless of the machine size. I have spent the better part of a decade using CNC routes to cut flat sheet materials, mainly plastics. While I have been factory trained and have industry experience I will not claim to be an expert in the field of CNC routing (or grammar). If you don’t agree or see errors in my thinking please feel free to speak up, we are all hear to learn.
Understanding the tool and material relationship
If we could slow down time and witness what happens then a spinning tool is moving linear through material we would see that the flute (the void just before the cutting edge of the tool) is where all the magic happens. This flute allows material to be pushed inside of the tool which is then immediately sliced away. This amount of material that is inside the tool to be cut away is the golden key to CNC heaven. The material cut away is called a chip, this chip is not only pushed into the flute after cutting and ejected, the chip also carries the heat of the tool out with it. Yes, the stuff that is making a mess of your shop is the same stuff that is stopping your bits from turning red hot after an hour long job. The thicker the chip that is sliced away the more heat the chip can be carried away from the bit. If you are not letting enough of a chip be created inside of the tool flute you are not removing any heat and you will burn your material and your tooling. You should be making nice uniform “C” shaped chips.
You should be able to touch your tool after a cut job and it should be warm but not burn you.
Why is correct chip load important? It results in the three best things we as CNC operators can ask for. Longest tool life, cleanest cut, and fastest possible job time. Remember though that Chip Load recommendations are just that - recommendations. They are the best place to start and then troubleshoot to refine from there.
Chip load is simply a measurement of the amount of material that is shaved off with each pass a cutting edge of the bit. This is not some mythical number, you can literally take the chips that are being cleared out of the cut and measure their thickness with calipers and it should match your calculations. Any reputable router bit manufacturer should be able to provide the Chip Load for a specific bit in a particular category of material (i.e. “Hard Plastics” or “Hard Woods”)
Here are the three possible ways to calculate chip load. You will need to know any two variables to solve the third (chip load, feed, and rpm). Typically you will know chip load and also know a RPM or a Feed that you wish to run out, and solve for the other.
If you know Feed and RPM……..Feed Rate / (RPM x # of cutting edges) = Chip Load
If you know RPM and Chip load ………….RPM x # of cutting edges x chip load = Feed Rate (IPM)
If you know feed and Chip Load ……..Feed Rate / (# of cutting edges x chip load) = Speed (RPM)
Example: We will use a single cutting edge tool that requires a .005” chip load. And we are running a spindle at 16,000 RPM.
16,000 (RPM) x 1 (cutting edges) x .005” (Chip Load) = 80 IPM (Feed)
RPM and Feed relationship
If you play with the formulas enough you will notice that you can get the same Chip Load with different Feed or RPM numbers. We are still using a Single Cutting edge tool.
80 IPM / (16,000 RPM x 1 Edge) = .005” Chip Load
200 IPM / (30,000 RPM x 1 Edge) = .005” Chip Load
Basically you can get the benefit of decreasing RPM by increasing the Feed and vise versa.
Number of Cutting Edges
The number of cutting edges you have is basically multiplying the RPM of the spindle. If you have one cutting edge then your tool is only slicing one chip away per revolution. If you have three cutting edges then our tool is slicing three chips away per revolution. This means if I we spun the three edge cutting tool at 1/3 the RPM as the single edge cutting tool we would get the same size chip and theoretically the same quality of cut.
Single Edge Tool ……80 IPM / (16,000 RPM x 1 Edge) = .005” Chip Load
Three Edge Tool ……80 IPM / (5,333 RPM x 3 Edge) = .005” Chip Load
Multi edge tooling is good for slower spindles as it allows the machine to act as if it has a much faster spindle and in relation allows for faster feeds and quicker job times. Think of cutting aluminum on a mill that has a 10,000 RPM max spindle, if you put in a three edge tool you have basically increased RPM to 30,000 as you are taking three cuts per revolution.
Single edge tooling is good for machines with faster spindles (Like a X-Carve with a trim router). You are cutting once per revolution, giving the tool time to take a larger chip out of the material.
Note: This is theory based on adequate horsepower and rigidity
Most tooling is typically designed to cut at least at a depth of 1 x the diameter of the tool. A 1/8” tool can get optimal heat and chip clearing by cutting at a depth of 1/8”. Tool manufactures typically post their chip load as 1 X D which means 1 x Tool Diameter. Please keep in mind that cut depth is very limited by the rigidity of the machine, if your machine is not rigid it can induce chatter which can easily break a bit (especially expensive solid carbide bits which will not flex like High Speed Steel HSS when things get sloppy)
Note that this info is just a generalization I have come up with over time.
Smaller tools are easier to spin and in my experience put less lateral force against your material allowing you to hold down you’re work piece with less force. Their cutting edge is also traveling at a slower speed than an larger tool at the same RPM (remember that the outside of a wheel is traveling faster than its hub). This reduction in speed means that a slower Feed or higher RPM is typically required than that of a larger tool. I have seen that smaller diameter cutters typically leave better edge finish along the top edge of the cut if the material is prone to chipping (think plywood, or a painted surface) as the amount of material being sliced away with each revolution is smaller.
I have learned the hard way that tools are rarely the diameter they claim to be. A .25” endmill can easily be .245” in diameter and result in parts that are larger than expected. If you are working with something that needs to be + - .01” tolerance and you start off with a tool that is .005” different in size than what you tell your CAM software you are already halfway to having you part out of spec. So always measure you tool.
When you are moving a tool downward in the Z axis into a material it is commonly called a Plunge Rate. Please keep in mind that while most flat endmills are designed to be able to plunge downward, they are still a poor design and inefficient when compared to say a drill bit that is only designed to move down and never side to side. All tools behave different in plunging based on their tip design. I have never used a formula (I am sure someone has one out there) for plunging but I typically start somewhere about 1/10 of my linear Feed Rate and see how that goes as a starting point. You will know if you are doing it right as you will get a nice continuous coil of material (if cutting plastic or aluminum) that is being removed from the hole. If you are going too fast you will get a horrible chattering sound and moaning sound. If you are going too slow it will burn or melt the material you are cutting into.
This is a topic I don’t know very much about and would like if someone with experience could chime in. This is typically measured as Cubic Inches per Minute. And has to do with the physical amount of material displaced. It is mainly calculated when milling and especially when doing aggressive high speed milling. (Not typically calculated for flat sheet cutting and mild pocketing).
Climb vs Conventional Cutting
This basically means are you feeding your tool around the tool path either clockwise or counter clockwise (this has nothing to do with the direction the spindle is spinning it is alway spinning the same). Some materials benefit from Climb cutting and will provide a superior edge finish. Basically when Conventional cutting you are running the tool clockwise around your path and the tool is spinning clockwise at the same time, thus, the tool is pushing its cutting edge toward the rear of the cut path. When Climb cutting the tool is spinning clockwise but is being driven around the path counter clockwise, which means the cutting edge is driving into the material in a forward motion. There is a very simple way to determine if you need to cut Climb or Conventional. Simply cut a line running from the front of the machine to the back that is a few inches long. If the cut is cleaner on the left side of the cut then you should run the material with a Climb and if the cut is cleaner on the Right of the line then cut with a Conventional. When in doubt Conventional cutting is well...conventional.
Melted/Burned material or charred endmill - Lower RPM or Increase your feed speed. You are generating too much heat in the tool as a result of not taking large enough chips of material away with each revolution. You need more material to make its way in the flute on the cutter before it gets sliced away by the cutting edge. Either moving the material into the flute faster will increase the chip size or slowing down the revolution to give more time for more material to enter the flute will decrease tool temperature. Remember you should be able to touch your tooling immediately after a cut and it should only be warm and not hot.
Chattering - Slow down your feed rate or increase RPM. This means there is too much lateral force against the tool and it is beginning to bend and deflect its cutting edges away from the proper cut path. Most likely you are trying to put more material into the flute per revolution than there is room for (you are basically gagging the tool by shoving too much material down its throat). When you are getting chattering and deflection you are dangerously close to breaking you tooling which can cost you money and is very dangerous. I have personally had a bit break and fly across the room passing between me and a coworker about two feet next to me and bury itself in a wall (wear your safety glasses). While I have typically only used solid carbide tooling (I was spending other peoples money on tools not my own) HSS or High Speed Steel has more flex to it and can tolerate more chatter and more force against it unlike Carbide which has superior long term sharpness but is very brittle.
Basic Rules of Thumb:
Your tool should not be hot
Your chips should not be dust, they should be slices
Your tool should not sound like Adam Savages “Duck Bomb” - if it sounds bad, there is a good chance it is not cutting optimally.
Now have fun and wear your eye protection or the CNC gods might throw a 1/8" Super-O Flute bit flying by your head.