Choosing the right end mill before turning on your CNC machine can be a pretty complex task. Many variables come into play, such as end mill length, geometry, profile type, material, while several trade-offs have to be considered: performance, cost, surface finish, tool life,...
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What is an end mill actually, is it like a drill bit? It is a usual question that comes up since terminologies are often mixed up, but the answer is no. A drill bit is designed to plunge directly into the material and create holes only, while an end mill can cut laterally into the material and create slots or profiles. Besides, most end mills are designed to be centre-cutting, meaning they can plunge into the material as well and making them far more versatile than drill bits.
This article aims at explaining the basics of end mill science while answering the simple question you might have: which end mill should I use on my CNC machine?
Three major factors will influence your choice :
Answering these questions will help you determine the desired tool geometry, based on your project type, the material being cut and on the desired surface finish. So let's dive into the theory and answer those questions later with some concrete examples.
The main features used to describe an end mill are illustrated below.
On top of these geometric features, end mills are characterised by their type, number of flutes, material (and coating). Thus, in a classical catalogue, the end mill above would be described as:
Each of these features has a direct influence on how the end mill will behave during a machining operation and on the cutting possibilities in terms of performance and shapes.
Imagine you want to mill a very deep pocket inside a high part. You will need a long tool to be able to machine the bottom without colliding your spindle mandrel with the stock of the material. Hence, the length of your end mill will be determined by how deep it must penetrate into the material. The associated concept with this observation is called 'stickout'. It is defined by the distance from the end of the tool holder to the tip of the end mill.
Also, keep in mind that the cutting depth should never exceed the flute length of your end mill. If cutting deeper than the length of the flutes, chips won't clear properly, heat will build up and you risk to damage your tool.
At this point, it might seem smart to buy end mills that are as long as possible to have the flexibility to use them in more situations, right? It's actually not the case, because the more stickout, the less rigid a tool is. If it sticks out too far and you work it too demanding, the cutting forces will make it bend, which is what is called 'tool deflection.'
Tool deflection can actually be very problematic since it induces:
In short, shorter end mills are more rigid and less expensive. So save the extra-long ones for operations where they are really needed
The diameter of your tool will have a direct incidence on the profiles you can cut. Let's say you want to make a box and assemble it using 90° interlocking joints.
Since your tool is a cylinder with a certain radius, it won't be possible to do it as such. Indeed, the tool will leave a circular profile in every internal corner, with a radius equal to half its diameter. The larger your end mill diameter, the bigger the radius of this circular profile. In order to overcome this problem, CNC operators use what is called 'dogbones'. Dog bone corners describe the shape of a corner that is extended outside the cut area to create a perfect 90º corner.
On the other hand, increasing the diameter of your tool offer two main advantages.
First, it makes your tool more rigid, which will allow you to make deeper cuts while minimizing tool deflection. A lot more rigid actually, since increasing diameter by 2x will increase its rigidity by a factor of 16x.
Secondly, it improves your MMR (Material Removal Rate) since the end mill can remove more material per unit of time when it is moving inside the material, allowing you to optimise certain operations and do the same job faster.
Flutes are the deep spiral grooves that allow chip formation and evacuation. They are the part of the end mill anatomy that create those sharp cutting edges (sometimes referred to as 'teeth').
The number of flutes on your end mill is a crucial parameter that depends mostly on the material you want to cut and on the capabilities of your machine. Indeed, the number of flutes on your end mill will impact:
Feed rate is indeed directly linked to the number of flutes of your end mill: if you add flutes, you will have to increase the feed rate, or decrease the rotational speed of your spindle, to keep a constant chip load. So depending on the speed capabilities of your CNC machine and of your spindle, you might have to choose an end mill with less/more flutes.
If you are not familiar with those concepts, we recommend you to have a look at our article on feeds and speeds.
Secondly, having more flutes on a tool creates smoother cuts, but it also leaves less space for chips to evacuate. This can be somehow overlooked if you are cutting soft materials, but not at all if you are cutting aluminium for instance. The reason is that aluminium produces very large chips compared to other materials. So when the end mill is cutting down in a hole or a slot, its flutes provide a crucial path for chips to escape. That explains why it is recommended to use 2 or 3 flutes end mills with aluminium since they have more chip clearance than with a 4-flutes one, where the chips will jam progressively, overlapping the cutting edges of your tool and making it break eventually.
To sum up, fewer flutes are best at chip clearing, while more flutes create a smoother surface finish.
General-purpose end mills typically have helix angles around 30°. Increasing the helix angle will reduce the cutting forces and the amount of heat and vibration generated during the milling process. Hence, end mills with a higher helix angle tend to produce a better surface finish on the workpiece.
It comes with a trade-off, unfortunately. The end mill will be weaker and won't sustain heavy depths of cut with high feed rates. So lower helix angle cutters are stronger, but they give a less smooth surface finish.
There are as many different end mill types as there are possible cutting operations: profiling, contouring, slotting, counterboring, drilling,... Here is a short overview of the main ones.
Square end mills are the most common ones and can be used for many milling applications, including slotting, profiling and plunge cutting.
Corner-radius end mills have slightly rounded corners that help distribute cutting forces evenly to prevent damage to the end mill and extend its life. They can create flat-bottomed grooves with slightly rounded inside corners.
Roughing end mills are used to quickly remove large amounts of material during heavy operations. Their design allows for little to no vibration but leaves a rougher finish.
Tapered end mills are centre-cutting tools that can be used for plunging, and are designed to machine angled slots. They are generally used in die-casts and moulds.
Ball end mills have rounded tips and are used to mill 3D shapes or rounded grooves.
T-slot end mills can easily cut accurate keyways and T-slots to create working tables or other similar applications.
Straight flutesend mills have a zero degrees helix. They work well for materials where the lifting effect of a spiral flute might cause unwanted results, such as wood, plastics and composites. For those materials, the straight flute minimizes the fraying of the edges and provides better surface finishes than helical general purpose end mills.
As your CNC router spins the cutter clockwise, the helical direction of the flutes determines if chips are ejected towards the top or bottom of the workpiece. Upcut end mills are the most conventional ones, pullingthe chips away from the material, which is a very important feature for most milling operations on many different materials. It has a downside if you want to cut laminated materials since it leaves a poorer surface finish on the top of the workpiece. A downcut end mill has the advantage to push chips down, leaving a cleaner cut on top, but then it will also fray the bottom edge.
Combine an upcut and a downcut and you have a compression cutter, where the flutes are carved one way for the bottom half of the flute length and the other way at the top. That feature makes them a very good candidate to cut plywood, composites, and laminates. Try to use one to cut a sheet of plywood in one pass, and you should obtain cleaner edges on both sides.
Let's answer our original questions with an example: I want to mill a small food tray.
Based on those observations, I know that I need a flat-end mill type to get a nice flat surface inside the pocket. I will go for a diameter of 8mm, quite large because I don't want to waste too much time while removing all that material and because the inside corners are already designed to be rounded. I'll use a short carbide end mill as well, like 30mm long, because oak is a hardwood and I will need the rigidity to keep a high feed rate. Since the pocket is pretty shallow and chips will evacuate easily, I will choose a 3-flutes end mill with a 45° helix angle to get the best surface finish possible and spend little time hand-finishing.
Let's assume you have a working CNC machine that you've just acquired, but that you know very little about CNC. Let's further assume it is a mill and that you will be focused on cutting metal. You're ready to start milling custom chopper parts, build a tool changer, or scratch build a Colt handgun. With CNC, you can build almost anything and you're chomping at the bit to get started on your pet projects.
Not so fast! Remember, you just got the machine and you're a beginner. You're not ready for those projects yet.
If you have some idea how to make your first CNC parts, charge ahead with these 10 suggestions. Otherwise, you need to see the whole process of making a part mapped out in detail.
Take a break and go read our Beginner's Step-By-Step Guide to Making CNC Parts. It will fill in those hazy details you've been wondering about, then, you're ready for this article.
If you're even more of a beginner, and likely don't have a CNC machine yet, don't worry! Start with our "What is CNC?" article.
After that, here are 10 things you should focus on to maximize your chances of becoming quickly successful:
1. Buy Some Decent Cutters
Don't get the package of assorted sizes of cheezy imported cutters. You also don't need the solid green unobtanium aerospace cutters from Men-In-Black-Cutter-Supply. Get some decent cutters from a reliable source that has reasonable prices. I like Lakeshore Carbide for example. Try for a name brand, one that's on sale.
If you don't know what the good brands are, check out our End Mill survey. It'll show you which brands are most popular and have the highest customer satisfaction.
Having a decent quality cutter eliminates a whole bunch of problems right up front. I would also go with HSS when you're just starting out. You'll eventually want carbide for a lot of uses, but HSS is cheaper and more forgiving.
The exception is high speed spindles such as CNC Routers commonly have. They need carbide to stand up to the higher speeds.
Get yourself a few sizes:
Nothing smaller at this stage until you've learned on less delicate cutters. Buy 2 or 3 flutes for aluminum and some 4 flutes for everything else. I'd get 3 of each in each size to start. You are going to break some cutters, so get over it and accept the idea. It's a good thing at this stage to remind you to wear your safety glasses because you will break some cutters!
While you're at it, buy a full twist drill assortment. HSS from a decent brand on sale will work fine. I like the idea of replacing the ones I break with cobalt over time. That way my most-used sizes wind up higher quality. And one more twist drill tip-buy screw machine length, not the usual jobber length bits.
The screw machine length bits are more rigid and you will seldom drill super deep holes anyway.
2. Get a Decent Vise, a Clamping Kit, and a set of Parallels
Yeah, I know, vises are expensive, but workholding is very important.
Get a decent Kurt-style vise for your mill. It'll be money well spent on a valuable piece of tooling that will last for years. In my shop I have used Kurts purchased from eBay and a couple of brand spankin' new Glacern vises. There is sneaky stuff that goes on when you clamp a workpiece into the vise. If you don't have a good one, the workpiece will shift and you'll be wondering what happened.
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You'll need a way to mount your vise to the T-slots of your table, so you may as well get a clamping kit too. Any old kit will do. Buy the one that is on sale at your favorite supplier. These are step clamp kits that give you a basic set of cnc clamps. There are many other types, and you can expand your collection as needed over time.
Lastly, you'll need a set of parallels, at least until you get proficient at making step jaws.
CNC Routers are a different story. You'll mostly be clamping things to your spoilboard. Do yourself a favor and go read up on how that's done so you can get a lot of good ideas before it's your turn to start clamping.
3. Get yourself a misting setup for coolant and use it while being paranoid about chip buildup
If your machine didn't come with flood coolant, and isn't set up for it (i.e. you don't have an enclosure to keep the wet inside), get yourself a misting setup. I got mine off eBay for about $100. It's a Noga. There are a lot of different brands. Get a jug of coolant to go with it. I use KoolMist, but again, there are a lot of brands.
Now train yourself to be paranoid about chip buildup.
Recutting chips is bad for cutters and in the worst case leads to breakage. Being paranoid means you're watching the cut like a hawk and you fiddle with your mister's nozzle until you figure out how to position it right the first time and every time thereafter to blow away the chips from the cut. Learn more about the different CNC Coolant options.
If you're a CNC Router user, the story is different, unless you plan to cut a lot of aluminum. If you're cutting aluminum, you still need a misting system. Otherwise, a ShopVac or even just blowing the chips out of the way with compressed air will serve as you cut wood or plastics.
4. Learn how to use MDI
Your next step is to learn to run your CNC as though it was a manual mill with power feeds and DRO's on every axis. In the process you'll learn some basic g-codes so that you have some idea what your program is doing the first time you run a real g-code program (that's a little ways away yet!). It's very convenient to be able to do this trick, and you can learn all about it through our article on this topic:
Don't stop there. When you've got a grip on MDI, start on the CNCCookbook G-Code Tutorial.
Start out with the cutter way high and don't try make any moves in the Z-axis so you won't crash the cutter into anything. Practice making moves in X and Y. Until the cutter goes where you want it to and you're not making mistakes.
One more trick: don't use G00 in MDI-that makes the machine move in rapids mode which is as fast as it can go. Use G01 and set a slow feedrate. "G01 F20" will make the machine move at 20 inches per minute (or metric units if you're in metric). You'll have a lot more time to react this way if it starts heading into trouble. Visualize pressing the red E-Stop many times so it'll be almost automatic when you need it.
5. Get yourself a feeds and speeds calculator and use it
You're getting close to making your first cuts.
Do yourself a favor and get a feeds and speeds calculator. Trying to figure out feeds and speeds as a beginner or asking people on forums is a recipe for frustration and faillure. And it is such a false economy to try to save a few bucks by skipping the calculator or using a cheezy free online calculator. Stuff will go wrong that you're just not equipped to understand. So, get a decent feeds and speeds calculator so your spindle speeds and feedrate are perfect for that first cut.
We recommend our own CNCCookbook G-Wizard feeds and speeds calculator (natch!). It doesn't cost much more than 1 single cutter to get started, and you can use it for 30-days completely free.
Best of all, there's a great deal just for beginners and hobbyists that we call "G-Wizard Lite":
When you subscribe for 1 year of G-Wizard, you get Lifetime use of the product for up to 1 horsepower. You can do a lot with 1 HP, especially if you're a student or hobbyist. In fact, it may be all the power you ever need!
So check it out, your cutters will thank you and your projects will go by faster and easier.
6. Get yourself a Z-height measuring gizmo and learn to use it to touch off your tool lengths. While you're at it, get an Edge Finder and use it to touch off your Part Zeros.
Your machine needs to know where the tip of the tool is or terrible things can happen.
As a beginner, you tell it by using a Z-height measuring gizmo of some kind. Have a look at our article on the subject to see what's available. Don't get wrapped up in the fancier stuff in that article. The first few options will get your going.
Learn how to use these to tell the machine what your tool length is. This process tells your machine where the tip is in Z. To establish the position of X and Y you'll need and Edge Finder. The first thing you'll do after sticking a piece of material in your vise and inserting a tool in the spindle is to set these zeros.
You can learn more about touching off and setting part zero from our G-Code Tutorial.
Also take a look at our 2-part series on Tool Data Management for information on Z-Height Gizmos (more properly called touch setters) and tool lengths. Use your newfound MDI skills from #4 to start using your Edge Finder and Z-Height Touch Setter.
7. Learn to Tram your Mill and Vise
How to square and tram your vise on your CNC mill...
Tramming is what machinists call getting things square by sweeping an indicator around. It's a basic skill everyone needs to learn.
When you first start machining, get into the habit of checking your mill's tram every time you walk out into the shop. You'll know later on whether you need to do it every time, but for now, you could use the practice. While you're at it, be sure you know how to tram your vise so the jaws are aligned properly with one axis or the other. Let's don't try to square you mill just yet, but tramming the mill and vise are a good step at this stage.
For details on how to do this, refer to our Mill Tips and Techniques page.
8. Start out with wood, aluminum, brass, and plastic. Avoid stainless.
Okay, the very next step involves making some cuts. Sorry if it seems like you had to do a lot before getting there, but I did say I wanted to tell you some things that will help you succeed and there's quite a few things in that category!
One more is to avoid difficult materials for your first cuts. Stick with aluminum, brass, or non-metals like plastics and wood.
BTW, nothing wrong with sticking a piece of 2×4 in the machine to learn on, even if the eventual part is going to be made of metal. I often use wood to prototype what I'm doing until I get everything right. It's a lot cheaper than metal!
When you're doing okay with softer materials, graduate to mild steel. Only after you feel like you've pretty well mastered cuts in these materials can you try tougher materials. Master means you're not breaking or wearing cutters out too quickly, and your surface finishes no longer look like the material was attacked by a pack of rabies-infected beavers.
One more thing. It'll be tempting to try some "mystery metal." This is metal you think you know what is, that you got essentially for free, and that is therefore expendable. Trouble is, a lot of mystery metal is hard to cut. It's nasty cheap stuff that will break cutters and generally confound you. Not worth it!
9. For your first project, learn how to square a block of material and make yourself a few sets of step jaws in aluminum
Let's quit procrastinating and make some chips!
For this step, you're going to start by learning how to square a block of material. Get your saw and cut out some pieces of material that are slightly oversized to serve as vise jaws. Google "Kurt Vise Jaw Dimensions" to find drawings of jaws if you need some. As mentioned, cut the stock slightly oversized from aluminum. Now you need to square those blocks. Squaring means you will make a series of milling cuts until all sides are properly parallel or perpendicular to one another, at which point the workpiece is "square".
The proper steps for squaring a block of material are described on my Turner's Cube page. The recipe described there is one I got from the great book, "Machine Shop Trade Secrets." Pick up a copy to continue your learning process.
One thing about the Turner's Cube material squaring: I use a Face Mill, but you should start out doing it with multiple passes and a 1/2" endmill. Why? Because Face Mills generate a lot of force. You can stall the spindle, yank the work out of the vise and throw it across the room, and other shenanigans a beginner could do without.
Leave the face mill in a drawer if you have one already and do it with a 1/2" endmill the first time until you know a little more what you're doing. Same goes for fly cutters, and for the CNC Router crowd, those big cutters you use to surface spoilboards. You're not ready for them yet.
Having squared the material, your next task is to cut it to size by continuing to mill it until it is a perfect fit for your vise jaws (you'll need 2 square pieces, one for each jaw). The last step is to drill and countersink the mounting holes.
Actually, I wasn't completely truthful. Once you have mounting holes, the last step is to mill a step along each jaw, maybe 1/8" square. Now you can use that step instead of parallels when you drop material into the vise. Over time, you'll learn a lot more tricks with aluminum jaws, but remember: it all started here with your first set.
10. Graduate to CAD/CAM
Friend, you're now able to do the basics. Sure, there's a lot more learning to do, but you're in a position to have a leg up on the manual machinist down the street who has a Bridgeport and no CNC. Your next step is to start pumping out the g-code. To do that, you'll need to be proficient with CAD/CAM. This is going to be your biggest step yet. Neither CAD nor CAM is very easy to learn.
I give myself 2 weeks to feel comfortable with a new program, and I've had the luxury of learning quite a few so far. If this is your first time, get ready to drink from the firehose.
I have put together a bunch of pointers to make it as easy as possible for you to choose the best CAD CAM software for your needs. Check out our Best CADCAM Software for Beginners article. It is chock full of buying guides, evaluation tips, how to learn ideas, and best of all, it has a guide to the secret deals on the most popular packages that can save you money. These are deals most people have no idea exist, including one from CNCCookbook.
Bonus: Delay the need for CAD/CAM a little while with Conversational CNC
Many talented manual machinists run afoul of CADCAM and it stalls their ability to be productive on CNC machines quickly. Even after they've learned CADCAM, they're still left with the feeling that manual machining can make simple parts faster than CNC, because they don't have to stop to make a CAD drawing and run it through the CAM package to get g-code.
Instead, they can just make a simple bracket with 4 holes shooting from the hip.
Conversational CNC makes it possible to do that with a CNC machine. Imagine changing the long essay test that is CADCAM to a quick multiple-choice quiz. That's what Conversational CNC is all about. Here's the pictorial menu of Conversational CNC operations that can be performed on a mill:
You pick the operation, a popup asks a few simple questions, you press the Insert button, and you've got your g-code. We've built Conversational CNC right into our G-Wizard Editor product so you won't have to purchase an expensive stand alone Conversational package.
You'll still need to learn CADCAM, but with Conversational CNC you can be making simple parts before finishing the CADCAM learning curve.
Congratulations, you've got some Basic CNC Skills!
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