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The Skinny on End Mills

Don’t be intimidated by CNC routing. Take the first step into machining by gaining a solid understanding of subtractive tooling basics.  

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Interested in CNC routing but clueless about tooling? Can’t tell an end mill from a drill bit? Here’s an overview of end mill anatomy, some basic cutter types, and tips on how to choose the correct tooling for basic wood or plastic jobs.

Drill Bits vs End Mills

DrillvsEndMill

Top: Drill Bit, Bottom: End mill

CNC machining is a subtractive process that uses rotational cutting tools called “end mills” to remove material. An end mill, while similar in appearance to a drill bit, is far more versatile. However, in practice the terms “bit” and “end mill” are often used interchangeably.

Figure-A

Drill plunging axially on left, endmill cutting laterally on right

Here’s the key difference. Drill bits are designed to plunge directly into material, cutting axially and creating cylindrical holes. End mills are typically used for horizontal carving and cut laterally. Additionally, most mills are “center-cutting,” meaning they are able to cut both axially and laterally. This is due to cutting flutes that extend to — and protrude from — the end face and enable plunge cutting. To minimize tool breakage and stress on the material being cut, most CNC software will “ramp” the end mill slowly into lateral cuts.

Figure-C

Endmill ramping into a cut.

The project type, material being cut, and desired surface finish determines the tool geometry. Key tooling features include the diameter, shank, flutes, teeth, tip shape, center cutting capability, helix angle, helix direction, length of cut, and overall tool length.

CorrectEndmillAnatomy3

Basic End Mill Anatomy

Tip Shapes and Applications

Each end mill tip shape is designed for a particular purpose. Some common cutter shapes are ballnose, fish tail, surface planing, v-carving, and straight.

Ballnose mills produce a rounded pass and are ideal for 3D contour work, while fish tail cutters will produce a flat surface. V-bits produce a “V” shaped pass and are used for engraving, particularly for making signs.

Endmills-Figure-G

The difference in clearing path shapes between fish tail, ballnose and a v-carve mills.

The diagram above shows the difference in clearing path shape between a fish tail, ball nose and V tools. Ball nose mills are often selected when doing 3D contouring because their rounded edge reduces jagged steps when cutting several stepped layers. Ball nose mills can also be used to cut wide paths with rounded edges by reducing the step over amount (overlapping distance between) between passes. By overlapping steps, the central scallop shown in the diagram is eliminated.

Flutes and Chipload

Flutes are the helical grooves that wrap around the sides of the end mill. Each flute has a single tooth with a sharp cutting edge (although there can be more than one) that runs along the edge of the flute.

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TOP: Four flute endmill  BOTTOM: two flute endmill

As the tooth cuts into the wood, each flute whisks away a small section or “chip”. The fewer the flutes, the more material that is ejected with each tool rotation.  The overall cutting depth should never exceed the length of the flutes on an end mill. If cutting deeper than the length of the flutes, the tops of the flutes will be blocked and chips won’t clear, building up heat and reducing tool life. Illo3-ChipDetail

Chipload is the thickness of a machined chip as cut by a specific tool type. More flutes create a smoother surface finish, while fewer flutes remove material fastest, but make rougher cuts.

Proper chipload is important because chips dissipate heat. Hot cutters can lead to suboptimal results, including burned wood, a poor edge finish and dull tooling.

If you’re machining a material like HDPE plastic, you want to use an “O” or single flute bit to clear the chips away as quickly as possible or heat will build up melting the plastic, which will “reweld” to the tool.

SingleOFlute5

Single flute endmill

To summarize:

  • More flutes create a smoother surface finish
  • Fewer flutes are best at chip clearing, keep heat from building up
  • Two or four flute cutters are the most common.

The direction, size, speed and amount of chips being ejected can also damage the surface of the work piece. We can control how the tooling effects the material through our end mill type selection (upcut, downcut or compression) and speed at which we cut.

Helical Direction, Chip Ejection, and Surfaces Produced

upDown_vertical

Downcut on left, upcut on right

A CNC router spins a cutter clockwise. The helical direction of the flutes as they wrap around the tool determine if chips are ejected towards the top or bottom of the workpiece.

True to their name, upcut mills eject chips towards the top of the workpiece, producing a cleanly cut bottom surface. The downside is possible surface splintering or “tearout” on the top surface as the chips are ejected upwards.

Downcut tools do the opposite, producing a smooth upper surface. They are ideal for pieces that have been previously engraved or v-carved and cannot be flipped to hide tearout. In addition, as downcut mills pack the chips into the cut path, they can be used instead of tabs to hold down a workpiece and keep it from moving.

Plywood tearout caused by downcut chip ejection,

Plywood tearout caused by downcut chip ejection,

Compression end mills feed chips upward from the base and downward from the top side of the end mill. This type of mill produces a smooth surface on both sides when cutting through material. The sides of the cutting path however can be damaged by the chips. To further reduce tearing in a wooden work piece, apply a layer of blue painters tape the surface. This will result in less surface splintering and can be easily removed when the milling operation finishes. If the desired finish is still not achieved, consider switching to wood stock that has a finer grain.

Feeds and Speeds

The speed at which we move a cutter across the material is called the “feed rate”. The rate of rotation is called the “speed” and is controlled by how fast the router or spindle turns the cutting tool. Both feed rate and spindle speed will vary based on the material being cut. A general rule of thumb is that you want to move the tool through the material as fast as possible, without sacrificing surface finish. The longer the tool rotates in any one place, the more heat that builds up. Heat is your enemy and can burn your material or radically decrease the life or your cutting tool. Feed rate vs spindle speed:

  • Spindle speed that is too fast paired with a slow feed rate can result in burning or melting.
  • Spindle speed that is too slow paired with a faster feed rate can result in dulling of the cutting edge, deflection of the end mill and possibility of breaking the end mill.

A good strategy when selecting a cutter is to attempt to balance feed rate and spindle speed by performing two passes on the work piece. The first pass, called the roughing pass, can be done by using an end mill that will eject a large number of chips at a high feed rate. The second pass, called the finishing pass, then won’t require as aggressive of a cut and can provide a smoother finish at a high speed.

Which Cutters to Buy First?

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If you are looking to purchase a great wood and plastic starter set, consider picking up a few of the following carbide tool types in 1/4″ and 1/8″ diameters:

  • 2 flute upcut and downcut end mills (great for hardwood and plywood)
  • 2 or 4 flute ballnose mill (great for 3D contours)
  • Single or “O” flute mill (great for plastics like HDPE and acrylic)
  • 60° or 90° v-bit (great for cutting hardwood signs)

The quality of your work can be significantly improved by selecting the right tooling for your project and materials —plus you’ll spend less time on hand-finishing.

Tyler Worman

Tyler Worman

I’m a maker living in Ann Arbor, MI! I enjoy listening to music, playing disc golf, and experimenting with rapid prototyping tools.


  • Colin Chapman

    Time for a MAKE book on milling. This article is a great start.

    • Ironwright .

      That would be a big book. Milling basics would be a good start though. Everything in machining is situation dependent. – tool stick out, machine rigidity, work piece rigidity, tool holder type, Coolant vs dry machining, end mill geometry, material composition and variance, cutting techniques, depths of cut, open vs closed contours, tool wear. These are the ones that beginners mess up on a lot. Many times a new machininst will get bad results and blame the tool when it really is how the tool is being used.

  • walt

    Thanks for a great article. I was working on a job using a 1/32″ carbide end mill to mill a slot in some aluminum (fairly soft, don’t know what alloy). I used 2 passes, 1/64″ deep, 2 IPS feed at (about) 10,000 RPM in a DIYCNC machine driven by a Dremel tool. It worked OK except that the cut filled with chips, and when it went back for the second pass it seemed like the old chips combined with the new ones and “welded” themselves together. It was a real chore to clean out the groove afterwards. Next time I’ll vacuum out the groove after each pass!

    A GREAT source for end mills is Kodiak Cutting Tools https://www.kodiakcuttingtools.com

    They have a great selection, pricing, and delivery.

    Walt

    • pcf11x

      A 1/32″cutter? Are you sure it was that small? Because if it was then 10,000 RPM is far too slow to spin such a small carbide tool cutting aluminum. Ten times that speed would just get you to the lower end of the speed range you should be in. I know this because I know things this article isn’t telling anyone.

      Very important things I might add :)

      • walt

        Yes, it is a 1/32″ cutter. Very fragile, I’ve broken 3 of them so far, actually given up on this particular project, and bought a commercial version of what I was trying to make :-(
        I built a CNC machine from a kit a year ago (mydiycnc.com) and the subject of feeds & speeds has been largely experimental for me. All of the cutters I use are less than 1/8″. My experience with milling dates back 50 to years ago with HSS cutters; we had some indexible-insert carbide cutters, but as the “new kid” I wasn’t allowed to use them. I realize 10,000 RPM equates to only about 80 SFM which is way too slow for carbide, but I’ve been gun-shy after breaking so many mills (at $8 each). I can get to 30,000 RPM on the Dremel tool that drives the cutter so I’ll try that, and increase the feed rate to keep the same chip load. Hopefully that will expel the chips from the cut. 100,000 RPM just won’t happen with my current equipment.
        I’d be grateful for any reference material you can point out to me. Everything I’ve found about F&S starts at 1/8″ cutters and goes up from there, so I’ve been extrapolating downwards, and as I said, experimenting.

        Thanks for the reply!

        Walt

        • pcf11x

          I had a really good day today so I’ll do Make’s job for free for them now. This is the surface speed formula:
          (PI * Dia. * RPM) / 12 = SFPM

          Carbide tools cutting aluminum is a range of 800-2,000 SFPM Whenever you run thin fragile tools you are always better off over driving them, and under feeding them because of their delicate nature. So for you with your 1/32″ diameter cutter you’re looking at 2,000+ SFPM which isn’t practical to do. You’re essentially trying to do the impossible, and that is why you’ve had less than success so far.

          Using that formula we can calculate that you would have to turn at a speed of 250,000 RPM in order to get a surface speed of 2,045 SFPM. That should just about do it. Yeah, ah no. That’s not gonna happen.

          I don’t know what you are trying to cut but if you can get a slitting saw there that is the better way to go.

          • walt

            Naah, it’s curved slots, so a slitting saw wouldn’t do it. (This is a jig for making the turnouts (switches) for a model railroad.) As I said, I bought the jig I was trying to make, but I need one or two other variations of it, so I’ll probably mill them out of wood and just make a new one when they wear out. Thanks for all the help!

            Walt

          • pcf11x

            Depends how tight a radius is required. Slitting saws can cut straight, but they don’t have to. I suppose you would need an extra axis of movement to make the turns though. I’m sure that is how the pros do it.

        • 48panhead

          Walt the best thing that you could do is get a Machinist Handbook it will help with milling and many other things. Also if you have a compressor add a air jet to blow the chips out and cool the tool. If you add a suction tube you can make it a mister with coolant. I’m a retired mold maker and had been at it for over 40 years it also sounds like pcf11x has some history.

      • Ironwright .

        Most CNC machines can’t crank the rpm necessary to go over 10,000 RPM. You can cut with a 1/32 end mill just fine in a lot of materials.

  • Paul Pyro

    Glad I had manufacturing growing up, taught me so much about all the bits/tools. :)

  • Wipop Bam Suppipat

    This is super useful, thank you!

  • http://the-nerds.org/ Jeff Del Papa

    A few more things: for steel, the rules change a bit. The norm is 4 flute, and they aren’t center cutting (so you can’t plunge cut). For smaller machines, consider using “roughing” end mills, where the flutes are scalloped, to reduce the cutting loads.

    For both aluminum, and steel, consider carbide insert mills. They cost more initially, but last “forever”, you just turn the insert to expose a new sharp corner, and replace the insert (cheap) when you run out of corners.

    Cutting metal is a complicated topic, a look at a catalog for milling cutters, and you are looking at over a hundred pages. Sure stuff comes in lots of sizes, and carbide formulas are tailored to saving the last $0.0001 per cut of tooling costs, but there is a lot of variety, because there is no one right answer.

  • Michael

    Hello all, I am new to wood milling and have a project I’m working on and would like some assistance choosing the right end mill bit. I am looking to mill a rectangular recessed space measuring 6″x11″ and 1-3/32″ deep into a solid piece of 2-1/4″ walnut. Two cuts are being made, one for the main cavity space which has corner radii of 1/2″ and a top ledge for a lid with a corner radii of 1/4″. I know that i want a square end finish and minimal sanding. I would greatly appreciate the help as I am very new to this craft.

    Best

    Michael

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  • Augusto Leao

    Very nice article!!! Thanks for sharing this!

  • AndyDaniel

    Great article. Really well written with useful information. Can yo comment a bit about mill lifespan and how to tell when a mill is no longer sharp? I’ve started to see rougher cuts on my mills after a while but to the touch they still seem sharp.

    • http://www.novaslp.net/ Tyler Worman

      Many end mills have a rating of generally the amount of cutting they are good for. Obviously based on heat or material being cut, that can vary greatly.

      One of the things I usually notice is that the end mills sound louder when cutting. If you don’t run your mills often that isn’t a great help.

      You can also drag them across something decently hard, I was taught to check using the edge of my fingernail (Warning: This may not be the safest thing to do). If it’s sharp it will easily pull up a piece. If it’s dull you’ll need some force.

      Outside of that, if you stick them under a microscope you’ll be able to easily see when the end mill is dead or just dull.

  • Douglas Brown

    Finally, someone that speaks my language! I have a friend that grew up in Missouri, so naturally he’s well versed in construction lingo. He would throw these terms out there like I SHOULD understand him already, but i would never interrupt. I, like Daniel, am a little curious as to the life span of end mills. Great read!
    http://richardsmicrotool.com/cobaltendmills.html

    • http://www.novaslp.net/ Tyler Worman

      Sorry for the delay. Just saw the comments now. Replied to Daniel’s post on the article below concerning life span of the end mills if you are still curious.

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