It's important to be familiar with and have access to the variety of different cutters. Cutters are commonly used on CNC Milling machines for many different reasons. Here we will provide a good overview of the different types of cutters available on the market so you will learn how to choose the right tool for each kind of job.
Let's start naming the different kinds of cutters milling uses.
End mills.The most common cutters are end mills. They come in a plethora of sizes, geometries, coatings, and even materials they're made out of. We'll just cover the basics here.
Carbide versus HSS. The first thing to explore is the material the end mills are made of, typically carbide or HSS, though there are a few more exotic alternatives such as powder metal. For a lot of production machine shops, the question of whether to use carbide end mills or HSS (high speed steel) seems silly. Carbide is always better, right? It turns out that it depends on the material to be cut as well as on the machine maximum spindle speed and potentially some other factors. If you're working with softer materials like Aluminum and your spindle is not capable of reaching the recommended speeds for carbide in aluminum, you might very well discover HSS end mills make more sense. For smaller mills whose spindle won't exceed 6,000 rpm that will be cutting aluminum, I typically recommend HSS for end mills larger than 1/2" diameter and carbide for smaller end mills. This saves a lot of cost on the larger end mills (which you might prefer to be indexable and using carbide inserts anyway) but keeps the rigidity up on the smaller end mills.
Sizes. End mills are available in a variety of sizes both metric and imperial. Even very tiny micro-mills can be had for machining extremely small features. Something surprising to know about end mill size is that the tolerances on diameter for most end mills are not especially precise. If your work demands precision, you may need to make a test cut with an end mill to determine what its real diameter is. You can also measure them directly, but the test cut is a more reliable method.
How many flutes? The number of flutes the end mill has is crucial depending on the material you want to cut and the capabilities of your machine. The most common options are 2, 3, or 4 flutes, (but even more flutes are available). Unless you know exactly what you're doing, you never want to use more than 3 flutes with aluminum. The reason is that aluminum produces very large chips relative to other materials. The flutes provide the path for chips to escape when the end mill is down in a hole or slot. 2 and 3 flutes end mills have a lot more chip clearance so they work with aluminum. Using a 4 flute where the chips are confined at all results in jammed chips and a broken cutter in aluminum almost every time. For most other materials 4 flutes is the norm. You want to use as many flutes as you can because the flutes act sort of like a "spindle multiplier". For a given spindle speed and chip load, a 4 flutes can be fed twice as fast as a 2 flute and it will tend to give a better surface finish. The 3 flutes was developed as a nice compromise between using a 2 flutes or a 4 flutes in aluminum. It has sufficient chip clearance for all but the gummiest aluminums, yet it can be fed about 50% faster than a 2 flutes at the same spindle rpm. There are quite a few other varieties available that are seen less often. End mills with more than 4 flutes are great for increasing productivity on tough materials like Titanium where the spindle just can't turn very fast. And there are a whole host of situations where a single flute end mill offers advantages.
Center cutting or not? Most 2 and 3 flutes end mills are center cutting. Some 4 flutes end mills are not. A center-cutting end mill is one that can be plunged straight down into the material. None center cutting end mills have a depression in the middle with no cutting edge, so they'll go down a very short distance and then can be pushed no further. The only good reason to buy a non-center cutting end mill is they're cheaper. I prefer to only have center cutting end mills around as I most often discover an end mill is not center-cutting the hard way. By the way, there is an argument to be made never to plunge end mills (well, almost never). It's the hardest possible way to get the end mill into the material, ramping or helixing are far more gentle. Given that perspective, perhaps one should always prefer non-center cutting so as to be incented to avoid plunging.
Single versus double ended and stickout. Speaking of cost, you can purchase your end mills as either double or single ended. For a little more money than one single ended end mill and a lot less money than two single ended end mills you can purchase a double ended end mill. When one end is dull, you just reverse it in the tool holder and you've got a brand new end mill ready to go. The downside is that most end mill crashes end with a broken end mill that you may or may not be able to flip around. Still, they could be an effective way to reduce your costs if you mostly dull them without breaking too many outright. This brings me to an important concept which is called "Stickout." Stickout is the distance from the end of the tool holder to the tip of the end mill. The more stickout the less rigid a tool is. If it sticks out too far without support and you work it too hard, the cutting forces will make it bend, which machinist's call "Deflection". The takeaway at this stage is that while it may seem smart to buy end mills that are as long as possible because you'll have flexibility to use them in more situations, that's probably not the best way to go. Shorter end mills are more rigid. Save the extra long ones for times when you have no other choice. We'll see something similar with Twist Drills as well.
End mill coatings. The right coating can dramatically increase the performance of an End mill (or other cutter, such as a twist drill). There are lots of different coatings available, some of which are amazing and some of which are very exotic and expensive. The choice to use coated end mills is a cost versus benefit choice. If your machining benefits enough from the extra performance of a premium coating, by all means, purchase end mills with a coating.
Bullnose, High Helix end mills and other geometries. Just as coatings can account for big differences in performance between budget and premium end mills, so too can geometry. The simple fact of the matter is that a lot of claims are made for various geometries and the proof is in the pudding. Some things are less controversial some are very subtle. When you're starting out, buy decent quality end mills but don't break the bank on fancy geometries. When every increment of productivity starts to be money in the bank, test some of the premium end mills to see what works for you. If you're curious to learn more about what sorts of geometry tricks these end mills play. Two geometries worth exploiting fairly early on are Bullnose End mills and High Helix End mills. Bullnose end mills have a slight radius on their edge so they can leave a nicer surface finish and can be less prone to chipping--both great attributes. High Helixes can help pull the chips out in materials like aluminum much better.
Roughing end mills. They have little serrations in the teeth. These serrations do a couple of useful things. First, they break up the chips making it easier to clear them out of the hole you're cutting in. Second, they are less prone to chatter. All those serrations produce a variety of vibrations that interfere with one another instead of creation a single ringing (more often screeching) tone. Roughing end mills are not very expensive and can provide a nice step up in productivity. I like having some around in a couple of sizes, especially sizes that are a little small for a decent indexable end mill.
Ball nosed end mills. So far we've talked about end mills that are largely used for what's called "2 1/2D" machining, sometimes called "Prismatic" machining. This is machining where the Z or height of the surface doesn't change in smoothly flowing contours. It may drop down for a pocket or two, but the floors of the pockets and the top of the part are generally smooth planes punctuated by holes and more pockets. The vast majority of parts have this characteristic, but for those that don't, you'll be doing true "3D" machining, and to do that, you'll want to use a Ball nosed End mill. Ball noses create scallops whose size depends on the diameter of the ball nose, the depth of cut, and the step over between successive passes.
Indexable end mills. Large end mills can remove a lot of material, but they're also very expensive. It didn't take the machining world long to decide they're too expensive and to come up with indexable tooling as the answer. With indexable tooling, the cutting edges are removable carbide inserts. A lot of different indexable tooling is available, but for this we will confine ourselves to indexable end mills and face mills.
For efficient roughing, it's convenient to have an indexable end mill that's 5/8's to 3/4 inches in diameter. That's the size where buying solid end mills starts to be expensive, but it is small enough that what's left for smaller solid end mills to clear is little enough that they can do that job quickly. So, you rough with the indexable and finish with the solid end mill. Depending on the jobs you have, larger indexable end mills may make sense too. It's basically a trade off between your machine capabilities, how tight the confines of your work piece are, and the cost to keep a bunch of indexable tooling and an inventory of suitable carbide inserts for them. The companion to indexable end mills are called "face mills" because they're used for "facing." Facing is surfacing a large flat area, typically the top of the part.
Twist drills. The ubiquitous twist drill was probably the first cutter on this page each of us used, perhaps in a handheld electric drill. While only machinists have end mills, most any home do it yourselfer has twist drills on hand. But there's more here than meets the eye. Statistics show that holes are by far the most common feature CNC machines make. In addition, the Material Removal Rate of twist drills is outstanding and usually better than equivalent sized end mills. It may even make sense to drill a bunch of holes in a grid over your pocket and then machine the web between the holes out with an end mill. Like end mills, twist drills come in different sizes, coatings, materials, and lengths.
Twist drill sizes. Twist Drills come in a lot more diameters and sizes than end mills. This should probably come as no surprise given how popular holes are and how hard it is to use the same twist drill to make different sized holes, whereas end mills can be a lot more flexible.
HSS, Cobalt, and Carbide + coated or uncoated. If you need the toughness and rigidity of carbide for working tough materials with high productivity, there's no substitute. For most others, buy a full index of either HSS or Cobalt. I heard of one shop that bought HSS and a box of Cobalts. The Cobalts went in the Tool Crib and were used to replace the HSS twist drills as needed. The assumption was that the ones that needed it were getting a lot of use and were worth upgrading. Coated or uncoated is a matter of preference. Coatings can help but seem to be a little less impressive than on End mills, probably because most twist drills are not carbide.
Screw machine or Jobber length. Twist drill lengths can vary as well. Most non-machinists are used to jobber-length twist drills. Because they're shorter, they're also much more rigid. The hole they make is likely to be truer and you're less likely to break one. Most machinists prefer to use screw machine length twist drills wherever possible for those advantages.
Silver and Deming drills. Typical sets of twist drills only go up to 1/2" in diameter. To drill bigger holes you may want to use what's called a "Silver and Deming" drill. These bits have a 1/2" shank and much larger flutes. The shank is kept small so they fit standard drill chucks. A couple of thoughts here. First, standard drill chucks are not very accurate and for CNC Machining we'll often use collet chucks instead. Second, big twist drills can soak up a lot of horsepower so make sure your machine is capable of driving one in the material and conditions you've chosen.
Parabolic flutes for deep holes. Drilling really deep holes is hard. Once the hole is deep enough, it's very hard to extract the chips while continuing to drill deeper. If they pile up and block the flutes, you're going to break the twist drill off in the hole, which is always a mess and a nuisance to clean up. One innovation that helps a lot for deeper holes is called "parabolic flutes." These bits are more expensive than conventional twist drills, but they can go quite a lot deeper and so they're darned well worth it if you application demands deep holes. "Deep", by the way, is all relative to the diameter of the twist drill.
Keep'em sharp. You'll be using the heck out of your Twist Drills and nothing is more annoying than a dull bit. There's a lot of life left in the twist drill if you can sharpen them. Drill bit sharpeners are available at every price range or you can sharpen by hand at the grinder.
Spot drills and Center drills. These two are special purpose drills, but their purpose is one we'll likely use a lot. In theory, there's no point in using a Center Drill on a mill. Center Drills are intended to create a hole in the end of stock for a tailstock on a lathe. Their secondary "pilot" tip makes them more delicate than spot drills. However, many machinists will grab one anyway if it is handy and use it as they would a spot drill. Neither has flutes that go very far and they are just used to "spot" a small depression for the twist drill to get a good start on. You don't always need to spot a hole, especially with screw machine length drills.
Indexable drills. Given just how often our machines need to make holes and the advantages of indexable tooling for end and face milling, it should come as no surprise that indexable drills are available too. These are generally best used for larger holes. Not a lot to know here at the basic level other than to keep in mind that they exist and can save you quite a lot of time on a job.
Chamfer tools. Chamfer Tools are used to put a chamfer (bevelled edge) on the edges of a part. Chamfers are one of the things that make CNC'd parts look so professionally made. You can chamfer an edge with a purpose-made chamfer tool, or in a pinch if you don't have one or are trying to save a tool changer slot you can use a spot drill for chamfering. There are even indexable chamfer, engraving, and spotting combination tools that are pretty slick. Chamfering is often preferred because it's harder to get a good finish without chatter marks with most corner rounding and gets worse the larger the radius. Chamfering cleans up the edge quickly without those complications.
V-Bits. V-Bits are commonly used for engraving. The simple geometry on the cutters prevents them from being useful for a lot else, but they will do a nice job engraving nice clear letters and figures.
Reamers. Reamers offer a quick and efficient way to clean up the sides of a hole, make sure it is round, and get it to a particular diameter with fairly high accuracy. They require a hole to be drilled first that is fairly close to the final size so that the reamer actually removes relatively little material. If a twist drill or interpolated hole with an end mill doesn't produce an accurate enough hole in terms of diameter and roundness or a hole with good enough surface finish, the primary alternatives are Reamers and Boring. For holes too small for a boring head, a Reamer can be the only choice. Use a reamer with helical flutes if the hole has a keyway or similar feature. The helical flute will bridge the keyway instead of getting caught in it. Reamers have a long shank so they'll "float," which basically means so they can deflect and seek the center of the hole being reamed. A Reamer with helical flutes may leave a better surface finish by evacuating chips better than a straight flute reamer. Re cutting chips is a common cause of wall finish issues. Use a collet chuck or other tool holder with low run out for a reamer. CNC'ers use a G85 rather than a drilling cycle for reamers. The drilling cycles rapid out of the hole which can mar the surface finish.
Tapping. Mills thread holes either using taps or by thread milling. There are advantages and disadvantages to each, but all things considered, tapping is usually faster and cheaper.
The two major categories of taps are Roll Form and Cutting Taps. Roll Form taps don't make any chips, which is very advantageous. Essentially, they forge the threads by cold forming the material, so they create stronger threads too. In addition, Roll Form Taps are stronger than Cut Taps, so they're less likely to break and they give longer tap life. Most of the time Roll Form taps are preferred over Cut Taps when they can be used, but they're not for every material. If the material produces a continuous chip when drilling, it's a good candidate for a Form Tap. Hardness is another criteria. While many machinists may think form taps are only for aluminum, the truth is you can form tap materials up until they have a hardness greater than 36 HRC, which is about 340 BHN. That actually covers a surprisingly wide range of materials including a lot of steels. There are also plug taps and bottoming taps. The latter have flat bottoms for tapping blind holes. Beware, blind holes are notorious hazards for breaking taps. A Roll Form Tap has the advantage in a blind hole because it makes no chips. If you can provide some extra depth in the blind hole beyond where the threads go, that'll give the chips a place to go with a Cut Tap. You can also get different geometries on Cut Taps, such as a Spiral Flute tap that will do a much better job pulling the chips up out of the hole.
Rigid tapping, Tapping heads, and Tension/Compression holders. Having selected the Tap you want to use, you're still not done. You need to arrange tool holding appropriate to your machine capabilities. Taps have to be driven in such a way that the feed rate downward is properly synchronized to the spindle rpm based on the threads that are being cut. Too much or too little puts pressure on the tap and the threads and will create a problem. The approaches to this depend on whether you machine can precisely synchronize the feed rate to the spindle rpm, which is called Rigid Tapping. If so, the Tap can be held in a rigid tool holder and higher rpms can be used. If not, you need a tool holder that has some play in it so that the tap can be drawn into the hole at its own rate as it cuts the thread. The play along the spinning axis prevents inordinate pressure from building so long as the feed rate is close. Blind Holes can be really tough to get right without Rigid Tapping as the spindle will often spin a bit after it is commanded to stop and it's hard to get it to spin just the right number of times for a perfect blind hole. A tool holder with the play described can help provided you don't run out of play. The two styles of tool holder used when Rigid Tapping is unavailable are called Tapping Heads and Tension/Compression Holders. Tapping Heads incorporate friction slip clutches, axial free play, and a reversing feature that automatically reverses the spin direction when the feed direction reverses. They were originally created to make tapping easy for drill presses and manual milling machines. They can be used on CNC machines, but they're not nearly as common as Rigid Tapping or Tension/Compression Holders. These holders are spring-loaded along the axis and allow the tap to seek its own position as it is cutting threads.
To avoid broken taps, drill the right sized hole to avoid excessive torque on the tap, and please note the hole size is not the one listed on most drill tap charts or on the tap itself. Important for beginners, avoid hardware store taps and dies! The ones made for machining don't cost much more and they sure do work a lot better. Use a form tap where possible, the tap is stronger (so less likely to break) and the threads are stronger too. Plus, they make no chips, so there is no possibility of chip bind up in a blind hole. Their only disadvantage is they're limited in the hardness of the material they can be used on.
Consider using a good tapping lubricant if your machine's coolant isn't good enough for tapping. You can even put a cup on the machine table and program the CNC in g-code to dip the tap in it before starting to tap. It beats the heck out of hanging in the enclosure door with a brush. Consider "peck tapping" difficult holes. For the most part, you will need rigid tapping to be able to peck tap because the tap has to get itself synchronized back to the same set of threads as it goes in and out of the hole. Peck tapping is only called for with a cutting tap–no benefit to pecking with a form tap. Peck tapping is also an excellent way of clearing the long stringy chips often found when machining plastics and some other materials. For the hardest materials, and especially when the cost of a broken tap is very high, consider thread milling. You’re much less likely to break a thread mill, and if you do, it won’t be stuck in the hole the way a tap would be.
Thread mills. We mentioned above that for the hardest materials, and especially when the cost of a broken tap is very high, consider thread milling. You’re much less likely to break a thread mill, and if you do, it won’t be stuck in the hole the way a tap would be. That's an excellent reason to Thread Mill instead of Tapping. A thread mill is a rotary cutter that has the form of a thread that is moved with a helical motion to create either an external or internal thread. They look a lot like taps, but are used entirely differently. For one thing, they're smaller than the hole diameter so that there's room to move them in a helix and get them back out without touching the just-cut threads. There are indexable single-point Thread Mills as well for larger threads.
Boring. Boring is a method of making a hole of a precise diameter, roundness and high quality surface finish. It's an alternative to reaming, especially for larger holes that would be prohibitive to ream. Boring is a precise analog to Turning on a lathe, except that the cutting tool spins instead of the work piece. It is typically used for internal diameter bores, but there are tools and setups available that can be used to make an outside diameter bores of a precise nature too. Typically, you'd drill or interpolate the initial hole and then use a Boring Head for a finish pass that actually removes minimal material, but assures the precision and surface finish of the bore. If this cans all be done in one pass, so much the better. If it requires multiple passes, things are going to be slower and more cumbersome. The most common boring heads have to be set for one diameter and then locked down. You could use an additional head for each pass, but that's going to get crazy pretty fast. Another solution is what's called an "Automatic Boring and Facing Head". These mechanical marvels expand the diameter through mechanical linkage so they can do multiple passes automatically. Unless your application requires really tight tolerances, perhaps to fit bearings, you're unlikely to need to resort to boring. Instead, you'll probably interpolate the hole with an end mill. It's important to understand what the limitations of your machine are with interpolation.
Saws. Saws can be extremely useful when milling. For example, they can be used to create deep but narrow slots or to slice apart individual parts from a single piece of work stock. Saws for milling use are typically called "Slitting Saws" and are installed on an arbor. It's important to choose a slitting saw with the correct number of teeth for your application and it's also important to get the correct feeds and speeds as slitting saws are fairly delicate.
Woodruff cutters. Also called Key seat and T-Slot Cutters, are most commonly used to cut a small slot in a shaft to hold a Woodruff Key. These are small semi-circular keys whose advantage is they can be made a little distance from the end of the shaft without intruding on shaft shoulders. This reduces stresses on the shaft, which is often a highly stressed part. Very similar or identical geometries are used to cut T-Slots as well. Woodruff cutters are listed with saws because Woodruff feeds and speeds are calculated identically to saw feeds and speeds.
Broaching. Is the operation of shaving a slot into something, typically with a vertical motion. Many don't know it but a broach can be fitted into a special tool holder and the CNC's ability to move the spindle up and down results in a broaching operation that can often save a lot of time and money.
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