The parameter that links these concepts and that is widely used as a standard metric to determine optimal feeds & speeds is called chip load.

Feed rate formula for milling

The three basic cutter configurations for spiral bits are up-cut, down-cut, and a combination of the two, known as a compression bit. ... A down-cut bit sends the chips downward; an up-cut bit sends them up to ward the shank. (On a router table, all direc-tions are reversed.)

THERE’S NO CLEAR WINNER.    Solid-carbide spiral bits are becoming increasingly popular because they make a very clean cut. But they have many limitations compared to the straight bits that have been popular for years.

Let's illustrate this concept and imagine you want to cut plywood with a 6mm 2-flutes end mill. In our case, the recommended chip load for plywood is around 0,1mm/tooth (cf. the Advanced chip load table at the end of this article).

Chip load, also called “feed per tooth”, is the thickness of material that is fed into each cutting edge as it moves through the work material.

As stated earlier in the article, we recommend that you start by setting the actual feedrate of your machine below the value from the table and gradually increase it. In general, you will find that your optimal feeds & speeds will be determined from experience or trial-and-error. For instance, for most materials, you can typically set the spindle speed between 15000-25000rpm and adjust your feed rate to obtain nice results with your machine.

Some woodworkers like to sharpen their straight bits, although I find it difficult to get it right and always send out my bits for sharpening. Carbide-tipped straight bits usually have enough carbide thickness to be reground four or five times, and the tech- nology to do so is common.

Always clamp your workpiece in the best possible way. A loose workpiece will vibrate while being cut and cause a bad surface finish. If you are not sure about your clamping, use wood screws to attach your workpiece in many points to the spoiler board. It is not the fanciest clamp in the world, but it is fast and efficient.

Now let's imagine that your spindle can't run faster than 10 000rpm. We can still increase the feed rate by using a 3-flutes end mill and keep a constant chip load:

Spiral bits work incredibly well in the production environment and especially in CNC (computer numerically controlled) router industrial applications. But in a hand router, their use sometimes imposes unusual risks not associated with the equivalent or bigger straight bits.The down-cut spiral bit's screw-driven forces are sufficient enough to pick the router up and twist it out of your hands—with no warning. I know, because it has happened to me. On end grain the spiral bit is getting even more traction, so the risk is even greater—a pity, too, because a sweet end-grain finish is attractive.

Based on this knowledge, we can now use tables that will allow us to calculate our feeds & speeds and achieve an optimal chip load for any given material.

So again, for these heavier milling operations, you will need to use a lower feedrate to allow your mill to stay cool, or simply reduce the depth of cut.

Feeds and speedschart

The increasingly popular spiral router bits borrow technology from the metalworking industry. Spiral bits look like drill bits and are most often made of solid carbide, so they are super sharp and leave a superior cut on wood. Two flutes ground around the body of a spiral bit smooth vibration by spreading the cutting action over a longer edge. With their drill-like point, spiral bits are also better for plunge-cutting. All of these advantages also mean less wear and tear on the router, but don’t throw out all of your old straight bits just yet.

You can plunge with both types of bits, so they’ll both work for, say cutting mortises. But because most spiral bits are ground on the tip end of the flute, somewhat like a drill, you can plunge straight down as far as you like, without stopping.You can’t really plunge any deeper than about 1/8 in. with a typical straight bit. Inspect the end, and you’ll see why (see the photo at left). On most straight bits there is a space above the web, between the cutters, where no cutting takes place during a straight plunge because there is no cutter overlap. Chuck a straight bit into your drill press and plunge it into a piece of wood. After about 3/32 in., the middle of the bit bottoms out. To go any farther, the bit has to abrade the wood away in this middle area.

Hence, getting your feeds & speeds right simply means finding the sweet spot where your tool is spinning at the perfect speed relative to its moving speed inside the material. That sweet spot can mean different things depending on your goal: achieving the best surface finish, machining your parts the fastest, or maximizing your tool life.

Besides directing the chips, the advantages of these configurations are best illustrated by the quality of cut, especially on veneered plywood (see the photos above).A down-cut bit will leave a clean edge on top but a ragged edge on the bottom; an up-cut bit will accomplish the opposite. This is great until you want to cut a dado with no tearout on the face. A down-cut bit will leave a clean top edge, but it sends the chips downward, into the dado where they have no place to go.You can make this cut, but you have to take it slower than usual to give the chips a chance to clear.

An important factor to consider while reading these tables is the tool diameter. A larger end mill will indeed be able to handle a larger chip load.

The new solid-carbide spiral bits come with some disadvantages. The first is that the cutters are expensive. A typical solid-carbide spiral bit is likely to cost at least $50. A similarly sized straight bit with carbide-tipped cutters will run somewhere in the range of $7 to $23.With a cost differential that large, you will want to know what you are going to do with this bit and that you will use it often enough to get your money’s worth. To highlight other differences, let’s compare the qualities of spiral bits and straight bits.

Feed rate formula

A general rule of thumb is to take passes that are around half the diameter of your mill. But remember, as mentioned above: some more complex or hard materials requires lower depth of cut (typical examples are aluminium and  plexiglas...).

The harder the piece, the more deflect your end mill will bear. This will cause chatter and vibrations. Be patient when milling hard material and use smaller steps or lower your feedrate.

Straight bits also have a huge advantage over spiral bits when it comes to template routing, because you can buy them with guide bearings. And those bearings can be mounted on the tip of the cutter or on the shank of the cutter, depending on your needs and your template. The bearings are made for a variety of cutter diameters and lengths. It is really too bad that solid-carbide spiral bits can’t accommodate bearings a little more readily. With their superior edge cut, spirals make great template cutters when used with collar guides. But when it comes to bearing-guided bits, spirals seem to be available only with bearings mounted on the end of the bit. There are some problems with this: The cost is high (about $80); it precludes cutting only partway through the work, which means full-thickness cuts only; and the template has to be under the work, an inconvenience. Shank-shod, bearing-guided, solid-carbide bits (spiral bits with the bearings on the shaft end of the bit), which would permit template routing with the template on top of the work and trim cutting through only part of the work face, are not available. For this type of routing, you’ll have to stick with straight bits.

Feedrates are found using the formula given earlier in this document, but Fusion360 embeds a very handy chip load calculator which gives you the mill’s chip load for given feed and speed.

The up-cut spiral bit can have the opposite effect. It wants to pick up the work. So you must secure the work in some kind of fixture or hold it by a clamp. (I never rout anything that is not secured or clamped, but some people do.) The up-cut bit’s tendency to pick up the work also happens quickly and without warning.

Lathefeeds and speedsChart

During some milling operations, more than ¼th of your tool’s circumference “touches” the material during the milling. As a result, the end mill can’t cool down properly and tends to overheat easily.

If you're using end mills that you bought at Mekanika, you can access the pre-parametered feeds and speeds for Fusion360 here on the product pages.

For woodworkers who work with A-grade veneers on both sides of the stock and must have a clean edge top, bottom, and middle, the compression bit is a good choice. It has an up-cut configuration on the tip of the bit and a down-cut spiral ground on the shank. By lining up the bit just right, you can get a superior edge across the entire thickness of the wood.This virtuosity comes at a hefty price:A typical compression bit will cost about $90.

Understanding how feeds and speeds work is critical if you want to improve your CNC skills. It will help you to optimize your machining speeds, to obtain a better surface finish and most importantly to have a longer tool life. Here's an overvie of what this article goes through:

Based on this mathematical relation, we observe that if we want to increase the feed rate to cut that plywood faster, we will have to increase the spindle rotational speed as well to keep a constant chip load :

Millingspeeds and feedschart pdf

Millingspeeds and feedsChart

Before diving into numbers and values, you need to be aware that the following variables will heavily influence the quality of your cuts and the achievable chip load on the same machine.

This doesn’t mean you can’t cut mortises or plunge with a straight bit. You just have to sweep the router while you are plunging. You should probably cut mortises in passes not much deeper than 1/8 in. anyway, but with a straight bit, such shallow passes are just about a must.

This is a great starting point if you are a novice. When you are feeling more confident, slowly increase the chip load towards the “Advanced chip load table” values.

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Cutting speed formula

As illustrated above, there are mainly two bad spots that you want to avoid. The first one happens when you reduce your spindle speed too much relative to the feed rate. Doing so, you’re forcing the flutes of your end mill to cut off too much material, which can lead to unwanted vibration or worse, a broken tool.

Router-bit manufacturers have difficulty making solid-carbide spiral bits with cutting diameters larger than their shanks. So for small-shop hand routers you won’t find many bits with a cutting diameter larger than 1/2 in., the size of the largest bit shank. Spiral bits also come pretty much in a few standard fractional sizes up to 1/2 in. Straight bits, on the other hand, go through dozens of fractional sizes, all the way up to 2-in. dia. cutters. Depending on the job you have in mind for your router bit, straight bits also come in a variety of cutter lengths. So you can buy close to exactly the length of cutter you need.

On the other side of the graphic, if you reduce the feed rate too much relative to spindle speed, the flutes of your end mill will start rubbing the material instead of cutting nice chips. This action will make your tool overheat, and thus soften. Its sharp edges will become dull and if you keep cutting with dull edges and you will start to see a very deteriorated surface finish on your material.

Aluminum milling speed chart

Similarly, we suggest you slowly increase the depth of your cuts while doing these tests. Indeed, excessive depth of cut will result in tool deflection (see this article to understand why that can be problematic).

All spiral bits make clean cuts. This veneered plywood shows the effects of the three types of spiral bits on the top and bottom edges.

These concepts can be visually summarized on a graphic, where the feedrate is plotted against the spindle rotational speed, and which helps us to identify 6 different zones.

An end mill is a cutting tool and with time, it will eventually get dull. As it gets worn out, you will need to take it easier and reduce the feedrate to keep a good surface finish. You can also just replace it or resharpen it.

Let’s define an arbitrary feedrate of 2 000 mm/min. Using the former equation, we find that the spindle has to rotate at 10 000 rpm to achieve the proper chip load:

A few services claim to be able to sharpen spiral bits. But I haven’t found anyone who can sharpen spiral carbide to factory standards. To me, this translates into a substantial loss. Here’s why: The spiral bit costs twice as much—or more—as a straight bit, and the straight bit can be reground up to five times. A sharpening service charges about $4 to regrind a straight bit, and the cutter of ten comes back sharper than it was from the factory. So even if I pay $23 for a straight bit and sharpen it five times, I still pay only $43. Spiral bits might stay sharp longer than straight bits, but even so, the cost of using spiral bits will always be higher.

If you are not yet familiar with your machine, we compiled a starter chip load table with lower values. They are intentionally low to help you get confident with the machine no matter the type of engagement, the hardness of your material, etc...

Because a spiral bit is designed much like a drill bit, it makes plunge cuts easily. The cutters of a straight bit do not overlap, so if you plunge straight down deeper than 3/32 in., you might burn away the wood in the middle, but you won’t cut it.

My teaching and woodworking are centered on routing, so I have a cabinet filled with more than the weekend woodworker’s supply of router bits. I do keep a few solid-carbide spiral bits because, when I want a beautiful face cut or I am cutting narrow mortises, and I have the money, there is just nothing better. But my cabinet is mostly full of a wide variety of straight bits. For general-purpose work, for template and pattern routing, and for those times when need a large-diameter bit, I still reach for one of my straight bits.

Depending on how deep you want your end mill to go inside the material, you will have to adapt your feed rate to spare it.