With each increase in size, the maximum depth of cut increases due to reduced tool deflection, allowing more of the cutter to be engaged - this both speeds up the cutting of course, but also lengthens the life of the tool since that cutting action is spread over more of the length.

The calculated toolpath for an end mill. Blue lines show cutting; yellow repositioning; and red are helical movements into the material so as to not overload the less-efficient-at-cutting end of the tool. All these “ancillary” movements also gives insight into why knowing max material removal rate and the amount of material to remove just provides a lower bound on the machining time.

Feet Per Minute (FPM) is a linear speed measurement that tells how far an object travels over a period of time, in this case, measured in feet. When dealing with rotating machinery, converting RPM to FPM helps you understand how fast the surface of the object is moving in a straight line.

RPM toinchespersecond

Accuracy: Provides precise conversions, which is crucial for performance, efficiency, and safety. Time-Saving: Automates the conversion process, eliminating the need for manual calculations. Error Reduction: Helps avoid mistakes in complex unit conversions, especially in high-stakes industrial applications. Versatility: Useful in various industries such as manufacturing, motorsports, and machining.

Drive motors can slip: The relatively low power stepper motors control motion in this machine via belts. The belts or the motors can slip if the cutter head gets too much resistance. Because the motion control is “open loop” - there’s no feedback to the controller to confirm the actual position - a slip means that the machine is no longer where the software thinks it is, so what you’re making starts getting errors, and these errors quickly compound as different layers of the cut no longer line up and the machine then experiences even more resistance.

This frame took hours to carve, and yet was still in want of more detail, and also required a lot of hand sanding. An automatic tool changer, in combination with flexible speed control, could get a higher quality result much more quickly.

RPM to feet per minuteCalculator

This one-of-a-kind lamp - which I kept for myself - took about 30 hours of machine time; between a faster machine and automatic tool changing, I can potentially get an even-more detailed carving in perhaps as little as a few hours.

In machining operations, RPM to FPM conversion helps determine the cutting speed, ensuring the material is machined at the right rate for both precision and safety. This is particularly important when working with metals, where the wrong speed can lead to overheating or tool damage.

So while the settings in relation to cut depth, overlap, and speed I used to initially cut this were far different (and I dulled my bit more quickly and got a lower quality finish that required more hand sanding due to my suboptimal settings), I won’t be saving a lot of machine time in the new machine if I stick with the same 4-flute 1/4” bit.

In factories and warehouses, conveyor belts move goods from one location to another. The speed of these belts is often specified in FPM to ensure the right throughput. Converting RPM to FPM helps ensure materials move smoothly without delays or damage.

But that’s just the intuition - of course, there’s a whole bunch of math and physics involved in determining the “best” speed; it depends on the multiple attributes of both the tool (type, material, coating, geometry, number of flutes) and the material you’re cutting (wood, aluminum, steel, plastic, etc.).

On the other hand, if your machine is going “too fast” - assuming that it can keep up with the speed you’ve asked of it - you’re potentially generating too much heat on the cutter, also a way to dull it prematurely. The chips being cut off may also be too big, giving a lower quality cut.

The 1/2” & 9/16” mill will fit in my spindle, but not in the automatic tool changer I’ve added on top of the spindle. Thus, the two largest mills here show me the tradeoff in speed I’m giving up by adding the ATC. As for spindle power, every 10 cu. in. per min of MRR requires about 1 horsepower when working in wood.

However, I can try a stiffer 2-flute bit, as well as larger (2-flute) bits up to the maximum 7/16” shank my new machine can handle. So I simulated a few different end mills up to and a little beyond the capability of my machine, all assuming a 2.5” tool “stick out” to get to the bottom inside corners of the tray.

The RPM to FPM conversion calculator simplifies what would otherwise be a complicated process involving math and unit conversions. It allows users to input the rotational speed (RPM) and diameter of the object, and the calculator handles the conversion quickly and efficiently.

So let’s try plugging in a few different end mill sizes into the calculator to get the right feeds and speeds. This gives me the ideal RPM & IPM to use, which I can then plug into Fusion360 to generate a tool path and a machine time estimate. I’ll summarize all these speeds and metrics into three stats: the power required of the spindle; the material removal rate; and the time estimated to do the carve.

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A larger diameter end mill will deflect less. So you can plunge it deeper into the material. So a greater length of its cutting edge can be put to work, also spreading the heat and wear of the cutting action across more of the bit, yielding a longer life. But a bigger diameter end mill also requires the RPM of the spindle to drop to maintain the same surface speed of the tool’s teeth against the material. And obviously, a bigger end mill also limits the detail that can be cut: it cannot cut any corners with a radius smaller than the diameter of the mill itself, of course. And a bigger cutter, removing more material at once, requires a spindle with more power; many motors are unable to deliver their full-rated power at when going more slowly than their max speed.

My current machine - an X-Carve by Inventables - definitely does not let me go too fast. The spindle is 1-1/4 horsepower, with a very coarse (and approximate) manual control for speed. And the largest diameter bit I can use is 1/4”.

Flexible frame: The frame is not very rigid. Even if the motors do not slip, the aluminum extrusions of the frame flex with the torque of the spindle or twist as the spindle acts as a “lever arm” cutting into the wood too deeply.

An RPM to FPM conversion calculator is an invaluable tool in many industries where understanding both rotational and linear speed is critical. Whether you are in manufacturing, motorsports, or precision machining, this calculator helps ensure efficiency, accuracy, and safety. It simplifies the conversion process, allowing professionals to focus on what matters most—optimizing their operations and achieving their goals.

By inputting these values into the calculator, it provides an instant conversion from RPM to FPM, giving users a more intuitive sense of speed in linear terms.

The number of flutes also has an interesting relationship: more flutes mean more cutting edges contact the material with every rotation. Doubling the flute count of an otherwise identical tool might lead you to think you’d be able to double the spindle speed and double the feed speed, assuming all your motors can keep up. But there are two other implications of more flutes that counter that. More flutes means there’s less metal in the tool supporting its rigidity, so it will deflect more. And as you squeeze more flutes into the same diameter tool, the channels of the flute get smaller, so they can clear fewer chips. In fact, given some speed limitations even on my newer CNC, two-flute tools will tend to be much more efficient for material removal for me than four-flute tools.

Or check out some of the mobile or desktop applications for the actual calculations - one that I’ve found very easy to use is the mobile app FSWizard, and I’ve seen many threads in woodworker forums refer to the desktop package CNC Cookbook.

FSWizard screenshots - choose the material; enter the tool attributes and physical setup; and see the results. It’s free, albeit with advertising.

Revolutions Per Minute (RPM) is a measurement that quantifies how many complete rotations a rotating object makes in one minute. It's a commonly used metric to understand the speed at which a motor, conveyor, or any rotary device operates.

Obviously, if your machine is going “too slow”, you waste machine (and your own) time. But you also potentially dull your tools more quickly and get a lower quality cut as the cutter head is essentially burnishing the workpiece rather than cutting it.

I made a laminated hardwood tray in 2018 as a commission; using a 1/4” four-flute end mill, taking off 1/16” of depth at a time, the tray took over 9 hours to carve.

This is all well and good, but what’s this mean for Branching Out Wood? Well, it’s hard to say definitively because I still do not have the new machine. I’ve pieced it together from multiple vendors and some of the key pieces won’t arrive for a few more weeks.

The need to convert RPM to FPM often arises in applications where linear speed is crucial for efficiency, safety, and performance. For instance, in conveyor systems, knowing the FPM is critical to ensure materials move at the right speed. Similarly, in motorsports or in machining operations, understanding the surface speed (FPM) can directly impact performance, safety, and the quality of work.

Fortunately, there are many online and app calculators that take care of all the calculations, but if you’re interested in those details beyond the rough intuition above, this PDF from AutoDesk starts at an introductory level but then gets quite deep.

And of course, you will soon reach a point where the machine just can’t keep up: either the drive motors or belts will slip, the spindle may stall, or the cutter will deflect, causing it to chatter and further degrade the quality, or just break.

RPM to FPM conversion calculators are used across various industries where the movement of materials or objects at a precise speed is critical. Some key areas include:

This milling was done with too-aggressive of a speed, causing the mill to “chatter” or vibrate against the edge as it deflects from the excessive cutting force.

SFMto RPM

Rotational Speed (RPM): The speed of the rotating object, often given by machinery specifications. Diameter of the Object: The size of the rotating object or pulley, usually measured in inches.

The RPM to FPM conversion calculator relies on the diameter of the rotating object and the RPM value. Once these two variables are entered, the tool performs the necessary calculations and provides an accurate linear speed in FPM. This saves time and reduces the chances of errors, especially in complex machinery operations.

RPM to feet persecond calculator

The 9/16” end mill with only 1” stick out has the highest material removal rate, but taking off this much material requires about 3.1HP, just beyond the power of my 2.95HP spindle, even if I could fit the bit into my automatic tool changer.

A shorter stick out meets a stiffer tool; not surprisingly, this suggests that if a tool change can be done quickly and repeatably enough, there is a performance benefit to gain by using a short fast tool for the middle where the machine does not need to avoid a vertical wall, but then to use a longer slower tool only for the edges that approach the walls.

My machine has been more than sufficient for many projects, but I thought it time to upgrade to address the above limitations, and also, to give me a lot more flexibility in tool selection.

But other than potentially being able to service clients that have a lot of material removal in their designs, applying the right feeds & speeds with my stiffer & stronger new machine will also allow me to cut or engrave aluminum for the first time, as well as revisit some of my own earlier sculptural pieces that just proved too much work for my current machine.

Understanding the surface speed of tires is crucial in motorsports to optimize performance. The RPM of the tire can be converted to FPM, which provides insights into how fast the car is moving in a straight line based on the tire’s rotation.

First, I tried using the calculator to figure out the “right” parameters to use for cutting with a 1/4” four-flute end mill rather than the default 16th of an inch at a time I typically use. This yields a fairly low maximum material removal rate (MRR) of 3.7 cu in per minute, requires a 0.37HP spindle, and takes about 8.5 hrs to carve.

One critical driver of the feeds and speeds calculations is tool deflection - how much the tool bends from perfectly straight based on the force from it cutting into the workpiece. Understanding this helps with initial intuition on the relationships in the calculations.

Though I’ve been using a CNC for several years for everything from jewelry box engraving to stair tread fabrication, given the limited power of my current machine, I’ve been able to largely ignore what is often a critical topic with larger machines: speed. More specifically, what speed (revolutions per minute, or RPM) should the spindle - or cutter head - turn at, and what rate should the cutter head “feed” on the material (inches per minute, or IPM) via the drive motors moving the spindle.

However, as I’m in the process of upgrading my machine to something with far more capabilities, just as a potential client project has come in that might leverage those capabilities, it’s now a topic worth some attention. My reading and exploration here came up with some numbers that I found surprising, but that also explained some of the problems I’ve seen with my current machine - so I thought it worthy of a post!