SPEED, RESOLUTION, and ACCURACY

This is the Salesman’s real ammunition. Please be aware… it’s not as important they would want you to think. What is really important is the speed and accuracy you actually need.

‘Speed and Feed’, ever heard of those before? Imagine you are whittling a stick with a pocket knife. The thickness that you cut down into the stick with each whittle, that is the Feed. Speed is obviously how fast you are pushing the pocket knife. A router bit (also called a tool) is essentially one, two, or more knifes on a rotary cutter and the same whittling action applies. Why is this important? There is an optimum speed and feed for cutting materials, wood is no exception.

The speed is limited by the material properties, heat generation, and tooling and for aluminum and steel and it’s about 100 feet per minute. As you whittle a chip, the material has to separate, deform, and dissipate heat and this does not happen instantaneously. Wood likely has a speed limitation too (probably by setting the wood on fire), but you will sooner hit other limitations and I’ll explain shortly.

Feed is the thickness of the chip generated and usually for milling aluminum is about 0.007 of an inch, for steel is about 0.004 of an inch, and for wood is somewhere in that range (based on the tool diameter). What is really important is that the thickness of the chip plays a huge role in how fast the tool gets dull. First; the chip actually acts a coolant. If you don’t take off enough material, there isn’t enough mass there to cool the tooling and the tool overheats. Second; if the tool can cut 0.008 of an inch just fine, a feed of 0.004 of an inch would result in twice as many cuts and wear the tool twice as fast.

So how fast does your router spin, 18,000 rpm or more? Using a 1/4″ bit, each revolution the cutting edge travels 0.79 inches or 0.0066 feet. With that bit spinning at 18,000 rpm this equates to 1,178 feet per minute. That’s more than ten times too fast for aluminum or steel. However, wood can handle that speed. Now let’s calculate feed: If we want a 0.008 inch chip from a single flute bit, the tool needs to advance 0.008 inches each revolution and at 18,000 revolutions per minute that equates to 144 inches per minute. But most bits have two flutes so the tool has to advance twice as fast so that each flute gets a 0.008 inch chip. This would result in 288 inches per minute. That’s pretty fast. I’m not saying that it can’t be done, but your limitation is now going to be router (or spindle) horsepower, how strongly the material is held down, and how strong the bit is.

My point is that it’s silly to think that a skinny entry level CNC with a three horsepower router is going to cut 1/2″ plywood at 288 inches per minute. If entry level is what you are considering, 100 to 150 inches per minute is pretty fast, seems to be the norm, and would likely make you very happy. (Yes, I know, if you were cutting foam you could go much faster, but most of us don’t build with foam.)

So then what’s the deal with maximum rates of 400, 600, 1,200 inches per minute?!

If you need to get from one side of the table to the other (without cutting) it makes since to go as fast as possible, right? Yes and no. Fast is good, but anything above 400 to 600 inches per minute on a 4 foot by 8 foot router is scary. But technically, a large portion of that time the machine will be accelerating and decelerating so it will spend a lot less time at maximum rate as you would think. And another point, why the heck are you trying to get all the way across the machine?! That’s just bad cutting practice. Your next cut normally is normally right next to the last cut. You will probably only hit maximum rates only a few seconds total on a sheet, so don’t get too caught up comparing maximum rates of different machines.

This is where resolution and accuracy becomes the topic. A driver motor (stepper or servo) will have a maximum speed at which it can produce useful torque, for conversation let’s say that it’s 300 rpm. Another property of the driver motor is the quantity of increments per revolution, which may be 200. So these two properties work in the same way to force the designer to make a choice between maximum speed and resolution. If want to move fast, the gearing on the motor will be high and the result is that each revolution will result in a longer distance traveled. Due to that a single motor increment will result in a longer minimum distance, this is called resolution. Resolution is the measure of the distance of movement as a result of a single increment of the motor. Smaller resolution is usually better due to the increased control that one has over movement.

What does this mean? For a system that is geared to provide 1,200 inches per minute at maximum speed, the resolution will be half that of 600 inches per minute. Common resolutions are usually 0.001 to 0.004 of an inch.

So what is accuracy? Accuracy is not the same as resolution (although salesmen will use the term interchangeably). Accuracy is the measurement of the machines ability to cut to the dimensions desired, and is initially based on resolution. If you need 1.000 inches of cut with a machine that has a resolution of 0.006, the closest it can come is 1.002 inches (one less increment would provide 0.996 inches, get it?) However, there is looseness in the gearing, the bearings all have slop, the material moves, the gantry flexes, the table flexes and those all add up to a cut inaccuracy of 1/32 of an inch. That’s the difference.

So what do I recommend that you look for? Don’t get sold on maximum feed rates (300 to 600 ipm is just fine). Don’t get sold on maximum resolution (0.00001″ vs 0.005″ is essentially the same when cutting wood). Focus more on the quality of the table, the bearings, and the drive system.

Updated: October 1, 2016 — 7:50 am
© 2017 Privacy Statement Facebook Building your CNC router