Chris Fitch is our resident CNC expert and designer. Check out all of the episodes of CNC Basecamp on the Woodsmith YouTube channel.

Our very own Chris Fitch has been hosting the CNC Basecamp on YouTube for over two years now, doling out new info each episode on utilizing your CNC to its fullest. If you’ve been following along the entire time, you’re well up to speed on the techniques Chris has demonstrated. But there are plenty of helpful tidbits of info Chris has picked up over the years that haven’t been collected in one place yet.

Before you jump into this pool of knowledge, you’re going to want to frame CNC machines correctly in your head. A CNC is a great addition to a shop that opens new doors for woodworking and more. It doesn’t replace traditional hand tools like chisels any more than a table saw does — on the contrary, a CNC can actually enhance your hand tool experience by taking care of material removal and rough shaping.

With all of that in mind, let’s get underway with our journey of using a CNC in the shop.

Let a CNC router rough out a carving, then you can add the fine details and bring the work to life with gouges and knives.	Small dust particles are usually the sign of an inefficient cut. Using a chip load calculator can help you make adjustments.	Large chips like these mean increased productivity, reduced heat buildup and therefore longer bit life.

MATERIALS

Most common sense advice here is focused on woodworking. There are some special considerations to take when working with materials other than wood. Here’s some high level advice and tips when expanding your CNC repertoire.

METALS.

Chip removal helps prolong cutter life. With a big compressor, you can deliver a continuous stream of air.	Brass is harder than aluminum and most woods, but can still be milled with a home shop CNC router.

With the right techniques and tooling your woodworking CNC router can mill brass and aluminum with dependability, accuracy, and consistency. Milling these materials on a home shop CNC router isn’t difficult, but milling metal requires a different and disciplined approach. Here is a basic formula to get you started:

1. Stick with easily machinable alloys like 360 brass and 6061 aluminum.
2. Choose the correct bit — two flutes for brass, one for aluminum.
3. Mill at a spindle speed and feed rate that is right for your machine. A feed rate of 50 IPM at a speed of 18,500 RPM is a combination that works well as a starting point.
4. Use a light depth of cut.
5. Lubricate the bit with wax, spray can oil, or a continuous oil mister.
6. Clear the chips. A compressed air nozzle on your router does the trick.

PLASTICS.

With the finishing pass, drop down to a smaller 1⁄16" tapered ball nose bit. This pass takes much longer than the roughing passes but leaves a smoother surface.	A variety of plastics are suited for CNC

Plastics have a lightweight grainless structure that gives them uniform strength in all directions, making them an ideal material for all sorts of parts and projects. When machining plastics the goal is an efficient cutting operation with no welding of chips (the biggest problem when cutting plastics) and an excellent surface finish.

1. Acrylic and polyethylene are the two most common types of plastic I recommend most people start with.
2. Soft plastic works best using a single flute “O” style bit; for hard plastics, choose a two flute “O” style bit instead.
3. A larger bit - ¼ " usually, works best. Use 1⁄8" for fine detail work.
4. For plastics the ideal chip load falls between .004 and .012.
5. When cutting plastics on your CNC router, increase the feed rate first, then try slowing the RPM to obtain a good cut.
6. Generally, half the bit diameter is a good place to start for depth of cut.

QUESTIONS & ANSWERS

When you want the best information you go to the source directly. Here are Chris’s answers to some of the most common questions about CNC woodworking.

THERE ARE A LOT OF DIFFERENT STYLES OF BITS OUT THERE, DO YOU HAVE A GO-TO SIZE AND STYLE?

If you’re not sure which bit to use, you can’t go wrong reaching for carbide straight bits like these.

I’ve tried a lot of different router bits with our CNC routers over the last few years: up cut, down cut, compression, single flute, two flute, three flute, coated, uncoated, large and small, long and short. You need specialty bits for certain materials and situations, but my go-to bits for 90 percent of cutting operations are a ¼" and a 1⁄8" straight bit, each with a short length of cut.

These bits produce smooth cuts, are versatile, tough, and reasonably priced. Check your bits before use and know when to say goodbye — they only work well when sharp. Top quality bits will be the diameter stated, however hobby-grade bits can be surprisingly smaller than the stated size. When a project demands accuracy, get out your calipers and verify bit diameter.

HOW DO YOU DIAL IN FEEDS AND SPEEDS?

It’s never a good feeling when you look at that expensive router bit in your CNC machine and see that it’s black from heat. Using the correct feed and speed is super important — it ensures the best possible finish on the cut and allows you to get the full life from an expensive router bit.

To arrive at the correct feed and speed numbers there are chip load formulas. I have tried to use the formulas and carefully apply the numbers, but it’s tough with hobby machines and small projects.

Experience taught me to use the rule of half the bit diameter for the depth of cut, use a spindle speed between 16,000 and 17,500 RPM and a feed rate of 90-120 IPM for solid wood, plywood and MDF — the chips are big, the bit stays cool, and the job gets done. So, no babying the bit, push it along and make it work!

WHAT STEPS DO YOU TAKE BEFORE EACH RUN TO ENSURE SUCCESS?

A calculator, like this one from Fablab, is an indespensible tool for calculating the feeds and speeds of your cuts.

Things can always go wrong and the outcome will not be what we planned. Anyone who thinks CNC woodworking is risk-free, brain-free woodworking has clearly never done it.

I’ve been able to eliminate some of these moments by going through a checklist before getting starting on run: 1. Use the preview feature in your toolpath software – make sure it‘s showing what you have planned. 2. Do I have the XY datum point set correctly? Sometimes I use the center, sometimes the lower left, and sometimes I forget which I chose. 3. Will the bit and dust boot clear the hold downs, screws, or registration blocks? Air cutting can help find trouble before it happens. 4. Is the material secure? Hold downs, screws, wood blocks and hot glue help. 5. Do I have the plywood’s grain oriented correctly? 6. Did I remember to re-zero the bit after changing it out?

WHAT’S THE BEST WAY TO IMPROVE EFFICIENCY?

After completing the chip load calculations and test cut, save the information into your CNC router’s software tool database. The same size and type of bit can be saved multiple times for different conditions.	Upfront planning will save you time and material. Maximize how many pieces you can cut out of a single sheet of plywood.

Watching a CNC machine dance over a workpiece and transform it is exciting, but after many hours of noise what’s exciting is when the machine shuts down and you can take off your hearing protection and get busy building.

1. Plan the order in which you cut parts to minimize bit changes.
2. Use the full format size of your machine and maximize the number of parts on a workpiece, doing so means less setup time, less startup time and less baby-sitting the machine.
3. Use the Time Estimator in your toolpath software. I am often surprised at how small tweaks in bit depth or percentage of overlap can cut machining times by over half — so work with the variables.

The best results are often not the most time consuming. Hopefully this helps you get off on the right foot when introducing a CNC to your shop and workflow!