19.Jul.2012 5 Rules Every Design Student Should Know About CNC Prototyping

5 Rules Every Design Student Should Know About CNC Prototyping

1. CNC integrated manufacturing does not exist to eliminate the need for technical skills.

It can produce designs with greater accuracy and repeatability than we are capable of, but it is still just a tool that enables accurate production.  Just because a CNC router comes with a price tag of $15,000+ and is hooked to a computer that moves the tool for you does not make it capable of magic.  In the end thoughtful selection of tools, materials and planning has almost the same impact on output quality as they do with hand tools.

2.       There is nothing rapid about rapid prototyping when you’re learning.

Rapid production is all about understanding technology and applying it efficiently.  The first few projects have a steep learning experience paved in rejected prototypes and likely a bit or two.  It is important to remember that although CNC is just there to make your life easier and your projects more accurate, it is often not the final objective when prototyping.  If you only need to make one, some things are better done by hand unless there is a specific reason you need to CNC them.   The learning process takes time and so does tooling orders. Sometimes you can get what you need locally, but sometimes your project will need something special.  You will not find a tapered ball nose bit at Home Depot the night before you need it.   Once you get used to the technology it gets faster but it is never a silver bullet that will save you on projects that should have been started weeks ago.  If it ever does save you on one of those, count your blessings and make sure you never try it again.  Some things don’t work twice.

3.       These machines have no brain so use your own

Image from MySafetySign.com

You may have already heard or seen the quote, “This machine has no brain so use your own”.  If you are familiar with this saying then most of the following will be a refresher course. If this is the first time you have heard it, burn it into your mind because it may just save your life and at the minimum will save you a lot of money, time and frustration.  Both successful projects and safety are never an accident.  Plan how things will be set up, how the tool will do it, test it, then set up your work and make sure it proceeds exactly as expected.

Between the router control systems and the CAM software used to generate your tool paths, you have more computational power at your fingertips than was used to put men on the moon. You can harness this technology at will to create complex objects with the push of a button, but never let that entire high technology lull you into a false sense of security.  A drunken squirrel has more accident avoidance logic than the most expensive CNC router or CAM system. This is why it is so important to always follow safety procedures, plan your projects in advance and watch closely for mistakes before they become problems.

The collision detection system on advanced cam programs is of little use to most beginning students.  Software collision detection is a valuable safety system for advanced projects that can help avoid mistakes, but relying on it is dangerous because it relies on the data you give it and the assumption it is accurate to ensure safety.  In professional production with a small number of highly trained operators ensuring proper setup these systems are reliable, but for our purposes they are at best an indicator of areas to watch.  In the academic environment where equipment is shared and everyone is learning, it is never safe to assume anything.  You never know who used the equipment before you and if they changed anything.  Small changes like pushing a bit in more than you told the CAM software it would be or using slightly thicker than expected material can render these systems useless.  They also do not track the movement of the dust collector, or the bottom of the spindle.  Since dust collectors are adjustable, large and protrude below the collet, the router will happily crush one into the work piece if ordered even if the CAM software never saw any collisions.  To avoid this, plan out your setup in advance, know your work area and know where the tool will go. Once your plan is verified with an air pass, you are as safe as you can be. Watch the first run extra attentively and be prepared to interrupt it if you spot any problems.

Software is not the only thing that can’t think for itself. Remember the CNC machine is just a machine, no different than a band saw or table saw, even though it is hooked to a computer.  While you spend less time with your fingers near spinning blades and you don’t have the same kickback risks, CNC machines are capable of moving on their own and they have no idea what is going around them.  Use your safety interlocks whenever you are with the CNC workspace or you are just inviting accidents.

The average low end CNC system you will likely encounter is an entirely dumb system with little or no two way communication.  Even expensive machines only have limited feedback and will not do much to stop impending mistakes, crashes or accidents so the responsibility for safety rests firmly with the operator.  While we use computer controlled  mills and routers to create parts for mankind’s most precision dependent applications, CNC technology traces its lineage to the era of punch cards and the protocols used to control them are usually just about as advanced.   They only focus on telling the machine where to go, how to go there and how fast to move.  The only feedback commonly available from most CNC machines is the limit switches at the extent of each axis designed to stop the gantry from moving off its tracks or crashing the router into the side of the machine and some older machines do not even have these basic safety devices.   Most machines cannot tell you if you’re pushing the  tool too hard or even the true location of the tool.  All the router usually knows is where the tool is supposed to be based on the commands that it has been given and the assumption they were successfully carried out accurately.   Your only method of ensuring you get the result you expect is planning, tests and careful observation.

4.       Never leave automated machines alone…EVER!

You may hear stories about professionals doing this, but remember you are not a professional yet and that expensive hardware your using is not yours either.  If they want to risk thousands of dollars of their money it is their choice, but this is never acceptable in the academic environment.  Advanced machines running proven programs are at best capable of semi attended operation and even this is a calculated risk.  The programs we run as industrial designers are used to create prototypes. They are often only used once and there is no way to know they will run error free and the machines we run them on are not usually particularly intelligent.  In addition, things just happen from time to time and if you’re not there to see it how will you stop it from destroying your work or the machine?

CNC Error

This happened when a collet loosened unexpectedly while cutting a finishing pass on a proven file. The error was caught within a minute and and the machine shut down. If the machine was left unattended, how would this have been stopped?   All it takes is a programing mistake or hitting the edge of a hold down screw to damage a cutter and cause it to run hot enough to start a fire.

If this sounds like extreme paranoia, please see these pictures I am borrowing form a Woodweb article located here that you really should read.

This was not a freak occurrence…  it happens.  Do you know where your fire extinguishers are?


5:  Garbage in, garbage out!

Prototyping can get expensive and we’re usually working on projects we need done yesterday, so we are always looking for places to save both money and time.  When we start out it is hard to know what you need to invest in and what is just a waste of money or time to get.  Bad choices on materials and tools can ruin an otherwise successful project or require enough redo’s to make you wish you had invested in higher quality materials to begin with.

Often when we think of CNC routers, the first thing that comes to mind is sheet goods like plywood and MDF.  This is not accidental since at least half of what we cut on CNC routers is usually sheet materials.  I am not going to start an exhaustive lecture on choosing the right plywood today as that is an essay in itself, and instead I am simply going to say that all plywood is not created equal.  You can safely get your MDF at the home improvement store and nothing bad will usually happen, however the same thing does not hold true for plywood.   Good plywood is expensive and usually purchased at a dedicated lumber vendor.  It has a consistent core with good adhesion that will handle fine details and CNC cutting with less chipping or weak spots.  This is where the standard home improvement store plywood tends to fall short. You might be able to get away with it on large parts but if you try producing an intricate design in it, you run the risk of finding a weak point in the lower grade core and having delamination or cracking occur.  It is a fine material for cutting cabinet sides out of on a table saw, but it doesn’t hold up when you start cutting it in 2” strips and routing pockets ¾ of the way through it like we tend to when designing highly refined objects that push the potential of plywood to the limits.  For this kind of performance, high quality plywood like Baltic Birch or ApplePly is essential.  Don’t be afraid to ask questions to your vendors since they are specialists in what they do and they can be an important component in successful projects.

Just because wood is the first thing we often think about with CNC routers does not mean you should limit your imagination.  You can also use routers to cut Renshape and some modeling foams.  These materials take time to acquire and usually cost more than alternatives such as creating a large lamination of MDF, but they have properties that make them extremely useful in some applications. Renshape for instance will hold mold details much better than MDF, require less finishing and is less sensitive to moisture.

Not only do materials count, but so do the tools.  Bits are an important part of CNC success, not an afterthought.  Bits need to be paired appropriately with both the material they are cutting and the application they are used for.  If you rely on only home improvement stores for your bits, you are limiting yourself to a small selection of spiral and straight flute tools intended for use in hand and table routers.   This is OK for simple projects but you will quickly find that you need specialty bits and more data.  If you do buy your bits from a home improvement store, buy the big name brands only and stay away from bargain bits and combo packs like they are the plague.  Under no circumstances should you buy your router bits from  bargain suppliers like Harbor Fright. They are great to use for the pack of forester bits you use once every blue moon or for the digital calipers you always have with you but not your router bits. CNC routing is hard on bits since you cut at high speeds for long periods of time with no breaks.  Additionally there is no change in pace between cutting with the grain or cross cutting and you don’t feel the router so you can’t slow down when encountering excessive resistance from dull tools or hard spots in the material.  Don’t take chances with your projects; buy good bits from trusted suppliers.

Good bit suppliers and manufacturers open up a new world of possibilities.  The options available are extensive and deserve their own essay, but suffice it to say that you can choose from hundreds of bits tailored for specific materials and applications.   Ball nosed bits will enable you to create curved 3D designs, angled bits are produced to create mitered corners and you can even find special form bits to produce edge details.

A few brands worth considering when ordering or buying router bits are:



Take your time when ordering your supplies in between projects and try to leave room in project schedules to get the good stuff.  The right materials can make your projects shine and good bits last a while if you take care of them.


Remember that at the end of the day, CNC technology is there to enhance your prototyping capabilities and free your mind to crisp, detailed and innovative designs.  As long as you’re working to push the envelope, there will be a learning curve and resistance but that is just a sign of progress.  Stay with it and see the places your mind can take you.

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