22.Jul.2012 A Bit About Bits: Introduction
A Bit About Bits: Introduction
Let’s talk a bit about bits. I enjoyed writing “5 Things Students Should Know About CNC” and with a new semester quickly approaching, I felt a follow up article about bits was in order. Bits are a deep topic, but over the next several articles we will cover the basics about the different bit types, what they do and why you want them. In addition I will share a few of my favorite basic bits that will serve you well starting out.
One thing I should warn you about up front though is that bit selection is heavily influenced by not only the application but by the machine they are used in. Ridged machines with powerful spindles can handle anything including big bits with aggressive cutting profiles. Moderately ridged machines with spindles are great prototyping and limited production tools; they just use smaller bits or less and are run slower. If you use a machine that is equipped with a router instead of a spindle or is not particularly ridged, then drive gently and be careful with large tools. Things may take a little bit longer but you’ll still get your projects done.
There are several ways you can categorize router bits. For our purposes, let’s first divide them into two categories: bits you can use in CNC routers and bits that you can’t. Any router bit with ball bearing guides or roller pin tips cannot be used in CNC machines. The ball bearings exist to guide stock consistently along the bit when used in hand routers and router tables, but CNC routers have no flexibility to follow the stock. When used in CNC systems, bits go where you tell it to go so the entire bit must be able to cut through anything it encounters.
Some specialty tools exist that only cut in specific areas. Examples of these tools include joining bits, grooving cutters and other similar tools. Most common versions of these cutters cannot be used safely in CNC machines, however some CNC specific versions do exist. They are commonly used in the cabinet and furniture making industries to speed up production but they are not for use in the student prototyping environment. Many of these tools cost several hundred dollars and require professional level skill and more powerful and full featured equipment than we commonly have available.
Please view the following videos to see a few of these tools in action but don’t feel limited or cheated by not preforming these tasks with a CNC mill for your projects. Everything done in both videos can be accomplished as secondary operations using a table router with regular bits and a drilling jig for the side holes created by an aggregate tool head used in the second video.
In a later article we will discuss some of the capabilities of professional grade equipment and the impact that it can have on your designs, but for the moment let’s just focus on creating our own prototypes.
In addition to unusual specialty tools that cannot center cut, some seemingly simple straight bits also lack center cutting ability. Whenever purchasing a router bit for CNC application, be sure that it can center cut or “plunge cut”. This lets you know you can push the bit down into the work piece to start a cut. Any non-center cutting bits have to approach a work piece from the side to begin cutting and may not be able to change cutting depth while inside the work piece.
One final detail that you must take into consideration is how you will hold the bit. Most systems students have access to do not have automatic tool changers so there is no need to worry about tool holders, but you still have to be sure that the shank of your bits match an available collet for your machine. Some machines use highly flexible ER collet systems allowing you to order a wide array of collet sizes, but systems equipped with routers usually have between 3 or 4 available collet sizes based on the most commonly available router bits. If you are using a regular router bit, this is usually never a problem, but if you ever find yourself needing something odd sized like a specialty end mill, this can become an issue. If your machine uses an ER collet system, you will likely be able to order the correct sized collet but quality collets are usually at least $20. If you are going to use handfuls of this bit then this may be the way to go but, most end mills can be ordered with a closely matching standard shank size like .125” and .25” and they only cost slightly more than regular end mills.
Now that we have covered some of the bits that do not work in CNC applications, let’s talk about what we want to use. The bulk CNC work is done using straight plunge cutting bits. These come in two main classes, straight flute and spiral flute. These basic bits can be broken down even further by the cutting edge material, number of flutes, length and diameter. Spiral bits also have one additional property, because their cutting edges act like a screw they create upward or downward directional forces. Up cut bits pull material up towards the router and away from the cutting surface resulting in better chip extraction but this same action can cause tearing around the top of the surfaces of the hole. In addition up cut bits can exert a surprising amount of pull on a part. This pull can become a problem in thin materials or in some vacuum work holding situations. Downcut bits force the work piece down. This results in cleaner edges around the top edges of the hole but this also directs chips into the cut. Because chips are pulled back towards the cutting area, downcut bits must be run slower than upcut bits to allow time for the chips to be removed and to prevent overloading the cutter. A third class of spiral bits also exists called compression bits. Compression bits are used for composite materials like plywood and some laminates. They are comprised of both upcut and downcut sections which pull material into itself to help reduce chipping.
For further clarification take a look at these articles discussing the basics of spiral bits. While they are primarily directed at traditional woodworking applications the basics still apply to CNC machining.
Besides the common straight cutting profiles, a few other useful bits are also commonly used. Ball nosed bits are used to create rounded edges and to smooth out 3d forms and angled bits are used to create smooth mitered angles and champers. In addition to creating miter joints, angle bits are used to enable a powerful finished technique called miter folding to create tight seamless angles and in a simple method of adding fast 3D details often used by the signage industry called V Carving.
One final bit exists that can have the greatest impact on the accuracy of your project. They are called spoilbord cutters or spoilbord surfacers and while they almost never touch your project they are essential to accurate work. Spoilbord surfacers are used to plain down the spoil board. This is done to ensure that the spoilboard is parallel to the x and y axis across the entire table. It is also done periodically during the life of the spoil board to remove groves left by through cuts. With vacuum systems this is essential because these groves cause a loss of holding power but they can also cause accuracy problems with other hold down systems as well, especially when they build up in heavily used sections of the table. Spoilboard surfacers are also used to remove the hard coating from the surfaces of MDF when used in vacuum table applications because it blocks the flow of air through the material.
Spoilbord surfacers come in a variety of sizes and two main styles. The most common and efficient has replaceable knives, however some versions are constructed like traditional router bits.
Amana Inset Tooling Spoilbord Surfacers:
Most Spoilboard cutters are expensive so don’t run out to buy your own personal cutter. Instead talk with your shop manager or technicians if you recognize problems in the spoil board and see if they can handle this for you. You only want to consider surfacing your OWN spoilboards which you KNOW are hardware free. In school shops, a lot of people still use screws for work holding and they frequently crack off leaving remnants in the spoil board. If they have not been removed or are broken off below the surface of the table, you will likely not know until you hit it with the spoilboard cutter which is both dangerous and will destroy or at the very least damage a very expensive bit. I am making this safety point very clear because when I was in school, we all used steel drywall screws for work holding and we had plenty break. I know that hitting a screw with a .25″ router bit is enough to be dangerous in the right conditions so I don’t want to imagine what hitting one with a 2 inch surfacer is like. I assume though that it will potentialy be far worse since the outside of large bits travel at much higher speeds and the bits themselves contain much more mass.