George’s Guide To CNC Bits

Chip Load

Chip Load refers to the size and amount what the end mills cut away. Larger chips are easier to clean out than smaller ones (Think snow vs ice chips). The RPMs of your Router or Spindle, Feed Rate of your Bit, and number of Flutes effect your chip load. In general we do have a smaller chipload overall since we use the snow, but if you make the chip load larger you will get ice chips that are easier to clean out.

Speeds and Rate

RPM Speed – The faster the bit spins the less it actually cuts at a given feed rate. This causes your Spindle or Router to create more heat also at higher speed, which can cause the snow that comes in contact with it to melt and drip onto your ice slap. You also increase the issues of “run out”, which is when the bit is not perfectly straight causing unwanted vibrations. This leads to “chatter” which is the sound you hear from the bit and can also cause a scalloping effect on the edges of the carving. High speeds will make finer snow

Feed Rate – This is the rate at which the bit moves in the XY plane (Left, Right, Forward, and Back). Moving the bit faster will create larger ships. In normal machining this is ideal, as you want to actually cut chips away, not just rub the surface you want to cut. However to high of a feed rate for the RPMs can cause the bit to bend, not giivng you a straight cut

Plunge Rate – This is the speed at which the bit moves in the Z plane (Up and Down). To high and the bit will bind up in the ice, but too low and you waste time. If you are doing full 3D carvings, Plunge Rate becomes just as important as Feed Rate since a low plunge rate can force your machine to slow down the feed rate while it waits for the bit to reach the correct depth

Flutes and Length

Flutes – Flutes effect the chip load, less flutes create a larger chip that is easier to clear. More flutes however lead to a larger core, making a stronger and longer lasting bit

Flute Length/Helix – A larger, tighter, helix will give you a more ridged bit, however may be bound up more in the ice as it ca not clear the chips as well to the surface

Bit Shape and Length

Bit Length – This is more important to ice carvers than it is in other forms of CNC manufacturing. We work with much thicker materials than many other industries for the type of CNCs we use. As such we work with longer bits, with our starting cut depth deep than most others maximum cut. This puts more strain on our spindles and routers, so always use the shortest bit you can. This is why an large heave bits like our 1/2″x1/2″ we have multiple sizes. Running nothing but an 8″+ long bit will put excessive strain on the bearings and shaft of your spindle

Bit Type – There are 2 main types of bit we currently use for end mills, though there are many specialty types out there. The main ones are Slot Mills, which are the ones will flat tips used by most carvers and are good for making flat surfaces such as seafood trays and logos. Ball Nose are for 3D carvings as the rounded tip allows for smooth curves. Always use largest ball nose possible as it has the most gentle curve creating a better finish

Bit Shape – There are 2 types of shape for the bit over all length, straight and tapered. Straight bits are a consistent with the entire length of the cutting area. Tapered bits get thicker the higher up the bit you go. For example, a tapered 1/8″ bit will be 1/8″ at the tip, but at 1″ deep, the part closer to the shank will be 3/16″ or 50% larger.

Step Over

Step Over – This refer to how far over the bit moves on each pass and how much over lap. A step over of 50% means that it will move half way over on the next pass – example, 1/2″ bit will move over 1/4″ inch with a over lap of 50%. We use normally an 80% step over, with a 20% over lap. If you do 100% step over, then you will risk having spots missed at corners. at 10% you will have ti go over some spots 10 times, creating a nicer finish, but taking far longer. When using a ball nose go for a smaller step over as a result