CNC Router Bits
Choosing the correct cutter for your CNC router can be a little daunting. Hopefully this blog can clear things up.
Before we start, there are a few things that need to be considered. Firstly, please ensure the correct size collet is being used. Do not use a ¼ inch collet with a 6mm bit. The small size difference between the cutter and collet could cause chatter, especially with longer bits. If you have discoloration on the end of your bit, your spindle speed is too fast or your feed rate is too slow. I will not discuss speeds and feeds in great depth but to make life simple, I follow these rules:
- 6mm Double flute end mill – 12000rpm, 3mm double flute end mill - 24000rpm. Using this guideline, it's easy to determine what spindle speeds are needed for other size end mills. Generally, the larger the bit diameter the slower the spindle speed. If there is screeching from the cutter or burn discoloration on the bit, then your spindle speed is to too high. If it sounds like your spindle is being put under a lot of load, then your spindle speed is too low.
- Pass depths are determined by the rigidity of your machine and the material being cut. If there is lots of chatter or a bad surface finish, consider reducing your pass depths. Small machines (not rigid) can generally make between 0.5mm and 1mm passes in wood whereas, larger, more rigid machines can make passes from 3mm to 16mm+ deep.
- Feed rate is also determined by machine rigidity and material being cut. It has the opposite effect to spindle speed in that if your cutter is screeching or you have burn discoloration on the bit, you can attempt to make your feed rate faster. If your surface finish is not good, consider slowing your feed rate. Feed rate can vary from 20mm/s to 100mm/s in wood depending on your machine.
- Plunge rate – I generally use half of the feed rate. If there is excessive vibration when the tool plunges into the material, then slow the plunge rate down. Ramping the plunge is good practice, especially with a down-cut or compression end mill. Ramping the plunge refers to the tool entering the cutting material at an angle instead of vertically like in a drilling operation.
If you know the chip-load of the material, the cutting speeds and feeds can be calculated. Cutting parameters is a balancing act between cutting time, machine capability and surface finish. Once you play around with different parameters for your machine, you will find out what works for you.
Up-cut end mills are the most common bits used for CNC routers. They are used for cutting profiles and pockets. Up-cut refers the flute of the bit evacuating the chips upwards, away from the work piece. This allows the bit to cut through material with ease and with low heat generation as there is little friction. Single flute bits can cut faster, and double flute bits produce a better surface finish. The only downside to having an up-cut end mill, is as the work piece is being pulled upwards, therefore the surface finish on the top surface is affected.
For cutting aluminium or non-ferrous metals, I recommend using the aluminium cutting end mill as it is manufactured using higher quality material to withstand the beating of cutting aluminium.
Single Flute Aluminium Cutting End Mill
Down-cut end mills evacuate chips downwards into the material. They are also used for cutting profiles and pockets. They are used for cutting operations where a good surface finish is required on the top surface of the work piece. Heat generation is higher, and these bits require a more rigid machine. As stated above, a ramp plunge is highly recommended.
Double Flute Down-Cut End Mill
Up/down or compression end mills combine the functionality of the above two bits, to give you a great surface finish on the top and bottom surfaces of your work piece. These bits are commonly used for cutting melamine or veneer boards that have delicate top and bottom surfaces. The first 10mm or so of the bit is up-cut and rest of the bit is down-cut. This means for the bit to be effective; your first pass must be deeper than 10mm. Again, this requires a more rigid machine. Your wood chips are directed towards the point where the up-cut portion and down-cut portion of the bit meet.
Double Flute Up/down (compression) End Mill
A ball nose cutter has a rounded tip. These bits are used for making a half moon shaped grooves in your work piece. They can also be used to make fillets on the inside of a pocket and for 3D carving. The tip is rounded and therefore dull, they should therefore not be used for clearing large pockets of material.
A Tapered ball nose is used primarily for 3D carving. The small nose allows for very detailed carvings while the tapered portion makes the bit very strong. This allows you to carve detailed pieces much faster. The smaller the radius on the nose the more detail you will be able to achieve but the longer the carve will take.
Double Flute Tapered Ball Nose
A v-groove bit is for making v shaped grooves or chamfers in your work piece. It can also be used for large scale text engraving or patterns. There are different angles for different groove thicknesses. The larger the angle, the larger the groove at the same pass depth.
A v-cutter engraving bit is like a v-groove bit but is much smaller. It is used for very fine text or patterns.
An MDF spoil board on a CNC router should be surfaced occasionally. The process of surfacing the spoil board keeps the bed true in relation to the spindle. In other words, if your gantry has a slope, your bed will follow that same slope. Therefore, when the work piece is on the bed, it is perpendicular to the bit. This produces more accurate cuts. A bottom surfacing bit is used to surface a bed. Small passes between 0.2mm and 0.5mm is recommend. When surfacing the bed take the shallowest pass possible for the entire bed to be surfaced.
A CNC Router can be used to create custom one-off circuit boards. If you have the technical knowledge to design a PCB, you can create it on your CNC router. A PCB endmill is used for cutting traces on a PCB. These bits have a very small diameter therefore they need VERY shallow passes and a slow feed rate otherwise they will break.
I will add to this blog as I receive more queries from clients regarding bits. Hopefully this information will help you make an informed decision when purchasing cutters. If you have any queries please feel free to drop me an email.
Happy cutting!!