Tech Talk
What are "Solid Carbide Spiral Router Bits"?
Insert tools, spirals, brazed tools, diamond bits, single flute, double flute, up shear, down shear, etc, etc, - the choices in router bits seem endless. With the variations in machines, materials, finish requirements and production requirements in different plants and shops, every type of bit has its place. A complete comparison of all the router bit styles available would be confusing at best, so in this article we will just deal with solid carbide spiral router bits. Spiral router bits made from high speed steel have long been available. They were primarily used because they did a good job of shearing the fibres in softwood routing. In the past few years, spiral router bits have been available in carbide, which widens the range of materials that can be worked with this design of tool.
Basically, solid carbide spirals can be used in any application that other straight flute bits are used, such as trimming, shaping or grooving. The primary difference have a cutting edge that is constantly in contact with the material - therefore the load on the work piece, the router bit and the router head is constant and uniform. The cut is a 'peeling' action rather than the chipping action that you get from a straight flute bit. One of the benefits is that spirals actually draw less power than the straight bits. This reduction in power requirement can translate into more stock removal, or faster feed rates. Another benefit, generally, is a much better finish. Spiral tools also run much quieter than standard bits.Spiral bits are at their best at high feed rates, so if the application calls for a lot of straight line or large radius routing, spirals excel. But keep in mind, higher feed rates require more hold?down force. In some applications, where you have a lot of turns in the cut, spirals may lose this advantage because high feed rates are difficult to achieve. It's not that the don't cut satisfactorily at the slower rate, but when determining tool cost, based on footage, the efficiency of a spiral is much better at the higher rates when compared to conventional straight router bits. Also, spirals prevent laminate chipping because, if used properly, they push the edge of the laminate towards the material where it has support.There are two basic spiral designs - up cut and down cut (sometimes called up shear and down shear). Up cuts shear up, pulling the chips up, which aids in dust collection. Up cut tools are used when you need good chip removal, and/or when the finished or laminated surface is on the underside. Also, up cuts are usually the best choice when grooving, because they won't pack the chips in the groove.. Up cuts, by their action, also apply a lifting effect to the work piece, so unless you have a good hold?down on the table, they cannot be used effectively.Down cuts are the reverse. They spiral down, pushing the material down. They are most effective when the finished side is up. Down cuts will also work at higher feed rates because they are working with the vacuum hold?down, not against it like the up cuts. For this reason they are generally recommended for small parts or hand tools where hold?down may not be as good. Down cuts tend to pack the chips in the cut, except in trimming applications, so attention must be paid to the dust collection.Compression spirals are a more complex tool in that they have down cut flutes on the top and up cut flutes on the bottom. These are primarily for applications where there is a laminated surface top and bottom, and both must have an excellent finish. Spiral routers can be used in hardwoods, particle board, MDF, laminates, other composites, plastics, fibreglass, and even aluminum. Once again, because of the variety in materials and machinery, a number of variations in the basic up cut/down cut designs are available. The following is meant as a general guideline that should help you narrow your choices.Single Flute: For applications where feed rate is of prime importance, hold?down is excellent and there is plenty of horsepower.Double Flute: Finish is the prime factor where two?flute bits are used.Three Flute: Used where an extra fine finish is required, or in materials that are more difficult to cut. Also three?flute bits can be used when the horsepower is a little light for single or even double flute tools.Chip Breaker: These tools are available in either two or three flute. Each wing has a series of notches in it that serve to stagger the cutting load. Chip breaker bits will work at higher feed rates than standard two or three flute spirals. The finish cut of a three flute chip breaker bit usually will be equal to a standard two flute spiral, but feed at a higher rate. When considering the spiral design options, cost and productivity as they relate to the job have to be factored in. Tool cost goes up with size, number of wings, and the addition of the chip breaker grind.There are a number of other engineering factors that can affect the performance of the tool - such as hook, helix, and relief angles, and the type of carbide. Also, there are coatings that can be applied that will improve the tool life. Before purchasing spiral tools an examination of these factors can help to determine the best tool for your job. When doing the arithmetic on the cost of routing, you may want to consider coated tools. Titanium Nitride (TiN) and Zirconium Nitride (ZrN) coatings can extend tool life considerably. by: Doug Reid, President, B.C. Saw
