A question that comes up on a regular basis, is how fast do I have the router for the different bits?
Obviously, this is only really applicable for variable speed routers. It doesn’t mean that the information here is not applicable for fixed speed routers, but instead indicates at what point (ie at what bit-size) that the fixed speed router should not be used.
What we are really concerned with is the tip speed – ie the speed at the outside edge of the bit.
For example, we have 2 bits – one 80mm diameter (approx 3″) (a panel raising bit), and a 13mm bit (approx 1/2″) (a straight cutter). Our router is set at 22,000RPM.
The tip speed of our 13mm bit will be around 54km/hr (33mph). Whereas our 80mm bit has a tip speed of 330km/hr (206mph).
The large bit is therefore running way too fast for optimum performance and safety. However, it is not a matter of running every bit at dead-slow. A router after all is a high rotary speed tool – the bits are meant to run fast. If you run too slow, you risk chatter, kickback, and tearout. If you run too fast, you risk burning of the wood, and catastrophic failure of the bit.
Here are some suggested (maximum) router speeds, which can be used as a starting point for determining the optimum bit speed. Now unlike any other table that I’ve seen for router bit speeds, mine gives plenty of overlap. This is because I have researched the various recommendations out there and the general consensus seems to lead to a suggested tip speed of 100 – 150 km/hr. If you are uncomfortable with higher speeds, or are going to make a heavier pass, removing large amounts of material, then tend towards the lower end of the scale. For final passes removing very little material, tend towards the upper range.
I have also based them around a router with a speed range of 8000 – 22000 RPM.
00mm – 35mm: 22000 RPM
20mm – 40mm: 20000 RPM
30mm – 45mm: 18000 RPM
35mm – 50mm: 16000 RPM
40mm – 60mm: 14000 RPM
45mm – 70mm: 12000 RPM
55mm – 80mm: 10000 RPM
65mm – 90mm: 8000 RPM
Determining the final speed also needs a few other things to be taken into consideration. Sharpness of the bit, quality of the bit, quality of the router (the bearings, and the collet), density of the material being routed, and material feed rate. (I refer to material here, rather than just wood, as it is feasible to use a router on some metals (such as aluminium), and plastics. However, I am not covering the other issues involved in routing different materials. Even wood has a phenomenal range in material characteristics that I can’t take into account here.)
It is generally a very good idea to make test cuts until you are familiar with the working properties of the material.
Another point is how much material to remove in any one pass. A straight 1/2″ cutter can generally handle a single pass (in softer material such as pine), but taking 2 passes leaves a better finish, and places less load on the router and the bit. In some instances (such as dovetails), multiple passes with the same bit isn’t possible, but even then, you can use a small straight cutter to remove the bulk of the material before switching in the final bit.
For something like a panel raising bit (80-90mm diameter), you may think that running it dead slow and going in one pass is the answer (with a slow feed rate). It isn’t though. The tip speed may be high (150km/hr at 8000RPM), but the centre of the bit is only running at 15km/hr – not enough for a good finish (and never mind the huge load placed on router and bit removing all that material in a single pass). The only answer then, is multiple passes (or removing some of the bulk material on the tablesaw or bandsaw).
A final hint? Listen to the bit – it sound ‘funny’ when at the wrong speed. You get used to the sound that a correctly set router bit makes.
Oh, and one last thing. Any bit over 50mm is a router table job only. Even smaller bits benefit significantly by being used in this manner. Sometimes handheld is the only way, but if I have a choice, I use the router table every time.