Tech Talk
How "Fast Can you Run Tools"?
There have been a number of responses to the article on Routing with Spirals (May 1996), most of them specifically wanting to know just how fast can you run these tools, and also what is the most efficient speed to run them.
Speed can refer to the RPM of the tool and/or the feed rate at which the tool head travels. These two are directly related, in other words - the faster the RPM, the faster the feed rate. The question then becomes: just what feed rate and RPM do you run spirals at? To determine this you need to know what the chip load factor for the tool is. Chip load is calculated by taking the rate of feed and dividing it by the product of the RPM multiplied by the number of wings. As an example consider a standard two-wing, 1/2-inch spiral tool sizing (routing) a piece of 3/4-inch particle board or MDF. The chip load for this type of tool and application is about 0.011. From here, choose an average RPM that you would normally run a 1/2-inch tool at. I assume that most would use about 18,000 RPM.Therefore, the feed rate for this speed and tool would be calculated as: Chip Load x RPM x Number of Wings, or 0.011 x 18,000 x 2 = 396 or 400 ins/min. If the feed rate needs to be faster (eg 500 in/min) increase the RPM accordingly: 500 ¸ (2 x 0.011) = 22,700 RPM. The chip load factor is important. Too often people assume that as long as they don't overfeed the bit, there won't be a problem. So, they think if they increase the RPM from the calculated figure and leave the feed alone, the finish cut will be improved. Wrong - this is only true if the machine or collets need service.By underfeeding or running at too high an RPM more heat is generated, the chips become fine sawdust, and you wear the cutting edge of the tool dull at a much faster rate. Because of the variables created by such things as horsepower at the router head, tool diameter and style, number of wings on the router, chip breakers, material being routed, desired finish, depth of cut and hold?down force, the chip load factor can change. The following are some guidelines:A. Single flute bits, with the change in helix and increased gullet capacity, have a much larger chip load (as much as 2-1/2 times). In other words, at the same RPM a single flute tool can run at about a 10% faster feed rate.B. Three-flute bits, conversely, have a chip load of about half that of a two-flute.C. As the depth of cut is increased, the chip load is decreased. For example, if the cutting depth is doubled, reduce the chip load by 25%.D. As the tool diameter gets larger so does the chip load factor - and vice versa. For example, if you increase the diameter by 50% (say from 1/2 inch to 3/4 inch) then the chip load must be increased by 35-40%. Similarly, if you reduce the tool diameter by 50% (1/2 inch to 1/4 inch), then reduce the chip load factor by 35-40%.E. Finish: If the finish isn't satisfactory, reduce the chip load until it is - being careful not to reduce it excessively. If you require an extremely small factor to get the desired finish then there are some other problems that could relate to anything from the wrong tool to a machine that needs service or collets that need replacing.F. Horsepower: If the spindle RPM is affected by more than about 10% when under load, then reduce the chip load.G. If using chip breaker bits, the chip load may increase by about 10 -15%.H. Cutting laminated board will reduce the chip load by about 10%.The above are guidelines only. Your tool supplier, together with your own experience with the conditions, will help you fine tune the correct feed and speed. In the past there was some rejection of these tools, primarily because they were often used like conventional router bits and found to be very inefficient. Recently, however, the market for spirals is growing at a very fast pace because both users and tools suppliers are realizing that, when used properly, these tools can be very productive. There have been questions about the feasibility of sharpening these tools. Spiral router bits can be re-sharpened, but there are a lot of components to a spiral - hook angle, helix angle, primary and secondary relief angles, flute size and sometimes chip relief. All of these must be maintained to original specifications if the tool is going to perform consistently.SOME HANDY FORMULASChip Load = Feed Rate / RPM x No.of WingsFeed Rate = RPM x No.of Wings x Chip LoadSpeed (RPM) = Feed Rate / No.of Wings x Chip LoadProblems cutting or profiling? If you have an area in your plant or shop where you are not satisfied with the quality or efficiency, let us know. We will research the problem and/or invite some reader feedback. Similarly, if you have some application ideas or solutions to share with other readers, we'd like to hear from you.by: Doug Reid, President, B.C. Saw
