Tech Talk
What is the "Relationship Between Feed Rate and Tool Wear" in woodworking tools?
Just in case you haven't read enough in this column about Feeds and Speeds - I've got one more. I know it may seem as though I beat this topic like an old rug, but we find that in a lot of applications the relationship of feed and speed is not given the attention it is due. In the past the subject has been, primarily, the adverse affects of over feeding. For the cautious, the problem is usually not going to fast, but under feeding; running the material through the blade, or cutter, too slow, or in the case of routers, running the RPM up without increasing the feed proportionally. The reasoning behind this is to reduce the load on the tool and get a better finish on the material. The problem, is that it is like running your car in low gear all the time because it's easier on the car on a steep grade, therefore it must make it really easy on it on a flat road - when, in fact, you will do more damage than good. While a better finish is inevitable at a nice slow feed, the chip load is reduced to the level of almost filing the material away, and the level of production per sharp edge declines drastically. By feeding slowly the amount of removal with the passing of each tooth is greatly diminished, but yet the tool wear is usually greater. Heat is the biggest enemy of sharp tools and slow feed rates create heat by increasing the friction between tool and work piece, whereas higher feed rates actually reduce the friction, therefore the tool wear is often much worse when you don't let the tool run at it's optimum rate of feed.
When feeding material through saw blades too slowly (or blade through material, in the case of Panel Saws) the excess heat can build up very quickly, and can cause a residue of burnt pitch and create warped blades, either of which will exaggerate the problem exponentially by increasing the friction at a very rapid rate . At best, this will give you bad blade performance and a poor finished product. At worst, you will do permanent damage to the blades, and possibly cause a fire in the dust collector.A long time ago we had a customer that decided he could eliminate a step in his manufacturing process by running his Band Resaw slower and get a "planed" finish. Any one who has used a Resaw knows that it is a sizing machine only and never would it be able to cut material and leave a finish that even remotely appears to be dressed, but there is always one. After doing this for about a week we got a call about poor blade life. On inspection we noticed the chips produced by the saw were as fine as baby powder, and we asked him to put a new blade on so we could watch it run. His production, per blade change, had dropped from 15,000 to 20,000 feet, to less than 5,000 feet. All because of increased friction on the tooth. On top of that, the finish wasn't all that good, and the hourly production had dropped to the point it more than offset any savings he had hoped to achieve.Many times we have seen the same thing in Router applications. I would estimate the number of users under feeding routers is probably twice the number that overfeed. As their supplier of routers we have to watch feed rates closely because extremes in either direction (too fast or too slow) results in, what we think is a pretty good product, looking bad because of it. The three ways to compare cutting tools are finish, edge life and price. You can usually get any tool to give a good finish if you slow the feed down enough, but when you have to do this, edge life suffers. If you have the right designed tool for the machine, and the material, and the tool is good quality, then running at too low a feed rate will make it perform like a poor quality tool, or one that is not matched to the job. As an example we recently had a customer with a brand new and very expensive state of the art CNC Router, and very little experience routing. The material he was cutting was tough, and the application required some trials to get the best tool for the job, but in the end everything was working well; for about a month. Suddenly tools were breaking, burning, and yes, even a fire was started. The ones that didn't break were coming off blue and he was changing them more than twice as often. Again, upon inspection, one of the shift operators had dropped the feed rate from 18 meters per minute to 2.5 meters per minute, and was very adamant that was as fast as these tools would go, because he felt 18 meters just seemed too fast and he felt more comfortable and safer at the lower feed rate, when in fact, the problems and the danger were occurring because of the low feed rate.The most extreme example happened recently when a machine was programmed at no feed rate. It was set up to fully stop the router 5 to 10 seconds before the rotation and/or the lift out started. They completely destroyed 6 expensive spiral routers, and started a fire, before realizing the cause. While most realize the importance of the relationship of Feed and Speed, when programming a tool path, we often find that when the feed has to be slowed, at a corner, for example, the RPM is not reduced proportionally, creating the friction wear that reduces the life of the tool.Depending on the machine, the operator, and, of course the tooling, you can usually get an excellent finish on a moulder. However, many times I have seen feed rates adjusted to compensate for deficiencies in the equipment. To site yet another example I was at a plant last year where the customer was getting less than 1000 feet between tool changes on his Moulder. He wasn't even complaining. He knew it should be better but really wasn't aware of how ridiculously poor this was. When asked why, he said he realized he should be running faster to get better tool life, but the finish was so poor at normal feed rates he had backed down to between 10 and 15 feet per minute, depending on the amount to be removed. This was causing such a bottle neck in the manufacturing process they were actually thinking about buying a second machine, when all they needed was to find out why they couldn't feed at a normal rate (for this machine 50 to 80 fpm). It turned out the tool heads were of very poor quality. That combined with the way they were grinding the knives created a finished product that would have needed tremendous amounts of unnecessary sanding if they ran at a normal feed.The cost to solve the problem and to get back to an acceptable standard of product was in the low thousands of dollars. Expensive, but nothing compared to purchasing a second machine, not to mentioned the expense saved in tool changes every 1000 feet. The same principles of feeding too slowly can apply to planing, bandsawing, drilling, and even hand feed applications. The consequences are the same - poor tool performance. by: Doug Reid, President, B.C. Saw
