Tech Talk
What are the "Causes of Tool Failure"?
Judging by some of the questions we are asked and by the condition of some of the router bits we see coming in for resharpening and repair, it seems that there are a lot of applications where the wrong router bit is being used - or they are being set up and used in an inefficient manner. One of the most repeated mistakes is buying a two-flute carbide tipped 3/4" diameter straight router bit to dado (or groove) for insertion of 3/4" shelving.
This tool will work perfectly when it is new, but its diameter is reduced slightly with sharpening. There are a surprising number of people who don't realize that sharpening removes material and therefore alters dimensions. The size reduction necessitates a very minute adjustment and a second pass. It makes more sense to use a 1/2" or 5/8" diameter bit and take two passes that are easy to set up right from the beginning.When purchasing a straight bit to either groove or size, always buy the shortest bit you can use. This will reduce strain on the holding collet, and reduce potential runout and vibration. It will also cost you less to buy and resharpen, and it reduces the chance of breakage. Shank size should always be as large as you can use - never purchase a 1/4" shank tool when you can use a 1/2". The cost saving is minimal and not worth the risk. Half-inch shanks rarely break, except in extreme circumstances, and the 1/2" collet has a surface area of over 1-1/2" (times the shank length). By contrast, the holding collet around a 1/4" tool is only gripping a surface of just over 3/4".With the larger grip surface there is obviously less chance of slippage, and collet wear is much less. Also, always grip the full length of the shank. Probably the most common cause of router bit failure is due to collet problems. If the tool is poorly gripped it can slip, sometimes as little as a quarter turn. It is this sudden slip and re-start at router speeds that can break carbides. Collet condition is critical to tool performance. As the collet gets worn it tends to open slightly at the top and bottom, caused by the constant strain put on it by the tool. We call this "belling." A belled collet has very little surface contact, and runout and slippage will result. To check for this look for lateral lines - black or shiny - on the shank of the router bit. These are caused by slippage. It may only mean that the collet is glazed from use. A piece of emery cloth on your finger tip, rubbed very lightly up and down the collet, could solve this problem. But if the problem persists, replace the collet.The rule of thumb is a new collet every 600 to 800 hours of operation, or at least once a year on tools used regularly. Never put a tool with a damaged shank in your router. It will run out, destroy your collet, likely damage your machine, and tool performance will be very poor. Keep the tools clean, on the shank as well as the cutting surfaces but, if you use an oil based solution, make sure that the bearings are blown out dry before using the bit. Never use an oil based lubricant on your router bit bearings because it will pick up dust and eventually seize or burn. If a lubricant is needed, use one of the dry ones available.Cleaning is particularly important if you are using insert tooling. When changing the knife, make sure the back support, gib, screws and screw holes are free of any particles or build-up. It is very important that everything seats smoothly and firmly. If you see signs of wear on the screws, replace them before they damage the tool. If loosening screws are a concern, use a light grip Loctite solution on the threads. Use dust collection where possible, in hand or table routing. You would be amazed at just how much it will save you in tool life.One of the misconceptions we run into is the use of router bits for deep grooving or mortising. If you decide to mortise using several passes, readjusting the depth in small increments to get a better cut, save the tool and not plug up the mortise, you are creating more work for yourself and causing unnecessary tool wear. If you are only going to cut in 1/4" depth increments, then only the bottom 1/4" of the tool is doing all the work - multiplied by the number of passes. Cut as deep as the machine and tool will handle. But, for this type of work, talk to your tooling supplier. There are options such as stagger tooth tools and spirals that may make the job easier. Also in deep cutting (or even shallow) if you are doing a lot of straight down plunging, you may want to ask your tool supplier about a simple modification to make this job easier and increase the life of your tool.The most common cause of tool failure and poor quality cutting is overloading. This is particularly evident in hand routing. If the machine bogs down in the cut, and this is easily detected by the sound, you either need to slow down the feed rate, use a more powerful machine, take a smaller bite, or change the bit. If you don't, everything from machine, to the collet, to the bit and your work, will suffer. But don't cut at too slow a feed rate, trying to be gentle. Router bits need to bite. Slow feed rates increase friction, heat, pitch build-up, burning and tool wear. With the current variety in router bit sizes, it is almost imperative to run variable speed hand routers so that the larger diameter bits can be run at a lower speed. A 1/2" diameter bit has a peripheral speed of about 28,000 in/min at 18,000 RPM. Put a panel raiser bit on with a 2-1/2" diameter, and it's running at 140,000 in/min. Obviously you can't reduce the router to 3000 RPM (and you shouldn't) but it should be cut down to the lower part of the speed scale. Although a lot of what has been said here may seem obvious and logical, the condition of many of the router bits we see coming in for sharpening shows that most are not being used as efficiently as they could be. by: Doug Reid, President, B.C. Saw
