Tech Talk
What are "Woodworking Insert Tools"?
One of the very first Insert Router Bits I ever sold was a dismal failure. They were cutting Pine and the bit tore up the material and moved it across the table. When I went to the factory we both went out to the machine whereupon the operator showed us the useless router bit. When I pointed out to him that he didn't have a knife in it, he was understandably embarrassed, as was his boss who had called me. At first, the urge was to gloat at their ineptitude, but the concept was new to them, and I knew he hadn't started the machine without a knife in the bit. It had obviously broken or fallen out and he hadn't realized what happened. It was at this point that I realized that I had not spent enough time explaining how the tool worked and how to look after it. I had taken a lot for granted, and assumed that it was obvious how to use an insert tool. Now, in the new millennium it is probably obvious to most, but I have found that there is still some ignorance, or maybe just bad habits, associated with insert tooling.
There are a basically two types of insert tools - shank type and bore tools, and these types are broken down to straight knife tools and profile tools. Different manufacturers use different mounting systems, and each of those can create their own handling methods, but the fundamentals are common to all.One of the primary advantages to insert tools is that they maintain constant profiles and dimensions between tool changes. It is equally important that the individual wings of the tools be at constant position relative to each other. This will guarantee the profile, dimensioning, and the knives will last longer. Every tool has a positioning system (it's a good idea to compare these positioning systems before deciding which one to buy, but that's another column) that should keep each knife in exactly the same spot, and help seat them in the pocket securely. This locating system could be machined in to the body, or could be a pin, or a screw, or even a combination of these. Whatever the system, it is important that the locator is in good condition. After a lot of use the pins can wear, break off, or fall out; or the screws can move, and if not maintained will defeat the main purpose of buying the insert tool. Similarly, if the seat and pocket are not cleaned with each change the knife will not seat properly. This is the primary cause of knife breakage, because carbide inserts will not flex at all, and if the positioning isn't smooth cracking and breakage occurs.Tools that are designed for multiple profiles use support plates that also have to seat properly or dust will get in and could cause knife breakage. I saw a head recently that had been dropped, and dented the body. Consequently the backer didn't mount properly but the knife was secure against it, so it didn't break, but it was not positioned in exactly the same place causing a line in the wood, which the customer assumed was from poorly made knives. Close examination revealed a small chink in the head. Ten seconds with a file solved the problem.Insert tool bodies last a lot longer than brazed tools and therefore things that would never really be an issue with the old style should be watched for. Router tools, for example, need to have shanks that are in good condition. Spinning in the collet can cause gouges and burrs that will impact on the centering of the router. Likewise, the shoulder around the bore, where the collars seat, of a spindle mount tool has to be clean and burr free. This includes punch marks often used in house to identify a tool.No matter which tool you have there are screws, and they are small and short. These screws often have Torx or Allen heads. The threads on the screw will wear and become sloppy, and sometimes this is barely detectable. The screws are supposed to be softer than the body, so they wear, and not the threads in the head. In other words, they are sacrificial and should be replaced at the first sign of wear on the thread or head. We often hear of screws coming lose. This could mean they are worn out or possibly there is wear in the tool threads. If, after new screws have been put in they still come lose we recommend a light grade of thread locking fluid.Keeping the tool clean not only applies to the pockets and seats at the time you change knives, but also at the edge of the knife. Pitch build up at the back of a knife will severely affect the life and performance of the cutting edge. Often this occurs in only part of the knife, and not over the whole cutting edge. By watching for this and removing the pitch the whole knife will last longer. There are several different grades of carbide in insert tooling and it is surprising that many users don't know this. They have been buying the same knives for years and no one has ever suggested anything different. I mention this because many grades of carbide work better in different applications and materials. Ask your tool supplier for options. You may be surprised by improved performance and overall cost reduction.Hardly anyone ever thinks of sharpening insert knives, and the way most are sharpened this is probably a good thing. I wouldn't recommend sharpening standard inserts because, for one thing I don't believe it would be cost effective. But many profile knives could be sharpened once or twice, depending on wear. The important thing is that all of the knives be the same thickness after sharpening. If not they will not be on the same swing circle when remounted and the tool performance will suffer. The knives should not have a step in them after sharpening, ie; the whole surface should be taken down equally. They should not be taken down below 2/3rds of their original thickness, for safety purposes. Sometimes they can be re-profiled, but this will reduce back clearance which may affect tool life, and it will change tool diameter, which may modify the finish size of your material.In theory, insert tool bodies should last indefinitely, and if they are kept clean; screws, pins, gibs and support plates kept in good condition, they will work efficiently for a long time. by: Doug Reid, President, B.C. Saw
