Tech Talk
What are the "Steps in Ordering Custom Tools"?
Ordering Custom Cutting Tools involves three steps - fact finding, designing, and manufacturing. Last month this space was taken up with the fact finding segment. This is the process of finding out as much as possible about the application, and what exactly is expected of the tool. Profile, type of tool, material being machined, tool diameter, type of machine the tool will run on, feed and speed rates have all been determined by now. From here it is up to the tool makers to design a tool, and give you all your options, and the pros and cons of those options. The first step is to produce a drawing. It does not necessarily have to be a machinists' detailed dimension drawing, but an outline of the tool indicating certain critical details. The first, and most important detail, of course, is the profile, showing how it starts and ends. It is also a good idea to show an outline of the wood beside the cutter. This will give you a clear idea of how the tool will set up to your profile and the position of the tool relative to the material it is cutting. The drawing should also show the rotation of the tool on the spindle or collet, and all pertinent dimensions, such as the diameter, at the widest and the narrowest points, and details of the bore or shank. From the information gathered from you, the manufacturer has had several decisions to make, and often several choices he has to present to you. There is no particular order, but often the various choices that are made can affect the decisions made on other options. One of the most basic, is the choice of material the tool is to be made of. Thirty-Five years ago the choice was simple. It was High Speed Steel. Now you have that, plus Brazed Carbide (and several grades of that), Inserted Tooth Carbide, Tantung, and Diamond (PCD). There is a great deal that could be written on this option alone, but for now let's just say you and your tool supplier should discuss the plus' and minus' of each of these in detail before you make your final choice. How many cutting edges (wings) should the tool have? The feed rate, the material the tool is made from, the material being cut, the type of machine, and the desired finish quality can all have an effect on how many wings you need. As the number of wings increase, the chip load on each decreases and you will get a better finish, or the feed rate can be increased. This does not work to infinity, however. There is an ideal chip load for each material, and over decreasing it can be completely ineffective and actually cause the cutter to wear faster.
Hook angle is the angle the cutting edge contacts the material at. It ranges from a high of 25 degrees to a low of 0 degrees, and can sometimes, but rarely, be a negative angle. The hook is determined by the material the tool is made from, the material being cut, and the direction of cut. Softer woods usually are machined at higher hook angles, with the angle decreasing in cross grain cutting. Tools for composite boards usually have less hook than tools for solid woods. The sharper the cutting edge generally, the higher the hook, with High Speed Steel having the sharpest edge. What it comes down to is, that all the other decisions that are made will lead the designer to the correct hook. If it is not correct it could greatly affect cut quality and tool life.There are many applications where it is in your best interest to build a shear angle into the tool. Shear is a top to bottom (or reverse) angle that can reduce cutting pressure and be very beneficial to finish. In applications where the cutting is on the side of the material, shear angle can improve the top (or bottom) edge finish by putting the cutting pressure on the wood, where it has support. In the case of double sided laminates, or veneers, for example, a compound, or alternating shear is necessary for a smooth finish at the edge. In some applications a shear angle may allow you to increase your feed rate. In routing or shaping applications, where the cutting action is on the bottom of the tool an upshear angle will definitely leave a smoother finish.Whenever a custom tool is cutting into the material leaving a straight edge (i.e. grooving in) it is a good idea to make sure that scoring spurs are built in. Without them you are risking a poor edge finish and tool life will definitely suffer.Clearance is the amount of space between the cutting edge and the body of the tool. Clearance angle is the angle behind the cutting edge. Without adequate clearance a tool will wear prematurely and may even leave a burnt finish on the material. On Brazed Tools the clearance must be enough so that the tool will still cut efficiently down to the last sharpening, but not so much that the tooth will be too weak. The amount of clearance varies, based on application and cutting edge material. It is up to the manufacturer to make the final decision on clearance, and the results after the tool is used is really the only way an end user will know if he has built enough clearance into his design.Some tools work much better if the cutting load is staggered. This is often the case with large profiles or tough materials. Staggering the load is accomplished by putting more wings on the tool with each wing cutting less of the profile. Where the profile is split between the wings is important. You have to make sure there won't be any indication of the staggering on the wood, when the tool is new, or after several sharpenings. Also, the wings doing the various sections of the profile must overlap for a smooth transition. There is another way to stagger the cut, and that is by making more than one cutter, with each cutter interlocking, and overlapping. This method is used when you may want to expand or shrink the profile with spacers to change the shape, or compensate for tool wear; or when the design makes this method more advantageous than staggering the teeth on one cutter. Staggering is necessary when compound shear angles are being incorporated into the design. It is in your best interest to discuss the staggering option with the manufacturer if you think it may be to your advantage. There is an initial cost increase, but where it is required the cost is more than offset by the improved performance.With most of these, and some other decisions that have to be made, you will probably follow the direction of your tool supplier, because ultimately he is the one who has to stand behind his tool, but the difference in getting a tool that works and one that saves you time and money may be in picking his brain thoroughly and weighing all the options.
