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Machining MDF with High Speed Steel Knives
Question
Forum Responses
From Gene Wengert, forum technical advisor: The adhesive in MDF makes HSS knives wear very fast. That is why you hear that HSS is not suggested. But, for a short, run, it is certainly less expensive to use HSS. (Lower density MDF will wear HSS less than higher density). In case some readers are not familiar with MDF manufacturing, it is basically just like the low density fiber board used for ceiling tiles, except that adhesive is added and higher pressure is used when making the board. A third similar product is high density fiber board (Masonite) that has no added glue, but uses the adhesive within the wood and also higher pressures and temperatures. The story goes that a man was operating a press making ceiling tiles (no adhesive; just intermingled fibers) and one lunch break he forgot to open the press and take out the tiles. So, over the lunch hour, the press heated the wood and continued to apply pressure. When he came back from lunch, what he had left in the press was a thin, dense panel that the heat had turned dark brown in color and that the heat had also glued together. His company allowed him to patent the product. The man's name was George Mason and the rest is history.
From contributor K: We run short runs of moldings with HSS on a regular basis. We do find that if we can remove the harder top or bottom surface of the MDF on the table saw prior to shaping we get longer life. The softer inner core shapes pretty well. I will say this though - your results may vary based on the steel you use. We don't specifically use anything special. From contributor J: Your best bet is to look for double refined MDF such as Ranger Board. It's more expensive but takes face machining a lot better than most brands out there. Forget how long the knives last but I'm not sure how you guys are producing MDF moulding that is clean looking and easy to finish with normal grade MDF. I've found that even without carbide tooling milling a premium double refined MDF will do less wear on your tooling. I think I usually find that my tools take twice as long to start getting dull with ranger than it does with even Trupan. Some companies that produce moulding for home centers I have seen use Trupan ultralight or the equivalent, sometimes on smaller crown/base profiles it's like flex moulding it's so flexible. From Gene Wengert, forum technical advisor: It is not the wood, but the adhesive in MDF that dulls. As different boards have different adhesive amounts, you will see a difference. Incidentally, particleboard is also dull, but a second factor is the amount of grit that is in the chips - chips can be stored on the ground, etc. From contributor R: 100 feet should not be a problem with hhs. The harder the rockwell the more you will get. Just keep the piece moving try not to stop on the product. Global tooling has a new Endurance steel that you can get about 500 lf of MDF. It is a coated steel and has a hardness of c2 grade carbide. From contributor A: I've considered running a couple of quick, cheap and dirty curves, but I've always hesitated with the HSS knives. From the original questioner: This is a short run of eyebrow arches for a holiday gift project. I'm going to paint them so the ease of throwing a sheet of MDf up on the router is tempting and I may still do it that way. However, I have a new joint assembly machine arriving this week and a run of about 25 test joints in poplar would be appropriate. I may just cut these segments in poplar and test out the machine. From contributor M: We have used a glue joint cutter for laying up radius blanks but have been receiving some negative feedback from customers once profiled out - this being in stain grade of course. From contributor A: To the original questioner: what type of joint assembly machine did you purchase? From the original questioner: I've got the Rangate finger joint press coming. If it works well we’ll be using it for that and other joints as well.
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