Machining MDF with High Speed Steel Knives

The adhesive in MDF is rough on tooling, but for short runs of custom MDF moulding it may be worth using HSS knives. June 26, 2009

I've heard a number of people over the years say that they've run MDF mouldings with standard high speed steel knives. I don't doubt that it will work, but wonder how many running feet they are getting before the knives turn into screw drivers. Also, if you've done this, what is the best type of MDF product to use? Ranger board, lightweight, etc.? I have a short run of about 100' to run with an existing tooling pattern and thought it would be nice just to use the HSS knives instead of having carbide knives made which is what I've done in the past.

Forum Responses
(Architectural Woodworking Forum)
From contributor K:
I ran a 1" deep profile into the edge of a maple veneered panel for about 24' and it destroyed the edge to the point it had to be re-sharpened. That small of a profile sanded out very easily and I would do it again if I had to. That said, I do feel that a plain board with no veneer like a Ranger board is a different composition than a coreboard that gets veneer. You can actually see metal particles in the edges of a veneered product but I have never actually seen metal in a raw board product, although that doesn't mean it's not in there. Lightweight would not be dense enough if it's going to be painted. I would use a standard weight board in a brand name of your choice. Just for the price of sharpening I think it's worth experimenting for 100'. Keep in mind the quality will get noticeably worse by the end of the run. Just curious, why are you running millwork out of MDF? I just installed my first job using an MDF moulding from Home Depot and I swear I will never touch it again, even to throw it in a fire!

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.