Causes of Grain Ridging in Moulder Finish

This thread includes a nice concise summary of issues that cause finish defects in mouldings. August 21, 2012

I am milling about 400 lin. feet of v grain fir to a 4.25 inch crown. I am using a Jet 14 molder/planer with three cutter heads at a feed rate of 10 feet per minute. I am progressing at about a half turn up each pass and I am getting a fair amount of grain ridging in the final product. I can scrape and sand them out if needed, but am I doing something wrong? The molder is only 1 1/2 hp. and the cutters are new. This just may be the best that this machine can produce or the best I can expect out of KD v grain fir?

Forum Responses
(Architectural Woodworking Forum)
From contributor L:
I suspect your problem is too slow of a feed rate and itís pounding the heck out of the wood. You would probably have better luck with a feed rate of 45'/min with a three knife head and taking enough of a chip load to prevent the rubbing you are now doing. It might be that between the excessively slow feed rate not taking enough of a chip load and the heating from all the rubbing you are doing causing the variation between the spring and summer wood.

From the original questioner:
The machine only has two speed settings 10 and 20 feet per minute. The manual says that molding is to be done at 10 feet per minute. I will try going to 20 feet.

From contributor L:
Chip load is the amount of wood removed by each cutting edge during one revolution. The energy being expended by the motor pushing the cutting edge through the wood creates heat at the cutting interface. That heat is either carried away by the wood chip being cut or absorbed by the cutting tool. Heat damages most tooling so it is better to have enough chip to carry away the energy.

When a cutting edge first hits the wood there is a short time that the wood is compressed before the actual cut starts. The slower the feed the more compression and it becomes visible. There is no such thing as a perfectly sharp edge, at least not for long. As the edge dulls the area behind the leading edge rubs the wood as it advances called healing.

Itís another source of compression and heating. Even though you have a three knife head most likely only one is doing the actual finish cut because not all three are exactly the same distance from the center of rotation. Some molders overcome this by having jointing stones that bring the knives to the same cutting circle.

From the original questioner:
I jumped the speed up and I will decrease the distance between the cutters and the chip guard.

From contributor M:
I would also take a good look at the tooling. A good sharp edge and the correct angle for the type of wood is important.

From contributor S:
Odd as it may seem, it is a truism that on softwoods, fir in particular, the inside of the tree will mill with noticeable ridging, while the outside of the tree will not. Run your hand over both sides of a fir/cypress/WRC board out of the planer and tell me I'm wrong. When running any of these I sort to put outside of the tree out, though with VG Fir the difference will be small and the tooling/feed may well be to blame.

From contributor L:
Tooling geometry is always a consideration when molding. Unfortunately you have no choices, whatever Jet decided is what you've got. That type of machine is not a molder! If you were running a molder you could change to a more aggressive hook angle for softer woods or a less aggressive one for very hard woods or wild grain. Molding heads come in a variety of hook angles, some four knife heads have two different angles available.

From the original questioner:
I have knives cut by a reputable tooling company. I believe that Contributor L is right about this not really being a molder - somewhere between hp and feeder speed. I am probably past the tolerance of the machining. Next time I build a house I will spring for the Williams Hussey or by paint grade moldings for everything.