Chip Removal and Planer Finish Defects
The knives are nice and sharp - they do a good job on curly maple - except for these marks. Because the hook angle of the head is so sharp (about 35°) we get a face bevel of 15° ground on them to help prevent tearout. We are then jointing and back grinding to 1/32" or less land.
Unfortunately I can't tell you what the actual knife protrusion is, because we can't calibrate our dial indicator; the base is too large to seat on the head without one leg or the other falling in a knife pocket. We have them set so that there is about 1/16"+ of knife below the back grind protruding. What should we be looking for?
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From contributor G:
Definitely inadequate dust collection and/or too wide of a secondary bevel on the knives.
From contributor M:
Close up a few blast gates, or clean out the filters on your dust collector. If that doesn't work, move up to a larger diameter hose and/or get a bigger dust collector.
From contributor A:
I agree that it is the recirculating wood chips causing the problem. The above suggestions are good. In my experience I've noticed a correlation between the knives being set too low in the head and having issues with the chips.
The height of the knives in the head can definitely affect the air flow in the suction area. So maybe try setting the knives a bit (1/16 - 1/8) higher in the head. Also take the time to develop a surefire method to set the knives at the same height every time so if you get lucky, you can repeat the results. Finally if you do change the height of your knives, don't forget to reset the chip breaker and feed rollers as well.
From contributor D:
You are not alone, but there appear to be different causes. I have consulted engineers on this throughout the industry, and all agree on two things: it is a problem, and no one thing is the fix. I have even run magnehelic gauges and such to measure airflow.
These are dents caused by the chip riding around in the hood and coming between the knife back and the planed wood. A wipe with a damp cloth will raise them, but that is no real fix.
Dust collection is the first thing to eliminate as it is the most frequent and obvious culprit. After that, I would methodically go through differing grind angles. My experience led me to suspect relative humidity, as it does affect water column (suction).
From the original questioner:
We definitely don't have a problem with suction at the hood, so it may well be a lack of space around the head because of the knife height. The guy that's taking care of this now has been sharpening in the past, but never set or raised the knives before and the previous person is no longer with us. So we will try raising the knives a bit more, as we've never had this problem until just now. Once it's figured out we'll mark the height in our specs.
From contributor J:
I have seen this myself. Here are a couple of things to look at. Make sure that everyone is clear of the machine.
1) Run material through the machine without the dust hood or the dust collector. Allow the dust to blow completely clear of the machine.
2) Monitor the path of the dust as it is being produced. This path will change in relation to the height of the knives. You want to have the dust blown towards the dust collector port.
3) Look at the surface of the board that was produced. Are the marks still present? If they are and you can confirm that dust is not being restricted from leaving the machine, then it is most likely an issue with the setting of the knife.
4) Raise the knives and repeat the above tests.
Hook angle is the angle of the cutting face of a knife in relationship to the material being cut. Different species require different hook angles. The harder the wood, the less the angle. An easy to see example of this is your tablesaw blades. Some blades have a positive hook to the teeth, and cut from the tip to the gullet. Others will have a negative hook, and cut from the gullet to the tip. (Just as an example.)
Here is where it gets complicated. The closer the cutting edge of a planer is to the head, the greater the angle, making it better for softwoods. Extending the knife tip will cause the hook angle to lessen and help with the finish on harder woods. Just make sure that they don't protrude too far and that the back grind is correct.
Unless you are sharpening these yourself on your planer, you need to make yourself familiar with the back grind. All tooling has a relief cut along the back of the cutting edge. The knife actually does not use this angle (directly) to cut. The cut is governed by the hook angle. The back grind supports the hook angle. If there is not enough of a relief, the knife will hit the board on the back side. If the angle is too great, the knife edge will be weak. There is such a thing as a dual angle grind - kind of the best of both worlds.
From the original questioner:
Some good ideas that we will follow up on. Yes, I am aware of the influence of hook angle, and do back grind on the machine, as mentioned in the original post:
"Because the hook angle of the head is so sharp (about 35°) we get a face bevel of 15° ground on them to help prevent tearout. We are then jointing and back grinding to 1/32" or less land."
Your statement that if the back grind is not sharp enough there will not be enough clearance reminds me to see if possibly the grinding wheel is getting too small, which reduces the radius of the back grind and therefore the clearance.
From contributor T:
Had the same problem years ago. My mill man told me he just put in new knives. Turned out to be the jointing of the knives. We changed jointer stones (Yates A20-12 planer) to the softer white stone and the problem disappeared. Shaving was hanging on the rough jointed edge of the knife. Also had this problem with carbide insert heads at moulders. Still have it. My heads are about 12 years old and do not have a large enough gullet for the shaving to be removed, and the knives have extremely little projection.
From contributor F:
I agree the posts that say "These marks are from wood chips staying on the knife and being beaten into the board."
Besides increasing dust collection, the amount of material you take off with each pass of the planer directly affects how many wood chips there are to stay around the cutterhead and for the dust collector to remove.
If you adjust the amount removed with each pass so the last pass on each side is very light, the problem will go away. If you make 4 passes and remove 1/16" each pass, make the first two a little heavier and remove 1/32" on the last pass on each side or make more passes with the last ones not removing much material. It will be more work at the planer, but it will save a lot of work at the sander and down the line.
From contributor L:
While these marks could be from chips pressed into the wood by the cutterhead, they sometimes seemed to be a chipout that doesn't quite happen. Are they still there if you feed from the other end of the board?
From contributor E:
I must respectfully disagree with the posts about high moisture content and chip out. This is definitely caused from shavings caught between the cutting edge and the material. High moisture content makes the wood fibers very pliable and will cause the grain to tear or pull up instead of cut and will create a fuzzy or ripped out finish. Chip out is usually from the lumber being too dry or grinding angle being too high. I don't see chip out in the picture. In fact, scrape one of the little specks with a pocket knife and you will find it will be slightly raised and will scrape off.
I agree that it's either dust collection or the joint on the knives (more likely the dust collection, however I would check to see if the width of the joint on the cutting edge is the same on all the knives). I don't know what type of dust collector you have, but you may want to try and hand squeeze or shake the bags to make sure they are clean or run some extra clean cycles. You could be experiencing clogged cloth. That is, the inter sock is clear of material but there is inadequate airflow through the bag preventing maximum suction and restricting the fan's ability to properly clean them. There are companies that will wash your bag if replacement is not due yet.
From contributor N:
If possible, use a strobe light. You may laugh, but if you can run and have a strobe light, you can see the exact direction of the chips as they leave the knife. We had to put a blind in to stop the chips from continuing around the head. Wasn't a fix, but very soon we realized the hood was backwards and the direction of the chip exit was changed. The dust people needed it to be a left instead of a right, funny. But the light was very helpful. Also, if you don't have air coming in from somewhere, you won't get any suction. Dust collection needs air to move the dust. Put dust on your hand, place vacuum on the dust sealing your hand against the hose, turn on, turn off, wait till the vacuum subsides, and then remove the hose, dust still there. We opened the end of the hood a bit with a clamp, allowed more flow, and the chips evacuate like they should.
From the original questioner:
It seems the problem was that the front bevel we had put on the knife to reduce the hook angle was too large. Now, I know that it shouldn't matter how big it is, since if the head was the right angle to start with, the face of the knife would extend all the way to the chipbreaker. So there may be other issues, but with the front bevel small enough, it's working pretty well. I was able to talk to the guy who used to maintain the machine and he said we always had this problem when we put freshly front-beveled knives in until they got ground down to the sweet spot. I had apparently exacerbated the problem by making the front bevel larger than it ever was, thinking it would mean less times out of the head.
My suspicion at this point is that there is either a fundamental design flaw or some defect that has come in, which is preventing proper extraction of the chips. With a sharp hook angle the chips are allowed to be pulled away from the surface and out the chute. However, with the hook angle reduced by the front bevel, there is less reason for them to leave the area, and they get sucked under the knife.
Thanks to all for your ideas and suggestions.
From contributor V:
Put an air hose (or 2) to blow away excess during the planing process and see what this does in terms of preventing the dents.
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