Chip Dents, Cutter Choices, and Dust Extraction
Dents made by chips being pressed into the wood by the planer or jointer knives are a common problem. Here's a thorough discussion of causes and remedies.April 15, 2013
Question
I started a thread a few years ago discussing a problem I had with the planer leaving dents in the wood. Nothing was ever fully resolved. Two weekends ago I completely burned down my woodworking shop and sawmill. Anyway, it is time to buy a new planer now that my 20" Powermatic is in the scrap heap. The original had straight knives and consistently left dents in the wood from chips that would stick to the knives. I am thinking about a spiral head with carbide inserts to get a better finish by eliminating the dents and perhaps causing less tearout in woods like maple, hickory and birch. If you any insight, I would appreciate hearing it.
Forum Responses
(Solid Wood Machining Forum)
From contributor M:
Very sorry to hear of the loss of your shop. That had to be catastrophic.
The chips are caused by insufficient dust removal. You need to increase your vacuum CFMs to eliminate this problem. It's got nothing to do with straight or spiral blades. Check your dust collection specs. A cyclone dust collector is best for chip removal (such as your planer, jointer, shapers or moulders). Hope this helps.
From contributor W:
Those are difficult to get rid of on most of the new planers sold today. While inadequate dust collection will contribute, too much air can also cause a problem. Normally they are small enough to sand out when finishing away knife marks. Sometimes, small tearout specs can look almost the same as chip dents. In the order mentioned experimenting with the chip deflector, chip breaker and blade grinding angle will solve the problem, only to re-appear when planing a different species.
From contributor B:
Get the planer with the insert teeth and spiral head. Way fabulous! I also got a jointer with the same style head - very nice.
From contributor D:
My old Makita 2040 15 1/2" had urethane feed rollers; never had chip dents with that planer. I am using a 15" shop fox spiral head planer that pulls grain less but leaves chip dents, except on the hardest of woods, same dust collector. I adjusted the outfeed roller to have less pressure thinking the dents come from the roller, but some say the knives produce the dents?
From Professor Gene Wengert, forum technical advisor:
Sorry to hear about the loss. The chip dents are caused because the dust system cannot remove the chips from the blade area fast enough. If you still had the planer, you would notice that if you closed the vacuum system on all the other machines, the problem at the planer would be gone. Get an adequate dust system. Spiral heads are much quieter, have less chipped grain and are probably easier to maintain. I would advise you to get them, if possible.
From the original questioner:
Thanks for the advice. I need to get replacement machines quicker than I would like and was hoping to convince myself to buy a spiral head. Perhaps I am getting a spiral head and a larger dust collector.
From contributor J:
Well the other guys have pretty much covered it. Only thing I can add is that if you're going entry level planer, I would recommend the spiral heads as they are much easier to deal with. If you're going more industrial, I like a planer with an on board grinder. The finish is comparable to what you get with an insert head. Over time, though, much cheaper as you just grind your one set of knives and get back to work. No turning inserts and no buying new inserts once the first set is worn out. A lot of guys also like the Tersa heads, but again you're buying new knives all the time so that can be costly.
From contributor R:
In my opinion the old Powermatics were the best overall design ever. Chip dent with these machines is related to poor dust collection, pure and simple. There are advantages to the Byrd style cutter heads as well as disadvantages. Generally speaking, get another 20" Powermatic with a jointer grinder attachment and hook it up to a good dust collector. You can't go wrong.
From contributor A:
The dudes who design planers never seem to think about chip removal. Most planers need to be slightly modified in order to facilitate proper chip extraction.
My first boss bought this beautiful euro (no idea of brand in 1995). It had one port out the back. It clogged constantly. We finally ended up cutting a big slot in the top and installed a proper full blade with plenum.
From the original questioner:
Tell me more about this full blade and plenum thing. It sounds like I really need to focus on dust extraction, and to this point I have never been able to get it adequate enough.
From contributor E:
Planers are not that hard to get proper dust collection. For a 20" planer you'll want a 6" port on the machine, nothing less. More importantly you need to make sure your dust collector has enough CFMs to handle everything that will be open at the same time.
From Professor Gene Wengert, forum technical advisor:
The key to dust removal is not so much the fine dust but the larger chips. The large chips have the most weight and so need the most CFM of air. Chip weight varies with planer feed speed and stock removal (depth of cut). Also, chip weight varies with wood species density and MC, as you might expect. As alluded to, the design within the machine is also important so that there is good air flow at the knife to wood interface.
From contributor Z:
Improper dust collection isn't always the problem. I can run my old Invicta Delta with and without dust collection, and still not get any chip dents. I think that improperly sharpened and/or set knives are just as likely to cause problems.
From the original questioner:
This is the direction the thread took last time. I could run my planer without dust collection (just shoot chips out the back) and get no dents or with dust collection and get no dents. Then one day I just started getting dents that come and go, but I can never totally get rid of them and haven't figured out the variable that produces them. I believe it is a combination of things. I don't think it will hurt to have more dust extraction and that is the one thing left that I haven't fully explored, so I am on board with checking that out. I need to buy a dust collector now anyway, so I might as well get a bigger one.
From Professor Gene Wengert, forum technical advisor:
Some planers are designed (perhaps by luck) so that the air currents encourage the chips to move away from the knives. However, the chip size and weight, especially MC, affect how the chip behaves. I suspect that with high stock removal, things will be worse as the chip will be bigger and heavier. Also, as the outer 1/32" of the wood gets drier, there will be more static electricity developed with the dry chip. This will cause the chip to cling to the knife even with the centripetal (rotational) forces trying to fling the chip away. I do recall one client that tried squirting a water mist on the surface just before planing and it instantly removed the chip mark problem. We suggested grounding the lumber and machine, but I never heard if this worked. Certainly a metal feed roller would be helpful in this case instead of rubber.
Having said this, I do know from experience that a larger dust system solves the problem in most cases. I can recall many times when we "proved" this to a client by having them shut down the other dust outlets so the dust system worked only on the planer... Problem solved.
From the original questioner:
Gene, thanks for the more in-depth discussion. I love to talk to people that have knowledge on a subject that I can trust and also have enough information to understand the logic behind it.
Everything you just said seems to be the case. The heavier the planing, the worse the dents. To eliminate them I take an ultra-light pass, but that kills me time-wise. I did try wetting the wood in the past, but the results didn't change. I suspect the dust collection was insignificant at the time, so the change in humidity on the surface of the lumber would have little effect on the outcome. From the entirety of the discussion so far, it sounds like either straight or spiral heads can produce chip dents, but in both cases they can also be eliminated. Thanks again everyone for the input.
From contributor Q:
Mr. Contrary, here. When I encountered this problem, running a 5 head Weinig with two different, first class dust collection systems, I never did get to a clear cause for what we called chipbeat. We just made our peace with it and when we had the problem show up, we would do a different grind on the knives and then a face grind. It was most prevalent on poplar, particularly S4S work.
In the process of trying to solve the problem, we eventually noticed two links. One was humidity (higher humidity, chipbeat was more likely), and the other was knife angle and grind angles.
We tried all the dust collector tricks: all on, all off but one head, added air via a compressed line, starved the air, changed the baffle shape, used a magnehelic gauge, had the dust techs come and check it, and so on. Talked to Purdue University, who involved Weinig and even engineers from Weyerhauser - with no real consensus, other than everyone saw it, hated it, but knew little about cause and effect.
I even wrote Jerry Metz, the man with all the answers, and he humorously asked me to go elsewhere - that he had battled the problem many years ago and never wanted to see it again!
While some things will add to the problem - poor dust collection being the largest single cause - even with all things engineered and perfect, sometimes it just shows up. And just as mysteriously, goes away.
From contributor N:
Extra clue to the mystery - I can run the same profile, same feed and rpm on a shaper, and then on a CNC. Chip dent shows up on soft white pine run fast conventional, never on climb cut on both. On other species, it never shows up on the CNC. On the CNC, the tool is out in the open, no fence or dust extraction baffles around it. On the shaper, just like a planer or moulder, it's pretty well trapped in a confined area. High speed chips overcome the vacuum and swirl around until they eventually get sucked up - some of those chips are getting recut, no doubt about it. The massive rooster tail the CNC is throwing, along with the clean path of a cutter which is climbing (chips not thrown in the direction of the oncoming cutter) evacuates the chips really well. Perhaps on those planers mentioned above where the dust collector is off, there is just enough volume in the dust hood to accommodate the thrown chips, which hit a baffle and get deflected back down to the stock outside the path of whirling knife. I would bet money those same planers, if set to take off a lot on a wide board, say full width of the table, with dust hood on and vacuum off, would damn sure show chip dents...
From contributor T:
Sorry to hear of your loss last month.
I recently replaced the heads in a 20" Delta Planer and a 12" Bridgewood jointer with spiral insert heads that were made for me by Hermance Machinery. I researched this for quite a while before purchasing. The Hermance head directs the chips to the center/main extraction point and does not have any problem with chip removal. I do have adequate dust collection but nothing elaborate. I am totally pleased with my Hermance Heads and would recommend them to anyone. I should have purchased them long before I did.
