Troubleshooting Planer Tearout and Snipe

      An intensive discussion of the causes and cures of tearout and snipe in planing work. July 13, 2006

Question
Our two man shop has a 15" Grizzly planer. We don't use it a lot since we do frameless boxes, but we're doing the 5 part panel doors on a job rather than outsourcing as usual. We have the snipe problem on soft and hard maple particularly. We're having a bear of a time with snipe and tearout. The knives are sharp fresh from the sharpening service. I forget the angle the owner had them put on them, but what angle is appropriate? What are some other ways to reduce snipe and tearout?

Forum Responses
(Solid Wood Machining Forum)
From Gene Wengert, technical advisor Sawing and Drying Forum:
Snipe is caused by the lack of a proper in-feed (front of the piece) and outfeed (rear end) support. Tear-out is caused by several factors including very low MC, high feed rate, dull knives, or knives with the wrong angle. If you go to the archives and search on the words "tear out" you will find 130 articles dealing with this. One article is referenced here. Related Web Page: Tear-out comments



From contributor B:
Grizzly has all their equipment manuals in Adobe. Start at the beginning and reset the entire machine giving definite attention to bed roller height.


From contributor C:
Bites on the leading end (within 3" or so of the end) are caused by the chip breaker not being properly set. If the bite is on the trailing end, it is a pressure bar adjustment. Roll height is important, but rather than causing a bite on the end, it usually shows up as miscut on the material a distance from the end equal to the distance from the cutter head to the roll. If the rolls are high enough, the work will be generally bad because the material is not properly in contact with and supported by the center table.


From the original questioner:
The shop owner did go through the motions with Grizzly's manuals and their tech support, and Grizzly chalked everything up to the low moisture content as mentioned above. So what does a crammed-for-space two man shop do to increase moisture content?


From contributor C:
The wrong moisture content will cause tear out, fuzz, etc., but it will not cause bites. As for how to improve your humidity, Gene Wengert is the expert on that.


From contributor Gene Wengert, technical advisor Sawing and Drying Forum:
I assume that you have a top head machine. Snipe on the leading end of the lumber is caused when the lumber is pushed upward into the head. The chip breakers (which are not exceptionally strong and not design to fight outside forces; they are located in front of the knife) cannot hold the lumber down against the bed plate. Therefore, the entering end of the lumber lifts up into the knives and it is planed too thin. As this very beginning or entering end is now thin, the pressure bar (which is behind the knife) cannot hold the piece down to the bed either. The result is snipe. Note: Certainly, the chip breaker must help to hold down the lumber to the bed, but its main job is to prevent chip out. That is why it must be set very close to the knives and as the knives wear or the chip breaker fingers wear, adjustment is required. Ask yourself why the end is being forced into the top head. The knives themselves actually push the lumber away. The answer is that the infeed support is too low. Gravity is pulling the tail end of the lumber away from the machine downward and the lumber is pivoting on the some point in the machine forcing the front end upward. To prevent front end snipe you must lift the tail end of the lumber upward slightly. That is, the lumber must approach the machine at an angle that is very slightly above level with the bed plate of the machine. The support table is indeed even with the machine where it joins the machine, but the other end is slightly elevated. It does not take much elevation as the chip breaker (slightly), knives and pressure bar are all seeking to prevent snipe, but gravity will win without this elevation. For short pieces that are fed by hand, you could be tempted to also elevate the piece slightly, but safety issues like getting your finger pinched between the lumber and bed plate make this an unacceptable procedure.

For snipe on the exit end, it is the same deal. If the pressure bar is slightly mis-set or if the wood is soft enough to compress slightly, then if there is inadequate support on the exit end of the piece, gravity will work to push the tail end upward. Once the end is cut a bit too thin, gravity will work to cut it thinner. Again, it the front end of the lumber is slightly elevated as it proceeds to exit the machine, that will eliminate the force of gravity pushing the tail end upward and, then without this extra force, the pressure bar will be able to do its job (unless it is mis-set).

As an added note: When the pressure bar is too tight, you will have feeding problems - the piece will not feed easily but will get hung up in the machine. If the pressure bar is too loose, you will see chatter and possible a little bit of snipe.

Note that moisture content was not mentioned in my description. It is not a factor in snipe, as Claude mentioned above. Also note that it is very easy to blame MC for chip out and ignore the other possible causes. In my experience MC often is not always the key factor. To increase the MC of wood is a difficult chore. The first step is to determine if you actually have wood that is too dry (under 5% MC for medium to high density hardwoods and under 9% MC for most softwoods). Then if you do have over-dry wood, you must store the wood at about 100 F and at a slightly higher RH than required to achieve the correct MC (38% RH for hardwoods and 55% RH for softwoods might be good settings). Then wait until the CORE reaches an acceptable MC level (which might be a month or more).

Incidentally, you asked about the correct angle. What we want to avoid is a slender knife that will act like a chisel and split the wood ahead of the knife. We want an angle that will make the knife act more like a plow. Often we use a 10 degree clearance angle so that the heel does not hit the wood. The knife angle might be about 60 degrees, leaving the rake angle (also called the hook angle for saw teeth) at 20 degrees (total is 90 degrees). Sometime the head of the planer is set for a certain rake angle (such as 30 degrees; sometimes the number is stamped on the end), so the knife must be back beveled (which means it is machined on the "wrong face") to achieve the correct angle.



From contributor C:
Thank you for exploring the manner in which lumber is fed into and taken away from the planer. I would add that I have seen bite on the tail end of lumber many times on the big machines such as the Newman S382, caused by the out feed conveyor not being the proper height in relation to the machine. Obviously if the outfeed rolls and pressure bar on the big machines such as the S382 can't overcome the tipping action, it is essential that the smaller machines such as the Grizzly be fed and tailed properly.

Both of us have neglected to mention that there will be the occasional board that is so cupped or bowed that the planer will bite it. If your lumber is poorly dried and does not lie relatively flat, no planer regardless of size, weight and cost will machine all ends perfectly.



From contributor D:
I have the 20" grizzly planer and have used it to plane 4/4 white ash 10' long (6 to 14 inch wide) for wide plank flooring in my log cabin. I found, as noted above, that holding up on the end of the boards as I start feeding and then again upon exit almost eliminates any snipe. I lift to just raise it off the infeed roller and exit roller. These rollers are extensions bolted on to the machine that came with it. The machine is set to the owner's manual specs, as I bought it used and then adjusted it myself.

I think you have to develop a feel for how to handle the wood during the infeed and outfeed process. Of course I picked up this information and technique previously from reading this forum.



From contributor E:
Elevating infeed and outfeed will help reduce end snipe. Sometimes you need to make your boards a little longer so you can saw it off. I like Grizzly planers but they do have snipe problems. Tearout can be caused by several things. Moisture content does have a lot to do with it but doesn't usually cause problems that a wide belt sander won't cure. Direction of grain when feeding can also cause tearout, but it is usually minimal. Make sure you have really good dust collection. Chips get trapped behind the knives and cause imbalance, then the head gets egg shaped, and you get gouge marks.

If you are using a bed board, make sure it fits flat and tight against the bed of the planer. If not, it will bounce and cause very bad tearout-oftentimes intermittent. Poorly balanced knives can cause problems, but usually show up as chatter. Bad bearings can do all the above.



From Gene Wengert, technical advisor Sawing and Drying Forum:
I hope nobody ever thinks about making their boards a little longer so that the snipe can be cut off. Snipe is typically 4 to 6" on each end, so that would be 8 to 12" longer. On an 8-foot piece of lumber this would be wasting 8 to 12% of the lumber, and increasing lumber costs by the same amount. Every reputable text about chip-out will indicate the high importance of low moisture causing this defect. Once the wood is over-dried, bringing the MC back to a higher level will not restore machinability very well at all.

All chip out is a result of planing against the grain. The defect develops when the wood splits ahead of the knife and follows the grain. When planing with the grain, the split goes into the part of the wood that will be machined off (and therefore no chip out); when planing against the grain, the split goes into the wood that will be the finished surface. Although grain is indeed critical, it is usually not possible to do much about the grain angle. In fact, in hard maple, the grain angle changes within one face with local pockets of grain deviation. Grain deviation also exists around knots in all species.



From contributor E:
I'm not sure if I understand Dr. Wengert's comment concerning end snipe.I have planed thousands of feet of lumber. Some planers produce much less snipe than others. A heavy chipbreaker on the infeed and a pressure bar on the outfeed will help hold the board down when it enters and exits the rollers. These features are not usually found on inexpensive planers. These features must also be properly adjusted to work. If you elevate the infeed and out feed tables or rollers, or just simply elevate the board on entry and lift up a little when it exits, you may greatly reduce or possibly eliminate snipe. I really hate to waste wood of any kind, but when you build custom cabinets and furniture, you can't send them out with snipe. You can sand your butt off, and still see it thru the finish. Then you have to burn the stuff, and that's even a bigger waste. We have achieved best results when planing maple to eliminate chip-out by using a planer where the knives are a little dull. It doesn't seem to tear into the grain near as bad. A faster feed rate also helps especially on hickory.

We have learned that woods such as Oak, having a loopy grain will chip out much less when you feed the loop into the planer first. It makes no difference on maple due to the configuration of its grain. Dull knives on wood such as poplar and cherry will cause bad chip out. We like the MC of our wood between 6 and 8 percent. Anything else is not accepted. You have to get to know your planer. It seems like no two are the same.



From Gene Wengert, technical advisor Sawing and Drying Forum:
Regarding snipe: A chip breaker on a large machine is often a series of fingers that has each one spring loaded separately. The idea is that a chip breaker will move as the incoming lumber thickness changes. If it were rigid, then if a slightly thicker piece of lumber came to the machine, it would not be able to enter. Because the chip breakers move, it is easy for a piece being fed with an unsupported end to have gravity pull the end down and then the front end would be forced up. This would open up the chip breakers and the piece would not be held tightly against the bed plate. This is what causes snipe on the front end. Sorry if my earlier explanation was not clear to you.

On the out feed end, the rigid pressure bar does indeed prevent snipe, except of the force of gravity on the unsupported end that has already been planed causes the wood being held by the pressure bar to compress slightly. For a long piece of lumber, the stress on the PB is indeed quite large. Note that the original questioner is using a 15" planer and there may not be a pressure bar that is extremely rigid. Hence, the force of the lumber may actually bend the PB slightly.

As far as proper end support controlling snipe, I have many customers using maple and oak that have zero snipe with a proper in-feed and out-feed support system (and all the other items in the machine working properly too). In fact, last week I was at a manufacturer that uses over 10 million feet a year and they have eliminated all snipe. They had noticed that the planer manufacturer had included long in-feed and out-feed tables with screw adjustments on the legs, but they never knew how the legs were supposed to be adjusted. Once adjusted properly, snipe disappeared.

Regarding chipout: Note that a dull knife has the cutting angles change as it dulls. The same is true for knives that are jointed and not sharpened. In your case, it is possible that when the knives dulled, the angles became closer to optimum. I believe it would be better to make the angles correct right from the beginning.



From contributor D:
To Gene Wengert: Your first explanation was easy to understand and also was right on the money. Good job. We had the snipe problem before and your advice then worked perfect for us.


From contributor C:
A two man shop should be looking for a good lumber supplier who will rough plane the lumber well enough that it almost looks like finish planed. With that quality, the light planer can finish the job. The better lumbermen today are using a Newman S-382 or perhaps a Newman S282 and are doing a good job. You could buy from them "hit or miss" if you have a heavy duty finish planer, or planed heavier if you have a light machine.

It is my opinion that shops larger than the two man shop should be looking at the bargains available today on the used market. The week before last, a Whitney S-290 was sold by Thomasville Furniture for $3,600. The carbide in the head is worth more than that. There are some unreal bargains today. I am not trying to sell planers here as I have no bargains. All of mine were purchased with the intent that I would rebuild them and they must be rebuilt, so they can't go cheap. You can not expect too much from a finish planer. If you set the rolls high to feed rough stock, it will not give you a smooth accurate part. If you try to feed rough lumber with the rolls set for finish work, it will not feed rough lumber properly. Even the heavy iron works that way. So find a good lumber source.



From contributor F:
Check the table gib screws- the table must be tight. I actually installed locking screws on our planer. They are a quick release screw with a handle that is in the same spot as the gib screws. The difference is rather than adjusting to a tension that will allow the table to be cranked up and down, they lock the table solid. No matter what else you do, if that table can move at all, you will have snipe. First the infeed side will dip down, as the feed roll puts pressure on the leading end of the board, and angles it up into the cutterhead, and then the reverse on exit. And the other thing you can do - run all your stock butted end to end, and use a piece of scrap the same thickness to lead the stock and to follow it. Any snipe will happen on the scrap and you can keep using it as it gets thinner. A side benefit is that you have a piece of scrap handy the exact thickness of your stock, for doing setups.

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