Q: We have a planer that has been giving us problems with snipe. How can we control this machining defect?
A: For those readers who are not aware of the description of this defect, it's a reduced thickness of the front end or trailing end of a piece of lumber, where the planer has taken off too much wood.
In the following description, I'm going to assume that you have a single head in your planer and this head is on the top of the lumber. If it's on the bottom side, then you'll have to reverse the description and the suggested cures.
Snipe on the leading end of the lumber is caused when the front end of the lumber is pushed upward into the head. The chip breakers (which are not exceptionally strong and not designed to fight outside forces; they're located in front of the knife) cannot hold the lumber down tightly against the bed plate of the machine. Therefore, the entering end of the lumber lifts up into the knives and it's planed too thin. Because this entering end is now thin, the pressure bar (which is behind the knife) can't hold the piece down to the bed either, as it is properly set for thicker lumber. 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, chipped grain, tear-out or torn grain -- all the same defect. That's why the chip breaker must be set very close to the knives. As the knives wear or are resharpened or jointed, or as the chip breaker fingers wear, adjustment is required.
Low infeed support
Ask yourself why the leading end is being forced into the top head. In fact, the knives and the chip breaker are trying to push the lumber against the bed plate, but they can't do it. The answer is that the infeed support for the lumber is too low. Gravity is pulling the tail end of the lumber downward and the lumber is pivoting on the same point in the machine, forcing the front end upward. So, 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's very slightly above level with the bed plate of the machine.
[Note: The support table is indeed even with the machine where it joins the machine, but the other end is slightly elevated.] It doesn't take much elevation, but once the effect of gravity is removed, the chip breakers, knives and pressure bar are all able to prevent snipe. Without this elevation, gravity will win, especially with longer lumber pieces. For short pieces that are fed by hand, you could be tempted to also elevate the piece slightly by hand, but safety issues (getting your finger pinched between the lumber and bed plate) make this an unacceptable procedure.
For snipe on the exit end, it's the same deal. If the pressure bar is slightly mis-set or if the wood is soft enough to compress slightly, and if there is inadequate support on the exit end of the piece of lumber, then gravity will work to pull the leading down and push the tail end upward into the knives. Snipe will occur. Once the end is cut a bit too thin, gravity will work to cut it thinner as the piece moves through the planer. Again, if 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'll have feeding problems. The piece will not feed easily, but will get hung up in the machine. Hence, many amateur planer operators will lift the pressure bar way too high to make sure that the lumber will never stall, even after the knives are sharpened or jointed. If the pressure bar is too loose, you'll see chatter and possibly a little bit of snipe. In truth, the pressure bar setting is quite critical and requires skill to set it at the correct opening.
Moisture content was not mentioned in my description. It is not a factor in snipe.
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