Snipe (wood machining)

A white cedar board showing snipe after passing through a thickness planer.

Snipe, in woodworking, is a noticeably deeper cut on the leading and/or trailing end of a board after having passed through a thickness planer or jointer. The term has its origin in forestry where it is applied to a sloping surface or bevel cut on the fore end of a log to ease dragging. (OED)

Snipe in Jointers

The cause of snipe in a jointer, is an out-feed table which is set too low relative to the cutter head.

Snipe in Thickness Planers

The predominant cause of snipe in a thickness planer is change in the downward force applied to the workpiece by the feed rollers and the resultant movement of the planer table.

The rollers are pulled down by springs. When the workpiece is absent, they rest on stops in the frame of the planer. When the workpiece passes under a roller, the roller is lifted from the stops and the spring force presses the workpiece down onto the table. This holds it steady as it passes under the cutter head and creates the friction which allows the rollers to move it through the planer.

The pressure of the roller(s) on the workpiece and thence onto the planer table will inevitably cause the table to move down, i.e. away from the roller / cutter head assembly. To save cost and weight, the table may be a fairly thin plate with reinforcing ribs. This can flex by several thousandths of an inch due to a pressure of some tens of pounds force per roller. Other parts of the mechanism may also exhibit strain.

A workpiece fed into the planer first meets the infeed roller, passes under it and continues towards the cutter head. When the cut begins, the table is being pressed down by the infeed roller only and is deflected downward accordingly. The cut then progresses to the point where the workpiece meets the outfeed roller. As it passes under, the downward force of this roller is added to the infeed roller and the table is deflected an additional amount.

The downward movement of the table increases its distance from the roller / cutter head assembly. The workpiece, riding on the table, moves away from the cutter head. From the point of view of the workpiece, the cutter head has moved up and cuts less deeply. The part of the workpiece that passed the cutter head before engagement of the outfeed roller was cut more deeply than when both rollers are engaged. This is snipe. The length of the leading edge affected by snipe will be approximately the horizontal separation of the cutter head and the outfeed roller (the truth of this observation is evidence of the correctness of this explanation).

The table remains at full downward deflection until the trailing end of the workpiece disengages from the infeed roller. Now only the outfeed roller is pressing down through the workpiece and the table springs back up. This brings the workpiece closer to the roller / cutter head assembly and increases the depth of cut, again causing snipe. The length of the trailing edge affected by snipe will be approximately the horizontal separation of the infeed roller and the cutter head.

Some planers have a fixed table at the bottom of the frame and the roller / cutter head assembly moves up and down to set the thickness. The cause of snipe is essentially the same except that the table is likely to be stiffer and it is easier to think of the cutter moving upwards due to the roller pressure.

Dealing with Snipe in Thickness Planers

In many applications, snipe may be of little or no consequence and can be ignored. With a small, moderately priced combination jointer/planer the change in depth of cut can be in the range 0.003 to 0.005 inches (0.08 to 0.13 mm). A larger, more expensive planer should provide a stiffer table and result in snipe of 0.002 inches or below. Some planers feature a lock which, when engaged, supports the bed or cutter head more firmly than the mechanism that raises and lowers it. The last pass of a workpiece through the machine establishes the final thickness and flatness. With a little planning, the depth of cut can be well below the maximum. Operating at reduced stress will allow the planer to do the best possible job.

For a short workpiece, it is feasible to make a sled on which it rides through the planer. If the sled is sturdy and as wide as the bed it may reduce movement in response to roller pressure and produce less snipe.

If the loss of thickness can be allowed for, snipe can be removed by a pass over a jointer or through a drum sander. With some loss of flatness, snipe can be sanded away by hand or with a hand-held power sander.

The workpiece can be milled longer than needed so that the ends exhibiting snipe can then be cut off and discarded.

A less wasteful procedure than discarding the snipe is to make up extra pieces of stock of the same thickness as the workpiece and feed them before and after to “collect” the snipe. What they’re really doing is picking up the pressure of the outfeed roller before the workpiece meets the cutter head and holding onto the pressure of the infeed roller before the workpiece leaves the cutter head. These pieces don’t have to be “fresh” for each workpiece in a run; that is, they don’t have to be of the unplaned thickness. They work well enough if they are of the final thickness and so can be reused. A run of several workpieces can start with an extra piece, then the work can be fed in quick succession and an extra piece (or the same extra piece) passed through at the end.

Depending on the design of the planer, feeding successive pieces nose-to-tail without gaps may be challenging. If the planer is wide enough, they can be fed side-by-side and overlapping, but the result should be checked for other problems due to the pieces not being on center. A better solution (although much more work) is to make up a “frame” with rails that run right and left of the workpiece, entering before it to pick up the infeed roller and remaining after it to hold the outfeed roller. As with the extra pieces, above, the frame will work well enough if it is of the planed thickness and so can be reused for all the work in a run.

Other Sources of Thickness Variation in Planers

Incorrect adjustment of a planer and poor technique can result in thickness variation that may be mistaken for snipe (or lumped together with snipe in discussion). To get the best results, manufacturers’ recommendations and established good practice should be followed.

Non-standard adjustments, such as raising infeed and outfeed tables above the main planer table, are unlikely to reduce snipe and may be the cause of problems, perhaps termed snipe, that are hard to understand and deal with. In this example, the consequence of a raised infeed table will be that the tail of the workpiece will snap suddenly down onto the main table as it nears the end of the pass. How this might manifest as thickness variation is hard to predict.

The same goes for modifications to feed technique, such as lifting the outside end of the workpiece up at an angle as it enters and leaves the planer.

The work should enter and leave the planer without being pushed down, up or to the side to any degree that affects the function of the feed rollers. Long pieces should be supported level with the planer table by any convenient method (stands / roller supports or by hand). If the planer has extension infeed and outfeed tables to help with this, they should be set at the same height as the main table under the cutter head.