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Pitch Rail and Knife Adjustment for Veneer Lathes

Pitch Rail and Knife Adjustment for Veneer Lathes

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How to Calculate Pitch Rail Slope Angle

The angle of the lathe knife largely determines uniformity of the veneer thickness as the log is peeled. The height and slope of the pitch rail, upon which the knife assembly and carriage ride, control the knife angle. Maximum veneer thickness uniformity can be obtained only over a narrow range of angles.

Baldwin (1981) suggests the following technique for setting adjustable slope pitch rails. The steps consist of the following .

  1. Determination of the optimum knife angle.
    The ideal knife angle for a particular bolt diameter is determined by observation and lathe operator experience. As a rule, the larger the initial diameter, the greater the change in optimum knife angle during peeling. An indication that knife angle is near optimum at the minimum diameter will be observed after the core drops from the carriage. A polish developed from friction against the log approximately 0.31 or 0.62 centimetres down from the edge when cutting hardwoods, or 0.62 to 1.25 centimetres on softwoods is about right.

  2. Determination of the change in elevation of the pitch rails needed to change the knife angle one degree.
    To determine this elevation, the lathe operator adjust the pitch rails to horizontal (zero degrees) so that the height of the rails results in a 90 degree knife angle. The operator then measures and records the distance between a reference point on the lathe and the top of the pitch rail. Next the operator lowers the pitch rails horizontally until a knife angle of 89 degrees is reached, being careful to keep the pitch rail angle at zero degrees. Re-measurement to the same reference point easily establishes the change of elevation required to change the knife angle one degree.

    Calculation of the Knife Slope Angle

    In calculating the pitch rail angle for the optimum knife angle the following formula developed by the Forest Products Laboratories of Canada is used.

  3. D/A = slope

    where

  4. D = the difference of elevation needed to change the knife angle one degree multiplied by the difference of the knife angle needed between the inner and outer diameter of the block.

  5. A= the length of carriage travel between inner and outer diameter of the block.

    Setting of the Pitch Rail Slope Angle

    The pitch rail slope is adjusted to the optimum setting. The carriage is then moved forward until the knife is 11.25 cm from the spindle centre. While the correct pitch angle is carefully maintained, the rails are raised or lowered equally until the knife angle at 11.25 cm from the spindle centre is correct. The computation and adjustments are checked by moving the carriage back to 30 cm from the spindle centre (or to the point at which the larger diameter is being calculated) and then measuring the knife angle. If it is correct, the compilation and lathe adjustments are also correct. If the knife angle is incorrect, recheck the procedures in the following order

    1. Angle of the pitch rails, if incorrect, adjust to the calculated pitch rail angle.

    2. Knife angle at the minimum calculated diameter based on distance from spindle centre.

    3. Knife angle at the larger block diameter

    At first this technique may appear complex, but Baldwin (1981). suggests these technique rapidly becomes easier with experience.

    Setting Fixed Slope Pitch Rail

    The technique for setting fixed slope pitch rails is similar in principal. The pitch rail elevation and slope angle are usually set during lathe construction or installation. A knife tilt mechanism allows the operator to make quick and easy knife angle adjustments. A simplified implant set-up can be described as follows.

    1. The desired knife angle for a specific log diameter is selected.

    2. The lathe carriage is brought forward until the knife is 30 cm from the spindle centre.

    3. A bevel protractor head, or similar instrument is preset and placed vertically against the clean face of the knife. The carriage tilt mechanism adjusts the slope angle, and the knife angle will decrease automatically as the log diameter decrease(Baldwin 1981) .

    Advantages

    These pitch rail setting techniques provide several advantages. They largely eliminate trail and error, as trail and error settings can have a substantial effect on recovery, partially at the small diameters. Lathe settings become increasingly critical as diameter decreases.

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