Kickback of the workpiece
Definition
A piece of wood can be rejected in the direction of rotation of the tool in a brutal and unexpected
manner. This can happen when only a part of the wood is machined, when feeding direction and
rotation direction are identical, and in several other cases.
Causes
This problem is mainly due to the design of the tool body which, by its construction, enables the
operator to take big passes. Other causes can also have an influence:
- number of inserts on the tool;
- cutting speed;
- inserts sticking out compared to tool body
- type of wood;
- adjustments (e.g. cutting depth).
Independent of the above mentioned, following factors have to be taken in account:
- working conditions: starting to cut in the middle of a piece of wood, machine running, working in the
same direction as the rotation sense of the tool, working without stops fixed to the table or spindle
fences.
- speed not adapted to the type and diameter of tool
- maintenance of the tools: blunt tools, bad installing of the knives on the tool-holder.
When the wood is rejected, it happens sudden and fiercely. A piece of wood with a square of 50mm
and a length of 1 m can be rejected speed that varies between 20 to 30 m/sec ( i.e. 70 tot 110 km/h).In
the mentioned case the average speed of 100 km/h is already attained after +/- 30 thousand of a
second (0,030) after rejection.
Possible solutions
In all cases:
- use tool-holders; nowadays there is a new generation of cutting tools which is equipped with a limited
pass. Although the rejection of a piece of wood is not ruled out completely, it is nevertheless easier to
control by the operator.
- check whether the tools are correctly sharpened,
- respect the speed of the tools as to type and diameter. The sticker on the machine indicates the
optimal speed to be chosen, considering the type of tool and the diameter.
- never exceed the maximum speed that is indicated on the tool.
Finally: always install various protections when working with the machine. The vertical and horizontal
wood-pushers must be adjusted so that there is complete freedom of movement to put the piece of
wood into place and to machine it without the risk of hands encountering the tool.
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