Precutting along the scribed lines for the recesses to be mortised in the wood ensures dimensional accuracy and straightness of recess edges
- which cannot be mortised accurately to dimension,
- which cannot be cut in by means of sawing tools.
Precutting is mainly done in case of recess edges across the grain. In doing so the wood fibres are cut on the surface by means of the ripping chisel.
Figure 8 Precutting of wood fibres along the edges of the recesses
1 wood fibres, 2 cutting edge of the ripping chisel
For precutting and the subsequent mortising of recesses
- smaller workpieces are clamped on a carpenters bench or on a bench plate.
- larger workpieces are fixed on horses or (not too high) supports.
Suitable cutting tools are firmer chisels and ripping chisels with steel blades of medium width and a wedge angle from 25° to 27°.
The cutting edge of the blade must be sharp in order to reliably cut the wood fibres. With a dull cutting edge no fine and straight cut can be achieved!
Precutting is carried out in the following steps:
- clamping the workpiece,
- checking the scribing: when scribing with a marking gauge pin or a scriber the wood fibres should be slightly cut in,
- selecting and checking the suitable cutting tool,
- cutting along the scribed line,
· For this purpose the firmer chisel blade should be grasped by the hand.
· The flank of the cutting edge is turned outwards.
· The blade is slightly tilted sidewards and back, so that the flank of the cutting edge ist guided at a right angle and the cutting edge itself is guided at an acute angle to the surface of the workpiece.
· During cutting the working direction is towards the trainee.
Figure 9 Precutting along the scribed line
Figure 10 Precutting across the grain
1 flank of the cutting edge, 2 right angle, 3 cutting edge, 4 acute angle, 5 surface of the workpiece, 6 working direction
- notching along the scribed line diagonally to the grain and cutting off wood chips.
· Here the face of the tools blade is turned outwards.
· The position and the cutting direction of the tool is the same as mentioned above.
Figure 11 Notching along the scribed line
1 scribed line, 2 wood chips, 3 face of the blade
What are the features of sharp and dull cutting edges of
mortising and ripping