Power Hacksaw

[tonyfoale]

The blade does not rise when the reverse movement is, and it constantly presses the weight.

Vyacheslav Nevolya,

Nice job, it is the best power hacksaw using handsaw blades that I have seen.
However, I think that with a simple modification its performance could be significantly improved. This sketch shows the basics.

The modification consists of raising the link mounting point on the sliding assembly and lengthening the link to suit. Alternatively you could lower the motor and crank, or both. Lowering the motor is probably a lot more work and not so simple .

Click for full size

On the sketch.
The hatched block represents the sliding assembly, the rest is obvious showing the crank in two positions, approximately half way through each stroke.
On the right of the sketch are some force diagrams assuming an anti-clockwise rotation of the crank. I have no idea which way yours rotates.
In the original case we can see that the tension force in A can be resolved into Fxa a horizontal force pulling the blade back AND Fya which is a vertical force pulling the blade downwards. All good.
Looking at the forces in case B we see that Fxb pushes the blade forward on the return stroke BUT Fyb is pushing the blade down. Which is not good. This causes increased blade wear, it takes more energy to move the blade and so heating is increased for no benefit.

The lower sketch shows the situation after raising the mounting point. In the A case the forces are as in the unmodified case except that the vertical force on the blade is increased. As long as this is not too severe it is a good thing as it will cut better. Looking at case B we can see that the situation has reversed itself. The vertical force Fyb is now upward, that is; relieving some of the downward weight force, this is a welcome change. So raising the link mounting point is beneficial on both strokes.

This was for anti-clockwise crank rotation. Clockwise rotation reverses everything. The purpose of the link in A and B positions is also reversed. A is now the return stroke and B is the cutting stroke, so the forces Fa and Fb are reversed. In the original case instead of the blade being pulled down more on the cutting stroke it will be subject to a lifting force acting against the weight force. Not good.