On eg. the Rapidor the blade is at a very slight angle to the frame guides. This has the effect of pushing up the frame on the cutting stroke. On the return stroke an adjustable dashpot dampens the fall of the frame, hence creating a small clearance between the blade and the workpiece.

On my Rapidor saw, as with most crank driven ones, its due to the relative angles of crank and saw bow motion combined with damping action form the oil filled dashpot.

Quote from the Rapidor sales literature:-

“Automatic Lift.

The Sawblade is lifted automatically on the return stroke by means of a simple oil dashpot in conjunctionwith the angular setting of the saw.”

Because the crank is below the saw bow guides there is an upward lifting force generated on the, backwards, return stroke offsetting the weight of the bow and guides causing it to lift. The oil in the dashpot supports enough of the weight to allow the smaller geometric force to lift the blade just clear of the cut. If the dashpot is empty or the bleed maladjusted the blade won’t lift properly. Adding weight to get more work out of a blunt blade also destroys the lift action.

On the forwards cutting stroke the crank generates a downwards force pulling the blade into the cut. The dashpot adjuster needs to be set to sufficiently dampen the pull down force so the blade cuts but isn’t pushing down too hard. In practice the adjustment isn’t that critical. Somewhere near does well enough but if you have lots of things the same size to cut a bit of optimisation makes things go less slowly.

No one in their right mind ever called a Rapidor fast. Effective, durable, a bit crude but long lived and hard to break. Yep. Fast. Nope wrong door.

I’ll bet that the percentage of Rapidors that ever had thier dashpots properly adjusted is vanishingly small.

With all crank driven saws its important the the crank rotates in the correct direction depending on whether the cut is on the forward or reverse stroke if the blade lift action is to work correctly. Basic rule is that the cut direction is towards the fixed jaw of the vice.

Clive

My Qualters & Smith saw has the arm lifted on the return stoke by an eccentric driven pump plunger at the bottom of the dashpot. The blade is parallel to the slide.

I must have been labouring under a mis-apprehension for most of my life. A power hack-saw blade cuts on the return stroke, my Qualters certainly does, and it’s the reason the blade is mounted with the teeth facing the opposite way to a manual hack-saw which face forward and cut on the forward stroke. Have I just mis-read or mis-interpreted what others have stated above?

On 2 April 2025 at 11:09 Chris Crew Said:

I must have been labouring under a mis-apprehension for most of my life. A power hack-saw blade cuts on the return stroke, my Qualters certainly does, and it’s the reason the blade is mounted with the teeth facing the opposite way to a manual hack-saw which face forward and cut on the forward stroke. Have I just mis-read or mis-interpreted what others have stated above?

By return stroke, I meant the non-cutting one. My Q&S cuts on the inward stroke, so I would call the outward stroke the return.

Hi Clive. yes my blade is in the correct way, and the saw works very well as it should, over the 20 years that I’ve had it, but it doesn’t have any adjustment on the dashpot, like others have said. Mine is the Manchester Light Duty one with a mechanical disengagement to the saw drive.

Regards Nick.

These things were ubiquitous at one time. My first part time job was with a guy running a light engineering workshop in his domestic garage. 2 fly presses, drill, mill, lathe and mechanical hacksaw. The trick for getting the last few miles out of worn out blades was a couple of bar ends fixed to the bow by a GCramp. Periodically there would be a loud clatter as the clamp let go.

The local factory boiler house had one that spent every night shift turning pallets into firewood.