Powered hacksaw – blade lift for the return stroke

[Sonic EscapeParticipant @sonicescape38234]

How is the blade lifted on the non-cutting stroke? I found a few descriptions but I couldn’t figure out the mechanism. I don’t understand what the reference is to raise the blade against. Since the angle of the blade is also variable during cutting.

[Clive Brown 1Participant @clivebrown1]

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.

[Sonic EscapeParticipant @sonicescape38234]

I understand, that is clever and simple!

[Clive FosterParticipant @clivefoster55965]

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

[Clive Brown 1Participant @clivebrown1]

Should say that my Rapidor seems to work satisfactorily with the dashpot missing. If it affects blade life then it’s not very noticeable.

[Charles LamontParticipant @charleslamont71117]

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.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, my Rapidor doesn’t have any adjustment on the dashpot, so it needs an oil of the correct viscosity to work correctly. OK mostly, but if the weather is quite warm or hot, the oil tends to seep by quicker, I could change the oil, but that’s a bit of a long winded job for the amount of time it would be needed.

Regards Nick.

[Chris CrewParticipant @chriscrew66644]

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?

[Chris Crew]

On 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?

No misinterpretation, it seems to depend on the manufacturer.
I’ve a Manchester Rapidor Junior, which cuts on the forward stroke, as do my little Kennedy Hexacut, and my larger Denbigh.
A visit to Tony’s site will confirm the Denbigh cutting direction, and the Kennedy manual is also clear on it.

The Manchester Junior, I’ve never been sure, as documentation refers to automatic relief on the return stroke, but doesn’t seem to give actual tooth direction, though some photos of larger machines appear to have the teeth pointing forwards, at least on a label anyway.
The dashpot on mine has never worked, so doesn’t help me work it out.

Bill

[Chris Crew]

On 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.

[Clive FosterParticipant @clivefoster55965]

Chris

The general rule with power hacksaws is that the blade cuts when travelling towards the fixed jaw of the vice.

Some cut on the forward stroke some on the backwards. On types producing blade lift geometrically by the mutual arrangement of pivots, crank and saw bow the crank direction of rotation is generally opposite between forward stroke cut and rear stroke cut machines.

Clive, Nick

Providing the blade is in the right way round Rapidor saws seem to work well enough, unless terminally abused, however poor their condition. Certainly mine goes noticeably better since I sorted the dashpot system out but I’d had it ten years before I got round to it.

Clive

[Nicholas FarrParticipant @nicholasfarr14254]

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.

[Clive FosterParticipant @clivefoster55965]

Hi Nick

The Rapidor I use is later model than yours on a sheet metal stand with adjustable dashpot and fixed 90° cut vice. Hafta say I’d not noticed the dashpot adjustment having any significant effect on cutting time

Came with an angle iron work support around 18″ long on the right side looking at the saw. Added another on teh other side. Between them a great help in balancing longer stock. Green cabinet is on wheels so with that out of the way I’ve around 6 ft clearance to the Bridgeport. 15 ft the other way yet I’ve still had bars hanging out of the door! One of Mr LiDLs fold up axel stands made a great basis for a long stock support. Foilds up to hang on the wall out of the way.

About a fortnight before being offered that one for £50, my wallet came out smoking, I’d picked up one similar to yours in needs a scrub and oil restoration condition. Came out of the weirdest workshop I’ve ever seen. Basically a long, house width, conservatory with an under drive shaft for the machines running in a gully down the floor. In front of the machines with a scattering of drain covers as a nod to safety. Clearly disused for some years everything was buried in dried leaves. Only the bow and half the flywheel of the saw were visible. Like yours that one has a fixed dashpot, fast and loose pulleys with mechanical clutch and a swivelling vice. Plain bow with no letters cast in.

Clive

[Martin KyteParticipant @martinkyte99762]

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.

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Clive, yours looks similar to the Major medium duty one, but I’ve never seen one, or seen illustrations/photos with the vice arrangement like the one you have, maybe it was a special build. Lathes have a fairly good writeup about these machines Rapidor Saws I’m guessing mine is an early model, as the adds say it has an adjustable dashpot, and as I have an old tool catalogue from around the late 30’s, that advertises the “New Manchester” one, which looks the same. When I first started work after leaving school, the shop I worked in had one like mine, but it was powered by overhead line shafting.

Regards Nick.

[Clive Brown 1Participant @clivebrown1]

AFAIK, the dashpot adjustment consists of a vertical rod which is screwed through the dashpot piston and projects slightly from the lower face of the piston. Varying this projection by turning the rod alters the rate of leakage of oil past the diaphragm on the piston’s lower face. Fairly crude and the rod could perhaps go AWOL fairly easily.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Clive Brown 1, thanks for your advice, and today I took off my dashpot and had a look, and sure enough there is one hole in the piston with a fine thread, and it turns out to be 1/4″ x 40 TPI, so it shouls be easy enough to make a replacement rod.

Regards Nick.

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