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  • Power Hacksaw Blade Tension

    I just bought an oldish Gaco power hacksaw, something like this: https://memax.nl/veiling_98/machines...agbladen_17368

    Nice compact little machine with no apparent play or wear. Built in hydraulic lift and coolant. It will cut 8" square, which is enough for 95% of what I do. It takes a 1" x 17" blade. When it came to me, it had no means of tensioning the blade, and the previous owner had shoehorned a 1 1/2" x 18" blade in. I have now made a screw tensioning device for it.

    My question is, how much tension does the blade need? I can apply a lot of tension with the jacking screw, but I'm afraid of bending the frame. This saw is the type where the slide is part of the hacksaw blade frame, so I'm concerned about bending the frame and causing the dovetails to bind. The frame is cast iron and fairly stout, but it doesn't take much of a bend to make things lock up.

    The saw cuts on the pull stroke. Do I need a lot of tension, or just enough to eliminate vibration / flexing on the return stroke? The (new) blade clamps will clamp the ends of the blade firmly and can be tightened to grip it at both ends.

    The new blades are on order - haven't arrived yet.

    Ian
    All of the gear, no idea...

  • #2
    I always thought power hacksaws cut on push stroke any chance previous owner fiddled with Hyd lift function.I had a 1"x16" OMNI that I just tensioned it by feel.

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    • #3
      Originally posted by Tundra Twin Track View Post
      I always thought power hacksaws cut on push stroke any chance previous owner fiddled with Hyd lift function.I had a 1"x16" OMNI that I just tensioned it by feel.
      Nah, a lot are on the pull stroke. Kasto's are push.

      My Keller has a little rocker which gives you about a 2-1 mechanical advantage and then has a 1.5" knob. I just tighten that to about as tight as I can get it by hand and that's good. I'd say take a 3" wrench and snug it up pretty good.

      BTW, nice saw Ian.
      21" Royersford Excelsior CamelBack Drillpress Restoration
      1943 Sidney 16x54 Lathe Restoration

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      • #4
        Starrett recommends 25000-35000 psi. for power hacksaws. So to calculate tension, you need your blade cross-sectional area. See: https://www.starrett.com/docs/other-...e.pdf?sfvrsn=6
        If you have access to a blade tension gauge, no calculations needed. If not, do the math to get the elongation corresponding to the desired tension and set up a test indicator.

        RWO

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        • #5
          Push stroke vs. pull stroke doesn’t really matter, because the frame is pulling the blade through the work. The tension is there to keep the blade cutting straight. Ever try one of those relatively new high-tension hand hacksaw frames? They work a lot better, don’t they?

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          • #6
            JohnMartin, my thoughts exactly! "Pull" stroke only comes into play in situations where the blade is held on just one end, e.g. the Japanese pull saws. ...and there's not enough rigidity to resist buckling.
            Lynn (Huntsville, AL)

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            • #7
              Assuming the blade is one inch wide and .1 inch thick, those figures would mean about 2500 lbs of tension or more- seems pretty high to me. If you're worried about it, adjust till you feel the friction, then back off some. But at even 1/10 of that, you still have 250 lbs pulling the blade on the return stroke- seems like more than enough to draw the blade back.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                On my Peerless I snug the pusher bolts (3/8” coarse) to maybe 10 ft lbs. 2:1 advantage, I haven’t done the math. Blade will give a mid range ring when tapped with ratchet handle. Usually snug it up after making some cuts.

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                • #9
                  Damn that's a nice saw SVS. Easily as nice as the 16" Racine I we ran at school. Too bad they can't be discussed at PM, that would never be used in a manufacturing environment.
                  21" Royersford Excelsior CamelBack Drillpress Restoration
                  1943 Sidney 16x54 Lathe Restoration

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                  • #10
                    Thanks all!

                    John, I know what you mean about both strokes being pull strokes. I don't know the logic of choosing one over the other, apart from a pull stroke putting the cutting forces on the fixed vice jaw. The saw has automatic hydraulic relief which lifts the blade on the push stroke, but it might be possible to retime this to the pull stroke if needed. But I'm fine with it as it is. And yes, the high tension hand hacksaws do work great.

                    To get a blade tension of 30,000 pounds per square inch, I'll need about 700Kg of pull. Seems like a lot for the frame, so I'll try building tension up until the blade doesn't deflect. If it sounds right and gives square cuts, I'll call it good.

                    Still waiting for the blades to arrive - it seems that the UK is further by mail from Holland than China...

                    Ian
                    All of the gear, no idea...

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                    • #11
                      Thanks TMB-weighs 3,500 lbs, cuts straight, lots of monkey motion for entertainment.

                      The good deals on NOS blades tend to be 4 or 5 TPI. I’m usually cutting tubes, and starting to run low on 10 TPI which I have better luck with. I’ve broken more coarse blades than I’ve worn out.

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                      • #12
                        The blades I have ordered atre all 10tpi, they should do most of what I need.

                        Ian
                        All of the gear, no idea...

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                        • #13
                          I've had no experience with power hacksaws, but it would seem a guy could readily make suitable blades from rolls of band saw blade stock. Anyone ever do that?
                          Lynn (Huntsville, AL)

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                          • #14
                            Pretty sure a power hacksaw blade is thicker and wider than you might find in a roll. I like the idea though. I don't have a power hacksaw, but I do have a couple blades. They are both more than an inch wide, and are .1 inch thick. Both are HSS. I've been cutting into one of them to make custom cutters.

                            .
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                            Comment


                            • #15
                              Originally posted by SVS View Post
                              Thanks TMB-weighs 3,500 lbs, cuts straight, lots of monkey motion for entertainment.

                              The good deals on NOS blades tend to be 4 or 5 TPI. I’m usually cutting tubes, and starting to run low on 10 TPI which I have better luck with. I’ve broken more coarse blades than I’ve worn out.
                              Ha. That's funny. Our Keller 3Ch hates 10 tooth blades, or at least the only one I've tried. It chatters so bad no matter what it's cutting unless the pressure is just zero, and even then some. I think it's some sort of resonant frequency thing. But it'll cut all day with a 4 tooth. I just stick to >3/4" metal. But it is funny to see a much smaller saw with a coarser blade.

                              Originally posted by lynnl View Post
                              I've had no experience with power hacksaws, but it would seem a guy could readily make suitable blades from rolls of band saw blade stock. Anyone ever do that?
                              Noooo, definitely not. Power hacksaw blades are HSS all the way through and much thicker. The blade cuts nearly an 1/8" kerf and is probably at least 50-75 thou thick.
                              21" Royersford Excelsior CamelBack Drillpress Restoration
                              1943 Sidney 16x54 Lathe Restoration

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