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There is a trade off between shear angle and force required. High shear angle requires less force, but causes fracturing along the length of the cut as well as across the thickness. Nothing restricts the direction of the fracture ahead of the cut which can veer off to either side in hard / brittle materials on a microscopic scale. If the cut is full length at the same time, as in a flat bottomed punch, all the fracturing is across the thickness both from the top at the edge of the punch and from the top edge of the die at a specific angle for each material and hardness. The ideal in a punch is to have these two fractures meet halfway through the sheet, giving a clean burr free blank. A full length cut requires much more force, but has very little risk of cracks.

On my punches, I chose to use a low shear angle so the center of the punch fully penetrates the material at the blank center just as the ends of the punch touch the surface. The cut is from the center out. You could reverse it and cut from the ends in.

Clever design of bench shear blades could probably be optimized for the hard reed material.

Dana

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An advantage of the Crabb-style press shear tool is that it has a constant fairly low shear angle, whereas a scissor-type bench shear starts out with a very steep angle and gets shallower along the length of the cut. Of course the temptation is always to place the work as close as possible to the pivot point because the steeper blade angle there reduces the force needed on the handle.

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That kind of shear blade is shaped to maintain the same shear angle from beginning to end. It is optimized for relatively soft material like mild steel. Those shears can cut through quite thick stock. On reed steel thicknesses, you could still cut easily with a much shallower curve. I am not sure, but I believe it is a exponential curve ( constant angle spiral ). If you scaled it by 2, it would be better suited to hard steels even though you couldn't easily cut 3mm stock.

Dana

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Guillotine bench shears like Di-Accro shears have a straight blade set at a constant angle ( very low) and work very well on hard steel in short ( reed size ) lengths but only up to .032 inches in thicknes. The Crabb type shear is quite powerful, and was designed for a wide range of thicknesses. ( mind you it worked quite well for them! ). Reed steel can benefit from a custom designed shear / punch, especially if you are aiming for production, where you want to need to do as little fitting as possible since reed making is the most time consuming part of making a concertina.

Dana

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That kind of shear blade is shaped to maintain the same shear angle from beginning to end. It is optimized for relatively soft material like mild steel. Those shears can cut through quite thick stock. On reed steel thicknesses, you could still cut easily with a much shallower curve. I am not sure, but I believe it is a exponential curve ( constant angle spiral ). If you scaled it by 2, it would be better suited to hard steels even though you couldn't easily cut 3mm stock.

Dana

You're right, I'd forgotten that the top blade is curved.

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