A Scribe or Center Punch From a King Wire




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I used to help build big clear span structures using post-tensioned steel technology. We made some fantastic buildings with spans up to 114m.

The technique we used to achieve such spans relied on a stressed cable made up of several strands running through the bottom chords of the trusses. These strands were stressed using hydraulic jacks and then the whole lot was locked into place by filling the chords with high strength grout. The result was an incredibly stiff but very light weight truss. See the picture below of a roof for cattle sale yards by s-squared. 70m clear span.

The strands used in these projects is the same as is used in post-tensioned concrete floors in large commercial building projects.

These strands are made of very tough steel, and this instructable is for just one way of reusing the off cuts.
  • All you need is about 200mm of stressing strand.
  • Twist the strand with pliers against the lay to break it apart.
  • You will notice the centre wire is straight. It is known as the king wire.
  • Carefully sharpen the king wire with a grinder. Don't let it get too hot.
You now have a scribe capable of easily marking steel. It can also be used as a centre punch. Put a handle on it and you have an ice pick.

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    8 Discussions


    Reply 7 years ago on Introduction

    Don't be so technical, as long as it is pointy it is good. I have 3 punches I use to center punch with, in each the angle gets shallower. If I try with the wide one first I cannot see what I am doing. Plus I always miss, so I have to knock the dimple to where I need it. Ha ha!


    Reply 7 years ago on Introduction

    I have four punches, Fred, but the best of them is that obtuse.

    It seems that the pit produced by the center punch must have preferably the same angle of the drill bit's edge.


    7 years ago on Introduction

    Unless you work in that industry, or know someone who does, how do you get your hands on those off cuts? Can you just call up a company and say 'Hey, can I have those?'

    2 replies

    Reply 7 years ago on Introduction

    Good question. Thanks for asking.
    If you were to Google search "post tensioning contractors" or "prestressed concrete" in your area you should come up with at least one good source of off-cuts. It would not hurt to call them up, and I'm sure they would be happy to let you have off-cuts.
    Remember that every strand they stress will provide about 1m of waste. In my experience this stuff is just thrown away.
    Good luck.


    Reply 7 years ago on Introduction

    Cool. So it really is that easy. I feel like you could do all sorts of interesting things with those strands, it's good to know about a source like this.