Balanced Boffers

All my kids took fencing lessons at some point in their upbringing, and outside fencing, they all enjoyed sparring with boffers (here, PVC pipe with foam pipe insulation over it) for years. One of them has discovered that “lightsaber club” for sparring with illuminated “lightsabers” is a thing, and would like to practice with boffers.

In order to do this, one of them is making a few new boffers out of 3/4" PVC pipe. However, they are older and more discriminating about quality than when they were about 10, and wanted to move the balance point nearer the hilt. These are two-handed swords, and we worked out that they wanted the balance point about 325mm from the hilt end on a 40" / 1018mm long weapon.

We used FreeCAD to model the pipe, measuring internal and external diameters, and a piece of 3/4" A36 hot-rolled steel inside it. We used the FreeCAD CenterOfMass macro to calculate the center of mass and iterate of the size of steel slug required to move the center of mass.

We calculated that a 130mm slug of 3/4" steel of 7.8g/cm³ density in the end of a 1018mm 3/4" PVC pipe of 1.38g/cm³ with external diameter 26.5mm would put the center of mass at about 325mm from the hilt end.

The steel slugs were already a close fit, but how to keep them from sliding out in play?

I knurled a section near the end, and then hammered them in. They won’t come out easily. You can’t see much hammered in, but here’s before doing the hammering, showing one inserted partway and the other not yet inserted:

Close-up of weighted end in model, showing the calculated center of mass in cm:

Zoomed out, you can see that it has calculated the center of mass for each of the parts (in gray) and the center of mass for the whole system (in red, with green and red axis marks).

Yes, we could have done the math ourselves. These are not hard shapes to do that for. But this way was more fun. :smiling_face:


I can only hope your kids see the dedication you’ve shown here and grow up to be equally as thorough as you!

Fair play.


Glad to see you taught your kids to come up with weighty solutions. :weight_lifting_man:

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