Softer pen flexure

I’ve been doing my fair share of plotter work lately and I decided I wanted a pen holder with gentler down-force.

I measured Ryan’s pen holder as having a stiffness of about 1 oz of force for each 0.2 mm of deflection. I had been drawing with a Z height of about -0.5 mm for drawing and +0.5 mm for non-drawing moves. Less downforce is possible with simply less deflection, but then I am subject to inaccuracies in the surface or in how precisely I set the starting Z position.

I really like the flexure concept because it has no sliding parts to create friction or play. Here is what I came up with:

This uses a flexure with thinner springs to reduce the spring constant. The thinnest version has a stiffness of about 0.2 oz of force per 1 mm of deflection. I can pre-load it with about 5 mm of deflection for about 1 oz of force, and even then the downforce changes only a little as the Z height changes. A huge Z error of 0.5 mm is only a 10% change in the force.

I rearranged the geometry to try to maintain as much stiffness as possible in the horizontal direction. And I had to split it into two parts to allow the spindly flexures to print flat.

[attachment file=103159]

I got some new ultra-fine-tip pens that feel like they “want” to be used with a light touch. I’m hoping this keeps them happy.

Holy bridging batman!

That would really be something! But no, it prints in three parts, each lies flat.

[attachment file=“pen_flexure_printable.png”]

How does your design perform in drawing dimensional accuracy as compared to Ryan’s or something like this ?

I have yet to fully put it through its paces. It “feels” rigid horizontally. It’s much stiffer than it looks. I have a few different versions with different stiffness. I think I might settle on medium stiffness with low deflection because higher deflection reduces sideways stiffness.

With low deflection the beams are nearly straight which in turn makes them really quite rigid in tension/compression. Then the truss-like geometry makes the structure stable. The tradeoff is more z-force variation at low deflection.