cuttermouse® blog & faq

This section is here to help answer questions and generally explain how Cutter Mouse™ works, in detail. If you have any questions not covered below, please email me. Note that some questions I get are quite technical, so the answers have to be too. There is no deliberate increase in difficulty as the blog goes on, or any particular order, so just skip the hard bits or use you browser’s “find” to get to the answers you want.

Brief History

Shearing cutters date back over 3500 years and were a lot like the modern blade shears pictured on the right, with a spring at the back and hand grips in the middle. The centre pivot and finger loops we see today were added about 1500 years later by the Romans.

So it’s been pretty quiet for hand-held shearing cutter development over the last 2000 years, no surprise then that few people understand how this new one works.

Inside Cuttermouse


Note that the diagrams below are based on the generic original minimum envelope design, which is functionally identical to the licensed version, made and sold by Westcott, called scissor mouse™.


Right side cutaway view, cutting checkerboard paper

The picture above shows a sheet of checked paper being cut. The front wheels on the right drive the rotary blade via the idler gear. All three gears have 26 teeth and so rotate at the same speed, but, the disc blade has a slightly larger diameter than the wheels, which means it’s cutting edge is moving (slowly) to the left at the cut point. This is very important, here’s why.

Conventional scissors can be used in two ways; normally, or zipping through paper (or cloth) by holding the blades open at a fixed angle and just pushing quickly. The latter often works, but if you don’t take a run at it, you get jams or tears because the friction between paper and blades holds the paper back. Scissors are more reliable when used normally because friction between blade and paper actually holds the paper in the cut point. Scissormouse has two blades, like scissors, but one works like the first method and one like the second.

The lower blade slides against the paper at full cutting speed, tending to hold it back (bad), while the top blades slides with the paper at about 10% of cutting speed, pulling it in (good). So the two blades are fighting, but the rotary (good) blade wins because it is sliding slower and therefore produces more friction. Yes, this does not make sense, but it’s fundamental to making many devices work. Your car probably has ABS (anti-lock braking system) fitted for the same reason, that is; when braking, a locked wheel slides along the road at full car speed has less friction than one that is controlled (by the computer in the ABS) to slide slowly. Between paper and blade, the difference in friction is about 2:1 for slow and fast sliding, so the disc blade reliably pulls the paper into the cut point, as long as it’s turning! So if the wheels aren’t turning, cuttermouse won’t cut.

Left side view, without body.


Cuttermouse, like scissors, needs the blades to be kept in proper alignment and biased so they stay together when cutting thin or thick paper. Scissors use the spring of the blades and/or a spring at the pivot which may be adjustable for different materials. Cuttermouse cannot have the blades biased enough to cut five sheets all the time, the high friction makes it too hard to push, the user relaxes, and the paper jams. So the blades are designed to push together more when cutting heavy paper and lees on tissue. The only spring holding the blades together at all times is similar to that in a retractable pen.

Right side cutaway detail view, cutting a 2mm wide from 0.2mm thick (160gsm) checkerboard paper


The blade bias spring aligns the horizontal blade in the other directions as well, so at rest everything is in place, ready to go. This produces perfect alignment, without the expense of making perfect blades, but also allows the blade to get out of the way when needed. If you drop your cuttermouse and it lands on the blade, the spring allows it to move out of the way without damage. If you forget to push down while cutting and get a paper jam, you can roll the cuttermouse over, push the horizontal blade away from the disc blade with your thumb, and clear the scraps easily. When finished, everything snaps back into perfect alignment. So don’t worry about the loose blade - it has to be like that.

Horizontal blade contact points for force balance analysis.

And for those who need to know everything, here is how the most important part in the cutter stays in place. The picture above shows the horizontal blade from both sides so you can see all the forces acting on it. The blade hangs from the light blue patch in the top right hole where the disc blade shaft (not shown) goes through along the roll axis. The hole is deliberately loose so the blade can twist a few degrees about the pitch, roll and yaw axes allowing the green patch to sit flat against the disc blade.

Gear train in action, body top removed.