DIY: Engraving Pen to Electric File

If your engraving pen has been idle for a long time and you rarely use it, how can you fully utilize its value? Why not transform it into a mini electric file? Let's DIY together.

Engraving Pen Design

Due to the very short stroke of the spindle, it can only be used for engraving, and it produces a tremendous amount of noise.

When disassembled, it's quite simple inside.

It's essentially an electromagnet with a swinging arm. At 220 volts, each cycle includes both positive and negative half waves. During the positive half wave, the arm is attracted upwards to perform work, and at zero crossing, it's pushed back by the spindle's spring to complete a stroke, with the negative half wave operating similarly.

The amplitude is controlled by an eccentric plastic limit pin. The larger the spacing, the greater the amplitude, but the maximum amplitude is only adequate for engraving and insufficient for modifying the electric file stroke.

As shown, the maximum stroke is approximately 1.5mm. The main modification goal is to increase this stroke.

Removing the limit pin allows for a stroke close to 4mm, which is barely sufficient. However, increasing the stroke amplifies the already substantial noise, and enlarging the electromagnet's arm movement decreases coil reactance, causing the current to increase several times over, leading to smoking in less than a minute. Further efforts are needed to lengthen the spindle, reduce noise, and control costs as much as possible.

Increasing the stroke brings more significant noise. Initially, noise reduction at each stage is necessary. Originally, the arm and torque transmission rod collided, creating considerable noise.

Drilling a hole and inserting the rod for fixed torque transmission reduces noise.

After modification, the arm moves directly with the rod, eliminating collisions and reducing noise.

There's still hard collision between the arm and the iron core that needs improvement.

Due to increased stroke and previous arm drilling to insert torque rods, the rod length may be insufficient, necessitating a cap extension. A gold-plated brass button cap from a speaker can be used, with a hole in the middle for shortening and fitting over the spindle. 

This round top reduces friction when transmitting torque rods, thereby lowering noise.

Inserting this pendulum...

The eccentric limit pin needs a reduced diameter to become a fixed concentric limit pin (fixed stroke).

Gluing a small 1mm ultra-thin silicone pad with 704 adhesive and then digging a hole.

Once assembled, this silicone layer reduces collision noise.

Due to increased motion stroke, the electromagnetic resistance increases with the arm, lowering coil reactance by a large amount, and the current surges. Originally around 20 watts, after increasing the stroke, it surged to 40-50 watts. This overloaded electromagnetic, which was already working at 10 watts, would smoke in about a minute without intervention. Adding a diode in series resolves this, with almost no cost increase and no reduction in instantaneous power (without affecting original torque output). This allows the electromagnetic to rest during half a cycle of 50 Hz, reducing temperature, while maintaining power consumption at the original level after increasing the stroke.

Once assembled...

Trimming the file end, after switching on, the stroke extends approximately 4mm.

Aluminum and iron plates are equally effective, especially for tasks that require deep stroke limits, as the file extends significantly.

End