As part of a larger prototyping project I needed to wind some special coils. The coils would be a few hundred turns of relatively thin copper wire (about AWG 35) on a specific piece of polysterene pipe. In order to bracket the proper parameters of the coil during development I would need a few variants of the coil to experiment with.
Winding the first coil by hand was a desaster. Even with another pair of hands helping me it was difficult to control the tension of the wire while rotating the pipe, messing up the coil. I also lost count on several occasions due to the distraction. The resulting coil was poorly wound and geometrically undefined. This also makes the DC resistance a poor estimator of the number of turns, not to mention inductance. Clearly, an alternative method of producing the coils was needed.
While there are companies offering coil-winding as a service (CWaaS), the closest one within an hour of driving, the setup costs are high and the turn-around time would slow down my development project. Buying a used winding machine and adapting it myself was an also alternative but it would take even longer, and it would be expensive. At that point I decided to build a specialized winding machine myself. This Hackaday project explaines my thinking and the resulting design.
Every design project should start from functional and non-functional requirements. It is not always necessary to write these down but they should be in the mind of the designer. Here is the list I came up with for this project:
- Machine to help me wind specific coils with greater precision and convenience. Semi-automatic operation is sufficient.
- Safe to use mechanically and electrically.
- Coils made from a few hundred turns of 0.15 mm enameled copper wire. Speed can be low.
- Carrier is a cut piece of polymer pipe, inner diameter 7 mm, outer 9 mm, length below 30 cm. For the application it is important that the coil is wound on the pipe itself.
- Counting the turns automatically.
- Tension of the wire adjustable.
- Forward and reverse winding.
- Rotational speed adjustable.
- Ready to roll (pun intended) as fast as possible.
- Fits on the table.
- Budget: at most 2 days of my time, at most 250 EUR, any part from the workshop that would not be missed.
Note that beauty is not among the requirements. So this thing will be ugly, rough cut and designed for a highly specialized purpose.
Given the set of requirements I scavanged the workshop for available parts and did some 'thinking with my hands' (as opposed to 3D CAD design) on the structure mechanics part. Suddenly, pieces of wood were in my hands.
The easiest way to make the machine mechanically safe was to have a low rotational speed, few turns per second max. The shaft could be driven by hand, for example with a crank, but this would occupy a hand. Better to use an electric motor. A foot switch will free both hands, which is why sewing machines have them. With slow rotation either a stepper motor is needed or a gearbox. For varying the speed some electronics is required. For holding the pipe some coupling and a bearing are needed. The tension in the wire should be controlled and adjustable. Counting windings can be done with a mechanical counter or an electronic rotary encoder.
There is an electronics shop close to where I live who have their inventory online. It turned out they have small electronic motors with gearboxes. I settled for a MFA/Como drills MFA 919D1481, with 148:1 single ratio metal gearbox. By varying the voltage between 4.5 and 15V the speed could be adjusted from 40 to 132 RPM nominally. The torque of 0.6 Nm would be more than sufficient for winding the coil but it is safe to use.
The motor is an inductive DC load of 2-3A at 4.5-15V. Drive electronics for this class exceeds the usual Arduino shields, even using Toshiba's TB6612. Circuitry for electric safety would complicate the drive electronics, and it would take many days to design, build, test and qualify it. Hm, ...
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