Steps for HOWTO Make a CNC pickup winder
Step 1  Step 2  Step 3  |
Step 3: Wire tensioner
What is wire tension?
Tension is the force required to pull the pickup wire against the accumulation of all resistance, forces and loads imposed on the wire as it moves, including any tension device. The forces imposed by the wire path before and after the tension device can be called pretension and are caused mostly by the friction of the wire moving over eyelets, pulleys, supply spool flange, etc. controlling the wire path from the supply spool to the pickup coil being wound.
Why is it important?
Tension is necessary to give the winding machine maximum control of the wire. Excessive tension can become a problem, though, if it damages the wire or its insulation. Damage to the insulation, scuffing scraping, cracking, can reduce its dielectric properties. Too much tension can stretch the wire reducing the diameter which reduces its strength making it more susceptible to breakage. Excessive tension or spikes in the tension can exceed the tensile strength of the wire and break it causing frequent machine stops and inefficient production. Resistance is a major characteristic of pickup coils and often a measurement equated to a pickup's performance and used for quality-control checks. Stretching the wire reduces the diameter and thus increases the resistance even if the stretch is only at one point. Tension controls the tightness or compactness of the coil. Coils with the same number of turns that are randomly wound have different wire lengths. If a coil is wound with good tension it is compact and it has less wire on it then a coil wound with incorrect low tension that results in a coil with longer total length of wire. The coil with the shorter wire has the lower resistance and the coil with the longer length of wire the higher resistance.
Optimal tension
The basic factor determining the optimal tension is the wire size. Industry standards have been established by National Electrical Manufacturers Association (NEMA) and wire manufacturers that provide recommended wire tension for each wire size related to the tensile strength. However, this” wire” tension is not necessarily winding tension. Optimal winding tension includes other factors such as bobbin size, bobbin shape, number of turns, winding speeds, as well as acceleration and deceleration rates.
Varying tension
Tensioning devices must be able to react to the various conditions that can occur when winding pickup coils. The shape of the bobbin is important. A round bobbin is the simplest to wind because the round shape creates an even pull on the wire path. The rectangular or elongated bobbins used for guitar pickups impose tension spikes during each revolution which the tension device must absorb and maintain control. An exaggerated rectangular bobbin can make it difficult to maintain good tension control with the difficulty increasing as the difference between the long and short side increase. Winding speed is typically limited and becomes a very critical factor.
Tensioning mechanisms
Commercial mechanical tension devices typically use a friction clutch assembly. They are usually manually adjusted by increasing the spring pressure on two disc with abrasive surfaces. Many home-made pickup winding machines have succesfully used two adjustable felt pads which can be tightened against each other to produce friction as the wire passes between them. Since the tension results from the friction of the felt pad against the wire, operation of these devices causes the pads to wear and periodic adjustment is required.
In addition many tensioning mechanisms use a take-up arm or lever (a dancer arm). A take up arm is a lever mechanism with one end mounted to the tension device and the other end extending up on an angle and it has an eyelet or roller at the end for the wire to pass through and continue to the wire guide and on to the bobbin coil The upper most point of travel of the arm is the home position and the forces imposed as the wire goes over the roller or eyelet pull the arm down. Spring pressure pulls the take up arm toward the upper position so when any slack is created the arm moves upward and takes up the slack. The take up arm typically has it own adjustment and needs only to apply sufficient force to pull back the wire and eliminate the slack. Tension devices can include a sensor to detect wire breaks, end of spool condition or excessive tension.
The current machine will use a tension device made of felt pads (for simplicity), with a dancer arm to take up slack and to indicate the level of winding tension. Microswitches at either end of the limits of travel of the dancer arm will act as machine emergency stops, to halt the machine when the wire breaks or when maximum tension is exceeded. Closed loop monitoring and adjustment of wire tension is possible, but probably not necessary for this machine.