The Spring Winder

The Spring Winder is based on the usual method of a rotating former and a wire dispenser moving along its length. For an ordinary spring, the wire would be moved at a constant rate, but for a fret-spaced spring it needs to be fast at one end and slow at the other. A linear actuator was obtained as they are popular in 3D printers. They are built from readily available extrusions with a stepper motor driven lead-screw. The spring length needs to be roughly 700mm, so a 1000mm actuator will allow some breathing space.

An Arduino Uno was purchased as I really wanted to try one out! No additional programmer box or pricey software is required, just a USB A-B lead and a (free) download of the Arduino IDE. The built-in functions make pin operations straightforward and makes it a great introduction to programmable hardware.

After successfully blinking a few LEDs on and off, the main circuitry was developed to PWM control the winding motor and use a DRV8825 module to run the stepper motor. An LCD alphanumeric display was also added to show useful info. 

The winding former was estimated to be 30mm when using 1.8mm diameter stainless steel wire and a 50mm inside diameter. The winding speed was set at a fairly pedestrian 12rpm and the position of the winding former is monitored by a rotary encoder. 

The whole fixture was built on a combination of plywood and MDF measuring about 1200mm x 400mm x 45mm and uses pillow block bearings to constrain the winding former and a brass clamp to provide wire tension. The first winding motor tried out didn't have enough torque, so a larger motor was sourced connected to a chunky 36V/10A PSU. To negate the effect of loading on the motor speed, the PWM controller uses a small amount of current feedback. 

The main software loop takes the rotary encoder count, calculates* the new actuator position and sets the number of actuator steps so that this can be achieved. Using the micro-stepping facility of the DRV8825 module gave a nice round 100 steps per mm and worked up to a maximum rate of 8000 steps per second. If the quantity of steps are not completed within 30ms an error flag is set and the loop repeated anyway. The slow (~12rpm) winding speed ensures that this overspeed condition does not occur. This process is repeated until the windings are complete. 

*The calculations use a simple linear ratio for the lead-in and lead-out at each end, and a more involved formula that comes from working out fret positions for the bit in the middle. 

distance = scalelength * (1.0 - 2.0 ^ (turns / -12.0))

An Unexpected Turn Of Events

Smart people will already have realised that turning the winding motor 12 times to get a whole octave of windings does not result in 12 finished coils! The ratio of spring ID to former ID had been overlooked. The spring is wound on a 30mm former, but when released (and yes, this was a scary bit) it rapidly returns to the more relaxed state of 50mm diameter. It is the same length of wire in both cases so the larger relaxed diameter has less turns. To correctly compensate for this it needs to rotate (50/30) more turns than I first thought. 

After this, I settled for 4 lead-in turns, 4 lead-out turns and 26 fret turns based on 50mm ID. This equates to around 7 lead-in turns, 7 lead-out turns and around 44 fret turns when mapped to a 30mm former. The 26 fret turns allows some freedom in choosing the best 22 (plus a zero fret). 

After several rounds of debugging it was ready to run. Winding the spring took around 5 minutes and apart from a bit of congestion on the lead-in, it nailed it first time! The video clip speeds up the action into just over a minute.

Doing The Rounds

In between working on the Spring Winder, other parts of the Tubular Bass also needed to be made. The nut and bridge assemblies are both made from aluminium, mostly 15mm thick, but the nut and saddle are only 6mm. To anchor these, 47mm discs are turned on a lathe** and fitted inside the tube. The visible parts of the nut and bridge are bolted to these through slotted holes to allow a certain amount of height adjustment but keeping things sturdy and rigid. 

**I don't have a lathe so I laid a benchdrill on its side instead. Each part was rough cut into an octagon shape with an 8mm centre hole, then turned lathe-style. It doesn't have quite the same bearing system as a proper lathe so there is more judder and it doesn't give a great finish but it's good enough. Don't try this at home - it makes a lot of swarf!

The Steelworks

 An unexpected problem of this bass build was the complete lack of any useful edges or datum lines. Before any holes are drilled or cutouts are made, the tube needs to be marked out. This can't be done freehand, or with a ruler so I had to think outside the, um, tube. Fortunately a bit of spreadsheet magic enabled me to plot some graphs that I could wrap around the tube and use as cutting guides. Once in place it was time to fire up the angle grinder - not a tool I normally use when making guitars!

The guide allowed the two cutouts for the headstock to be made and marked the tuner holes for drilling too. 

Making holes in stainless with ordinary HSS drills is often less than successful, cobalt ones seem to fare much better. 

Fretwire?

Bashing fret wire into wood is quite satisfying. Bashing them into perspex as I have done on the last two builds is not great - it doesn't have the same give and one bash too hard could end up cracking the whole fboard. For the perspex fboards I had to make wider slots then rely on glue doing the rest. There has been one or two dislodged moments over the years, but on the whole they have lasted well. Cutting accurate slots on the stainless tube could have been an option, but I think it would look 'bitty'. Besides I had another daft idea that could only work on a tube...

Curly Wurly

Wouldn't it be great to use a specially wound spring fitted snugly on the tube to act as frets. It would start off with a wide coil spacing and reduces each turn. Between frets 12 and 13, the coil distance would be half that between coil 0 (the nut?) and 1. I drafted out a spec, supplied the equations and sent it off to a couple of spring companies for a quote. 

The coil winding fboard approach had a couple of side effects:

1. the frets won't be conventionally straight (perpendicular to the neck), they will be angled

Dingwall guitars make basses with fan frets. I've never played one, but apparently they combine good tone with playability. As long as the frets under each string are accurate to a single scale length, it doesn't matter that an adjacent string has a different scale length. Using a coil spring for the frets should be exactly the same. To capitalise on the thicker-strings-needing-a-longer-scale concept, so I'll need to ensure the spring is wound in the right direction!

2. the frets will also be on the back of the neck

Mmm, I suppose they will, but I'm going to do it anyway

Meanwhile, the spring company replies came back. One wanted £250 + VAT just to develop a spring with no guarantees of success, the other just wished me good luck...

The sensible option here would be to make the design more conventional, cut nice perpendicular slots and glue individual pre-radiused frets in. So I decided to make a purpose built spring winder instead*. Boing said Zebedee!

*To be honest I did try winding a spring manually first just to see how difficult it was. Stainless steel likes to fight back so this prompted the spring winder to be pretty chunky. As can be seen from the pic, manually winding a spring results in a very irregular string spacing which is no use at all.

Useful info gleaned from this exercise was the diameter of the former on which the spring is wound. Empirical equations exist, but this application is a way outside the normal wire-diameter-to-spring-diameter ratio. Two or three trial attempts showed that with a former of 30mm, a finished spring ID of approx 49mm was produced which was ideal.