Project: keyhead changer on knife-edges

[sheffield steel]

Wow Georg, THIS IS SPACE AGE TECHNOLOGY, THIS IS WAY BEYOND ME, I'm JUST A SIMPLE GUY! PLEASE KEEP US POSTED.

Dave.

[Georg]

Well Dave, I think maybe my drawings make it look more "space age" and "different" to traditional PSG builders, than need be. In the end my contraption will look like "just another pedal steel guitar", and definitely not as nice as yours.

I am sure a proficient builder like yourself can make nice-looking wooden body-covers completed with polished end-plates for my skeleton, if looks matters. Nothing in my design that will get in the way.


At the moment I am checking the balancing effect of the changer scissors' knife edge connection with the tuning gear slider under normal string tension, to see if the rollers for the sliders can be replaced by 6.35 millimeter (1/2in) ceramic balls between opposing "L" shaped aluminum profiles - see drawing.



If the connection ensure balance under tension, one ball can replace two rollers with knife edge components for each slider - simplifying the slider design a lot. Testing indicates that it might work.

As all can see: such "ball balancing" will fail without string tension, but can't play without strings anyway so that is not a real problem, IMO. I just have to make sure the keyhead can't fall apart without strings, which is no different whether I use rollers or balls.

[Bent]

Georg,
Just being smart-alecky here...6.35 mm equals about 1/4", not 1/2.
The ball rollers look neat, save for one thing. I am wondering if the 4 contact points will create unnecessary friction? As an alternative, would a half-round shallow track milled into the bottom of slider and top of lower angle arm be better? And would ball bearing balls be smoother and most friction-free? Think: the balls and inner and outer race of a ball bearing.

Doing it this way might possibly eliminate the need for the upright arm on the angle. I envision hardened steel in the slider and bottom plate ...same as ball bearing material. Lasts forever and rolls very very freely.
Gives me a great feeling to finally start understanding a bit about your system

[Georg]

OK Bent, you caught a typo. Missed it while proofreading
BTW, I used this ball as example.


I have been, and am, thinking "friction" of a ball in an aluminum square, and may go for more contact surfaces. But, with the right angle of the string leaving the tuning peg vs angle of the slider - something that can be tuned, the downward pressure will be almost zero. Sideways pressure will be slightly higher, and can't be tuned much.

The knife edge connecting the slider to the changer scissors will take up the full string tension, and the ball will be held close to the other end of the slider and more or less only act as a guide and not cause much friction neither horizontally nor vertically. I need a tiny bit of friction to make the ball stay neutral between two "raise/lower blocks" inside there, so I am not sure if profiles rounded and hardened for the ball will make any real and positive difference for the maximum +/- 4 millimeter linear movement I need. Naturally corroded aluminum surface will probably work best, and entire sliders with guide and all are quick and easy to replace using a screwdriver if wear becomes a problem one day (*).


The "upright arm on the slider" is an essential part in my keyhead design, as it is this "arm" that makes it possible for individual tuning gears to move linearly past each other a given length and give the appearance of a normal, stepped, keyhead.

Without that raised arm I will have to go keyless since each tuning gear then can't be wider than the slider itself. Keyless is an easy solution and will definitely simplify and shorten the keyhead, but I am not fond of keyless so I will leave that solution to others.



(*) Ease of maintenance is a major point in the way I design. For instance: I want to be able to take out, service or replace nearly all parts for individual strings with simple tools - a screwdriver and a hex wrench, without having to worry about parts for other strings.

[Georg]

More details for keyhead sliders...

...with full-size drawing on my site.

The slider guide with ball is at one end of the slider, and changer scissors knife edge at the other. Changer scissors take up full string tension.

Changer scissors have fixed height, but slider guide can in theory be set at any height/angle and be given enough adjustment range to fine-tune exact pressure on the guide ball and the screwed-down guide's angle relative to the slider. The minimal friction between ball and slider will make the ball stay and roll free of its end-stoppers once pedals has been operated once, and a tiny (around 0.1 degree) smaller angle at the (black) guide seat than on the (yellow) slider will counteract the tendency of the ball to move towards the changer scissors during operation/playing.

As can be seen I am designing in "loose", almost "rattling", low pressure contact points where 1/100 millimeter mechanical precision, or not, does not make a difference for tuning precision or cause string noise. The idea is that string tension has to be as precise as the strings allow for for neutral and any change, and that there should be no "mechanical hysteresis" tendencies.



Just want to mention that at a certain point "pragmatism" kicks in and says that "enough is enough and more isn't better", and (hopefully) prevent meaningless and costly over-engineering of details. As long as the mechanics is made precise enough to ensure less than 1 cent detuning in itself, my "mechanical precision" goals have been reached, and I believe that comes down to how sharp and precise I can shape those knife edge connections.

For instance: absolute precision and tight fits in the keyhead slider guide system will be more that upset by not being able to align the string "perfectly" on the tuning pegs every time I change strings, resulting in slight variations in sideways pressure since the tuning pegs go horizontal inwards (as on most PSGs). Unless the ball gets worn too non-circular or the inevitable grooves in the slider/guide aluminum surfaces become too uneven, one 3d string width variation in the slider guide system won't make a 0.1 cent difference to overall tuning precision.

Another point - that I have touched on earlier in this thread - is the intentional vertical "rattle" between neck and "backbone" near the bridge-end I will design, build and tune for. It is an important "tone" detail, but it will cause at least 0.25 cent detuning no matter how well it gets tuned.

Thanks for looking in and commenting.

[Georg]

Just a note to state that I will probably end up with a much smaller string angle over the nut rollers. After having thought it through for the n'th time over the last 20 years or so, I can't really see any advantage in having an angle that is much more than what the bar pressed hard down on the strings creates. That angle is obviously far less than the 7 degrees I have used as starting point for keyhead / nut rollers.

As a result the keyhead will be flatter but not flat, and effectively vibrating strings be more even (same length) because of less vertical bending. All this will be reflected in my drawings when I have gotten through the seasonal flu and can think and draw straight again.

[richard37066]

Georg -

Point well made on the string angle even though the manner of winding the string on the tuning peg might alter that by a tad.

Question, though - Would it not be more advantageous to have the string angle equal the slider angle? According to your drawing, there will be an upward - albeit small - pull on the slider in the vertical direction. If they were equal, would this not ease the pain in regards any captive hardware needed to keep the slider in some sort of sliding equilibrium? If they were precisely (that's unrealistic) the same then the slider could just lay on a ramp and slide merrily up and down - or isn't any of that going to matter?

Get well, my friend. I had my bout with some form of creeping crud about a month ago. I know what you're going through.

Richard

[Georg]

Richard,

Draw a straight line between knife edge at the left end and nut roller on right, and see that the string pulls the slider downwards despite the apparent upwards string-angle ... tuning peg is above the straight line.
String will also pull the slider sideways - outwards ... tuning peg is inside the straight line.

String will leave the horizontal tuning peg on top, and have just enough of an angle to pass above the end of any tuning peg between its own and nut rollers. The 3-point guide system - knife edge and the ball for horizontal and vertical guidance, will keep the slider and thereby the string angle steady and friction minimal. Zero friction at the guiding ball will counteract guide-action and precision, so I definitely want the small but pretty constant 45 degree force caused by offset string pull on slider to steady it.

With all sliders mounted I extend the neck over the slider ends with a 1 - 2 millimeter clearance, which will keep them vertically in place when there's no string tension on them. Horizontally the sliders can't go anywhere since there will only be around 0.2 - 0.3 millimeter between them.

[Georg]

To give a more complete picture, here are the basic drawings showing what I want my PSG to look like.



10 strings, 5 floor-pedals, 4 knee-levers, 2 vertical switch levers.

Left-knee vertical switch lever acts as a half-note/full-note stopper switch for E strings raise/lowers, same as I have on my modified Dekley. This means this vertical lever itself doesn't pull on the changer.

Right-knee vertical switch lever acts as a "gear-switch", modifying action for right-knee levers and a couple of pedals. This means this vertical lever itself doesn't pull on the changer, so it doesn't conflict with the VP since it can be but normally isn't operated while actually playing.

Pull-rods only go from keyhead end to center of frame - with no bellcranks coming down beyond this point, so I will raise the back-side body-cover to provide plenty of room for the VP foot/knee.

No end-plates apart from the frame around pull-rod tuners at the keyhead end. Ladder-frame supporting rod-pulling bellcrank axles and all else, with 3 inch deep tubes with locking-mechanism for inserting legs.


The finished contraption will have dimensions and look more or less the same as any other PSG, apart from being black and have a slightly sloped and curved body-cover.


I intend to place a quadruple pre-amp v/mixer and eq on a screened PCB inside the PSG frame near the bridge-end, which will add around a pound to its weight. This is necessary since I will have at least 3 PUs in it, and I don't want loads of cables and boxes and whatnot hooked up to the PSG just to be able to balance the tone sent to amp/mixer.

[Georg]

The skeleton...