Gram Gage for Measuring Action Center Friction

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Dustin Golden
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1 Gram Gage for Measuring Action Center Friction Greg Graham, RPT, March 11, 2011 If you have been measuring action center friction with nothing but your hands and a Correx gage, there is a better way. This is my first post to the new system, intended to be a discussion entry in the new community I created, called "Touchweight". I've been working with Chris Solliday on the new carbon fiber action parts from WNG. Their directions for measuring action center friction made us step back and think about how we had been using the Correx Gram Gage to date. I built a jig to measure the critical grand piano action centers: Hammer shank, wippen flange, jack flange, and rep lever. One photo is attached. Others will be posted in the "Touchweight" community, if I can figure out how to do that. Here is the critical nugget I learned from all this: For accurate measurements, you need to control two variables: 1. The "feeler" of the gram gage must point directly at the action center. In piano language, this puts the tip of the feeler on the "magic line" between the action center and the pivot point of the feeler. In engineering terms, the feeler will be acting tangentially at the required measurement radius, eliminating radial force vectors. 2. The tip of the feeler must be at a specific distance from the action center, and that radius must be mentioned any time gram measurements are discussed, or they are meaningless. WNG specifies 20 mm from the action center for hammer and wip flanges. David Stanwood specifies 32 mm, which works on Steinway style flanges (20 mm does not). 5 grams at 20 mm is the same as grams at 32 mm. Jacks are usually measured at the toe, and rep levers at the drop screw buckskin pad. I used to measure by hand, holding the part and moving it past the gram gage which was sitting on a non-slip pad on the edge of the bench. Readings were fleeting, but I worked up a technique that was sort-of consistent, measuring at the tip of the flange. Well, if the flange length changes, so do the friction measurements. You can't measure at the tip of the flange and expect precision. Unless you measure at the same distance from the action center everytime, for every different style parts, it is of no use to say "I want my hammer flanges to have 4 grams". It don't mean a thing if it ain't got that radius. I'll dig into a description of the jig in this pages that follow. Let me just say that my friction measurements are WAY WAY better than they used to be. And almost as fast.

HOW IT WORKS The goal is to keep the tip of the gram gage at a fixed distance from the action center while measuring friction IN MOTION (kinetic friction). The part being measured moves.

2 HOW IT WORKS The goal is to keep the tip of the gram gage at a fixed distance from the action center while measuring friction IN MOTION (kinetic friction). The part being measured moves. If we mount the gage on a sliding arm that pivots at the same point as the action center, the distance between the gage and action center will stay constant. But, we must also keep the gage feeler arm pointing directly at the action center, along the Magic Line. If we do so, the gage tip measures only the friction force tangential to arc of the action center s rotation, eliminate errors from any unwanted radial forces. To keep the feeler pointing at the action center, we need to be able to rotate the gage as loading increases and the feeler moves. Center of rotation for gram gage and it s mounting plate Center of rotation for action center AND for main pivot arm which carries the gram gage mounting plate Photo 1 Measuring a rep lever center at the drop screw buckskin

HOW TO USE IT In practice, you hold the action part on the jig with your left hand, and move gage with your right, starting below (toward you) the part as shown in photo 2. 1.

3 HOW TO USE IT In practice, you hold the action part on the jig with your left hand, and move gage with your right, starting below (toward you) the part as shown in photo Move the pivot arm counter clockwise until the feeler touches the part (in photo 1, the drop screw buckskin). Stop momentarily, just touching the action part while you do step Rotate the gram gage mounting plate clockwise, keeping the feeler pointed at the action center, until the part starts to move. 3. Leave the gage mounting plate where it is set and resume rotating the main pivot arm counter clockwise to take the reading. Unlike the handheld method, the needle stays very still, and the reading is solid and repeatable. After you have tried it a few times, it becomes a quick, fluid motion pivoting the gage and swinging it at the same time. Photo 2 Main pivot arm at rest, away from action part. This is the starting position.

THE GRAM GAGE MOUNTING PLATE I used some eighth inch thick plywood to make a mounting plate large enough to surround the gage on three sides with locator blocks.

4 THE GRAM GAGE MOUNTING PLATE I used some eighth inch thick plywood to make a mounting plate large enough to surround the gage on three sides with locator blocks. I used Titebond Molding glue and rubbed the blocks next to the gage until they grabbed. No clamps needed. They are snug enough to hold the gage reliably, but I can still pop off the gage if I want to. My plate is about three inches square, with the front corners cut off. Drill a hole for a piece of hammer flange directly below the gage feeler arm pivot point. It is about a quarter of an inch from the edge of the gage I m using. Check yours from several angles and get as close as you can. Glue the pivot dowel to the plywood plate. Sand a small chamfer on the dowel to make placement on the main pivot arm easier. Photo 3 underside of Correx gage mounting plate

5 THE MAIN PIVOT ARM I used a piece of 1/4 inch plywood about three inches by five inches. The first three inches of the length are full width, then I cut it down to an inch wide for the last two inches. Drill a shank-sized hole ½ inch in from the narrow end of the arm, and radius the arm around it. I glued a three by three piece of 3/4 ply on top of the arm to make a pad for elevating the gage mounting plate, as shown in photo 3. You will eventually drill holes at various radius locations, but not yet. Photo 4 Main pivot arm with Correx gage mounting plate removed, showing various radius mounts

6 THE FIXTURE MOUNTING BLOCK and MAIN PIVOT I cut a shim of ¼ ply 3 by 5 inches. The mounting and pivot block is ¾ ply, 3.5 by 7 inches, with one end cut to a point at 45 degrees from each side. Screw the block to a base of ¾ ply about 10 inches square, with the ¼ ply shim between the block and base, held back at the end of the block opposite the pivot. Drill a hole ½ inch in from the point of the block for a hammer shank pivot dowel. Drill through the block, the ¼ gap, and down about ½ inch into the base. The Main Pivot Arm slides into the gap between the block and the base. Insert a 1.5 inch long piece of shank dowel through the block, the arm, and into the base, and trim it off flush with the block. No need to glue it. The Main Pivot Arm should rotate freely. A little paste wax on the base would be nice to reduce friction. NOW you can measure and drill holes for the gage mounting plate pivot. I used dial calipers to measure from the gage pivot to the tip of the feeler. Add 20 mm to that dimension and place one point of the calipers on the main pivot dowel center, and the other point on the Main Pivot Arm to mark your 20mm hole. Do another at 32 mm plus the feeler length. You will need to have your jack and rep lever in hand to measure for those holes (which are different for various types of wippens). Measure from the action center to the feeler contact point, and add the feeler length. You can drill your gage pivot holes anywhere on the Main Pivot Arm. As long as the radius is measured from the main pivot dowel, the geometry will be correct, even if the gage sits off center on the arm. See photo 5. I suppose it would be possible to have some sort of infinitely adjustable, sliding gage pivot point and avoid drilling holes, but then you d have to measure each time you changed the measuring distance. Maybe a scale and cursor would work, like a tablesaw fence. Hmmmm. Maybe on Mark II.

7 Photo 5 ready to place gage mounting plate on main pivot arm

FIXTURE PLATES Each unique part design will require some sort of fixture to hold it motionless on the jig with the action center directly over the main pivot dowel.

8 FIXTURE PLATES Each unique part design will require some sort of fixture to hold it motionless on the jig with the action center directly over the main pivot dowel. I ve been using ¼ ply plates, screwed to the mounting block with ½ inch flathead wood screws. There is probably a better way, but this was enough to get things working. For the WNG wippen in photo 6, it was easy to make a plate of ¼ ply, mark the location of a few locator dowels, and glue on a few wood shims to level the wippen and provide clearance for the disengaged butterfly spring. The wippen plate can be mounted in three different places, so that the wippen flange center, jack center, and rep lever center can all be placed over the main pivot when needed. I measure a whole set of jacks (photo 6), then relocate the plate and measure the wip flanges (photo 7), then relocate again to do the reps (photo 1). Photo 6 measuring jack center friction at jack toe (butterfly spring disengaged from jack and rep)

9 Photo 7 measuring wippen flange at 20 mm

For the hammer flange fixture, I cut ¼ ply a little shorter than the distance between the action center and the hammer, mounted a block to touch the underside of the shank and the back of the knuckle.

10 For the hammer flange fixture, I cut ¼ ply a little shorter than the distance between the action center and the hammer, mounted a block to touch the underside of the shank and the back of the knuckle. A block on the opposite side of the shank, closer to the hammer, locks the shank in position. I suppose dowels would have been easier, but this was the first thing that came to mind. I used some 1/8 th wood shims to support the shank level with the width at the bushings. See photo 10 for a clear view without the human fingers in the way. Photo 8 measuring hammer flange at 20 mm

WHY THE MAGIC LINE IS IMPORTANT Photo 9 shows the gage feeler touching the hammer flange at a point which is not on the Magic Line between the action center and the gage feeler pivot.

11 WHY THE MAGIC LINE IS IMPORTANT Photo 9 shows the gage feeler touching the hammer flange at a point which is not on the Magic Line between the action center and the gage feeler pivot. Several things are happening that we don t want: 1. The distance between the feeler tip and the action center is no longer the desired 20 mm. 2. The force of the gage spring is no longer acting tangentially on the radius line from the action center. 3. Friction between the flange and the feeler can produce very large forces without moving the flange at all. The extreme example would be pressing the tip against the end of the flange, pressing toward the center pin. Readings would be meaningless. Even at this low angle shown in Photo 9, the tip could get stuck on the injection molding bumps on the flange while trying to slide toward the action center. The friction readings would be inaccurate, sometimes very inaccurate. The MAGIC LINE Photo 9 closeup of hammer flange measurement, not quite on the Magic Line

12 For the Physics nuts Figure 9A, from a Wikipedia article on Torque, shows how a Force can be broken down into it s components, acting along the lever arm (radial) or perpendicular to it (tangential). If the feeler arm points toward the action center, (the red circle in this Figure) and forms a straight line, the only force applied is tangential. If the feeler is at some angle other than zero, part of the feeler s spring force is diverted radially along the flange. Remember the sine function? The greater the angle of misalignment, the greater the radial force, and the greater the error of our gage measurement. Figure 9A Forces acting on a radius to create a Torque To get really picky, Friction should really be stated as torque, such as mm-grams. The familiar Foot-Pound unit is well known to the auto-mechanic crowd. One pound of force applied at the end of a wrench one foot long produces one foot-pound of torque on a bolt. Piano people often talk about tuning pin friction in terms of Inch-Pounds. Action center friction could be expressed as grams times measuring radius in millimeters. For example, four grams applied at 20 mm produces 80 mm-grams, or gram-mm, or g-mm. The standard metric system unit for Torque is the Newton-Meter. A Newton is a metric measurement of Force. Technically, grams are a measure of Mass, not force. Our gram gages are really gram force gages. Gram force is the force one gram exerts on a scale when acted on by the acceleration of gravity. F=MA, or Force equals Mass times Acceleration. We would have to go through conversions to get to Newton-Meters, and the resulting number would be very small. So just forget all that and use grams times radius in millimeters. Compare the tightness of these action centers: Hammer flange of 4 grams x 20 mm = 80 g-mm Hammer flange of 2.5 grams x 32 mm = 80 g-mm Rep lever of 6 grams x 54 mm = 324 g-mm

SO BE CAREFUL WHERE YOU MEASURE THINGS Photos 8 and 10 make it clear how different friction measurements would be if measured at the tip of the flange.

13 SO BE CAREFUL WHERE YOU MEASURE THINGS Photos 8 and 10 make it clear how different friction measurements would be if measured at the tip of the flange. WNG specifies a 20 mm radius from the action center. If measured at the tip of the WNG Mason and Hamlin flange using my old methods, that would be a 42 mm radius. The resulting measurements would show HALF the friction because the lever arm would be twice as long. Using the Stanwood distance of 32 mm, appropriate for Steinway hammer flanges, would result in yet a different measurement. Photo 10 measuring a Steinway hammer flange at 32 mm It ain t rocket science, but it is something we should understand if we want to get predictable results from our touchweight designs.

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