Jump to content

why are reed tongue slot walls not parallel ?


Recommended Posts

 

Is everyone here even talking about the same thing?

  1. There's "taper" from the base of the reed to its tip, i.e., being normally wider at the base (where it's fastened) than at the tip (the freely swinging end).
  2. And there's the "bevel" or "undercut", in which the slot of the reed frame is wider at its "under" side than at the "top".

I'm a bit confused, but I think you two are talking about the second, while aybee and some others are talking about the first.

 

LOL I was thinking about the second - but thought both were included!

Link to comment
Share on other sites

  • Replies 36
  • Created
  • Last Reply

Top Posters In This Topic

 

Is everyone here even talking about the same thing?

  1. There's "taper" from the base of the reed to its tip, i.e., being normally wider at the base (where it's fastened) than at the tip (the freely swinging end).
  2. And there's the "bevel" or "undercut", in which the slot of the reed frame is wider at its "under" side than at the "top".

I'm a bit confused, but I think you two are talking about the second, while aybee and some others are talking about the first.

 

LOL I was thinking about the second - but thought both were included!

 

Me too!

 

Chris, I apologise for jumping into this without realising the sort of dimensions involved; I was only intrigued as to whether this feature was due to a notionally acoustic reason, or because of original ease of manufacture. But I’ve measured up a couple of reeds this morning. One’s a Jeffries reed, which has had its slot filed to size, the other was a Lachenal reed of about the same size, which appears to have been simply stamped out. The rake angle of the slot (or Jim’s 2) is about 2-3 degrees on the Jeffries reed and less than 1 degree on the Lachenal. However there is a lot more taper (Jim’s 1) on the Lachenal, which seems to confirm your post that the two are inter-related. Both reeds are about 29mm long and the Lachenal tapers in width from 2.58 at the tip to 3mm at the clamp, while the Jeffries taper is only from 3.04mm to 3.1mm.

I have no idea if these are typical figures for the given makers production, but it does seem to suggest that mass production, even then favoured less of a rake angle. I could imagine that stamping reed frames with a fly-press would be easier if there was a little taper, otherwise the pressed frame would surely have the tendency to jam in the tool? Perhaps the tools themselves were more complicated than I imagine them – does anyone have experience with stamping out reed frames using a fly-press?

 

Adrian

Link to comment
Share on other sites

I have not used press tools but I have seen a set. They are expensive to have made, one of the reasons some concertina makers have been people with the skills to make them. The ones I saw involve two operations per frame. The first stamps out the overall blank, the second holds that blank and broaches the slot in the centre, at the same time, at the end of the stroke, opening the aperture more at the bottom, the taper that has been the subject of this thread, while simultaneously allowing the brass that is displaced by this part of the operation to flow outward and create the outside bevel that secures the frame in the reedpan.

 

There would be no great push to create a taper just to allow the frame to come off the tool easily, disengaging the product from the die is routine in such operations, usually done by a device called a stripper plate.

 

The frames produced by the dies I saw needed work after the pressing operation, mainly filing of the top and bottom surfaces to remove buckling.

 

I don't know how much it would cost to get, say, 15 sizes of diesets made (Jeffries had about 9 sizes, Wheatstones maybe 15), but I'd be surprised if it was less than $30,000.

 

It can certainly be done with a fly press, Richard Evans once stamped a couple out to show me, and all of the factories would have used flypresses, but a small flywheel press is an easier bet and they can be cheap secondhand these days. If you want to know about these things in greater depth and more veracity, Dana who often posts in these pages has a huge depth of knowledge about press and diesets, and made his own set.

 

Chris

Link to comment
Share on other sites

Hi Chris,

 

I'm confused by your explanation of the die stamping.

 

The ones I saw involve two operations per frame. The first stamps out the overall blank, the second holds that blank and broaches the slot in the centre, at the same time, at the end of the stroke, opening the aperture more at the bottom, the taper that has been the subject of this thread...

I don't understand how a broach can open up the aperture (slot) more at the bottom than at the top, unless the broach is tapered and moves in a relative upward direction. This would require the broach to stop at a precise axial position. Although not impossible, I haven't heard of a broach being used in such a way, but I'm not an expert on this. The normal broach I'm familiar with that uses one stroke is a stepped cutter, moves in a direction perpendicular to the work piece, and cuts a slot with parallel sides.

 

I also understand that any stamping operation will produce a tapered hole, simply because the punch fits loosely within the die. The loose fit is required so that the punch does not jam, and the softer the metal work, the looser the fit. Thus, the angle of the bevel of the slot, after rough stamping, depends upon the mechanical properties of the plate material. Because of this, I can appreciate Adrian's general point that perhaps the bevel in the slot was originally a manufacturing artifact, which perhaps could've been taken advantage of, as a way to conveniently fit the tongue with precision, since it's only the top side of the slot that then required the most precise shape.

 

...while simultaneously allowing the brass that is displaced by this part of the operation to flow outward and create the outside bevel that secures the frame in the reedpan.

I'm really confused here. It's my understanding that a broach cuts metal and does not simply push it out of the way. Perhaps you're describing some kind of operation that molds metal, by deformation?

 

In thinking about all this, another question arises. Have reed makers used a metal working operation that, after rough cutting the slot in the reed plate by stamping, the slot is subsequently shaped to precision by inserting a mandrel, which has the (near) precise slot geometry, and then squeezing the entire plate from the outside, around the mandrel? The mandrel is then removed. With tapered mandrels, this process can conceivably result in a beveled, and/or tapered slot.

 

Your linking a taper on the tongue to a bevel in the slot intrigues me. I understand your basis for this, that both cause a wider air flow passage as the tongue descends through the slot, but this view is entirely a steady state view, and there may be unsteady effects that result in different acoustical behavior.

 

In your earlier post:

When there is relief in the window the returning reed does not come under so much pressure until the moment it breasts the narrowest point. This would effect lowest reeds least because they bend far enough to clear the slot completely and come under return pressure for a shorter period of their swing. Highest reeds are always in the frame.

Doesn't your reasoning cause one to conclude that a bevel in the slot would then more likely affect the sound of the smaller reeds, since, their tongues experience the greater changing air flow in the slot for a greater portion of their period of oscillation? The larger reeds spend less of their time in the slot region, and so are less affected by the changing flow area produced within the slot. This conclusion is not entirely different from my own, which examines basically the relative time portion the oscillating air particles occupy in the region within and near the slot. For the large reeds, air jets are formed, requiring that these air particles move far away from the slot, but for the smaller reeds, air particles vibrate in the vicinity of the slot for much more time, compared to the period of oscillation.

 

Best regards,

Tom

Edited by ttonon
Link to comment
Share on other sites

Oh dear, this will take a while..!

 

Hi Chris,

 

I'm confused by your explanation of the die stamping.

 

The ones I saw involve two operations per frame. The first stamps out the overall blank, the second holds that blank and broaches the slot in the centre, at the same time, at the end of the stroke, opening the aperture more at the bottom, the taper that has been the subject of this thread...

I don't understand how a broach can open up the aperture (slot) more at the bottom than at the top, unless the broach is tapered and moves in a relative upward direction. This would require the broach to stop at a precise axial position. Although not impossible, I haven't heard of a broach being used in such a way, but I'm not an expert on this. The normal broach I'm familiar with that uses one stroke is a stepped cutter, moves in a direction perpendicular to the work piece, and cuts a slot with parallel sides.

 

Tom,

 

I'm no expert either, though I have been tremendously impressed by what is possible with a dieset. Most of my information comes from Dana, and if you want good accurate information about frame stamping an email to him would get you more accuracy than I can provide. My reply related to the taper (I call it the relief) in the reed slot, and I described, though obviously not too well, the operation of the group of diesets owned by Richard Evans. In his operation the broach, which is largely as you describe it, is followed on the same shaft by a taper which widens the slot at the bottom just before it stops (the frame is upside down when broached). The broach does stop at a precise place every time, and this sort of precise operation is normal in diesets. The material displaced by the taper forming has to go somewhere, and it flows (yes it is very sudden but still it can be called flowing) sideways forming the bevel on the edge of the frame which is used to secure the frame in the reedpan. This is done relatively precisely because the part of the die holding the frame is shaped so as to control the flowed metal in the right shape.

 

 

I also understand that any stamping operation will produce a tapered hole, simply because the punch fits loosely within the die. The loose fit is required so that the punch does not jam, and the softer the metal work, the looser the fit. Thus, the angle of the bevel of the slot, after rough stamping, depends upon the mechanical properties of the plate material. Because of this, I can appreciate Adrian's general point that perhaps the bevel in the slot was originally a manufacturing artifact, which perhaps could've been taken advantage of, as a way to conveniently fit the tongue with precision, since it's only the top side of the slot that then required the most precise shape.

I think this underestimates what a good toolmaker can do with a dieset. The clearance between the male and female parts of a die is governed by the thickness of the material to be cut and the nature of that material, and there will be edge deformation but I don't think it inevitably results in a taper. My recollection is in a single toothed tool it is usual to aim for 1/3rd cut and 2/3rds shear. A broach by its nature is more like a series of progressive shavings.

 

Perhaps the best thing I can say is, a good toolmaker ends up with the shape he wants, and the relief we are talking about is way outside anything that is an accidental by-product. There is no way it is anything other than a desired shape.

 

 

 

In thinking about all this, another question arises. Have reed makers used a metal working operation that, after rough cutting the slot in the reed plate by stamping, the slot is subsequently shaped to precision by inserting a mandrel, which has the (near) precise slot geometry, and then squeezing the entire plate from the outside, around the mandrel? The mandrel is then removed. With tapered mandrels, this process can conceivably result in a beveled, and/or tapered slot.

That sounds like it would work, and I would be surprised if someone has not done it that way.

 

 

Your linking a taper on the tongue to a bevel in the slot intrigues me. I understand your basis for this, that both cause a wider air flow passage as the tongue descends through the slot, but this view is entirely a steady state view, and there may be unsteady effects that result in different acoustical behavior.

Granted; I was making a rough suggestion of parallel effect without expecting it to be exactly the same in all respects.

 

 

In your earlier post:

When there is relief in the window the returning reed does not come under so much pressure until the moment it breasts the narrowest point. This would effect lowest reeds least because they bend far enough to clear the slot completely and come under return pressure for a shorter period of their swing. Highest reeds are always in the frame.

Doesn't your reasoning cause one to conclude that a bevel in the slot would then more likely affect the sound of the smaller reeds, since, their tongues experience the greater changing air flow in the slot for a greater portion of their period of oscillation? The larger reeds spend less of their time in the slot region, and so are less affected by the changing flow area produced within the slot. This conclusion is not entirely different from my own, which examines basically the relative time portion the oscillating air particles occupy in the region within and near the slot. For the large reeds, air jets are formed, requiring that these air particles move far away from the slot, but for the smaller reeds, air particles vibrate in the vicinity of the slot for much more time, compared to the period of oscillation.

Tom, though it is obscured by the manifold subjects of these posts, the point I was making, I think, is that the shape of the slot is going to make a difference to all of the reeds, and I agree it will make a more difference to the shorter ones.

 

All of the talk about the possible accidental shape of the relief is specious, nothing about the design of concertinas is accidental. Opinion alert..! (No thorough testing done..!) The degree of relief is an adjustment on the operation of the reed, an adjustment which leads to different properties of volume, not just overall amplitude, but dynamic volume if you will, and to tone differences.

Edited by Chris Ghent
Link to comment
Share on other sites

Whilst not attempting to answer the original question or to become embroiled in discussion or to apply my usual kiss of death to a topic, I offer the attached which may be of interest.

 

 

 

 

Geoff

 

December 2008 you posted details of a new 12 key Midget, accompanied by the all-time understatement 'I didn't have one so thought I should knock one out'. Hurry up with the book and then we can all 'knock one out' !

Link to comment
Share on other sites

Might it be to allow the reed to return more easily?

 

On the downwind stroke the air flow is pushing the reed and supplying energy.

On the upwind stroke the reed has to return against air flow. If it returns to a parallel slot it faces close to full air pressure as soon as it reaches the bottom of the slot and loses energy in overcoming this until it clears the slot.

 

I imagine this would make it more likely to choke, and also introduce more harmonics due to asymmetry in the swing cycle.

 

With a taper, air can bleed around the reed until it is almost back to start position thus the reed loses less energy.

 

On the downwind stroke the air pressure on the reed will also drop off more rapidly once it starts to move, as air bleeds around it. This will result in a less sustained push and less energy on the downwind stroke, again improving symmetry and tendency to choke.

My jew's-harp has a slot with an edge in the middle, and a bit of relief on both sides. My understanding for the rationale (at least partially) is that having sharp edges in the airflow (on the reed & in the slot) creates more turbulence in the stream, producing more high harmonics for a fuller, brighter tone.

Is everyone here even talking about the same thing?

  1. There's "taper" from the base of the reed to its tip, i.e., being normally wider at the base (where it's fastened) than at the tip (the freely swinging end).
  2. And there's the "bevel" or "undercut", in which the slot of the reed frame is wider at its "under" side than at the "top".

I'm a bit confused, but I think you two are talking about the second, while aybee and some others are talking about the first.

 

One point that I don't think has been addressed here is whether the first has an effect on the quality of the sound. Seems to me that it does, as I understand that the rare "clarionet" concertinas -- with a sound intended to imitate a clarinet, bassoon, or other woodwind reed instrument -- have reeds that are wider at the tip than at the base, often described as "spade-shaped". And I remember seeing in the book The American Reed Organ drawings of experimental reeds of even more complex design, including one in the shape (looking from above) of a cross and one like a forked snake's tongue. No idea what those latter two sounded like, though I believe they were intended to produce unusual sound qualities.

 

Jim, I just measured a set of Wheatstone English reeds and frames very accurately and it turned out that some of the reeds were tapered and some were parallel. Some "pairs" of the same note would have a tapered one and a parallel one, not always the same way. It's my conclusion that, as the frames were stamped and irregular, the reeds were just hand filed to fit the slot and sometimes they would end up tapered. I don't believe there's any relevance in the shape of them, although I'm happy to be proved wrong!

Andrew

Link to comment
Share on other sites

  • 3 years later...

well I'm bringing this very informative thread back to life because I'm stuck in a similar place in my work. Here are a few observations I've made, and I hope that someone can confirm and possibly explain them:

 

1. The trapezoid shape of the reed arrangements makes for an about 5° angle between the outsides of the frames, which means that there is roughly a 2.5° angle between either side of the reed frame and the center line of the tongue.

2. One side of the reed shoe is normally aligned with the reed chamber wall.

3. The two taken together imply that the tongue slot does indeed not run parallel to the chamber walls but have an angle of about the 2°5 mentioned above.

4. I noticed this because I found that the width of a reed chamber wall is typically less than the width of the two widest spaces of the reeds added onto each other to provide a numerical example, reed shoes frequently have a width of 8 mms at the longer end of the trapezoid, but reed chambers may be smaller im width than 16mms.

 

You can also verify this by looking at the reed plate from the side - the inside edges of the two reeds sort of overlap at the outside end (a very crude sketch is attached to clarify). Due to the trapezoid shape, the tongues will over the length of the reed chamber "run away from each other towrads their respective ends of the chamber wall."

 

The question then is - is there tolerance in the physics that allows the arrangment to be driven by the measurements, or is the design of the reed frames an evolutionary result of this arrangement being the best one acoustically?

 

Another question arises with respect to the dimensions of the slot between the valve and the reed shoe. I believe these two always hold:

 

1. The slot always has the width of the tongue. Makes sense, doesn't it?

2. The slot always originates below the tip of the tongue. Appears to make sense as well.

 

However, the length of the slot to me is a mystery; with short (= high pitched) reeds, the length appears to be mostly the length of the tongue as well, whereas slots belonging to long (= low pitched) reeds may range anything from about half to almost all of the tongue length. My suspicion here is that the actual length may also be driven by the space constraints of the reed chamber. As discussed above, the paired tongues sort of "run away from each other;" could it be that at times there is simply not enough space in the arrangement for a slot to be full length because otherwise it would interfere with the neighboring reed in the same chamber? Would the acoustics tolerate that?

 

Thanks a bunch, and I promise not to add public brain dumps about things that may or may not be possible into this discussion (capable of learning I am at times).

post-8903-0-06859400-1427093250_thumb.jpg

Edited by Ruediger R. Asche
Link to comment
Share on other sites

The question then is - is there tolerance in the physics that allows the arrangement to be driven by the measurements, or is the design of the reed frames an evolutionary result of this arrangement being the best one acoustically?

 

 

Sadly, the answer usually lies in an uneasy compromise between the two. On rare but happy occasions both are true.

Link to comment
Share on other sites

Being unable to see Geoff's attachment, my understanding is that the back draft of the reed window was formed at the end of the fly press stroke where the punch was wider than it was at the part that punched out the window. This forced the relatively soft metal of the shoe blank aside, swaging the back draft into the slot and I believe also forcing the shoe approximately into the dovetail form on the edges of the die. You can see the rounded edge of the thusly forged metal in many Wheatstone Reed shoes. The broach was a small slip of steel with cutting edges on the side ( like a normal broach ) that was pushed through the window to accurately size it. ( having no effect on the draft angle other than creating a narrow vertical band at the top) I watched this bring done when Steve Dickenson kindly demonstrated the process for me in 1995 using Wheatstone factory equipment. There are more than one way to skin the cat though and other companies may have done it differently than ay the Wheatstone factory.

Link to comment
Share on other sites

While tapered reeds ( narrow at tip and wider at root ) will have the effect of allowing air to pass by the sides of the Reed increasingly as the angle of the reed diverges from parallel to the top of the reed shoe and the reed is effectively forshortened in the slot, this is no substitute for the much greater gap that develops at the sides of even a paralell sided reed in a normal concertina reed with varing degrees of draft ( to use Geoff's perfect term ) That rational also doesn't explain fishtail reeds ( reverse taper) which you would expect to jam as they swung ito ever tighter portions of their window. Over the distance normal reeds swing in a reed plate of normal thickness, I would be very surprised if the gap changed by more than a thousandth of an inch due to the taper of a reed.

I won't speculate on the mechanism at work, but after having done a number of tests on concertina reeds, varying the draft angle as well as experiments with bellmouthing the reed windows ( curved side walls rather than just angled straight ones ), I hav found a few results. First is that for a given reed, the draft angle strongly affects the pressure /volume curve, with increasing angle increasing the rate at which the reed reached full volume, and even the actual amplitude the reed maxed out at. Above a certain amount which varied depending on the pitch of the reed in question, increasing the draft angle reduced the power of the reed. This is similar to having a reed window of insufficient depth. Lower pitched reeds did best with only a few degrees of draft while very high ones could use quite a lot. Curved walls on the reed window created a steeper pressure / volume curve than a straight sided taper.

The second result I found was that in order to have the high reeds feel the same as lower reeds when playing ( so that you didn't have to exert extra pressure to get them to be loud enough ) they needed increasing amounts of draft angle as their pitch went up. Even an extended vertical band in a reed window can cause a reed to require extra force to start. It seems to be important that the draft angle start within 5-10 thousandths of the upper edge of the window for best results. This matters more for high reeds than for low.

Lastly, reed stiffness ( as in light medium or heavy for a given length and pitch ) seems to determine the right amount of draft for that reed. Light reeds need less, heavy more. Light reeds however can't reach the ultimate volume of a heavier reed even though they require less pressure to play, so reed weight and draft angle should probably be chosen based on the playing charastics you desire.

Steve Dickenson once said to me that the draft angle allowed the reed to cycle at lower pressure than it would without it. That has been my experience.

 

What surprised me the most in all my tests was that having the right draft angle could have the effect of doubling the maximum amplitude of the reed over some other angle regardless of playing pressure used. Of course since our ears perceive a doubling of volume in a logarythmic scale, not a linear one, doubling the linear amplitude of a reed amouunts to a minor increase in audible volume, rather than the miracle it seems on the meter. Even so, it points to some dynamic that effects the actual efficiency of the reed at turning the compressed air into sound.

When I think of paralell sided reeds ( all the Wheatstone reeds I've seen Which are quite a few, but certainly not all of them ) or Tapered reeds ( most Accordion reeds I've seen and all the Jeffries reeds I've seen ) I don't think about how they pass the air so much as the effect the taper has on the reed profile. A tapered reed is somewhat equivalent to a paralell reed that is thicker at the root, though since the reed can be thinner overall, its working mass is located closer to the center of bending, so will have a lower spring coeficient than the same materiel piled up on a paralell reed. This may affect how close to sinusoidal the reed's vibration is, but I don't know. Alas, I never upgraded my modeling software and it won't run on OS X. Since all the tapered accordion reeds I've seen increased their taper with increasing pitch, it essentially matches the way paralell reed profiles work with Low reeds thick at the tip and thinner at the root and high reeds thick at the root and thin at the tip. If you were to squash a set of paralell reeds flat so they were the same thiickness over all, they would look a bit like a tapered reed set, except that the sides wouldn't be straight lines. Since tapered reeds are still profiled, they may simply need less of it.

Dana

Link to comment
Share on other sites

To Ruediger's last post above,The taper of the outside of the reed shoes I believe is a combination of conveniences. The first is that included angles less than 3 degrees are for metalic things like drill chick arbors or lathe centers, a class called self holding tapers. the side force generated by pressing them home creates enough friction to hold them in place where a steeper taperr would tent to force it back out ( as on a milling arbor ). The self holding angle may be different for wood which is much softer than steel. The second convenience is that a tapered reed shoe works well with a radial reed pan. allowing more reeds than paralell sided reed shoes would. Some if not all reed organs have parallel sided reed shoes ( still mounted in dovetails but with felt bottomed slots ). These rely on the compression of the felt to keep them tight in their dovetails instead of the handy dandy tapered dovetail slot.

Inside the chamber, it is a good idea to keep the reed as far as possible from the reed port on the press side reed, hence jamming it up against the wall. Since the dovetail generally cuts into the bottom of the wall, it also means the reed shoe has one edge along long grain wood regardless on the orientation of the chamber on the reed pan. No Ideal how helpful that might be. Since the press side reed doesn't have to be within the bounds of the chamber walls, it often overhangs the other wall helping to keep it's port at a distance from the inside reed making it less likely that its flap valve will be affected by the vibration of the inside reed.

Regarding the length of reed ports ( the hole in the reed pan )I've seen various treatmenst of this. Shorter ports are easier to valve I supose, but given the nature of reeds, a port 50% of the length of the reed will pass enough air, and most of what I've seen is between 50% and 70% of the length of the reeds. What is important is that the vibrating part of the reed, or at least the percentage of it that can come anywhere close to the port, not be influenced by the surface below it or it can have a cushioning effect. I haven't noticed long ports causing any problem ( and I have played with them ).. Thin reed shoes may need longer ports tto cope with the distance the reed swings. I haven't noticed any sensitivity to length that might point ot some acoustic property. That doesn't mean the port can't have some acoustic property, just that what ever property it might have seems to be not operating at the pitches involved. Perhaps others might have more specific information regarding the port sizing.

dana

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...