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Set-up Part 6: Pot Assembly Fit

Posted by Banjophobic on Thursday, January 22, 2015

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Welcome to part 6 in the series on set-up. Sorry it's been so long between these blog posts... life got in the way, haha!

Another aspect of set up, and one that is crucial to your banjo's tone and volume is the fit of the "pot assembly." This refers to how the three main components of your banjo's round body fit. Those three main components are the tonering, rim and flange. There are many other parts in the complete assembly, like hooks, tailpiece, tension hoop, etc but I'm speaking about the parts that make up the core structure of the banjo.

The fit of these 3 main parts are vital to the overall tone and power/projection of the instrument. Let's look at the rim to tonering fit. The typical tonerings, like "flathead and archtop", need to be well fitted to the rim in certain ways. The flathead ring is much more temperamental in regards to fit, so let's focus on that ring for now. The two main areas of proper fit is the top ledge of the ring to the rim top, and the fit of the skirt to the rim's diameter surfaces.  The top ledge of the tonering is sometimes call the 'bearing surface' and a few other names. Just remember that this is the area where the top portion of the tonering rests/seats onto the top portion of the wooden rim. Here's a shot of the wood ring, with flathead ring seated, from Greg Earnest's site:

The second picture is a good close-up shot of the ring seated on top of the wood rim. For a proper fit here, there should be no major gaps in the fit. In other words, you should not be able to get anything between that tonering ledge and the top of the ring, once the two are seated together. Even with no tension on the ring, there should be no gaps and full contact all the way around with the ring and rim surface. Many times players interchange rings to try them out in their banjos, thinking all flathead rings are 'about the same size'/ While some rings have very, very close dimensions, others do not and only a few thousandths can make the difference between well seated ring and an improperly seated one.

Sometimes excess prying of rings off of the rim can mar the rim's top surface (it is wood after all and can't take but so much well intentioned abuse). If it gets excessive, that too can make for an improper fit on the top ledge of the rim. This will need to be corrected by re-surfacing the top of the rim by lathe or jig designed for this job. It can also mean a re-fitting of the lower turned portion of the rim where the rings outer portion rests (the ring's "skirt), as the flattened rim top changes the overall specs of the ring's fit in relation to those two areas. Years ago the idea of the 'tonebell" system came into vogue. This method of tone ring fitting actually introduces a sizeable gap in the tonering to rim fit, as shown here: 

The result of this fit is extra brightness of tone, usually. While many players like the tone of this system, I am not a fan and discourage folks from trying it on thier rims. If it must be tried, i recommend buying a new rim to modify, particularly on valuable or vintage banjos. Once you original rim is altered in this way, its done and theres no going back without major surgery, plus you have destroyed its originality. There are other ways to fit tonerings to rims, as in Geoff Stelling 'wedge fit' system. This one utilizes a triangular, solid bronze ring, that slides down and angles machined area of his wood rim, securing it as a 'wedging effect' under head and string load. compare this picture, showing a cross section of the traditional flathead rim to tonering fit (Gibson style) to his 'wedge' system:


The resulting tone and volume of this system has won Stelling's design many fans and high praise. There are a few other alterations to the way a tonering rests on top of the wood rim, but they move into old time and or ragtime tenor banjo realms, so i will skip that for this article.

The second way the tonering should be well seated is in how tightly/loosely the skirt is fitted to the rim's circumference. What has become accepted as 'optimal' fit for the rings outer 'skirt (the body of the tonering) is referred to as a 'slip fit'. A 'slip fit' refers to having the ring fit just tightly enough to keep the ring in place on the rim, once fitted, so that there is no lateral movement and the ring will stay put, even if the rim is turned upside down. But if the rim/ring combo is shaken a little, the ring will fall off. You can also check for a 'slip fit' by placing the ring in your hand and trying to pull up on it to remove it (most rings have a hole through which the necks upper lag bolt go through, so the neck will need to be removed to do this, if that is the case).

While this is the 'ideal' fit for a flathead ring, it is hard to maintain as the tolerances between the rim and ring are very small, and the wood rim is subject to changes in shape because it is wood. Wood is affected by climate changes/heat-cold/moisture, so what starts as a slip fit can turn into a very tight fit. Having the ring very tightly fitted usually means loss of bass response, excess treble overtones, brashness of tone and a loss of power, particularly up the neck. There are always exceptions to every 'rule' in banjo set-up, and some banjos may sound great with a tightly-fitted ring, but what's generally agreed in the banjo set-up world today is that a slip fit is best. Many builders hedge possible changes in the tonering tightness/fit from climate changes by taking a few thousandths off the rim during fitting, so that it is a 'very slip' fit. It can even get to the point that the ring has some small amount of lateral movement from this very slipped fit. Shims can be installed to keep the ring centered if that happens. In general, a slip or very slip fit is better than "too tight."

The third component of the big 3 is the "flange." There are a few types of flanges in use in banjos today, but the two main ones are the two piece and one piece flange designs. They attach to the outer area of the rim, below the tonering. This first picture shows a one piece flange, which is sand cast and stamped out of a mold. 

The other type is actually two pieces: a flat 'plate' of metal with a metal ring sitting on top:


These pictures show the differences, with the main one being the two piece flange has holes present where the hooks holding the head tension pass through the tube and plate. The one piece flange has holes too, just no round tube of metal on top. 

two piece:

One piece below:

 

The flanges both must be well fitted, not being excessively loose or extremely tight and they must be mounted to the rim on center, so there is no off-axis orientation on the rim, once the pot assembly is under full tension and load. The metals of choice for these flanges are brass for the two piece version and brass or Zamac (a zinc alloy, also called 'pot metal') for the one piece. There are always conjectures and opinions in the banjo set-up world about which flange 'sounds better'. I wont get into that debate other than to say each design has its own strengths and weaknesses, tone wise and it is all subjective. The main thing here is to properly fit the flange to your rim. 

These three main components: rim, tonering and flange, make up the heart of your banjo's construction and are the major players in tone and volume.

Well, I hope this article has been enlightening and I encourage you to give me feedback and comments if you like.  Next time, I will look at the banjo's rim in more detail... see you next installment!



12 comments on “Set-up Part 6: Pot Assembly Fit”

banjoman717 Says:
Sunday, January 25, 2015 @7:57:09 AM

I Like this article! The banjo I am working on has a big slip fit . I can move the tone ring from side to side about .030 to .050 . What do you use for shims here?

Banjophobic Says:
Monday, January 26, 2015 @7:54:58 AM

If the 'slop' is excessive, I prefer to use hard maple veneer to build up the whole mating surface under the skirt, to get a perfect slip fit. But some set-up folks use various materials..even tape, to make small shims. I prefer maple veneer, again, for shims. I would place them at 10-2-4-8 o'clock positions to keep the ring centered.

casey050 Says:
Thursday, January 29, 2015 @3:38:53 PM

Yeah, cant wait for your next install. Fascinating reading, it is always good to know everything you can about your chosen instrument and your articles are an excellent source, casey.

banjovet Says:
Thursday, January 29, 2015 @5:07:30 PM

John: Good, informative article.
My only comment would be about the Stelling wedge fit design.
More of the mass of the tone ring is in contact with the rim as opposed to the standard Gibson flathead tone ring. Not a bad thing. But over time, tension from co-ordinator rods can force a rim out of shape slightly.
The same thing can also happen with any rim. But it seems to me that the wedge fit design would be affected much more because of the amount of the tone ring on an out-of-round rim. A Gibson-style flathead ring does not make contact as much and the underneath part would still be flush with the top of the ring. The thin, outside lip could have less contact because of warping but the sound of the regular flat head ring does not depend on total contact like the wedge fit design.
Adjusting the tension of the co-ordinator rods is sometimes used to change the neck angle I prefer to change the bridge height instead and leave the snug heel-to-neck fit alone.
I'd be interested in knowing if co-ordinator rod pressure has been a problem for wedge fit tone rings.

Bullard Says:
Thursday, January 29, 2015 @5:55:40 PM

Great article...thanks!

rockil Says:
Friday, January 30, 2015 @5:46:23 AM

Nice article. I have made a couple of banjos now I prefer to leave a 1/16 to 1/8 gape from the top of the rim to the tone ring but i also do this, on the skirt I slip fit the tone ring to the rim with only 1/8 inches touching the skirt for me this gives me better lows and still maintains my mids and high notes what do you all think of this. one other thing i have found on banjos with a resinator i keep the bottom of the rim at least 1/2 away from the resinator any closer will choke the sound
Vern

rondes1 Says:
Friday, January 30, 2015 @7:50:58 AM

Will you be covering the Nechville design in future articles ?

Slapstick_inc Says:
Friday, January 30, 2015 @1:17:40 PM

I want to address 2 questions. First, I have played a Stelling Golden Cross since 1997, and I've neither had nor have I even heard of coordinator rod pressure causing any problems for wedge fit tone rings. The wood in entirely too hard and thick for that to happen. The second question is in support of the author's view that each banjo set-up will be different. A couple of years ago I changed both my Deering GDL's and my Stelling's tone rings to the carbon fiber tone rings produced in Australia, and I couldn't be happier with the results! Not only are both banjo significantly lighter, but they both have the same tonal qualities and power they did before. The best part is that now I have more control of the volume. It doesn't take much to get full power, and varying degrees of touch are easily discernible. As for slip, the manufacturer indicates that the tone rings has to have a very tight fit. Check out the sound examples on Nickerson's website. Hope this helps!

Banjophobic Says:
Friday, January 30, 2015 @1:52:14 PM

No, I have not seen any issues with the wedge fit design myself, but I don't think Stelling owners tinker as much with the internal components as mastertone and master lone owners do. Of course if one cranks the co rods tight enough on any banjo equipped with them you would damage something. Either the rim shape, the neck heel and or lag bolts.

Banjophobic Says:
Friday, January 30, 2015 @1:53:46 PM

The bottom line for anyone is does your banjo sound good "to you". But no, I am not a fan of the tone bell system. If one must try this, use another rim and do not alter your factory one.

Banjophobic Says:
Friday, January 30, 2015 @1:54:48 PM

No, I won't be covering the Nechville design here, but Tom has tons of info about his designs on his website. It's a very unique take on the classic mastertone pot idea.

Banjophobic Says:
Friday, January 30, 2015 @1:55:32 PM

I appreciate the input everyone. Please keep and comments and question coming.

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