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Double Bass Adjuster screw thread question

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draftsmann

Apr 30, 2006
272
3
60
Malta (Europe) and Britain
#1
I have a pair of aluminium adjusters that I intend fitting to a bridge I'm using for my EUB project. They were bought from the US, and so will, I am sure, have an Imperial thread. I need to beg, borrow or (most probably) buy a tap to thread the bridge legs. Remember I am on the metric side of the Atlantic, and even Britain now seems to use the Napoleonic weights and measures for everything!

I have one of Bob Gollihur's adjustable bridges on my double bass, and the threads on the adjusters I'm using for the project are 100% identical.

Therefore, this ought (hopefully) to be a straightforward question - can anyone tell me what thread the Gollihur adjusters have, so that I can make sure I search on eBay for exactly the right tap?

Many thanks,

Adrian :)
 
#5
I might urge a bit of caution with the Whitworth threads. The pitch angle and some other microscopic details are different:

http://en.wikipedia.org/wiki/British_Standard_Whitworth

I can't fathom how this might be a problem, but instead I recommend threading some similar wood and making sure the screw fits nicely. You'd hate to split your bridge. Also, if you haven't tapped wood before, this will give you some practice before you try on your bridge. If the machinist has a complete shop, he will hopefully offer you both tapered and "bottoming" taps so you can fully thread the hole without having to drill it deeper than necessary. And if it's a really top flight shop, see if he has two-flute taps rather than three or four.

Note that I am not a luthier, but have done a lot of machining.
 
#7
fdeck said:
I might urge a bit of caution with the Whitworth threads. The pitch angle and some other microscopic details are different:...
Click to expand...
True. They are not identical. But as you said, it shouldn't be a problem here. I personally wouldn't worry about overdrilling the hole depth (there's evidence that it might actually be a good thing acoustically), but yes, practice tapping a piece of scrap maple in the same grain direction first. The normal tap drill size is a #8, but I have no idea what luthiers prefer!
 
#8
wayne holmes said:
Adrian, go to TOOLRAGE.com - you can order one for less than $3 -I think- then the handle is another $2 or $3 dollars--good luck --Most likely Bob would have 1/4-20- you could take it to any hardware store and match it up with a 1/4-20 to make sure.
Click to expand...
I think Draftsmann means Malta, like in the Mediterranean Sea, not Malta Illinois. Hardware stores in Malta are unlikely to stock American tap sizes:D
 
#9
Yes there is--right next to McDonalds

BTW-overdrilling a hole for an adjuster stem - well - there "might" be some evidence that it is a good thing acoustically, but it is not good when you want to turn the adjusters. Robo- you impressed me that you know that there is Malta, IL, but to suggest that a void will help the sound when the sound depends on vibration... - stick to Geography!
 
#10
wayne holmes said:
Yes there is--right next to McDonalds

BTW-overdrilling a hole for an adjuster stem - well - there "might" be some evidence that it is a good thing acoustically, but it is not good when you want to turn the adjusters. Robo- you impressed me that you know that there is Malta, IL, but to suggest that a void will help the sound when the sound depends on vibration... - stick to Geography!
Click to expand...

I cite Molly Kay's hollow bridge experiments. Although more research is needed, removing mass from the center of the leg would make it lighter, while not impacting stiffness very much - especially when we're only talking a 1/4" hole. Also, I do not see how it would affect turning of the adjusters at all.
 
#11
Robo-it will affect the turning of the adjusters because all the tension of the strings and the top part of the bridge are in solid contact with the adjusters.(I realize that some pick-ups require solid contact). Maybe it's my personal preference to leave a very small space between the upper part of the leg and the adjuster and 5 to 7 mm of thread showing. It makes it easy to make adjustments without loosening the strings very much. If the bridge top sits on the adjuster over a period of time, as you know, they can stick. If one is not aware of this and forces it loose, then there is a chance of damage to the top or to the bridge. Just mho. I am not married to these spaces and no void in the legs and I am open to anything that improves the vibration. Maybe others have opinions on this.
 
#12
Wayne,
There are different ways to mount adjusters, but I believe the most standard way is to drill the holes first from the bottom, cut the bridge, and then thread the upper part of the bridge legs. This leaves the weight on the thread, and on the bottom disc surface. If you want to leave a gap, and have the bottom of the pin take the weight, then it will certainly turn more easily, but then you need a flat bottom hole, it will be less stable, and how do you drill straight holes? With your setup the bridge would be more flexible, which might be a good thing, but it seems like it would be a pretty difficult installation. Can you describe it further?
 
#14
draftsmann said:
Blimey! I didn't realise I'd opened such a can of worms!

I have read that some luthiers prefer to install the adjusters thread-down. I'd be curious to know why.

On most 2-post guitar bridges (both tune-o-matic and wooden jazz guitar bridges) it is the lower part that is threaded.

Adrian
Click to expand...

I stuck my foot in my mouth. I just noticed that the adjusters on one of my basses are thread down, and on the other they are thread up. Interesting, since I'm pretty sure the same guy did both bridges! With the standard approach, i.e. loading the weight on the disc surface, I don't think up or down matters much. With Wayne's idea of offsetting the disc so that the end of the pin carries the weight, then you would certainly want to go with "thread down". I stand my ground, though, in saying that doing this would be a tricky installation, and might have stability issues.

I just play the damn thing. Where's Arnold anyway?
 
#17
Robo, I use the steps in Threagers book, in part, when I install adjusters. I built a jig that I put the the bridge in which helps keep the bridge in the correct angle position on my drill press. I start with a very small bit. With the bridge upside down, level and plumb with the vertical lines on the bridge that I drew while I had the bridge in place on the top of the bass. (I made a 1" by 2" board with feet on each end that I place on the top plate lined up with the f notches.) Using a small square I draw vertical lines centered in the feet. After I drill the holes a little less than the overall length of the adjuster stems(allowing for the thickness of the adjuster wheel) , I cut the feet off 3/4 the distance up the feet. If for some reason the bit didn't make it to the middle of the upper part of the bridge foot, I can drill again to center it, since I only have a hole no more than 2mm. If I have to redrill, then I put the feet back in place in a jig that keeps it all together and in correct position. It is very rare that I have to redrill, but I always start with a small bit and increase the bit size until I reach the 1/4" dia. for the stems. For the upper part where I leave a hair width space, I reach it with a hand drill.

But, I have found an easier way- I get most of my bridges from Connelly- they ship them to John Sprague at CSC Products and John has his guy to install their wood adjusters with aluminum stems. They do a great job and leave a space above the top of the foot which I finish. It saves me time and money. I think that they only charge like $10 to install their adjuster and the adjusters are $16. Hope this helps Robo- hope it makes sense.
 
#19
Thread down question---
I had a bass come in my shop once that had a bridge with one adjuster thread down and the other up. Not really-I just made that up.

I think that most of us like them down because- well, I'll just speak for myself. I like the thread down because(1)if the female thread becomes stripped, then it is a matter of replacing the foot(2)it's more logical for my simple mind to turn the wheel clockwise into the foot to lower the bridge and to back it out to raise the bridge(3)My mentor did it thread down(4) the stem is longer on the thread part which I would think would give the feet more stability(5) some do this- I don't, but some have the thread down, and thus the top part of the stem can be intalled loose and it makes fitting the feet a little faster but not better(Maybe it's my house building experience speaking, but starting at the bottom and building up is the way it's done, even though we have the anchor bolts with threads up so we can fasten the foundation seal to the block foundation)(6)ah, and probably the best reason of all.., When I am doing the final fitting of the top of the bridge, branding it, or putting my initial in it, I can just left the top part of the bridge off the feet leaving them in their place on the top of the bass. Then I can put the top part of the bridge, by itself, in a vice and work on it. This prevents the dropping of the feet which can damage them. (7) I just enjoy screwing them into the feet more that screwing them into the upper part.(8) you have more room(length) in the feet. I think that I have seen some uppers that were cut off, otherwise, they would have been screwed through the roof. I can't think of any other reasons why I put the threads down, but if I do, I will share them with you. Mr. Draftman -- you can ask some very good questions- could you make the next one a little easier?
 

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