Double Bass Fill and resize bridge adjusters?

[Jeremy Darrow]

Is it possible to fill and resize an existing bridge for a different adjuster? I've got a bridge that I'd prefer to use 1/4-20 adjusters on. Is this a thing? Or should I replace the bridge?

 

[robobass]

Anything is possible if you have the proper tools and are willing to invest the time I think that filling and re-tapping adjuster holes is fairly common, though not free of problems. For example, you can just glue in a hardware store hardwood dowel, but then the grain direction and material is wrong. If you want to fill with a maple dowel, matching the grain direction, then you need to cut cross-grain dowels on a lathe. This isn't exactly easy, as they tend to break off during turning if you even play your music too loud. You can fill the holes with a thin mixing but very hard-setting epoxy. This is not difficult, but the wood purists probably won't like it. Last is using a metal thread insert. Probably the easiest of all, and maybe the most common, but I personally hate it. I will agree to have an aluminum adjuster in my bridge, but absolutely no steel or brass

 

[Ortsom]

Yes, that should be possible. I have never done it, and I suspect that drilling out the old thread, gluing in a dowel & re-threading will not work: the direction of the grain is wrong then, and you'll get no strength. You might have to resort to epoxy and/or a sleeve, or drill & cutting oversize thread and use a helicoil.

 

[Ortsom]

Sorry Rob, our posts crossed (but we say similar).

 

[Jeremy Darrow]

Thank you both, that's really helpful.

 

[robobass]

I suppose using a helicoil isn't the end of the world. I just get frustrated when certain people I know insist upon using a brass threaded insert instead of tapping a 1/4"-20 thread directly into the maple. Totally unnecessary. If your adjusters don't turn nicely, then maybe it's your jig and technique that you should look at. Don't make every bass you work on suffer from an over-weighted bridge because you are too cheap to buy a proper drill press! Arrgh! Sorry for the rant, everyone

 

[neilG]

How about an insert of something man-made? Delrin comes to mind as a tough and easy machining material.

 

[robobass]

Delrin would be ideal as far as ease of turning. Might affect the sound, though. The purity police would certainly make this claim, but I suppose you are breaking the rules already by just talking about adjusters

 

[Chris Fitzgerald]

Back in the day, Nick Lloyd did this to the bridge on my LaScala so I could put a Full Circle pickup on it. I know he used a dowel, but not sure what it was made of. He managed to do it in one day so I only had to make one trip up to Cincy. Most of that was drying time, IIRC. That same bridge is still on the bass.

 

[vejesse]

If you can find some I would suggest boxwood for your dowel plug. This wood is so tough that you can usually drill and tap it without paying attention to the grain orientation.

 

[neilG]

Delrin would be ideal as far as ease of turning. Might affect the sound, though. The purity police would certainly make this claim, but I suppose you are breaking the rules already by just talking about adjusters

There are rules?

 

[keiranohara]

Delrin doesnt play well with adhesives. Another concern to keep aware of: once the holes are filled it can be difficult to drill the pilot for the threaded inline (co-axial) with the hole on the opposing side of the adjuster. I have in the past filled all holes with fine grained hardwood and superglued the feet to the bridge. Then drilled and tapped as though installing the adjusters on a new bridge (you end up losing about a bandsaws kerf in bridge height, but hey, that's what the adj are for).

 

[neilG]

There are adhesives that will work with Delrin, other acetal products and plastics in general. I know Loctite makes one.

 

[robobass]

If I were to tread into delrin inserts I would want the outsides to be ribbed, so you aren't relying too much on adhesive. I haven't seen such a thing, but it would be pretty easy for someone like me to make. Should I do a short run?

 

[vejesse]

Here's another solution: drill out and tap the existing holes for a 3/8-16 or maybe M10 1.5 thread. Then bond aluminum (or brass or whatever material you choose) turned adapters with both a corresponding inside and outside thread to the bridge height adjusters. I've done this for the last ten years with complete success. With the adapters you can match any adjuster thread to any larger thread in the bridge. And you don't have to temporarily glue the feet back onto the bridge doing it this way, just start the drill from the inside. For accuracy I use a drill press vise with the ability to change angles so I can square up the holes with the drill press spindle. Note: if you do temporarily superglue the feet to the top part of the bridge to improve the fit of the bridge to the top for instance, soaking the joint with superglue remover will dissolve the glue.

It works a lot better to use a carbide tipped moldmaking drill for this rather than a normal twist drill which will wander around at the start of the cut. A straight flute drill works too. Or, start the hole with a step drill to center and get the size right, then finish with a normal drill. I realize all this may be more than the "home shop mechanic" wants to get into - just putting it out there as possibilities.

 

[neilG]

If I were to tread into delrin inserts I would want the outsides to be ribbed, so you aren't relying too much on adhesive. I haven't seen such a thing, but it would be pretty easy for someone like me to make. Should I do a short run?

Now that I've suggested Delrin, I have a good argument against it. Even with the proper glue, you're only going to get a good mechanical bond between Delrin and wood, which would be okay if the insert is big enough so that all the new threads are in the Delrin, with good support. Going from 1/4" to 3/8" doesn't leave much Delrin left after the new threads are cut. Can you put a 1/2" hole in a bridge foot with any confidence? I don't know, but it seems like one's energy might be put to better use making 3/8" face-grain dowels. With wood there can be a much stronger bond and the repair will be more monolithic, if that's the right word.

Really the easiest thing is for you to make and sell replacement adjusters that have the next size thread.

 

[vejesse]

Right, and for 1/4-20 the next size up that would clean everything up would be 3/8 - 16, as that tap drill is 5/16". And adjusters with that thread are already commercially available.

 

[neilG]

Right, and for 1/4-20 the next size up that would clean everything up would be 3/8 - 16, as that tap drill is 5/16". And adjusters with that thread are already commercially available.

Did not know that. Found some in 7/16 -14 also, which would use a letter U or 23/64 pilot. In Delrin, too.

 

[robobass]

You need only go up to 5/16. The pilot hole is slightly over 1/4".

 

[robobass]

Really the easiest thing is for you to make and sell replacement adjusters that have the next size thread.

Right, except that I think most people consider 1/4-20 ideal, at least for aluminum adjusters. I imagine someone has some 5/16 adjusters in a drawer somewhere. I doubt they are ever used except in this situation.

Note: if you do temporarily glue the feet with superglue to the top part of the bridge to refit it, soaking the joint with superglue remover will dissolve the glue.

Why not just use hide glue?