Lowering Insert (Saddle) on Non-Adjustable Bridge

[mandrian]

Hi,

Tried to research this, but could not find answer. One for the luthiers or mathematicians!

Thinking about sanding the bottom of the bone saddle on my Sobell to lower the action marginally. Would take it to repairman, but not available in lockdown.

Basic question is what to you have to take off the bottom of the insert to achieve the desired result at the 12th fret? Do you take twice as much off, that is if you wanted to lower the action by 0.2mm at the 12th fret, you would take off 0.4mm off the insert?

Could just do it by trial and error, but use Thomastik flatwounds which the repairman told me previously did not react well to detunings.

Thanks.

Regards,

[Richard500]

Yes, your geometry is correct. You don’t need to restring to test progress, using a long straightedge will allow you to see how much you’ve dropped (not what the actual string clearance is, because of the slots. And also top deflection under string load.)
If you’ve never sanded or ground bone, be advised, like Proust’s bite of madeline (not mandolin), you will instantly be transported to the dentist’s chair. A fond memory for very few.

[pops1]

Also make sure you keep the bottom of the saddle flat and square with the sides.

[mandrian]

Hi,

Thanks for the replies. Not done it before, but some good guitar videos online. Should be manageable if I go slowly, now that I know approximately how much to take off.


Regards

[rcc56]

I prefer to take the material off the top, and then re-compensate if necessary.
That's because when I was first learning how to do this work, I would lose the square edge if I took it off the bottom, and then would have to take too much more off when I tried to restore the square edge.

[mandrian]

I prefer to take the material off the top, and then re-compensate if necessary.
That's because when I was first learning how to do this work, I would lose the square edge if I took it off the bottom, and then would have to take too much more off when I tried to restore the square edge.

Hi,

I think to modify the top would be much harder to someone of my limited ability as the top is compensated with the bass side being higher. See photo. If that’s the advice I wait until I can take it to the repairman (September if I’m lucky).

Thanks.

Regards

[rcc56]

The job would be easy for me, but I understand. It's important for you to stay within the bounds of your ability and experience.
You can work from the bottom if you can find a way to keep it square.

You might try to clamp the saddle very securely to a block of wood in a vise, and let it over-hang the block by the amount that you want to take off. Then file or sand away material until it is flush with the block.

The problem with this method is that the saddle is so small that it is hard to keep it from slipping while you work on it.

[darylcrisp]

as others have mentioned, be sure to keep the bottom of the saddle as square as possible. when I started doing this sort of thing and did not have the more proper tools that I have now, I used a regular mill file laid flat on a table, then used something(like a wood or metal ruler that has some thickness to it) that laid on top of the file also. I clamped(or held but its harder to hold)the wood/metal ruler to the mill file at the table edge. then I would hold the saddle and pull it in one direction only(to keep from wobbling and rounding the tips or edges as is highly probable if you go back and forth). pull the saddle in one direction, apply some pressure but don't bear down really hard, keep it level and against your "guide"(the clamped ruler). as RCC mentioned above, its hard to work at times due to being so small, but you can do it in a few minutes since its only one, and you don't have a lot to remove.

if this makes no sense, I can post a picture here for you, then it will be immediately understood, but I'm at work and it will be about 8 hours before I can do that.

every so often, clean/brush/blow the dust away from the file(don't suck it in while breathing-wear a mask if you have one, or tie a bandana around your nose/mouth as you work). take your time. check it in the bridge after about every 10-15 pulls, stop short as to what you think you want it to be, string up and put tension on and check it. all this is slow methodical work, but it will result in a good job.

put on some good music(slower the better so it keeps you in a slow rhythm), and take your time.
tell us how it turns out.

d

[ajh]

Been struggling to stay away from this one. This is all premised on the old mating surfaces being perfectly matched. Perfectly. One imperfection in either mating surface throws the whole thing off. YMMV.

[Richard500]

Mandrian, I’m rethinking this. As I’m not in the trade, I may be wrong, but here goes: It could be much easier to lower the slot in the wood part than to accurately deal with the bone saddle. The only one I have is just about the thickness of a backsaw, so the slot itself would maintain squareness and the sawblade is definitely flat enough. If the slot is wider, whatever else you have on hand that could abrade or shave the wood will work. Wood cuts a bunch easier than bone, and if you screw up, easy enough to repair. Much easier to hold that rather large bridge too, if you have a little vise, or a small clamp. The small adjustment you mention may only be a few strokes, so be careful. You can use the bone part as a gauge, drawing a pencil line at the existing wood/bone junction and a second line the desired lowering amount above it. Take a stroke, put the saddle in, take another, until the upper pencil line is even with the wood.
Luthier guys - a boneheaded idea?

[rcc56]

This is a blind end saddle slot, so deepening the slot is not practical in this case.

If it came to me, I would take it off the top. To me, it's the easiest and most practical solution.

Mandrian, it's not as hard to so as you might think. 5 minutes to take it down with a good sharp file or sanding block, a few strokes to bevel the edges, than some fine sandpaper [400 grit in the US] to polish the surface. If re-compensating it is what's worrying you, you can let that part wait until your luthier's shop re-opens.

[mandrian]

Hi,

Thanks for all the advice, much appreciated.

I managed the task fairly successfully. I went with the sanding the bottom of the insert route. The slanting top and compensation at the A string put adjustment there out of my comfort range.

I used a marble pastry board and taped 320 grit sandpaper to that. On top of that I then clamped a large ruler that had a nice straightedge. Used this to hold the insert upright when sanding along it. I had marked the bottom of the insert with a felt pen to ensure I was sanding the bottom of the insert flat. I had also marked the side of the bottom of the insert to indicate the 0.5mm that I needed to take off. This latter marking was probably the most difficult part of the job. Working with such small measurements is not easy.

I was paranoid about taking too much off and in the end when I refitted to bridge the action at the 12th fret had reduced the action from 1.75mm to 1.6mm rather than the 1.5mm I was aiming for. The good news was that I managed to save the existing the existing Thomastik strings and hopefully they have not suffered any adverse reaction to the slackening off to allow removal of the bridge.

The mandolin is playing noticeably easier on the G string. I think the wide nut on the Sobell ( 1 7/16 inches ) makes playing consecutive notes on the G string less easy if the action is too high there. That was where I had noticed the high action, the other strings had always been fine.

Anyway will play away and if it still seems a little high, I’ll take it down to 1.5mm when I next change the strings, but first impressions are that it will be fine.


Regards,