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Bridge slot cutting jig? (Read 4184 times)
Bob_C
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Bridge slot cutting jig?
May 12th, 2009 at 11:38am
 
Hi gang,    I'm just finishing up a mahogany tenor uke for my son (my first build! Cool). I've reached the point where I need to cut the compensated slot in the unshaped bridge blank. Any hints or jig ideas to help ensure a good job on this step?

                                  Thanks!    Bob
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Matt Blacka
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MAKAIO Ukuleles Australia

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Re: Bridge slot cutting jig?
Reply #1 - May 12th, 2009 at 3:08pm
 
Hi Bob,

First of all, I would say to be careful of using the angled bridge slot. I set up a compensation testing jig a little while ago and tested Aquila strings (both high and low G) for the compensation requirements. For both low G and high G string sets, a square saddle was better than the angled one, with the overall string length between nut face and saddle peak of approximately 434 mm the best. This is damn handy if one wants to change between the two as well. Other posts on this forum have indicated the exact same results. Not sure how this goes for other string brands though.

I'll try to explain my jig as best possible (it is built for cutting a square saddle slot).

Take a piece of ply or MDF about 11 mm thick (thicker than your bridge blank by about 1 mm, and not more than about 4 mm) and 200 mm by 200 mm square.

Draw the shape of your bridge blank onto the piece of ply. Now drill a hole through the ply inside the shape you have drawn. Get a jigsaw (or some type of hand saw) and cut out the rest of the bridge blank shape from your piece of ply. Be careful to cut no wider than the inside of your line. What you want to end up with is a window in your piece of ply that is a fraction smaller all round then your bridge blank.

Now take out a file, and file the edges of the window so that your your bridge blank fits snuggly inside. Ok, now make some marks on either end of the window where you would like the saddle slot to run out (I think this is about 7mm back from the front edge of the bridge, but you should check). Now get your router/dremel, put the appropriate sized bit into it, and measure the distance from the square edge guide to the edge of the cutter bit. Go back to your piece of ply and mark this same distance away from the front edge of your saddle location marks. Take a piece of stright edged wood, and screw it to your piece of ply so that its straight edge is along the guide offset line.
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Matt Blacka
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Re: Bridge slot cutting jig?
Reply #2 - May 12th, 2009 at 3:13pm
 
Ok, now clamp your piece of ply to a flat bench and drop your bridge blank into the window. Push it down so that it seats against the bench. get your router and sit it on the piece of ply with the edge guide against the straight piece of wood (using the piece of wood like a guide fence). The cutter bit should be lined up with where you want the saddle slot to be.

Check everything is lined up ok. Adjust the jig if necesarry. Set the depth of your plunge and/or cutter bit to give you the appropriate saddle slot depth, and slot away.

My piece of wood has multiple holes for different bridges, and uses a single fence guide for all (windows are just offset differently from the guide). If you want to go with an angled saddle, you will have to angle the wooden guide block accordingly.

Matt
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Bob_C
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Re: Bridge slot cutting jig?
Reply #3 - May 12th, 2009 at 3:26pm
 
Hi Matt, Thanks for all your effort replying to my question! As a first time builder do you think it advisable that I just go with the straight slot on htis uke? I'm willing to go either route but if a straight slot will get me a playable well intonated uke then I'm game.

                                    Bob
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Uncle_Bill
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Re: Bridge slot cutting jig?
Reply #4 - May 12th, 2009 at 4:59pm
 
Aloha Kakou - I  have just finished my first build (a modified low g tenor) and I also used the compensated slot in the bridge. I strung it up with new Aquila strings and to my horror there was a terrible buzz on the 1st string. I checked everything, all the measurments, fret height, etc. and nothing would stop it. Finally I decided to try some different strings. I switched to low g Kamaka strings and the buzz went away. I measured both string diameters and there is a slight difference in size. All I can think of what happened is that  the length of the sine wave created when the string is plucked is a different shape and is creating the buzz off some of the frets. (I could hold down the string at different fret locations and the buzz would disappear.) That is all my meagher mind could come up with. Obviously form now on, I will be using Kamaka strings on this 'Ukulele!
Good luck on your build!   Uncle Bill
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Matt Blacka
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MAKAIO Ukuleles Australia

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Re: Bridge slot cutting jig?
Reply #5 - May 12th, 2009 at 6:21pm
 
Bob, I have posted some data back in the other section of the forum about compensation, so you should be able to get something from that. Hopefully others will chime in with some data of their own.

Uncle Bill, if the buzz doesn't happen when the open strings are played, but does happen on different frets, it's likely to be uneven frets causing the troubles. Different string tension (between different string brands) will allow the strings to vibrate differently, and this could be why you've found this problem with some string brands and not others. You could check with a short straight edge to see how flat your frets are. Maybe level them off and re-crown them if you have the tools, or go for a higher saddle to give more string clearance above the frets. Or just keep the strings that work.....

Matt
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jack
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Re: Bridge slot cutting jig?
Reply #6 - May 28th, 2009 at 2:04am
 
Bob,
I make all of my own bridges for guitars and ukes.  While the bridge is still in rough shape, I usually cut the bridge slot, by simply runnung it through a table saw, with a plywood blade (1/8" wide and square cut) which does not go all the way through the material, then shape bridge around slot.  For a compensated saddle (guitar) I just add a small wedge with tape to the edge of the rough blank, and run it through the same saw.  The only exception is on my Kasha Baritone, I use a jig to cut the slot with a 1/8" downcut bit.
Jack
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