Showing posts with label Bridges. Show all posts
Showing posts with label Bridges. Show all posts

Sunday, March 29, 2015

Positioning the Bridge

There are several handy gauges and gadgets available that help the guitar builder determine the position of the bridge, but the method I've adopted works well enough that I haven't felt the need to try them out. To give credit where it's due, I believe I first saw the method I'm about to describe on Matt Mustapick's now defunct and sorely missed workshop blog.


On this particular Port Orford Cedar soundboard, the grain is incredibly fine and the centre join is extremely difficult to detect. I've lightly pencilled a line along the join to highlight it, then taped a sheet of graph paper to the soundboard with the centre grid line aligned with the pencil line. With the neck securely attached, I measure from the nut end of the fret board and mark the notional scale length on the graph paper - it's the longer of the three marks in the picture below.


I make a second mark a little under four millimetres further along the centreline to allow adequately for compensation, acknowledging that with wider saddles gaining favour among many builders these days - myself included - there's plenty of scope for fine-tuning of intonation once the guitar is strung up.

I measure the distance from the centre of  the saddle slot to the front of the bridge at its mid-point, then measure back from the compensated mark on the paper towards the nut by the same distance, placing a third mark on the graph paper to represent the front edge of the bridge. I extend this mark outwards in either direction.

The underside of the bridge has been sanded to conform to the dome of the soundboard, initally using a domed platform to which I've stuck some 120-grit sandpaper, and then, by way of fine tuning, by taping some 240-grit sandpaper to the soundboard itself then moving the bridge to and fro across the sandpaper until the white pen marks I covered the bridge underside with have been removed.

I insert a clamp through the soundhole, positioning a piece of MDF that conforms to the bridge plate outline inside the body and over the bridge plate as I do so. With the bridge clamped lightly in place over the graph paper, I wriggle it into position, using my third mark to align the bridge's front edge at its mid-point, and the grid lines that run perpendicular to the soundboard centreline to align the bridge's front corners. The clamp is tightened, and I drill through the first and sixth bridge pin holes so that the bridge can be accurately pinned in position when the time comes to permanently attach it to the soundboard. The MDF inside the body prevents chipping when the point of the drill bit exits the bridge plate.


Removing the bridge, I replace the graph paper with a thin, low-tack adhesive film known as frisket film. I give the film a light scuff with fine sandpaper so it's easier to mark. I pin the bridge into position through the first and sixth bridge pin holes, and lightly mark the bridge outline onto the frisket. I remove the bridge and, using a craft knife, carefully cut through the frisket around 3/16" inboard of the pencil line, being careful not to cut into the top wood. I can then peel off the frisket that sits outside the scored line. I've found that by leaving a generous margin between the edge of the remaining film and the actual bridge outline, it's much easier to level-sand and buff the finish in the area of the bridge perimeter, bearing in mind that the frisket film is removed only when the finish has been buffed and I'm ready to remove the remainder of the finish within the bridge outline prior to permanently gluing the bridge.



In the interests of a neatness, I'll leave around 1/16" of finish inboard of the bridge outline, and rout a ledge fractionally over 1/16" wide around the underside of the bridge perimeter to a depth equivalent to the target finish thickness. To do so, I clamp my laminate trimmer to a MDF platform that's been domed to replicate the curvature of my soundboard. To prepare the MDF, I laminated two 3mm layers of MDF in my go-bar deck using a 25' radius dish as a base, then drilled a hole in the centre large enough to accept my chosen router bit. As previously described, the underside of the bridge has been sanded to conform to the dome of the soundboard before I rout the ledge.

As always, I'm open to suggestions and I welcome any comments.

Cheers
Pete

Sunday, September 25, 2011

Saddle Slots and Bridge Pin Holes

Although I'm nowhere near ready to make use of them, I've spent some time lately working on the bridges destined for the two guitars I have under construction for no other reason than it's a job I can devote short periods of time to in a spare moment, or treat as light relief between more demanding or time-consuming tasks.



To facilitate drilling of the bridge pin holes in a perfectly straight line, parallel to the saddle slot I've previously routed out, I glue a small wooden block to the front edge of the bridge blank on the treble end prior to cutting the bridge outline. I trim the block carefully using my disk sander so that as the bridge is slid along a makeshift fence attached to my bench drill's table, the centre of each hole will be equidistant from the slot. Having guaranteed alignment in that direction, I need only take care then to align the bit with the string spacings marked on the bridge as I prepare to drill each hole (a white ball-point pen is a godsend when marking dark woods like this ebony). A brad-point bit makes accurate alignment much easier and produces a perfectly clean hole provided it breaks out through the lower surface of the bridge into a backing board of MDF or scrap timber.


With the bridge pin holes drilled parallel to the line of the saddle, there's a consistent break angle for the strings as they pass over it. While some reason that this guarantees an equal downward string pressure along the saddle's length, the fact that string gauges and tensions vary from string to string casts doubt on the validity of that argument. A slight increase in the saddle's height towards its centre and the potential use of radically altered tunings such as "Orkney" tuning (CGDGCD low to highcomplicate the matter still further. Whatever its other merits though, it's a neat look and I'm assured of adequate downward pressure on the saddle and therefore good transference of the strings' energy to the soundboard.


It seems there's no hard and fast rule where bridge weights are concerned; popular wisdom has values falling within a fairly wide range. Given that there are a multitude of factors influencing the guitar's sound, I somehow doubt I'll ever build enough instruments to draw any meaningful conclusions where small variations in bridge weight are concerned; there are so many other variables likely to have a more significant and measurable effect should I feel the urge to experiment. That being the case, I'm happy to aim for a more or less consistent bridge weight from one guitar to the next. With ebony bridges such as these, I shoot for a weight of between 30 and 35 grams, making use of a cheap digital gram scale as I shape and shave the bridge wings to their final thickness.

Useful links:
Routing the Saddle Slot

Cheers
Pete

Wednesday, November 17, 2010

Routing the Saddle Slot

None of my jigs are particularly attractive, polished or professional, but they're certainly functional and take the guesswork and inconsistency out of many tasks.

Routing a slot in the bridge blank into which the bone saddle will be fitted is one job I've given over to the ugly but effective jig pictured below.


The Macassar ebony bridge blank is affixed to the base of the jig using double-sided tape (what would I do without it?); the blank's positioning isn't really critical. The blank is bedded down with a few taps from a soft-faced hammer which more than adequately secures it for the duration of the process - the tape is tenacious stuff!


As you can see, two adjustable guides sit atop the jig and, with the plunge router riding snugly between them and the router bit sitting just proud of the wood surface, are adjusted so that the bit will accurately track the line of the saddle marked on the bridge blank.

I rout the slot in several passes, each one a little deeper than the last, until I hit the depth stop set on the router body.

I'm tempted to add sliding stops to either end of the jig to limit the router's travel, but for now, a couple of strategically positioned panel pins serve this purpose well enough, which perhaps explains why I haven't been in any great hurry to make that modification - I'd rather be building guitars than building jigs.


I'll show the next stage of the process in a future post.

Cheers
Pete

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