Please remember to set your cutting depth at least somewhat close to the actual thickness of the finish.
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| Author: | David Collins [ Tue Dec 22, 2015 8:10 pm ] |
| Post subject: | For those who route finish off for their bridge footprints |
Please remember to set your cutting depth at least somewhat close to the actual thickness of the finish. Attachment: image.jpeg Attachment: image.jpeg
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| Author: | Alain Lambert [ Tue Dec 22, 2015 8:15 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Ouch! |
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| Author: | Alex Kleon [ Tue Dec 22, 2015 8:43 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Man, that's some thick finish! Alex |
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| Author: | DannyV [ Tue Dec 22, 2015 8:52 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I thought the dado was an acceptable wood joint. |
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| Author: | meddlingfool [ Tue Dec 22, 2015 8:56 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Yet it's amazing how long they'll stick like that... |
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| Author: | jack [ Tue Dec 22, 2015 9:26 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
So, is there a repair for this, short of replacing the top, David? |
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| Author: | David Collins [ Tue Dec 22, 2015 11:42 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
jack wrote: So, is there a repair for this, short of replacing the top, David? There are a few options, one being to inlay spruce. This of course adds cost (which was not in the owner's budget for this instrument), still does not eliminate the stress riser from the cut end grain, and since it still leaves possibility for the patch joint to be compromised in heating during future bridge reglues, the benefits can be debatable. The other alternative is to rabbit the bridge (assuming it is thick enough to allow for this), and inlay it in to the route to create good wood to wood contact. This is the method we chose for this guitar, and although much deeper than we prefer to usually do this, it should make for a solid joint that will last a few decades at least. When it does come up again (all bridge joints will come apart eventually) it will of course be a bit more challenging to get apart, and may require further repairs. Given the circumstances though, rabbiting and inlaying the bridge was the most appropriately reliable course for this particular case. |
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| Author: | Shaw [ Wed Dec 23, 2015 2:49 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Bondo! Sent from my HTC Desire 626s using Tapatalk |
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| Author: | TonyKarol [ Wed Dec 23, 2015 4:23 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
yeah .. and try to get a little closer to the bridge outline ... no wonder it lifted ... I have had to reglue a couple newer Martins lately and was astonished how much finish they left under there ... again, not surprised they lifted either .. |
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| Author: | Hesh [ Wed Dec 23, 2015 4:45 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
On some of the worst examples that I've seen I've calculated over 40% of the bridge foot print having finish under it.... And... since most don't rabbit the ledge on their bridges there is also the issue of spanning the ledge where the finish ends and the wood is exposed. For example if the finish is .003 - .005" thick as the bridge is smashed down with no rabbit there won't be any real wood-to-wood contact at the finish ledge but some distance inward further reducing the available bridge foot print that could have been used for a better joint and wasn't.... Maybe though we all have it wrong and instead we should just cut a bridge shaped hole in our tops and make our bridges with an integral bridge plate installed from the underside of the top.....  Kidding of course.... kind of..... I like the bondo idea too, tone bondo!!!!
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| Author: | Barry Daniels [ Wed Dec 23, 2015 5:34 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Maybe we should all go back to Gibson style plastic bridges installed with lots of bolts. Those worked great! |
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| Author: | Hesh [ Thu Dec 24, 2015 7:39 am ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Barry Daniels wrote: Maybe we should all go back to Gibson style plastic bridges installed with lots of bolts. Those worked great! Oh the horror....... Happy Holidays Barry!!!! |
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| Author: | DennisK [ Thu Dec 24, 2015 9:41 am ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Well it is called a bridge... it's supposed to span over a valley of some sort, isn't it? 
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| Author: | Tai Fu [ Sat Dec 26, 2015 12:40 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I used to rabbet under the bridge but I found that I could not really do so accurately (it would create an unsightly gap, while structurally sound it causes customers to panic). So instead of doing that I would basically score around the actual bridge footprint (I believe it to be the traditional method) and simply glue the bridge down without trying to create a rabbet. Makers such as Taylor and Collings have highly precise CNC equipment that they can take exactly .003" off the edge of the bridge so that when glued on it will not have any gap, but the rest of us using router jigs will have a variance of at least .005" which is not acceptable. The underside of the bridge must be contoured to match the top. I don't even try to route the finish off anymore, but I would use a router to start the cut (because finish can be tougher than you think, it is hard to scrap it off if it's unbroken), and once the finish broke through, they can be chipped off until I reach the scored edge. For polyester/polyurethane I must score harder to break through the thick finish but I actually find them easier to deal with than lacquer, they chip off the undercoat like glass. The undercoat must be scrapped through. In the OP's circumstance I think the best way to proceed is a spruce patch over the old rabbet. If the OP made the mistake then he should fix it at no cost to the customer, but if it's the customer's mistake (or done by another unqualified person) then it is a different story. The problem with making such a pronounced rabbet is how do you make sure there's no .005" gap under the rabbet (enough to stick paper through it, and cause panic), but also in the future bridge removal will be nearly impossible without damage because the rabbet will prevent you from slipping any tool between the joint. |
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| Author: | JasonM [ Sat Dec 26, 2015 3:45 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
That looks like an old guitar. How long did that hold for? How about a view of the headstock? |
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| Author: | David Collins [ Sat Dec 26, 2015 4:39 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
bad post - iPhone emoticons apparently wreak havoc in this software. |
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| Author: | David Collins [ Sat Dec 26, 2015 4:41 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Hey Jason - sorry, no headstock shots. This model is over 20 years old, so it lasted a very surprising amount of time for such a bad joint. We do have friends with the company that made it, and since this problem existed for a relatively brief period and has been long since corrected, I'm going to avoid naming names that could tarnish their public image (I actually love the guitars made by this maker today).Tai, our tools and methods must differ substantially, as we actually have no problem achieving and verifying a tight wood to wood joint with the rabbeting method, while leaving no gap around the perimeter where the ledge rests snugly on the finish all around. I am strongly averse to cutting full footprints out of the finish for reasons too numerous to go in to here, and would hope someday to be able to look back at this method as one long abandoned. As to our choice of methods, considering all aspects and circumstances of the instrument and client, I feel quite comfortable in authoritatively stating that our route to solution was the most appropriate and ideal. |
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| Author: | Tai Fu [ Sat Dec 26, 2015 7:39 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I'm not sure what you use (again without CNC machines) but I used a router jig. It doesn't account for the radius under the bridge and the variance tends to be at the corner of the bridge, a place that is always noticed by anyone paying attention. When I glue bridges I have to pay particular attention to the wing corners because that area has the highest tendency to lift up compared to any other area of the bridge. |
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| Author: | David Collins [ Sat Dec 26, 2015 9:12 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Not clear if you're referring to a router jig to cut a rabbet on the bridge, or finish from the top. We do all our top prep with chisels, blades, and scrapers, no routing (though flex base templates can be made to account for top radius and deliver a consistent depth all around). For bridge rabbets we do use a rotary cutter (dremel) with a radiused base allowing us to cut a very consistent depth rabbet after the bridge base has been radiused to match the top. Then if there are any inconsistencies in depth of the pocket from prior damage or work, we can easily cut the depth of the rabbet a few thousandths over, and selectively scrape the base until the perimeter sets down flush all the way around. Very accurate fitting, and with practice can easily be done in just a few minutes. |
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| Author: | Ken Jones [ Sat Dec 26, 2015 9:27 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
David, I'd actually be very interested as to why you're averse to scraping the full bridge footprint. Though it's much more tedious and more work, I feel strongly about scraping the full footprint. I'd say 90% of lifted bridges we reglue are let-in. I've seen many Martins and several Collings with a surprisingly large lip of finish under the bridge. Many different boutique builders as well. I can count on one hand how many of those bridges were rabbeted to receive the finish lip. Very rarely have I seen this done. Sent from my iPhone using Tapatalk |
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| Author: | Pat Hawley [ Sat Dec 26, 2015 11:11 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I question the original premise of this post, i.e. that you have to have the depth close to the the thickness of the finish. Assuming it was done right with only a small amount of finish under the bridge and the bridge rabbeted correctly so there was good contact between the gluing surfaces, why would a pocket with a little depth be any weaker than a pocket that was close to the thickness of the finish? Pat |
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| Author: | david farmer [ Sat Dec 26, 2015 11:23 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I am with you Ken, I cut as small a rabbet as I can muster on pristine instruments and warranty work because that is what the customers and companies are looking for but I don't like it. On everything I can, I stick the whole dam thing down. I would be interested in any reasons, other than aesthetic, for not clearing the full bridge footprint. If more bridge weight is desired, why not put it in the service of strength and longevity rather than an un-attached, decorative trim ring? I see this as a negative effect from the arms race between manufacturers for bridge installations that are fast and risk free, but visually perfect. Some manufacturers supply replacement bridges with a rabbet that makes me downright uncomfortable. Should I thin the bridge and cut a smaller one? As a repair person, I have an opportunity to offer some things even high end manufacturers can't. One of them is a bridge glue up that has the very best chance of surviving the long haul, another would be fretwork quality they just don't seem to be able incorporate into their bottom line. I really find it baffling how much effort goes into the exact dimensioning of tops, braces, finish thickness, etc. and yet a useless ring of heavy bridge wood doesn't seem to be questioned. Like Ken I'm very interested in your thoughts David. I have the highest respect for your opinions. |
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| Author: | David Collins [ Sat Dec 26, 2015 11:37 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Yeah, I may bit a bit overzealous in saying I'd wish the method would disappear. I do have a great many of respected colleagues who continue this practice with fine results. The main reasons I do not prefer it are largely centered on the fragility of finish at corners, and ability to perform routine service over time while preserving the edges where they are cut. Obviously the modern factory standard of cutting a smaller inset and laying the flat bridge on top has serious shortcomings. The bridge is suspended on the perimeter finish leaving a significant gap between wood components (which leads to predictable joint failures with the poor gap filling strengths of most glues. With full footprint clearings however, I see very high occurrence of finish chipping at the front edge from stress as it begins to give, and occasionally at the sides from cyclical expansion/contraction of the top through repetitive seasonal cycles. In addition there is potential damage to the corners when a bridge needs removal. While the inset rabbet does not fully eliminate either of these factors, it does allow for a bit of a buffer for the visible area, and in the rabbeted bridges I've had to reglue (although very few) they seem to allow for cleaner service with less risk to the visible edge, and just as strong a joint as full footprint gluing. Of course I've only been using this method for about 15 years, so we'll have to wait at least another 15-30 before I can start seeing enough data on reglue durability to make a truly informed judgement.
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| Author: | David Collins [ Sat Dec 26, 2015 11:42 pm ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
Pat - on this one the cut measured a full .030-.032". A few thousandths may be inconsequential, but here they routed through roughly 1/4 of the top, then just layed the bridge down on the finish with that 1/32" gap beneath filled with glue. It's really quite remarkable that the joint lasted so long as it did. |
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| Author: | Tai Fu [ Sun Dec 27, 2015 2:11 am ] |
| Post subject: | Re: For those who route finish off for their bridge footprin |
I guess it depends on the glue being used... I had a Taylor 416 that needed a bridge reglue, I rabbeted the bridge as the manufacturer had and there is always a gap beneath the wing tip (Taylor bridge wing tip is so thin and soft that this happens quite often if the clamping/fitting is not exactly perfect). You wouldn't believe how flexible a wood bridge can be, if you were to hold the bridge in the wrong way while cutting the rabbet the rabbet will be off by enough that there will be a sizable gap no matter what. Anyways after doing this about 5 times with unsatisfactory result (always gap under that darned wing) I gave up and just sanded the rabbet out, removed ALL finish under the entire footprint of the bridge, glued it in and the job was more than satisfactory. Cosmetically it doesn't really look any worse, because the tiny amount of glue fillet will just about hide any ragged edge of the finish anyways. Furthermore the UV cured finish of a Taylor is so durable that it is impossible to scratch it, and flexible enough that I really don't have to worry about it cracking in the future. I've done a few more Taylor reglues this way later in my career and it all worked out. Like I said, Taylor got such a complex CNC setup (and enough personnel to use it properly) that they can finish a guitar with a very precise film thickness and route the rabbet to just the right thickness that it fits every time. The only reason to do this is because it supposedly looks better, and I guess in the world of mass produced guitar, that is very important. By the way there are also several more budget guitar manufacturers who do things this way but I've had them in my shop for bridge reglues. Most of them actually have the bridge CA'ed to the finish itself (of course polyester, which is actually strong enough that you can do that) which would later come off for whatever reason. I would actually have to route part of the footprint off because the finish is too hard to scrape. Once I do that though, I take a sharp knife and do a heavy score and the finish chips away until it reaches the line. After that treatment the bridge will NEVER come loose unless the guitar has been obviously mistreated. None of them have ever come back since. I'm not doubting that you can produce an accurate rabbet with a Dremel type setup, BUT you will need to do this on a bridge blank that has been precisely profiled to its 2D shape but the top not yet profiled (no wing profile basically, just a 3/8" thick bridge shaped piece of Ebony) because if you were to even put light pressure on the bridge wing it will distort more than .003" |
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