| Official Luthiers Forum! http://mowrystrings.luthiersforum.com/forum/ |
|
| Pyramid bridge lifting again http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=50015 |
Page 1 of 1 |
| Author: | roby [ Sun Dec 03, 2017 5:27 am ] |
| Post subject: | Pyramid bridge lifting again |
This is the second time i've glued this bridge and i suspect it's lifting again, i've got two ideas why this may be happening , to much saddle showing (.150 above bridge in center) and pin holes too close to saddle .Glued with fresh titebond and clamped with four clamps and shaped cauls on the pyramid ,thoughts/opinions please. . Attachment: DSC_0002-1.jpg Attachment: DSC_0003-2.jpg
|
|
| Author: | Woodie G [ Sun Dec 03, 2017 6:32 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
I am not certain that the height of exposed saddle would be a factor here, as more exposure - versus higher string height above top - does not equate to higher stress on the bridge/top joint. If the saddle slot was failing structurally, excessive saddle height and poor fitting of the saddle might be suspected, but it appears as though your bridge is intact. As to pin placement, we see some old Gibson and Kalamazoo instruments where the pin holes at the back edge of the bridge - essentially where glue line stress is at or near maximum value - that have released in that area decades ago. The bridge remains on the instrument due to load shedding to the bolts and pins, as well as well-glued bridge wings. I suspect any perceived advantage that a more rearward pin placement might have as the joint begins to fail and the pins begin to take the glue line shear loading is nullified by the loss of gluing area critical to keeping the bridge on the instrument. When we see bridges peeling off the top, we look for poor preparation and fit of the bridge/top joint, improper or inadequate clamping at the wings and rear edge of the bridge, and poor time management when hot hide glue is used, leading to early gelling of the adhesive and thick, weak glue lines. |
|
| Author: | Clay S. [ Sun Dec 03, 2017 6:50 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
Sometimes if the top is bellying too much it can cause the bridge to peel. How high are the strings off the top and what gauge are you using? did it take any wood with it when it let go? cutting string slots in the bridge to allow the ball ends to seat against the bridge patch might take some stress off the bridge. Pyramid bridges with their small footprint had their heyday in the gut strung guitar era. Although bridges with a similar footprint have been used with steel strings they don't have as much peel resistance as a "belly" bridge. Some woods (less oily) glue better than others. |
|
| Author: | Hesh [ Sun Dec 03, 2017 7:12 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
Some additional things to check: 1). Bridge plate and pin hole condition. Worn pin holes where the string balls are trying to migrate up through the holes can cause a "crease" in the plate resulting in the top no longer having a uniform surface for the bridge to attempt to hold onto. Result, bridge can't hold on and lifts. I see Clay just brought up the idea of the surface condition too, I agree. 2). Loose braces also creating top distortion making it again difficult for the bridge to have a homogeneous surface to live on. 3). Since this bridge has a little foot print it's even more critical to clear as much finish under it as possible AND have both surfaces clean of any debris from the prior lift and old glue or finish. 4). Not overly concerned about the saddle height unless the string height exceeds about 1/2" off the top in front of the bridge OR as Woodie suggested the saddle is a sloppy fit in the slot causing that slight lean that I believe that I see in the pics. 5). When using Titebond we read date codes and since we guarantee our work and more important want to do superb work we make sure our Titebond is no older than one year. This is a stretch but we tend to be pretty anal about bridge reglues especially if someone has been in there before and it failed again. 6). Clamping can be suspect too and dry runs are your friend. Too many clamps that are unsupported as they hang in the sound hole can weight the bridge so very much toward the sound hole that the trailing edge is actually not where most of the clamping is concentrated. We support our clamp(s) with blocks between the bridge front and sound hole or use clamps with adjustable stabilizers for the same purpose. When letting glue dry and cure we hang the instrument and are sure to have beefed up hangers that can handle the added weight of the clamps. 7). I visually inspect my glue joints with a glued bridge all around with a flash light and at times if I have any doubt a .0015 feeler strip prodding around in there before saying..... next. Back to surface prep, clearing finish and of course fitting the bridge. We spend most of our time on the fitting process not viewing clamps as a remedy for forcing a poor fit. Instead we want only light finger tip pressure to seat the bridge in all areas all the way around. Our final fit we scrape both surfaces nearly immediately before we deploy the HHG. Lastly for now I have to ask since it's a possibility, not only can old glue be a problem but the wrong glue can be a problem too. When I speak of Titebond I speak of Titebond original or extend (original) not any of the II or III varieties. And really lastly since this one has been uppty and is peeling once again if you have HHG chops and are familiar with it's use, care and feeding I would favor HHG. Woodie's crew and some others these days are using higher gram strength for bridges. I've not tried it and not seen the need yet but it's a good idea and likely a great idea when there have been problems with small footprint and prior lifting. Might be a good place, if you are again familure with HHG use to deploy this solution for perhaps added insurance. The parameters for use are different and more specific than Titebond in terms of open time, prep and I'll add perhaps.... preparation of surfaces but none of this ever hurts. Good luck. |
|
| Author: | roby [ Sun Dec 03, 2017 7:33 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
Thanks guys the string height is 1/2 inch above top and the bridge is slotted for unslotted pins and i was very meticulous about prep and clamping my thinking is if i made a new thicker bridge to reduce saddle height which is 0.150 in the center at the moment and shifted the pins back slightly this would result in a less severe break angle and reduce the leverage on the back of the bridge, am i right? |
|
| Author: | Woodie G [ Sun Dec 03, 2017 8:13 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
While reducing the break angle might help prevent structural failure of the bridge - usually seen at the treble end of the slot - I do not believe it will have any real impact on the bridge-to-top glue line joint. Likewise, any reduction in glue line stress due to moving the string balls rearwards may be more than undone by the reduction in gluing area near the rearward edge of the bridge where the glue line loading is the most adverse. I believe that Mr. Breakstone may have the order of concern in hand - first, preparation and execution (e.g., bridge path and bridge clean and closely fitted, properly clamped, etc.), then concern for other structural issues adding additional tensile or shearing loads to the joint (also as mentioned by Mr. S.), and finally, choice of adhesive (e.g., old/expired versus new, fresh, and suitable to the job). For what it might be worth, we consider 0.140"-0.150" of exposed saddle (after the instrument settles in) to be an ideal outcome where the bridge design will allow - our target bridge thickness for new or replacement bridges is usually 0.350", and we want to see 0.500" of string height above the top on larger bodied instruments, whether after a reset or on new construction. |
|
| Author: | Rocky Road [ Sun Dec 03, 2017 8:23 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
In my possession is a parlor size guitar with a rectangular bridge. The bridge is 3/8” thick. The saddle exposure is at least .150, without measuring it probably a bit more. The width of the saddle is 1 1/8”. It was glued on with HHG, the saddle and soundboard were carefully prepped. The guitar has been strung and tuned to pitch for a few years. No bridge lifting. Ever. I’m almost positive that the problem you are having is not the physical size and layout of your pyramid bridge. Probably one or a combination of the things Hesh and Woody wrote about above. Sent from my iPhone using Tapatalk |
|
| Author: | Hesh [ Sun Dec 03, 2017 8:32 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
roby wrote: Thanks guys the string height is 1/2 inch above top and the bridge is slotted for unslotted pins and i was very meticulous about prep and clamping my thinking is if i made a new thicker bridge to reduce saddle height which is 0.150 in the center at the moment and shifted the pins back slightly this would result in a less severe break angle and reduce the leverage on the back of the bridge, am i right? You're very welcome. I don't see any difference in increasing bridge height for a 1/2" saddle height with a taller bridge from what you have with a 1/2" saddle height with a lower bridge IF.... the saddle has a snug, not tight, but snug with no tilting or wiggling fit. The rotational forces on the trailing edge of the bridge should be the same. Filling the pin holes and moving them does change things but we see plenty of 1940's instruments with steep break angles such as your's and much of the time these bridges are still on and original. If you know the builder Mario Proulx who is a member here some of his bridges are no larger than your pyramid bridge and have a very steep break angle. I've never heard of one of Mario's lifting. His instruments are coveted by many including the Bluegrass crowd who favor mediums and competing with a banjo for volume. Or in other words they are player guitars that get played hard and completely enjoyed. If I was to move the pin holes and we do this from time to time we carefully make a composite of the bridge plate material and the top material and then drill the pin holes to make them uniform before plugging with the two layer plugs that we turn on our mini lathe. Because we can and we tend to be smart arses we often match the grain direction of the plugs with the spruce upper layer grain of the plug matching the grain direction of the top.... This has NO functional value, the grain direction matching other than simply having some fun with our work. But the two layer plugs we believe won't be an impediment to the same degree as one layer plugs going through the top and plate IF the bridge plate ever needs to be serviced and removed. We always try to have solutions that do not decrease serviceability going forward. So to summarize I see no value in the taller bridge IF the saddle fit is correct and snug with no wiggle room. I do see some value in moving the pins but still don't suspect this as the primary causation. Instead even when sure that we did things perfectly prior I would still suspect the same things previously mentioned. Most of all if I had to guess I would be clearing finish to very near the perimeter of the bridge for maximum foot print AND being really certain that we are not leaving any residue of prior glue on either surface. I would also be closely refining the fit and here is a new one for you. If the finish is very thick and the bridge has to span the finish ledge there is no wood to wood contact for a bit inside the gluing area as the bridge is propped up on the finish. We rabbit and closely fit, some here chamfer also to gain more direct wood to wood contact around the perimeter. Either practice if the finish is thick will gain you some precious wood to wood gluing surface and would be advisable. Lastly I would take a closer look at how a dry run is clamped up. Regardless of the small foot print bridges don't usually come off unless there is a reason and most reasons are rather obvious. As such I'm still inclined to double check the fundamentals. |
|
| Author: | roby [ Sun Dec 03, 2017 8:54 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
Thanks guys thats given me a lot to think about. |
|
| Author: | Clay S. [ Sun Dec 03, 2017 9:11 am ] |
| Post subject: | Re: Pyramid bridge lifting again |
If there was no wood tearout from the soundboard I would suspect bad glue. Even with a poorly fitted and clamped bridge if the glue is good you will usually get some fibers torn from the soundboard in a few places. |
|
| Author: | AndyB [ Sun Dec 03, 2017 9:25 pm ] |
| Post subject: | Re: Pyramid bridge lifting again |
Pyramid bridges don't have as much surface area as many other steel string bridge shapes, as well as less span aft the saddle pivot point. Their installation details are most critical when it comes down to all aspects of gluing them down - bridge radius matching bridge plate radius, no finish pushing up, proper clamping with even pressure across the bridge, and good glue (fresh if white/yellow, at the right temp and not too thin if HHG). Since you said fresh titebond and such, I would ask how you cleaned the first titebond glue-up (since that's critical)? Also what did you do differently the second time, that you thought would fix it from the first time? My preference is HHG 300g on pyramid bridges though plenty of them glue up with titebond no problem. Andy |
|
| Author: | Mike Collins [ Tue Dec 05, 2017 3:29 pm ] |
| Post subject: | Re: Pyramid bridge lifting again |
I had this problem with a guitar I sent to a dealer. It took a year for it to surface-he sent the guitar back-I removed the bridge only to find out the carved out area at the end of the saddle block on one side was thin & FLEXIBLE-ready to break. In fact it did when I flexed the bridge by hand. A bad piece to use for this style of bridge. I bought it from a very rep. supplier. I replaced that one. Checking the new one for this problem! NO problems with the new bridge!! Mike |
|
| Page 1 of 1 | All times are UTC - 5 hours |
| Powered by phpBB® Forum Software © phpBB Group http://www.phpbb.com/ |
|