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Back Strip Installation
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=44882		 Page 1 of 2
Author:  KThomas [ Fri Jan 09, 2015 9:13 pm ]
Post subject:  Back Strip Installation
I just started working on my next build tonight, it's going to follow the Martin Herringbone Dred plans from Stew Mac. I plan on installing a decorative back strip on this guitar, and was wondering, do I just joint the back halves, glue them up and rout out the channel for the back strip or do I place the back strip down and glue the two sides directly to that. It seems to me the routing idea makes the most sense. Thanks
Author:  meddlingfool [ Fri Jan 09, 2015 9:38 pm ]
Post subject:  Re: Back Strip Installation
You can do it both ways, but I prefer the routing method. I don't know why it makes me feel better, but it does.
Author:  Eric Reid [ Fri Jan 09, 2015 9:43 pm ]
Post subject:  Re: Back Strip Installation
Assuming that the decorative strip is thick enough, either method can work. Routing offers a little more support for the strip, but getting a good fit is crucial. However you do it, I recommend saturating the strip and joint with thin super glue before finishing. Movement at this interface is pretty common as guitars age.
Author:  meddlingfool [ Fri Jan 09, 2015 10:11 pm ]
Post subject:  Re: Back Strip Installation
Yup. Also, if you're using wood glue for the strip, don't level for 24 hrs. If you level it right away,it can sink and be a pain at finishing.
Author:  KThomas [ Fri Jan 09, 2015 10:24 pm ]
Post subject:  Re: Back Strip Installation
OK, hey thanks for the advice. I am using fish glue on this build, just to try it out. For the most part I used tite bond on my first few. So sounds like the way I am going to go is rout it and fish glue it, wait for it to dry 24 hours and scrape it down. Should I rout the depth so the strip is a little deeper than flush and sand down to it, like I normally put in my purfling and bindings a little recessed which I thought was correct.
Author:  Mike OMelia [ Fri Jan 09, 2015 10:57 pm ]
Post subject:  Re: Back Strip Installation
Back strips are not strong. I would not glue them between two boards and hope it will work. Joint and join your back set, then rout a channel to accept the "decorative" strip.
Author:  Terence Kennedy [ Fri Jan 09, 2015 11:27 pm ]
Post subject:  Re: Back Strip Installation
I usually rout and install. If I use a bwb fiber purfling on either side of a wood strip as I usually do (with Tightbond) it is definitely a weak point even after running thin CA in it. After cutting out the back on the band saw I usually stress the off cuts to see where they break and they break pretty easily along the purfling line.

When I have just inlayed a strip of wood with no purfling It's actually pretty hard to break.

I still use the purfling in many cases and have not seen one fail to date but handle the back pretty carefully until the center strip reinforcement is well glued.

I've never used laminated wood purfling for this, have you guys found it to be stronger than fiber?
Author:  Mike OMelia [ Fri Jan 09, 2015 11:43 pm ]
Post subject:  Re: Back Strip Installation
I used to like fiber, and I can still find uses for it. But it's tendency to "bloom" when sanded is very annoying. Good news is, a scraper will fix that. But still, it's added work. Depends on where it's used. For me,rosettes and flat surfaces are out
Author:  Tim Mullin [ Sat Jan 10, 2015 1:12 am ]
Post subject:  Re: Back Strip Installation
KThomas wrote:
Should I rout the depth so the strip is a little deeper than flush and sand down to it, like I normally put in my purfling and bindings a little recessed which I thought was correct.

No. The time to establish surface flatness of any plate is BEFORE inlay, whether it be a back strip, purfling, decorative inlay or whatever. Trying to do this after inlay is inviting issues with dips and surface irregularities from overworking the inlaid portions of the plate, while trying to get down to the level of the inlay. The sole exception (for some folks) is installation of edge binding so that sides are sanded down to the level of the binding, preserving the full thickness of the binding, rather than scraping the bindings themselves. So, your back strip should be left proud (how much doesn't make much difference) and planed/scraped/sanded carefully once fully set to preserve flatness of surrounding surfaces.
Author:  meddlingfool [ Sat Jan 10, 2015 2:38 am ]
Post subject:  Re: Back Strip Installation
Yup. Leave center strip tall and sand flush post glue.
Author:  dnf777 [ Sat Jan 10, 2015 6:57 am ]
Post subject:  Re: Back Strip Installation
I can't give you any sage advice from a position of great experience, but I can give you a fellow newbie's advice and share some of the mistakes I made, so hopefully you won't! ;-)

I agree with the comments above that say join plates and level FIRST, then route channel and leave strip proud and sand/scrape flush. Getting the excellent joinery on two sides of a strip is not easy. But when you route your channel (and pick your strip) Don't make it too deep, or you'll sacrifice the good joining surfaces of the plates for what won't be as precise or strong with in inlayed channel joint.

I made a beginner's mistake and routed to the outside edge of my pencil lines (without trial fittings between passes of the router!) and was a little wide. Z-poxy was superb at filling the gap, and once sanded actually added a nice accent on both sides of the strip. Time will have to tell if its a durable joint or not. I also added some sawdust to tint the epoxy first.

Lastly, this is a fundamental rule of all woodworking, but I'll mention it anyway since I bunged it up: Measure 20 times, cut once! Specifically, mark your center seam well and work off of this, on both sides of the plate. Remember you'll be doing a lot of work and measuring off the other side of the back, and I found it easy to misalign the centerstrip in regards to the heel and end stripe, even though that's not supposed to happen, when you get focused on bracing, and other aspects, etc... ;-)
Author:  bluescreek [ Sat Jan 10, 2015 7:15 am ]
Post subject:  Re: Back Strip Installation
I have made over 200 guitars. You are using a cross grain support between the plates , you can do it either way. I set them between the plate. The back strips are actually a lot stronger than you think. I use Gurian for my back strips he makes them for us. Here is how I do it with rosewood.
https://www.youtube.com/watch?v=3asM32m0S1U
This is showing using CA and it is one of the few places that I will use it. I also use fish glue.
why add a step to the process you don't really need. Martin has been gluing them between forever.
Author:  Hesh [ Sat Jan 10, 2015 7:37 am ]
Post subject:  Re: Back Strip Installation
It can be done either way. There is a tendency on Luthier forums that include woodworking hobbyists and even woodworking pros to go for the method that is likely stronger such as inlaying the back strip. Inlaying is stronger because the glue joint is larger when inlaying in terms of more surface gluing area but does it matter...

On the inside there is a BJR (back joint reinforcement) that is a full length cleat of sorts complimented by cross grain braces with gluing surface area too. Considering this, what's on the inside either butting and gluing or inlaying is plenty strong enough.

Most of the failures that I've seen have been cracking next to the BJR or off to the side of the decorative inlay but not the inlay glue joint itself.

I install proud and scrape to flush. I've never experienced any issues from scraping too soon with say Titebond original but we will and do see issues when using the old solvent based glues and plastic or celluloid bindings where solvent glues are well known to gas off and shrink for a couple of days making waiting for leveling important in these instances.

Personally I've always inlaid mine mostly because I want to have fun with the router.... :D but again I see no reason why either method won't work and work well.
Author:  Clinchriver [ Sat Jan 10, 2015 8:03 am ]
Post subject:  Re: Back Strip Installation
Glue between the plates
Author:  KThomas [ Sat Jan 10, 2015 10:49 am ]
Post subject:  Re: Back Strip Installation
Thanks again guys, your help, responsiveness and experience always amaze me. Well I'm doing the routing thing because I jointed and glued it up last night. I would have never thicknesses it first before installing the strip so you saved my bacon. Yup Hesh, I got some new router bits from Mike O I've been wanting to put to work and have some fun. I got one of Rigid laminate trimmers with the LED light that comes on when you fire up which is neat. And John, back strips I have I got from you at the 2014 Woodstock Show, I guess Martin glues right to the strip then, but what do they know [WINKING FACE].


Sent from my iPhone using Tapatalk
Author:  Tim Mullin [ Sat Jan 10, 2015 11:13 am ]
Post subject:  Back Strip Installation
KThomas wrote:
I would have never thicknesses it first before installing the strip so you saved my bacon.

Hold the phone. I didn't recommend "thicknessing" first, but rather "flattening" the surface to be inlaid first -- not quite the same thing. Once the plate is inlaid, scraped and outside surface made perfect, then I thickness (or finalise the thickness) by removing material from the inside surface. Exactly the same sequence when making the top plate (establish flat top surface, inlay rosette, level rosette and make top surface perfect, then thickness).

BTW, on the strip joining versus inlay question, I'd be quite happy with John's joining method, provided there is marriage strip (joint reinforcing strip) used inside, and I believe that's the way he makes 'em (Martin style). But I also think you can dispense with the reinforcement strip entirely if you use a typical inlaid back strip, as there is significantly more joint surface glue area. Some folks prefer that aesthetic. Many times, however, I prefer the look of no back strip and let the wood do the talking, and in those circumstances I think the interior marriage strip is essential to guard against joint separation.
Author:  Joe Beaver [ Sat Jan 10, 2015 11:46 am ]
Post subject:  Re: Back Strip Installation
I haven't used back strips in the last few builds but when I did....

I always joined the backs before thickness, then level sanded the show surface. Then routed and added the back strip.

After it was good and dry I would scrap level then thickness the back from the inside surface.

I would never consider adding a back strip between the plates unless it was at least as thick as the finished back. To me a .10-.09 back joined with a .06 back strip is not a good idea
Author:  bluescreek [ Sat Jan 10, 2015 12:02 pm ]
Post subject:  Re: Back Strip Installation
A back strip that is less then then the back plate is nothing I would use. China makes them thin like that. Martin uses .135 then they get sanded. You should finish 2 sides glue then thickness. Finish means making the surface flat , thicknessing is taking to your size. I allow .005 for sanding. So if I want a .095 back I finish often about .150 glue then thin to .100.
Author:  Tom West [ Sat Jan 10, 2015 12:05 pm ]
Post subject:  Re: Back Strip Installation
I install them during glue up...........level........then hit both sides with CA glue if it is more then a single strip. With the cross grain support installed I think they are plenty strong and think it is just extra work and anther chance for something to go wrong by routing.
Tom
Author:  kencierp [ Sat Jan 10, 2015 12:11 pm ]
Post subject:  Re: Back Strip Installation
Yep --- get the full thickness strips (.125"+/-) glue in a sandwich, ditto Tom, drizzle thin CA glue on the inlay to add a bit of strength. But you still need to take care when handling the assembly.
Author:  Hesh [ Sat Jan 10, 2015 1:57 pm ]
Post subject:  Re: Back Strip Installation
Good thread guys and yet another example that there are lots of ways to do much of this stuff.

Related to how you install a back strip is the issue of using them at all.... More specifically when to use one and when, perhaps not to use one...

Early on since I had never made anything before in my life besides reservations and certainly had not done any woodworking I was absolutely in love with some of the killer wood that we would see available for sale. From highly flamed Koa to Tazz Tiger Myrtle I was really taken by how absolutely beautiful some of the wood that is available to us is/was....

It occurred to me one evening when sitting around my shop looking at a back and side set (I used to take all of the parts for the next build and prop them against the wall and sit there and look at it all for a bit...). It occurred to me that it kind of sort of seemed counter to using certain woods to install any back strip at all. From highly flamed Koa with patterns that looked cool to the snake like patterns of snakewood or Tiger Myrtle it occurred to me that breaking up the patterns was an eye draw and in this case not a good thing....

Where I like to use a back strip, personally is with more uniform, plain Jane woods that are not all that interesting. Where I decided that I would not use back strips is with wood with interesting patterns, etc. Of course YMMV but to me this is a question of the artist's eye since the back strip is not a necessary structural addition. There is also tradition if one is building say Martinesque reproductions and wishing to remain true to the heritage of a certain model.

One last thing too that I wanted to mention without naming names I have seen two instruments now from the same maker with no back joint reinforcement on the inside. The reason that these guitars were bought to our shop was split center seams and yep it was the glue joint that gave way....

As such if butt joining a center strip the same parameters as joining plates should always apply. The joint should be tight with full material to material contact and not relying on clampage to make up for a poor fitting joint.

Although I would and do either butt or inlay a back strip I would not recommend not using a back joint reinforcement in the box no matter who does this and promotes not using one. A split center seam to me is a totally avoidable failure and as such if it happens to one of ours we only have ourselves to blame....
Author:  Mike OMelia [ Sat Jan 10, 2015 7:07 pm ]
Post subject:  Re: Back Strip Installation
To be fair, the one that failed on me (during the sanding stage) was one from Stew Mac, and def thinner than plates. Honestly, this is an opinion. Almost emotional. Yes, the cross grain strip and braces will make up for it... If u make it that far... And most of u who do this will. And a failure at this stage is not even close to a tragedy. But once was enough for me. I like the essence of all the extra glued surfaces. Sorry if I came off as dogmatic.
Author:  KThomas [ Sat Jan 10, 2015 8:29 pm ]
Post subject:  Re: Back Strip Installation
Well it's done. I was downstairs working on it so I didn't see the more recent posts, so I misunderstood the point about thicknessing as opposed to flattening out the area prior to the strip being installed. What I did was joint the back, glue it up and drum sanded to 3mm or about .120. The strip that I got from John at Blues Creek, was about as thick as the back at this point. So I took the strip to the drum sander and took it down to a little over 2mm. Then I routed my channel in the back 2 mm deep so the strip was just a hair proud, I sealed the routed channel with some 1Lb cut of U-beat shellac, let it dry, and put the strip in. I flooded the area with thin super glue, allowed that to dry, and scraped it flush. Now I have a back with the strip at .120 thick at this point, I think I am shooting for .110-.115 for the final thickness. I keep referring to mm because my thickness caliper gauge is in mm. It looks pretty good, I should be OK, no?
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Author:  Mike OMelia [ Sat Jan 10, 2015 10:44 pm ]
Post subject:  Re: Back Strip Installation
I don't think I would have ever bothered sanding the strip down, that can always be fine after install, when it is securely held by the channel. If the strip is high detail marquetry, I would fear destruction under sanding forces. But regardless, it worked for u.

Looks good! Btw, be aware that CA is not a universal goto product. Especially around spruce. In hardwoods, no real issues. But in unsealed spruce, CA flooding can cause wicking stains u won't believe.

Mike
Author:  DannyV [ Sat Jan 10, 2015 11:03 pm ]
Post subject:  Re: Back Strip Installation
That is a very nice looking back strip. It's going to look great under finish. Something to consider. If you are doing an easy inlay, like that one, one piece, a water based glue like Titebond will swell the wood. Doing so will help eliminate any tiny gaps that don't show up until you start to finish, When it comes time to finish the fewer gaps the better. I use Titebond whenever I can for this application. Of course sometimes if you are doing something a little more complex CA is the way to go.
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