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I've used this tower on 2-3 guitars, and have had the same issue each time, and wondered if someone could shed some light on it.

The problem:
When I'm routing the channel toward the neck on the back, especially, the channel becomes shallow.
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The cause:
Because the back is at a radius the bit cuts in at an angle. The result is an angled cut, meaning that the guitar and the bit are not parallel. The top of the bit hits but the bottom begins kicking outward because of the angle.
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The solution?
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Is this a flaw in my building, or am I failing to compensate some way in using the jig?

I appreciate any help with this. I'll include a pro rendered drawing below to further illustrate what I mean.

Behold



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In pondering this… perhaps the isn’t isn’t the contour of the back, but rather the top of the body, at the neck joint. Unfortunately I don’t have a guitar I’ve made here to measure things (they are all in happy homes).
What would cause me to be building w a slant in this area? Here’s another professionally rendered illustration…




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Are you positioning the body in the cradle in a way where the sides are as vertical (compared to the bench, not anything on the rest of the guitar) as possible? That's one thing that can go wrong. The point of the tower is to hold the bit vertical, as compared to the bench. You need to do the same with the sides of the guitar. Ignore what the rest of the guitar is doing. Make the sides vertical to the bench.

If you have "undulation" of the sides, so that you can never have all parts of the sides vertical to the bench at the same time, you might need to use something other than a tower to cut the channels. Something that registers 100% off the sides, like the Elevate jig, would accommodate that problem better. Or just use a gramil to mark the channels, use the tower to cut most of the waste, but use a chisel to get the rest.

Using the mold for as long as possible during construction helps avoid the undulation I'm talking about.

Does your router base have a donut so that it rides only on the edge of the back.

Just saw your second post. If you are gluing the tops and/or backs on without the help of a mold, that can definitely cause slanting. Like I said before, using the mold for as long as you can during construction, for the gluing on of both plates, helps you wind up with sides that all stay in the same plane, if that makes sense.

Yes sir I understand what you're talking about. I do get my sides darn close to flat with an elevate rolling pin sander, so I don't think undulation is the issue. Yes the self aligning thing rests on the sides, and I see where that could be an issue, but It has happened several times in the exact same spot. This is leading me to believe that when I use my mold the neck block is getting tilted so that the top is more forward than the back, thus making a shallower cut.

makes perfect sense. Maybe I need a top to bottom spreader instead of just the waist caul spreader to keep things parallel

yes indeed.

Honestly this is an issue I should have rectified by now. Now that I'm in between builds I feel like i can tighten some tolerances

I do bolt-on necks. I finally started using the holes, and the endpin hole in the tailblock, to bolt my heel and tail blocks to the mold. It sure tightens things up for many of the processes through the gluing of the tops and backs to the rim.

Not a terrible idea. I know Hall screws it down too


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Yeah, the John Hall drywall screw method is pretty useful. It locks the blocks to the vertical position very well. Add decent spreaders for the waist, and you should be OK. Again (beating that dead horse), keep the body in the mold for both the top and back gluing jobs. Only pull the drywall screws once the whole body is together.

Does your mold have perfectly vertical interior surfaces? Something to confirm and fix, if not right.

With a tower type jig I align the body in the holding jig with a right angle on the table to get the sides and head and tail block as perpendicular to the table as possible.

Then I put the router on the guitar with the bit protruding through the doughnut maybe 1/4+ inches and squat down and look at how parallel to the sides it is all around the guitar. It usually is not absolutely perfect everywhere but I get it as close as I can. A slight out of parallel on the head vs. the tail block is most common. Mild differences don’t seem to be too big a deal.

Good to check and be sure the bit is perpendicular to the table too. Sometimes you have to shim the base of the tower slightly.

A lengthwise spreader is always a good idea. I like the screw idea. I know Laurant Brundell does that.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

So, do you guys install the end wedge after the top and back are glued on the rims? I've always made the sides to the blocks, radiused them w my dishes then removed from the mold to install the end wedge.

I use a tower setup to route the binding channels too.

I use the edge of the top as the reference for positioning the body in the cradle to get even channels. To route the top channels, I put the body in the cradle and adjust the support blocks of the cradle until the edge of the top is the same distance from the bench top surface at both the tail block and neck block, and the same distance from the bench top at both tips of the lower bout. This ensures that the sides of the body are perpendicular to the bench top.

To route the back channels, I flip the body in the cradle and again use the edge of the top as the reference for positioning the body in the cradle. I repeat the same steps above with the cradle support blocks and adjust them so that the edge of the top is the same distance from the bench top surface at both the tail block and neck block, and the same distance from the bench top at both tips of the lower bout.

This is the method I was taught and it has always worked. The one thing I always have to be careful of when I'm setting the body up in the cradle to route the back channels is to remember to measure off the edge of the top to the bench, and not off the edge of the back. If the body has a tail-to-neck taper in body depth, then measuring off the edge of the back will result in the sides at the neck and tail ends not being perpendicular to the benchtop.


I install the end wedge before gluing the top and back on to the sides.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I do the tail wedge after I glue the sides to the blocks and before I sand the rims to the radius dishes. I don’t drill the endpin hole until after I have sanded the rims to the radius dishes, installed the linings, and then resanded the rims to the forms. Then, I know exactly where the center is. Then, I bolt the blocks into the forms. I use 1/4” bolts because I can knock them out without having to unscrew them inside the guitar.

Just to add another dimension to the conversation: I’ve done somewhere in excess of 125 instruments with bindings and never used a jig. I use the StewMac bit with the bearing and guide the router freehand using my eyes and feel to keep it parallel to the side. Where I have any unevenness, I adjust it with a file or a chisel. The towers and jigs just seemed too complicated for me.

Jigs that guide for depth with a donut-style arrangement will have depth issues where back slope becomes extreme (the slope at the neck block will be 6-7 degrees on a typical Martin, depending on the difference in tail block and neck block depth). This causes router bit cutter depth to guide off the 'up-slope' side of the edge, reducing depth of cut... usually by a noticeable amount.

Re-leveling the back sounds like the solution, but that angles the cutter relative to the neck and tail blocks, resulting in a channel that is angled... this results in a thinner binding width at the neck and thicker at the tail, with that difference in width being proportional to the depth of the binding channel (less error in shallow binding and greater for taller binding). One way to avoid that issue is to take the first pass with body leveled using the top as is normally done, then take a second pass from the waist to neck block with that area of the back leveled. The resulting tiny ledge in the milled channel can be removed with use of a chisel.

The solution we applied at Greenridge was to mill the channels with router bit shaft aligned with the sides (using the top to level), then using a wheel-style cutting gauge to adjust channel depth above the waist by setting depth of cut at or close to the tailblock and making an adjustment pass from where the cutter first starts taking material off (usually just below the waist) to the neck block. A good gauge such as the Titemark style will at make quick work of the job, but as we all know there is no such thing as a free lunch, the slight added angle at the bottom of the channel will need to be addressed - we did this with a 50% channel-width back chamfer of the binding material from waist to neck on the back bindings (we usually did this anyway, so no real increase in work) and a quick pass with an edge-cutting curved riffler from waist to neck to re-square the channel.

Careful card scraper work would also address correction to the channel for squareness, but at the usual 0.060"-0.070" width and a back chamfer, there is little to correct.

Living with the error appears to be an option as well for some builders, as I've seen that uneven back binding depth issue on a few small-shop made custom guitars in for repair.

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A constellation only takes shape when one maps the whole.
- Beth Brower

After years using a tower jig, I eventually bought a jig that indexes off the sides because of this issue. As noted, if your sides and end blocks are all nicely square and fit is good in the mould, a tower style jig works perfectly.

A tower jig actually can do a good job even on a guitar with totally undulating/slanting sides. However, the process involves adjusting the body cradle so one section of the sides is at 90 degrees to bench, milling that section of the channel, readjusting the body so the next section is at 90 to the bench, mill that section, and so on. A few minutes of sanding or chisel work blends the various sections. I tried this on an instrument where a bonehead error when gluing the back (forgot a spreader) led to our of square sides. I ended up with accurate channels but it was so time consuming that I’d have been better off free handing the lam trimmer.

Now that I’ve got both styles of jigs, I use whichever is most suited to the instrument at hand - tower for those instruments where I achieved perfectly square sides, side-registering jig for anything else.

So I guess what I’m really saying is, buy more tools

The gentlemen I worked with at Greenridge were engineers, and one of them a career military pilot and commander, so root cause was a major part of how they did business. Making a mistake was acceptable, but not dissecting that mistake to determine root cause/contributing factors was very definitely unacceptable.

A few ideas on where to look:

- Are your neck and tail blocks glued in when the sides are held in perfect vertical alignment in the mold? In other words, are you leveraging ALL of the advantages of the outside mold in your construction, or only taking advantage of a few?

- Are the sides held firmly in the mold during all phases of assembly up to closing the box? Once the neck and tail blocks are in, we removed the sides from the mold only to mill the corner post on the cutaway, and only when using tapered versus flush, square heels. We also radiused the top prior to removal so we could ensure the rim could be registered on the radius dish and the mold blocked at heel and neck to ensure alignment.

- What issues are keeping you from using the mold to maintain perfect shape when the plates go on? As mentioned, this does a lot to keep sides from twisting or bowing, and maintains design shape.

- Stepping back further, is your radius dish true, and is your mold inner surface dead square?

Working from the apparent issue back through equipment, technique, materials, and order of build can help pin down root causes and contributors, which can then be assessed for either correction or to be minimized in terms of impacts. Don't stop at the first apparent cause of your problem, as you want to identify as many as possible, and see if there is a common origin for what may be related issues in your workflow, procedures, equipment, or whether you ate a good breakfast. This also prevents/minimizes throwing money at the issue, which while satisfying from the viewpoint of retail therapy may not actually be the real problem.

A repair-related example:

An early 2000's custom came in with 70 running inches of cracks in the Brazilian rosewood back, with the edges of the cracks curled downwards. There were two small top cracks as well, but they were both tight and represented a simple fix. While low humidity might seem like the obvious issue, the wood used in the back was largely flat-sawn stump wood, with an abundance of reaction wood and one very prominent area of what appeared to be a large area of reversed-grain knot shadow.

In terms of root cause for the cracks, wood selection tops the list, but there were several other choices made in construction that were contributing factors. The back bracing was a modified X with added light-weight fans which ran under the legs of the X through large cutaway openings. The cutaway 'tunnel' areas were aligned with the flawed areas of the back, focusing cross-grain stresses from drying and the natural tendency of reaction wood to move/crack once thinned and partially constrained in a structure. Another contributing factor was lack of any real support in between the upper and lower legs of the X, promoting the unconstrained downward curl of the cracked, structurally flawed areas.

In other words, as the Fixx opined, one thing leads to another...

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

I may be missing something, but I'm not getting why a bunch of added steps are needed after routing the binding channels to end up with constant binding height around the back edge of the body. I do the setup I described a couple posts up from this one, cut the channels, make sure they are free of debris, and done. My guitar bodies have a 15' radius back and the same tail-to-neck taper as my HD-28VR. I just measured the height of the back binding on the most recent guitar I built and it's exactly the same at the neck end, at the waist, and the tail end. Maybe the shape of the donut I made for the router carrier on my tower allows it to register right at the edge of the back all the way around the back? I don't know.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

If you are satisfied with what your particular rig produces, then life is good...but that does not seem to be the case with the OP, thus my comments addressed the known errors produced in the channel milling process associated with the Ribbeke and Williams style jigs/standard binding sets.

Tower-style (really, Ribbeke) and articulated arm (Fleishman/Williams-style) jigs which use the StewMac and LMII binding cutter sets (or similar) reference width of channel cut off the sides. Leveling the back (on a tapered acoustic body) prior to routing the channels with this setup may help address the depth issue, but it mills a wider channel at the tail block and narrower at the neck block because of the router bit bearing location below the channel being milled.

More generally, the closer the fit of the guide donut to bit and the smaller the diameter of the bit used, the less depth error is seen. One of the reasons for using something like the side-referenced/wedged top guide jigs such as the LuthierTOOL Pro line or ELEVATE is to minimize both the offset of the bit from the guide point and address out-of-square sides. Both jigs may still produce a channel that is not perfectly squared to the vertical as measured from the center of the top, but they are close enough that the depth error on back binding is not noticeable.

Let me see if I can dig out some of the illustrations from the binding section of Mr. Stock's building guide and confirm that I can post them...they do a better job of explaining the issue in a few drawings than my lamentably opaque prose.

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

" the closer the fit of the guide donut to bit "
I agree (it's simple geometry)
I keep as close to the bit as possible and also I like a relatively steep angle on the nose of the donut and a very small radiused inner edge to it.
I level the bottom of the ledge (which is curved in places where the cutter is descending or ascending) with a scraper made from a 2mm x 6mm section file.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

If you have the same setup as I the first problem I encountered was the same as mentioned earlier. I use a drafting square to adjust the sides square to the table (the tower needs to be square also). But the second problem I had was the aluminum plate holding the router was riding on the back near the neck. I bought a piece of 1" plastic on ebay and turned a new doughnut that was deeper than the original. now it works great.

I believe my issue is
1 the neck block not being perfectly square in the mold
2 I need to verify and reverify that the routed top is perpendicular to the bench top

. I’ll screw make sure the block is strait this time and screw it in.

I’ll need to install the end wedge before doing this so I don’t have to take it out before glue up


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On the end wedge, I personally find it easier to fully close the box, then deal with the end wedge before I cut the binding/purfling ledges. This allows me to use drywall screws to hold the tail block to the mold until the box is closed, because those holes get covered up by the end wedge.

The rest of your precautions look sound to me.