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CONTROL CAVITY ROUTING FOR DUMMIES

DANELECTRO

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Feb 24, 2003
Messages
6,340
There’s been a lot of chatter and analysis of control cavity routs lately, so I thought I'd offer some information that may explain some of the specific details seen in routed features.

I've never built a guitar, nor visited the Gibson factory to see their process, but I did work in a production woodworking shop 27 years ago. I made hundreds of cutlery blocks which have knife slots routed into slabs of wood. These slabs were later laminated together to make up the completed cutlery block. The process for routing the slots would be the same as for routing guitar bodies.

Although every cutlery block was "handmade", the routs were very consistent from one to the next because they were made on an overarm pin router. This is a machine which has the router bit chucked above the workpiece in a vertical spindle. There is a large table with which to slide the workpiece under the bit. The table has a guide pin that is the same diameter as the bit and is located directly under and inline with the overhead bit. For every different configuration of cutlery block that we made, there was a specific jig that had all of the necessary rout patterns cut into the underside of the jig. The jig had locating features and clamps on the topside for the workpiece. To rout a part, you simply clamp the slab of wood into the jig, and then guide the jig into the high-speed router bit. The guide pin under the jig prevented you from routing anywhere that you weren't supposed to. It was possible to under-rout a cut if you weren't careful to keep the jig pushed against the guide pin, but you could never over-rout a slot. The pedal allows you to plunge the spindle for blind pockets.

Onsrud.jpg


I can say with near certainty that Gibson used a similar, if not the same machine. It is may still be in use today. Jigs would be made with guide slots on the bottom of the jig for whatever topside feature was to be routed (control cavity, pickup pockets, wire channels, etc). Each rout would require a separate jig.

For instance, routing the control cavity on a carved top Les Paul would require three routing operations with three different jigs:

- One for the coverplate rout (shown in yellow).
- One for the primary control pocket routing (shown in green)
- One for the angled routing (shown in blue). This jig would have the body slab mounted at angle to the table surface.

Color%20Coded%20Cavity.jpg


Depending on how well each of the jigs was constructed, or how accurately a guitar body slab was cut, there could be some variability between each of the rout processes. In other words, if the guitar body wasn’t located and clamped in exactly the same position between each jig, then each of the rout operations could “float” relative to each other. This explains why the flat edge between the two volume knobs varies in length, because it is the result of two different setups and operations. Same goes with the depth of the "chew marks". I believe that the reason for the rough surface of the chew marks may be because a smaller diameter router bit was used. The cutting edge of a smaller bit has a lower velocity than a larger bit, so it might have a tendency to tear at the wood. The Onsrud router had a 20,000 rpm spindle which just cut like butter. It took over 5 minutes for the spindle to wind down after shutting off the machine.

The actual profile of each rout process should be identical between all guitars manufactured, unless:

- The operator did not keep the jig pressed against the guide pin. This could allow for some edges to be under-routed.
- More than one jig existed and there were differences between them. There may have been a second jig made for backup, or one may have simply worn out and had been replaced.

The holes are drilled in separate operations (using drill jigs with guide bushings), so its possible for the holes to float relative to the routs, again depending on the accuracy with which the tooling was built. It’s likely that the drill jig for the backplate holes located itself off of the routed pocket.


Anyway, that’s my take on the control cavity routing based on my knowledge of manufacturing processes, and examining the cavities of my Historic reissues and the numerous ‘Burst cavity photos I've seen here on the Forum. I don’t have a ‘Burst to analize first hand, so if anybody would like to loan me one, I’ll be happy to look it over and maybe offer some more insight. ;)

- Dan

(Your Friendly Neighborhood Forensic Control Cavity Analyst)
 
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Wilko

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Mar 11, 2002
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21,046
That's a nice piece of work there.

That scenario works for all the vintage and most reissues, but not my favourite Les Pauls. The 1968 cavity is different.

bwaa ha ha Haaaa
 

mikeslub

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Jul 15, 2001
Messages
15,352
Great job, Dan!

Key statement in your analysis:
Allowing for some variability, "the actual profile of each rout process should be identical between all guitars manufactured..."

:)
 

John Catto

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Jul 15, 2001
Messages
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Gibson certainly used a large pinrouter to do the cavities, this is why they are so consistent with each other, to believe or convince yourself otherwise is heading way in the wrong direction.

I'll add one more thing here. I'm pretty certain that Gibson used the same template for both the first router cut on carved top LPs and also the main cavity cut on Specials. If you look carefully you can see that the basic starting shape is identical but routing the last angled pass removed and squared off the rounded areas.

lpspecialroute.jpg


In fact there's a very few examples that I've only seen photos of (in Burst Gang and a couple other places plus verbal reports from people who's opinion I trust) where they appear to have done the first "Special Style" route but then rather than cutting the angled route, just hogged (technical term! ;) ) wood out from the floor of the cavity till the pots fitted. Perhaps the jig that did the angled cut got damaged for a day or 2 or the operator who could do the job well got sick? Who knows, but the fact is that these "aberrant" cavities started out with the correct first step so I'm inclined to think they may be correct. On the other hand one of the things that makes the cavity on the S**y guitar ring so wrong is that it's obvious that the routing Doesn't derive from this important first step
 

Joe Ganzler

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Jul 18, 2001
Messages
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Well said, John - typical of your attention to detail! I had the opportunity to examine FIVE original '58-60 Bursts last night; specifically their respective electronics cavities. One of them, a '58, had the EXACT procedure you describe done to it - a "first pass" with the correct router, and a subsequent "hand fitting" of the pots and also the 3-way switch. When looked at from up top, the knobs were all at different angles to each other! Absolutely NO doubt in my mind of this guitar's originality, BTW. . .

I also looked at the wire channel route on each (including Gladys) - each and every one came out in a little different "place" in the large cavity! Actually, I think it's more of a function of where the channel LOOKS like it comes out, vis a vis the westerly "shoulder" of the routed wall. The "bevel" on the floor of each was in a slightly different orientation on each guitar as well. . .One man's findings. :ahem
 

Lily

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Jul 13, 2001
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This is a great thread! Control cavity 101. :ahem

Thanks guys for all the explanations and illustrations. :yay
 

John Catto

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Jul 15, 2001
Messages
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Here's an example of the oddball route with the first cut but no angle. This is 9_2174 from Burst Gang (can anyone read what it says in Japanese?), I have couple more shots of different guitars (no serials) all done exactly the same way. This is a '59 so if Joe saw a 58 with this it must of happened on a few separate occasions.

Oddball-Routing-9_2174s.jpg


What's really interesting about these is that all the guitars I have shots of with it including this one ALL have the chew-marks so that must have happened not when they cut the angle but rather during some final fitting procedure.
 

58burst

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May 11, 2002
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I would imagine the "chew marks" are an artifact of a drill chuck rubbing against the wood when respective control holes are drilled in the top from the back- Their usual location would imply this, and the non-sharpened surface of a chuck ought to nicely produce the "chewed" effect-
 

DANELECTRO

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Feb 24, 2003
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58burst said:
I would imagine the "chew marks" are an artifact of a drill chuck rubbing against the wood when respective control holes are drilled in the top from the back- Their usual location would imply this, and the non-sharpened surface of a chuck ought to nicely produce the "chewed" effect-

I never thought of this possibility. If you were to remove the pots, I wonder if there's enough meat in the hole to tell if the holes are perpendicular to the horizontal bottom surface or the angled surface. Again, if anybody would like to send me their 'Burst to look over, I'll see if I can verify this hypothesis. Might take a couple of 'em to tell for sure....

:couch
 

Joe Ganzler

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58burst said:
I would imagine the "chew marks" are an artifact of a drill chuck rubbing against the wood when respective control holes are drilled in the top from the back- Their usual location would imply this, and the non-sharpened surface of a chuck ought to nicely produce the "chewed" effect-

It DOES look like that, until you try to "reproduce" that chew mark the same way. . .Terry has tried with all manner of drill (chucks), with 4 and 6-sided bolts attached, etc. . .That chew mark is very unique in that if you look at it closely, there are really deep "pores" to it - almost like fine needle pricks. I just can't figure out what manner of tool would have made such a mark. . . :wha
 

John Catto

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It's a weird mark isn't it. The one thing that seems obvious is that it didn't happen when they cut the angled route which is the usual explanation.
 

Tom Wittrock

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Could it be the result of the first entry of the bit, when making the cavity rout?

Many of us have assumed the bit "shifted" as it left the rout, upon completion.

:hmm
 

plaintop

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58burst said:
I would imagine the "chew marks" are an artifact of a drill chuck rubbing against the wood when respective control holes are drilled in the top from the back- Their usual location would imply this, and the non-sharpened surface of a chuck ought to nicely produce the "chewed" effect-

Almost like a low-speed grind. If you look at the pot vs. marks in relation to how close to the wall of the cavity they are, this theory seems logical. The further away the pot towards the center of the cavity, the less of...or even no mark. No matter what, the pot always appears to be in relation to the chew mark, no? We need to understand the orientation and equipment used for drilling the pot pilot hole. What a great thread!




2410_p39002.jpg

2410_p39003.jpg
 
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Tim

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That chew mark is very unique in that if you look at it closely, there are really deep "pores" to it - almost like fine needle pricks.

Maybe a rat-tail file or something similar would have that texture.
 
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plaintop

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Maybe the control's were piloted from the top, but due to the potential of top damage were bored through from the back? Making the piece of machinery doing the control drill to rub up against the side varying slightly from guitar to guitar?

Mr. Speculation
 

Joe Desperado

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Jun 8, 2004
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Interesting! The chew mark is very much related to where the pots are mounted. The closer to the sidewall, the larger the chew mark. Maybe it was not a standard chuck, but one of those that takes a slip bit. Meaning no tightening key is used. Each bit has the same size shaft that fits into the chuck. I forget what this is called, but many of the pro shops I have been in use this style chuck.

Hmmmm


JD
 

BCR/Greg

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I have always thought the chew mark was from the keyed area on the drill chuck.
 
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