# Routing a cavity with a close fitting lid?



## digitalrust (Sep 12, 2009)

Hi I need some advice from the router forum experts. 

There is a drawing attached to this post that shows what I want to do. Basically, I need to rout out an electronics cavity in the back of a solid body guitar. 

I have access to a porter cable router, various bits, and a scroll saw to cut templates. But I'm stuck trying to decide which bits to use, bushings, bearings, templates, etc. 

Is there someone who can look at my drawing and give me some "step-by-step" advice on how to accomplish the task?


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## Grumpy Old Tom (Feb 25, 2011)

*Well, I'm a newbie myself but...*

I'd say you'll need a template first. You'll do the deep route first, using a plunge router, going down in steps using a straight bit and the appropriate bushing. Don't take too much at once, hopefully others can advise based on the wood, router speed and the bit size.

Say you have a straight 1/2" dia. bit, and a 5/8" dia. guide bushing. Then you trace your template 1/16" larger than the outline of the deep cut. (If the 5/8 bushing won't go over the 1/2" bit, and you have to use a 3/4", then the template is 1/8" larger than the opening, etc.) You then obviously can't make any corners tighter than 1/4" radius (half the bit dia.)

Once the deep section is routed, it's fairly easy to use a rabbet bit with a bearing to cut the 3/16 lip for your cover.

Hope it helps!

Tom


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## bobj3 (Jan 17, 2006)

HI

I'm a big fan of the template and guide way but in this case I would suggest a pattern bit, no math needed just a true size with a 3/4" thick template, the smaller the bit the better.
Once you have the pocket done you can use the same template and a inlay kit to make the lid to fit just right.

Pattern/Flush Trim Bits

MLCS Flush Trim and Shear Angle Flush Trim Router Bits
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## Grumpy Old Tom (Feb 25, 2011)

*Pattern bit!*

Ah, I see! The bearing on top! I knew a better mind than mine would be along. Thank you, I've "larnt" something new today.

Tom


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## harrysin (Jan 15, 2007)

digitalrust said:


> Hi I need some advice from the router forum experts.
> 
> There is a drawing attached to this post that shows what I want to do. Basically, I need to rout out an electronics cavity in the back of a solid body guitar.
> 
> ...


This is how I do it. To calculate the size of the cut-out in the template, choose a template guide and cutter, for instance a 1" guide and a 1/2" bit, guide diameter minus bit diameter plus size of the finished routed opening.
If the cut-out in the guitar was to be 2" x 4", then: 1" - 1/2" + 2" = 2 1/2"
1" - 1/2" + 4" = 4 1/2"
So the cut-out in the template would be 2 1/2" x 4 1/2"
This pdf should make the method clearer


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## jschaben (Jun 21, 2009)

Grumpy Old Tom said:


> Ah, I see! The bearing on top! I knew a better mind than mine would be along. Thank you, I've "larnt" something new today.
> 
> Tom


HI Tom- one thing about template bits. You need to watch the thickness of the template as well as the cutting length of the bit. The bearing has to be in contact with the guide material in order to have any control so it is already pretty well extended before you start cutting. Cutting length also needs to be long enough to achieve the desired depth of cut, that or you will need to change bits. 
Don't get me wrong - it is a very useful technique, there are just a couple of quirks to be aware of.


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## Ralph Barker (Dec 15, 2008)

My first question, J, would be how many of these are you going to need to do? If just one, it might actually be less expensive to take the guitar body to a cabinet shop and have them do the job to your specs, since they will already have the tooling needed to do the job.

If you're building more than one guitar, it starts making more sense to tool-up, and do it yourself. 

The next question is whether the electronics package actually requires the outline shown in your drawing. Or, was that shape determined more on the basis of the aesthetics of the "look" of the guitar back, once completed? Plus, how deep does the cavity need to be? (the depth isn't shown on your drawing)

Then, there's the question of whether the Porter Cable router you have access to is properly set up to do template routing, assuming that's the method you choose. (The hole for the template guide bushing needs to be _precisely_ centered over the center of the bit, so every thing lines up properly.)

For the template approach, you'll need two templates - one for the cavity, and another for the cover-plate rabbet. (Commonly-available rabbeting bits won't have a small enough diameter for the corners of the cover.)

You'd also need two templates if you decide on the pattern-bit approach that Bob suggested, and, perhaps, more than one bit to achieve the depth you need for the cavity.


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## bobj3 (Jan 17, 2006)

Hi

All done with one template and one bit with the inlay kit to make the lid that would sit on a lip on the inside of the pocket (insert type), you need to think about that one..but a very easy job. 
By the way here's a little tip, put in some fishing line in the pocket b/4 you put the lid in place, in that way you have a way to lift it out b/4 the sanding is done on the lid to get that nice fit on the lid..

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Ralph Barker said:


> My first question, J, would be how many of these are you going to need to do? If just one, it might actually be less expensive to take the guitar body to a cabinet shop and have them do the job to your specs, since they will already have the tooling needed to do the job.
> 
> If you're building more than one guitar, it starts making more sense to tool-up, and do it yourself.
> 
> ...


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## Phil P (Jul 25, 2010)

Ralph Barker said:


> You'd also need two templates if you decide on the pattern-bit approach that Bob suggested, and, perhaps, more than one bit to achieve the depth you need for the cavity.


Then I'll suggest an alternative variation. Make a "master" template for the cover plate in something like 6mm MDF or 1/4in hardwood plywood using a jigsaw or the like then adjust with files, sandpaper, etc until the cover plate passes through very snugly without much light visible around the edges. If the template is loose at points it can always be filled with 2-pack car body filler and resanded. 

Once this master template has been done make a production template by screwing the master onto a similarly sized piece of 18mm MDF or 3/4in hardwood plywood and routing out with a template bit. This gives you a disposable template which will allow shallow cuts to be made with the bearing running against the inside of the template.

Fit a guide bush to the router 3/8in (i.e. 2 x 3/16in) larger in diameter than the template bit's diameter (so 7/8in for a 1/2in diameter cutter). With the template fixed onto the guitar body make a first pass around the perimeter of the cut out and make subsequent passes to deepen until the guide bearing can run against the internal face of this "trench". The main downside to using template bits is that they generally don't come in diameters much less than 3/8in

Remove the guide bush and the template. Hog away the inside of the cut-out in steps using the template bit.

Swap to a bearing rebate bit which will produce a 3/16in rebate and form the rebate this way. Using this second cutter will allow the depth of the recess to be precisely controlled in a way which is difficult to achieve without a plunge router and the thickness of the cover plate may or may not be that precise.

Two cutters I know, but the production approach involves the use of something like an overhead or inverted pin router

The use of a master template allows a "pristine" template to be kept (but never used for routing) which gets away from the need to recreate a production template from scratch when it eventually wears out, gets damaged, etc.

When all said and done the approach I take to a given task is dependent on the tools/cutters I already have wherever possible. I have a good selection of pattern and template bits, hence my "weapons" of choice.

Regards

Phil


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## bobj3 (Jan 17, 2006)

Hi Phil

I like it, do a photo shoot and post it so we can see how to do it.. 
Don't forget, it's a guitar 
===



Phil P said:


> Then I'll suggest an alternative variation. Make a "master" template for the cover plate in something like 6mm MDF or 1/4in hardwood plywood using a jigsaw or the like then adjust with files, sandpaper, etc until the cover plate passes through very snugly without much light visible around the edges. If the template is loose at points it can always be filled with 2-pack car body filler and resanded.
> 
> Once this master template has been done make a production template by screwing the master onto a similarly sized piece of 18mm MDF or 3/4in hardwood plywood and routing out with a template bit. This gives you a disposable template which will allow shallow cuts to be made with the bearing running against the inside of the template.
> 
> ...


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## digitalrust (Sep 12, 2009)

*Routing a cavity with a close fitting lid - followup*

Thanks everyone for all of your help and suggestions. 
This is what I think I have learned from your suggestions about how to solve my problem in the first post. 

The following are formulas for inlaying a "lid" to sit on a "ledge" that surrounds a deeper cavity:

BUSHINGS AND BITS
The inside diameter of the bushing used to cut the inlay must be just large enough to fit the bit.
Three rules that work: 
The inlay bushing outer diameter = Bit diameter *2
The ledge bushing outer diameter = (the size of the inlay bushing + (2 * the diameter of the bit)
The cavity bushing diameter = (the size of the OD of the ledge bushing + (2 * the width of the ledge) 

Ex 1: 
1/8 bit
Inlay bushing 1/4" outer
Ledge bushing 1/2"
Cavity bushing 7/8" for a 3/16" ledge

Ex 2:
1/4 bit
inlay bushing 1/2" outer
ledge bushing 1"
Cavity bushing 1 1/4" for a 1/8" ledge

Sand the inlay/lid to fit. Put fishing line or thread across opening to be able to get the inlay/lid out when test fitting. 

TEMPLATES
Now to figure how large the opening in the template needs to be for a certain bushing bit combination 
use one of these formulas: 

(a) For irregular cavity or cutout shapes: 
Template offset formula = (Bushing guide outer diameter - the diameter of bit) / 2
Example: If your guide bushing is 1 inch outer diameter and the bit is 1/2 then the off set would be 1/4"

(b) For rectangle or square cavity or cutout shapes: 
Rectangular cavity template cutout formula = (Bushing guide outer diameter - diameter of bit) + size of finished routed cavity or cutout..
So, if the cavity or cutout is to be 2" x 4", with a one inch bushing and a 1/2 inch bit
(1" bushing minus 1/2" bit) plus 2" = 2 1/2"
(1" bushing minus 1/2" bit) plus 4" = 4 1/2"
So, the template should have a 2 1/2" x 4 1/2" rectangle cut in it.

Please check my work. 

Thanks again!


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## Mike Wingate (Apr 26, 2009)

When I do this, I use a template and a guide bush. I remove most of the waste with suitable sized Forstner bits, almaost to depth, then use the template, sticking it down with hot glue. Route the cavity. Then use a bearing guided rebate bit to route the ledge.


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## digitalrust (Sep 12, 2009)

I follow your procedure but once you get the cavity and the ledge cut what do you do to make a lid that fits the rabbeted ledge?


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## Mike Wingate (Apr 26, 2009)

Use the template and suitable bit/guide bush combo.


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