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Discussion Starter · #1 ·
Hi all, first post from a routing newbie. I have a tricky job to do and am looking for advice from the experts.

I have a pair of speaker cabinets that ive had cnc cut from 21mm birch plywood. Ive now glued them up and done the first pass of sanding. At some point soon i need to rout speaker openings in the stacked side of the cabinets.

These will be recessed drivers, so the front edge of the cut needs to be clean as a whistle. Ive had a stainless steel circle jig fabricated for my cheap palm router with plunge base.

Hole needs to be 14 cm dia. down to 1cm depth, then an inner hole of 12 cm dia. all the way through 20mm

Im looking for advice on the best method and bits to use. I figure all the experts are here! Ive already been advised on a spiral downcut bit for the edge, and one person said an o-flute will give the cleanest edge. (brands whiteside and freud suggested) Local carpenter when asked for a quote said he wouldnt touch the job with a bargepole.

Ive attached images showing the cabinets, the circle jig, the internal cabinet structure, and a 3d model showing the hole placement.

Any advice greatly appreciated, im a noob, and these cabinets are basically irreplaceable. 馃槙
 

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Hey, Robin; welcome! I'm not sure if you were using the term 'carpenter' loosely or not, but this is more in the realm of cabinet making or furniture making. Even an experienced finish carpenter might not want to tackle this.
In theory it's not really that complicated but the work that's already been done on it, not to mention the to-date expense, is a bit daunting.
So far they look pretty impressive!
If you fill in your profile with a bit of info about yourself, other members will get a better sense of your level of experience, and generally where you're located.
 

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Paul
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One thing that I'd be concerned about is when you've cut through, your center pin will be loose. Maybe you can clamp something in behind the face to keep your center. I would make very shallow passes on the exposed part of the hole. Do you have any scrap you could test cut?
 

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Paul's point is a good one. Your circle jig requires that there be something under the piece you are routing to hold it together (a sacrificial substrate). I'm not sure that's possible in this instance. The alternative to that is to use the circle jig to cut a template that you would clamp to the box and then use an appropriate bit to rout with. I would jigsaw or use a hole cutter to get close to the finished size before routing because when the loose center falls in while routing it can hit the bit and cause it to wobble which will leave a mark on the rim of the hole.

There are two ways to use a template like I described. One is to use a bearing guided bit directly against the edge of the template. The second is to use a template guide installed in the base of the router and the template guide follows the edge of the template. In this method you have to account for the difference between the diameter of the guide bushing compared to the diameter of the bit you are using but the advantage of this method is that you can use straight bits or spirals that don't have a guide bearing.

A down shear spiral will give you the cleanest top edge but I wouldn't try to cut the hole out with it. When the spiral makes contact with the wood it wants to force the router up off the surface so the more wood you cut, the stronger the upward forces. I would just want to clean up an edge with one because of that. Buy a good bit to do this with. Whiteside, Freud, CMT, or Amana. Amana has some Chinese made ones that aren't great quality, but the Israeli made ones are good. I hear that Freud does too but their Italian made bits are good. Not sure about CMT. I believe that Whiteside is strictly US made in North Carolina and may be your best bet.
 

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Welcome. If I'm reading this right, you're needing to cut a rabbet, which is something like a mini shelf. A rabbeting bit, preferable with a changeable bearing is a good way to start. Cut the 1cm depth of the larger opening using your circle cutting jig. You might have to make two passes to get the width of the rabbet with your trim router. For the second pass, you will cut the full smaller circle using the same jig, but this time you will route down through the entire thickness of the material. With a trim router's limited power, you will not want to cut more than 3-4 mm at a time.

I'm assuming the pieces are not yet assembled, at least I hope they aren't. This should also produce a nice finished opening.

The other alternative is to make a template of MDF the size of the larger opening. Cut the smaller opening through with your circle jig. Then tape the template in place and use a rabbeting bit with a bearing to cut the width of the rabbet. Go very slow because your router isn't very powerful and that's a heavy bit. I like the first method better.

The comment about cutting the circle, you can use several pieces of double sided (carpet) tape to keep everything stable while you make your cuts. Make the final cut through slowly and in small increments to limit lateral forces.

Very interesting looking speaker enclosures.
 

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Discussion Starter · #6 ·
Thanks for all the kind replies.

ive glued up the cabinets in sections, with the central parts glued in stacks, and the wider elements separate for now. this is to allow sanding of the recessed areas.. ive almost finished that phase of the sanding. after that i will glue them up completely, apart from the top panel, to allow continued access, and sand the whole speaker, probably with the top clamped on temporarily.

regarding the circle jig, my idea was to rout the outer circle down to the depth of the lip (circa 1cm) then rout the inner circle through, apart from a few tabs around the circumference which i would leave a couple of mm of wood, to hold the centre in place. after routing i would cut out the tabs using a hand saw or stanley knife.

i wasnt sure wether to rout the clean outer edge first (mm depth by mm depth) using the downcut bit, then do the rest with a decent straight cut bit, or, rout out the outer edge a few mm inside the final edge using the straight cut, then use the downcut to slowly enlarge the hole out to the final dimension.

i have been advised the second option, but to my (inexperienced) mind, the first cut with the straight edge risks weakening/damaging the end-grain of the ply for a good few mm either side of the cut, which could compromise the cleanliness of the finishing cuts with the downcut spiral...

what do we think here?


the carpenter (yes im using the term loosely, im translating from the italian "falegname".. these guys do everything from window frames to furniture restoration... there is also an "ebanista" in town, which i believe translates to "cabinet maker" but ive no idea of his skills) suggested cutting inner circle with a hole saw, and then use one of those bits with the bearing on ( im not sure of the name) to run around the edge of that to get the lip.. however a) the roughness of the inner circle would cause imperfections in the edge i am following it to produce,and b) ive only seen those types of bit with a straight edge, which i know is inferior for producing a clean edge..

finally, if i buy a decent straight bit and a spiral downcut bit, and use them with my circle jig, they will be useful for basically any other project.. useful given the cost of decent bits.. if i buy a specific bit for "following" a hole, it will only be useful for other projects where i need that exact size lip again..


thanks for the pointers re. bits.. for the spiral bit i was stuck deciding between a whiteside standard downcut spiral, or a freud o-flute downcut, which have basically the same price. One guy on the diyaudio forums swore by the o-flute as he said it makes a cleaner cut on plywood.. ive read conflicting stuff after which says the more flutes, the cleaner the cut, so im confused...!

my main fear is the unusual nature of the wood to cut.. its all basically end grain, with a change of direction every 2mm, and any number of potentially weak laminations or areas of thicker glue... it will be finished with a transparent coating, so anything more than small single lamination damage will look very ugly.
 

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Robin one of the issues with a hole in any material except mdf is that you are going to go crossways to the grain at least twice. With layers of ply it's more than that. Anytime I build with ply I like to cap the visible laminations with a strip of solid wood. It's easy to finish. I'm still not 100% sure what you need but if the hole is getting covered then your biggest worry is not leaving a ragged upper edge which a downshear bit will help solve. If you need a lipped hole then as someone else pointed out, use a rabbeting bit. I think Freud and CMT have them with downshear cutting edges. That's something you could check on. They should be easy to come by there.

If you need a smooth bore I would be tempted to drill it with a hole saw if you can match the bit to the diameter you need. Sanding sleeves like these: Sanding Drum Kits - Lee Valley Tools will smooth the hole but you have to watch or you'll get the hole out of round with them as they have no guide.
 
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Discussion Starter · #8 ·
Thanks for the further reply. The hole will contain a recessed speaker driver, so the top edge of the cut is the critical bit. (see my attached images earlier..) Although, unless i make the hole exactly the same size as the speaker, there will be a small gap around the speaker, where you will see the wall of the hole.

However the critical thing is the front face (which is *all* plywood end grain, and that is the defining feature of the design of the cabinets, so covering it with veneer is a no-no)

so, question is, as above: is it better to "rough out" the hole with a straight edged bit a couple of mm small, then finish it with a full depth (1cm) pass with the downcut bit to take those last 2mm off cleanly, or is to better to cut the hole in very (1mm or so) shallow passes with the downcut bit? - my concerns are in my previous post.. Cheers!
 

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You might consider covering the whole face with painters masking tape to protect it from tearing out around the end grain cuts, and also for ease of laying out the concentric circles.
Herb
 

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The speakers don't come with a trim ring? Anyway, general rule when plunging is to take a depth no deeper than the bit's diameter. Cleaning up an edge (flush trimming) you can go more but you have to watch because a long bit will chatter, even in only trimming a light pass. That compounds when you are using small shank bits like 1/4 or 6mm. Larger diameter shafts are more stable but won't fit your trim router. One of the advantages of using a guide bushing with an external template is that you could keep making successively deeper passes with one bit but to go any depth with a top bearing pattern bit you might need several different lengths.
 

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Discussion Starter · #11 ·
Thanks again everyone. Yes the speakers have a trim ring but its an ugly thing that also wraps around the side of the speaker a few mm, meaning to have the speakers sitting on the face of the cabinet, they would stick out 1cm. Not very pretty on a small bookshelf speaker. Also, the curves at the sides of the cabinet start before the edges of the driver, so id need to rout the place where the speaker sits flat anyway.. Good advice on the taping up.

Regarding bit chatter, i can also do the cleanup pass in multiple passes of a couple of mm depth each time. My circle jig is quite precise. My main concern is if doing the pass first with the straight cut will weaken/damage the plys surrounding the cut making breakout on the cleanup pass more likely? If so maybe doing the edge cut just with the downcut bit as the first cut i do, but in very shallow passes, might be better..?
 

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Discussion Starter · #12 ·
One final question, would a pva based glue size help with holding the edge together when routing? I was thinking of doing it anyway before final sanding..
 

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Maybe a cyanoacrylate but not a pva. Pva is a plastic and will likely gum up the bit.
 

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Discussion Starter · #14 ·
Well i received my whiteside downcut router bits, glued up a test piece of translaminated plywood, and gave my speaker cut a test run! im quite happy with the result. with the new bits, my little router cuts this beech plywood like butter, and it needed exactly *no* finishing after.

thanks for all the advice, im now excited to do the final cuts, rather than terrified.
 

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Paul
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Your test looks amazing, Robin. Looking forward to seeing the progress on the speakers.
 
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