How to cut holes in the soundboard for a resonator?

Posted by Rand Moore on August 17, 2011 at 12:32am

Hi All.

 

I was trying to install a home-made resonator in a cigar box, but was unable to successfully cut a round hole in the box for the resonator. So, I thought I'd open this topic up as an area of discussion. I know there are a variety of techniques from using routers with a circle cutting jig and hole cutting attachments for rotary cutting tools (like Dremel tools), to drill attachments (both fixed diameter and adjustable diameter attachments (up to 200mm or 4.75")).You may know of other ways to cut these circular holes, so please add to our discussion by describing your method.

 

What I Tried:

As I have no router or rotary cutting tools, and the options for drills wouldn't give me the 5.75" diameter hole I needed, I decided to devise a home-made hole cutter using a small board, a screw and nut, and a carving chisel with a side facing blade. Here is a diagram of what I was thinking:

 

And when I tried it, it seemed like it might work okay, but as the circle cutting progressed, I broke the small board holding the knife, so I decided to do it free hand and started making sloppy mistakes, and then the hole cut through the sound board, I got rushed and broke the rest of the hole thru, splintering the sound board pretty badly. Not liking my sloppy work, I decided to remove the sound board and replace it with 2mm plywood veneer. And, I decided to hold off a while longer at building a Reso-CBG until I got better cutting holes. I did take a couple photos before things started going wrong. Here are those 2 photos:

And:

Again, this technique could have worked if my "rotating beam" was a thicker/stronger piece of wood. What I used was a scrap of 2mm plywood veneer, and that proved not strong enough. Notice that you have to cut around and around the circle several times with a fair amount of force to cut through the sound board. When it finally breaks through, the wood will likely splinter. To prevent this you should place some scrap wood between the soundboard and your work surface. If you are working on a separate sound board that you will glue to your box latter on, this extra support won't be needed as your soundboard will be resting directly on your work surface. You also should take your time and be very patient. It's too easy to want to rush through the job and mess it up.

 

Well, that's all I have for now.

 

-Rand.

 

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Replies

  • Ron "Oily" Sprague September 12, 2011 at 2:50pm
    Rand,

    I have a very similar tool floating around here somewhere, that has the cutting blade on only one side, on a shorter bar, which allows about a 5 or 6" diameter circle. I've only used it a couple of times, and yeah, they shake like the dickens! Not sure I'd recommend it for a soundboard, unless it was braced or clamped to a much thicker piece of wood. The last time I used it was several years ago on 3/4" plywood, before I acquired the Rotozip and various cylindrical hole saws. It worked OK for what I needed, but still was subject to a lot of vibration at high speed. IIRC, setting the variable speed drill to a much slower speed gave better results. If you have some scrap, test it on that first at a fairly slow speed. Also, make sure the set screws clamping the blade to the holder, and the holder to the bar, are really tight. I found mine had a tendency to vibrate loose pretty quickly; the slower rotation speed helped that problem as well.
  • Rand Moore September 3, 2011 at 11:15pm

    Hi All,

    A couple of days ago I bought a circle/hole cutter attachment for a hand drill. I bought it from a local Mom & Pop hardware store for 40 RMB ($6.40 US), which seemed to be a pretty good deal. Anyway, I took a few photos of it and decided to post it here. This first set of 4 photos show it before I actually try to use it. I discovered I would need a 2" thick work surface with a center hole in the middle so I can cut holes without drilling a hole in my desk. So, I plan to find some MDF or similar wood about 12" x 12" x 2" to cut my 6" circles/holes.  Maybe I'll have to buy a longer, thinner board and laminate them together to form this special work surface. You'll have to wait till I figure something out for photos on how to use this cutter and to see the results. In the mean time, here are the 4 photos I have...

    305747247?profile=originalHere's the circle cutter attachment which I'll use with my Black & Decker hand drill. I had to use an allen wrench to adjust the two cutting blades so it would cut a 6" diameter circle.

    305748426?profile=RESIZE_1024x1024The chuck had to be opened up all the way before it could hold the center piece of the circle cutter attachment.

    305749076?profile=RESIZE_1024x1024The combination looks like a pretty lethal tool. Maybe that's why I don't see them any more in the hardware stores in the States - OSHA has probably banned the sale for safety reasons. Well, I guess I'll have to be real careful using the tool. Wish I had a drill press. Here's the last photo showing how deep the center bit cut into the work surface -- at least 1.5".

    305750121?profile=originalAnyone have experience using this kind of cutting tool. Is is suicidal to use with a hand drill? Let me know.

     

    Woohoo! That shakes like the dickens when you plug in the drill and fire it up.Looks like I'll need to build a jig to hold the drill steady as well. Or, maybe use a less powerful drill on a low speed setting. Any ideas / recommendations?

     

    -Rand.

  • Rand Moore September 3, 2011 at 9:56pm

    Hi Bo,

    I had not thought about X-acto as a source for circle cutters. Thanks for the tip. I visited the X-acto web site and found the have a number of different tools that could be useful for making your own boxes and cigar box guitars. Here is a image file showing several I thought would be particularly useful. Has anyone had experience with these?

    305746754?profile=RESIZE_1024x1024The circle cutter you are talking about (I assume) is the X7753 Compass Cutter:

    305747900?profile=RESIZE_1024x1024They also have another compass cutter, but they don't say that it can cut soft wood (as the one above can). Here's info on that one:

    305748773?profile=RESIZE_1024x1024I think with both of these circle cutters we'd be pushing the cutting limits to cut holes through cigar box lids. I think 2 or 3 mm plywood veneer might be an easier cut. Certainly Balsa wood would be easy, but then you'd have to add additional internal bracing to support the string tension. Would be interesting to hear from people who have tried these tools on wood.

    The razor saw also sounds interesting for making box joints. I have a razor saw from another company and it serves that function really well. X-acto also makes a medium blade for their razor saw. I wonder what the kerf width is on that blade? Would be great if it could be used for fretting. They have the miter box as well.  

     

    -Rand.

     

     

     

     

     

  • Bo September 3, 2011 at 3:26pm
    Hobbytown USA stores carry a hole cutter for wood - looks similar to your design and has an xacto blade
    Cutter- under 10 bucks I believe. I have a rotozip with a circle cutter attachment- I have mixed results with it because as you cut deeper the bit changes angle slightly and makes a larger hole.
  • Ron "Oily" Sprague August 24, 2011 at 10:03am

    Rand,

     

    No worries. However, a thought occurs to me: you do have Internet access, and the Chinese supplier links I found can do business over the Web, even for 1 piece. What about contacting them via their e-mail links, tell 'em your situation, and see if you can somehow find a bus / subway to them, assuming they are willing to deal? Or does the issue then become the language barrier?

     

    Re cutting curves with a coping saw: With a thin coping saw blade, it's pretty easy to cut for a relatively short distance along a curved line; if you drill enough holes, then you're just making lots of small straight cuts along the chord of a very small curve.Then you shape those out more curved with the file (although my guess is that the lip of the reso cone would cover this). Just a thought - hey, it worked well with the very detailed Halloween pumpkins I used to do for my kids: poke lots of holes along the outline, then link the holes with cuts from a small straight saw.

     

    Good luck, and post pics when your reso is done (or even a build blog).

     

    Oily

  • Chuck Dubman August 24, 2011 at 3:43am
    A broken drill bit of the right size (3/32" I think) can be ground into a cutter if you're cheap.

  • Rand Moore August 23, 2011 at 8:58pm

    Hi Oilyfool,

    Thanks for all the detailed info on Dremel flexible shaft attachments. It makes sense that third party rotary tool manufacturers would try to design their products to be compatible with the market leader. But, I guess I will have to locate a store that sells them. You should know that much of what can be easily bought at any store in the States can't be easily found in China because most of their products are designed and marketed for the foreign markets. And what you could find probably "fell off the truck". Well, things are changing, and more products are available in the Chinese market, but things that tend to be sold for the DIY market still aren't very common, as most Chinese don't have the time and luxury to have a hobby where they can build things on their own. That, coupled with a sizable language barrier, makes it difficult for me to locate sellers/sources in China. I prefer to see and touch things before buying, so I can be sure they will work, but you don't get that when buying over the Internet. So, it will likely be a while before I buy. I'm thinking of checking out new sources in the coming months, as the new extensions to the Shenzhen subway have opened up a lot more travel destinations... they have more than doubled the size of the subway and that opened up just this summer. I need to do this as my wood source (B&Q) seems to have dried up. Both Home Depot (in Northern China) and B&Q aren't doing that well because their traditional DIY markets are still virtually non-existent in China. The independent contractors that they use to build out the cement shells of homes here also don't buy much from B&Q, and for that reason, B&Q hasn't been re-ordering quality wood. The remaining wood is very crappy. Rumor has it that there are good quality wood suppliers in Bao-An between there and the airport. Hopefully they have a subway stop nearby. In the mean time I have wood for 3 more necks and several cigar boxes to play with. So, I'm experimenting with more traditional CBG designs including Reso-CBGs as opposed to building my own body styles or building other kinds of instruments.

     

    I have used your technique of using a drill, saw and files to cut holes in sides of cigar boxes, and I surprised I didn't consider that for cutting a circle. I probably didn't see it because cutting a 6" diameter hole is a much bigger job and involves circular curves as compared to the straight line cuts made when cutting a hole for a neck. You are right, this technique would work if you take your time and have a lot of patience. I have been thinking of practicing more with my coping saw as they also come in handy for cutting f-holes, something that would make my simple instruments look more sophisticated. I guess I should get busy working some wood, rather than spend so much time on CBN.

     

    Well, thanks again for all the info. If I don't use it, at least its posted on CBN for other novice builders to access.

     

    -Rand.

     

     

  • Ron "Oily" Sprague August 23, 2011 at 10:11am

    Rand,

    In answer to your question "Do non-Dremel flexible shaft attachments work with Dremel tools (or vice versa),?" the short answer is a qualified "maybe." First, you'd have to have the rotary tool in hand, or get the info from the manufacturer (some of them do provide this right on the links I looked at) to see what chuck diameters it has, and what attachment spindle diameters it can accept. The flexible shaft attachments I have, have seen, and / or used, typically have a pretty thin, whippy, square profile cable or metal shaft running down the center. This is thin enough to be gripped by both 1/8" Dremel and non- Dremel chucks. The Dremel I have has both 1/4" and 1/8" chuck inserts for various tool spindle diameters, and from what I can see from the pics from the Chinese 3d party manufacturers, at least some of them appear to as well. There seems to be some effort amongst 3rd party manufacturers to at least partially ensure that Dremel (tm) attachments will work with their 3d party rotary tools.

    Reason I sent you the "non-Dremel flexible shaft" link is that it seems from my research that there are tons of Dremel-like tools where you live, and they make flexible shaft attachments for them, just like Dremel does for their tools, figuring it would be less expensive for you to acquire an almost-Dremel locally. China pretty much makes any and everything nowadays.

    Another way of cutting the reso insert, if you have even just a hand drill, a fine file, and a fine coping saw, is to drill a bunch of equally-spaced holes around the circumference of your planned hole, then take the coping saw blade and link up the holes along the outline you've drawn. The fine file can be used to pretty up the edge, although the lip of the reso would probably cover it. It's tedious, but it does work; I've cut various hole sizes this way for other projects when all I had was the above tools. You'd have to do it with the soundboard detached from the box. The key to this method is not to rush it.

    Chuck's method using the Grifhold blade and a compass, and cutting on each side of the board alternately, will work too.

    But I'd rather use a rotary tool.

  • Rand Moore August 23, 2011 at 8:09am

    Hi Chuck Dubman,

    I didn't know that they made cutting blades for bow compasses. That's a great idea. I'll have to give it a try. I think this technique (or my original one) would have worked better if the sound board was not yet attached to the cigar box. Maybe, as a standard practice, I'll remove the cigar box top, and replace it with a 2mm plywood veneer. But before doing that, cut the hole for the resonator (and any sound holes) before gluing the sound board back onto the box. Your idea of cutting half way thru on one side, and then half way thru on the other side also makes good sense. Thanks for you input. I have sumarized what we've been talking about in the following image file:

    305736956?profile=RESIZE_1024x1024Again, Chuck. Thanks for the idea.

    -Rand.

  • Chuck Dubman August 23, 2011 at 3:02am

    A knife-type cutter should work fine.  You probably need a relief cut about 1/16" in from the finished circle diameter to reduce the cutting force needed and prevent splintering upon breakthrough.  Helps also to cut in about halfway from the bottom side first. 

    Grifhold 24-F blades are also useful.  Replace the lead in your bow compass with one and use it as a circle cutter. 

    http://www.learningsea.info/product/Grifhold-24-F-Compass-Blades-pa...

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