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Discussion Starter · #1 ·
I am working on a project where I need to cut some rectangles inside a piece of 3/4" MDF. I made a few of them using a Forstner bit and Skil saw, cleaning it up with a file and sanding block. The first one was tough, the 3rd one went smoother and turned out better. However, I can't help but think there should be a way to use a plunge router with a jig and pin or something along those lines.

What is the best practice on something like this? Can a jig be built using lumber or is is better to purchase something that is easily adjusted? Can you show me an example of something like this being used in a picture or video?

See pics of my current technique and results. The results are pretty good, and perfectly acceptable as these will not be seen, but want to learn.

Cheers,

Gavin

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Theo
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I don't know what the best way is, but as far as I am concerned, I just have what I call a master, you would likely call it a template. I don't cut out squares, but it would be done the same way, squaring up the corners after, if desired. I run a pencil around the outside, and inside - because I do the outside at the same time. Rough cut around the outside, route it nice and pretty. Inside I drill a 1" hole, rough cut around the inside, then rout the inside, following the master. Then do the same with any more pieces I may want. Works for me. Oh yes, my master is tacked down, I use a straight 1/2" bit, with a bearing on the end.
 
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Try a corner cutting chisel. Line up with your rounded corner and use a hammer to tap your way through. Place the MDF workpiece on a piece of hard wood to avoid tearing out the corner. Pix is of a Wood River Quick Cutting Corner Chisel, $22 on Woodcraft. Tap it, don't whack it.
WoodRiver - Quick Cutting Corner Chisel
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@WoodFrog,
I'm with Theo. If you need to make replicas of one shape only, you can do without and adjustable jig. Take your best piece so far, and use it as your template - do any final smoothing of the inside edges if needed. Then on a workpiece, mark the outer and inner squares/rectangles, and cut out the inner squares/rectangles close to the line with you jigsaw, but leave a bit of waste. The starter hole needs to be just wider in diameter than your jigsaw blade - you may not even need a Forstner bit. Do this before you cut the outer dimensions, so your router will have plenty of support.
Cutting with a jigsaw is generally faster, but more importantly, especially on mdf, it generates less dust. Mdf is, as you know, compressed and resin-bonded wood dust. When cutting or routing, you are pulverising it - the amount you pulverise is proportional to the width of your cut, i.e. thickness of the jigsaw blade, vs thickness of router bit. Further, you will have to make several passes, at increasing depth, with your router.
Once you have made the cut-out, stick on the template with double-sided tape (lining up with the scribed lines), invert the board, and support the edges with some scrap mdf (same thickness as the template) to prevent tipping. Then use your router with a straight bit (or compression bit, if you want to get fancy), with a bearing (same diameter as the bit) on the end furthest away from the router. Set the height so that the bearing rides on the template, which is under the workpiece, and you can get a very good replica in one pass.
Thereafter, remove the template and cut the outer dimensions - you should get a good enough cut with a decently sharp blade in a stationary saw, no router needed - maybe a touch of sanding. Do you have a table saw or radial arm saw?
If you are using a largish mdf board to start with, you could establish quite an efficient workflow: mark multiple workpieces with your template, drill them all, cut all insides, rout all insides, then cut the workpieces loose from the "mother" board.
 

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Thanks for the tips, I think I followed what you are saying, will see what I can do.

Edit: I think maybe what I was looking for is something like these jigs.

Woodpecker's
Trend Varijig
DIY Jig

Anybody have experience with these?

Gavin :geek:
You can do something similar with four pieces of wood. By using fairly wide pieces, you can provide extra support for the router.
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Discussion Starter · #8 ·
The application is a rectangular hole, with a recess, for a ribbon tweeter in a set of floor-standing speakers. So, no cut-cut-glue-glue and must be very precise. I decided I am going to build a jig and do some testing, then give it a go once I get satisfactory results on my test piece. I will definitely report back, stay tuned...

FYI, the pictures above are of the internal braces, so the quality I got with the Skil saw are excellent, but that technique will not work well enough for the visible baffle cuts.

I appreciate all the ideas and support! 📐📏;)

Gavin
 

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Welcome to the forum, @Saomai

You may be able to follow the pictures to see how it was made.

Not sure if @WoodFrog is still on the forum.
 
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Looks great, I am doing something similar.
Are the inside corners rounded over and then cut out with another tool?
Not sure you are accomplishing everything with one bit and one template?
 

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Welcome to the forum, Keliot.

Using this jig, the corners will be round due to the rounded cutter. If you want square corners, they coud be trimmed with a coping saw or jig saw. Or you could make the jig large enough to run a jig saw inside the actual jig.
 

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Looks great, I am doing something similar.
Are the inside corners rounded over and then cut out with another tool?
Not sure you are accomplishing everything with one bit and one template?
You can use the four pieces shown above to make a quick pattern, then drill a half inch diameter hole maybe 1/8 from the inside edge, then use a jig saw to rough cut the inside shape. With the four pieces in place as before, use a flush trim bit with bearing riding on the four pieces to get the four sides nice and straight and smooth. Then you can remove the four boards, and square off the corners. The result is a pattern you can use to repeat the process as often as you like. For example a picture frame with multiple same-size openings.

If you use a 3/8ths trim bit the rounded corners won't be a big deal. I wouldn't use a router to cut the bulk of the opening out. You'll burn through bits much faster. Cut with a jig saw, just outside the frame line so the trim bit doesn't have to hog out so much stuff. Make sure you get the right kind of jigsaw blades for what you're cutting when hogging out the solid center. I'd probably use my corner chisel if possible, but you might also consider using a regular chisel, or even a Japanese pull saw to cut really clean square inside corners. Be careful, chisels and Japanese pull saws are both very sharp.
Sleeve Tool Blade Waist Cutting tool
 
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