Acoustic Guitar Construction in the small shop -- Kenneth Michael Guitars LLC

Information, ideas and help for those constructing scratch built acoustic guitars or acoustic guitar kits - © 2017
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PostPosted: Fri Sep 01, 2017 6:38 pm 
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Here's the problem:

I want to project a 2 1/4" circle onto a 45 degree slanted surface. IOW I want to cut a hole into a board that is tilted 45 degrees, that will accept a 2 1/4" shop-vac hose. Like this (below)

Tried various things, hoping for a formula that will allow me to print to scale, the resultant hole shape.
Or maybe there's something my level - i.e., caveman simple - that you can share?
Thanks


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PostPosted: Fri Sep 01, 2017 8:13 pm 
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Location: Seattle
Well I only gave it a little thought. As you are tilting in one direction With 45 degrees I think you end up with an oval that the long axis is the square root of 2(diameter^2) and the short axis is the diameter. you can wiki ovals do find how to draw on with those parameters.

I am a bit confused. I would try to drill the hole with a large bit or hole saw. I would have the bit at a 45 degree angle from the board.

Also as a nutty idea is shine a light through a tube that is the right diameter on a board tilted 45 degrees to the tube and draw the shape.

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PostPosted: Fri Sep 01, 2017 8:51 pm 
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I'd already tried the nutty idea and some other ones. :-)

Thanks John - I don't have a hole cutter that large but perhaps I can get hold of one.

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PostPosted: Fri Sep 01, 2017 10:47 pm 
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How about starting with a "cube" of wood, otherwise known as a square block. (glued up from a few pieces if you like) Use any size of forstner bit to drill holes around the perimeter of the circle you've drawn on it, using your vacuum hose as the trusty tracing pattern. Then use your Ridgid spindle sander to get right up to the 2 1/2" line. Now you have a 2.5" diameter "well" in your block of wood.
Now take that block to your bandsaw and cut a slice out of it at the 45 degree angle.
Maybe this won't work if your drill bits aren't too long.

I'm sure there's a better way. Why do you need it to be at 45 deg?


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PostPosted: Sat Sep 02, 2017 12:15 am 
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That would work. DD, and I have the bits. Maybe that's my best option.
I'm making a special kind of fence for a new-ish tool that is now in my possession, and the dust-collecting portion of the fence as I've designed it just necessitates that 45 degree-er. Perhaps I'm going too fancy-pants with the design, but I'll try another day on it before I curse like a drunken sailor, throw tools around the shop in disgust, and drown my sorrows in sarsparilla....

edit - this was the nutty idea I tried earlier. Not actually that nutty, but I was unable to trace the projected dark oblong shape with any accuracy. I still think this is a good approach.


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PostPosted: Sat Sep 02, 2017 10:03 am 
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get short bit of pipe the right diameter. Cut it at a 45 degree angle. Use it to trace the outline on the board. Or even ink the cut side of the pipe and use it as a stamp.

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PostPosted: Sat Sep 02, 2017 10:30 am 
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Very good, after 10 or so cuppa Joe's to wake up, I will commute out to the barn and get 'er done.
Thanks John.
And dd.

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PostPosted: Sat Sep 02, 2017 12:12 pm 
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http://www.rockler.com/universal-dust-port

I clearly have no idea what you are trying to accomplish. More pixs of the actual device? Perhaps there is a different approach to the goal?

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PostPosted: Sun Sep 03, 2017 10:55 am 
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Location: Marshall, MI
Dave,

This is a simple surface development problem using orthographic projection and draftsmen's triangles. I keep a 30-60 and a 45 degree triangle on a nail between my table saw and jointer to set blade and miter guide angles and square the fence to the jointer table.

Draw a 2.25” diameter circle. Divide it into 30 degree segments. Project these circle/segment intersections parallel to a horizontal centerline through the circle onto a 45 degree line representing your slanted surface. Carry these projection intersections down at 90 degrees to your 45 degree line.

At 90 degrees to your 45 degree line construct a half circle divided into 30 degree segments. Project lines from the half-circle/30 degree segment intersections parallel to the 45 degree line. The intersections of these projections are points that can be connected using an appropriate size French curve, or by bending a thin piece of wood. 15 degree projections are more accurate than 30 degree but probably needlessly complicated for the job.

Get someone to hold the thin strip of wood so you can draw a smooth curve through all 4 points of a quarter of the oval at one time.

If your 45 degree surface to be penetrated by a pipe has more thickness than sheet metal (i.e.: plywood) you will have to rasp the upper half to remove the thickness interference created by cutting out the oval at 90 degrees to the surface.


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PostPosted: Sun Sep 03, 2017 12:23 pm 
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Ah, very nice. The intersection of the north-south projection and the east-west projections yield the to-scale oval shape. I would not have thought it up, but figuring why it works was mucho fun.
Thanks Will. Mas Fino.

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