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Diagonally cutting across a plywood cylinder? Answered

i want to make a couple of identical circular wedged speaker enclosures by cutting a large drum shell (16in by 16in floor tom) in half diagonally and then capping the open sides with discs, what would be the best/easiest method of holding the shell in place and making an accurate straight diagonal cut?

i have access to a maker space but i haven't been in since their recent refit so i'm not sure what cutting machines they now have...

Discussions

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bassbindevil
bassbindevil

2 months ago

I'd make a rough cut around the drum with a sabre saw, then fit a piece of plywood (or mdf or particle board) in the end (somehow), then position my router to trim the rough edge flat (space the router (which has an extended base) off the plywood with a scrap of mdf).

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Jack A Lopez
Jack A Lopez

2 months ago

I did not know plywood was available in cylinder shapes. How about that?

https://duckduckgo.com/?q=plywood+cylinder&iax=ima...

https://duckduckgo.com/?q=floor+tom+shell&ia=image...

I am not quite picturing what you mean by, cutting "in half diagonally and then capping the open sides with discs,"

I am guessing the cut you want is the intersection of a plane with a cylinder, and the, what you call "discs", are ellipses, or circles, for the cut and uncut ends respectively.

http://mathworld.wolfram.com/CylindricSection.html

Because the dimensions of your cylinder are large, compared to ordinary sized saws and miter-boxes, I think you will have to calculate where the cut needs to go. Then map those points onto the surface of the cylinder.

I suggest using Cartesian coordinates, with the axis of the cylinder centered on the z-axis. Or a combination of Cartesian and cylindrical coordinates, to calculate where the cut goes, and mark those points on the surface of the cylinder.

http://mathworld.wolfram.com/CylindricalCoordinate...

Then make several cuts, slowly and carefully, using a hand saw.

To keep the cylinder from moving around, maybe you could strap it to something, like a pallet, or just some boards resting on the floor, using some of those ratcheting tie down straps.

https://en.wikipedia.org/wiki/Tie_down_straps

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ambientvoid
ambientvoid

Reply 2 months ago

the enclosures are for mounting onto a sidecar type trailer, hopefully this rough model i did in Tinkercad gives you a better idea of the shapes i intend to end up with

rotate backwards 2.jpg
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Jack A Lopez
Jack A Lopez

Reply 2 months ago

Oh wow! Thanks for the picture. I guess that is not too different from what I was imagining.

The cut is as I imagined it.

I guess the caps for the "open sides" are circle-shaped, and the ellipse-shaped sides rest against the flat bed of that trailer.

It might be fortunate the edges of the ellipse-shaped cut are facing down, since that will make imperfections in that cut harder to see.

You know, if you attach your drawing picture to the main topic, then the rest of the class could easily see it. Although it might be the case, I am the only one here who really "needed" to see it, because of my complaint about, "I am not quite picturing what you mean..."

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-BALES-
-BALES-

2 months ago

This sounds tricky, but a few thoughts come to my mind:

1. If you can find a large enough bandsaw, you could make a sled with a toggle clamp on each side to hold the shell. You'd end up putting exponentially more time into making the sled than making the cut, which might not be worth it unless you plan on making several cuts.

2. With a long enough hand saw, you could clamp the shell to a table and make the cut manually.

3. Draw your cut line onto the shell - I'd probably make a poster board template, which could slide onto the shell for repeatable tracing. Make the cut using an oscillating multi tool or a jig saw, but don't cut right to the line since you'll want to sneak up on that by sanding.

For sanding, unless your makerspace has a very large disc sander, I'd get a very flat surface -- 3/4" melamine/plywood or solid stone countertop cut off. Adhere sheets of sandpaper to this surface with spray adhesive and then use it like a truing table. AKA .. run the cut edge of your drum shell across it until you get a flat plane.

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ambientvoid
ambientvoid

Reply 2 months ago

i reckon your 3rd option is my best bet, i'm not sure how to go about doing an accurate template but i think i'll have more than enough space to mark a precise line at least a quarter of an inch either side of a central cutting line that i can sand down to...