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21st Jul 2019, 08:28 PM #1
Notching RHS to a cylinder- how to calculate angles
I'm building a BBQ, the big, obnoxious, American kind- AKA an Offset smoker.
I'm using a cylinder of approx 530mm dia. I don't own a spring caliper, so 530mm is as best as I could get with the tools I have.
The frame is a truncated isosceles triangle which intersects the cylinder at a point (a chord of the circle).
Is there an easier (maybe mathematical) way to calculate rough angles to allow a bevel to be cut then scalloped , say, for an edge of the RHS to meet a given chord of said circle?
I reverted to the old cardboard cutout, and transposed the shape after laying out the design on my welding table with chalk, compass and ruler. Worked, but a lot of messing about.
20190721_154233.jpg
Pic of the project. Disregard the plumb line which is clearly not plumb. Also disregard the dirt floor fab area I'm working in. It's a bit too big for inside my little shed.
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21st Jul 2019, 08:33 PM #2
... also disregard the fact that gravity clearly doesn't apply in my workshop.
Explains all the missing 10mm sockets...
Hindsight being 20/20... I could have worked out radius / diameter of cylinder using its circumference.
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21st Jul 2019, 08:52 PM #3Philomath in training
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The angle of bevel will be the tangent of the circle where the leg touches. The tangent is at 90 degrees to a radial line, so using some trig if you can work out the angle of a radial line to the contact point, the tangent angle should be easy.
Michael
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22nd Jul 2019, 08:59 AM #4Most Valued Member
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If you are making more just stick with the template.
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24th Jul 2019, 08:41 PM #5
I'm going to brush up on my trig, and bust out the year 9 math skills...
I don't know why, but this job bamboozled me. The whole time as I was man-handling these lumps of RHS on and off the bench, I was thinking ' there has to be a way to at least get the cut close first time'.
I'll need to work out some examples, but in the instance pictured above, the cylinder is not centered on the apex of the triangle. Not sure how this will affect the calculations steps, but in the end the fun is in working it out.
The next one I build will be a bit more balcony friendly. 100L as opposed to 400L water capacity
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24th Jul 2019, 09:28 PM #6Pink 10EE owner
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- Aug 2008
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- near Rockhampton
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I generally just draw stuff in one of the free CAD programs and measure from there.
Gold, the colour of choice for the discerning person.
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25th Jul 2019, 07:26 AM #7Golden Member
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25th Jul 2019, 07:46 AM #8Intermediate Member
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- Jul 2019
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- Caboolture
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- 47
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- 28
place the leg against the cylinder where you want it
place a marker in the middle of a washer
Run washer around outside of cylinder while pen marks the leg.
Should give you the profile to cut.
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