is there a formula for making a cap?

brian · #1 10-12-2018, 10:02 AM

I'm about to make a sheet metal cap to cover an open cylinder. While I could just eyeball it and make random notches to prevent overlap when folding the flat sheet metal down over the sides of the cylinder, I'l bet there's some sort of formula to calculate just how many notches there should be and how many degrees of angle (or total length of notch cutouts) there shoudl be for no overlap and no gaps. While the "no gaps" isn't critical, I thought it would be nice to make it so the portions of the sheet metal that get folded over the top meet nicely on the sides.

I hope I've accurately described what I am making; just a round cap from flat sheet metal to go over and cover a cylinder, with the flaps folded down over the sides, to form a cup.

Is there a formula to get this just right, without a lot of trial and error?

Thanks,
Brian

BillL · #2 10-12-2018, 10:22 AM

Quote:

Originally Posted by brian

I'm about to make a sheet metal cap to cover an open cylinder. While I could just eyeball it and make random notches to prevent overlap when folding the flat sheet metal down over the sides of the cylinder, I'l bet there's some sort of formula to calculate just how many notches there should be and how many degrees of angle (or total length of notch cutouts) there shoudl be for no overlap and no gaps. While the "no gaps" isn't critical, I thought it would be nice to make it so the portions of the sheet metal that get folded over the top meet nicely on the sides.

I hope I've accurately described what I am making; just a round cap from flat sheet metal to go over and cover a cylinder, with the flaps folded down over the sides, to form a cup.

Is there a formula to get this just right, without a lot of trial and error?

Thanks,
Brian

Google for a tinners handbook. They have all the cool layouts for making all sorts fo shapes.

Here is one example , you can use it online for a month by signing up. I've seen better, but it was well guarded by its owner. It had amazing layouts.
https://www.scribd.com/doc/105666556...ct-Layout-Book

BobTurner · #3 10-12-2018, 10:23 AM

Count the number of flaps you?re planning, call it N.
The notch between each flap should be 360/N deg.
e.g., 4 flaps, 360/4 = 90 deg.

jacoby · #4 10-12-2018, 10:40 AM

How accurate does this need to be? And you want straight sides?

The general way is to decide how many flaps you want. I'd do something that is easy to lay out. Then you compute the delta of the inner and outer circumferences and divide that by the number of flaps to get your notch size at each segment. 8 is probably a minimum but again it depends on what you want.

Notch width = ((OD*3.141)-(ID*3.141))/N

In other words, you want to cut off everything from the OD until what's left matches the ID.

For a 2" ID with a 1/2" flange (3" OD) and 8 notches you end up with a .39" notch width.

If accuracy matters then you need to calculate your ID based on a triangular segment of the circle since your folds will go from the point of each notch and will clip the circle a bit. For example, if you laid a 2" ID circle with 8 notches, the actual ID will be around 1.9"

spark68 · #5 10-12-2018, 09:34 PM

Im a bit old school and simple...

If I were doing something like this I would make a cardboard template out of poster board and cut to fit. When you are happy with the fit you can flatten it out and use it to mark the layout on the sheet metal. It will depend on your bend radius matching the cardboard to get the same fit.