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Reduction of inside diameter of a steel hoop.

 

 

Hello, I need to reduce the inside diameter of the 5mm thick, flat bar, steel tuning hoop on a conga drum and would like to know how to calculate exactly how much material I need to remove from the hoop when I cut it without disturbing the allignment of the 5 tuning-lug points of the hoop to the tuning-lug plates on the drum shell?

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Let me get this straight:   you have a flat bar of 5mm thick metal with five holes drilled in it.  You bend the bar into a circle (hoop), but when the ends meet, completing the circle, it's too big for your purposes, but the holes align with the holes on the drum.  And you want to know how much to cut from the end of the bar, so that it fits the drum better, and maintain the position of the 5 holes relative to the holes on the conga drum?  Does that state your problem?  If so, you are in deep doo-doo.  First, we'd need to know by how much you want to decrease the diameter of the hoop.... a foot?  1/10th of an inch?  But the basic formula is that circumference = diameter x pi.  In your case,  circumference is also the length of your steel bar.  Cutting off 3.14 or so inches from the bar will reduce the diameter of your hoop by an inch.   BUT your problem is with the holes.  Their positions are fixed, but the distances between them will vary with the diameter of the hoop.  Therefore, as you change the size of the hoop, you gotta drill the holes in different locations.  The hole arrangement that works for a 12" hoop will not be the same as for a 10" hoop.  Save yourself the headaches and just buy what you need retail.

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I'm still a little confused.  If the hoop goes around the drum and all the holes line up, but it's too long (at the ends), measure how much the hoop overlaps and cut half that amount from each end.

Reread the problem as presented.  No mention of an overlap, just a "hoop" with an overlarge diameter but CURRENTLY correctly positioned holes.  Reduce the diameter and the position of the hoop holes, relative to the holes in the drum, change.  Try it with a piece of paper.  

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