FLSTEAM wrote:
For the purpose of making the side sheets and the firebox sheets line up you need to also remember that metal bends on its centerline.
John B.
John,
Correct me if I'm wrong, but what I think you are saying is that the length of the sheets, and resulting position of the holes, will be determined (approximately) by the centerline of the sheets after forming. Correct? My assumption is that the inside width of the firebox will be determined by the outside width of the former.
BTW, thanks for posting the photo and explanation of the forming process you used.
FLSTEAM wrote:Or you could do a lot more work and build a forming jig. Just tack weld the sheet to the jig and start on one side with two burners ( Rose Bud and weed burner )......John B.
Rats. Just the other day I was at Harbor Freight, and they had a propane weed burner on the half-price table. I thought to myself, "what in the devil would I use that for?
Metal bends on the inside edge and stretches the outside of the sheet. Metal doesnt compress.
Redneck you might want to do a little online research on your statement. There are several formulas online to calculate the bend allowance. There is a neutral line ( not exactly but close to center ) that metal bends on.
The first firebox I made the stay holes did not line up with the side sheets as drawn. I recalculated the location for the firebox holes based on the center line and that fixed the problem. The local sheet metal shop found the problem.
John B.
You are correct John. I edited my post to reflect my new found knowledge.
The problem is not the bend. The problem is laying out drilled holes in the flat sheet before the bend.
For example draw two circles one inside the other. Now draw a vertical line and a horizontal one thru the center of the circles. This will represent the bent part and there is a hole located at each intersection of the lines and circles. But remember we have to lay out and drill the holes before the bend so lets do the math. Lets say the inside circle has a diameter of 10 inches. If I remember right the circumference is Pi x Diameter which equals 31.4159 or about 31.4inches. The holes are at 90 deg. from each other or 1/4 the circumference so the distance is 7.85". Lets do the same calculation on the outer circle and lets say it is 10.5" Diameter. Now the 1/4 distance is 8.25". So which dimension do I use? Neither, I need to make my calculations using a 10.25" circle.
it may be easier to just talk to your local fab shop. hydraulic brakes are common in large shops and the men who run them are rather good at what they do. a $20 bill and a 6 pack will go a long way if you go at the right time of day.I had a keyhole crown sheet made of 1/4 sheet on a brake during lunch. if nothing else you may find a place to snag scraps that are prefect for the next project ( there always perfect if the price is right).
I tend to form the two sheets, then clamp / screw them in place with the mud-ring, put the lot in the mill or drill press and use the outer wrapper as a drill guide to find the holes for the inside.
plenty of space to get in there and countersink after the fact.
