DIY Backplate webbing slot sizes/angles

[redacted]

I saw a link to a backplate design in another thread.

http://www.scubaboard.com/attachment.php?s=&postid=112280

I was able to pick up a cutoff of 1/8" aluminum for peanuts at the local metal shop and I'm going to try make something mostly following that design.

My question is on the size and angles of the upper webbing slots. How do you get away putting 2 inch webbing through the top 1 3/4 inch slot? I know it is on the bend but I still can't figure out how you can get away with less than a 2 inch slot.

PS. It's one hell of a lot easier to make a slot a little larger than to try to make it a little smaller.

Also, it seems that some production backplages have the shoulder straps angled a bit at the top. Is this advisable?

-Andrew

Those are 2 inch slots, not 1.75. Slots are cut by drilling a 1/4 inch hole at each end and the distance between the center of the holes is 1.75 inches. The drill radius then produces a 2 inch slot. When I made mine, I got the slots a bit too wide and had some webbing slippage problem. I'd suggest you stick with the 1/4 inch drilled holes but cut the slots a bit narrower - like 1/8 inch wide. As you deburr and smooth the cuts they widen a bit and fit the webbing fine.

Angles are on the drawing. Upper slots are 45 degrees off vertical and the 4 lower slots are 20 degrees off vertical. My upper slots are parallel to the top of the plate and I am getting some wear on the inside of the straps as they come over the top of the plate. A 5 to 10 degree angle in the upper slots would probably have allievated that wear.

EDIT: The 45 degree slots in the drawing are 2.1 inches from drill center to center giving them a total length of 2.35 inches.

PS. It is one hell of a lot easier to make a slot a littler larger than to make it a little smaller.

 

[aalbinger]

Those are 2 inch slots, not 1.75. Slots are cut by drilling a 1/4 inch hole at each end and the distance between the center of the holes is 1.75 inches. ... ... ... The 45 degree slots in the drawing are 2.1 inches from drill center to center giving them a total length of 2.35 inches.

PS. It is one hell of a lot easier to make a slot a littler larger than to make it a little smaller.

indeed. Makes sense now. Thanks!

I've started cutting / shaping. Got the end holes for the slots done and maybe will get them connected tonight yet.

On to tomorrow. Has anyone here bent an aluminum plate at home without the benefit of a machine shop with a really nice brake? I'm thinking of annealing the aluminum (heat to ~800 f and then quench quickly) and then using angle iron in a vise and a piece of ply clamped to the side I'm bending from. Going to test this on some of the left over scrap first of course. Any suggestions?

-Andrew

 

[jonnythan]

There's no problem with that 1.75" measurement. It's 1.75" in the horizontal plane, but the webbing is not normal to that plane. If you measure the width of the slot directly from end to end it will be 2", however, looking from the top down, it can be less.

Imagine a 2" slot at a 45 degree angle to the viewer's angle. The measurement from above would be about 1.4 inches.

 

[JeffG]

indeed. Makes sense now. Thanks!

I've started cutting / shaping. Got the end holes for the slots done and maybe will get them connected tonight yet.

On to tomorrow. Has anyone here bent an aluminum plate at home without the benefit of a machine shop with a really nice brake? I'm thinking of annealing the aluminum (heat to ~800 f and then quench quickly) and then using angle iron in a vise and a piece of ply clamped to the side I'm bending from. Going to test this on some of the left over scrap first of course. Any suggestions?

-Andrew

Mine was bent without heating, but a round rod was used to "bend around" (1/4 " bend radius IIRC...might of been 1/8").

 

[redacted]

I used 1/4 inch T6 aluminum. Without heat, I couldn't even bend a 2 inch wide strip to make a STA. Heat will definately soften it up but I'm afraid the bend may come out pretty ugly. I took mine to a machine shop to get the bends. It only took about 10 minutes. It was at work so it only cost me a 6-pack.

 

[Dive Doctor]

I saw a link to a backplate design in another thread.

http://www.scubaboard.com/attachment.php?s=&postid=112280

I was able to pick up a cutoff of 1/8" aluminum for peanuts at the local metal shop and I'm going to try make something mostly following that design.

My question is on the size and angles of the upper webbing slots. How do you get away putting 2 inch webbing through the top 1 3/4 inch slot? I know it is on the bend but I still can't figure out how you can get away with less than a 2 inch slot.

Also, it seems that some production backplages have the shoulder straps angled a bit at the top. Is this advisable?

-Andrew

The 1-3/4" dimension is the center-to-center distance for the the two 1/4" diameter holes that make the radius ends of the slot. Total width is 1/2 of the diameter of the 2 holes (1/2 of 1/4") added to the center-to-center distance; 1-3/4" + 1/8" +1/8" = 2" total slot width.