Sheet metal work in crossbow building

Good Day to all my fellow forumers,

Haven't been on much lately, mainly due to work and a few old and new projects that I'm trying to manage on a budget(not because I can't afford it, but because it's a challenge...you get me )

So today I would like break the routine and finally sit down and ask for your help (some of you still believe the myth that I'm a crossbow guru )

So I was working on this one bracket design and in CAD everything folds perfectly, however approaching the actual sheet metal business turned out to be a bit problematic.

The material is 16ga steel (is that still considered sheet or plate? ) that will be polished and case hardened once bending is complete. Before I continue, this is what I have in mind.

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The quirky part is the bend on the inside of the bracket...if you guys have any advice on the issue and specifics of bend allowance(compensation?) for 16ga range metal...I would greatly appreciate your input.

Thanks,

Ivo

PS: I hope this topic doesn't stop on this one question's resolution and we can discuss sheet metal further as I believe it has great potential in crossbow building that not many people realize.

Ivo,

For 90degree bends, I think the usual allowance is 1/2 the metal thickness, but it depends on the radius of the bend.

What I would do is this.....

-Make up a mandrel that is the size and shape of the thing which is to go into the socket.

-Make the tab which is going to be bent back be the exact width you need, but longer than you think it has to be.

-Bend the tab around the mandrel using the vice and a hammer.

-Bend the sides up using the vice, a C-clamp, and a hammer.

-Trim the tab to length

With this method, you don't have to know the allowance; you just have to make the mandrel accurately.

Mac

I join the MAC.
First bend. Then drill.

Here is the little I know about sheet metal bending. I believe the minimum bend radius should be 1*thickness, 1.5*thickness is better. Of course if your not using a bender then that isn't as likely to be exact. Figuring out how much extra to allow for the bend is actully not a trivial question as steel deforms and stretches when deformed. Depending on the 3d Modeling software it may have a feature to create a flat sheet metal pattern from a 3D model. How much it deforms depends on the thickness of the material and how hard it is and yes it can matter if its cold rold or hot rolled steel.

I second (or third) the suggestion to make the bend sides a little longer and file back if your worried about it. once you have made one and have a grasp on the how much it will deform you can make the others closer to correct size. One of the nice (or terriable) things about sheet metal is that it does deform so if things aren't quite right you can deform it a bit more to make it fit.

As for the holes. Ideally they should be at least 3*thickness + bend radius away from the bend so that they won't be deformed. But in your case with the low production numbers and, I assume desire, to have the holes inline with one anouther then drilling them after bending might not be a bad idea. the shape seems strong enought to take being drilled with out deforming too much. But placing a piece of wood between the two sides to give it extra strenght during drilling would be a good idea as well.

If bending on a mandrel consider some degree of radius on the corners. This should be less stressful on the metal.

Todd

Big Thanks guys!

It was really new to me, so I didn't really have a good angle on the approach. With your help I was able to research it better given the new keywords and points of attention.

The videos on you tube showing the operation of electro-magentic sheet metal brakes/benders I got a good picture of what a mandrels you were talking about(about 2 minutes into the video they bend some cool stuff with those little mandrels)



I have a pretty cool idea of a "bum" sheet metal break on my mind, I'll dig around in my parts bin and be back in a few days with an update...you guys are going to love it(or hate it ).

Again Big thanks to all...more to come.

Ivo

For these small parts a steel bar as a mandrel like Igora has shown might just be quicker but then again having a little brake press around the shop might be kind of handy.

And thanks for the video on the Electromagnet Brake Press. That was really much more interesting than I thought it was going to be. First thought was "Why make some thing simple more complicated?" but seeing what they can do with that. When I win the lottery I might have to pick one of those up
Doesn't everyone has their ideal shop design floating around in their heads for when they come into large sums of money?

Third time's a charm...right? ... - damn right.

Three times I approached this thing and three times I messed up(well 3rd was really close and I was able to pull out of it)... 3AM will do it, however today I can safely say that this was the last of it...feel like a hero here.

Here it is, my "bum"(sheet metal)break in it's basic form.



Weapon of choice...



Man, I really gained some respect for the flame with this little project...this isn't some MIG welder - point and click deal...you really have to watch what you're doing there...even though it's just a simple braze joint. And I thought that with years of centrifuge casting experience i had it all figured out...well, it's good to learn new stuff.

Anyways, just thought I'd share my progress with you guys...still a bit of work to do here and there(like getting proper bolts for axels, so that the hinge isn't riding on the threads)...and I know it's a brutal looking braze, but I'm happy so far - the most pleasant squeaky hinge I've ever heard ...next step - I'll bolt it down to a much wider metal plate(to which I will also bolt down the finger/plate mangrells) and bolt the whole thing to the bench. Damn, it better hold together for 16ga.

Ivo

So I made my first few bends on the Quasimodo sheet-metal brake.

The very first one had almost a "cm" radius and I was pretty
pissed..didn't even take a picture(sorry), then after a few adjustments I
started seeing some improvement.





It's nice and straight and I can even press it with all the holes
aligning on both sides, but a sharper angle is still left to be wished
for. Even after staying up all night, making and brazing in the
reinforcing brackets...




...the angle iron still flexes a tiny bit, which is unfortunately "enough" slack to fail in achieving a nice sharp 90 degree bend.

For now I still have a few choices...

Heat the line of the bend with an oxy torch and bend it while it's hot

...or...

Build a new, smaller, beefier press, And just keep this bender for thinner metal.

Ivo

Hey the press looks good.

Heating the bend might work but the rest of the press could act as a giant heat sink and prevent the sheet metal from staying hot enought to help.

I think the smaller, stouter press might be a better option as long as you don't have some unrealistic idea of what good bend should look like.

Thats all I can think of right now but I have lunch on the brain. so good lunch and I'm off to find food

Basilisk120 wrote:Hey the press looks good.

Thanks.

It's an ugly bastard compared to the heavy versatile little box/pan brake monsters out there, but I can't complain since it cost me practically nothing to make, the only investments being the brazing rods/gas..rest is salvaged angle iron and some misc parts I had laying around...compared to the commercially made little presses going for $800-$1000.

Basilisk120 wrote:Heating the bend might work but the rest of the press could act as a giant heat sink and prevent the sheet metal from staying hot enought to help.

True, still I managed to get it working the way I wanted. The heat sink issue wasn't the worst part. I preheated the press, slipped in the sheet under the pressure plate, heated the line of the bend and all went pretty well, BUT it took allot of gas to get there, so even though it worked out well there is still a bit of a problem with cost effectiveness of the process. (yeh, keeping in mind that I'm on a budget here ) Oh well, learning has and allays will have a price.

Basilisk120 wrote:I think the smaller, stouter press might be a better option as long as you don't have some unrealistic idea of what good bend should look like.

That's the next step.

As for my idea of a good bend...well I'm being as realistic as I can and push off of things I've seen on factory made components. This is what I'm happy with.




Nott too much difference compared to the one I did before, except that it's a cleaner bend. No bending this thing out of shape by hand...I gave it to everyone in my house and at work and had them try to unbend it...or at least bend it slightly out of shape.
Haha...no luck doing any of those.

...It's solid.

If I can get this kind of bend with a heavier little press without heating or hammering over mandrels... I'll be happy.

Thanks for the help, BTW that little box above is a trigger housing for the little crossbow I'm building...ohhh I'm close my friend, sooo close.

Ivo

That's looking really good Ivo. Maybe more supports or heat treating on the bearing edge? For your purposes it seems functionally adequate.

It's an ugly bastard compared to the heavy versatile little box/pan brake monsters out there, but I can't complain since it cost me practically nothing to make, the only investments being the brazing rods/gas..rest is salvaged angle iron and some misc parts I had laying around...compared to the commercially made little presses going for $800-$1000.

Well I often get accused of having no fashion sense. I guess I just prefer function over form.

the second piece does look like it has some nicer bends in it and that is a goal worth shooting for. I would buy those parts.

One quick question since I didn't see the brake in operation. Is it the angle iron that is flexing or the flat, bolted down mandril. It kind of looks like the mandril is flexing up hence the unevenness in the leg not the base.

What grade of bolt did you use to bolt it down with?

Sorry to leave you hanging there for a while

The grade of bolt is probably the lowest homedepo had to offer...I bought a big bag on Sale one day when I was building all metal workbenches in the shop (drilled and tapped every hole by hand...unbelievable )

Nothing off the top of the head, have to double check when I get home. I'll just make a video and show you how this thing works, flexes under the load...and actually I think I may even have a solution to my problem. We'll see.

Ivo