Leverage laser-cutting and tab-and-slot fabrication

[Moderator]

(This could fit in the Suspension sub-forum too. It's kinda on the edge.)


I was reluctant to start designing parts for laser cutting. I'd worn out copies of the sacred scrolls: Carroll Smith's "Prepare to Win" and Ron Fournier's "Metal Fabricators Handbook". Those books are truly classic. It'd be a far better world if every kid were brought up with them. After studying them carefully and lots of practice, I reckon I'm pretty good at cutting, filing, bending, and hammering.

But what I didn't appreciate until this year is how much better my welded fabrications could be with tabbed-and-slotted subcomponents for quicker/easier fixturing and for better alignment. The precision of laser cutting has made tab-and-slot fabrication viable.

If you guys already have or are starting to experiment with these techniques, let's talk about it!

[Moderator]

Where to start?

I'm not even a little bit reluctant to recommend you visit https://sendcutsend.com/ 

Go there. Read their pitch. Get excited. SendCutSend is awesome! Can't recommend them strongly enough.

But then doubt may hit you. "Oh no! I'm gonna have to learn Computer Aided Design! It's gonna cost me a fortune!"

Catch your breath. You're gonna be fine. CAD isn't as difficult as you probably think. There are great two-dimensional CAD options, and that's all you need to get started. Two dimensional drawing is simpler than 3D. You can move up later if you want.

For 2D CAD software, I heartily recommend QCAD: https://www.qcad.org/en/  It's what I use. Comes in both Windows and Mac versions. I played around with the free version for a minute or two, then decided to pay $48 for "QCAD Professional". No brainer. No regrets.

Let's see some example screenshots!

Here, I've used QCAD Professional to draw out an elaborate part to be laser-cut from 0.059" (1.5mm) cold-rolled mild steel:
   


Notes:
(1) the two red lines are where there will ultimately be bends. You don't have to worry much about radius or angle yet.
(2) the six broad rectangles near the top slots where this thin part will connect to tabs around three sides of a 3/16" thick plate.
(3) the two skinny rectangles near the bottom are tabs where another sheetmetal part mates up.
(4) the 12 tabs around three sides plug into another part's slots.

Okay, I'll confess that there is a tricky thing about this part. Bend allowance. I did have to learn how to calculate how bend radius and stretching would affect how the tabs line-up with mating parts. I found this video tutorial most helpful: https://www.youtube.com/watch?v=Ig8mTICj-r4

Here, I've uploaded my part to SendCutSend.com and I've selected a material:


Notes:
(1) If you look carefully, you can see that SendCutSend lets me enter the bend info here. This is where I finally see a 3D rendering. I can use my mouse or touchpad to spin the part around and inspect it from all angles, zoom in/out, etc.  Great fun.
(2) All throughout the process I can see menu options and pricing. Of course the material selected, the gage, the area of metal used, and the cutting time affect price. There's also a cost per bend. The savings for ordering multiple parts is pretty substantial. (In this case I saved money for using the same blank for both lefthand and righthand parts, even though they were folded opposite ways.) They offer all sorts of finishing options. For this part, I don't think I used any of those so the price probably really was $24.55 per side. (Note: when you have a blank that gets folded into lefthand and righthand versions, there's a way to flag your order to retain the quantity discount on the laser-cutting aspect of your order.)

[Moderator]

Oh snap! There was something I meant to post a little earlier.

Assuming you have a printer, CAD is even more awesome because you can use it to make excellent paper or cardstock mock-up parts! (Excellent but not perfect, since bend radius and stretch aren't easily modeled.)

The part I showed you above was mocked-up and trial fitted through several design iterations.

Here's what the final one looked like, taped to two other components of a weldment.



An upper mount for a coilover shock absorber in an MGB GT.  I needed this mock-up to be accurate and stiff enough to verify adequate clearance all around the coil spring.

[Moderator]

From order to delivery from SendCutSend takes about one week.

With tab-and-slot fabrications, I can never wait to snap the parts together!

Here's what the parts looked like minutes after the delivery truck came:

:

There's one bend I deliberately didn't have SendCutSend make for me. You can see it in the bottom righthand corner of the pic. I waited to fold that flange until after putting some weld behind it.
 
I actually screwed-up one dimension. One of my tabs was misaligned with its mating slot, and that's why there's a tiny gap between (a) the part I showed you in a CAD screenshot above and (b) the part it sits on top of. I fixed that with a tiny file. But I'm really proud that I did't need to redraw my design and order a second round of parts.

After fixing that one slot, the part was ready to be welded. But frankly, not by me. I'm pretty confident now with my MIG welder, but this assembly called for TIG. Thin metal is harder to weld without warping. And thin metal (0.059") is especially hard to weld to thick metal (0.188") without issues. So I called up my friend Michael at Spriso Motorsports.

[Moderator]

Historically, I've spent a ton of time fixturing up parts for welding. Clamp here. Magnet there. All around. And then the clamps are usually in my way. Not this time! No clamps or magnets were used at all. Michael just set the tabbed-and-slotted assembly on his welding table and placed a heavy block on top of it to keep it from shifting around. Welded for a while, slid it a bit to a more convenient angle, and welded some more.



Here are some other notes:
(1) where thick and thin metal are TIG welded together... we decided not to weld all the way around the tabs. Instead, Michael just TIG'd one side. Then we put braze over everything there because my aim was to have a vapor tight part.
(2) in this view, the coilover bracket assembly is shown sitting on top of the "foundation" I made for it. I put that foundation underneath the MGB package shelf (i.e. where the battery boxes normally are) to provide a whole lot of stiffness and strength - more than just a simple backing plate.
(3) All the round holes you see are 8mm diameter, and they're for plug welds. Where I was welding together three layers (i.e. where the package shelf was sandwiched between), I drilled 8mm holes through it too. Total number of plug welds per coilover: 36.
(4) Weld-thru primer used liberally...

[Moderator]

Not too much more to say, but maybe you'd like to see it with paint on it?

Driver-side bracket viewed from the passenger seat side, looking rearward:


(1) The knob is for 24 steps of rebound dampening adjustment.
(2) The stem mount uses a nylon ball between aluminum cups rather than a rubber bushing.

Passenger-side bracket viewed from the rear hatch:


(1) The silver-colored cover in the foreground is for a built-in toolbox.
(2) The unpainted aluminum cover to the left is for two hanging storage cubbies.
(3) Seats and safety harnesses were uninstalled at the time of these photos.

[BlownMGB-V8]

Pretty freakin' awesome Curtis. Looks like some high tech struts too from the adjuster on top.

I'm thinking something like that top joint would be custom made for oven brazing provided a kiln was available. It'd make for a sick finished part. I'm definitely interested in hearing more about this method.

Jim

[mgb260]

Curtis, Excellent! Now I see why you wanted to wait on the top coil over mount question!

[MGBV8]

Wow.  Quite the tutorial, Curtis!