CNC Fishmouths for Welders

[BobWarfield]

CNC machines are excellent in terms of precision, repeatability, and doing tasks that would otherwise be difficult to get right manually. Take the cutting of arbitrary "fish mouths" as needed when welding tubing together to make frameworks. Imagine you're welding a bicycle frame, for example. Tubing of different sizes is coming together at various angles. The "fish mouth" is the shape one piece of tubing must have to be fit closely to the curve of another before welding can commence. Given the possibility of different angles, different diameters, and even different shapes (square tubing meets round tubing), these shapes can be odd. The name "fish mouth" (and I can't seem to decide if it is one word or two) comes from the shape's resemblance to a fish's mouth.

There are a variety of jigs and fixtures available to facilitate fishmouthing (if I may use that oddly contrived word). They hold the tubing in the desired configuration while an annular or other cutter comes down vertically. I didn't have such an apparatus at hand and needed to make some fishmouths as part of my tapping arm project. It seemed immediately obvious that my CNC mill would be a good choice for the job. The first task was to make up the proper curves for the fishmouths so my CAM program could generate g-code. That was also very easy in my favorite CAD program, Rhino3D. I started by drawing the configuration that needed the fishmouths:



Since Rhino3D is a solid modeller, we can "subtract" shapes from one another. I merely drew up the configuration and then "subtracted" the two blue rounds from the red square tubing. What was left is a perfect solid model of the desired fishmouth:



Now I made 2 g-code programs using OneCNC. The first was to cut the diagonal side profile. "Why use CNC for something that simple?" you ask. Because its fast and easy. I have been fixturing using my Kurt-style vise. I set the part zero as being the left corner of the fixed rear jaw on the vise. I insert the square tubing 1" past that edge. Close enough is good enough--so even a tape measure will do on the diagonal. Insert tubing in vise, clamp, press green button, reverse tubing, repeat, done! CNC is great for this kind of repetition. Yes, you as easily set up a chop saw, but this was faster and more precise for me. In fact, I've seen a number of accounts of people using their CNC's largely just to saw pieces to size, especially lathes with bar feeds.

Okay, the second program cuts the upper and lower fishmouth. Its a quick and easy program. The curves are, in fact, elliptical and not circles, but that didn't phase OneCNC. Here's what the toolpath looks like:



The whole process took maybe one hour. If I'd had a fish-mouthing fixture, I could have done it faster, but probably not a lot faster. For this approach, I needed no special tooling and I can run off as many of these tubes as I like very quickly. The best news was not only that this was fast, but the fit was really nice. A little deburring with a file and I was ready to Tig together the parts.

Cheers,

BW

PS For those interested in the tapping arm this is part of, here is the page:

http://www.cnccookbook.com/CCTappingArm.htm