View Full Version : CNC Fishmouths for Welders

06-05-2010, 03:18 PM
Just some thoughts about why its great to have a CNC in the home shop, even when it comes to the blunt trauma that is my welding jobs, LOL!

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, my CAM software. 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:


For the non-CNC'ers, those red and green lines describe the motion of the 1/2" roughing ("corncob") endmill I used for the operation.

The whole process took maybe one hour, including the CAD/CAM, CNC, and welding. 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.



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


John Stevenson
06-05-2010, 03:53 PM
I did mine with an angle grinder :D




06-05-2010, 05:05 PM
I did mine with an angle grinder :D

Been there, done that. Usually after the first method didn't fit right though.



John Stevenson
06-05-2010, 05:51 PM
Been there, done that. Usually after the first method didn't fit right though.



Hint - bigger welding rods :rolleyes:

06-05-2010, 06:08 PM
I cope tube like that all the time at work. we cope 2 pieces of 2in 4130 1/4 wall tube in about 12 sec :cool:

06-05-2010, 10:58 PM
Nice job all round Bob.

I haven't actually laid hands or eyes on the type you describe:

or John Stevenson's:

I think that both are very clever.

I have used a large (manual - me!!) "Deckel" pantograph milling machine as well as engravers (also part of the "Milling" section) but both were following masters/profiles in the horizontal plane. I have used pantograph drafting machines as well. There are many uses for pantographs - electric trains and trams/street-cars are but two of many.

I have just one concern about the pantographs used on tapping heads which are pantographs that operate in the vertical plane and in that sense they are similar to the "Deckel" and the engraving machines.

Once over the job the vertical/base pivot is locked into position and while the vertical axis of the tapping head in the pantograph will remain vertical it will swing/describe an arc about the end of the pantograph arm remote from the tapping head. The longer the pantograph arms and the nearer the pantograph arm is to horizontal and the shorter the tapping stroke is the closer the arc swung by the tapping head will be to the chord of that arc.

If that is so there will be a side-thrust on the tapping head that must be resisted either by the tap and the job it is tapping and/or by a counter horizontal force applied by the machine operator.

I was going to make one but had a good think about it and so went and bought a couple to use in my pedestal drill (MT2) and my HF-45 and Sieg X3 mills (3MT).

This one (MT2) has a reversal mechanism but no adjustable slipping clutch:

And this one (MT3) for larger taps and which is reversible and has an adjustable clutch.

My Sieg Super X3 has a "tapping" function which is a thumb-operated switch on the ends of the 3 arms on the quill feed but my pedestal drill and my HF-45 and Sieg X3 do not.

All my milling machine tapping options require me to place the job under the mill quill axis either by use of the "X" and "Y" feeds or physically pushing the job under the work (as on a pedestal drill) but all operate in a true vertical straight line/axis - ie with no arc or lateral imposed forces.

The pantograph-type tapping machines seem to use a pneumatic tapping head rather than an electrical one. I'd have thought that low tapping speeds - especially with higher torques would be a problem for an air-driven device (read drill or die-grinder) which seem to function better at higher speeds without stalling.

I have a completely open mind on this topic and I have no fixed opinions on it at all.

06-05-2010, 11:38 PM
Tiffie, the charm of the devices is they're quick and easy to just leave set up and you needn't trouble your mill or drillpress. If it's no trouble, you may prefer to have at the mill and drillpress. I've always liked the tapping arms and peeps I talk to who own one wouldn't be without them. They're well proven and seem to sell for inordinate sums, even used.

I've also seen shops with a row of little drill presses set up so they could just walk down the line and drill, chamfer, and then tap in a couple different sizes. To each their own.

I've kept a right angle air drill with a zero flute deburring tool handy for some time just because it's quick and easy. That's something else I may leave chucked in the tapping arm, which BTW, will actually wind up with two tools attached. More details as progress allows.

As for the air tools, that is really the question. Air tappers are fairly expensive as air tools go. They come up sometimes on eBay and sometimes can be had new, but never really very cheap. I've used a little butterfly impact wrench quite successfully, but wish it had a torque limiter on it.

As to your query on the speeds, the air tappers often have tandem planetary gears to slow them down and multiply their torque.

I'm still debating whether to acquire one, continue my evil ways with the impact wrench, or take a look at building or buying a torque limiter.



PS John, I find the bigger welding rods also lead me to the die grinder to clean up the mess I've made welding, LOL!

06-06-2010, 12:35 AM

Thanks for the "heads up" re the use and popularity of those pantograph tappers as I've never seen one in use or used one. The best I've seen which set me off on that road was an excellent post/thread by John Stevenson where he tapped heaps of (I think about M6 or M8) holes in next to no time and with minimum effort and no breakages. It was quite an eye-opener.

I was really impressed with the air-strut balancing mechanism on John's tapper as its elegant, solves a problem and is much neater and nicer than springs.

The "mechanics" of a panto-graph concerned me and that, amongst things, was why I went for the tapping heads for my pedestal drill and mills. But the way John's machine worked was the reason I went for them. I bought a full set (3mm > 12mm) of "gun" taps for mine. I have yet to use them but I am looking forward to it.

I have posted this power-tapping pic before but it might be of use to some who have not seen it.


I had thought about using my very good drill which has a good slow speed as well as lottsa toque and a good adjustable slipping clutch and a good forward-reverse arrangement used in a clamp/adaptor like this:



I have not looked into air-driven two/multi-speed heads as I've never had the need to - but I will - thanks.

John Stevenson
06-06-2010, 07:48 AM
Bit of history on mine.
For years I had an Arboga milling machine that came out of a school because it had no chuck or chuck guard.
It was 3 morse taper so I fitted an Archer tapping head to it and locked the head to a bracket on the quill so it didn't need a torque arm.

This sat next to the CNC and was just used for 2nd op tapping after the CNC had done all the drilling. Good machine, very quick, in fact it was always waiting for the CNC.

I had seen the import tapping heads on Ebay, about 800 [ $1400 at that time ] and couldn't justify that when the Arboga was doing what it did.

Then I started getting some steel straps in 30mm x 8 in section and 330mm to 400 mm long.
The CNC could just drill these but the Arboga couldn't because of the hight, the Bridgy couldn't hold the Archer head and the strap, again because of hight. At first these came in in 20's so not hard to tap by hand but the supply started getting bigger.

I took another look at the import arms but the arm itself was only a pressed steel pressing. I contacted the importers and found that a head with al the torque collets in a box , 8 or 9 ?, was 320 which was a lot better than 800.

So I arranged to buy one at the upcoming show we were both going to. Just before the show they ordered 100 straps, wow now worth it, then it was 150, then 200 and I went to the show.

On my return I had orders for 320, what an uncanny number ? So I set to to make the frame from 40mm box section and a big handful of sealed bearings I had kicking about.

The frame took a full day start to finish and I cut corners on the head bracket as I was short of material.
The parts arrived, all now 400 of them and between the CNC and this arm it did all 400 in a morning. It cost two drills and one tap which broke for some unknown reason only a bit into the hole ?

It now lives permanently on the back of the bench so it can reach anywhere on the tooling plate fitted there or reach both vises. Tapping blind 4mm holes into alloy until it stalls then backing out is priceless.


06-06-2010, 08:15 AM
i used to work in the fabrication field, and we had a lot of architectural features that were made from large round pipes and tubes. we also used a solid modeling package to detail the jobs, but one of the capabilities it had was creating an unfolded template of the end of pipe with the cut shape on it. you could then just wrap the template around the pipe and mark it, then torch cut it. not perfect like an annular cutter or mill, but for 6+ inch pipe and fabrication tolerances, it worked slick. do any of the less expensive modeling packages do templates like that?

06-06-2010, 12:12 PM
i used to work in the fabrication field, and we had a lot of architectural features that were made from large round pipes and tubes. we also used a solid modeling package to detail the jobs, but one of the capabilities it had was creating an unfolded template of the end of pipe with the cut shape on it. you could then just wrap the template around the pipe and mark it, then torch cut it. not perfect like an annular cutter or mill, but for 6+ inch pipe and fabrication tolerances, it worked slick. do any of the less expensive modeling packages do templates like that?

No worries, lostcause, its very easy to do what you say. This drawing took me about 3 minutes:


Print that out 1:1 and you have your template. This cad program is Rhino3D, which is cheap compared to Solidworks and expensive compared to free. I like it a lot. YMMV.



Michael Moore
06-06-2010, 04:07 PM
Bob, I use that "unroll surface" in Rhino to make templates for mitering/coping the tubing ends. The numbers in the middle show the actual distance that should be set between the patterns.


If you keep track of things at each end you can get the templates clocked into each other pretty accurately by drawing a reference line down the tube and then aligning the edges of the template with it.

There is one drawback, and that is you get the outside surface which is going to leave the tips feathered down to nothing so you have to trim them off before you weld.

There's freeware called "TubeMiter" which can be downloaded from


and an online pattern generator here:


I like the laser tube cutting in this video:



06-06-2010, 08:29 PM

Love the laser video. Looks pretty doable with a plasma cutter. Who knew I needed a 4th axis plasma table? LOL.

Seriously, I got a deal on a high capacity ESAB plasma on eBay, and I love having the thing around for cutting plate. It's so fast to clamp a guide on some plate and slice out a chunk. Gonna have to break down and do a table some day.



06-06-2010, 08:47 PM
I have used this one.Does everything tubemiter does,but with bigger pipe.


For repeat jobs I make a permanent template out of PVC pipe,split it lengthwise and add a couple small hinges.

06-07-2010, 10:39 PM
This cad program is Rhino3D, which is cheap compared to Solidworks and expensive compared to free. I like it a lot. YMMV.



looks pretty good. the price of rhino3d is about 1/10 of the stuff i used to use, but problem is, if it were something i needed for productive work, it would probably be cheaper to buy the $15k package, as i am proficient with it. i've never had much luck learning new packages on my own. i've tried a couple autocad clones and never had any luck with them. i stick with drafix/autosketch and get a lot more done. oh well, maybe someday i'll find the time to spend with some other software.