Technical Laying out a fishmouth cut in tubing

Yesterday, I made a crossover H pipe for my new exhaust on my 57 Chevy. It just so happended that I had the right size holesaws to cut the fishmouth joint and all is good.

It got me to wondering, however. How do you manually lay out a fishmouth cut? There has to be a fairly simple way because pipefitters do it all the time on large pipe, etc.

I would like to know how just for future reference. I'm sure the holesaw is still the easiest way.

 

There is a template. One you can buy and another you can print on computer

 

Look up metalgeek.com/static/cope_big.pcgI It has a calculator that lets you enter tubing diameter, wall thickness and so on and you can print out a template, wrap it around your tube, mark and cut. Pretty slick

 

years ago I learned how to make a simple 90 degree fishmouth using a chop saw, set at 45 degrees. It gets pretty close.

If it weren't so cold I'd go to the shop and take pictures.

 

A friend taught me a trick . He made a tube holder for his lathe tool post. He placed the holesaw in the chuck and could cut a fishmouth at any angle. Much the same as the commercially available units.

 

I just rough it with a sawsall and then hit it with the proper size sanding roll .You actually remove less metal than you think when its done. After a couple of them its pretty fast.

 

One of the first things I learned was how to make fishmouth templates without computers....
let's say you need to land an 8" pipe on a 12" pipe at 30*. Oh and the 12" needs a nice clean smooth hole too
It's kind of fun and easy if you take it one step at a time- it develops right before your eyes like magic.

The same thing and same way works for 2" exhaust or 1-5/8 roll cage. I can get you the marks pretty quick- The tricky part is cutting the small stuff light gauge stuff clean quick and accurately. That's where the hole saw cutters shine.

 

We used a torch for all our stock car roll cage pieces... definitely the hard way since the clean up with a grinder was tedious. I did have a fixture on my mill at work that was pretty good, but hard to steal shop time to use it. Now that I'm a little more prosperous, I can buy whatever makes it go quicker.

 

I have often used a porta band saw to make pipe copes in the
field when building or repairing hand and safety railing. In a
nuclear cooling tower they are often deteriorated from the
constant wet environment. A little touch up with a grinder
and they are ready to weld. Also an inexpensive heavy cardboard
type wraparound is handy to mark pipe. They are inexpensive and
can be obtained at most welding or industrial supply places.

 

Just a suggestion. If you decide to go with using a torch fabbing a stock car roll cage, NEVER stick the lit torch down inside your welding glove. Just sayin'.

 

That's bad, I saw it happen and it's ugly.

 

You can buy a tool that has metal rods in a holder . You slide the tool over the tubing/pipe and push up against the pipe and the rods form the shape. Mark around the rods and cut. Easy peasy . Race car shops use them building roll cages.

 

it is just a straight line you cut , at what ever angle you need

 

If the tube is to be welded to a flat surface, you're correct. If you are joining it to another tube, the only way that will work is to hammer the end of your tube flat.

 

no, go back and look at the pic above , you will see it is a straight cut . it can be done with a band saw or cut off wheel . I have done hundreds of them

 

it warmed up a bit, it's in the 50s now, safe to go outside.

two cuts with the chop saw, 45 degrees, but you'll need to play with the exact amount to cut off (and make sure to get them both the same, which I failed at)

 

there you go

 

Thanks guys ! The template is interesting and so are the other tips. I was doing Stainless, so the fit up for TIG welding needed to be pretty tight. The holesaw and a sanding roll did it for this job. It just happened that my belt sander had 2 1/4" diameter rolls.

 

You have no idea how sorry I feel for you to have to endure the hardship of 50 degree weather! It's going to be -9 tonight here in St Paul MN with a 20 MPH wind. Wind chill around -15 to 25 below.

 

high of -5 next week.....opps, giving false info here, just heard on the radio daytime high of -13 next tues &weds

 

Greetings from south jersey Squirrel!...soooo, how cold is it in Arizona....all the way down in the 70s?!!...

 

Lets all pick on Squirrel because he is warm and we are not.... Guess which of us is smarter?

 

This pipe style contour gauge works well.

 

Look at the template laid out flat and you'll see compound curves which exactly fit the contour of the tube. The calculator will give you the exact template for the cut on any size tube at any angle to the same size tube and larger or smaller tube. Much more precise than cutting, grinding and guessing.

 

I knew I missed something this week. I just noticed on my phone's calendar that Monday, the 21st, was officially designated Squirrel Appreciation Day. I entered it in my calendar several months back when someone else said that was your day. Sorry I missed it but here's to you, Squirrel, Happy Squirrel Appreciation Day.

 

The exact tool i mentioned. I believe a few different sizes you can buy.

 

I use a hole saw for fitting up roll cage tube, etc. But a chop saw works fine for mild steel exhaust pipe, and stuff like that where the fit is not quite so critical.

The other day I had to fit a piece of square tube up against a round tube for a non car project, I just spent a little time with the bench grinder, and it fit close enough.

Hey, if you live in paradise, and the temp drops to 25 in the morning, something is wrong! I stay inside on those mornings

 

would putting a flashlight inside a pipe, held at the proper angle, shine a beam onto the joining pipe that can be traced?