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Workshop know-how · Pipe layout

How to notch pipe: the saddle cut from template to weld

Notching (coping) means cutting the end of a pipe so that it sits on the surface of a second pipe without a gap. The cut line is a saddle curve; unrolled on paper it becomes a template that is wrapped around the pipe and scribed. Three dimensions decide the shape: the diameter of the main pipe, the diameter of the branch pipe and the angle between them.

Dipl.-Ing. (FH) Thorsten W. Recklebe · Updated

Where saddle cuts are needed

Wherever a round pipe butts against another: posts under a handrail, rails between posts, braces in frames, branches in pipework. A pipe sawn off square would touch at two points only and leave large gaps on both sides. The saddle cut closes them – the weld becomes even, the joint is strong and looks clean.

The geometry in one sentence

For every point on the circumference of the branch pipe you calculate how far it reaches until it meets the surface of the main pipe. With R as the radius of the main pipe, r as the radius of the branch pipe, the angle α between the axes and the angle θ around the branch pipe, the cut height is:

h(θ) = √(R² − (r · sin θ)²) ÷ sin α − r · cos θ ÷ tan α

At 90° the second term disappears and the curve is symmetrical with two equal valleys. At a slant it becomes one-sided: one long point and one short saddle. Plot h over the unrolled circumference and you have the template.

Case Shape of the curve
equal diameters, 90° deep saddle, two peaks running to a sharp point
small pipe on large pipe, 90° shallow wave, little material removed
angled joint (for example 45°) one long and one short side, template not symmetrical
Flat pattern of a saddle cut: 33.7 mm pipe on 42.4 mm pipe at 90°, circumference 105.9 mm, cut depth 8.3 mm
This is what the template looks like: the unrolled saddle curve for a 33.7 mm pipe on a 42.4 mm pipe. The height is exaggerated for clarity.

Marking out

  1. Measure both diameters – do not take them from the catalog.
  2. Print the template, check it against the control ruler and cut along the outer contour.
  3. Scribe a straight line along the pipe (lay a piece of angle on it). The 0° line of the template is aligned to it.
  4. Wrap the template tightly; start and end must meet exactly. If they do not, the diameter is wrong.
  5. Transfer the contour with a scriber or a fine marker.

With several branches on one pipe the scribed lines must match each other, otherwise the branches end up twisted.

Cutting and clean-up

  • Angle grinder with a thin cutting disc: for one-offs; rough cut, then grind to the line with a flap disc.
  • Belt grinder with a contact wheel in pipe diameter: fast and accurate for 90° joints of equal pipes.
  • Hole saw in a tube notcher: clean, if the saw diameter matches the main pipe.
  • Plasma or laser: for series; here the flat pattern is used as the cutting contour.

From about 3 mm (1/8 in) wall thickness the inside edge is dressed until the pipe also seats on the inside. An even, small gap is better than a pipe that sits hard in one place and gapes in another.

Common mistakes

  • Template printed at the wrong scale
  • Template applied turned by 180° (on an angled joint the long side then sits on the wrong side)
  • Nominal size used instead of the measured size
  • Stainless steel worked with tools that have ground mild steel before – this leads to rust spots later

The pipe notching template generates the pattern at 1:1. For corners without a branch, use a miter cut.

Questions

Short answers

Why does the template not fit although the dimensions are right?

Usually it was not printed at 100 %. Many print dialogs shrink the page to "fit". Print at actual size and measure the control ruler. The second cause is the pipe itself – welded pipe deviates from its nominal size, so always measure the outside diameter.

What is the inner contour on the template for?

The outer contour is the scribe line on the pipe surface. The inner contour shows where the inside edge of the wall ends. On thick-walled pipe the strip in between is ground at a bevel so the pipe seats inside as well as outside.

Can the branch pipe be larger than the main pipe?

No. A larger pipe would wrap around the smaller one and the curve would no longer form a closed saddle. In that case swap the roles of the two pipes or use an adapter piece.