How to Make a Pipe Saddle Cut Template by Hand

When a branch pipe meets a main pipe at an angle, the end of the branch needs to be cut in a curved profile — called a saddle cut or fish-mouth cut — so it sits flush against the main pipe before welding. Get this wrong and you have gaps, bad welds, and a joint that won't pass inspection.

Most fabricators either eyeball it (and grind forever to fix it) or pay for software they use once a month. This guide shows you how to do it properly by hand using nothing more than a steel rule, a compass, some paper, and basic arithmetic. I've used this method on gate frames, handrails, exhaust systems, and structural frames.

If your pipes are a standard size and you're at a computer, you can also generate the template automatically using the free Pipe Notching Calculator at MetalWeightPro (https://www.metalweightpro.com/pipe-notching.html) — it draws and prints the exact profile for any OD and angle. But knowing the manual method means you're never stuck on site without a printer.

Gather Your Tools and Materials

What you need:

1. Steel rule or Vernier calipers (to measure pipe OD accurately)
2. Pencil and paper (A4 or larger — graph paper is ideal)
3. Compass or a piece of string and a nail
4. Scissors
5. Scriber or soapstone chalk
6. Angle grinder or hacksaw
7. The two pipes you're joining

The key measurement you need before anything else:

1. Main pipe OD (outer diameter) — the pipe the branch will connect to
2. Branch pipe OD — the pipe you will be cutting
3. Branch pipe wall thickness — needed if you want a precise fit
4. Join angle — the angle at which the branch meets the main (90° is simplest; this guide covers 90° in detail)

Tip: Always measure OD with calipers, not a tape. Nominal pipe sizes in India (IS 1239) are slightly different from actual OD — a "25mm" light-class pipe has an actual OD of 33.7mm. Use the real number.

Before you draw anything, it helps to understand what shape you're actually making.

When the branch pipe meets the main pipe at 90°, the cut profile is a wave — it's deepest at the sides (where the branch touches the top and bottom of the main pipe) and shallowest at the top and bottom (where the branch meets the sides of the main).

The mathematical formula behind the profile is:

z(θ) = R_main − √(R_main² − (r_branch × cos θ)²)

Where:

1. R_main = outer radius of the main pipe (OD ÷ 2)
2. r_branch = outer radius of the branch pipe (OD ÷ 2)
3. θ = angle around the branch circumference (0° to 360°)
4. z(θ) = cut depth at that point around the branch

You don't need to calculate every point by hand. In the next steps I'll show you a simple geometric construction that gives you the same result.

Quick check: The maximum cut depth (at the sides) = R_main − √(R_main² − r_branch²). For a 60mm OD branch into a 114mm OD main pipe: max depth = 57 − √(57² − 30²) = 57 − √(3249 − 900) = 57 − √2349 = 57 − 48.5 = 8.5mm. That's how much material comes off at the deepest point.

On your paper, draw a circle with radius = R_main (half the main pipe OD). This is the end-on cross-section of the main pipe.

Example numbers I'll use throughout this guide:

1. Main pipe OD: 114mm → R_main = 57mm
2. Branch pipe OD: 60mm → r_branch = 30mm
3. Join angle: 90°

So draw a circle with radius 57mm. Mark the centre point — call it O.

Now draw a horizontal line through O (this represents the centre line of the main pipe). Mark where the line crosses the circle on both sides — call these points A (left) and B (right).

The branch pipe circumference is: π × branch OD = π × 60 = 188.5mm. You'll need this in Step 5.

IMAGE PROMPT 3:

Hand drawing a circle with a compass on graph paper on a workshop bench. The circle has a centre point marked O, a horizontal diameter line, and handwritten measurements (57mm radius, 114mm diameter) noted in pencil beside it. A steel rule lies alongside the paper. Realistic photo, natural light, no digital overlays.

This is the core of the geometric construction.

1. Divide the right half of your circle (from top to bottom, through point B) into 6 equal segments. Mark these points 1 through 6 going clockwise from the top.
2. Draw horizontal lines from each of these 6 points across to the left side of the circle. These lines represent the depth of the main pipe surface at each point around the branch.
3. For each point, measure the vertical distance from the horizontal centre line (line O–A–B) to that point. Write down these 6 distances. These are your z values — the cut depths at those positions around the branch.

For our 114mm OD main and 60mm OD branch, the 6 distances (approximately) are:

1. Point 1 (top): 0mm
2. Point 2: 3.2mm
3. Point 3: 6.1mm
4. Point 4: 8.0mm
5. Point 5: 8.5mm (deepest — this is the max depth from Step 2)
6. Point 6 (side): 8.5mm

The circle tells you how deep the cut is at each position around the branch. You're essentially "unrolling" the intersection.

Now you'll draw the actual cut template — the shape you'll print, cut out, and wrap around the branch pipe.

1. On a new piece of paper, draw a horizontal baseline.
2. The template width = branch pipe circumference = 188.5mm (for our example). Mark this length along the baseline, and divide it into 12 equal sections (each section = 188.5 ÷ 12 = 15.7mm). Number them 0 to 12.
3. The left half of the template (sections 0–6) mirrors one side of the pipe going from top to side. The right half (sections 6–12) mirrors back from side to top.
4. At each numbered point, draw a vertical line upward with the height equal to the z-value for that position from Step 4. Use the same 6 z-values, then mirror them:
5. Point 0: 0mm
6. Point 1: 3.2mm
7. Point 2: 6.1mm
8. Point 3: 8.0mm
9. Point 4: 8.5mm
10. Point 5: 8.5mm
11. Point 6 (midpoint): 8.5mm
12. Then mirror back to 0mm at point 12
13. Connect all the top points with a smooth curve. This wave-shaped curve is your saddle cut profile.

Shortcut: If your branch pipe is a standard IS 1239 size, you can generate and print this exact template using the free calculator at metalweightpro.com/pipe-notching.html — enter your ODs and angle, and it draws the template to scale for printing.

Cut along the curved profile line you drew in Step 5 with scissors. Cut carefully — any wobble in the paper becomes a wobble in your weld line.

You now have a paper template that looks like a rectangle with one wavy edge. The straight bottom edge is your reference line. The curved top edge is the cut profile.

Before you wrap it: Hold the template up and check it looks symmetrical — the curve should rise to the same height on both sides and peak in the middle. If one side is higher than the other, recheck your measurements from Step 4.

Also check: the total width of the template should match the circumference of your branch pipe exactly. Roll it loosely around the pipe — it should meet end-to-end with no gap or overlap.

1. Wrap the paper template tightly around the end of the branch pipe, with the straight bottom edge aligned to the pipe end.
2. Secure it with masking tape — make sure the two ends of the template meet exactly and the template sits flat with no buckling.
3. The curved edge of the template now traces the cut line directly on the pipe surface.
4. Using a scriber, soapstone, or permanent marker, carefully trace along the curved top edge of the template all the way around the pipe.
5. Remove the template.

You should now have a clean curved line marked around the end of the branch pipe — this is exactly where your cut will go.

Check: The marked line should be deepest on the two sides of the pipe (at the 3 o'clock and 9 o'clock positions when you look end-on) and shallowest at 12 o'clock and 6 o'clock. If it's the other way around, the template was wrapped upside down — re-mark it.

Use an angle grinder with a cutting disc, a hacksaw, or a jigsaw with a metal blade depending on your pipe wall thickness.

Tips for a clean cut:

1. For thin-wall pipe (up to 3mm): a hacksaw gives good control along the curved line
2. For medium wall (3–6mm): angle grinder, cutting disc, take it slow along the curve
3. For heavy wall (6mm+): plasma cutter or oxy-acetylene if available

Cut on the waste side of the line — always cut slightly outside the marked line and grind back to it. Never cut inside the line; you can't add material back.

After the main cut, use a grinding disc or file to clean up the edge to the exact scribed line. The goal is a smooth curved edge with no notches or flat spots.

Hold or clamp the branch pipe against the main pipe at your required angle (90° for this guide) and check the fit.

What a good fit looks like:

1. The cut end of the branch pipe sits flush against the curved surface of the main pipe with no rocking
2. There are no large gaps — gaps under 1.5mm are fine for welding; anything larger needs more grinding
3. The branch pipe sits at the correct angle

How to find high spots:

1. Apply a thin coat of engineers' blue (Prussian blue) or even a marker pen to the cut end of the branch pipe
2. Press it against the main pipe and twist slightly
3. Where the blue transfers to the main pipe, that's where the branch is touching — the areas with no transfer are gaps

Grind the high spots (where it touches) gently, recheck, repeat until the fit is even all around.

Once the fit is clean and even, you're ready to tack weld and then run your full weld.

Before welding:

1. Clean both surfaces with a wire brush or flap disc — remove any mill scale, rust, or marker pen
2. Tack weld at 4 points (12, 3, 6, 9 o'clock positions) before running the full weld, to prevent distortion
3. For structural applications, check IS 800 and your WPS (Welding Procedure Specification) for joint preparation requirements
4. A correctly fitted saddle joint requires minimal root gap and gives a full-penetration weld with minimal filler

That's it. A properly fitted saddle cut takes 30–45 minutes start to finish once you've done it a few times. The first time always takes longer — the geometry feels strange until you've wrapped a template once and seen how it works.

The manual template method works for any pipe size and any angle from 15° to 90°. For non-90° joints the geometry is slightly more complex — the profile becomes asymmetric — but the wrapping and marking process is identical.

If you want to skip the drawing and go straight to a printed, to-scale template for any combination of pipe ODs and join angles (including metric and imperial), the free Pipe Notching Calculator at MetalWeightPro generates and prints the exact profile in seconds:

→ https://www.metalweightpro.com/pipe-notching.html

The site also has a metal weight calculator, a pipe cut list optimizer, and reference charts for IS 1239, IS 808, and IS 2062 sections — all free, no login.

Article prepared for posting to Instructables.com Link back: https://www.metalweightpro.com/pipe-notching.html