Printable pipe saddle and mitre template
Enter the diameters and the angle — you get a wrap-around template that prints at full size. Wrap it round the pipe, scribe the line and cut. Three layouts on one page: the saddle (fishmouth) on the end of the branch, the hole in the run pipe and the mitre (US: miter) cut for a segmented bend.
How to use the template
- Print at 100 %. Switch off “fit to page” in the print dialog. Before anything else put a rule on the check line on the sheet — if it is not exactly 100 mm, print again until it is.
- Mark a line round the pipe. Measure the “measure from the end” figure back from the end of the branch and draw a line square round the pipe — wrap a straight-edged strip of paper round it and scribe along the edge.
- Trim and wrap. Cut the template along the heavy marking line and wrap it so that the trimmed edge sits exactly on the line on the pipe. With several strips, wrap them in order, edge to edge.
- Scribe and cut. Trace the red cut line, take the paper off and cut away the hatched part. Leave a millimetre or so and dress it with the grinder until the branch seats without a gap.
The scale along the marking line is in degrees round the branch. The 0° and 180° marks have to lie in the plane of the two pipe axes — on the lines of the branch that face along the run pipe — and 90° and 270° at its sides. Scribe a centre line along the branch before wrapping and set the 0° mark on it. On a branch at less than 90° the 0° mark goes on the side of the obtuse angle, the back of the leaning pipe.
What is calculated
The cut line is the intersection of two cylinders unrolled onto a flat sheet. For a branch of radius r meeting a run pipe of radius R at an angle θ, the distance along the branch to the cut at the angle φ round it is
L(φ) = [√(R² − r²·cos²φ) − r·sin φ·cos θ] / sin θ
The same line, seen from the run pipe, gives the outline of the hole; its length in space is the length of the weld. For a mitre the line is a plain sine wave: h(φ) = r·tan β·(1 + cos φ), where β is the cut angle.
Unrolling a cylinder is an exact construction, not an approximation: the surface rolls out flat without stretching. The only errors are the printer and the width of the scriber.
Worked example
A 60.3 mm branch into a 108 mm run pipe at 90°. The wrap is π × 60.3 = 189.4 mm long, which is two strips on A4 portrait. The saddle is 9.2 mm deep: 54 − √(54² − 30.15²). That is the figure to measure from the end of the branch for the marking line.
With both pipes at 108 mm the saddle is exactly one radius deep, 54 mm, and the weld runs to 413 mm — about a fifth longer than the 339 mm circumference of the pipe. The deeper the saddle, the more filler and time the joint takes; the cost per metre of weld calculator turns that length into money.
For a 90° bend in three pieces of the same pipe the cut angle is 22.5° and the cut rises 108 × tan 22.5° = 44.7 mm across the diameter.
What the template does not know
- Gap and bevel. The line is where the outside surfaces meet. A root gap, a bevel and the root face are prepared by hand afterwards.
- Ovality. Pipe is rarely perfectly round; a template wrapped on an oval pipe closes with a small mismatch. Split the error evenly between both ends of the wrap.
- Offset branches. Only intersecting axes and a straight run pipe are covered.
Welding the joint itself — positions round a fixed pipe and how the technique changes on the way — is covered in pipe welding positions. For a conical reducer between two diameters use the cone flat pattern calculator.
Frequently asked questions
How do I print the template at full size?
In the print dialog set the scale to 100 % and switch off “fit to page” or “shrink to fit”. Then lay a rule on the check line printed on every strip: it has to read exactly 100 mm. If it reads 97 or 103, the printer has rescaled the sheet and the template is wrong by the same three per cent — print again.
The wrap is longer than an A4 sheet. What then?
The template is split into strips of equal width that fit the printable area. Wrap them one after another, edge to edge, in the order printed on them. A 108 mm pipe has a circumference of 339 mm and comes out as three strips on A4 portrait or two on A4 landscape.
Which diameter do I enter — nominal or outside?
The outside diameter as measured with a caliper. DN 50 pipe is 60.3 mm outside, DN 100 is 114.3 mm; a nominal size typed into the field gives a template that is short all the way round. The list under the field holds the common EN 10220 sizes, but any value can be entered.
Does the template allow for wall thickness and bevel?
The saddle is drawn on the outside surface of the branch, so its outside edge meets the run pipe all the way round. On thin-walled tube that is all that is needed. On a heavy wall the inside edge fouls at the sides of the saddle — grind it back until the branch seats, and prepare the bevel for a full-penetration weld separately.
What is the cut angle for a segmented (lobster-back) bend?
The bend angle divided by twice the number of joints. A 90° bend in two pieces has one joint and is cut at 45°; in three pieces it has two joints and each cut is 22.5°; in four pieces, 15°. The middle segments are cut on both ends, the end ones on one end only.
Can it work out an offset branch or a branch into an elbow?
No. The template covers a branch whose axis crosses the axis of a straight run pipe, at any angle from 30° to 90°. An offset (eccentric) branch and a branch into a bend follow a different curve and are not calculated here.
Updated: