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BP0116 Simple Saddle Cuts In Pipe

IForgeIron Blueprints
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BP0116 Simple Saddle Cuts In Pipe
Original Demo by Bill Wilda, retired welder, master pipe fitter, & mentor

Text and Photos by Jim Carothers

I recently ask my friend and mentor Bill if he could teach me a reasonably simple way to layout saddle cuts to fit one pipe to another at 90 degrees. I got an offer to bring my camera and come up to his shop for hands on training. Thanks for your patience Bill.

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Here's what I wanted to learn how to do. Saddle cut a piece of 3 inch (3.5 inch OD) steel pipe to fit on to a 4 inch pipe (4-1/2 inch OD) at 90 degrees.

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The basic tools Bill used for layout include a light tape measure, compass, square, and his welding table. It is not seen in this photo, but Bill has done so much pipe layout that he has a nice center punch mark in his welding table just for a compass pivot point.

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The demo layout that goes with these notes is for a 4 inch (4-1/2 inch OD) pipe saddle cut to fit at 90 degrees onto a 6 inch (6-5/8 inch OD) pipe. Bill and I both thought that the 4 inch pipe size layout would show up better in these photos.

A layout arc having a Radius of 1/2 x (6-5/8 inch) pipe size OD = 3-5/16 inch is struck on the table. The square is placed so that the left end is just touching the arc and the 4-1/2 inch (4 inch pipe size OD) division on the square is touching the arc. The top leg of the square as show is the tight Chord of 4-1/2 inch on an Arc of 3-5/16 inch Radius. For a little better accuracy, you could “cut” an inch and measure from the 1 inch mark on the square and read 5-1/2 inch where the square is touching the arc.Â

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The Middle Ordinate or Versine is read directly from the center of the tight Chord to the Arc. Bill called this his "cut-back" dimension. This is the maximum depth of cut for the saddle shape. The full cut-back dimension will be used in two places that are 180 degrees apart on the pipe to be cut.

Our cut-back dimension from the top of the square at 2-1/4 inch from the end (center of the tight Chord) up to the radius layout (chalk) line measured 7/8 inch. For those of who like to do the math; the cut-back dimension calculates to be 0.8814 inch. That accuracy is not really necessary for hand cut saddles in pipe.

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Since the subject 4 inch pipe in this demo was rather roughly cut at the open end (my torch work), Bill put a First line around the pipe using a light cardboard strip wrapped around the pipe. The wrap around cardboard strip needs to be wide enough so that it will sit flat on the pipe ensuring the circumferential layout line is square with the pipe centerline; use of 4 to 6 inch width is OK. A strip cut from a manila folder would be about right for small pipe.

If your pipe had a nicely cut, square, end, there would be no need for the First circumferential layout line. You would simply work from the end of the pipe.

A Second circumferential line was spaced back 7/8 inch (cut-back dimension) from the First line. Again the wrap around strip was carefully placed and the edges aligned to keep the First and Second lines parallel to each other and square with the centerline of the pipe.

An axial line along the top of the pipe and 90 degrees to the two circumferential lines was drawn and marked ("S"). This is the starting point for more layout work.

If you were planning to saddle cut both ends of a pipe, this starting line would extend straight down the full length of the pipe being worked on. You would then have a starting line ("S") on both ends of the pipe - a timing mark so to speak. A chalk line works well for this.

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The circumference, distance around, the pipe to be saddle cut is measured. For the 4� pipe shown (4-1/2 inch OD) the circumference is = 14-1/8 inch. You can check your measured dimension by multiplying the Outside Diameter of the pipe to be saddle cut by 3.14 (Pi): 4.50 x 3.14 = 14.13 inch.

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From the "S" line measure around and divide the circumference of the pipe into quarter sections and eighth sections. These lines are seen as the short lines along the axis of the pipe - that is parallel to the "S" line but spaced around the pipe at 1/4 or 1/8 the circumference.

For the 4 inch pipe shown here:
Quarter section dimensions = 14-1/8 divided by 4 = 3-17/32 inch

Eighth section dimensions = 14-1/8 divided by 8 = 1-49/64 inch (use 1-3/4 inch)

When "S" is taken as being at 12 o'clock, the quarter section lines will be at 3, 6 & 9 o'clock. Each eighth section line is half way between the quarter section lines.

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On the 4 eighth section lines, measure back from the First ( see photo) circumferential layout line 1/2 of the cut-back dimension; in our case that is 1/2 x 7/8 = 7/16 inch. Two of these lines can bee seen in the photo.

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Using the cardboard strip, place it on edge against the pipe as shown. Align the strip from the "S" line to the two nearest 1/8 circumference layout marks as shown. Using soapstone or Silver Pencil mark that part of the saddle cut.

Note that the arc created by the strip ends at the 1/2 cut-back dimension; it does not go all the way back to the full cut-back dimension.

Rotate the pipe 180 degrees so that the "S" layout line is down and the 1/2 circumference line (6 o'clock before rotation) is up. Use the cardboard strip again in the same way as noted above to connect the two adjacent 1/8 circumference lines on either side.

Again, note that the arc created by the strip ends at the 1/2 cut-back dimension; it does not go all the way back to the full cut-back dimension.

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Rotate the pipe 90 degrees so that a quarter circumference line is up ( 3 o'clock or 9 o'clock from starting). Note that the tilt of the cardboard strip has been reversed from the position shown in the previous photo.

Align the strip so that the quarter circumference mark at the full cut-back dimension is connected to the adjacent 1/8 circumference marks (one on each side of this mark) at the 1/2 cut-back dimension. Use soapstone or Silver Pencil to make this part of the cut out.

Rotate the pipe 180 degrees and repeat this layout at the remaining quarter circumference line.

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Your layout should now look like what is shown here. The "S" line is out of sight in the left of the photo. You should have a nice blend of the two arcs at the 1/2 cut-back dimension at each 1/8 point.

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A finished saddle cut in 3 inch pipe. All that was required for a very close fit, was some minor grinding of the two long edges - pencil tip on one of these edges.

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Why did I need a saddle cut? To bring an old forge back to life. The tuyere in the photo is a 4 inch pipe with a 3 inch pipe saddle cut and welded on to make the air inlet pipe.

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The fire pot is an old brake drum with a 3/8 inch thick plate in the bottom. The 4 inch pipe of the tuyere is only welded to the 3/8 inch plate.

Can I get a hand to turn this forge over so I can cement in the new fire pot from the bottom?


Greetings , Nice layout,which is basic Steamfitter technique .would you do a n example of your layout off an offsize branch on an angle ? (wye branch)?.