Cómo doblar una silla

To bend a three-point saddle over a pipe: measure to the centre of the obstruction and add one offset's worth of shrink — that is the centre mark. Put an outer mark depth × 2.613 either side of it. Bend the centre 45° with the centre mark on your bender's saddle-centre mark, then roll the pipe a half turn and bend each outer 22.5° on the arrow, so the outers pull the opposite way to the hump. All three in one plane, centre first. The three-point saddle calculator works the figures.

Measure: the centre and the depth

The centre measurement is from the end of your pipe to the centre of what you are crossing — not its near edge. Mark the centre on the wall first, then transfer it to the pipe. The depth is how far the run has to lift: for a run on a wall crossing a pipe on the same wall, that pipe's outside diameter plus the gap you want.

A duct or a beam wants a four-point saddle instead — 3-point vs 4-point sorts out which.

The angles: 45 with 22.5s, or 60 with 30s

The outers are always half the centre: 45° with 22.5° outers is the usual saddle; 60° with 30° outers is shorter but harder to pull wire through. The figures are ratios, so they work in inches or millimetres.

The shop's ×2.5 is not the geometry. On a 3" (76 mm) saddle it puts the outers 5/16" (8.6 mm) closer than 2.613 does, and the hump comes out about 1/8" (3.3 mm) low. Fine with clearance to spare, not over a pipe you must just clear.

Lay out the three marks

Which mark goes on which bender mark

• C goes on the saddle-centre mark on your bender — often a notch in the rim or a teardrop on the side of the shoe. Some shoes have none, and some are not quite true: test 3 on measure your bender finds out from one scrap.
• Each outer goes on the arrow, with the hook toward the nearer end of the stick — pointing away from the hump. So the pipe is turned end for end between them.
• Not the star. The star is the back of a 90 and has no job on a saddle.

Bending the saddle, step by step

1. Measure. From the end of the pipe to the centre of the obstruction: the centre measurement. And the depth: how far the run has to lift to clear it, plus the clearance you want.
2. Work the figures. Outer angle = half the centre angle. Centre mark C = centre measurement + depth × the outer angle’s shrink figure. Centre to each outer = depth × the outer angle’s multiplier. Add twice the shrink to the stick length.
3. Mark. C, then an outer mark either side of it, every one taped from the same end as a running total. Lines all the way round, and a pencil line along the top of the pipe.
4. Centre bend. C on the saddle-centre mark on your bender, often a notch or teardrop, and pull the centre angle: 45° on the usual saddle.
5. First outer. Roll the pipe a half turn so the hump points the other way. Put the outer mark nearer the end on the arrow, hook toward that end, and pull the outer angle.
6. Second outer. Take the pipe out and turn it end for end, the hump still pointing the same way. Arrow on the other outer mark, hook toward the other end, and pull the same outer angle.
7. Check. Lay it on the floor. Both ends flat with no rock, the hump standing the depth, and its peak at the centre measurement from the end.

Centre first, always: bend an outer first and the pipe will not sit in the shoe for the centre. With the hump pointing down, most hands bend the outers in the air — see the hand bender guide — and sight down the pipe before each one.

Worked examples

Checking the hump

Hump up on the floor, both ends sit flat with no rock (a rock is an outer out of plane) and the legs sight in one line. Floor to the underside of the pipe at the peak is the depth. A square at the peak, taped along the floor from the end, should read your centre measurement.

A hump that leans has one outer short: back on the arrow and pull a touch more. Off centre by the same amount every time means your saddle-centre mark is not a true centre — find the offset once and shift C by it.

What goes wrong

Preguntas

How do you bend a 3 point saddle?

Measure to the centre of the obstruction and add one offset’s worth of shrink for the centre mark. Put the outer marks depth × 2.613 each side of it for a 45° saddle. Bend the centre 45° on the saddle-centre mark first, then each outer 22.5° the opposite way on the arrow, all three in one plane.

What is the multiplier for a 3 point saddle?

For a 45° centre with 22.5° outers it is 2.613 (1 ÷ sin 22.5°), which the shop rounds to 2.5. For a 60° centre with 30° outers it is exactly 2. Multiply the depth by it for the distance from the centre mark to each outer mark.

How much does a three-point saddle shrink?

At 45° with 22.5° outers, 0.199 × the depth for each of its two halves — the shop's rule is 3/16 per inch of depth. The centre mark moves along by one of them; the stick gets both. A 3" (76 mm) deep saddle moves C by 5/8" (15 mm) and needs a 1 3/16" (30 mm) longer stick.

Which mark on the bender does the centre of a saddle go on?

The saddle-centre mark on your bender, often a notch in the rim or a teardrop on the side of the shoe. If your shoe has none, or you doubt it, test 3 on the measure-your-bender page finds where the centre really is.

Why did my saddle come out as a Z?

One outer bend went the same way as the centre. After the centre bend, roll the pipe a half turn before the outers, so they pull the opposite way to the hump.

Related

• Three-point saddle calculator — C, the outers and the shrink for any depth.
• 3-point vs 4-point saddle — which one the obstruction wants.
• How to bend a four-point saddle — over a duct or a beam.
• Measure your bender — find out whether your saddle-centre mark is true.
• Common mistakes — Z saddles, leaning humps and the rest.