1. **How to draw a 90 degree bend?**
* **Method 1 (Using Tools):** Draw two straight lines meeting at a point (the vertex). Using a protractor, measure 90 degrees from one line and mark a point. Draw the second line through the vertex and this mark. For a curved bend (like pipe/duct), draw two perpendicular straight lines (legs). Determine your bend radius (R). Draw a quadrant (quarter-circle) connecting the ends of the legs, with its center at the imaginary intersection point of the legs extended. The radius of this arc is R.
* **Method 2 (Compass & Straightedge - Perpendicular Lines):** Draw the first straight line (AB). Pick a point on it (C) as the vertex. Using a compass, draw arcs of equal radius from C on both sides of AB. From where those arcs cross AB, draw two larger intersecting arcs above/below AB. Draw a straight line from C through the intersection of these larger arcs. This line is perpendicular (90 degrees) to AB. Add a connecting arc for a curved bend as in Method 1.
2. **What does a 90 degree bend look like?**
* A 90-degree bend resembles the **shape of the letter "L"**. It consists of two straight sections (legs) meeting at a corner. The key feature is that the **direction changes exactly by a right angle (90 degrees)**.
* *For pipes, tubes, sheet metal, or ducts:* The bend usually has a **smooth, constant-radius curve** connecting the two straight legs. It looks like a quarter-circle arc joining two perpendicular lines. The sharpness of the curve depends on the bend radius.
3. **How to calculate a 90-degree angle?**
* **Verification (Not Calculation):** A 90-degree angle is a fundamental right angle. You don't typically "calculate" the angle itself as 90 degrees; you **verify** or **construct** it. Use tools like:
* **Protractor:** Align the baseline with one leg; the 90-degree mark points along the other leg.
* **Try Square or Carpenter's Square:** Place the stock against one leg; the blade should perfectly align with the other leg.
* **3-4-5 Triangle Method (Pythagorean):** Measure 3 units along one leg from the vertex, 4 units along the other leg. The straight-line distance between these two points should be exactly 5 units if the angle is 90 degrees. (Works with multiples like 6-8-10).
* **Geometry:** Use properties of perpendicular lines, rectangles, or circles to establish a 90-degree angle.
4. **How to calculate the radius of a 90 degree bend?**
* **Method 1 (Measure Existing Bend):**
* Measure the **Outside Setback (OSB)** or **Inside Setback (ISB)** of the bend.
* Measure the **Material Thickness (T)**.
* Use the formula: `Bend Radius (R) = OSB - T` (for outside measurement) or `R = ISB + T` (for inside measurement). The most common formula using OSB is `R = OSB - T`.
* **Method 2 (Using Developed Length):**
* Measure the **Straight Length (L1)** of one leg *before* the bend starts.
* Measure the **Straight Length (L2)** of the other leg *before* the bend starts.
* Measure the **Total Developed Length (DL)** - the total length of the flat pattern before bending.
* Calculate the **Length of the Bend (BA):** `BA = DL - L1 - L2`.
* For a 90-degree bend, the Bend Allowance (BA) formula is: `BA = π * (R + K*T) * (90/360) = (π/2) * (R + K*T)` (Where `K` is the K-factor, typically ~0.3-0.5 for sheet metal). **Solve this equation for R.** This requires knowing T and estimating K.
* **Method 3 (Using Die or Mandrel):** The bend radius is often determined by the tooling (die or mandrel radius) used to make the bend. Consult the tooling specifications. The radius is typically specified as the *inside radius* (Ri) or *centerline radius* (Rc).
5. **How to calculate a 90 bend? (Interpreted as Bend Allowance/Developed Length)**
* The key calculation for a 90-degree bend is usually the **Bend Allowance (BA)** - the length of material consumed within the bend itself. This is needed to find the **Total Developed Length (DL)** of the flat pattern.
* **Formula:** `Bend Allowance (BA) = (π/2) * (R + K*T)` OR `BA = (π * A / 180) * (R + K*T)` (Where A = Bend Angle = 90°).
* `R` = Inside Bend Radius (usually specified by tooling/material)
* `T` = Material Thickness
* `K` = K-factor (empirical constant, ~0.3-0.5 for sheet metal, depends on material/bending method)
* **Developed Length (DL):** `DL = Leg Length 1 + Leg Length 2 + BA`
* *(Sometimes "calculate a 90 bend" refers to determining the required bend radius (R), which is covered in question 4 above).*
