---
title: "Overview: Manual Round Tube Bending Device"
slug: "overview-manual-round-tube-bending-device"
date: "2026-04-05"
author: "PolyMech"
---

# Table of Contents

- [Overview: Manual Round Tube Bending Device](#overview-manual-round-tube-bending-device)
  - [1. Functional Principle](#1-functional-principle)
  - [2. Main Components](#2-main-components)
    - [2.1 Frame and Base](#21-frame-and-base)
    - [2.2 Main Bend Die (Form Die)](#22-main-bend-die-form-die)
    - [2.3 Clamp Block / Clamp Die](#23-clamp-block-clamp-die)
    - [2.4 Follower / Pressure Die Assembly](#24-follower-pressure-die-assembly)
    - [2.5 Bending Arm / Handle](#25-bending-arm-handle)
    - [2.6 Optional Accessories](#26-optional-accessories)
  - [3. Dies: Types and Design Notes](#3-dies-types-and-design-notes)
    - [3.1 Bend Die Geometry](#31-bend-die-geometry)
    - [3.2 Groove Shape](#32-groove-shape)
    - [3.3 Materials](#33-materials)
  - [4. Typical Limits for a Manual Tube Bender](#4-typical-limits-for-a-manual-tube-bender)
    - [4.1 Tube Size Range (Common DIY)](#41-tube-size-range-common-diy)
    - [4.2 Minimum Bend Radius](#42-minimum-bend-radius)
    - [4.3 Bend Angle Limits](#43-bend-angle-limits)
    - [4.4 Repeatability and Springback](#44-repeatability-and-springback)
  - [5. Sketches and Concept Layout for Building One](#5-sketches-and-concept-layout-for-building-one)
    - [5.1 Side View – Overall Arrangement](#51-side-view-overall-arrangement)
    - [5.2 Front View – Die Stack and Clamp](#52-front-view-die-stack-and-clamp)
    - [5.3 Simple Dimensions to Start From (Example)](#53-simple-dimensions-to-start-from-example)
    - [5.4 Simple Build Sequence](#54-simple-build-sequence)
  - [6. Practical Tips and Safety](#6-practical-tips-and-safety)
  - [7. Production: machining, stock, and tolerances (30 mm × 2 mm, 130°)](#7-production-machining-stock-and-tolerances-30-mm--2-mm-130)
    - [7.1 Design basis for this bill](#71-design-basis-for-this-bill)
    - [7.2 Master table — stock, machining, tolerances](#72-master-table--stock-machining-tolerances)
    - [7.3 Purchased / shop-stock tolerance (typical)](#73-purchased--shop-stock-tolerance-typical)
    - [7.4 Order of operations and 130° clearance checks](#74-order-of-operations-and-130-clearance-checks)
  - [8. Summary](#8-summary)

---

*Figure 1 – Conceptual side view of a manual round tube bender with main components labeled.*

![Selected image](https://ytoadlpbdguriiccjnip.supabase.co/storage/v1/object/public/pictures/cache/919bafebbb1c7a6480a763d3fbe13c7056afc911259f0bbbac0042911d967d6b.jpeg)

![image](https://ytoadlpbdguriiccjnip.supabase.co/storage/v1/object/public/pictures/cache/41483eaa457dcebbd72118d0d0e50cbae535344a37cf98307d95329d6e6f7527.jpeg "image")

[Youtube Source](https://www.youtube.com/watch?v=k1hlAYbiNHY "Youtube Source")

# Overview: Manual Round Tube Bending Device

This page gives a practical overview of a **manual round tube bender** suitable for a small workshop or DIY fabrication. It covers:

* Functional principle
* Main components and dies
* Typical limits (diameter, wall, bend radius)
* Simple design sketches and build notes (concept-level, not certified drawings)
* A **Section 7** production-style machining and stock list for one target configuration (30 mm tube, 130°)

***

## 1. Functional Principle

A manual round tube bender for smooth, repeatable bends usually works on the **rotary draw** principle:

1. The tube is clamped against a **form die** (also called bend die) that has a groove matching the tube OD.
2. The die rotates around its center, pulling the tube with it.
3. A **pressure/follower die** supports the tube on the tangent side and keeps it in the groove.
4. Optionally, a **wiper** or roller can be used to reduce flattening and wrinkling.

Compared to simple "pipe hickey" benders or crude pinch benders, a die-based rotary bender gives:

* Better control of bend radius
* Less flattening of the tube cross‑section
* Repeatable angles

***

## 2. Main Components

A typical manual rotary tube bender you can build in a small shop includes:

### 2.1 Frame and Base

* **Base plate**: Thick steel plate or welded frame that bolts to the floor, a large bench, or a receiver tube on a stand.
* **Vertical post / side plates**:
  * Support the pivot shaft of the main bend die
  * Take bending loads; use adequate section (e.g., 10–15 mm thick plates or heavy channel/box)
* **Reinforcement gussets** between base and posts to resist twisting.

### 2.2 Main Bend Die (Form Die)

Function: Sets the **centerline bend radius** and supports the tube during bending.

Key features:

* **Outside diameter and profile**:
  * Large circular form with a **groove** machined or turned into the rim to match the tube OD.
  * Groove profile: usually a **semi‑circular or slightly open U** shape, just larger than tube OD.
* **Center bore / hub**:
  * Fits on a strong **pivot shaft** (e.g., 30–40 mm or more, depending on tube size and force).
  * May have a **keyway** or bolt‑on interface to the bending arm.
* **Detent or hole pattern** (optional):
  * For a spring pin to lock the die at standard angles (0°, 45°, 90°, etc.).

You will typically need **multiple bend dies**, each tailored to:

* Tube OD (e.g., 19, 25, 32, 38, 50 mm)
* Desired bend radius (e.g., 2×D, 3×D, 4×D)

### 2.3 Clamp Block / Clamp Die

Function: Grips the tube to the main die so the tube rotates with the die.

* Shaped with a matching groove for the tube.
* Bolts or pins into the main die.
* Uses a **clamping bolt** or lever to tighten onto the tube.
* Often split into two halves for quick loading/unloading.

### 2.4 Follower / Pressure Die Assembly

Function: Supports the straight tube just as it enters the bend, minimizing flattening and kinks.

Typical DIY options:

1. **Sliding block pressure die**:
   * A block with a matching groove riding on guide rails.
   * Spring‑loaded or cam‑adjusted to press the tube into the main die.
2. **Roller pressure die**:
   * A hardened or machined roller with a groove.
   * Mounted on a pivoting arm with adjustable pressure.

Key considerations:

* Low friction (use bronze bushings, bearings, or good lubrication).
* Sufficient adjustment range for different tube diameters and wall thicknesses.

### 2.5 Bending Arm / Handle

* Long lever arm attached to the main die hub.
* Provides mechanical advantage; typical length: 1–1.5 m for hand power on smaller tubes.
* May be detachable for storage.
* Often includes:
  * **Angle scale** (protractor, engraved scale, or pointer on a stationary reference plate).
  * **Handle grip** or crossbar for two‑handed operation.

### 2.6 Optional Accessories

* **Angle stop**: Adjustable mechanical stop to repeat a given bend angle.
* **Backstop / start position gauge**: For consistent bend location from the tube end.
* **Wiper die**: Small, sharpened block near the tangent to reduce inside‑radius wrinkling.

***

## 3. Dies: Types and Design Notes

### 3.1 Bend Die Geometry

For a round tube of **outside diameter D**:

* **Centerline radius (CLR)** \= distance from die center to tube centerline.
* Outer diameter of die OD\_die ≈ 2 × (CLR + D/2).
* Groove:
  * Depth ≈ 0.55–0.65 × D
  * Width a bit larger than D to allow easy loading but still support the tube.

**What sets CLR?** The die’s CLR is the **centerline radius of the bend you are producing**. You choose it to be **at or above the minimum** the **tube (material, OD, wall)** can handle in **rotary draw without a mandrel**, and above what would **wreck a coating** (e.g. zinc) or the surface. Shop practice usually expresses that minimum as a **multiple of OD** (CLR/D). The table rows (2.5×D, 3×D, …) are **convenient radii** that line up with those experience bands—not a separate physics formula.

**Sweet spot (stainless + galvanized, 30 mm OD, 2 mm wall, up to ~130°):** For **one** main die to serve **both austenitic stainless (e.g. 304/316)** and **galvanized carbon tube**, use a **slightly open** ratio so stainless work-hardening and **zinc** (avoid the tightest possible radius) are both happier. A good **single** starting CLR is **4×D = 120 mm** (see table A, 120 / 4.0 row): enough margin for hand work through about **130°** if the **frame, handle, and long tube** are laid out to clear. If the die would be too large, **3.5×D = 105 mm** is a second option—validate on **stainless scrap** first, as it is closer to the floor for kinking. Galvanized often tracks near carbon-steel minima but **treat the coating as a reason to avoid “minimum for bare steel”** radii.

Example: For 25 mm tube, CLR \= 75 mm (≈3×D):

* OD\_die ≈ 2 × (75 + 12.5) \= 175 mm
* Groove depth \~ 14–16 mm, width slightly larger than 25 mm.

**Sample tables — 30 mm OD, 2 mm wall (mild / stainless / galvanized, general hand bending):** The **main bend die** sets the bend radius. The **follower / pressure die** (roller or block) uses the **same tube groove** but does *not* use the large OD\_die formula; it only needs to support and press the straight leg. A **clamp block (clamp die)** also uses that groove and bolts to the main die. A **2 mm** wall is within the common DIY **1–3 mm** range (Section 4.1); for tight bends, favor a higher CLR/D to reduce kinking. Wall thickness does not change the OD-based main die size; it matters for force and wrinkling limits.

**A — Main bend die (form die, pivots with the handle)**

| CLR (mm) | CLR / D | Approx. main die rim OD (mm) (2×(CLR + D/2)) | Groove depth (mm) | Groove width (mm) |
| :--: | :--: | :--: | :--: | :--: |
| 75 | 2.5× | 180 | 17–20 (≈0.55–0.65×D) | Slightly over 30 (e.g. 31) |
| 90 | 3.0× | 210 | (same) | (same) |
| 105 | 3.5× | 240 | (same) | (same) |
| 120 | 4.0× | 270 | (same) | (same) |

**B — Follower / pressure die and optional base guide (same 30 mm groove on all rows)**

| Role | Type | Groove (match 30 mm OD) | Suggested size (indicative) | Notes |
| :-- | :-- | :-- | :-- | :-- |
| Tangent / straight leg | **Pressure die — block** (Section 2.4) | Depth 17–20, width as main die | e.g. **50 × 75 × 75** to **50 × 80 × 80** mm (same idea as Section 5.3, scaled from 32 mm) | Mount so the face stays **tangent** to the main die; add adjustment/slide. |
| Tangent / straight leg | **Pressure die — roller** (Section 2.4) | Same groove on roller OD | Roller OD often **~50–90 mm** in DIY builds (enough wall around the groove; fit bearings/bushings) | Smaller than main die; not another form-die radius. |
| Infeed (optional) | **Fixed guide roller** on base | Same profile | Roller OD e.g. **~40–70** mm; height aligns straight tube to groove | Stabilizes the incoming straight section; not every layout has one. |

**C — Clamp block / clamp die (Section 2.3)**

| | |
| :-- | :-- |
| Groove | **Same 30 mm profile** as the main die (depth / width as in table A for that build). |
| Thickness (into clamp force) | Often **~20–30** mm, depending on bolt and stock. |
| Plan area (indicative) | Large enough to **grip the tube** and **clear 2+ bolts** into the main die, e.g. on the order of **~50 × 40** mm and up. |
| Fasteners | **M10–M12** into the main die face is a typical range (Section 5.2). |

**D — Practical limits (same tube: 30 mm OD, 2 mm wall, hand bender, no hydraulics)**

| Topic | Guideline | Detail (see Section 4) |
| :-- | :-- | :-- |
| OD in DIY range | 30 is inside **~10–38** mm | Section 4.1 |
| Wall in DIY range | **2** is inside **~1–3** mm | Section 4.1 |
| Minimum CLR (mild steel, no mandrel) | Prefer **~75–90** mm (≈**2.5–3**×D) as a general floor; below that, risk of kink/oval rises | Section 4.2 |
| Tight / cosmetic / thin-wall | Safer at **~90–120** mm (≈**3–4**×D) or use internal support | Section 4.2 |
| Bend angle (by hand) | **~0–130°** is realistic for strong layout; **0–120°** is a common comfort band; **0–180°** needs full clearance | Section 4.3 |
| Springback (steel) | Often **2–5°**; set stops/overbend by test | Section 4.4 |
| Avoid | **Heavy / thick-wall pipe** (e.g. “schedule” pipe) | Bending force jumps; Section 4.1 |
| If flattening or wrinkling | Increase CLR, improve clamp and pressure-die contact, lubricate groove | Sections 2.3, 2.4, 6 |

*Indicative only, not shop drawings. Pick one CLR, machine matching main + clamp + pressure tooling; check clearances and refine on scrap.*

### 3.2 Groove Shape

* **Best**: Proper machined radius matching the tube OD, with smooth surface finish.
* **DIY compromise**: Multi‑pass machining with a ball end mill, or turning on a lathe using form tools.
* Avoid sharp edges; always **chamfer or radius** the edges of the groove to avoid scoring the tube.

### 3.3 Materials

* **Dies**: Medium‑carbon steel (e.g., C45/1045) or similar. For light duty, mild steel is often acceptable.
* **Clamp blocks**: Same as main die, optionally with **replaceable aluminum or nylon inserts** to reduce marking.

**Rollers**: Hardened or case‑hardened if high wear is expected.

***

## 4. Typical Limits for a Manual Tube Bender

Exact capabilities depend on materials, dimensions, and workmanship, but for a well‑built manual bender (no hydraulic assist):

### 4.1 Tube Size Range (Common DIY)

* OD: \~10–38 mm (3/8"–1 1/2") is practical by hand.
* Material: Mild steel, stainless, or aluminum tube (not heavy schedule pipe).
* Wall thickness: \~1–3 mm.

Above these, bending force increases quickly; you may need **hydraulic assistance** or longer handles.

### 4.2 Minimum Bend Radius

**Basis:** The die CLR must be no tighter than the **bending minimum** for that **OD, wall, alloy,** and process (here: manual rotary draw, no mandrel). That minimum is still often stated as a **range of CLR/D** from experience (Section 3.1). **Wall** mainly changes force and wrinkle tendency; it does not replace material rules.

* Carbon / mild **steel** tube: **CLR ≥ 2.5–3 × OD** for general work without a mandrel.
* **Austenitic stainless** (e.g. 304/316): work‑hardens; without a mandrel, treat **~3.5–4.5 × OD** as a more comfortable starting band for DIY than the bare minimum—use **test bends** in scrap of the *same* stock.
* **Galvanized** tube: bending behavior is like **carbon steel** underneath, but **very tight** bends can **damage the zinc** (flaking, cracks in the coating at the **outside** of the bend). Favor a **less aggressive** CLR than the absolute minimum you might risk on bare steel, and keep the groove **smooth**; hot‑dip pieces can be a bit stiffer in feel.
* Thin‑wall or tight cosmetic work may require:
  * **CLR ≥ 3–4 × OD**, or
  * Internal support (packing sand, low‑melt alloy, or a proper internal mandrel).

**Stainless and galvanized on one bender (30 mm × 2 mm, bends to ~130°):** Size the die for the **worse** case. Here that usually means **do not** pick the minimum carbon‑steel radius; a practical shared target is about **3.5–4 × OD** (see Section 3.1, “Sweet spot” paragraph and table A at **105 / 3.5×** and **120 / 4.0×**). At **~130°**, strain and overbend torque build—**120 mm (4D)** is a good first CLR if a **240–270** mm class main die is acceptable; step down in CLR only with scrap testing on stainless.

### 4.3 Bend Angle Limits

* Mechanically, you can design for 0–180°.
* Common useful range: **0–120°** for hand‑operated, as force gets high at large angles.
* **0–~130°** is still a realistic spec if you design **pivot height**, **handle length**, and **tube stick‑out** so nothing clashes through the full sweep, and you accept **higher** effort and **tighter** clamping needs than at 90°. **Springback** on stainless is also trickier to dial in at big angles—good lubrication in the **groove** and a **generous** CLR (Section 4.2) help.
* Ensure frame clearance: the tube and bending arm must not hit the base or floor as you approach maximum angle.

### 4.4 Repeatability and Springback

* Metals will **spring back**, so you must over‑bend slightly.
* Typical springback for steel tube: **2–5°**, more for harder materials.
* A simple **angle scale** and test bends in scrap will help you dial in accurate angles.

***

## 5. Sketches and Concept Layout for Building One

Below is a conceptual layout. Dimensions are **indicative** only; adapt to your materials and tube sizes.

### 5.1 Side View – Overall Arrangement

Key elements in a side view sketch:

* Base plate
* Vertical side plates with pivot hole
* Main die on pivot shaft
* Bending arm fixed to die hub
* Pressure die arm supporting the straight tube

> Build tip: Start the design around your **largest bend die** and required **frame clearances**. Ensure that at full bend angle the handle and tube do not clash with the floor or base.

### 5.2 Front View – Die Stack and Clamp

Elements to show in a front view sketch:

* Tube running horizontally through the die groove.
* Clamp block on the front face of the die, bolted with two or more M10–M12 bolts.
* Pivot shaft through die and side plates, retained with nut or collar.
* Pressure die block or roller just tangent to the die.

### 5.3 Simple Dimensions to Start From (Example)

For a bender intended for up to 32 mm OD mild‑steel tube:

* **Base plate**: 300 × 400 × 12 mm
* **Side plates**: 250 mm high, 12–15 mm thick, spaced 60–70 mm
* **Pivot shaft**: 35–40 mm diameter, medium‑carbon steel
* **Largest bend die**: OD ≈ 220–250 mm (e.g., CLR ≈ 100–110 mm for 32 mm tube)
* **Bending arm**: 1.2–1.4 m long, 40 × 20 rectangular tube or 40 mm round solid/pipe
* **Pressure die block**: 50 × 80 × 80 mm with a 32 mm groove and adjustable mount

These values are a **starting point**; verify against:

* Available stock sizes
* Intended tube material
* Your own strength / acceptable effort

### 5.4 Simple Build Sequence

1. **Design dies first**:
   * Choose tube sizes and CLRs.
   * Draw each die full scale (even on paper/card) to check clearances.
2. **Design frame around dies**:
   * Set pivot height so that your longest tube can swing without hitting the floor.
   * Confirm maximum bend angle.
3. **Build frame and mount pivot**:
   * Weld base and side plates.
   * Drill/ream pivot holes, fit shaft and bushings.
4. **Machine main die and clamp block**:
   * Turn OD and face on lathe.
   * Cut hub bore and keyway or bolt pattern.
   * Machine groove.
5. **Make pressure die and arm**:
   * Simple block on adjustable slide or a roller on a pivot.
   * Ensure smooth movement and easy adjustment.
6. **Add handle, angle pointer, and stops**:
   * Fit long bending arm.
   * Add a pointer and fixed scale or mark common angles.
7. **Test and refine**:
   * Make test bends, note springback and any flattening.
   * Adjust pressure die, add lubrication, or increase radius if necessary.

***

## 6. Practical Tips and Safety

* **Clamping**: The most common DIY issue is tube slipping in the clamp. Use enough bolt area, roughen clamp surface slightly, or add a soft insert with high friction.
* **Lubrication**: Use bending lubricant (wax/grease) in the groove to reduce marking and flattening.
* **Fasteners**: Use high‑strength bolts for clamp and handle connections.
* **Welding**: If frame is welded, brace and cool slowly to minimize distortion; check pivot bores afterwards.
* **Safety**:
  * Keep fingers away from pinch points between tube, dies, and frame.
  * Long tubes can swing with force—keep the area clear.
  * Wear eye and hand protection.

***

## 7. Production: machining, stock, and tolerances (30 mm × 2 mm, 130°)

**Not a certified drawing pack.** This section is a **shop checklist**: what to buy, what to leave on for first cuts, and what tolerances usually matter for a **30 mm OD, 2 mm wall** tube, **stainless / galvanized–friendly** setup, **maximum bend about 130°**, using the **4×D sweet spot** for the main die (Section 3.1, Section 4.2). If your layout differs (pivot height, arm length), re-check **stack length** and **swing clearance** before cutting finish sizes.

### 7.1 Design basis for this bill

| Item | Value / note |
| :-- | :-- |
| Tube | **30 mm** OD, **2 mm** wall (target process: manual rotary draw, no mandrel) |
| Main die **CLR** | **120 mm** (4×D) — main die **rim OD** **270 mm** (2×(120+15)) |
| Groove (all matching tooling) | Depth **~18** mm nominal (0.6×D target band 17–20); width **30.5–31** mm; **semicircular or form radius ~15.25** mm to match tube (Section 3.1–3.2) |
| Max bend angle | **~130°** — design frame and **long-tube** path so handle and stock **do not** hit base or floor (Section 4.3) |
| Pivot | Example: **Ø 40** mm finished shaft; side plate **inner gap 65** mm, plates **12–15** mm (Section 5.3; see `cad/base_plate.scad` defaults) |
| Base | **400 × 300 × 12** mm plate (matches CAD default); adapt if your stand differs |

### 7.2 Master table — stock, machining, tolerances

**How to read:** **Stock** = material to order / saw from (include allowance). **Key machined** = after roughing, before deburr. **Tol. class** = A = tight (groove / bore), B = fit-up, C = general saw/plasma. *Qty* is for a single complete machine; weldments count **per part**, not per assembly step.

**Frame and structure**

| ID | Part | Qty | Suggested material | Stock to buy (typical; add shop allowance) | Key machined / finish (nominal) | Tol. class |
| :--: | :-- | :--: | :-- | :-- | :-- | :--: |
| F-1 | Base plate | 1 | **Structural or mild steel** plate, HR or laser cut | **420 × 320 × 16** (finish **400 × 300 × 12**) — leave **+3–5** mm/l edge and **+2–4** mm on thickness to flatten | Faced to **12** mm; corners **R 8**; **M12** bench holes per pattern in `base_plate.scad` (or your bench); through holes **+0.3/0** vs drill size | C face / C holes |
| F-2 | Side plate | 2 | **Same** as F-1, 12–15 mm | Each **~270 × 200 × 15** (finish **250** high **× 180** wide **× 12** or 15) | Perpendicular; **pivot hole Ø 40 +0.1/0** reamed to match shaft (or **+0.2** for bush OD); any side-plate tie holes with F-1 | A pivot bore; B other |
| F-3 | Gusset / stiffener | 0–4 | Plate strip, same as frame | As needed, **+10** mm/leg | 45° mitres or fillet-weld prep | C |
| F-4 | Bending **arm** (hollow) | 1 | **RHS** or similar | **40 × 20 × 1300** (long; trim) | **1250** mm arm + handle detail; end mitre/trim **±2**; holes for **hub** per pattern | C length; B hole pattern to hub |

**Pivot and hardware (machined or bought finished)**

| ID | Part | Qty | Suggested material | Stock to buy (typical; add shop allowance) | Key machined / finish (nominal) | Tol. class |
| :--: | :-- | :--: | :-- | :-- | :-- | :--: |
| P-1 | **Pivot shaft** (main) | 1 | **C45/1045** or **SS** round | **Ø 45 × 300** (turn to **Ø 40**; length **+15** for cut/test) | **Ø 40 h8/f7** to mate F-2 (or ground **Ø 40** to match bush); one **key** flat; shoulders for die hub + handle | A diameter on bearing/journal; B overall length |
| P-2 | **Bush** (optional) | 2 | **Phosphor bronze** or sintered bronze | **Ø 50** OD × **L 20–25** ring stock | **Ø 40** ID press/slip fit to P-1, **Ø 50** OD to F-2, or use **purchased flanged** bush | A ID/OD; match supplier chart |
| P-3 | Collar / **nut** system | as req. | Commercial | — | Trued faces if shop-made; otherwise **M40×?** or split collar **—** buy match to P-1 | B |

**Tooling (dies and followers — 30 mm groove; match Section 3.1 table A, CLR 120 row)**

| ID | Part | Qty | Suggested material | Stock to buy (typical; add shop allowance) | Key machined / finish (nominal) | Tol. class |
| :--: | :-- | :--: | :-- | :-- | :-- | :--: |
| T-1 | **Main bend die** (disc) | 1 | **C45/1045** (or 4140 pre-hardened if you prefer) | **Ø 280–290 × 55** **round** or **slug** (finish **Ø 270** face, **40–50** total thickness typical) | Turn **rim Ø 270 ±0.2**; faces square; **hub bore Ø 40** line-bored with F-2; **keyway** or bolt pattern to arm; **groove** width/depth to **Table 7.1**; lead-in **chamfer 0.5–1** | A groove profile & bore; B OD |
| T-2 | **Clamp block** | 1 (or 2 halves) | Same as T-1; optional **aluminum** faces | **55 × 45 × 32** (finish **~50 × 40 × 25** net) | Groove **same as T-1**; **2× M10–M12** bolt holes to **main die** face, depth **safe**; clamp screw pocket | A groove; B location |
| T-3 | **Pressure die** (block) | 1 | Same as T-1 (or **bronze** insert for groove) | **55 × 85 × 90** (finish **50 × 80 × 80**) | **Same 30 mm groove**; **flat** pressure face; **slide** or pin holes for **adjust** | A groove; C block |
| T-4 | **Pressure** roller (alt. to T-3) | 1 | 1045 or bearing steel | **Ø 60 × 100** (turn) | **Roller body Ø 55–75**; **30 mm** groove; **bore** for **needle/roller** bearing; faces square | A groove; A bearing bores |
| T-5 | **Fixed guide** roller (optional) | 0–1 | Same as T-4 or bronze | **Ø 50 × 60** | Smaller **30 mm** groove; mount holes | B |
| T-6 | **Follower** arm / bracket | 1 | Bar + plate | As layout | Pivots at base or side; **tangent** adjust **±3–5** mm; stop **pin** holes | B |

*Hub axial length on T-1:* machine so the die is **captured** between the side plates with the **65 mm** clear gap; add **0.1–0.2** feeler room or shim pack—**measure your stack** before final facing.

**Hardware (purchased — not bar stock; counts depend on your hole pattern)**

| ID | Item | Qty (typical) | Notes |
| :--: | :-- | :--: | :-- |
| H-1 | SHCS, **T-2** to **T-1** and **T-3** (or T-4) to bracket | 4–8+ | **M10–M12**; **10.9** or **8.8**; length **clears** stack + **1–1.5D** thread |
| H-2 | **Bench** bolts, **F-1** | 4 | **M12** (or match **14 mm** drill in `base_plate.scad` if unchanged); flat washers |
| H-3 | **Parallel key** P-1 ↔ **T-1** hub (if keyed) | 1 | **Width/height** per keyway; length **< hub** |
| H-4 | **Pivot** retainers (nut + washer or split **collar**) | 1 set | **Match** P-1 end thread or smooth journal |
| H-5 | Lubrication | 1 | EP **grease** for bushes; **sulfur-free** for stainless if you want to be fussy; wax/grease in **groove** (Section 6) |

### 7.3 Purchased / shop-stock tolerance (typical)

**Purpose:** so the first pass does not cut “exact finish” from undersized or crooked bar stock. Values are **typical mill/saw** bands, not guarantees—**measure** each piece before programming CAM.

| Stock form | What to expect (rule of thumb) | What to do before finish passes |
| :-- | :-- | :-- |
| Hot-rolled **plate** (12–16 mm) | Thickness **+0–+1** mm, sometimes more on HR; width/length per supplier; edge **not** square to ±0.1 | Face mill or grind reference face; work from one scribed **datum** |
| **Round** bar (pivot, die slugs) | Diameter **+0.1–+0.5** vs nominal; not always straight for long bar | For **Ø 40** journal: start **Ø 42–45**; turn between centers for long P-1 |
| Laser/water **cut** plate | Better **edge**; heat-affected; tap holes from cut may need ream for precision | De-slag, measure thickness post-cut |
| **Saw** cut (bandsaw) | **+2–5** mm for shop stock length; miter not perfect | Add **+3** stock on critical lengths in Table 7.2 unless you are duplicating a proven part |

**Tolerance class (suggested, workshop):**

| Class | Linear / diameter | Use for |
| :-- | :-- | :-- |
| **A** (groove, pivot mate) | **±0.05** on **groove** bottom radius / width; **+0.05/0** on reamed bores; smooth **Ra 1.6** or better in groove if possible | Tube contact, **pivot** fit |
| **B** | **±0.1–0.2** on **bolt** circles, key flats, **die OD** (270) | **Assembly** that still adjusts |
| **C** | **±0.5** on **plate** outline, **arm** length, non-critical **slots** | Frame, first welding prep |

*Groove* is the highest leverage: a loose groove **drops** the tube; a too-narrow one **galls** stainless. Match **D = 30** and **+0.5** width, then **dial on scrap** before hardening (if any).

### 7.4 Order of operations and 130° clearance checks

**Suggested build order (aligns with Section 5.4, with production detail):**

1. **Frame square:** Weld **F-1** + **F-2** + gussets; check **diagonals** and that **pivot bores** are co-axial after weld (bore/ream together if possible).
2. **Pivot in:** Fit **P-1** and bushings; spin test—no **bind**; add **lubrication** points.
3. **Dies in order:** Turn **T-1** (main die) OD/face, then **hub** bore, then **groove** last; match **T-2** and **T-3/4** grooves to the **same** radius gauge or template.
4. **130° / full swing:** With **arm** and **a straight stick** the length of your longest workpiece, **rotate the arm** to **~130°**; confirm **arm**, **handle**, and **long tube** clear **F-1** and the floor. Raise **pivot** or trim **arm** if you get **clash**—a common fix when moving from 90° thinking to 130°.

**Inspection (minimum before release to assembly):**

* **Groove** on T-1 / T-2 / T-3(4) checked with a **Ø 30** pin or test tube—**light** drag, no **rocking**.
* **P-1** runout at die hub **< 0.1** TIR if you care about cosmetic bends (not always critical for hand tool).
* **Clamping:** **T-2** must **not** slip a **2 mm wall** 30 **tube** at 90°; if it slips at 130°, increase **normal force** (bolt size, serrate faces **lightly**, or **insert**), not only torque.

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## 8. Summary

A manual round tube bender with **proper dies** and a **robust frame** can reliably bend mild‑steel and aluminum tubing for roll cages, furniture, handrails, and general fabrication.

Key design steps:

1. Choose your target **tube sizes** and **minimum bend radii**.
2. Design and dimension **main dies** first.
3. Build a **stiff frame** and **long handle** around those dies.
4. Add a **pressure die** and **good clamping** for clean, repeatable bends.

Use the conceptual sketches and guidelines above as a base, then adapt details to your tools, materials, and accuracy requirements.