Sheet metal: bending, cutting and the SendCutSend deep dive (2026-11-01)

This page is for Valpatel/cad-agent#283, which follows the first sweep of #118. Part A is a deep dive into one shop, SendCutSend: what it cuts and bends, the limits it enforces, and how it prices. Part B records the generic rules that named fabricators, practitioners and CAD vendors publish, where they agree and where they disagree.

The guide survey, sheet-metal-guides.md, decides which source leads each topic. PEM hardware and blind rivets are on sheet-metal-joining.md. The numbers that checks use live in process-profiles.yaml: the generic rules, and SendCutSend's own rules and tables under shop: sendcutsend.

Diagram drawn by tools/process_profile_figures.py from process-profiles.yaml: SendCutSend's tables for 1.6 mm 5052 (ALU-063) and the rivet rules for a 1/8 in rivet. A test redraws it and fails if the committed file differs.

How this was checked.

• Part A was read on 2026-11-01. It draws on SendCutSend's two machine feeds, the specs file [ref:sendcutsend-specs] and the ordering catalogue [ref:sendcutsend-catalog]; its ordering guide [ref:sendcutsend-ordering-guide]; and its guideline, service, FAQ and pricing pages.
• SendCutSend asks crawlers not to scrape its material pages, so this page takes per-material numbers from the feeds only. Requests were spaced at least a second apart.
• Part B was read on 2026-10-31, except where it says otherwise.
• [derived] marks a number calculated here from cited data; the method is stated beside it.
• Guides are summarised, not copied. Paid standards are named by number and title only.

Symbols: T is the sheet thickness, R the inside bend radius, and K the K-factor (where the neutral sheet sits, as a fraction of T from the inside face). BA is the bend allowance, and BD the bend deduction.

Part A. SendCutSend

A1. What it is, and how cad-agent uses it

SendCutSend is an online shop. You upload a flat DXF or a STEP of one sheet-metal body. It cuts the part by fibre laser, waterjet or router, then can bend it, press in PEM hardware, tap, countersink, form dimples, and anodise, plate or powder coat.

It bends by air bending on fixed tooling. That gives one radius and one K per stock, and it offers no custom radii [ref:sendcutsend-bending-service]. A part must therefore be drawn with its R and K, or the flat pattern will not match [ref:sendcutsend-sheet-cutting].

In cad-agent, SendCutSend is a named shop profile (shops: sendcutsend in the YAML):

• its per-stock tooling and limits are shop_tables;
• its shop-wide limits are rules marked shop: sendcutsend;
• cadagent.manufacturing.process leaves them out unless a caller names the shop, so the generic rules stay supplier-neutral;
• a project's chosen suppliers (#284) will select the profile.

flowchart TD
    G["generic rules<br/>(no shop: field)"] --> Q["rules_for(shop=...)"]
    S["shop: sendcutsend<br/>rules"] --> Q
    Q --> N["no shop named:<br/>generic only"]
    Q --> Y["shop named:<br/>generic + its own"]
    Q --> E["misspelt shop:<br/>an error"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class G,S near
    class Q here
    class N,Y,E out

How cadagent.manufacturing.process picks the rules a check sees. Each arrow means "feeds". A rule marked shop: reaches a check only when the caller names that shop, and a shop the data file does not list raises an error. The per-stock tooling rows (shop_tables) are different: sheet_metal.bend and the sheet DFM check read them by default (shop=True), because a bend is always made on some shop's tooling, and shop=False drops them.

The vendored text-to-cad sendcutsend skill [ref:text-to-cad] reads the same feeds for a preflight.

A2. Materials and thicknesses

What the feed holds [ref:sendcutsend-specs]. The specs file was generated 2026-04-29 and lists 178 materials. For each stock it gives:

• the thickness, with its plus and minus tolerance (5052 0.063 in: ±0.004 in);
• the cutting process and the smallest hole, bridge and hole-to-edge distance;
• for a bendable stock, R, K, the bend deduction, the die width, the shortest flange and the relief depth;
• for hardware, the least hole-to-edge, hole-to-bend and same-part distances, and the thread range;
• Brinell hardness and strength.

The YAML keeps the rows a bent plate with hardware needs: 23 bend rows (5052, mild steel, 304, 316, G90) and 27 stocks with cutting and hardware limits.

Diagram drawn by tools/process_profile_figures.py from the SendCutSend tooling tables in process-profiles.yaml (shop_tables). A test redraws it and fails if the committed chart differs.

A3. Cutting

From the cutting guidelines [ref:sendcutsend-sheet-cutting]:

flowchart TD
    U["one flat part,<br/>one file"] --> L["fibre laser<br/>hole 0.5T<br/>±0.005 in"]
    U --> W["waterjet<br/>hole 0.070 in<br/>±0.009 in"]
    U --> R["router<br/>hole 0.125 in<br/>±0.005 in"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class U near
    class L,W,R here

The three cutting processes SendCutSend's cutting guidelines name, with the smallest hole and the tolerance each holds [ref:sendcutsend-sheet-cutting]; the feed also lists a CO2 laser for some stocks. The process follows the material and thickness, not the buyer's choice.

• Smallest hole or cut-out:

  • laser: at least 50 % of T;
  • waterjet: 0.070 in;
  • router: 0.125 in.

  The feed's per-stock min_hole_size is smaller, at 0.35T for 0.063 in 5052 [derived].

• Inside corners: about the kerf for the laser, 0.032 in for the waterjet and 0.063 in for the router.

• Tolerances: ±0.005 in for laser and router, ±0.009 in for waterjet. Taper adds 0.001–0.002 in under 0.250 in stock, and up to 0.010 in at 0.500 in.

• Nesting: one part per file. There is no common-line cutting; a real gap of 0.006–0.012 in stays between parts.

• 3D files: one solid sheet-metal body of uniform thickness. Model holes as plain through holes; countersinks and taps are ordered as services. Use square bend reliefs, because rounded ones give open-contour errors.

A4. Bending

From the bending guidelines [ref:sendcutsend-bending-service] and the deformation page [ref:sendcutsend-bend-deformation]:

flowchart TD
    C["cut: laser, waterjet<br/>or router"] --> T["tap · countersink ·<br/>deburr · tumble"]
    T --> D["dimples"]
    D --> B["bend"]
    B --> F["anodise or plate"]
    F --> H["press in PEM<br/>hardware"]
    H --> P["powder coat, with the<br/>hardware capped"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class C near
    class T,D,B,F,H here
    class P out

The order SendCutSend runs its processes, from its dimple, hardware and finishing pages [ref:sendcutsend-dimple-forming] [ref:sendcutsend-hardware-service] [ref:sendcutsend-finishing]. Each arrow means "comes before". Hardware goes in after anodising and plating, but before powder coat. PEM's own handbook says never to install after paint or powder (sheet-metal-joining.md).

• Part and bend size: the flat part from 0.375 × 1.5 in to 30 × 44 in. A bend is 16–44 in long, depending on the thickness.
• Angle: at most 130°, and at least 5°. It does no hems, curls, rolls or coining.
• Tolerances:
  • bend to edge ±0.015 in, plus at least 0.015 in for each further bend;
  • angle ±1° up to 24 in long and ±2° above.
• Flanges:
  • each stock has its own minimum flange (shop_tables);
  • a U channel's base is at least twice its flange;
  • a box with hardware has flanges of 3.00 in at most, and 5 in without;
  • flanges 3.5–5 in long must be at least 1.1 in wide.
• Features near a bend: keep cut features at least half the die width from the bend line, or they distort. The dies run 0.472–1.575 in. Example: 0.104 in mild steel bends on a 0.630 in die, so keep features 0.315 in clear.
• Unsupported bends: a flange tapering below the minimum length distorts, so relieve it along the bend. From 0.187 in up, the minimum flange may be needed on both sides along the whole bend.

Its bend allowance reproduces [derived]. The bending calculator [ref:sendcutsend-bending-calculator] prints K = t/T, BA = π(R + K·T)·A/180 and BD = 2·OSSB − BA. Those formulas, applied to each stock's R, K and T from the feed, reproduce its published 90° bend deduction within 0.005 in for 53 of its 56 bendable SKUs. The other three differ by −0.021 in (brass 0.187 in), −0.017 in (brass 0.250 in) and +0.012 in (hot-rolled 0.188 in). So a flat pattern built from the feed's R and K matches the shop's own, except on those three stocks. (The calculator page's own worked example prints two different flat lengths, 17.612 and 17.765 in, for the same part.)

A5. Hardware, taps, countersinks and dimples

• PEM hardware [ref:sendcutsend-hardware-service] [ref:sendcutsend-hardware-catalog]:
  • The catalogue feed lists 151 nuts, flush nuts, standoffs and studs, from $0.77 to $5.73 each (median $1.23) [derived from unit_price].
  • For each part it gives the hole H (+0.003/−0 in), the edge distance K, the bend distance N and the same-part spacing M.
  • A hardware hole's centre must be at least BA/2 + N from the bend centre.
  • No hardware goes on a part with an acute bend.
  • The full analysis is on sheet-metal-joining.md.

Drawn by tools/process_profile_figures.py from SendCutSend's sendcutsend-cut-and-hardware-limits table in process-profiles.yaml: the per-stock minimums of its specs feed, across 27 stocks.

• Tapping [ref:sendcutsend-tapping]:
  • sheet from 0.059 in;
  • holes drawn at the tap-drill size (M3 0.102 in, M4 0.134 in, #10-32 0.164 in);
  • a tapped hole's centre at least half the tap hole plus the stock's tap hole-to-edge from any other cut;
  • at least half the die width from a bend.
• Countersinks [ref:sendcutsend-countersinking-service]:
  • depth at most 60 % of T;
  • 82° for inch screws and 90° for metric;
  • ±0.015 in;
  • aluminium and plastics only, from 0.125 in.
• Dimples [ref:sendcutsend-dimple-forming]:
  • 0.5–3 in across, in 0.025–0.125 in stock;
  • a dimple's centre at least its listed distance plus BA/2 from a bend;
  • formed before bending and hardware.

A6. Finishes

Add powder build-up to slots and tab fits on both faces.

flowchart TD
    A["Type II anodise<br/>0.0002–0.0009 in"] --> F["a slot or tab fit:<br/>add both coated<br/>faces' build-up"]
    Z["zinc plate<br/>about 0.0003 in"] --> F
    P["powder coat<br/>0.002–0.005 in"] --> F
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class A,Z near
    class P out
    class F here

Finish build-up per side from SendCutSend's finish-thickness FAQ [ref:sendcutsend-finish-thickness]. Powder is 5–25 times the others, so it is the one that closes a fit.

A7. Pricing and lead time

From the pricing page [ref:sendcutsend-pricing]:

• Terms: no minimum quantity, volume discounts up to 80 %, and free US shipping from $39.
• What sets the price: size, material and thickness, quantity, cut complexity, the production method and post-processing. No formula is published.
• Quantity: the page's example prices fall 60–75 % per part from 1 to 100 parts [derived]. For example, 0.100 in 5052 cut, bent and tapped costs $37.49 for one and $11.06 each for 100. That drop is the visible set-up share.
• Lead time: standard orders ship in 2–4 business days. Bending adds 1–2 days and hardware 1–3.

A8. Where SendCutSend's own pages disagree

The hardware feed also has rows that contradict their own SKU. SP-440-1 is labelled #6-32, FH4-M3-12 is labelled M4 × 0.7, and SO4-440-12 is labelled #6-32. The catalogue page prints "A236" where PEM's A286 is meant. sheet-metal-joining.md lists them. Re-fetch the feeds before relying on any number here.

Part B. Generic sheet-metal rules (the #118 sweep)

The rest of this page is the generic sweep drafted in #119, read on 2026-10-31. Its rules are in process-profiles.yaml without a shop: field.

B1. Minimum inside bend radius

Agree. R ≥ T is the common generic default, and everyone says the material and the tooling decide.

flowchart TD
    Q["which inside radius?"] --> G["first: the<br/>caller's R"]
    Q --> S["then the shop's<br/>row: its<br/>tooling's R"]
    Q --> N["else the largest<br/>generic minimum<br/>(T: 5052, steel)"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class Q near
    class G,S here
    class N out

The order cadagent.manufacturing.sheet_metal.bend reads a radius in: a given one, then the shop's row (read by default; shop=False skips it), then the strictest generic rule. A shop's K and shortest flange hold only for its own radius, so a different R drops to the generic rules for those too.

Disagree.

• SendCutSend's own tooling bends at R < T for half its 5052 SKUs, four of five brass and copper SKUs and seven of nine mild-steel SKUs, and bends thin stainless at R > T [derived]. A blanket "R ≥ T" check would fail parts a shop makes every day, and pass a thin stainless part drawn at R = T that the shop will make at 2.7T.
• R ≥ T is not safe for hard aluminium. Davis gives 4–8T for 6061 and 2024, and SendCutSend bends none of its twelve 6061 or three 7075 SKUs, only 5052 [derived from which SKUs have bending_specs].
• Benson's rule over-predicts SendCutSend's listed radii [derived]: 16 % of its 0.472 in die is 0.076 in, against 0.045–0.063 in listed for mild steel on that die; 20–22 % of its 0.984 in die is 0.197–0.216 in, against 0.130–0.187 in for stainless. Benson says himself the rule is only a first estimate.

B2. K-factor and bend allowance

• Definition. K = t/T, with t the distance from the inside face to the neutral sheet. Bend allowance BA = π(R + K·T)·A/180 with A in degrees (SOLIDWORKS help [ref:solidworks-k-factor]). Hubs gives the same ratio [ref:hubs-sheet-metal].
• Ranges.
  • Hubs: 0.3–0.5, "average 0.4468". The page writes the K-factor in mm, but it is a ratio with no unit. Its chart gives K by forming method (air bending, bottoming, coining), material hardness and R/T band, from 0.33 to 0.50. It suggests 3–5 test bends for a real value [ref:hubs-sheet-metal].
  • OSH Cut: about 0.4 for soft metals such as aluminium, about 0.45 for steel and stainless [ref:oshcut-material-304].
  • SendCutSend: per SKU, 0.32–0.48 for metals and 0.33 for polycarbonate [ref:sendcutsend-specs].
• Defaults in CAD.
  • Onshape: 0.45, and 0.5 for rolled walls [ref:onshape-sheet-metal].
  • build123d BuildSheet: 0.5 [ref:build123d-sheet-metal].
  • FreeCAD SheetMetal: the README's example is 0.38 (ANSI). Its example table (R/T 1 → 0.38, 3 → 0.43, above → 0.5) is the same as the Hubs row for air bending a medium material such as steel [ref:freecad-sheetmetal].
• Two conventions. FreeCAD SheetMetal accepts an "ANSI" or a "DIN" K-factor, and its unfolder halves a DIN value to get the ANSI one [ref:freecad-sheetmetal]. DIN 6935 is the German standard for cold bending flat steel [ref:din-6935]; its content was not read here. A K-factor without its convention can be off by a factor of two.

Disagree. Hubs and OSH Cut put K higher for harder metals. SendCutSend's measured values run the other way: 0.37–0.48 for 5052 against 0.34–0.38 for 304 [derived]. Use the shop's measured K, or a test bend, over any rule.

flowchart TD
    I["R, K, T and<br/>the angle A"] --> BA["BA = π(R + K·T)·A/180"]
    BA --> BD["BD = 2·OSSB − BA"]
    BD --> F["flat length =<br/>outside flanges − BD"]
    F --> C["the cut part"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class I near
    class BA,BD,F here
    class C out

From the bend's R, K and T to the flat pattern the laser cuts (SOLIDWORKS help [ref:solidworks-k-factor]; SendCutSend's calculator [ref:sendcutsend-bending-calculator]). OSSB is the outside setback, (R + T)·tan(A/2). A wrong K moves every flat length by the change in BA.

Why it matters [derived]: a 90° bend in 1.5 mm sheet at R = 1.5 mm has BA = 3.13 mm at K = 0.33 and 3.53 mm at K = 0.5. That is 0.4 mm per bend, more than the cut tolerance, from the default alone.

B3. Minimum flange length

SendCutSend's minimum flange after the bend is 0.286 in on 0.030–0.040 in stock (7–9.5T), about 3.8T on 0.125 in 5052, and 1.37–1.38 in on 0.250 in stock (5.5T) [derived]. It follows the die width (about 0.6–0.9 × the die), not the thickness alone [derived].

Disagree. 4T is too short for thin sheet on SendCutSend's tooling, by up to 2.4×. Century Tywood's 3.5 × (T + R) comes closer for thin stock (0.26 in against 0.286 in for 0.030 in mild steel) but is longer than SendCutSend needs at 0.250 in [derived]. No generic rule fits every shop; the die decides.

Diagram drawn by tools/process_profile_figures.py from the sheet-metal compare rules in process-profiles.yaml, with SendCutSend's tooling table in orange. A test redraws it and fails if the committed chart differs.

B4. Holes: size, edge distance, spacing, distance to a bend

Minimum diameter.

• ≥ T: Protolabs; Hubs (smaller is possible but not advised); Greg Paulsen (more for high-strength material); Century Tywood.
• Gerald Davis explains why: a punch thinner than about T bends or breaks, so smaller holes are laser-cut.
• Fibre laser shops go smaller. OSH Cut gives a minimum of 0.25T and a recommended 0.5T for 304 [ref:oshcut-material-304]. SendCutSend's fibre-laser minimum holes are 0.22–0.6T, median 0.37T [derived from min_hole_size over 92 SKUs].

Hole to edge.

• Protolabs: 0.062 in for stock up to 0.036 in, else 0.125 in [ref:protolabs-sheet-metal].
• Century Tywood: 1/16 in for stock up to 1/32 in; thicker stock 2T but not under 1/8 in [ref:century-tywood-sheet-metal-reference].
• Boothroyd Dewhurst and Greg Paulsen: ≥ 2T.
• Hubs: ≥ T.
• SendCutSend (fibre laser): 0.22–0.67T per SKU, median 0.3T [derived].
• Protolabs and Century Tywood give nearly the same rule. The others differ by a factor of two.

Hole to hole. ≥ 2T (Hubs; Century Tywood for punched holes) against ≥ 6t (Greg Paulsen).

Diagram drawn by tools/process_profile_figures.py from the sheet-metal hole, edge, bend and hem compare rules in process-profiles.yaml, at a sheet thickness of 1.5 mm. A test redraws it and fails if the committed chart differs.

Hole to bend. Every source uses a different datum:

The vendored text-to-cad DFM checklist already says to state the convention (tangent, centreline or edge) before comparing [ref:text-to-cad]. These sources show why.

B5. Bend relief and corner relief

• SendCutSend's specs file agrees with its own article: its bend_relief_depth equals T + R + 0.020 in for the SKUs checked (0.030 in mild steel, 0.063 in 5052, 0.100 in 304) [derived].
• Davis adds two reasons for reliefs: a relief keeps the part from slipping in the tooling, and a relief cut with curves, not sharp inside corners, stops cracks from growing under vibration.

Disagree on width: T/2 (SendCutSend) against T (Hubs, Paulsen).

flowchart TD
    B["a bend that stops<br/>short of the edge"] --> D["relief depth:<br/>T + R past the<br/>bend line"]
    B --> W["relief width:<br/>T/2 or T"]
    B --> C["corner gap:<br/>0.015 in"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class B near
    class D,C here
    class W out

The three relief sizes of §B5 for one bend. SendCutSend adds 0.020 in to the depth and gives the corner gap. The width is the one the sources disagree on (T/2 SendCutSend, T Hubs and Paulsen), so it is drawn in orange.

B6. Hems, curls, jogs and countersinks

Hems.

• Protolabs: inside diameter ≥ T, return 6T [ref:protolabs-sheet-metal].
• Greg Paulsen: open hems, inside diameter ≥ t and return ≥ 4t; tear-drop hems, inside diameter ≥ t, opening ≥ ¼t and return ≥ 4t [ref:paulsen-2017-machine-design-sheet-metal].
• Steve Benson: a closed hem has zero radius, and its bend deduction is about 43 % of T "under perfect conditions", depending on the operator [ref:benson-2015-fabricator-inside-radius].
• Not every shop hems. SendCutSend bends to 120–130° at most [ref:sendcutsend-ordering-guide]. Xometry quotes hems and curls by hand [ref:xometry-manufacturing-standards].
• Disagree on the return: 6T (Protolabs) against 4t (Paulsen).

flowchart TD
    H["a hem: does the<br/>shop form one?"] --> O["open hem:<br/>inside dia ≥ T<br/>return 4–6T"]
    H --> T["tear-drop:<br/>opening ≥ T/4<br/>return ≥ 4T"]
    H --> N["SendCutSend:<br/>none, 130°<br/>at most"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class H near
    class O,T here
    class N out

The hem rules above, by kind (Protolabs [ref:protolabs-sheet-metal], Paulsen [ref:paulsen-2017-machine-design-sheet-metal]). A shop that only air-bends cannot form one, so check the shop first.

Curls (Greg Paulsen): outside radius ≥ 2t; holes at least the curl radius plus t away. Under the same heading the article gives bends at least 6t plus the curl radius away.

Jogs and offsets.

• Protolabs: recommended offset 0.030 in, standard heights 0.060–0.312 in, height tolerance ±0.012 in.
• SendCutSend gives a minimum and maximum joggle per SKU, from 0.288 in on 0.030 in mild steel to 1.425 in on 0.250 in stock [ref:sendcutsend-specs].

Countersinks.

• Protolabs: major diameter 0.090–0.500 in; 82, 90, 100 or 120°; formed countersinks +0.020/−0.010 in.
• Greg Paulsen: at least 8t apart, 4t from an edge, 3t from a bend. He also gives "maximum depth 3.5t", which cannot be right as printed, since a countersink cannot be deeper than the sheet. Do not use that number.
• SendCutSend: edge distance per SKU (0.781 in in its 0.25 in 5052 example), and half the die width from the bend centre [ref:sendcutsend-countersink]. It also forms dimples, with a size table per SKU [ref:sendcutsend-specs].

B7. PEM and other self-clinching hardware

This section moved to sheet-metal-joining.md, which reads PEM's handbook and bulletins in full.

One finding from the first sweep stays here: "a stainless nut is fine in stainless sheet" is the wrong rule.

• PEM: only its SP and SMPP nuts go into stainless sheet, and 300-series stainless fasteners such as CLS never do, because they are not hard enough [ref:pem-bulletin-cl]. Its handbook gives the reason: the fastener must be about 20 HRB harder than the sheet [ref:pem-handbook].
• SendCutSend lists 304 at HB 123 and 316 at HB 149 [ref:sendcutsend-specs]. Against CLS's rating of HRB 70 (HB 125) that leaves no margin in 304, and 316 is harder than CLS's limit.
• An SP nut, rated to HRB 90, covers both [ref:pem-catalog-self-clinching-nuts].

Drawn by tools/process_profile_figures.py from the compare: sm-clinch-sheet-hardness rules in process-profiles.yaml.

B8. Tabs, slots and notches

The word "tab" means two features here. Protolabs and Paulsen mean a finger in the cut outline. SendCutSend and Fictiv mean a tab that locates in a slot in another part.

• Outline tabs and notches.
  • Protolabs: tab at least the larger of 2T and 0.126 in; notch at least the larger of T and 0.04 in; either no longer than 5× its width [ref:protolabs-sheet-metal].
  • Greg Paulsen gives the same numbers, plus notch spacing ≥ 1/8 in and a notch ≥ 3t + r from a bend.
• Tab-and-slot joints.
  • SendCutSend: slot width = tab width + 2 × the tab material's cut tolerance. Slot height = tab thickness + its plus thickness tolerance + 2 × the slot material's cut tolerance. Allow 0.002–0.005 in per powder-coated surface. Make tabs 0.5–0.75 of the mating part's depth [ref:sendcutsend-tab-slot]. The specs file gives a tab_slot_tolerance per SKU; most are 0.010–0.035 in [ref:sendcutsend-specs].
  • David Willson (Fictiv): tab width 1.0–1.5T (1.2–1.8T for a press fit) and length 1.0–1.2T. Clearance 0.05–0.1 mm per side for laser, 0.15–0.2 mm for waterjet. Slot corner radius 0.1–0.2 mm (laser) or 0.2–0.3 mm (waterjet). At least 2T from the sheet edge and 1.5T from the bend tangent; tabs 3–6T apart; 2–4 tabs per joint [ref:fictiv-tabs-slots].

flowchart TD
    T["tab: width w,<br/>thickness t"] --> SW["slot width =<br/>w + 2 × cut tolerance"]
    T --> SH["slot height = t + its<br/>plus tolerance +<br/>2 × cut tolerance"]
    SW --> P["+ 0.002–0.005 in per<br/>powder-coated face"]
    SH --> P
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class T near
    class SW,SH here
    class P out

SendCutSend's tab-and-slot sizing [ref:sendcutsend-tab-slot]: the slot grows by each part's cut tolerance, and by the powder on every coated face.

• Disagree on tab length: 1.0–1.2T (Willson) against 0.5–0.75 of the mating depth (SendCutSend). They may measure different things; neither defines the datum.

B9. Laser and waterjet: kerf and the smallest features

Kerf.

• Hubs: about 0.3 mm on average for laser, 0.5–1 mm for waterjet [ref:hubs-sheet-metal].
• OSH Cut: 0.006 in (0.15 mm) on 304 at every thickness [ref:oshcut-material-304].
• David Willson (Fictiv): 0.1–0.3 mm [ref:fictiv-tabs-slots].
• Compensate for mating parts. Hubs adds half the kerf to the inner part and takes half off the outer. Willson enlarges slots by half the kerf per side.
• Disagree: 0.15 mm against 0.3 mm. Ask the shop, or measure.

Smallest features.

• Laser corners: fillet T/2 (Hubs).
• OSH Cut, 304: minimum channel about 0.2T and wall about 0.33T [derived from its table].
• SendCutSend: minimum bridge per SKU.
• Protolabs: smallest flat part 0.25 × 0.25 in; smallest formed part 0.5 × 0.5 in.

B10. Grain direction

• Gerald Davis: aluminium "tends to crack, particularly when bending parallel to the natural material grain"; the harder alloys need the larger radii in §B1 [ref:davis-2001-fabricator-design-tips].
• Century Tywood: bend across the grain, or at 45° to it, whenever the part allows [ref:century-tywood-sheet-metal-reference].
• No source read here gives a number for how much larger the radius must be when bending with the grain. The candidate tables (Machinery's Handbook, the Aluminum Association) were not opened (§B14).

B11. Tolerances by process

Agree. Protolabs, Xometry and SendCutSend all give bend angle ±1°. Tolerance grows with each bend between two features, so dimension critical features from one flat face.

flowchart TD
    F["two features on<br/>one flat face<br/>± 0.005 in"] --> A["across 1 bend<br/>± 0.010 in"]
    A --> B["across 2 bends<br/>± 0.015 in"]
    B --> C["across 3 or more<br/>± 0.030 in"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class F near
    class A here
    class B,C out

Protolabs' tolerance between two features in stock under 0.13 in, by the bends between them (its guidelines [ref:protolabs-sheet-metal]; the one-pager gives the 2- and 3-bend values [ref:protolabs-sheet-metal-tolerances]). In thicker stock the steps are ±0.015, ±0.020, ±0.025 and ±0.040 in. The band grows with every bend between them, so a hole pattern that must line up belongs on one face.

B12. What this changes in cad-agent

Since the #118 sweep, cadagent.manufacturing.sheet_metal computes bends from the profiles: a shop row first, then the generic K table and the strictest generic rule. With #283 the YAML adds four things:

• SendCutSend as a named shop profile, with its own limits as rules;
• its 304, 316 and G90 bend rows;
• its per-stock cutting and hardware limits;
• supplier-neutral rules for clinch hardware and blind rivets.

flowchart TD
    Y["process-profiles.yaml<br/>rules + shop_tables"] --> P["manufacturing.process<br/>rules_for · limits"]
    P --> B["sheet_metal.bend<br/>flat_length"]
    P --> D["the sheet DFM<br/>check"]
    P --> J["clinch and rivet<br/>checks (PR B)"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class Y near
    class P,B,D here
    class J out

Where the rules go. Each arrow means "read by". The DFM check takes only the generic rules, beside the shop's bend row for the stock (read by default; shop=False drops it). The orange box is the next step, not yet merged.

B13. Open-source tools for bent parts

The adopt, wrap, borrow or build decision for every tool, fastener libraries included, is in sheet-metal-guides.md §7. It was re-checked on 2026-11-01: build123d sheet metal is still unreleased, and FreeCAD SheetMetal is at 0.8.26. The table below gives the bent-part detail from gh api and the READMEs on 2026-10-31. build123d and FreeCAD SheetMetal were checked twice (registry entries); the other rows come from a separate survey on the same day and were not re-opened.

No open-source tool measures a sheet-metal part against DFM rules (hole to bend, flange length, radius against a shop table). That checker, on top of build123d's face map and the cited rules above, is the real gap. The small pieces cad-agent would add are a K table by R/T with an ANSI/DIN switch, and the shop-profile reader.

B14. Not verified, and next steps

Not opened, so not cited (see unverified in references.yaml):

• Boothroyd, Dewhurst and Knight, Product Design for Manufacture and Assembly (print).
• Machinery's Handbook bend-allowance and minimum-radius tables (print).
• Xometry's and Fictiv's sheet-metal design eBooks (behind forms). Xometry's rules here come from Greg Paulsen's article instead.
• Steve Benson's other columns: die width, minimum flange, 6061 (bot-blocked, no archive copy found).
• Standards content: DIN 6935, ISO 2768-1 (paid). The Aluminum Association bend-radius tables were not found.
• SendCutSend's material pages and its min/max charts: their numbers come from the feeds instead.

Next (#283):

• The clinch and rivet checks with golden vectors (PR B), and SHEET_MATERIALS entries for 304, 316 and G90 so sheet_metal.bend reads their rows.
• A parts catalogue and a skill for the bent plate + PEM + rivet pattern (PR C).
• Re-fetch the SendCutSend feeds before relying on any value here; they change.