# Flap discs

Coated-abrasive flap-disc construction, related abrasive forms, supplier shape and grade labels, mounting dimensions, and speed limits.

Reviewed: 2026-10-29

Tags: abrasives, grinding, finishing

## Identity and related abrasive forms

3M’s 769F sheet lists **Type 27 and Type 29** in 115, 125 and 180 mm and prohibits trimming the fiberglass plate. Klingspor’s **SMT 624 Supra** specifies a convex **12°** glass-fiber plate and zirconia alumina. [3M 769F sheet](https://multimedia.3m.com/mws/media/1886915O/3m-flap-disc-769f-technical-data-sheet-0220.pdf) [Klingspor SMT 624 sheet](https://www.klingspor.de/en/products/abrasive-mop-discs/smt-624-supra/pdf) The grain is on overlapping cloth flaps. The backing plate supports them. [Norton construction comparison](https://www.nortonabrasives.com/en-us/resources/expertise/right-angle-grinding-abrasives-know-your-options)

Klingspor also calls these tools abrasive mop discs. [Klingspor abrasive mop discs](https://www.klingspor.de/en/products/abrasive-mop-discs)

| Related form | Construction distinction |
| --- | --- |
| Radial flap wheel or mop wheel | Flaps extend around a central core and present a circumferential working surface. Klingspor describes radial flaps anchored in a resin core. This is a different geometry from overlapping flaps on a disc plate. [Klingspor mop-wheel description](https://www.klingspor.de/en/products/vorteile/schleifmop?set_language=en-de) |
| Resin-fiber disc | A coated-abrasive disc with a fiber backing. Norton's Red Heat example uses vulcanized fiber and resin-over-resin construction. It has no ring of overlapping cloth flaps. [Norton Red Heat](https://www.nortonabrasives.com/en-us/product/norton-red-heat-fiber-disc) |
| Bonded grinding wheel | Norton's resin-bonded right-angle example is a formed, cured mixture of grain, resin, and fillers rather than coated cloth flaps. Norton distinguishes its construction and use from flap and fiber discs. [Norton comparison](https://www.nortonabrasives.com/en-us/resources/expertise/right-angle-grinding-abrasives-know-your-options) |
| Nonwoven flap disc | The flaps themselves are nonwoven. Klingspor SMT 800 Special has aluminum-oxide abrasive, a glass-fiber plate, and a convex 12° design. [SMT 800 sheet](https://www.klingspor.de/en/products/non-woven-products/smt-800-special/pdf) |
| Mixed flap disc | Klingspor SMT 850 interleaves coated and nonwoven abrasives. Its US medium item lists aluminum oxide, resin bond, fiberglass backing, "80" grit, and "Medium" grade. Those are separate descriptions, not a conversion rule. [SMT 850 item 850080115022](https://www.klingspor.com/shop/312557-850080115022-162256) |
| Unitized disc | Compressed nonwoven abrasive web forms a consolidated body rather than discrete overlapping flaps. Klingspor lists unitized Type 27 angle-grinder discs separately. [Klingspor 2026 metalworking catalog, p. 44](https://www.klingspor.com/web/content/documents.downloads/45/documents_file) |

Norton's comparison describes fiber discs as requiring a separate backup pad, while flap discs have their supporting plate. An integral plate does not remove mounting-flange, guard, or machine requirements. [Norton comparison](https://www.nortonabrasives.com/en-us/resources/expertise/right-angle-grinding-abrasives-know-your-options), [Norton flap-disc safety](https://www.nortonabrasives.com/en-us/resources/expertise/dos-and-donts-flap-discs)

## Construction belongs to the exact product

| Documented example | Grain and flap construction | Plate or hub and form |
| --- | --- | --- |
| 3M 769F, European product data sheet, February 2020 | Aluminum oxide blended with proprietary ceramic Precision Shaped Grain. Resin bond, Y-weight polyester backing, and grades 40+, 60+, 80+, 120+. | Fiberglass hub construction, Type 27 and Type 29, nominal 115, 125, and 180 mm. The sheet prohibits trimming the fiberglass support plate. [769F European sheet, both pages](https://multimedia.3m.com/mws/media/1886915O/3m-flap-disc-769f-technical-data-sheet-0220.pdf) |
| Klingspor SMT 624 Supra, German-site downloadable sheet | Zirconia alumina. The sheet does not specify the cloth weight or bond chemistry. | Glass-fiber plate, convex 12°. The table identifies diameter, bore, grain number, maximum peripheral speed, and maximum RPM separately. [SMT 624 sheet, both pages](https://www.klingspor.de/en/products/abrasive-mop-discs/smt-624-supra/pdf) |

The Canadian 769F sheet, effective March 1, 2019, lists fiberglass and non-trimmable plastic hub constructions and inch dimensions. The European sheet lists fiberglass and metric dimensions. These are regional, dated descriptions, not proof that every 769F item has both hub materials. Neither sheet gives a numerical ceramic fraction within the mineral blend. [769F Canadian sheet](https://multimedia.3m.com/mws/media/1702856O/3m-flap-disc-769f-technical-data-sheet-english.pdf%26fn%3D769F%20Tech%20Data%20Sheet%20-%20English.pdf), [European sheet](https://multimedia.3m.com/mws/media/1886915O/3m-flap-disc-769f-technical-data-sheet-0220.pdf)

Trimmability is a construction-specific property. United Abrasives distinguishes a soft ABS backing intended for trimming from a hard plastic backing. "Plastic" alone therefore does not establish permission to trim. [United Abrasives backing descriptions](https://www.unitedabrasives.com/resources/product-safety-literature/product-technical-information/flap-discs/)

The 769F US SDS, document 38-9907-7, version 3.01, issued October 3, 2024, separately reports fiberglass plate, cloth, cured resin, and flap adhesive. Its mineral-blend interval is 5–25% by weight of the product, not the purity of the grain. The composition table includes fillers and fluorides and withholds exact composition as trade secret information. These intervals do not supply a complete recipe, surface grain coverage, or ceramic-to-conventional-alumina ratio. [769F SDS, sections 1–3](https://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSuUn_zu8l00xmYtGPxt1ov70k17zHvu9lxtD7SSSSSS--)

## Shape, density, and grade describe different properties

United Abrasives calls its flat flap discs Type 27 and its conical discs Type 29. These are flap-disc shape labels. They do not turn coated cloth into a bonded grinding-wheel construction. [United Abrasives shape descriptions](https://www.unitedabrasives.com/resources/product-safety-literature/product-technical-information/flap-discs/)

Source guidance is not identical. Norton's January 5, 2023 safety article gives Type 29 flap discs a 15°–25° working-angle range. Weiler's guide gives its conical flap-disc discussion 15°–35°. Both describe Type 27 at 0°–15°. These statements do not establish a universal operating angle. Norton also warns against flat or edge use. A flat product shape is not permission to place every part of the disc against the workpiece. [Norton safety article and illustration](https://www.nortonabrasives.com/en-us/resources/expertise/dos-and-donts-flap-discs), [Weiler shape guide](https://www.weilerabrasives.com/en/na-articles/flap-disc-guide)

Klingspor's SMT 624 sheet states convex 12°. Its 2026 US catalog labels the same family 12° convex but describes a "slight 6° convex form" in the body text. The conflict remains unresolved. A construction angle is also distinct from the angle between a running tool and the workpiece. [SMT 624 sheet](https://www.klingspor.de/en/products/abrasive-mop-discs/smt-624-supra/pdf), [US catalog, p. 5](https://www.klingspor.com/web/content/documents.downloads/45/documents_file)

United Abrasives describes high-density discs as containing approximately 40% more material than regular-density discs. It supplies no abrasive-mass measurement method on that page. Weiler describes extra abrasive cloth and allows high density in either Type 27 or Type 29. Neither description establishes a universal flap count, mass per area, or matched life ranking across suppliers. "Long life" is not a measured quantity without its comparison and test conditions. [United Abrasives density description](https://www.unitedabrasives.com/resources/product-safety-literature/product-technical-information/flap-discs/), [Weiler density discussion](https://www.weilerabrasives.com/en/na-articles/flap-disc-guide)

Grain grading describes a particle-size distribution, not one particle diameter or achieved surface roughness. FEPA P grades for coated abrasives and ANSI grading have different requirements. UAMA's overview includes historical standards and explains why matching numbers alone do not establish equivalence. The plus signs in 3M's 40+ and 120+ remain part of the supplier designations. The SMT 624 sheet's "Form" column contains a shape drawing, not a FEPA P prefix. [UAMA characterization and size standards](https://uama.org/abrasives-101/), [769F grades](https://multimedia.3m.com/mws/media/1702856O/3m-flap-disc-769f-technical-data-sheet-english.pdf%26fn%3D769F%20Tech%20Data%20Sheet%20-%20English.pdf), [SMT 624 illustrated table](https://www.klingspor.de/en/products/abrasive-mop-discs/smt-624-supra/pdf)

## Dimensions, attachment, and speed

Disc diameter and bore are lengths. A threaded attachment additionally has a thread designation. Klingspor's catalog distinguishes 7/8-inch bore SMT discs from an SMT 940 with a built-in 5/8-inch-11 thread. Its CMT system instead uses replaceable abrasive loadings and a dedicated mounting plate. These are different assemblies. [Klingspor catalog, pp. 4–6](https://www.klingspor.com/web/content/documents.downloads/45/documents_file)

**Synthesis.** Nominal diameter, a visible hub, and a bore or thread label do not establish complete machine fit. Flanges, engagement, clearance, guards, assembly ratings, and the exact instructions remain separate requirements. Changing units does not add a dimensional tolerance or establish interchangeability. [Norton mounting and speed guidance](https://www.nortonabrasives.com/en-us/resources/expertise/dos-and-donts-flap-discs)

Maximum RPM is a rotational-speed limit. Peripheral speed in m/s describes motion at the circumference. The SMT 624 sheet lists 80 m/s alongside size-specific maximum RPM. For example, its 125 × 22.23 mm, grain-60 item 322774 lists 12,250 RPM. The table does not identify that maximum as a suitable process setting. [SMT 624 size table](https://www.klingspor.de/en/products/abrasive-mop-discs/smt-624-supra/pdf)

The nonwoven SMT 800 sheet makes the distinction explicit. Its 115 × 22.23 mm rows list maximum 13,300 RPM and 80 m/s, alongside optimum 7,000 RPM and 42 m/s. The 2026 US catalog instead labels 7,000 as maximum RPM for its 4-1/2-inch SMT 800 listings. The documents differ in region and speed labeling. Exact-item correspondence is unestablished because the US table supplies no SMT 800 item identifiers. They do not justify substituting one rating for the other. [SMT 800 sheet](https://www.klingspor.de/en/products/non-woven-products/smt-800-special/pdf), [US catalog, p. 7](https://www.klingspor.com/web/content/documents.downloads/45/documents_file)

**Derived relationship.** For an outside diameter `D` in millimeters and rotational speed `n` in RPM, circumference gives `v = πDn / 60,000` in m/s. At 115 mm and 7,000 RPM, the result is about 42.1 m/s, consistent with the SMT 800 sheet's rounded optimum value. Arithmetic relates the units. It does not determine a safe assembly rating or optimum speed for another product.

## Applications and evidence limits

The European 769F sheet names carbon steel, stainless steel, and nickel-chromium alloys as its primary substrates and describes medium-to-high unit-pressure applications. It gives no universal force, feed rate, removal rate, or surface-roughness result. Its second-page notice makes suitability application-dependent. [769F European sheet](https://multimedia.3m.com/mws/media/1886915O/3m-flap-disc-769f-technical-data-sheet-0220.pdf)

**Synthesis.** A listed substrate does not establish residue-free processing, compatibility with later welding or coating, or equivalent results on every alloy. Grain, resin, adhesive, treatments, prior disc use, and the workpiece all matter to that assessment. Wet-use suitability, edge contact, pressure, speed, and finish remain exact-product and process questions, not properties established by the words "flap disc."

The 769F SDS addresses dust from both abrasive use and the material being abraded. It also identifies hazardous decomposition under extreme heat. Its hazard classification is not evidence of cutting performance, contamination limits, or compliance of a grinding operation. [769F SDS, sections 7–11 and 16](https://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSuUn_zu8l00xmYtGPxt1ov70k17zHvu9lxtD7SSSSSS--)

Coverage is limited to the dated 3M sheets and SDS, the undated Klingspor downloadable product sheets, catalog PUB260201MWG marked 2026, and the cited public manufacturer and UAMA pages accessed October 29, 2026 PDT. The catalog's shape and speed conflicts remain visible above. The sources do not establish a universal density test, current stock availability, complete formulations, customer capability, or an approved operating recipe.

Related references cover [coated-abrasive belts](coated-abrasive-belts.md) and [Finishing process references](../../processes/finishing/index.md).