Answered on 4 May, 2014 06:50 PM. M8 means metric thread 8mm of ISO standard and 6H refers to tolerance of threaded hole (know more about it in ISO 2768) Furthermore, what is G and H in thread? 'G' designates Ground Thread. 'H' designates the pitch diameter is on high side of basic. These two letters (GH) are followed by a numeral indicating. Tolerance class. Name for a combination of a fundamental deviation and a tolerance grade, e.g. Fundamental tolerance. A tolerance assigned to a fundamental tolerance grade, e.g. IT7 and a nominal dimension range, e.g. Planned joining condition between hole and shaft.
There is variability in any manufacturing process and, as such, tolerances are used to set permissible limits on this variability. Machining is often selected when tolerances are close, as it is more accurate than most other shape-making processes.
Tighter tolerances usually mean higher costs. For example, if the product designer specifies a tolerance of ±0.10 mm on a hole diameter of 6.0 mm, this tolerance could be achieved by a drilling operation. However, if the designer specifies a tolerance of ±0.025 mm, then an additional reaming operation is needed to satisfy this tighter requirement.
This is not to suggest that looser tolerances are always good. It often happens that closer tolerances and lower variability in the machining of the individual components will lead to fewer problems in assembly, final product testing, field service, and customer acceptance. Although these costs are not always as easy to quantify as direct manufacturing costs, they can nevertheless be significant. Tighter tolerances that push a factory to achieve better control over its manufacturing processes may lead to lower total operating costs for the company over the long run.
ISO 2768 is a standard for general tolerances, specifically covering linear dimensions, angular dimensions and geometrical tolerances. It is issued by the International Organisation for standardisation – ISO in association with Deutsches Institut für Normung – DIN.
The ISO 2768 standard comes in two parts, namely ISO 2768-1 and ISO 2768-2. The first part covers linear and angular dimensions, as well as radius of curvature and chamfer height according to four classes of levels: f (fine), m (medium), c (coarse), v (very coarse). The second parts covers geometrical tolerances in regard of form and position according to three classes or levels: H (fine), K (medium), L (coarse).
At CNC Proto we specialise in the machining of non-ferrous metals, mostly 6000 and 7000 series aluminium alloys, and engineering plastics. On our Quote Request form, we offer by default 3 different choices to our customers for linear and radii dimensions general tolerances (+/- 0.10mm; +/- 0.07mm; +/- 0.05mm). Of course, if you prefer to specify your general part tolerances according to ISO-2768, feel free to let us know in the additional comments field of the form!
For more articles on CNC machining, materials and design of machined parts, be sure to check our Resource Library.
With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing and building the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost. Learn more About Us.
Iso 2768 Tolerances
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Relevant Standards
BS EN 20286-1:1993(ISO 286-1:1988 )...... ISO system of limits and fits. Bases of tolerances, deviations and fitsBS EN 20286-2 : 1993(ISO 286-2:1988 ).... ISO system of limits and fits. Tables of standard tolerance grades and limit deviations for holes and shafts
Notes
The tolerance of size is normally defined as the difference between the upper and lower dimensions.
The need for tolerances to be identified on drawings is vital to allow assembly of parts in the desiredway and interchangeability of parts as required in modern manufacturing methods.
ISO 286 implements 20 grades of accuracy to satisfy the requirements of different industries.
- IT01, IT0, IT1, IT2, IT3, IT4, IT5, IT6.. Production of gauges and instruments.
- IT 5, IT6, IT7, IT8, IT9, IT10, IT11, IT12...Precision and general Industry.
- IT11, IT14, IT15, IT16..Semi finished products
- IT16, IT17, IT18 ..Structural Engineering
Hole and Shaft tolerance tables- Using Fundamental Deviations and Tolerance Band T
| ISO Tolerance Band T (3-315mm size) | ISO Shaft limit nearest Zero (3-500mm shaft size) | ISO Hole Nearest limit nearest Zero (0-500mm hole size) |
| ISO Tolerance Band T (315-3150mm size) | ISO Shaft limit nearest Zero (500-3150mm shaft size) | ISO Hole Nearest limit nearest Zero (500-3150mm hole size) |
Examples in using above tables..
Note: The tolerance band T is an absolute length value it does not have a negative value.
Hole Dia 110mm H11,...
Nearest zero ( El )= 110mm + 0mm = 110,000mm ...
Furthest from zero ( ES )= 110mm + (0+ T=0,220)=110,220mm
Resulting limits 110,000/110,220
Shaft 110mm e9...
Nearest zero ( es ) = 110mm - 0,072=109,928mm...
Furthest from zero ( ei ) = 110mm - (0,072 + T=0,087) = 109,841mm
Resulting limits 109,928/109,841
Note for the tolerance bands which can be either side of zero, (J,K,N,j )
The diagramsprovided with the table clearly show calculation needed.e.g. consider Hole 300K7.
T= 52 micrometres (0,052mm).Limit nearest zero ( K ) ES = 16 micrometres = (0,016mm)
Limit furthest from zero ( K ) El = (0,016- 0,052)= -0,036
Limits therefore (300 -0,036) = 299,964..and ...300+0,016= 300,016
limits 299,964/300,016
| ISO Hole Limits (3-400mm Hole Size) | ISO Shaft Tolerance(3-400mm Hole Size) |
| ISO Hole Limits (400-3150mm Hole Size) | ISO Shaft Tolerance(400-3150 mm Hole Size) |
Example limits and fits using hole basis
| Description | Hole | Shaft |
| Loose Running | H11 | c11 |
| Free Running | H9 | d9 |
| Loose Running | H11 | c11 |
| Easy Running - Good quality easy to do- | H8 | f8 |
| Sliding | H7 | g6 |
| Close Clearance - Spigots and locations | H8 | f7 |
| Location/Clearance | H7 | h6 |
| Location- slight interference | H7 | k6 |
| Location/Transition | H7 | n6 |
| Location/Interference- Press fit which can be separated | H7 | p6 |
| Medium Drive | H7 | s6 |
| Force | H7 | u6 |
Machining Process associated with ISO IT Tolerance Grade..
Din Iso 2768 Hole Tolerance
| IT Grade | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
| Lapping | |||||||||||||||
| Honing | |||||||||||||||
| Superfinishing | |||||||||||||||
| Cylinderical grinding | |||||||||||||||
| Diamond turning | |||||||||||||||
| Plan grinding | |||||||||||||||
| Broaching | |||||||||||||||
| Reaming | |||||||||||||||
| Boring, Turning | |||||||||||||||
| Sawing | |||||||||||||||
| Milling | |||||||||||||||
| Planing, Shaping | |||||||||||||||
| Extruding | |||||||||||||||
| Cold Rolling, Drawing | |||||||||||||||
| Drilling | |||||||||||||||
| Die Casting | |||||||||||||||
| Forging | |||||||||||||||
| Sand Casting | |||||||||||||||
| Hot rolling, Flame cutting |