Glossary

Compact explanations of manufacturing tolerancing and process related terms.

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σ, Standard Deviation
(Greek letter, small sigma) A statistical measure of the variation within a set of data values.
6σ, Six Sigma
A statistical technique for refining manufacturing processes to result in approximately 3.4 defective parts per million produced.
ABS, acrylonitrile butadiene styrene (thermoplastic)
A tough, impact resistant thermoplastic. A polymer comprised of three monomers. With a glass transition temperature ≈105°C ABS is useful for parts functioning in the range −20 to 80°C. ABS has good water, acid/alkali and oil resistance.
Lego bricks and the faceplates of wall sockets are familiar forms of ABS. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of ABS.
ANSI B4.1 Standard, also known as ASME B4.1 Standard
Preferred Limits and Fits for Cylindrical Parts
An American National Standards Institute / American Society of Mechanical Engineers standard relating to running, sliding and interference fits.
ANSI B4.2 Standard, also known as ASME B4.2 Standard
Preferred Metric Limits and Fits
A metric version of ANSI/ASME B4.1. Use Tolcap to help you achieve the standards recommended.
APQP, Advanced Product Quality Planning
A quality management system for developing new products. Widely used in the automotive industry but applicable to any industry. Similarities to DFSS.
ASME Y14.5-2009 Standard
Dimensioning and Tolerancing
An 2009 American Society of Mechanical Engineers standard that describes how to set out geometric tolerances and explains what they mean. In this interesting video GD&T experts from Tec-Ease talk about the changes introduced by the ASME Y14.5-2009 Standard.
BS 8888:2017 Standard
Technical product documentation and specification
This British Standard for engineering drawings and geometrical tolerancing describes how to set out geometric tolerances and explains what they mean. Tolcap's Richard Batchelor gives a brief overview of Tolerance related standards in his Sept 2017 Blog
BS 308 Standard
British Standard superseded by BS 8888.
BS EN 20286 Standard
ISO system of limits and fits. ISO system of limits and fits. Bases of tolerances, deviations and fits
The British Standard equivalent of ISO 286. Use Tolcap to help you achieve the standards recommended.
BS EN ISO 2692 Standard
Geometrical product specifications (GPS). Geometrical tolerancing. Maximum material requirement (MMR), least material requirement (LMR) and reciprocity requirement (RPR).
This British Standard for engineering drawings and geometrical tolerancing describes how to set out geometric tolerances and explains what they mean. It also treats Geometrical Product Specifications - how dimensions on drawings must show the proper relationships and alignment of features.
CA, cellulose acetate (thermoplastic)
An early synthetic thermoplastic fibre created from wood pulp or cotton. CA materials wick and drys quickly, they drape and dye well, but the fibres are weak and the materials produced are not very durable. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of CA.
Capable
See Process Capable
Capability
See Process Capability
Capability Analysis
A statistical determination of the ability of a manufacturing process to meet design specifications.
Control Charts, Shewhart Charts
A statistical manufacturing process control tool.
The graph to the right shows a control chart: A plot of how process data changed over time: Example of a Control Chart The central (green) line indicates the data average,
the top (red) line is the upper acceptable control limit
and the bottom (blue) line is the lower acceptable control limit.
Average, upper and lower limits are determined from previous data.
Cp
See Process Capability Indices (centred, typically unrealistic)
CP, cellulose propionate (thermoplastic)
Stronger, lighter and more flexible than cellulose acetate, CP is a particularly transparent and glossy thermoplastic. CP is familiar in cutlery and tool handles, spectacle frames and googles. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of CP.
Cpk
See Process Capability Indices (real world adjusted)
CSSBB, Certified Six Sigma Black Belt
See Six Sigma Black Belt
CTQ, Critical-To-Quality
Critical product characteristics whose specification must be met if customer requirements are to be supplied.
Design for Assembly, DFA
Designing products with ease of assembly in mind e.g. by reducing the number of components or creating easily gripped features.
Design for Manufacture,
Design for Manufacturing,
Design for Manufacturability, DFM
The practice of creating product designs that simplify manufacture.
Design for Manufacture and Assembly, DFMA
Combines DFA and DFM.
Design for Six Sigma, DFSS
Manufacturing design that employs Six Sigma techniques from the outset. This aims to produce at most 3.4 defective parts per million by early consideration of, and allowance for, potential manufacturing process and material issues.
DFMEA, Design FMEA, Design Failure Mode & Effect Analysis
FMEA aiming to uncover design failings effecting product safety, usability, and durability.
DIN 16742 Standard
Plastics Moulded Parts - Tolerances and Acceptance Conditions
Deutsches Institut für Normung e.V. standard related to tolerancing plastic components - particularly injection mouldings. Use Tolcap to help you achieve the standards recommended.
DIN 1688-4 Standard
Light metal alloy raw castings; pressure die castings; general tolerances, machining allowances
Deutsches Institut für Normung e.V. standard. Now WITHDRAWN.
DIN 16901 Standard
Deutsches Institut für Normung e.V. standard superseded by DIN 16742.
DMAIC, Define Measure Analyze Improve & Control
A data driven Six Sigma quality improving cycle:
  • Define process, limitations and expectations
  • Measure results
  • Analyse measured data collected against defined expectation
  • Improve inadequacies detected in analysis
  • Control the process via monitoring and documentation
DPMO, Defects Per Million Opportunities
DPMO= Total number of defects in the samplex1,000,000 Number of units in the samplexNumber of opportunities for a defect per unit
= DPOx1,000,000 DPMO = ( Total number of defects in the sample x 1,000,000 ) / ( Number of units in the sample x Number of opportunities for a defect per unit )
= DPO x 1,000,000

e.g. In a sample of 50 simple cylindrical metal units each unit is judged to have 5 opportunities for defect (length, width, straightness, smoothness, hardness).
10 units are found to have 1 defect, 6 have 2 defects, 1 has 3 defects:
(10+6x2+3)x1,000,000 50x5 = 25x1,000,000 50x5 =100,000 1,000,000( 10 + 6x2 + 3 ) / ( 50 x 5 )
= 25,000,000 / ( 50 x 5 ) = 100,000
defects per million opportunities

DPO is commonly used in Six Sigma Metrics.

See also PPM.
DPO, Defects Per Opportunity
DPO= Total number of defects in the sample Number of units in the samplexNumber of opportunities for a defect per unit
= DPU Number of opportunities for a defect per unit DPO = Total number of defects in the sample / ( Number of units in the sample x Number of opportunities for a defect per unit )
= DPU / Number of opportunities for a defect per unit

e.g. In a sample of 50 simple cylindrical metal units each unit is judged to have 5 opportunities for defect (length, width, straightness, smoothness, hardness).
10 units are found to have 1 defect, 6 have 2 defects, 1 has 3 defects:
10+6x2+3 50x5 = 25 50x5 =0.1 ( 10 + 6x2 + 3 ) / ( 50 x 5 ) = 25 / ( 50 x 5 ) = 0.1 defects per opportunity

DPO is commonly used in Six Sigma Metrics.
DPU, Defects Per Unit
DPU= Total number of defects in the sample Number of units in the sample DPU = Total number of defects in the sample / Number of units in the sample
e.g. In a sample of 50 units, 10 have 1 defect, 6 have 2 defects, 1 has 3 defects:
10+6x2+3 50 = 25 50 =0.5 ( 10 + 6x2 + 3 ) / 50 = 25 / 50 = 0.5 defects per unit

DPU is commonly used in Six Sigma Metrics.
EBM, Electron Beam Machining
A narrow beam of high energy electrons create areas of intense heat vaporizing the parts material. Typically used for accurate cutting or boring of metals. Tolcap can help you establish suitable process capable tolerances for Electron Beam Machined dimensions.
ECM, Electrochemical Machining
Typically used for extremely hard metals ECM is a method of removing metal by passing a high electric current between an electrode and the processed part through a conductive fluid.
ECM can create intricate shapes and fine mirror finishes. Tolcap can help you establish suitable process capable tolerances for Electrochemically Machined dimensions.
EDM, Electrical Discharge Machining, Spark Machining, Spark Eroding, Burning, Die-Sinking, Wire Burning, Wire Erosion
Pulsed electrical discharges between an electrode and the workpiece generate heat (and sparks) which precisely melt and remove material. Tolcap can help you establish suitable process capable tolerances for Electrical Discharge Machined parts.
Epoxy, polyepoxides (thermoset)
A range of strong thermoset polymers with high temperature tolerance and chemical resistance.
A good electrical insulator but with good heat conduction epoxy is used in circuit boards. With high strength and low weight epoxy is popular for marine and aerospace applications. Often reinforced with glass or carbon fibres. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving injection compression of Epoxy.
GD&T, GDT, Geometric Dimensioning and Tolerancing
Symbols used on mechanical engineering drawings to specify the acceptable range of dimensions. Tec-Ease website maintain a good list of GD&T symbols.
FMEA, Failure Mode & Effect Analysis
A highly structured technique for finding potential failures in a design, manufacturing process or product.
FMECA, Failure Mode Effect & Criticality Analysis
See FMEA
ISO 14405 Standard
Geometrical product specifications (GPS) - Dimensional tolerancing -
Part 1: Linear sizes
Part 2: Dimensions other than linear sizes

This standard from the International Organization for Standardization treats Geometrical Product Specifications. This is about how dimensions on drawings must show the proper relationships and alignment of features.
ISO 1829 Standard
Produced by the International Organization for Standardization now superseded by ISO 286.
ISO 2768 Standard
General tolerances —
Part 1: Tolerances for linear and angular dimensions without individual tolerance indications
Part 2: Geometrical tolerances for features without individual tolerance indications

This 2017 reviewed and confirmed standard from the International Organization for Standardization describes how to set out geometric tolerances and explains what they mean.
ISO 286 Standard
Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes -
Part 1: Basis of tolerances, deviations and fits
Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts

Fits and Limits standards from the International Organization for Standardization. Use Tolcap to help you achieve the standards recommended.
ISO 8026 Standard
Geometrical product specifications (GPS) — Dimensional and geometrical tolerances for moulded parts —
Part 3: General dimensional and geometrical tolerances and machining allowances for castings

Process specific standards for castings (sand through to investment castings) from the International Organization for Standardization. Use Tolcap to help you achieve the standards recommended.
JIT, Just-In-Time Manufacturing, Just-In-Time Production
A Japanese methodology. The aim is to reduce stocks of raw materials, works-in-process and finished products by having the required materials arrive where needed and be removed just-in-time.
Toyato Production System & Lean Production System.
Lean Production System
See JIT
Lean Manufacturing
A systematic manufacturing methodology that aims to reduce waste in design, manufacturing process, distribution and customer service.
Lean Six Sigma
A combination of Six Sigma and Lean Manufacturing methodologies.
Lower Specification Limit, LSL
See LTL.
Lower Tolerance Limit, LTL
e.g. A rods length specified as 10.0 ± 0.2cm has a LTL 9.8cm.
Pareto principle, or 80 / 20 rule
For a large sample, roughly 80% of the outcomes come from 20% of the causes.
PA, polyamide (thermoset)
Polyamides such as silk occur naturally. Man made polyamides, such as Nylon, are thermoset polymers. Typically durable and strong. Common uses include textiles, vehicle plastics, food packaging films and carpets. Use Tolcap to find process capable dimensional tolerances for injection compressed polyamide parts.
PBTP, PBT, polybutylene terephthalate (thermoplastic)
An electrically insulating, impact resisting polyester thermoplastic. PBT has electrical and electronic uses and can be spotted in households as showerheads and toothbrush fibres. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression (filled and unfilled) of PBT.
PC, polycarbonate (thermoplastic)
Easily moulded and thermoformed thermoplastic polycarbonates are typically tough, transparent and heat resistant. Familiar uses include spectacle lenses, DVDs and transparent lightweight greenhouse sheeting. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of polycarbonates.
PCTFE, PTFCE, polychlorotrifluoroethylene (thermoplastic)
Strong under tension, generally chemical resistant, water proof, transparent. A thermoplastic with a glass transition temperature ≈45°C and heat resistant to 175°C.
Used for transparent films, medical blister packs and water resisting covering for LCD panels. Use Tolcap to find process capable dimensional tolerances for parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PCTFE.
PE, polyethylene (thermoplastic)
A very commonly used plastic. Water resistant, largely gas impermeable, impact resisting, a good electrical insulator, it can be made almost transparent. A thermoplastic with melting point between 115–135°C.
Used for packaging, bottles, bags and films. Use Tolcap to work out process capable dimensional tolerances for parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of polyethylene.
PESU, polyethersulphone, polyethersulfone (thermoplastic)
A rigid thermoplastic, stable at high temperatures, flame retardant, transparent and chemical resistant. Useful for electrical and electronic parts, and cookware. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PESU.
PET, PETE, polyethylene terephthalate, PET-P (obsolete), PETP (obsolete) (thermoplastic)
A common, strong, impact resistant thermoplastic that has good fluid containing properties. Typically spun into clothing fibres or blow moulded into bottles. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PET.
PF, phenol formaldehyde resin, phenolic resin (thermoset & thermoplastic)
A class of plastics including the first commercial plastic Bakelite. PF's can be thermoset or thermoplastic depending on production treatments. Commonly used in adhesives for bonding plywood layers and in electrical components. Use Tolcap to find process capable dimensional tolerances for injection compressed PF parts.
PMMA, polymethylmethacrylate (thermoplastic)
A rigid, transparent, often low cost thermoplastic. Used in producing a shatterproof alternative to glass for skylights and aircraft glazing. Familiar under tradenames such as Perspex. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PMMA.
POM, polyoxymethylene (thermoplastic)
A strong, stiff, hard, opaque thermoplastic. Often injection moulded for high precision components such as small gears wheels, lock mechanisms and ball bearings. Use Tolcap to work out process capable dimensional tolerances on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of POM.
PP, polypropylene (thermoplastic)
A heat resistant, tough, translucent, chemical resistant thermoplastic. Used in a wide variety of applications: carpet fibres, crisp packages, kitchen appliances casings and plastic crates. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of polypropylene.
PPM, Parts Per Million (Defective)
PPM= Total number of defective units in the samplex1,000,000 Number of units in the sample PPM = ( Total number of defective units in the sample x 1,000,000 ) / Number of units in the sample
e.g. In a sample of 50 units 17 are found to be defective
(in PPM a failed unit is counted only once, even if it has multiple defects, unlike DPMO):
17x1,000,000 50 =340,000 1,000,000( 10 + 6 + 1 ) / 50
= 17,000,000 / 50 = 340,000
parts per million defective

PPM is commonly used in Six Sigma Metrics.

See also Cp to PPM relationship table.
PPS, polyphenylene sulfide, polyphenylene sulphide (thermoplastic)
A chemical resistant, high temperature tolerating, burn resisting, electrical insulating, opaque thermoplastic. PPS can be moulded, extruded or machined precisely and used in cooking appliances, motor housings and medical equipment. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PPS.
Process Capable
A manufacturing process is considered capable if almost all off the dimensions of the parts it produces fall within the ranges specified on the design.
Process Capable Tolerancing, PCT
A methodology for designing successful components by allocating acceptable dimensional variation taking into account manufacturing process and materials variations.
Process Capability, Cp, Cpk
A number reflecting the success of a manufacturing process.
High process capability indicates low numbers of rejected parts.
 
Process Capability, Cp Rejected parts per million (ppm)
0.33 317,300
0.67 45,500
1 2,700
1.33 63
1.67 0.6
2 0.002
There are a number of Process Capability Indices with varying reflections of real manufacturing process.
For a comprehensive mathematical consideration of process capability watch our Tolerance Stacks video.
Process Capability Index / Indices, Cp, Cpk
Process capability can be specified by a number of different indices (they assume normal distribution of a reasonably large sample):
Cp: A comparison of specified output to actual manufactured output.
Cpk: As Cp but takes into account real world variations in manufacturing process, materials and human limitation which result in a non-centred distribution.
Process Control
Monitoring and consequent improvement of a manufacturing process.
PFMEA, Process FMEA, Process Failure Mode & Effect Analysis
FMEA aiming to uncover process failings effecting product quality, production reliability, worker or environmental safety.
PS, polystyrene (thermoplastic)
Inexpensive, transparent, hard, widely used thermoplastic. Typically extruded, moulded and vacuum formed. Familiar as a foamed plastic for packaging and in a solid form as disposable cutlery, yoghurt tubs, lids etc. Use Tolcap to work out process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of polystrene.
PSU, polysulphone, polysulfone (thermoplastic)
A tough, rigid, opaque, chemical resistant, thermally stable, but relatively expensive thermoplastic. Typically extruded, injection or blow moulded. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PSU.
PVC-U, unplasticised polyvinylchloride (thermoplastic)
Odourless and tasteless PVC-U is ideal for containing food and drinks. A good electrically and thermally insulating, very chemically resistant thermoplastic. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of PVC-U.
Right First Time Manufacturing, RFT
A quality management premise suggesting that maximum effort should be expended on design and process before the first manufacturing run. RFT aims to avoid financial and reputational cost from rework and product failure.
SAN, styrene-acrylonitrile resin, styrene acrylonitrile plastic (thermoplastic)
Good durability, heat and chemical resistant properties. A thermoplastic commonly used for food containers, water bottles, hygiene and sanitary products. Use Tolcap to work out suitable process capable tolerances for dimensions on parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of SAN.
Sigma Quality Level
Manufacturing processes functioning at sigma quality level produce at most 3.4 defective parts per million produced.
Six Sigma
See .
Six Sigma Green Belt
Non specialist employees responsible for implementing Six Sigma within manufacturing projects under the guidance of Six Sigma Black Belt overseers.
Six Sigma Black Belt
Six Sigma specialists responsible for overseeing the application of Six Sigma methodology to specific manufacturing projects.
SPC, Statistical Process Control
A method of statistically monitoring and controlling manufacturing processes to maintain quality. Tolcap's predicted values of Cpk have proved to have very close correlation (98%) with SPC results.
Stack
The interacting components of a manufactured part.
Stack Tolerance
See Tolerance Stack
Standard Deviation
See σ.
Thermoplastic
A plastic material (polymer) that at specific temperatures becomes pliable and can be reshaped. On cooling the thermoplastic retains its new shape. If reheated thermoplastics will tend to regain their original shape.
Thermoplastics include ABS, CA, CP, PBT, PC, PCTFE, PE, PESU, PET, PF, PMMA, POM, PP, PPS, PS, PSU. Tolcap can be used to work out process capable dimensional tolerances for parts involving extrusion, blow moulding, rotational moulding, precision injection moulding or injection compression of thermoplastics.
Thermoset
Plastics that can melted, or heated until pliable, and reshaped. On cooling the thermoset plastic retains its new shape. Due to new cross-links that form during curing thermoset plastics do not revert to their original shape on reheating (unlike thermoplastics).
Thermoset plastics include Epoxy, PA, PF, UP. Use Tolcap to work out process capable tolerances for dimensions on parts involving injection compression of thermosets.
Tolerance Capability Expert, TCE
Tolerance Capability Expert (TCE) was the earliest, spreadsheet based, form of the software now known as Tolcap.
Tolerance
An expression of the acceptable variation in a manufactured parts dimension
e.g. a rods length might be specified as 10.0 ± 0.2cm meaning:
minimum acceptable rod length 9.8cm,
maximum acceptable rod length 10.2cm.
Tolerance Stack,
Tolerance Stackup,
Tolerance Stack-up
The accumulated tolerances in a manufactured part that comprises a number of individually toleranced interacting components.
TPS, Toyota Production System
See JIT
UP, unsaturated polyester (thermoset)
Hard durable thermoset plastics. Typically used in larger engineered items such as boat hulls, vehicle parts, shower trays and appliances. Use Tolcap to work out process capable tolerances for dimensions on parts involving injection compression of unsaturated polyester.
Upper Specification Limit, USL
See UTL.
Upper Tolerance Limit, UTL
e.g. A rods length specified as 10.0 ± 0.2cm has a UTL 10.2cm.
Worst Case Tolerance Stack
The tolerances of stacked manufactured components accumulated in such a way to present the worst possible mechanical outcomes
e.g. A rod needs to fit in a hole:
 
Component Diameter
(cm)
Worst
Cases (cm)
 
Rod 1.0±0.1 0.9
(LTL)
1.1
(UTL)
Hole 1.2±0.1 1.3
(UTL)
1.1
(LTL)
Note   Too
loose?
Uncomfortably
tight?

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