How to Design Parts for Tolerances: Press Fits, Slip Fits, and Assemblies

Tolerances are the specified allowable variation in a machined dimension that still allows the final part to function correctly and assemble properly with other components. Every machined dimension on a part has some measurable variation from the nominal design value no matter how carefully it is cut. The specified tolerance value defines how much of this natural variation is acceptable for the part to function correctly. Understanding tolerances is essential for designing parts that fit together in assemblies.

When I designed my very first multi-part assembly as a new machinist, I specified the same ±0.005 inch tolerance on absolutely every dimension. The individual parts were all within tolerance but the assembly would not fit together correctly no matter how hard I tried to assemble the parts. The tolerances stacked up and the cumulative error made the parts too tight. I had to go back to the drawing board and redesign the entire assembly with proper tolerance analysis applied to the critical mating dimensions.

This detailed guide covers the types of fits used in assemblies, tolerance standards, tolerance stacking analysis methods, and practical guidelines for correctly specifying tolerances on engineering drawings for machined parts.

Press Fits, Slip Fits, and Clearance

Category	Example	Purpose
Type A	Example 1	Purpose 1
Type B	Example 2	Purpose 2
Type C	Example 3	Purpose 3

Fits

Every mechanical assembly fit in machining falls into one of three basic categories based on the relationship between the shaft and hole dimensions. The category determines how the two parts engage with each other and how much force is required to assemble them.

Press fits require diameter tolerances of ±0.0005 inches or better on both parts. The shaft is machined 0.0005 to 0.001 inches larger than the hole. The parts are assembled with a press or by heating the outer part and cooling the inner part. The interference creates a permanent joint that does not require additional fasteners. Press fits are commonly used for installing bearings in housings, locating pins in linkage assemblies, and creating permanent structural joints that do not require welding or fasteners. The interference fit creates a joint that is stronger than most threaded fasteners for the same size.

Slip fits require diameter tolerances of ±0.001 to ±0.002 inches. The shaft is machined 0.0005 to 0.001 inches smaller than the hole. The parts slide together with light hand pressure or light tapping. Slip fits are used for parts that must be assembled and disassembled regularly such as alignment pins, locating dowels, fixture components, and removable covers. The parts fit together securely when assembled but can be separated without damaging either component.

Clearance fits require larger gaps of 0.002 to 0.010 inches depending on the part size and the specific application requirements. The shaft is machined 0.002 inches or more smaller than the hole. The parts assemble easily with no binding or force required. Clearance fits are used for moving parts like rotating shafts in bearings, sliding components, and fasteners.

Tolerance Symbols and Standards

The ISO 2768 standard defines general tolerances for machined parts and is the most widely used tolerance standard in CNC machining. The standard has three precision classes: fine, medium, and coarse. The fine class provides the tightest tolerances and requires more precise machining and careful setup.

ISO 2768 fine class specifies linear tolerances of ±0.05mm for dimensions up to 120mm and ±0.15mm for dimensions up to 400mm. Angular tolerances are ±0.3 degrees. These tolerances are readily achievable on most well-maintained CNC machines without special effort or additional operations.

ISO 2768 medium class specifies linear tolerances of ±0.1mm for dimensions up to 120mm and ±0.3mm for dimensions up to 400mm. This is the default tolerance class for most general machining work and is achievable with standard machining practices.

The ISO system also defines tolerance grades from IT01 which is the tightest to IT18 which is the loosest. Each grade represents a standard tolerance value for a given nominal dimension range. Common machining grades are IT7 for precision work, IT8 for general machining, and IT11 for rough work where tolerances are not critical.

The standard fits system uses letter and number designations. A capital letter indicates the hole tolerance and a lowercase letter indicates the shaft tolerance. An H7 fit means the hole is machined to IT7 grade with a basic tolerance zone. A g6 shaft fits into an H7 hole for a close sliding fit. The H7/g6 combination is the most common precision fit for machined assemblies.

Tolerance Stacking in Assemblies

Tolerance stacking occurs when multiple parts in an assembly each contribute their individual tolerances to a cumulative total. The tolerances add up linearly which can cause the assembly to not fit correctly even though every individual part is within its specified tolerance range.

The worst-case tolerance stack is the simple arithmetic sum of all the individual tolerances in the stack path. If three parts in a stack each have ±0.005 inch tolerances on the critical dimension, the worst-case stack variation is ±0.015 inches. The assembly may not fit if the total stack variation exceeds the available clearance at the assembly level.

Statistical tolerance analysis uses the root-sum-square or RSS method for a more realistic estimate of the expected stack variation. The statistical stack is the square root of the sum of the squares of all the individual tolerances. For the same three parts with ±0.005 inches each, the statistical stack is ±0.0087 inches which is significantly less than the worst-case value.

Use statistical tolerance analysis for assemblies with four or more parts where the worst-case stack analysis shows the assembly will not fit. The statistical approach allows the use of looser individual tolerances which are easier and cheaper to machine while still maintaining the required assembly fit. The risk is that a small percentage of assemblies will not fit due to the statistical distribution.

Practical Tolerance Guidelines

The following guidelines apply to common machining situations and provide reliable fits for most applications.

Holes for precision locating dowel pins should use a reamed hole with a tolerance of +0.0002 to +0.0005 inches above the nominal pin diameter. The dowel pin itself should be manufactured within ±0.0001 inches of the nominal diameter. The resulting fit is a light press fit that centers the two parts accurately for alignment.

Holes for standard cap screws and bolts should use a clearance hole that is 0.010 to 0.020 inches larger than the nominal screw diameter. The extra clearance allows for normal position variation between the clearance hole and the tapped hole in the mating part. Specifying tight clearance on cap screw holes makes assembly very difficult without precision alignment fixtures.

Bearing bores for press-fit installation should use a transition fit that allows the bearing to be pressed in with moderate, consistent force. The bore diameter should be 0.0005 to 0.001 inches smaller than the bearing outer ring diameter. A bore that is too tight will distort the bearing race and cause premature failure.

Shafts for rotating applications should use a running fit with 0.001 to 0.003 inches of clearance depending on the shaft diameter and speed. Higher speed applications require more clearance to allow for thermal expansion. The shaft surface finish affects the running fit with smoother surfaces allowing tighter clearance.

Communicating Tolerances on Drawings

Every dimension shown on an engineering drawing should have an associated tolerance that tells the machinist how much variation is acceptable. General tolerance notes in the drawing title block cover all dimensions that do not have an explicit tolerance callout. Specific tolerances are shown directly next to the dimension.

The general tolerance note for ISO 2768 fine class reads simply: “General tolerances ISO 2768-f.” The note appears in the title block of the drawing and applies to every dimension that does not have its own tolerance callout. Dimensions that require tighter tolerances than the general standard must have the tolerance value written directly next to the dimension on the drawing.

Critical tolerances for mating assembly features should always be called out explicitly on the drawing so the machinist knows they are important. A press fit bore dimension on a drawing would read: “1.000 +0.0005 / -0.0000” to indicate the bore can be 0.0005 inches larger than the nominal diameter but must not be any smaller than nominal.

Geometric dimensioning and tolerancing commonly called GD&T uses standard symbols to define the tolerance zone shape, orientation, and location for critical features on a machined part. A position tolerance symbol with an associated datum reference defines an acceptable zone where a hole center can be located relative to other features on the same part. GD&T is significantly more precise than standard coordinate tolerancing for defining critical features because it uses a round tolerance zone instead of a square zone. The round zone gives the machinist 57 percent more allowable position variation while maintaining the same functional fit.

Measuring Tolerances

Measuring machined tolerances on a finished part requires selecting the correct inspection tool for the specific tolerance range that needs to be verified. Using the wrong measurement tool produces unreliable results. Parts with ±0.005 inch tolerances can be accurately measured with digital calipers that have 0.001 inch resolution. Parts with ±0.001 inch tolerances require a micrometer that has 0.0001 inch resolution for accurate measurement.

Parts with ±0.0005 inch tolerances require a micrometer or bore gauge with 0.0001 inch resolution. Temperature at the time of measurement significantly affects dimensions at this tolerance level. A 10-degree Fahrenheit temperature change on a 2-inch aluminum part changes the measured dimension by approximately 0.0003 inches.

Temperature control during inspection is essential when measuring tight tolerances. Always measure parts at the same stable temperature as the inspection equipment for consistent and accurate results. Allow machined parts to cool to a stable room temperature after machining before taking any final quality measurements. Aluminum parts that are fresh from machining can be 10 to 20 degrees Fahrenheit above room temperature which will cause the measured dimension to be slightly larger than the actual dimension at room temperature.

Common Design Mistakes

Specifying tolerances that are tighter than functionally necessary is the single most common design mistake made by new designers. Tight tolerances increase machining cost exponentially as the tolerance range decreases. A ±0.005 inch general tolerance is easily and consistently achievable on any well-maintained CNC machine. A very tight ±0.0005 inch tolerance requires careful setup and may require additional grinding operations.

Using the same unnecessarily tight tolerance on all dimensions increases manufacturing cost without improving part function. Only the critical mating dimensions that affect assembly fit need tight tolerances. All other non-critical dimensions can safely use the general tolerance note specified in the drawing title block.

Not considering the available measurement method in the shop when specifying a tight tolerance on a drawing is another common and costly design mistake that leads to inspection problems. A tolerance of ±0.0002 inches requires measurement with a coordinate measuring machine or comparator. The machine shop may not have this specialized measurement equipment available. Always specify tolerances that can be reliably measured with the inspection equipment available in the shop.

Understanding how tolerances work and how to specify them correctly is an essential engineering skill for designing machined parts that assemble correctly and function as intended in their final application or product. Without proper tolerance specification, even the best-designed parts will not fit together reliably in production. The key principles for good tolerance design best practice are to always use the loosest tolerance that still allows the part to function correctly, to correctly specify tolerances on a drawing using standard ISO engineering drawing practices, to properly analyze tolerance stacks in multi-part assemblies, and to choose the right measurement tools for the tolerance level required.

The cost of machining increases significantly as tolerances get tighter. A good designer specifies tight tolerances only on the specific mating dimensions where they are functionally necessary for the assembly and allows standard general tolerances everywhere else on the drawing. This approach produces parts that fit correctly and cost less to machine.

Tolerance, Material, and Cost Considerations

The material being machined significantly affects the achievable tolerance and the appropriate measurement method. Different engineering materials respond differently to cutting forces, temperature changes, and environmental conditions.

Aluminum expands at approximately 12.3 microinches per inch per degree Fahrenheit. A 12-inch aluminum part measured at 70 degrees and assembled at 90 degrees changes dimension by 0.003 inches due to thermal expansion alone. Design tolerances must account for the full operating temperature range of the assembly in use.

Steel expands at approximately 6.5 microinches per inch per degree Fahrenheit which is about half the expansion rate of aluminum. Steel parts are much more dimensionally stable across temperature changes. Steel is the preferred material choice for precision fixtures, gauge components, and parts that must maintain tight tolerances over a range of temperatures.

Plastic materials like nylon, Delrin, and polycarbonate have high thermal expansion rates and moisture absorption that significantly affect dimensions over time. Tolerances on plastic machined parts should be ±0.005 inches or looser for reliable manufacturing. Tight tolerances on plastic parts are very difficult to maintain as the material changes dimension with temperature and humidity.

Cost Impact of Tolerance Selection

The cost of machining increases significantly as tolerances get tighter and the relationship is highly nonlinear. A part with ±0.005 inch general tolerances costs a baseline amount to produce. The exact same part with ±0.001 inch tolerances costs 2 to 3 times more. The same part with ±0.0005 inch tolerances costs 5 to 10 times more than the baseline.

The significant cost increase comes from several compounding factors that affect every stage of production from setup to final inspection. Tighter tolerances require slower cutting speeds and lighter finishing passes. The setup time increases because the machine must indicate work offsets more carefully. Additional inspection time is needed to verify each tight dimension. Cutting tools must be newer and in better condition with less allowable wear before replacement.

The smart designer always specifies the loosest tolerance that still allows the part to function correctly in the final assembly. A typical ±0.005 inch general tolerance is perfectly adequate for the vast majority of non-critical dimensions on typical machined parts. Tighten the tolerance only on the specific mating dimensions where the press fit, slip fit, or clearance fit assembly function requires it. Every other dimension on the part should use the loose general tolerance.

The single most important practical thing to remember about tolerances when designing machined parts is that tighter is not always better. The absolute best tolerance for any dimension on a drawing is always the loosest possible one that still allows the part to function correctly and properly in its assembly. This smart design approach produces parts that cost significantly less to make while still performing correctly in their assemblies and meeting all functional requirements for fit and function.

For more information on CNC machining tolerances, see our [CNC Machining Tolerances Guide ISO 2768](

G90 G54 G00 X0 Y0
G01 Z-0.1 F10
G01 X1.0 F20
M30

/posts/cnc-machining-tolerances-guide/). For design guidelines, see our Design for CNC Machining DFM Checklist. For measurement tools, see our CNC Shop Starter Kit Guide.