Skip to content 
What Are Slip Fit Tolerances?
Slip fit tolerances refer to the clearance you’ve established in two mating parts so that they can slide together easily, without creating friction that would automatically bind one part to the other. In most cases, this means that the hole size was purposely built to be larger than the shaft, dowel pin, or bolt diameter that goes inside. The reason for the clearance is to allow for easy assembly and disassembly.
Tolerance does not happen by accident. Designers follow rules, charts, and references like Machinery’s Handbook to contribute to thoughtful design decisions. The size of tolerance chosen is generally dictated by the type of materials used, diameter, and thickness, and then the intended service life of the connection; i.e., if the connection is an assembly, the slip fit would have to hold the position without having any wobble, but also allow the parts to disassemble or work effortlessly if needed.
However, slip fit tolerances are not the same thing as press fit tolerances, which utilize interference in a connection to create a tight or permanent connection. In other words, when using slip fit tolerances, the purpose is to establish controlled clearance, and not pressure. For that reason, this type of tolerance is optimal when a bearing, bushing, or assembly must be located in a specific position in the end use but later disassembled.
How Slip Fit Tolerances Work in Practice
Slip fit tolerances indicate a minimal tolerance in clearance between the hole and its matching part, which can be within the shaft or pin. In application, the clearance needs to be enough to allow for ease of assembly while keeping the parts aligned. It’s relevant to use tolerances because designers are managing the dimensions of both the hole and its matching part at the same time.
The clearance will ultimately depend upon the diameter of the shaft, the thickness of the material, and the type of fit needed. For example, a 0.5-inch shaft might have a hole just a few thousandths larger. That small tolerance would allow the shaft to slip in without a loose fit pattern.
Standard references, such as Machinery’s Handbook, have charts that list acceptable ranges of clearance for different types of fits. These charts are useful for designers and machinists because they take the guesswork out of engineering design and give actual values to use that are tested in the field.
Common Applications of Slip Fit Tolerances
Below are the common applications of slip fit tolerances:
Dowel Pins and Locating Parts
Slip fits are essential in assemblies that use dowel pins for maintaining part alignment. Unlike a press fit, which locks the pin into the assembly permanently, slip fits allow the insertion and removal of pins without imposing significant force. It is helpful in applications requiring the components to be held in place precisely while still allowing the disassembly of parts for inspection and service.
For example, a CNC-machined fixture may incorporate dowel pins with slip fits at its base plate and top plate to maintain the same alignment each time the fixture is used. It provides repeatability without wearing the holes in the base plate.
Shafts and Bearings
Slip fit tolerances provide the correct clearance for a shaft to rotate in a bearing assembly. The fit allows for a rotating shaft to turn smoothly within the bearing with minimal friction. Usually, if the tolerance is too tight, it adds heat and wear to the assembly, and if the clearance of the fit is too loose, it allows for additional ride, vibration, and noise.
Slip fits stabilize these applications by providing consistent motion while increasing the life expectancy of the bearings in CNC applications, like spindle assemblies or rotating tools. Designers rely on slip fits to provide the right amount of stability and freedom of motion for these situations.
Gears and Hubs
When gears need to be attached to hubs or shafts, slip fits make assembly easy without a lot of pressing and expensive fixtures. The slip fit provides wiggle space to make sure the gear is firmly seated and easily available for maintenance or replacement. In environments where downtime is corrosive to profits, the ability to simply and correctly slide on gears saves significant time.
Bolts and Fasteners
Slip fit tolerances are often applicable for holes for bolts and fasteners as well. If the hole fits too tightly, it becomes challenging to install the bolt; if the hole fits too loosely, it loses alignment. The slip fit fits between tight and loose, allowing the bolt to have a smooth travel and be stable when torqued and finished.
Difference Between Slip Fit and Press Fit Tolerances
Slip fit tolerances and press fit tolerances are two alternative methods of connecting mating parts, as well as serving a different functional variation. A slip fit allows for clearance between mating parts. The pin or shaft slides into the hole without force. This means that slip fits can be easily assembled and disassembled, which is simple for adjustments, inspections, or replacement throughout the part’s life cycle.
A press fit, often referred to as an overlap fit or interference fit, is a tighter connection without clearance between mating pieces; the shaft (or pin) is marginally larger than the hole. Force or pressure is required to insert it into place. This creates a connection that is strong and permanent, with some resistance to loosening from vibration or a load.
The two methods vary in the basic concept of clearance versus interference. Slip fits require some space between parts to allow motion; press fits require material overlap to create enough friction, holding the parts tightly together. Designers select slip fit tolerance for ease and flexibility, and press fit tolerances if rigidity, strength, and permanence are expected.
Factors Affecting Slip Fit Tolerances
Here are the various factors affecting slip fit tolerances:
Material Properties
The type of material is critical to determining the appropriate slip fit. Metals such as steel or aluminum hold tight tolerances well, while plastic and softer materials may experience expansion, flex, or deformation. Designers have to factor in how strong or flexible a particular material is before applying slips and fits.
Manufacturing Methods
Machining methods such as drilling, reaming, or CNC milling yield a different level of accuracy, which affects the clearance on a slip fit. For example, the tolerances on reamed holes can be much tighter than those on drilled holes. The method of fabrication will have a direct impact on whether or not the slip fit meets the desired clearance.
Surface Finish
Parts slide together more easily when the mating surfaces are smooth. If the parts have a rough surface, there may be friction that creates an assembly challenge even if there is clearance. CNC machining makes it possible to achieve a fine surface finish to make slip fits more consistent and reliable.
Part Dimensions and Size Tolerances
The required clearance between mating parts will be based on the diameter and thickness of the parts. This can usually be followed with larger parts needing a little more allowance built into the assembly in case the part was machined improperly. Designers need to design for size tolerances and allowance when functional size requirements are not met to create misalignment or excessive play in the parts or assembly.
Assembly Requirements
Some assemblies require parts to be removable after assembly, while the assembly allows for movement of the parts while remaining in an aligned position, without disassembly. These assembly requirements will dictate how tight or loose the slip fit of the particular assembly. An example of this is if the assembly required the distance tolerances of a bolt hole to be looser than those of a dowel pin fixture that would require close tolerances.
Benefits of Slip Fit Tolerances
🔗 Best Practice
Define slip fit tolerances early using H7/g6 or similar standard fits to ensure smooth assembly without excessive play, reducing rework and maintaining dimensional consistency between mating parts.
There are many benefits of slip fit tolerances, and some of them include:
Easy Assembly
Slip fit tolerances allow parts to slide together without pressing, hammering, or heating, leading to a faster and lower-labor assembly process. When it comes to CNC machining, the worker’s ability to simply align and locate pieces by hand will offer numerous time savings during mass production runs. The urgent need to apply force is reduced, thus the risk of breaking delicate pieces during assembly is also reduced.
Reduced Friction
Slip fits also provide a small clearance between mated parts when the parts are aligned to create the fit. Reduced contact pressure leads to reduced friction. Reduced friction is used in applications like shafts and bushings, where smoother motion prolongs the service life of the components. CNC-machined rotating assemblies, slip fits prevent binding while maintaining their fixed location.
Working with Many Materials
Slip fit tolerances also function well with differing materials, such as metals and non-metals. A slip fit can permit a CNC-machined metal shaft to be effectively mounted within a slip fit in a plastic housing, without the risk of cracks or stress on this softer material. The versatile nature of slip fits makes them useful for mixed materials, such as lightweight components for automotive and aerospace applications.
Concerns of Slip Fit Tolerances
🔗 Best Practice
Use precision reamers or boring operations for slip fit holes instead of drilling alone to achieve tighter roundness and surface finish, ensuring reliable assembly and repeatable performance.
While slip fit tolerances have many benefits, there are still some concerns to be worried about, and they are:
Excessive Clearance
When clearance is excessive, it generally leads to rattling and unpredictable movement by parts. Excessive clearance can cause vibration, noise, and less accuracy in high-speed machinery. Specifically, for example, in a CNC spindle assembly, an oversized slip fit between the shaft and the bearing can result in tool chatter and poor surface finish.
Tolerance Control
Slip fits require accurate control over tolerancing during the machining operation. If tolerances for the hole and shaft are not kept within specifications, it might end up maligned and/or have an uneven connection. This creates more demands on the quality control teams since they will be required to verify tolerance with tools such as coordinate measuring machines (CMMs).
Thermal Expansion
Materials will expand when heated. If a slip fit was not sized to accommodate thermal expansion, it would collapse to a larger clearance. At high temperatures, for example, in automotive or aerospace components, when materials are subject to the heat of the engine, these clearance fits are potentially unstable if not appropriately considered by the designer.
Conclusion
Slip fit tolerances provide an ideal balance between precision and function. They allow parts to be assembled easily, run smoothly, and be disassembled without damage, while remaining tolerant enough so the assemblies can be relied upon to function. When you have to design any mating part, knowing how to set the required slip fit tolerances makes life much easier for the designers, machinists, and maintenance technicians.
Here at AT-Machining, we have a firm grasp of the art and science behind tolerances. Whether you require prototyping, low-volume production, or precision parts with tight slip fit tolerances, we combine our CNC machining expertise with requirements for tolerance. Our services are all assembled around the premise that we will provide parts that follow the design intent, including proper slip fit tolerances. Contact us today to get started!
