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In many industries, from mechanical engineering to tool making and precision mechanics, the precisely tolerated positioning of components determines the quality and longevity of a product. Conventional threaded joints are often insufficient to precisely align and securely join components. This is where reamer bolts come into play – they ensure a play-free guide in bores and ensure repeatable assembly with the highest precision. In this article, you will learn why reamer bolts are the better choice for aligned joints, and the role of tolerances, thread runouts, and undercuts for optimal assembly.
Reamer bolts, like screws with shoulder, also called shoulder bolts, are special types of bolts used for precise positioning and fastening tasks. They differ from conventional bolts by a special geometry that performs specific mechanical functions. The cylindrical section, acting as a spacer element and bearing pin, also permits mounting a component such that it can move while simultaneously mounting the screw in a rotation-proof manner. This option is shown here in the figure using the example of bolts with shoulder (CBBD and CBDBR).
Reamer bolts have a cylindrical, precisely manufactured shaft - also called fitted shaft - which creates a precise fit in bores. The region machined to fit can have a diameter identical to the thread diameter or a larger diameter. If the diameter is larger than the thread diameter, it is referred to as a bolt with shoulder. Reamer bolts are mainly used for precise positioning of components with respect to each other. In addition, they can withstand not only axial forces but also lateral forces. Reamer bolts are used in mechanical engineering, tooling, and wherever precise alignment is required.
The term bolt with shoulder or shoulder bolt is a generic term used to refer to bolts that have a stepped region - either as a shoulder, a collar or a fitted section. A bolt with shoulder has an additional, typically cylindrical region below the bolt head. This step, also known as the shoulder, is larger in diameter than the bolt thread and often act as an alignment surface to precisely guide the screw into a bore or as a spacer.
Shoulder bolts or bolts with shoulder can have different head shapes and undercuts. The shapes and dimensions of these bolts are as varied as the applications and head shapes. Some commonly used shapes are therefore standardized, such as flat head screws with slot and shoulder according to DIN 923, square bolts with short lugs and core shoulder according to DIN 479, or socket head screws with hex socket and shoulder shank according to ISO 7379.
Example illustration - hex socket reamer bolt with shoulder and thread of the same diameter
Example illustration - hex socket reamer bolt with shoulder
Example illustration - reamer bolt with knurling
The thread run-out describes the region where the full thread transitions into the smooth shaft or end of the screw. It is therefore the region where the thread is no longer fully formed since the threads gradually flatten out. For reamer bolts, a clean transition from the thread to the accurately fitted shaft is critical to avoid stress risers and burr formation at the thread run-out. A correctly executed thread run-out with an undercut, e.g. according to DIN 76-1, helps to minimize notch stresses. Avoiding notch stresses and stress risers is important because they can increase the risk of material fatigue or breakage. The thread run-out creates a flat, uniform power transmission, which makes threaded connections more stable. The reduced stress in the transition area between the thread and the shaft increases the fatigue strength and thus the service life of the screw.
The DIN 76-1 standard defines various forms of run-out:
An undercut, such as the undercut according to DIN 76-1 A or B, is essential to ensure that the threaded region on bolts with shoulder can be fully threaded in. In order to ensure full-surface contact of the bolt head, an undercut can also be used at the transition between the bolt head and the fitted bolt shaft. For fitted bolts without a stop, the undercut ensures a uniform and secure transition from the thread run-out to the fitted bolt shaft.
An undercut in accordance with DIN 509 (also called back turning) is a recessed area in the runout zone of the tools for turned parts. The undercut is therefore a machined area that creates relief space for adjacent components.
In many engineering applications, especially in mechanical engineering and precision mechanics, the exact alignment of components is crucial for reliable function. Improper assembly can cause wear, unwanted play, or even mechanical failure. Due to their special design, reamer bolts provide a low-play guide in the bore and can reliably absorb lateral forces. They eliminate the typical inaccuracies that occur with standard threaded joints and ensure repeatable, highly-accurate positioning of components.
This method joins components to each other using only bolts, without additional elements for positioning. This does not ensure precise alignment of the components, and the position of the parts can easily shift as the bolts are tightened. This approach is particularly suited for simple joints where the exact position of the components is not of crucial importance.
In addition to bolts, this method uses dowel pins that are inserted into precisely machined bores in order to precisely position the components. While the bolts are primarily responsible for power transmission, the dowel pins are responsible for precise alignment. This method enables high positioning accuracy and is often used in mechanical engineering, mold making, and in applications that require repeatable assembly or component replacement.
Reamer bolts combine the functions of bolts and dowel pins in a single element. Their precisely manufactured shank ensures tightly toleranced alignment of the components, while the thread reliably joins the parts together. This solution offers very high fit accuracy while also making assembly easy. Reamer bolts are primarily used in precision joints.
Reamer bolts are designed to precisely position components and absorb lateral forces by fitting into precisely machined bores without play. However, reamer bolts and their bores are also subject to tolerances that must be taken into account during assembly.
Tolerances are critical for the function of reamer bolts. They ensure precise guidance while still allowing for easy assembly. Common fit systems for reamer bolts are based on ISO fits. Choosing the right tolerance combination depends on the requirements of the joint: For maximum precision, for example, h6/g6 is selected, while f7 allows for easier assembly. Without defined tolerances, either assembly would be difficult or precise positioning would be compromised. Therefore, reamer bolts do not simply have an “exact fit”, but are made with matched tolerances to ensure an optimal balance between fit, ease of assembly and mechanical stability.
