02.07.2021

Broaching tools: Produce complex internal profiles very easily

Broaching can be used to produce demanding internal and external profiles in series production. Broaching is mainly used in areas that require profiles that are difficult to produce on external or internal features with high surface quality and form accuracy. These include: Internal splines, brake calipers, hub keyways, bores on connecting rods, bushings, shaft and sliding sleeves, wheel and drive hubs, clutch carriers, splines on gears, racks, tongs and cylinder lock cores.

About 50 years ago, broaching was still a typical manufacturing process for finishing, which has now found its way into almost all production stages. Particularly in the field of drive technology, broaching has become indispensable.

Broaching is one of the most economical production processes in large-scale machining of workpieces with complex profiles when cutting performance and reproducibility are required. The advantages of broaching are obvious, because the profile is produced in a single operation with a multi-tooth tool in which the cutting edges are arranged one behind the other and staggered by one chip thickness.

Production of broaches

Almost all surfaces of broaches are ground, including the shank and end section, due to the high precision requirements. However, in the production of broaches, complete machining has not yet become widely accepted. In some cases, the hardened semi-finished products of broaching tools are further processed on up to 4 different machines before they are used in the broaching machine after application of the coating. This is not only associated with a high space requirement, but is also not particularly advantageous with regard to tool life and the achievable accuracies. 

Grinding machines from the Multigrind® series meet the high requirements for profile accuracy, which are usually very different. In addition, they offer the advantage that the broaches can be produced in a single clamping operation.

The advantages of complete machining are well illustrated by the example of a broach.

Broaches: Complete production with the Multigrind® series - quite simple

Extremely high demands are placed on broaches, as they are used in broaching machines for internal broaching. To meet these requirements, a wide variety of steels are used - from case-hardening steel to high-alloy tool steel. Broaches can only be used for the production of the mold for which they were made. For this purpose, the elongated, slightly tapered broach is pulled through an existing bore on a broaching machine.

When manufacturing broaches, the grinding machines from the Multigrind® series can simplify production considerably. After all, grinding intricate geometries is one of the core competencies of Haas Schleifmaschinen.

The thermostable Multicube design, symmetrically built, with linear axes aligned orthogonally to each other and the grinding machining in the center give the machines from the Multigrind® series a high level of stability and rigidity: Ideal for machining long narrow workpieces. Also crucial for high repeatability in the production of broaches is the clamping device as well as the support steady rests. This is all about perfect alignment of the workpiece during machining.

Straight and spiral grooves, spiral profile.

Each sprocket of a broach is different from the following for proper broaching.

Broaches: Resharpening with the Multigrind® series - as easy as making them

The question of how long a broach can be used always causes animated discussions. On the Internet, a wide variety of meter specifications can be found regarding the service life. However, the actual wear, and thus the optimum time for regrinding, depends on several influencing factors. Among other things, the material properties, the cooling lubricant used and the machine settings as well as the coating of the workpiece have an influence on the service life.

Since broaches are expensive to manufacture, it is obvious that they should be treated gently, which is why even the dulling of a tooth requires the needle to be replaced. If the tool is not replaced, it is highly probable that the next tooth will break off immediately due to overload. If the pressure gauge of the broaching machine shows an increased pressure or if the dimension of the profile is no longer maintained, it is recommended to replace the tool immediately. Other indicators that point to an immediate replacement are a strong heating of the workpieces as well as a deterioration of the surface quality of the broached geometries.

Prolonging the service life of a broach is usually achieved by re-sharpening the cutting surfaces. This reduces the diameter of each individual tooth due to the clearance angle, but since broaches generally have several so-called reserve or calibration teeth with the maximum diameter, of which only the front tooth is engaged, the required diameter can be maintained despite regrinding. The rake angle must be precisely maintained, and any offsets in the tooth root radius must be avoided. In addition to the grinding strategy, the perfect condition of the grinding wheels is critical to success.  

Tool measurement and the process design for dressing the grinding wheels are of decisive importance here. Multigrind® Horizon and Multigrind® Styx offer the possibility of simulating tool wear and displaying it with pixel accuracy. In this way, the high demands placed on the process can be taken into account even more effectively, and it is particularly easy to achieve optimum tooth resetting without changing the actual cutting profile.

 

The author

Zita Bader

Zita Bader works in the marketing and communication department of Haas Schleifmaschinen GmbH.

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