Finishing of holes

At the moment, it is difficult to imagine any complex details, overall assemblies, a manufactured product without using various holes in it. The global industry strives to optimize production processes, and therefore the blanks are becoming closer to the final shape (without unnecessary transfer of material into chips). The use of additive technologies (3D printing, etc.) has increased 30-fold over the past 5 years (sales of industrial 3D printers increased from 219,168 in 2015 to 6,700,000 pcs in the first quarter of 2020). 

It should be kept objective and noted that at the moment the accuracy achievable by modeling on 3D printers is insufficient for the formation of functional surfaces. In this regard, we list the principal possibilities to increase the diameter of the existing hole: reaming, boring with a non-rotating tool (with a conventional cutter / holder), boring with a rotating tool, countersink, sweep, milling by screw interpolation, etc. Not all of these methods allow you to get the required hole with the necessary technical characteristics (tolerance, ellipsis, roughness, etc.). 

Below we describe the standard methods of finishing holes and their inherent characteristics.

1. Drilling.

Drills made according to GOST and HSS drills (high-speed) allow you to get a hole IT11-13 (11-13 quality), roughness Rz=20-80mkm. The use of high-performance (usually imported) carbide drills allows you to achieve holes IT9-10. 

There are special carbide drills for processing precision holes (rifle, cannon and similar structures) —hole IT8-9.

Drilling is usually referred to as a preliminary processing operation.

2. Countersinking.

Zenker — a multi-pronged blade tool to improve the accuracy of the shape and direction of the axis of cylindrical holes. It is often used as an intermediate tool between a drill and a reamer (it allows you to extend the service life of the latter. The countersink according to GOST and various high-speed ones allow you to get 10-12 quality. High-performance and imported carbide countersinks allow you to get a hole IT 7, Rz=20-50 microns. They are rarely used in modern machining.

3. Hole deployment.

The reamer is an axial multi—blade cutting tool for increasing the accuracy of the shape and size of the hole, reducing the roughness of the treated surface. Scans according to GOST allow you to achieve 6-12 quality, Rz=20-40 microns. Increasing the purity and accuracy of the hole is achieved by simultaneous cutting with several cutting edges. Due to the large number of cutting edges and the minimum allowance, centering is improved and the forces causing axial deviation are reduced. The sweep corrects the shape of the hole, but, as a rule, cannot correct the direction of its axis (the allowance is too small).

High-performance carbide reamers allow you to get holes up to 5 quality. The use of an uneven step allows you to achieve errors on the shape of the hole 1-2 microns. The roughness range for carbide reamers is Rz=2-15 microns

Advantages: high quality of the hole with a large minute feed, the possibility of use in mass production. 

Expediency of application: large series of parts, small diameter, performance.

4. Boring with a rotating tool:

The process of finishing boring allows you to achieve holes IT6-9 . The complexity of this type of tool has led to the emergence of whole "boring systems". At the moment, there are systems that combine mechanical and electronic adjustment directly in the machine spindle ("analog and digital"). A wide variety of systems can be conditionally divided into multi-blade and single-blade boring systems. Multi-blade boring: rough boring, accuracy class by IT9 and more. It is used when the main criterion is the speed of metal removal.

Single-blade boring systems - hole quality by IT6, the ability to adjust with an accuracy of 0.002 mm (2 microns). Allows processing small diameters (usually from 5mm). It can combine mechanical adjustment with digital display.

The boring process is carried out on machining centers and horizontal boring machines, the rotating tool works with axial feed.

When choosing boring systems and assigning cutting modes, it is necessary to take into account the power of the equipment. The formula shows the relationship of torque Mwith (Nm) , equipment power consumption Pc (kW), spindle speed n (rpm)

Expediency of application: versatility (one boring system "overlaps" a large range of parts), tight tolerance, large diameter range (approximately 6mm — 1000mm), the ability to process holes with ellipsis (up to a certain point), the low cost of the consumable (the cutting element itself), the systems also withstand positioning errors (inaccuracy up to 0.01 mm),

Conclusions: when choosing the method of finishing the hole, it is necessary to take into account, first of all, the requirements imposed on it (tolerance, quality, roughness, alignment, etc.), the design features of the part, the specifics of production (seriality or versatility), equipment capabilities.  

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References:

1) Feldstein E.F. "Materials processing and tools"

2) Ryzhkin A.A. "Processing of materials by cutting"

3) Technical reference books of tool manufacturers

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