The 3 Common Use Hole Machining Methods in CNC Machining

    Hole Machining Methods

    Hole machining methods is the most common processing method in CNC machining. It can be said that it is rare to see a mechanical part without holes. For example, in the cars we use every day, about 5,000 holes are machined. Although not all these holes are processed by cutting out, hole machining occupies a very important proportion in the machining industry. Generally speaking, the proportion of hole processing accounts for about 40% of the cutting process.

    Hole machining can be done on almost all machine tools such as lathes, milling machines, and drilling machines.  The CNC turning process can also allow the generation of complex external geometries and internal holes, including the machining of various threads. It can also be seen from this point that hole machining is a very large type of machining. Most of the hole machining is general machining. But there are also some complex machining. There are many methods for processing holes during machining. Such as drilling, reaming, boring, grinding, broaching, and so on. Among them, here are 4 commonly used hole machining methods.


    Drilling is a process that uses a drill to make a hole in the solid workpiece. The drill is a drilled blade. It is attached to the main axis of the machine and rotates. Drilling is controlled by the spindle speed, digging speed and depth of each digging. Drilling is rough machining that precision is low and surface relatively rough. This is because drilling is divided by several time, it will drill up and dig. 

    When drilling on a drilling machine, the drill is easy to deflect and cause the axis of hole to be deflected and not straight. But there is no significant change in the hole diameter. While, when drilling on lathe, it’s opposite. The hole diameter is easy to change but the axis of the hole is still straight.

    The hole diameter is easy to be enlarged when drilling. The main reasons are: The radial force of two cutting edges of the drill is not equal, deflection happens when pitching in, and radial run out of the drill. 

    Generally, the diameter of drilling hole has a limitation at max 80mm. There are two types of drilling. One is rotating the drill and the other one is rotating workpiece. In the first type of rotating drill, the hole centerline will deflect due to the cutting edge is asymmetric and the drill lack of rigidity. But the hole diameter will not be influenced. It’s opposite in another drilling type. The drill deflection will lead to hole diameter changes. But the hole centerline will remain straight.

    structure limitation, the drill bending stiffness and torsional stiffness are low. Plus, the poor centering of drill lead to low accuracy of drilling processing. But the material removal efficiency of drilling is high

    It’s not hard to find that drilling is the basic hole machining method. It is mostly applied for machining holes which has low quality requirements. For holes with high precise and surface quality requirements, other precise hole machining methods are necessary like reaming, boring or grinding.


    Reaming is a process to enlarge an existing hole. It uses reamer to remove slight material layers from the original hole wall to improve dimension accuracy and surface roughness.

    1. Reamer

    Reamer, as its name suggests, it is tool used for reaming. It has two types: manual reamer and machine reamer. The manual reamer has a straight shank, long working part and good guiding effect. Machine reamer include two types. One is with shanks and the other one with sleeves. With reamer, it can not only machining round holes, but also tapered holes.

    2. Reaming process and application

    The quality of reaming is mainly influenced by the reaming allowance. If the allowance is too large, the reamer load will be large and cutting edge become blunt soon. Then,it neither to achieve a bright and clean surface, nor to assure the dimension tolerance. Accordingly, if the allowance is too small, it’s hard to remove tool marks left by the previous drilling process. Then the hole machining quality can’t be improved. General, 0.35~0.15mm allowance should be taken into consideration for rough reaming. For fine reaming it’s 0.15~0.05mm.

    To avoid built-up edge, reaming is usually done at a low cutting speed. The larger the of the hole diameter, the larger of the feed value.

    When reaming, cooling, lubricating and clean with the proper cutting fluid are necessary to prevent built-up edge and timely chip removal. Compared with grinding and boring holes, reaming is more efficient and easier to assure the hole tolerance. However, reaming cannot modify the position tolerance of the hole axis. The position tolerance should be controlled by the previous process. 

    Reaming is one of the finishing methods for hole machining. It is widely used in CNC machining services. Reaming is especially suitable for small and precise hole machining.


    Boring is a machining method to enlarge a prefabricated hole with a cutting tool. It can be done either on boring machine or on lathe.

    1. Boring method

    Boring includes three different process methods.

    The first method is work-piece rotates and tool makes feed motion. This is general way for boring on lathe. After boring, the hole axis is same as the axis of workpiece rotation. The rotate accuracy of the machine tool spindle has direct influence on the hole roundness. The axial geometry tolerance of the hole are controlled by the position tolerance of the tool feed direction relative to the workpiece rotation axis. This method suits for machining holes required concentricity with the external surface.

    The second method is tool rotates and workpiece makes feed motion. In this method, the boring tool rotates by the machine spindle. The workpiece feeding motion is driven by workbench.

    The third method is tool rotates and makes feed motion. In this method, the overhang of the boring bar varies. So does the force deflection of the bar. The hole diameter close to the spindle box is large. The hole diameter is small when away from the spindle box. With the overhang of the boring bar increases, the deflection of the spindle also increases. The boring hole axis will have deflection accordingly. This method suits for short holes only.

    2. Boring process and application

    Compared with drilling and reaming, the hole size is not limited by boring tool size. Boring can correct hole tolerance well. By multiple cutting, boring can not only correct the hole axis deviation, but also assure tight bore tolerance and position tolerance.

    Compared with turning ex-circle, the quality and productivity of boring is lower. Because boring tool bar system has poor rigidity, larger deformation, cooling and chip removal of boring is not well.

    As we can see, boring can be done in a wide range of different sizes and accuracy levels. It is almost the only method for holes with large diameters but high requirements of dimension and position tolerances. Boring is particularly suitable for machining large parts with complex structures such as box and chassis.

    Except above 3 common hole machining methods, there are also other methods like broaching and honing. For more details, please visit


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