Countersinking is what?

The technological process of changing the dimensions and shapes of parts, as a rule, does not do without this kind of machining, like sinking. Translated from German, the word means "go through", "deepen". More precisely, this is a process during which the diameter of the holes is increased. It can be compared with reaming. Countersinking is a mechanical operation, during which boring of holes is made, in order to improve the quality of their surface and accuracy.

Getting holes

In order to thoroughly understand what a countersink is, it is necessary to have an idea of how holes are made in detail. Suppose, in the workpiece, it is necessary to drill a hole of the fifth accuracy class with a diameter of 12 mm.

To do this, you need a drilling machine and 3 working tools: drill, countersink and reamer. Each of them is designed for the processing of passes, because drilling drilling and reaming are operations very similar to each other. First of all, a hole is made in the workpiece with a drill, but its diameter will be much less than the required diameter, for example, in half - 6 mm.

This will be roughing. Further, it is reamed with a countersink to the required diameter of 12 mm (semi-finished operation). In addition, the countersink provides the 7th grade of accuracy.

In order for the hole to be of class 5, it is necessary to use a sweep. It will provide the necessary maximum dimensions and allowance - finishing and finishing the hole. The countersinking and deployment are generally performed on the same diameter, only the limiting dimensions differ, and since the sweep provides a lower accuracy class, the deployment allowance is selected with high accuracy.

Countersinking and countersinking

Very often these concepts are confused with each other because they are very similar in names. However, if the countersink is a process during which the countersink renders the hole to the full depth, counting is called counting the upper part of the hole.

This is necessary for the formation of recesses under the hidden heads of the fastening elements (bolts, rivets, screws). Countersink - a tool for countersinking, differs from a sinker in the form of a working part.

Material and types of countersink

Like any other cutting tool, the countersink is classified according to the type of structure, the shape and type of processing, and also the material from which it is made.

As this tool has high rigidity, as a rule, it is made of high-speed steel, but it is often possible to find a sinker from structural alloyed (40X) and carbon (St45) alloy.

Countersinking is smoothing and cleaning the surface, so the tool itself has a large number of cutting edges. The countersink can be tail assembly with inserted knives, tail integral, breech blocks and integral ones.

If viewed from the point of view of economy, then, of course, prefabricated tools have a higher cost, but their service life is much longer, since the failed edges can be replaced.

Length and diameter

Since drilling, reaming - the processes are quite similar, then the drill (especially for the spiral drills) and the countersink have almost the same structure. Each of the tools has a cutting part that directly makes a hole in the part.

The difference is that the countersink does not have a transverse cutting edge, but there are three to six teeth. Its working part consists of the calibrating and cutting components. The length of the second depends on the depth of the core. The calibrating part is the longitudinal ribbons along the cutting edge. Their width depends on the diameter of the countersink (0.8-2.5 mm), and the height - 0.2-0.9 mm.

Countersinking is a mechanical operation, similar to reaming. If its purpose is to bore the hole for later deployment, then the diameter of the countersink is chosen less, taking into account the allowance for the last finishing operation. In addition, since the required accuracy of this tool is lower than for the sweep, the absolute values of the deviations can be much larger.

In the other case, the countersinking of the holes is the final finishing required for the part with a tolerance of 11, 12 in the class. Then the tool is chosen with regard to the breaking and the stock for wear, and its diameter must correspond to the diameter of the hole.

Design and calculation

In order for the diameter of the required hole to be as precise as possible, the design of the countersink is performed. Thus, you can find the maximum and minimum diameter of the required tool, determine the material of its cutting part and perform calculation of cutting modes.

An important indicator that affects the maximum and minimum values is the required qualification. For example, it is necessary to perform a full countersinking of a hole with a diameter of 85 mm with a H11 grade. Based on the tables of the tolerances of the holes at nominal sizes from 1 to 500 mm, for the 11th grade (for diameters from 80 mm to 120 mm) the tolerance field is: the upper value is "+220" and the lower one is "0", that is 85 ⁺²²⁰ mm. The maximum diameter of the boring hole can not exceed 85.22 mm, and the minimum - 85 mm.

In this case, the tolerance for size is the difference between D _max and D _min , that is, it will be 0.22 mm. If we talk about marriage, then for the hole, the diameter above the value of 85.22 mm, and the correctable one - less than 85 mm, will be considered irreparable.