Countersinking - what is it?

The technological process of changing the sizes and shapes of parts, as a rule, cannot do without such a type of machining as countersinking. Translated from German, the word means "pass", "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 holes are drilled to improve their surface quality and accuracy.

Getting holes

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

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

It will be roughing. Then it is drilled with a vertical drill to the required diameter of 12 mm (semi-finished operation). In addition, countersinking will provide 7 class accuracy.

In order for the hole to be grade 5, you must use a scan. It will provide the necessary size limits and allowance - finishing and finishing of the hole. Reaming and deployment, as a rule, are performed at the same diameter, only the limiting dimensions are different, and since the development 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 due to the fact that they are very similar in names. However, if countersinking is a process during which the countersink bores the hole to the full depth, then countersinking is the processing of its upper part.

This is necessary for the formation of recesses for countersunk heads of fasteners (bolts, rivets, screws). Countersink - a tool for countersinking, differs from the countersink in the form of the working part.

Material and types of countersinks

Like any other cutting tool, the countersink is classified by type of construction, by shape and type of processing, as well as by the material from which it is made.

Since this tool has high rigidity, as a rule, it is made of high-speed steel, however, one can often find a countersink 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. Countersinks can be tail teams with insert knives, tail one-piece, mounted teams and mounted one-piece.

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

Length and diameter

Since drilling and countersinking are quite similar processes, the drill (in particular, for spiral drills) and the core drill have almost the same structure. Each of the tools has a cutting part, which directly makes a hole in the part.

The difference is that the countersink does not have a transverse cutting edge, but there are from three to six teeth. Its working part consists of 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 is 0.2-0.9 mm.

Countersinking is a mechanical operation similar to drilling. If its purpose is to bore a hole for subsequent deployment, then the diameter of the core drill is chosen less, taking into account the allowance for the last finishing operation. Moreover, since the required accuracy of this tool is lower than for sweep, the absolute values ​​of the deviations can be much larger.

In another case, countersinking of holes is the last finishing required for a part with a tolerance of 11, 12 quality. Then the tool is selected taking into account the breakdown and wear margin, and its diameter should correspond to the diameter of the hole.

Design and Calculation

In order for the diameter of the required hole to be as accurate as possible, design the core drill. Thus, you can find the maximum and minimum diameter of the required tool, determine the material of its cutting part and calculate the cutting conditions.

An important indicator that affects the maximum and minimum values ​​is the required quality. For example, it is necessary to carry out the final hole counting with a diameter of 85 mm with a grade of H11. Based on the table of the tolerance fields of holes with nominal sizes from 1 to 500 mm, for 11 qualifications (for diameters from 80 mm to 120 mm) the tolerance field is: the upper value is “+220”, and the lower value is “0”, that is 85 ⁺²²⁰ mm. The maximum diameter of the drilled hole cannot exceed 85.22 mm, and the minimum - 85 mm.

The size tolerance is the difference between D _max and D _min , that is, it will be 0.22 mm. If we talk about marriage, then for a hole, a diameter above 85.22 mm will be considered incorrigible, and less than 85 mm will be considered correctable.