How to choose a drill? Part 2
the Spiral drill is a core of a cylindrical form with two cutting edges at one of end faces. Any spiral drill accurately is divided into three zones: the cutting part calibrating and a shaft. The working end of a drill with the cutting edges has the cone-shaped form.
The calibrating zone determines the accuracy of the size of an opening and a roughness of its walls. For removal of shaving in it there are two spiral flutes. The part of a drill on which there are no spiral flutes is called a shaft, it is intended for fixing of a drill.
If earlier main sizes of drills were defined by state standard specifications, then now in the course there are much more normative documents published by the most different countries. However the general regularities of a ratio of the sizes in general coincide. In the drill mainly with a diameter from 0,3 to 20,0 mm course with a step 0,05 (to diameter of 10 mm) and 0,1 mm (for diameters more than 10 mm).
The sizes of spiral drills are standardized. As the main size of a drill it is considered to be diameter. Therefore when drilling choose such sizes of openings for which there is corresponding diameter of a drill.
On length of a drill divide into several standard sizes. There are groups of drills with the extended shaft, is with shortened. In drills with a cylindrical shaft length of working part of a drill, depends on diameter, and is, as a rule, equal to 50 mm plus diameter of a drill.
You should not go deep into the description of difficult geometry of drills and calculation under what corner this or that edge is located. For practical purposes it is only important to us to know that the corner at top of a drill is chosen depending on the hardness of sverlimy material.
It has to make:
- for soft metals 80 - 90 °;
- for steel and cast iron of average hardness 116 - 118 °;
- for very solid metals 130 - 140 °.
The cutting drill edges - the most acting their part. That the rest of an end face did not interfere with the drilling process, the drill is sharpened so that the drill end face surface behind the cutting edge formed with a material surface certain, so-called “ back corner “. For the drills intended for metal processing, “ back corner “ it is usually equal 5 - 15 °, and it is a variable.
The inclination of screw flutes to an axis of a drill makes from 10 to 45 °. Depending on the direction of flutes of a drill subdivide into the right - and left rotating. The right rotating drills - the most widespread, rotation during their work happens clockwise. The left rotating drills in life do not apply.
To reduce friction of a lateral surface of a drill about opening walls, chamfer it. The rest of a sidewall forms a narrow strip which is called a ribbon along a screw flute. It provides preservation of the direction of a drill during the work. Even more to reduce friction about walls, diameter of the calibrating zone slightly reduce from the cutting edges to a shaft.
Shafts happen cylindrical, conic six-sided. The most running drills with a diameter up to 12 mm have, usually cylindrical shaft.
The material of a working edge of a drill is more solid, the cutting ability and wear resistance of the tool is higher than it. However with increase of hardness of material also its fragility increases. It increases probability of breakage of a drill. Optimum hardness of material of spiral drills - 63 - 69 HRS. Hardness depends on brand of steel and its heat treatment.
Domestic drills make from tool carbonaceous (U10A), alloyed (9HS) or quick cutting steels (P6M5, P6M5K5).
On some “ advanced “ drills apply the oxidic covering facilitating removal of shaving on a flute.
In import drills apply special polishing of an edge of a drill thanks to what the cross edge zatuplyatsya. It is done for the improved centering at the beginning of drilling and reduction of necessary speed of drilling. The self-aligning properties of an edge of a drill do excessive preliminary drilling and even a kerneniye for diameters to 10 mm.