Groove rolling and back grinding technology of the

2022-09-22
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Groove rolling and back grinding process of straight shank fried dough twist drill

at present, the rolling process is widely used in the formation of straight shank fried dough twist drill in China. The biggest advantage of this process is that it has high production efficiency and can make full use of raw materials; The internal structure of the processed bit body has fiber continuity, fine grain, uniform carbide distribution and high red hardness. However, the rolling process also has obvious defects, that is, the bit blank is very easy to crack. Taking the production of our factory as an example, under normal circumstances, the cracking rate of straight shank fried dough twist drills is 5% - 10%, sometimes as high as 20%, and the excessive growth momentum of production capacity has been curbed ~ 40%. Every year, enterprises lose hundreds of thousands of yuan only because of the cracking of drill bits. Especially in the current situation that high-speed steel materials are expensive, solving the problem of bit cracking and reducing the cracking rate will enable enterprises to obtain greater economic benefits

1 the proposal of groove rolling and back grinding technology

there are many factors leading to bit cracking. For different reasons of cracking, different methods can be taken to reduce the cracking rate. However, the effect of the existing methods is not ideal, which can not fundamentally solve the problem of rolling crack. Through observation, it is found that more than 98% of the cracks of the cracking bit occur at the intersection of the edge band and the edge groove. From this point of view, the generation of cracks is directly related to the formation of blade bands. The common rolling process is to simultaneously roll out the edge groove, edge back and edge belt of the drill bit on the four high mill, that is, the rolling groove and the rolling back are synchronized. In view of this feature, we put forward the process scheme of forming the edge groove and the edge belt respectively, that is, first rolling out the edge groove of the drill bit on the four high mill, and then grinding out the outer circle of the edge back of the drill bit on the special grinder (the edge belt is also formed at the same time). This is the process of groove rolling and back grinding to be introduced in this paper. Its main process route is: groove rolling → through grinding excircle → cutting and chamfering → heat treatment → through grinding excircle → fine grinding excircle → back grinding → grinding drill tip

2 principle of groove rolling and back grinding process

when groove rolling and back grinding process is adopted, the original four high mill can still be used, but the sectional shape of the two edge back sector plates changes, that is, there is no edge belt groove. At this time, the function of the two blade back sector plates is only to fix the drill bit profile and adjust the charging amount of the blade back. The pass pattern of four high mill with two rolling processes is shown in the figure below. It can be seen from the figure that when using groove rolling and back grinding to automatically complete the whole cycle test environment process of gas spring, because there is no blade groove in the blade back sector plate, the drill bit has a simple section, so the generation of cracks at the blade belt is avoided

(a) grooving and back grinding (b) grooving and back grinding

1 Blade groove sector plate 2 The reasons for the cracks caused by the blade back sector plate

are as follows: when rolling the groove and back, due to the different sizes of the bit edge band, the width and depth of the edge band groove on the blade back sector plate are also different, usually 0.45 ~ 0.95mm wide and 0.55 ~ 1.15mm deep. During hot rolling, under the rolling action of the sector plate, the blank of the drill bit gradually approaches the load of 50kN and flows and extends along the pass of the mill when it is heated to the near melting state, and the metal flowing into the groove of the blade band forms the blade band of the drill bit. However, the narrow blade belt groove makes it difficult for the metal to flow smoothly, so it is easy to produce blade belt defects. What are the characteristics of special insulation material testing machine? When the green body is cooled, the narrow and high blade belt is at the edge of the drill green body, which has the fastest cooling speed and is easy to produce stress concentration. Due to the influence of material, heating temperature, time and speed, machining accuracy of sector plate, it is very easy to produce cracks at the edge band. When the groove rolling and back grinding process is adopted, the pass of the rolling mill is simple, the structure is reasonable, and it conforms to the metal flow characteristics, so the above phenomena can be avoided; At the same time, because the blade belt is formed separately, the adjustment difficulty of the four high mill is greatly reduced, and the accuracy of drill core symmetry (one of the important inspection items of fried dough twist drill) is improved. Specification (mm) f3f4.2f5f6.8f8.5f9f10 processing method rolling crack rate (%) rolling groove rolling back rolling groove grinding back

after using the new process, the bit blank after groove rolling is subject to heat treatment, fine grinding of the excircle and other processes, and then the blade back is ground on the special grinder. This process can also improve the radial circular runout accuracy of the working part of the drill (which is also one of the important inspection items of fried dough twist drill)

3 application effect of groove rolling and back grinding process

whether to adopt a new process should depend on the actual situation in production. For example, if the rolling crack rate is less than 5% after trial rolling and the production batch is large, the groove rolling and back grinding process can still be used to process straight shank fried dough twist drills. However, when the rolling crack rate is higher than 5%, the process of rolling groove and back grinding should be adopted. The following table shows the comparison of rolling crack rate of some specifications of drill bits processed by our factory with two different process schemes. It can be seen from the table that the groove rolling and back grinding process has a very obvious effect on reducing the rolling crack rate of the drill bit. After adopting this process, the qualified rate of drill core symmetry of our drill bit is increased from 75% to 80%, and the qualified rate of radial circular runout is increased from 85% to 90%

author unit: Harbin Measuring Tool and cutting tool factory

originally published in tool technology (end)

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