Honing Principle And Selection Of Process Parameters

1. Introduction

In honing, the selection of honing process parameters has a great impact on the accuracy of machined holes, surface roughness, machining efficiency and the service life of honing oilstone.

2. Honing principle

Honing is to use the principle of mutual grinding of three flat plates to process a precise surface. In grinding, the cutting surface of honing oilstone and the surface of machined parts are regarded as a process of mutual trimming of flat plates.

3. Dressing of honing oilstone

Due to the manufacturing errors of honing oilstone, oilstone seat and grinding head body, it is impossible for the honing oilstone of honing head to form a normalized discontinuous cylindrical surface after assembly, so as to ensure good contact between honing oilstone and machined surface. Although the honing oilstone can be trimmed with the workpiece in the honing process, the grinding amount of the workpiece is small, so it is impossible to get sufficient trimming in the initial honing process. Especially the honing oilstone with superhard abrasive can not be fully trimmed because of its wear resistance. Therefore, it is impossible to obtain an ideal machined surface in machining, and the accuracy can not be guaranteed. Therefore, when a new honing stone is used, the honing stone must be repaired (also known as rounding) before machining. The dressing of ordinary honing oilstone is to directly install the honing oilstone on the grinding head used and take it to the cylindrical grinder for rounding, which is the most ideal. However, due to the structure of some grinding heads and other reasons, it is necessary to use special fixtures to trim their outer diameter with grinding wheels on the cylindrical grinder. If the accuracy requirement of honing workpiece is low, the honing head is floating connection, or the workpiece hole with waste or large machining allowance can be used to directly correct the rounding on the honing machine tool used. The dressing of superhard honing oilstone can be carried out on the cylindrical grinder with carbonized silica sand wheel. The grinding wheel speed is 18-25m / s, the grinding head speed is 1-3m / min, and the feed depth is generally 0.02-0.04mm/stroke for grinding and 0.01mm/stroke for finishing. At the same time, a large amount of coolant is required.

4. Constant pressure expansion feed form

In the constant pressure expansion and feeding, although the honing head expansion and contraction mechanism presses the hole wall of the workpiece with a constant honing oilstone working pressure, in the grinding, with the increase of time, various elements are not cut with a fixed value, but the amount of metal grinding and honing oilstone grinding loss gradually decrease with the increase of honing time, The surface quality becomes smooth with the increase of honing time. The reason for this situation is that honing and cutting are in surface contact, and the constant pressure expansion feed has strong follow-up. In grinding, the size of cutting amount depends on the abrasive particles and their sharpness, as well as the removal of abrasive particles and cutting under grinding.

5. Selection of honing process parameters

5.1 selection of cutting speed and cutting intersection angle

5.1. 1 speed and cutting cross angle

Cutting speed V is composed of rotation (circumferential) speed V and reciprocating speed v. In the process of honing, as a result of cutting movement, the abrasive particles of honing oilstone cut cross reticulation on the machined surface, and the angle formed by reticulation is called cutting cross angle θ 。 The angle between the rising anilox and the horizontal line is called the cutting rise angle α 1. The angle formed by the falling anilox and the horizontal line is called cutting angle reduction α 2。

5.1. 2 Calculation of honing cutting speed

The honing cutting speed can be calculated according to the following formula: V rotation = π d n / 1000 (M / min), where: D - machined diameter mm n - spindle speed r / min

5.1. 3 Effect of cutting speed on cutting quantity

In honing, the cutting speed has little effect on the cutting amount (cutting amount / work cut - indicating the sharpness of honing oilstone). No matter honing castings or honing steel parts, the specific cutting amount changes little with the change of cutting speed.

5.1. 4 Influence of cutting cross angle on cutting amount, honing oil stone wear amount and surface roughness

In honing, the cutting cross angle has a great influence on the cutting amount, honing oil stone wear and surface roughness. With the increase of cutting cross angle, the reduction of specific grindstone increases. Of course, the roughness of the machined surface also coarsens. This is because in the honing and cutting process, maintaining the sharpness of the honing oilstone depends on the machined surface roughness. Only the cutting amount of the sharp honing oilstone can be improved, and the oilstone must be worn if it is sharp. When the cutting cross angle is 0 °, the cutting efficiency is low; When the cutting cross angle is 90 °, the honing oilstone is worn greatly and the machined surface roughness is coarse, which makes the cutting amount small. Through experiments, when the cutting cross angle is about 45 °, the cutting amount is the largest. Therefore, in honing, the cutting cross angle of about 45 ° should be used to improve the productivity, and 20-30 ° can be used in finishing. 5.2 working pressure of honing oilstone and its influence

5.2. 1 working pressure of honing oilstone

The so-called honing oilstone working pressure refers to the pressure of honing oilstone on the machined surface - the pressure per unit area. Not the value on the machine pressure gauge or other pressure values. This paper discusses the honing machine tool with constant pressure expansion feed. The working pressure of honing oilstone can be calculated according to the following formula (regardless of friction and other forces). P = (d2πp/4Bln )ctg β/ 2 where p is the working pressure of honing oilstone, kgf / cm2 ≈ bar, B is the width of honing oilstone, M M L is the length of honing oilstone, m n is the number of honing oilstones

5.2. 2 selection of working pressure of honing oilstone

The working pressure of honing oilstone has a great influence on honing cutting performance, which directly affects cutting efficiency, honing oilstone wear loss, workpiece accuracy and roughness. When the honing working pressure is large, the honing cutting amount and honing oil stone wear amount are large, and the machining accuracy and surface roughness are also poor. When the honing working pressure is small, the cutting amount and honing oil stone grinding loss are small, and the machining accuracy and surface roughness are good. When selecting the working pressure of honing oilstone, in addition to the above conditions, it should also be considered according to the power of the machine tool used, the rigidity of honing head and honing fixture, the strength of honing oilstone and honing; The actual contact surface between the oilstone and the machined surface, as well as the material, size and shape of the workpiece shall be fully considered. Generally, the honing oilstone working pressure of production honing machine can be selected according to table 1. For repair honing machine, due to its low power, the rigidity of honing head and fixture is relatively low, and the working pressure of honing oilstone should be reduced, generally 2-5 kgf / cm2.

6. Calculation and adjustment of honing stroke

In order to process holes with the same diameter and good cylindricity, the honing stroke and the corresponding overtravel must be adjusted. As shown in Figure 2, if the honing oilstone length is 1, the hole length is l, the stroke length is L1, the overtravel of the upper end is L2 and the overtravel of the lower end is L2, the honing working stroke length is calculated according to the following formula: l = L + L1 + L2 - l the small overtravel L1 and L2 of the honing oilstone in the machined hole are generally 1 / 3-1 / 5 of the honing oilstone length. It is related to the support point of the oilstone seat of the honing head, the reciprocating motion of the honing machine tool, the residence time at both ends, and the material and shape of the processed parts. When the overtravel of one end is large, the workpiece hole is easy to produce horn mouth shape: when the overtravel of one end is small, the workpiece hole is easy to produce small mouth; If the overtravel amount at both ends is large or small, the workpiece hole is easy to produce waist drum shape: if the overtravel amount at one end is large and the overtravel amount at one end is small, the workpiece hole is easy to produce taper. Therefore, after the stroke adjustment machine tool is calculated according to the above calculation formula, it is also necessary to carry out trial honing, and then readjust the overtravel amount according to the actual size of the measuring hole until it is qualified. Honing is a commonly used finishing method in the process. This processing method has been widely used in the mechanical manufacturing industry. Through the understanding of honing principle and process parameter selection, it can help technicians in the same industry to master the key points of the processing technology well and faster, and learn and discuss it together in the future practical work.