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4.3 SELECTION OF POSITIONING REFERENCE(DATUM)
Those points,lines or surface of the part which are used to define the position of other points, lines or surface of the part are called the positioning reference datum
4.3.1 Classification of positioning reference
Positioning reference can be classified into two categories according to application purpose
4.3.1.1 Design reference
The points,lines or surfaces that are used by designers to determine the dimensions or relative positions of other points,lines or surfaces are called the design reference.For example,in Fig .4-3(A),section C is design reference of side A and B,the center line O-O is the design reference of the cylindrical surface ФD and Фd ,and the center point O is the design reference of surface E.
4.3.1.2 Process reference
The reference which is used in the process of operation or assembling is called the process reference or manufacturing reference. According to its application purpose, it can be further classified into 4 subcategories:
(1) Operation reference
In engineering drawings, the points, lines or surface that are used to determine the operation dimension or positions relative to the machined surface are called the operation reference.For example .the part to be machined is shown in Fig 4-3(a),if Fig ,4-3(b)is the operation drawing for machining surface B, and the operation dimension is l4,then the operation reference is side surface A, while the design reference is side C
(2) Positioning reference
The positioning reference is defined as the points, lines or surface which are used to determine the location of points, lines or surfaces that are to be machined. The process that determines the location is called positioning. For example ,in Fig.4-3(c),the positioning surface which is used to machine surface E is the cylindrical surface of Фd by using a V-shaped block ,while the positioning reference is the axis line of Фd. When the part to be machined is axial, a centered hole is usually used as the positioning reference, and if the surface to be machined is the cylindrical surface or teeth of a gear, its inner hole or end surface is usually used as positioning reference. As a surface is usually used as positioning reference , positioning reference is also called positioning surface, and the letter “v” is usually used to represent it, with the bottom of the V directed towards the positioning surface. Figure 4-4 is an example to show the way of describing a positioning surface when a gear is to be machined.
(3) Inspection reference
The points, lines or surfaces which are used as reference to inspect the location of machined surfaces are called the inspection reference. In most cases ,the design reference is used as the inspection reference. For example, in Fig. 4-3(a),when side A and B are to be machined, and dimensions l1and l2 must be satisfied, the inspection reference is the design reference of side C. But if the design reference is inconvenient or impossible to be used as an inspection reference, other surfaces can also be used as the inspection reference. For example,in Fig 4-3(d),the design reference of surface E is the central point O, while the inspection reference is the cylindrical line F of external cylindrical ФD, and the inspected dimension is l.
(4) Assembling reference
In the process of assembling, the points, lines or surface which are used as a reference to determine the locations of the parts or components relative to the product are called the assembling reference. For example, when a gear is to be installed on shaft, its hole is used as the assembling reference, whereas, if the shaft is to be installed in the hole of a box, the support part of the shaft becomes the assembling reference. When the spindle box is to be installed onto a machine bed, the base of the box becomes the assembling reference.
4.3.2 Clamping method and means to obtain the desired precision
4.3.2.1 Positioning and clamping by direct location
When putting the workpiece directly on the machined bed, the operator can locate the machined surface by using a micrometer inspection instrument, marking dish or straight rules to adjust and find the desired machined position relative to the cutting tool.
For example, in Fig.4-5,when a gear is to be machined in a large gear machine tool, if the index circle of the gear to be machined has a very high center position precision requirement relative to the machined external cylindrical surface, when the workpiece is placed on the support base, a dial gauge can be used to adjust the location so as to make both the centers of the gear blank and the rotational worktable fall onto the same point before clamping. This kind of clamping method has many shortcomings such as difficult location, time consuming, and precision dependence both on operator’s experience and the precision of inspecting instrument. It is usually used in single or small batch type production, or in the condition of high precision machining requirement to relative position when it is hard to meet by a fixture.
4.3.2.2 Clamping according to the pre-marked trail of outline
Before the work is machined, the outline of the surface to be machined on the raw material is drawn beforehand, then located and clamped on the machine tool according to the predrawn lines. When drawing the outline to be machined, the relative position among different surfaces must be taken into account and sufficient metal removal must be ensured.
This kind of clamping method is widely used in single or small batch type production, especially in machining large geometrically complicated rough castings or forgings. The limitation for this method is that one more operation of outline drawing is added, also, as the line itself has a definite width, and the error of the operation itself exists, the clamping precision is low, usually in the range 0.2~0.4mm.
4.3.2.3 Clamped with a fixture
A fixture is clamped in the machine tool. The fixture itself is a device to make the work located and clamped. After the work clamped, it achieves the proper position in relative to the cutting tool.
This kind method is convenient ,fast precise and stable, so is widely used in batch type production and large quantity production. For example, in Fig.4-1,it is an operation of matching a key in a stepped shaft. The workpiece is the directly put onto the V-shaped block of the fixture, and it can achieve the right position relative to the cutting tool without manual locating. Forced down with a clamping board, the workpiece can be machined. For some parts in small batches, such as the connecting rod or crank shafts, even though the batch quantity is not large, some special fixtures are still required in order to meet the special machining requirement.
Obviously, the relative positional precision among different surfaces in mechanical machining, such as parallel ,the perpendicular and coaxial degrees of accuracy, can be obtained with the above clamping method using several clamping steps, and also it can be obtained by arranging the different surfaces to be machined in the same clamping. These two methods are both commonly used to obtain the relative positional precision in mechanical machining. There are four kinds of methods to obtain the dimensional precision in mechanical machining；
(1) Trial cutting method-firstly, a tiny part of the surface to be machined is pre-cut. Secondly the dimensions are inspected. Thirdly, proper adjustment is made according to the desired requirement, and try cutting is tried again. Fourthly, this is inspected again and adjusted further till the desired requirement is satisfied. This process is usually repeated twice or three times to meet the final need. Finally, the whole surface is machined according to the final adjustment.
(2) Setting dimension method-The precision of the machined surface is ensured by applying a cutting tool with the required accuracy, such as a reamer, twist drill or core drill.
(3) Adjust method-precision is ensured by the cutting route setting device or preadjusted carriages.
(4) Automatic control method-some special automatic control device are used to sure the precision. The specific methods include:
①Automatic inspection- the inspection device is integrated into the machine tool. when the dimension are satisfied, an instruction will be sent from the automatic inspection device to make the cutting tool recede or stop.
②numerical control –a stepping motor, rolling screw or a whole numerical controlled device is used to control the movement of cutting tool or worktable precisely. The dimensions are achieved by pre-programmed instructions to control the movement of carriages or worktable through a computer controlled numerical device.
There are three kinds of methods to obtain the desired geometrical shape by mechanical machining:
(1) Path method---the geometrical shape of the part is formed by the movement of the cutting tool tip. The precision of this method is mainly dependent on the precision of the cutter’s movement .
(2) Forming method- the geometrical shape of workpiece is an exact counterpart of the cutting tool. The precision of this machining method is mainly depended on the geometrical precision of the cutting tool and its clamping precision.
(3) Generating method---the shape of the workpiece is formed by synthesis of two movements, one of which is the movement of the cutting tool, and anther is the movement of the workpiece. The surface formed by the path of the cutting edge moved on the work surface is the generated surface. The precision of this machine method depends on the precision of machine transmission and the manufactured precision of the cutting tool.
4.3.3. Selection of locating surface
The function of a locating surface is to serve as datum or dimensional reference for the manufacturing of features in the part. Their selection is of primary importance for the execution of correct process operations and the fulfillment of dimension requirement. The order of precedence in choosing datum surfaces has already been considered in 4.3.1.2,However ,special considerations may be necessary when selecting datum surfaces in relation to location requirements.
There are threes kinds of locating surfaces, which are called the first datum surfaces, the accurate datum surfaces and the auxiliary datum surfaces. The general rules applied in these cases are now briefly reviewed. The most commonly used criteria in these cases are as follows:
1.The first datum surfaces, also called starting surfaces, make use of the surfaces existing on the raw blank. They should be of sufficient size to ensure stability and be free of holes and slots. In many cases, the drawings give indications called “l(fā)iaison with raw material”, which are a great help when positioning the first surfaces to be processed in relation to the raw material. As a sequence, different liaisons with the raw blank can result in dimensional conditions on the raw part.
2. The datum surfaces defined by accurate dimensions should be given priority to avoid the stacking up of tolerances in following phases.
3 The choice of a hole as a datum is less accurate than the choice of a plane
4 The stability of fixturing has to be guaranteed against the actions of cutting forces and other external disturbances. Fastening on surfaces with defects or welding joints should be avoided.
5 Good accessibility to the machine tool is necessary so that the part can be positioned in its fixture properly.
6 Finally, the choice of fixture should be made on economic grounds. Where possible, modular elements should be used which can be combined in suitable clamping devices.
4.3.3.1 The criteria of selecting the accurate datum surfaces
When selecting the accurate datum surfaces, some factors, such as convenience, accuracy and stability must be taken into consideration. The following general principle should be followed.
(1) Principle of coincident locating surfaces
Select the operation datum surfaces (or design datum surface) as locating datum surfaces as best as possible .It can reduce the manufacturing error caused by inaccurate location.
Figure 4-7(a) is an operation drawing which shows a hole being machined in a drilling machine organized in batch type production. Surface N is the operation datum surface for dimension B. If surface N is selected as the locating surface to machine surface B, contacts with surface 1 on the fixture, and the position of the twist drill in relation to surface 1 is adjusted and kept fixed(shown in Fig.(b)),then dimension B is not affected by the variation of dimension A. Therefore, the precision of dimension B is increased. Otherwise, if surface M is selected as locating datum surface and contacts with surface 2 on the fixture, dimension B is affected by dimension A. It will cause the precision of dimension B decreased
（2）Principle of locating datum surfaces unchangeable
Make different operation datum surfaces the same as far as possible. For example, in shaft maching, the two centered holes at the two ends of the shaft are often used as the datum surfaces for most operation. In gear machining, the inner hole and one end surface of the gear are often used as the main operation datum surfaces. In box part machining, if the patch is of large quantity, a plane and two pin holes are used as the common locating datum surface for most operations.
The advantage of datum surfaces remaining unchanged is that different operations can be carried out using the same fixture. Thus it reduces the time and cost of fixture design and manufacture, speeds up the process of production preparation, and reduces the production cost. If most surfaces use the same group of datum surfaces, the error caused by datum surfaces transition will be reduced, and the position precision among different surfaces can also be ensured.
(3)Principle of being datum surfaces with each other and machining time after time
If the position precision requirement between two surfaces is high, it can be ensured by using this principle. For instance, as the coaxial degree between inner and out cylindrical surfaces is required to be very high when a sleeve is machined, the external cylindrical surface is usually machined first by using the inner cylindrical surface as the datum surface, and then using the external surface as the datum surface to machine the hole . After machining several times, the coaxial degree can be greatly improved.
(4)Principle of self-datum surface
When the material removal is small and uniform during precise or finishing machining, the machined surface itself can be selected as the precise datum surface to ensure the manufacturing quality and raise the product efficiency. For instance, in finish reaming a hole, the reamer and the spindle are connected in floating way. The hole itself is the datum surface. By using the machined surface itself as the datum surface, the relative positional precision among surfaces can not be raised but can be ensured by preceding operations.
(5)Principle of stable and simple clamping
Ususally, some surfaces with large area, high precision and low roughness are used as the precise datum surfaces.as most assembly surfaces have large area and stable and convenient clamping, they are often used as precise datum surfaces in box-like or bracket-like parts . For instance, an operational sketch for machining the headstock of a machine tool is shown in Fig .4-8. The general machining sequence is, machining the assemble datum surface A first, then machining the spindle hole B and other inner surfaces by using surface A as the datum surface.
4.3.2.2 selection of first datum surface
In the first operation, the datum surface can be nothing but a rough surfaces, which is also called first datum surface. Some points must be taken into account when selecting first datum surfaces;
(1) Select those surfaces, which have small and uniform material removal, as the first datum surface so as to ensure sufficient and uniform material removal when machining the surface itself.
For instance, the guide way surface is the main working surface in machine tool. In order to keep the uniform and fine-grained metallurgied structure produced in casting to enhance its wear-resistant capability, the guide ways surface itself is often selected as the first datum surface. In addition, small and uniform material removal makes the cutting force small and uniform, as well as causing less vibration. This raises the geometrical precision and reduces the surface roughness. In fig 4-9(a),the guide way surface is first used as the datum surface to machine the bottom surface of the lathe bed, then the guide way surface is machined by using the bottom surface as the datum surface to ensure small and uniform material removal, which is shown in Fig 4-9(b), If the bottom surface were first used as the datum surface, which is shown in Fig.4-9(c),the error of the bottom surface would be fully reflected onto the surface of the guide way so that the metal removal would be less uniform.
(2) Select those surfaces, which do not need machining but have position precision in relation to machined surfaces, as the first datum surface.
For instance, in fig.4-10(a) ,in order to ensure the uniform thickness of the flange of the pulley wheel, select the free machining surface 1 as the first datum surface to machine the external cylindrical surface. Another example is shown in Fig.4-10(b),where in order to ensure the uniform wall thickness of the part, the external cylindrical surface A should be selected as the first datum surface to bore the hole.
(3) Select a smooth, flat and large area surface as the first datum surface to ensure safety, security and small error instead of surfaces with voids and flanges caused by casting.
(4) First datum surfaces can be used only once and only in the first operation. Do not reuse it as far as possible, because both its precision and finish are low. It is hard to ensure the consistency of workpiece position in relation to the cutting tool when reusing the first datum face, thus it willaffect the precision of manufacture.
The principles mentioned above can only be considered one at a time. In practice, some times it is impossible to give attention to two or more things. A comprehensive consideration should be taken into account according to production type, process route and specific condition to make the correct decision.
4.4 DESIGN OF PROCESS PLANNING
When designing the process plan for part, the main problem involves the selection of the machining method, the selection of manufacturing equipment, the establishing of the process phases, the arrangement of centralized or decentralized process and the sequence of operation,
4.4.1 Selection of machining method and manufacturing equipment
4.4.1.1 Selection of machining method
(1)The economical precision and surface finish for different machining methods
Different machining methods have been used for different purposes, such as turning, grinding, planning, milling, drilling or boring. The precision and surface finish which can be achieved by different machining methods are also different. Even with the same machining method ,the surface finish and precision may be different under different machining conditions. This is caused by many factors in machining, such as the operator’s skill, cutting variables, quality of cutting tool resharpening or the adjustment of the machine of the machine tool etc.
According to the statistic information, the relation between a machining method and machining error is shown in Fig4-11. In segment 1 of the curve, when the required machining precision is very high, the cost of the part will be greatly increased until such a threshold when the cost rises but the precision can not be improved further. There exists a precision limit, whose