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Design Guide for general machining parts

Objective: to describe the design guide of general machining parts

A machined part must first meet the general machining requirements, and then meet the requirements of its own processing technology.
It should be noted here that in addition to the general design criteria of machining, different machining processes put forward different requirements for part design. The designer needs to understand different machining processes in detail, and can estimate the combination order of machining processes in the initial design. After the design is completed, it also needs to check with the manufacturer in detail. Optimize and complete machined parts before formal manufacturing.

Design Guidelines for General Machined Parts

Avoid machining as much as possible

The design of machined parts needs to consider the design requirements of the machining process for the part. Under the premise of meeting the function, appearance and reliability of the product, the design of machined parts should make the machining process simple, efficient, short processing cycle, low processing cost and high quality.
Due to the above-mentioned characteristics of machining, such as high cost of parts, low processing efficiency, and inability to process complex-shaped parts, machining is being replaced by other processing methods, such as injection processing, stamping processing, and die-casting processing, in more and more industries. Therefore, when designing machined parts, the first question that comes to the mind of the product design engineer is: Can other machining methods be used to replace machining? When possible, try to avoid the use of mechanical processing.
In this blog, some examples have been given to illustrate how to use injection processing, stamping, die-casting, etc. instead of machining.
This one is a very ridiculous one, but is listed in the first principle of machining design, because of the times. The author encountered a question about machining principles during an interview, and after answering this one, he felt a little embarrassed in the face of a bunch of old engineers.

Selection of blanks

A blank is a production object made according to the required shape and process dimensions of a part (or product) for further processing. The type, shape, size and accuracy of the blank has a direct impact on the machining process, product quality, material consumption, processing cycle and manufacturing costs. Therefore, in the design of the product, it is necessary to correctly select the type of blank and determine the shape of the blank.

Types of blanks

There are many types of blanks commonly used in machining, and there are many manufacturing methods for the same type of blank.
(1) Castings
The shape of complex parts blanks, it is appropriate to use casting methods of manufacture. Most of the current castings with sand casting, which is divided into wooden mold hand molding and metal mold machine molding. Wooden mold hand-shaped castings with low accuracy, processing surface allowance, low productivity, suitable for single-piece small batch production or large parts of the casting. Metal mold machine modeling high productivity, casting accuracy, but the high cost of equipment, the weight of the casting is also limited, suitable for mass production of small and medium-sized castings. Second, a small number of small castings with high quality requirements can be used for special casting (such as pressure casting, centrifugal manufacturing and fusion casting, etc.).
(2) Forgings
Mechanical strength requirements of high steel parts, generally to use forging blanks. Forgings are free forging forgings and die forgings of two kinds. Free forging forgings can be manually forged (small blanks), mechanical hammer forging (medium-sized blanks) or press forging (large blanks) and other methods to obtain. The accuracy of such forgings is low, productivity is not high, the machining allowance is large, and the structure of the parts must be simple; suitable for single and small batch production, as well as the manufacture of large forgings.
The accuracy and surface quality of die forgings are better than that of free forgings, and the shape of forgings can be more complex, thus reducing the machining allowance. The productivity of die forging is much higher than that of free forging, but it needs special equipment and forging die, so it is suitable for small and medium-sized forgings with larger batches.
(3) Profile
Profiles can be divided into: round bar, square bar, hexagonal bar, flat bar, angle bar, channel bar and other special cross-sectional profiles according to the shape of the section. Profiles have two types of hot-rolled and cold-drawn. Hot-rolled profiles have low accuracy but are cheap and used for general parts blanks; cold-drawn profiles are smaller in size and high in accuracy, easy to realize automatic feeding, but higher in price, mostly used for larger batch production and suitable for automatic machine processing.
(4) Welded parts
Welded parts are obtained by welding method, the advantages of welded parts are simple manufacturing, short cycle time, material savings, the disadvantage is poor vibration resistance, deformation, need to be processed by aging before mechanical processing.
In addition, there are other blanks such as stamping parts, cold extrusion parts and powder metallurgy.
In fact, after learning the advanced chapter: the selection of materials and processes.
chapter, we will understand that machining belongs to the secondary process, then the so-called blanks are the parts that are the products of the primary process: forming process. The sequence of secondary processes needs to be followed by primary processes. There are few formed parts that cannot be machined by machining, so it can be said that the process of the blank is close to infinite.

Functional principle

Functional principle requirements are specifically reflected in the working conditions of the parts, force and shape, size, accuracy and other aspects, only to meet the requirements of the use of the blank, the actual value. To ensure the use of functional requirements is the primary principle of the choice of blank. Such as the most commonly used gear parts, due to the working conditions, the use of different requirements, the type of blank, material selection and manufacturing methods are not the same.

  • 1) Agricultural machinery and construction machinery with gears: low-speed operation, the force is not large, meshing and vibration requirements are low, often using gray cast iron or alloy cast iron casting, no processing or simple processing can be used.
  • 2) Machine tool gears: smooth transmission, vibration, stable forces, speed change at rest, and require good lubrication, often choose carbon steel or low alloy steel forging and machining, and heat treatment (overall normalizing or tempering, high-frequency quenching of tooth surfaces) before use.
  • 3) Automotive gears: require high wear resistance and impact resistance, and dynamic under the variable speed, often choose a good hardenability of low-carbon alloy steel, such as 20CrMn and other materials, by forging, machining, carburizing, quenching forging, machining, carburizing, quenching and other processing.

Blank shape and size should be as close as possible to the shape and size of the parts

Due to the limitations of the blank manufacturing technology and cost, and parts of the machining accuracy and surface quality requirements are increasingly high, so some of the surface of the blank still need to leave a certain amount of parts machining allowance, in order to achieve the technical requirements of the parts through mechanical processing.
One of the trends in the development of modern machinery manufacturing is to refine the blank, so that its shape and size as close as possible to the part, so as to carry out a small amount of chip processing or even chip-free processing, reduce material costs and machining costs.

Try to use standard profiles

As long as the use requirements can be met, the parts of the blank as far as possible using standard profiles, not only can reduce the workload of blank manufacturing, but also due to the good performance of the profile, can reduce the cutting processing hours and save materials.

Consider the batch and production cycle

Production batch and production cycle have a great influence on the choice of blank type.
In general, single-piece, small batch production and short production cycle, should be selected commonly used materials, common equipment and tools, low precision and low productivity of the blank production method.
In mass production conditions, special equipment and tools should be used and high productivity of the blank production methods, such as precision castings, precision die forgings. This can make the manufacturing cost of the blank down, while saving a lot of metal materials, and can reduce the cost of machining. For example, in a lathe using 1 ton of precision castings can save 3500 man-hours of machining, with very significant economic benefits.

Consider combining multiple parts of the blank into a whole blank

In order to ensure the processing quality of the parts, easy clamping and improve the productivity of machining, often multiple parts of the blank into a whole blank, the blank of the planes are processed after cutting separation for a single piece, and then a single piece for processing.
For semi-circular parts should generally be combined into a full circle of the blank; for some small, thin parts (such as bushings, washers and nuts, etc.), can be a number of parts into a blank, to be processed to a certain stage and then cut and separated. As shown in Figure 6-6, the open and close nut shell in the lathe feed system is made into a whole blank, and then cut and separated after the parts are processed to a certain stage.

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Figure 6-6

The shape of the blank needs to consider the stability of the workpiece during machining

As shown in Figure 6-7, some casting blanks need to cast process bosses to ensure the stability of the clamp during machining, and the process bosses can be cut off after the parts are processed.
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Figure 6-7

Lax part tolerance requirements

The relationship between part tolerance and cost: the tighter the tolerance, the higher the cost, and this is especially true for machined parts. As the accuracy of the part increases, more precise machining processes are required, accompanied by a reduction in machining efficiency, so the cost of machining rises significantly.
Therefore, for machined parts, the more lenient the part tolerance the better, to avoid strict dimensional tolerance requirements. How to do this, you can start from two aspects:

  • 1) Start from the overall structure of the product: avoid the strict dimensional tolerance requirements of the parts by designing reasonable clearance, simplifying the product assembly relationship, using positioning features, and using point or line and plane fits instead of plane and plane fits, etc. See DFA chapter for details.
  • 2) Start from the parts: surfaces with low dimensional accuracy and surface quality requirements should not be designed for high precision and high surface roughness requirements; in addition, surfaces that do not need to be machined should not be designed as machined surfaces.

The principle of choosing the machining allowance between processes

  • a) Should use the minimum machining allowance, in order to shorten the processing time, reduce the manufacturing cost of the parts.
  • b) Should ensure that each process has sufficient machining allowance to ensure the accuracy and surface roughness required by the drawings in the final process.
  • c) Should take into account the deformation caused by the heat treatment of the part.
  • d) Should take into account the equipment and processing methods used to process the parts, as well as the possible deformation of the parts in the process.
  • e) Should take into account the size of the part being processed, the larger the part, the larger the required machining allowance should also be.

Select the principle of inter-process tolerance

  • a) Tolerances should not exceed the economic range of machining accuracy;
  • b) The choice of tolerance should take into account the size of the machining allowance, the limit of tolerance to determine the limit size of the machining allowance;
  • c) The choice of tolerance should be based on the final accuracy of the part;
  • d) The selection of tolerances should take into account the size of the production batch, and allow the selection of large values for a single small batch of parts.

Simplification of product and part structure

In the design of machined parts, not only from the perspective of individual parts to design, but also from the perspective of the product as a whole and the overall situation, taking into account the feasibility of mechanical processing of each part, processing efficiency and processing costs, etc., through a reasonable split and combination of products, simplify the product and part structure, making the product as a whole easy to mechanical processing, high processing quality, low processing costs.
As shown in Figure 6-9, in the original design, the shape of the parts is complex and time-consuming to process; in the improved design, the shape of the parts is simple, which helps to reduce processing costs.

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Reduce the difficulty of machining

Machining the inner surface is generally more difficult than machining the outer surface, so the part should be designed reasonably to reduce the machining difficulty. As shown in Figure 6-10, in the original design, the inner annular groove is narrower and more difficult to machine. In the improved design, the outer surface is machined instead, which does not affect the use and is easy to process.
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Position accuracy assurance

The surfaces with mutual position accuracy requirements should be machined in a single clamping, so that the position accuracy between the machined surfaces can be guaranteed and the number of clamping times can be reduced.
The part shown in Figure 6-12 has coaxial accuracy requirements for the inner and outer surfaces. In the original design, the outer surface and the inner hole were machined in two clamping operations, which made it difficult to meet the coaxial accuracy requirements; in the improved design, by adding a cam structure, the inner and outer surfaces can be machined in one clamping operation, which makes it easy to meet the coaxiality requirements and reduces the machining cost.

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// Here the author would like to give a reminder that many machining methods are very flexible, and one has to speculate on the machining process, but one also has to confirm with the supplier one by one, otherwise one will only design some useless features. For example, the author marked up the clamping surface in the picture above, but if the threaded hole is the initial clamping surface, then there is no need to optimize who, or, the outer surface is a long shaft of centerless grinding, that is another matter.
So in machining, the DFM must definitely: infer the machining process by itself → confirm by the machining manufacturer → optimize again by itself. At least a three-step approach.
20220414132507 84062 - Design Guide for general machining parts
In the part shown in Figure 6-13, the two inner circular surfaces on the left and right side have coaxiality accuracy requirements. In the original design, the part must be clamped twice and processed twice, so the coaxiality accuracy of the two inner circular surfaces cannot be easily satisfied; in the optimized design, the part can be clamped once and processed once, which is conducive to ensuring the coaxiality accuracy requirement.
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Dimension labeling for easy measurement

When labeling the machined parts, the labeled dimensions should be easy to measure. As shown in Figure 6-14, in the original design, the dimensional measurement reference is A surface, inconvenient to measure; in the optimized design, the dimensional measurement reference is B surface, easy to measure.
// There is nothing wrong with this design guideline, but care should be taken when using it in practice. The development of the benchmark is most important based on the design requirements, followed by taking into account the requirements of manufacturing, assembly, testing, etc.. The design requirements should not be discarded simply because of testing.
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Ensure the quality of parts after heat treatment

In the design of machined parts, you need to consider the quality of subsequent processes (such as heat treatment, etc.). Sharp edges and sharp corners of parts are prone to stress concentration during quenching, resulting in cracking. Therefore, the root of the shoulder of a heavy stepped shaft should be designed to be rounded before quenching, and the shaft end and shoulder should be chamfered, as shown in Figure 6-15.
// Avoid sharp corners, this one in the injection molding, sheet metal, die-casting, machining and other design requirements are very important one, but often ignored by the designer.
The wall thickness of the part is not uniform, and it is easy to produce deformation during heat treatment. As shown in Figure 6-16, an additional process hole is provided to make the wall thickness of the part uniform and to prevent deformation during heat treatment.
// Similarly, the wall thickness is uniform, is also an important one. In the process of machining with a mold is even more important.
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The part structure should have enough rigidity

Part structure should have enough rigidity to reduce its deformation under the action of clamping force or cutting force, to ensure processing accuracy and processing quality.
As shown in Figure 6-17, in the original design, the part wall thickness is thin, easy to deformation due to clamping force and cutting force; in the optimized design, after adding the flange, the part stiffness is improved, not because of clamping force and cutting force and deformation.

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Pipe sleeve

// Pay attention to the bearing of some machining process.
As shown in Figure 6-18, in the original design, the box structure stiffness is poor, planing the upper plane is easy to cause deformation of the workpiece due to cutting force; in the optimized design, after increasing the rib plate, improve the stiffness, you can use a larger depth of cut and feed processing, easy to ensure the quality of the processed workpiece and improve productivity.
20220414132835 70674 - Design Guide for general machining parts
// Feel that this can increase the strength of the part more, in order to manufacture is secondary to the purpose. Also, planing?

Adopt standardized parameters

Try to use standardized parameters, the same kind of parameters as much as possible consistent although to parts of the hole diameter, taper, threaded hole diameter and pitch, gear modulus and pressure angle, radius of the arc, groove and other parameters as far as possible to use the value recommended by the relevant standards, so that you can use standard tools, clamps, gauges, reduce the design of special tooling, manufacturing cycle and manufacturing costs.
As shown in Figure 6-19, standard parameters are used in the design of threads so that standard taps and slabs can be used.
20220414133134 27580 - Design Guide for general machining parts

As shown in Figure 6-20, in the original design, the width of the undercut on the shaft is inconsistent. During turning, it is necessary to prepare and replace the grooving tools with different widths, which increases the times of tool change and tool setting; In the optimized design, the width of these slots is changed to the same size, and all slots are processed with one tool, which reduces the types of tools and the number of tool changes and saves the auxiliary time.

20220414133210 86768 - Design Guide for general machining parts

As shown in Figure 6-21, the transition fillet on the shaft shall be as consistent as possible to facilitate processing.

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As shown in Figure 6-22, if the design keyway dimensions are consistent, all keyways can be processed with the same tool.

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As shown in Figure 6-23, threaded holes with similar dimensions on the same end face are changed to threaded holes with the same size to facilitate processing and assembly.

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Parts should be easy to clamp

Parts should be easy to clamping, that is, accurate positioning, reliable clamping, which can simplify the parts clamping time, improve processing efficiency, to ensure the quality of processing.

20220414134503 45954 - Design Guide for general machining parts
Figure 6-24 shows the bearing cover, to process the outer circle and end face, in the original design, if clamped at A, the general length of the jaws is not enough, B surface and inconvenient clamping; in the optimized design, the original tapered surface B surface into a cylindrical surface C surface, it can be easily clamped in C surface, or add a process cylindrical surface D for clamping.

20220414134513 94478 - Design Guide for general machining parts
As shown in Figure 6-25, in the original design, when the part is clamped on the three-jaw chuck, the part is in point contact with the jaws, and the workpiece cannot be clamped firmly; in the optimized design, by adding a section of cylindrical surface, the contact area between the workpiece and the jaws is increased, and the clamping is easier, and the clamping reliability can be improved.

20220414134526 86531 - Design Guide for general machining parts
As shown in Figure 6-26, in the original design, the large flat workpiece is not easy to be clamped during processing; in the optimized design, the clamping process flange or process hole is added for clamping with screws and pressure plates, and it is easy to lift and carry.
Planing larger parts, often the parts directly clamped on the table, in order to ensure that the processing surface level in order to facilitate the clamping and positioning, can be added to the parts on the process tabs, if necessary, after the finish machining to remove. As shown in Figure 6-27, in the original design, there is no process tab, and the part is difficult to be clamped and positioned; in the optimized design, the process tab is added to the part, and it can be accurately clamped and positioned.
 20220414134538 54373 - Design Guide for general machining parts

Reduce the number of clamping operations

Minimize the number of clamping, reduce clamping errors and reduce auxiliary work time, improve cutting efficiency and ensure accuracy.

As shown in Figure 6-28, in the original design, the inclined machining surface increases the number of clamping times; in the optimized design, the inclined machining surface is changed to a horizontal surface, and several surfaces can be machined simultaneously in one clamping.

20220414134615 28555 - Design Guide for general machining parts

As shown in Figure 6-29, before the optimization, two clamping times are required, but after the optimization, only one clamping is needed to grind two surfaces.

20220414134625 66894 - Design Guide for general machining parts
As shown in Figure 6-30, in the original design, the keyway on the shaft is not in the same direction, which requires repeated clamping and tool setting during milling; in the optimized design, the keyway is arranged in the same direction, which can reduce the number of clamping and adjustment, and also ensure the position accuracy.

20220414134637 80161 - Design Guide for general machining parts
As shown in Figure 6-31, in the original design, the machining of the A and B surfaces need to be adjusted separately; in the improved design, the height of the A and B surfaces is changed to the same, so that the machining of the A and B surfaces can be completed in one adjustment of the machine.

20220414134714 90478 - Design Guide for general machining parts
As shown in Figure 6-32, before the optimization, the tool needs to be fed from both ends separately, which requires two clamping, after the optimization, only one clamping is needed to complete the machining of the two inner surfaces.
20220414134745 66145 - Design Guide for general machining parts

Reduction of machining area

By reducing the machining area, the machining time can be reduced, and the machining cost can be reduced.
//This is very effective for the blank parts of die-casting and other processes.

20220414134813 29739 - Design Guide for general machining parts
As shown in Figure 6-33, in the original design, the machining area of the bottom surface of the support part is large; in the improved design, the machining area is reduced by the optimized design, thus reducing the machining volume and tool consumption, improving the machining efficiency and reducing the machining cost.

20220414135220 59719 - Design Guide for general machining parts
Figure 6-34 and Figure 6-35 show two other examples of how the part design was optimized to reduce the machined area by adding tabs or recesses.

Reducing the number of tool passes

By reducing the number of tool passes, the machining cost can be reduced by reducing the number of machining hours.
As shown in Figure 6-36, in the original design, the table is raised or lowered one by one when machining tabs of different heights, increasing machining time; in the optimized design, several tabs are designed to be of equal height, so that all tabs can be machined in one tooling run, increasing productivity and making it easy to maintain relative positional accuracy.

20220414135311 59090 - Design Guide for general machining parts

The structure of the part should be easy for the tool to work

The structure of the part should be easy for the tool to cut in and out correctly.
As shown in Figure 6-37, in the original design, the distance between the hole and the vertical wall of the part is too close, resulting in interference between the drill chuck and the vertical wall, which can only be used with non-standard extension bits and poor tool rigidity; in the optimized design, the distance between the hole and the vertical wall of the part is increased, so that a standard tool can be used, which can ensure machining accuracy.

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As shown in Figure 6-38, there should be an empty tool slot when shaping, so that the large gear can be hobbed or shaped, and the small gear can be shaped.

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As shown in Figure 6-39, when planing, the front end of the plane should have the part to let the tool, that is, let the tool slot.

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As shown in Fig. 6-40, when grinding, the transition part between the surfaces should be designed with overrun grooves.

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Clear separation of surfaces with different requirements

The surfaces with different requirements for machining and the machined surfaces should be clearly separated from the non-machined surfaces in order to improve the working conditions of the tool.

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As shown in Figure 6-41, in the optimized design, the bottom surface of the groove does not overlap with other machined surfaces, so that the part can be easily machined and damage to other machined surfaces can be avoided.

Source: China Machining Solution Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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