I worked as an engine designer at a certain automobile manufacturer for 15 years (I also researched things like design reform).

I regret not being able to fully pass on my knowledge, experience, and wisdom to my juniors because I retired after 15 years.

However, since I've left the company, I can no longer teach directly, so I would like to use the internet to widely share my experience with young engineers around the world.

Let's get back to the main topic: the drawings.

As of the time of writing (early 2021), design drawings are probably no longer 2D, but mostly 3D, and I think 3D drawings have become the mainstream.

However, the author"Mechanical design cannot be done with 3D drawings alone.I think

There are several reasons, but the main difference is that 3D can represent things clearly (mainly shape), while 2D drawings can represent things clearly (tolerances, roughness, manufacturing information, etc.).

In other words, 3D and 2D are not in conflict; they should simply be used effectively together.

SoThe ability to create 3D drawings and 3D models freely, and to read and draw 2D drawings, is absolutely essential.I think it will be like this (one side alone won't work).

Therefore, based on my own experience, I would like to start by introducing a practical way to view basic 2D drawings.

What I want to emphasize here is that while university textbooks and other specialized books tend to start with details like "the outline should be 6 mm," I want to introduce a more macroscopic perspective and a way of thinking that can be immediately applied in practice.

Drawing structure

First, let's look at the structure of drawings based on JIS standards.

I think many of them are made up of basic forms.

Normally, you'd want to see the picture (shape) right away, but please be patient.

There are many items that you can easily make a terrible mistake on if you don't check them before looking at the picture (shape).

So firstWhere to beginShould I watch it?

that isLook at the frame in diagram ①! ! Never look at the shape with dimensions first.

I will explain the reasons in order.

Regarding the diagram frame

First, why start with the diagram frame?

that isThe size of the drawing frame allows us to roughly grasp the scale of the equipment needed for manufacturing, the size of the molds, and the manufacturing process, based on the size of the object being designed.

To put it in extreme terms, in the past, only the size of the drawing mattered.Understanding the overall scale is so important that it's said to determine the approximate cost.

For example, from the manufacturer's perspective:The size of the frame in the drawing can determine the size of the mold to some extent for casting, forging, and injection molding, and it can also give an idea of ​​the scale of the processing equipment and processes involved in machining.

When I first joined the company, the cost would vary depending on the size of the drawing, even if the content was the same, and my superiors would dislike me just for having a large drawing frame.

In other words, by examining the drawing frame, one can roughly grasp the approximate size of the parts in the drawing, the scale required for manufacturing, and the manufacturing method.

Just to clarify, if the drawing frame is based on JIS standards, the sizes are fixed as A0, A1, A2, A3, and A4 (I believe this is the same for ISO standards outside of Japan).

Now, the frame of the diagramNext, we'll look at ② the title section!

Title block

First, let me give you a general explanation of what is written in the title field.

I think 99% of decent Japanese companies are the same.

(There are many others, but I think the five I've mentioned are definitely relevant.)

Regarding point ⑤, the rules vary greatly from company to company, and this information is somewhat different from understanding the drawings, so we will only touch upon it briefly in this explanation.

① Part name

Parts nameThe reason why this is important is,A single word that describes what the drawing represents.That's why.

One thing to pay particular attention to is when the part name includes terms like "COMP" or "ASSY." If these terms are present in the part name, the drawing is not a simple part diagram.

In the first place, there are many types of drawings, such as part drawings that show all the dimensions in detail, COMP drawings that show the joining (press-fit, welding) of two or three parts, ASSY drawings that show how many parts are assembled, and assembly drawings that show how the entire product is assembled. The content of each drawing differs depending on its purpose.

Therefore, before looking at the shape and dimensionsIf you don't always check the part names, you won't understand the purpose of the drawing, which will lead to confusion.

For example, if you mistake a COMP drawing for a part drawing without checking the part names, you'll be surprised to find that the dimensions listed are significantly fewer than those in a typical part drawing.

Furthermore, in many cases, there are rules for naming parts according to each company's own standards, so it is generally clear what role and purpose each part serves.

I think you now understand the importance of knowing part names.

② Part number

The next thing I want you to look at is the part number (often abbreviated as "part number").

In fact, part numbers contain even more information than part names.

As far as I know from my experience, the information hidden in the part number is

So much information can be found out.

Naturally, each company has its own strict rules for determining part numbers, and these are top secret.

However, once you learn the system within your own company, other companies in the same industry tend to use similar system definitions, so you can generally understand the meaning of other companies' department numbers.

Therefore, when looking at drawings, if they are your own company's drawings, make sure you thoroughly understand your company's part numbering system before reading them.

If it's a drawing from another company, I think it's a good idea to ask them to tell you the rules for part numbers as much as possible while keeping it confidential, and then look at the part numbers (if you have a confidentiality agreement, they should tell you quite a lot of details).

Understanding part numbers in this way provides essential background information for interpreting drawings, allowing you to read them more accurately and without misunderstanding.

Furthermore, if you understand the rules for part numbering, it can sometimes be easier to understand than the part name.

The author can understand roughly what a part is, where it should be installed, and how it should be secured, based on its part number.

Also, many other companies' part numbers follow similar rules, so while it's not perfect, it's understandable.

Incidentally, when you join a company as a designer, one of the first things you're drilled on is the rules of department numbering, which shows just how important they are. It's extremely important; designers can't do their job if they don't know the department numbers.

③Materials

IngredientsThis may seem obvious, but it's quite important; you can learn a lot by looking at the material before looking at the shape.

If the materials are selected appropriately, it is possible to determine how much load a component depicted in a drawing will experience during operation.

For example, for parts subjected to high loads, iron-based materials, special forged aluminum, or titanium are often chosen, while for parts subjected to light loads, aluminum alloys, magnesium alloys, sheet metal, and resins are more commonly used.

In other words, if you know the material, you can predict to some extent the conditions (harshness) in which the part will be used.

Knowing the part name and material beforehand, and understanding the context in which the part will be used, is quite important because it allows you to roughly grasp the dimensions of the part shown in the drawing.

On the other handFrom the manufacturer's perspective, it's safe to say that more than 6% of the manufacturing process is determined by the materials and heat treatment listed in the ingredients section..

For example, with high-carbon steel like SCM420, the process involves forging and heat treatment (carburizing, nitriding, etc.), and with aluminum, if you use duralumin (7000, 5000, 2000 grades), you can see that it's almost entirely done by machining.

If you know the materials used, you can roughly understand the manufacturing method, and if you understand the manufacturing method, you can predict the appropriate range of dimensional tolerances.

Of course, this will naturally have a significant impact on costs, and it can be predicted.

By examining the material first, you can understand the magnitude of the load the part will experience, the manufacturing method, the appropriate dimensional tolerance range, and the cost before seeing the shape. This significantly reduces misunderstandings and misinterpretations when looking at the shape of the part in the drawing.

Conversely, knowing this much allows you to quickly spot flaws in the drawings.

④ Scale

scaleThe reason why this is important isThis is because if you don't understand the scale, you often misjudge the size of parts in drawings.

Especially with small parts, a 2x scale can result in little visual change, making it easy to mistake them for the original size..

According to JISThe scale is determined by a standard, such as 1:2 or 1:5, and while a 5x scale is rarely a mistake, a 2x scale is questionable..

For example, if you design a washer (disc) with a diameter of 4 mm and a width of 2 mm, and then draw the drawing at twice the scale, the size of the drawing will naturally be 8 mm in diameter and 4 mm in width. Since the size doesn't change that much, you might misunderstand the shape just by looking at it.

I once did something similar while I was still working, drawing a diagram at twice the normal size, but the person who saw the diagram looked at it at normal size and manufactured a part that was twice the size of what I intended.

Of course, it's nothing but garbage.

It's nothing but a waste of money and time.

This isn't about how to read drawings, but rather a tip for creating them: I strongly recommend drawing at 1:1 scale if possible. If you absolutely must enlarge it, use 5x magnification (2x is fine for detailed drawings or arrow-indicated diagrams within a larger drawing).

Now, ② Title fieldAs you might have guessed, the next thing we'll look at is ③ Processing grade and processing symbol.

Processing grade, processing symbol

First, let me explain specifically what is written there.

Processing grade

FirstProcessing gradeHowever, thisTolerances for dimensions not specified in the drawings are determined.

In this example, we used JIS Class 1, but I believe each company has its own unique engineering standards.

However, I don't think the basics are far removed from JIS standards.

For JIS Class 1, the general tolerance is specified as ±2, etc. (By the way, when I wrote "processing," I didn't just mean machining or cutting; casting, forging, and pressing are also considered processing.)

If you're interested, you can easily find out more by searching for "JIS Class 1 General Tolerances."

In other words, If you don't check the machining grade before looking at the shape in the drawing, you won't know the tolerances for dimensions that aren't specified in the drawing (there are no dimensions without tolerances).

Therefore, if you don't check the machining grade here, you won't know the general tolerances for dimensions that aren't listed.

Surface roughness

next加工記号However, by processing symbolsThis allows you to determine how precise the parts depicted in the drawings are.

First, let me briefly explain the meaning of the symbols. The leftmost checkmark in the processing symbol with "100" written inside means "basically, for parts that are not specified, a surface roughness of 100s is fine, or leave it as is with no machining and make it 100s."

Incidentally, to explain surface roughness of 100s without fear of misunderstanding, it means that it's acceptable if the difference in surface irregularities is 100s (100 μm = 0.1 mm) or less.

In other words, any surface roughness (surface finish) within 100s is acceptable for areas where dimensions are not specified in the drawing (similar to tolerances, there are no surfaces without roughness specifications).

Next, the instructions in parentheses mean "the surface roughness of the indicated surface should be machined to the precision specified in the parentheses."

Since the Rz values ​​are Rz25 and Rz12.5 respectively, the surface roughness of the areas indicated in this drawing will be 025 mm and 0125 mm.

Conversely, this means that there are absolutely no surface roughness values ​​not listed in parentheses (for example, 6.3s, 12.5s, 50s).

It is a contradiction to require or be required to have a tolerance greater than this.

Well, I'll create a separate section for surface roughness and explain it in detail later. (It's a very complex topic.)

③ Processing grade, processing symbolAs you can probably guess, the next thing we'll look at is ④ the Note sentence.

Note statement

This is quite troublesome. Let me start by giving you a specific example.

The Note section is a free-for-all, allowing you to write anything that can't be explained by the shape (diagram) and dimensions, detailed material specifications that couldn't be included in the material section, heat treatment instructions, and more.

IfIf you look at the shape without looking at the notes in the diagram, such as "Unspecified radius is 3" or "General wall thickness is 3," you'll end up searching for unspecified radius dimensions and unspecified wall thickness dimensions within the shape indefinitely, and you'll never understand them.

This is not so bad, but it's really bad."I've only written dimensions for the parts that differ from the reference drawing."It might say something like that.

Therefore, before looking at the shapeIt's extremely important to carefully examine the notes, because without doing so, you'll almost certainly be unable to accurately determine the shape and dimensions.

Finally, after coming this farLet's start with the front view of shape ⑤.

The interpretation of the front view and subsequent views is closely related to the drawing method, so I will explain it on a separate page later.

Summary

When people glance at a drawing, their eyes tend to be drawn to the shape (the picture) first, but try to resist that urge.

I think you've understood by now that there are many prerequisites written in the drawings that you need to consider before looking at the shape (picture).

My own style, I call itKazubara's way of looking at drawingsIt is,

of courseWhen you're not used to it, it's difficult to remember ① through ④ while looking at the shape, so just refer back to ① through ④ each time if you forget or get curious.I think it's a good idea to look at the drawings while checking them.

If you watch it even once, the content will stick in your mind, and when you look at the shapes and get confused..."Come to think of it, I remember seeing something written there."I often find myself thinking that.

AnywaysWhen reading mechanical drawings, it's important to understand that there's a lot of crucial information beyond just shape and dimensions; everything written on the drawing has meaning.That's why it's crucial to first read information other than shape.

I really hope that not only designers, but also anyone who handles drawings, technical sales representatives, and everyone else will try using this method.

next timeLet's explain tolerances.

To those who found this article helpful in understanding design:

Since we're on the subject, I'd like to recommend a book that's essential for mechanical design.

To be honest, the content is extremely unhelpful, but it can be used like a dictionary when you forget the details. If you read this article, you should be able to understand the content and use it effectively. It also includes commonly used standards, making it quite useful.

If you don't already own one, I highly recommend getting one, even though it's a bit pricey. However, new ones are expensive, so if you're considering buying a used one, I strongly recommend checking that the surface roughness conforms to the new JIS standard.