Reading a technical drawing on the shop floor

Reading a technical drawing on the shop floor comes down mainly to three things: dimension, tolerance and symbols, including weld symbols. To know what they refer to, you also need to read the title block and put the views together into a solid shape. The rest of the information is used mainly by the designer and the process engineer.

Reading a technical drawing: the order at your workstation

A drawing is read in a fixed order, before you make the first piece:

1. Title block: part name, drawing number, material, scale.
2. Views and sections: which side of the part you are looking at.
3. Dimensions with tolerance: how big the part should be and with what deviation.
4. General notes near the title block: general tolerance, roughness, machining.
5. Weld symbols, if the part is welded.

This order is useful at every workstation described in the text about work in production, from machining to the welding shop.

Where the title block is and what to read from it

The title block is usually in the bottom right corner of the sheet. It contains the part name, drawing number, material, scale, mass, dates and signatures. There are basic, bill-of-materials and simplified title blocks.

Two items matter most at the workstation. You compare the drawing number with the number on the work order or card, because it confirms that you have the right version. The scale tells you whether the drawing is reduced: a dimension is not measured with a ruler on the paper, you read the number at the dimension line. The base sheet format is A4, that is 210x297 mm.

View, projection and section: how to assemble the solid

In Poland European projection is used. The main view is the front view, and the other views show the same part from other sides. You assemble the solid by matching an edge in one view with the same edge in another.

A section shows the inside of the part as if it had been cut. The cutting place is marked by hatching. Line types have fixed meanings:

Dimension lines are thin, continuous and do not cross each other.

Dimensions and tolerances: what ±0.1, H7 and ISO 2768-m mean

The sign Ø means diameter, R radius and M a metric thread. The notation ±0.1 next to a dimension is a deviation: for a dimension of 20 ±0.1 a good part measures from 19.9 to 20.1 mm.

The notation H7 is the fit tolerance zone. The letter H for a hole means the tolerance runs upward from the nominal dimension, and the digit is the accuracy class. The specific deviation for a given diameter is given in the fits table or on the workstation inspection card.

An inscription such as ISO 2768-m near the title block is the general tolerance. It applies to all dimensions that have no deviation of their own, in machined parts and parts bent from sheet metal. There are four classes: f (fine), m (medium), c (coarse), v (very coarse). An example for a linear dimension from 30 to 120 mm:

Dimensions below 0.5 mm must have the tolerance written next to the dimension. In automotive plants, described in the text about work in the automotive industry, there are often their own inspection cards with limits set by the plant.

Weld symbols: how to read the marking

A weld symbol consists of a reference line (thin, continuous, bent), an arrow leading to the joint, a shelf with an identification line, the weld sign and dimensions. Example: a triangle on the shelf and the notation a5 mean a fillet weld with a throat thickness of 5 mm. A circle at the bend of the line means a weld all around, and a flag means a site (assembly) weld.

The standard on weld marking allows two systems for indicating the side of the joint: with a double and with a single reference line. The same sign above and below the shelf can therefore mean something different in two plants. Before the first joint, ask the team leader which system the drawing uses. The notation on the drawing takes priority, not your memory from a previous job.

Roughness: the sign with the Ra value

The roughness symbol looks like a bent "tick" with a number. The number is the Ra value in µm (1 µm = 0.001 mm). The smaller it is, the smoother the surface should be. Next to the sign, the machining method may be written out, for example grinding.

Frequently asked questions

What should I do if the dimensions on the drawing do not add up?

Do not guess and do not work out the dimension from other dimensions on your own. Stop work on this part and show the drawing to the team leader, who will pass the matter to the process engineer.

Do I need to know technical drawing on the first day of work?

At the start, reading the title block, the dimension and the tolerance is enough. Questions about the drawing fit the list from the text about the first day at a new job, together with the question about the measuring instruments at the workstation.

Can I measure a dimension with a ruler on the printout?

No. A drawing is sometimes in a scale other than 1:1, and the printout can also change the size. The number at the dimension line is what applies.

How do I know which version of the drawing to use?

Compare the drawing number from the title block with the number on the work order or the workstation card. If they differ, report it to the team leader before you start work.

This text describes the general rules. What you are entitled to depends on your type of contract: an employment contract (umowa o pracę) or a contract of mandate (umowa zlecenie). Your own contract and payslip are what count.