Reading Manufacturing Prints
Read a part print: title block, orthographic views, dimensions, tolerances and revision control.
Table of contents
A part print is a contract between the person who designed a part and the person who makes it. Everything you need to make and check that part should be on the print: the material, every size, how close each size must be, the finish and the current revision. If you misread one number, you can end up with a scrap part, a broken tool or a failed inspection. Machinists, inspectors, assemblers, welders and sheet metal workers all use these prints, so they all need to read them the same way.
What You Will Be Able to Do
- Pull the key facts from a title block before you touch a part.
- Work out the allowed range for any dimension.
- Read hole and thread callouts.
- Read a surface finish mark and a parts list.
- Use revision control to stay on the right version.
Start With the Title Block
The title block sits in the lower right corner, and it controls everything else on the print. Read it first.
| Field | Why it matters |
|---|---|
| Part name and number | Confirms you have the right print for the job |
| Revision | Must match your work order |
| Material | For example, 6061-T6 aluminum, 304 stainless or ASTM A36, a common structural carbon steel |
| Units | Inches or millimeters |
| Scale | The ratio of drawing size to part size, such as 2:1 |
| Default tolerances | Apply to any dimension without its own tolerance |
| Projection symbol | Shows first-angle or third-angle views |
| Finish or treatment | Default surface finish, plating, heat treat |
Most US prints use third-angle projection, with the top view above the front view and the right side view to the right. Prints from Europe and Asia often use first-angle projection, where those positions swap. If the views do not seem to make sense, check the projection symbol.
Tolerances
No part comes out exactly on size. A tolerance states how far off it can be and still pass. The print shows this in one of four ways.
| Style | Written as | Acceptable range |
|---|---|---|
| Bilateral | 2.000 +/- .005 | 1.995 to 2.005 |
| Unilateral | 2.000 +.010 / -.000 | 2.000 to 2.010 |
| Limit | 2.005 / 1.995 | 1.995 to 2.005 |
| Default | No tolerance on the dimension | Comes from the title block |
Default Tolerances
Most title blocks tie the default tolerance to the number of decimal places in the dimension. A common example:
| Dimension format | Default |
|---|---|
| .X | +/- .1 |
| .XX | +/- .01 |
| .XXX | +/- .005 |
| Angles | +/- 1 degree |
Those numbers are only examples. Every company sets its own, so read the block on your print.
Worked example: The title block says .XXX = +/- .005, and a hole is dimensioned 0.500.
- Upper limit: 0.500 + 0.005 = 0.505
- Lower limit: 0.500 - 0.005 = 0.495
- Any hole from 0.495 to 0.505 passes.
The number of decimals carries meaning. On that same print, 0.50 would mean +/- .01, so 0.49 to 0.51. Do not add or drop decimal places when you copy a dimension.
How Tolerances Stack
When dimensions run in a chain, feature to feature, the tolerances add up. Five spaces at +/- .005 can drift as much as +/- .025 from end to end. When they all run from one baseline, each feature keeps its own tolerance. This affects your setup: work from the same surface the print dimensions from, or you can use up the tolerance by accident.
Datums and Geometric Controls
Many prints name certain surfaces as datums, shown by a letter in a box with a triangle. These are the surfaces the part is located from. Prints may also use geometric tolerances, shown in a framed box with symbols. Those are a language of their own, and they follow ASME Y14.5. For now, know that a boxed dimension is a basic dimension, meaning it has no tolerance of its own, and that the frame connected to the feature gives the tolerance.
Hole Callouts
Holes are described with a short note, using symbols or abbreviations.
| Callout | Meaning |
|---|---|
| dia .500 THRU | A hole all the way through |
| dia .500 x .750 DEEP | A blind hole, .750 deep to the full diameter, not counting the drill point |
| dia .500 THRU, counterbore dia .875 x .375 DEEP | A through hole with a flat-bottomed larger hole at the top |
| dia .500 THRU, countersink dia .875 x 82 degrees | A through hole with a cone at the top, 82 degrees included |
The 82-degree countersink is standard for US flat-head screws. Metric flat-head screws are usually 90 degrees, so do not assume.
Thread Callouts
Unified inch threads: 1/4-20 UNC-2B
| Part | Meaning |
|---|---|
| 1/4 | Nominal diameter, in inches |
| 20 | Threads per inch |
| UNC | Unified National Coarse (UNF is fine) |
| 2 | Class of fit, with 2 the standard |
| B | Internal thread (A is external) |
Metric threads: M10 x 1.5-6H
| Part | Meaning |
|---|---|
| M10 | Metric, 10 mm nominal diameter |
| 1.5 | Pitch, in millimeters |
| 6H | Tolerance class. Upper case is internal, lower case is external |
A tapped hole often gives the thread and a depth, such as 1/4-20 UNC-2B x .625 DEEP. Check whether the depth means full thread.
Surface Finish
A check-mark symbol on a surface shows how smooth it must be. In US prints, the number is roughness average, Ra, in microinches. A lower number means smoother.
| Ra (microinches) | Typical surface |
|---|---|
| 250 | Rough machined |
| 125 | Standard machined, often the default |
| 63 | Fine machined |
| 32 | Very fine, may need grinding |
| 16 | Ground or lapped |
Notes
Read every note. General notes apply to the whole part, while a local note uses a leader line to point at one feature. Common examples include "BREAK ALL SHARP EDGES .010 MAX", "REMOVE ALL BURRS" and "INTERPRET PER ASME Y14.5." Spec callouts send you to outside standards for heat treat or plating. A note can change how the whole part is made.
Weld Symbols
Welded parts carry a symbol built on a horizontal reference line with an arrow pointing at the joint. A small triangle on that line means a fillet weld, the common weld in a corner or T joint. If the symbol sits below the line, the weld goes on the arrow side. Above the line, it goes on the other side. Numbers beside the symbol give the weld's size and length.
Assembly Prints
A detail drawing shows one part, with everything needed to make it. An assembly print shows how parts go together, and usually does not give sizes for each part.
- Parts list, or bill of materials (BOM): lists each part with its item number, part number, description and quantity.
- Balloons: circled numbers that point to each part and match the BOM.
- Assembly notes: torque, adhesive and inspection steps.
Each part's dimensions are on its own detail print, found by part number.
Revision Control
Every change to the print gets a new revision letter or number and a line in the revision block, showing the date and what changed.
- Compare the print revision to your work order or traveler.
- If they do not match, stop and ask. Do not run parts on an old print.
- Remove or mark old copies so nobody picks them up.
- Read what changed before you restart a repeat job. It may be the feature you remember the most.
- Do not mark up a controlled print. Get a change approved.
Before You Run the First Part
- Confirm part number, revision and material.
- Note the units and default tolerances.
- Study all the views, then trace each feature from one view to another.
- Work out the limits on the tightest dimensions.
- Read every note.
- Plan how you will measure each critical feature.
- Ask if anything is unclear. A question costs nothing, while a scrapped batch costs a lot.
Common Mistakes
| Mistake | What it costs | Better habit |
|---|---|---|
| Running from an old revision | Parts made to the wrong size | Match the print to the traveler |
| Using the wrong default tolerance | Bad parts pass, or good parts fail | Read the decimals and the block |
| Measuring the print with a ruler | Wrong size | Use written dimensions only |
| Confusing inches and millimeters | Scrap parts | Check the units |
| Skipping the notes | Missed burr, finish or heat treat steps | Read every note |
Check Your Understanding
1. A shaft is dimensioned 1.250 +.000 / -.002. What range passes?
1.248 to 1.250. The tolerance only allows it to go smaller.
2. The title block says .XX = +/- .01 and .XXX = +/- .005. A slot is dimensioned .75. What is the range?
.74 to .76, because it has two decimal places.
3. What is 3/8-16 UNC-2A?
An external coarse thread, 3/8 inch in diameter, 16 threads per inch, standard class of fit.
4. Your print is revision C, and the traveler says revision D. The part is due today. What do you do?
Stop and get the revision D print. Running to the old print can scrap the whole job.
Key Takeaways
- Read the title block first, for revision, material, units and default tolerances.
- Every dimension has limits. If it shows no tolerance, get it from the title block.
- Decimal places can change the tolerance, so copy them exactly.
- Hole and thread callouts tell you the process as well as the size.
- Notes carry requirements that do not appear in the views.
- Match the print to the work order, and stop if they differ.