Here are some key things students should remember when using center lines in technical drawings:

1. When to Use Center Lines

• For marking centers of circles and holes

• For showing symmetry in round parts

• Along the center of patterns (like a row of holes)

• To indicate the center of cylindrical shapes

2. How to Draw Center Lines

• Use alternating long and short dashes

• Long dash should be about 3/4 inch (19mm)

• Short dash should be about 1/8 inch (3mm)

• Make them thin - thinner than object lines

• Center lines should cross at their midpoints

3. Proper Extension

• Extend center lines just a bit beyond the feature

• Don't extend them too far - usually 1/4 to 1/2 inch past the edge

• For circles, extend center lines slightly beyond the circle

• For patterns, extend past the first and last feature

4. Common Mistakes to Avoid

• Don't make them too bold

• Don't use solid lines

• Don't make long and short dashes the same length

• Don't stop center lines at the edge of a circle or hole

• Don't forget to include both horizontal and vertical center lines for circles

5. Line Priority

• Center lines should be less visible than object lines

• When crossing other lines, center lines usually break

• When center lines cross each other, they should cross at 90 degrees when possible

Here are some key things students should remember when using center marks in technical drawings:

1. What Center Marks Are For

• Show the exact center of small circles and holes

• Mark precise locations for drilling or machining

• Indicate center points for measurement

• Help locate features quickly on a drawing

2. How to Draw Center Marks

• Make two short crossed lines

• Keep lines thin and light

• Make lines equal in length

• Lines should cross at 90 degrees

• Use a sharp pencil for accuracy

3. Proper Size Guidelines

• Keep marks small but visible

• Lines should extend slightly beyond circle edges

• Typically about 1/8 inch (3mm) to 1/4 inch (6mm) long

• Use same size marks consistently throughout drawing

4. Common Mistakes to Avoid

• Don't make marks too large - they can clutter the drawing

• Don't draw them freehand - use straight edges

• Don't make one line longer than the other

• Don't forget to align them perfectly at 90 degrees

• Don't make them as bold as object lines

5. When to Use Them

• For circles smaller than about 1/2 inch (12mm)

• When full center lines would crowd the drawing

• For marking hole locations

• When precise center location is critical

Practice Challenge

Reflection

"Why is it important to mark the centers of circles and holes?"

"What do you do if you have many circles close together? How do you keep the center marks from getting messy?"

"Why do we use center marks and center lines?"

Review of Line Types