Marking Out Notes

Marking Out

Marking out involves scribing lines on a workpiece surface to indicate outlines, hole positions, slots, etc., typically done on individual or small batches of workpieces.

Purposes:
- Indicate workpiece outline and positions of features.
- Guide for material removal (hacksawing, filing).
- Guide for setting up the workpiece on a machine, especially for establishing a datum on castings and forgings.
Scribed lines serve as a guide only; final dimensions must be verified by measuring.

Datum

A datum is a reference position (point, edge, or centerline) from which all dimensions and measurements are taken.

For plane surfaces, two datums are needed, usually at right angles.
Datums are established by the draughtsman during dimensioning on drawings and are transferred to the workpiece during marking out.

Co-ordinates

Drawings can be dimensioned using:

Cartesian (Rectangular) Co-ordinates: Dimensions taken relative to datums at right angles, forming a rectangular pattern.
Polar Co-ordinates: Dimensions measured along a radial line from the datum. Accurate angle and dimension are crucial; angular errors increase inaccuracy with distance.
Conversion to Rectangular Co-ordinates: Polar co-ordinates can be converted to rectangular to reduce potential errors.

Marking Out Equipment

Surface Table and Surface Plate

A reference surface is required to establish a datum for measurements.
Surface Table: A large, flat surface used as a reference.
Surface Plate: A smaller reference surface placed on a bench for smaller workpieces.
Materials: Cast iron (general use), granite (high-accuracy inspection).

Parallels

Used to raise a workpiece off the reference surface while maintaining parallelism.

Made in pairs with precise, identical dimensions from hardened steel, finish ground.

Jacks and Wedges

Used to support forgings or castings with uneven surfaces, maintaining the datum relative to the reference surface.

Wedges: Thin steel or wooden wedges prevent rocking on uneven surfaces.
Adjustable Jacks: Support awkward shapes in the correct position.

Angle Plate

Used to position workpieces at 90° to the reference surface.

Made from cast iron with accurately machined flat, square, and parallel faces.
Slots facilitate easy clamping of the workpiece.
Types: Plain or adjustable.

Vee Blocks

Simplify holding circular workpieces for marking out or machining.

Materials: Cast iron (larger sizes), hardened and ground steel (smaller sizes), often supplied with a clamp.
High degree of accuracy in flatness, squareness, and parallelism; 90° vee is central and parallel to faces.

Engineer’s Square

Used to set workpieces square to the reference surface or to scribe lines square to a datum edge.

Consists of a stock and blade made from hardened steel, ground on all faces and edges for accuracy.

Combination Set

Includes a graduated hardened steel rule with mountable protractor, square, or center heads that slide and lock along its length.

Protractor Head: Graduated from 0° to 180°, adjustable for scribing lines at an angle.
Square Head: Used like an engineer’s square but less accurate due to adjustability; includes a 45° face and a spirit level, which can also be used as a depth gauge.
Center Head: Used to find and mark the center of circular workpieces.

Marking Dye

Applied to surfaces (especially non-bright metals) to enhance the visibility of scribed lines by providing contrast.

Scriber

Used to scribe lines on metal surfaces; made from hardened and tempered steel with a sharp point.

The point should be kept sharp for well-defined lines.

Surface Gauge

Used with a scriber to mark lines parallel to the reference surface; scriber height is adjustable.

Expected accuracy: around 0.3 mm, but can be improved with care.

Vernier Height Gauge

Provides greater accuracy than a surface gauge for marking out.

Features a vernier scale with a jaw for clamping attachments, such as a chisel-pointed scribing blade for marking out.
Care must be taken to account for jaw thickness.
Accuracy: up to 0.02 mm; available in sizes up to 1000mm.

Dividers and Trammels

Used to scribe circles or arcs and to mark off lengths.

Dividers: Spring bow construction with hardened steel legs, scribing circles up to ~150mm diameter.
Trammels: Used for larger circles, with adjustable scribing points along a beam.
Setting: With a steel rule by coinciding points with graduation lines at the correct distance.

Hermaphrodite Calipers

Also known as ‘odd-legs’ or ‘jennies’; used to scribe a line parallel to the edge of a workpiece, combining a divider leg with a caliper leg.

Precision Steel Rule

Made from hardened and tempered stainless steel, photo-etched for accuracy, with a non-glare satin chrome finish.

Lengths: 150mm and 300mm.
Graduations: Millimeters and half millimeters along each edge.
Accuracy: Depends on rule quality and operator skill. High-quality rules can achieve accuracies of around 0.15mm, but 0.3mm is more realistic.

Centre Punch

Used to create a center location for dividers and trammels or to mark the position of a scribed line with center dots; also used as a starting point for small drills.

Made from high carbon steel, hardened and tempered.
Point angle: 30° (for dividers), 90° (for other purposes).

Clamps

Used to securely fix a workpiece to equipment, such as an angle plate.

Toolmaker’s Clamps: Adjustable within ~100mm, for clamping parallel surfaces.
'G' Clamps: Clamp greater thicknesses and non-parallel surfaces due to a swivel pad.

Examples of Marking Out

Example 3.1

Marking out the position of steps on a plate filed to length and width with square edges.

Use a square on one datum edge and measure from the other datum edge with a precision steel rule. Scribe lines.
Repeat with the square on the second datum edge and scribe lines to intersect.

Example 3.2

Marking out a plate cut 2mm oversize on length and width with sawn edges.

Measure from each long edge to find the center using a precision steel rule. Scribe the center line using the edge of the rule. Find the center of the small radius by measuring from one end the size of the radius plus 1mm. Center dot where the lines intersect.
Using dividers, set the distance from the center of the small radius to the center of the first small hole. Scribe an arc. Repeat for the second small hole and the large radius. Center dot at the intersection of the center lines. The dividers are set using the graduations of a precision steel rule.
Set dividers to the small radius. Locate on the center dot and scribe the radius. Repeat for the large radius, and if necessary, the two holes.
Complete the profile by scribing a line tangential to the two radii using the edge of a precision steel rule as a guide.

Example 3.3

Marking out the profile and holes on a plate roughly cut to size.

Clamp the plate to the face of an angle plate, ensuring that the clamps will not interfere with marking out. Use a scriber in a surface gauge and set the heights in conjunction with a precision steel rule. Scribe the datum line. Scribe each horizontal line the correct distance from the datum.
Without unclamping the plate, swing the angle plate on to its side. Scribe the datum center line. Scribe each horizontal line the correct distance from the datum to intersect the vertical lines.

Example 3.4

Marking out angled faces on a plate produced from bright rolled strip, sawn 2mm oversize on length.

Using a precision steel rule measure from two adjacent edges to determine the datum point. Centre dot the datum point.
Set protractor at required angle and scribe line through datum point.
Reset protractor at second angle and scribe one line through datum point. Scribe the remaining two lines parallel to and the correct distance from the first line using the protractor at the same setting.
Set dividers at correct distances, locate in datum centre dot and mark positions along scribed line.
Reset protractor and scribe lines through marked positions.

Example 3.5

Marking out a keyway on a shaft along its center line for a required length.

Scribe line on the end face through the center of the shaft using the center head of a combination set.
Clamp shaft in a vee block ensuring that the line marked at step 1 is lying horizontal. This can be checked using a scriber in a surface gauge. Transfer the centre line along the required length of shaft. Scribe two further lines to indicate the width of slot.
The required length of slot can be marked without removing it simply by turning the vee block on its end and scribing a horizontal line at the correct distance from the end of the shaft.

Review Questions

Two uses of a center punch when marking out:
- Provide a center location for dividers and trammels.
- Show permanently the position of a scribed line by a row of centre dots.
Trammels are used for scribing large circles beyond the capacity of standard dividers.
Three heads of a combination set:
- Protractor head: Measuring and marking angles.
- Square head: Checking squareness and marking lines at 90 or 45 degrees.
- Center head: Finding the center of circular workpieces.
A vee block is used to hold circular workpieces during marking out or machining.
A marking dye is used when marking out on surfaces of metal other than bright metals.
Two main purposes of marking out engineering components:
- Indicate workpiece outline and positions of features.
- Guide for setting up the workpiece on a machine.
Surface tables and plates are used when marking out to provide a reference surface from which accurate measurements can be made.
It is necessary to create a datum when marking out to establish a reference position from which all dimensions and measurements are taken.
Jacks and wedges are used when marking out to support workpieces with uneven surfaces or awkward shapes, maintaining the datum relative to the reference surface.
Difference between rectangular and polar co-ordinates:
- Rectangular co-ordinates: Dimensions are taken relative to datums at right angles.
- Polar co-ordinates: Dimensions are measured along a radial line from the datum.