Craftsman Computer Aided Mechanical Drawing common mistakes
Craftsman Computer Aided Mechanical Drawing · Understand the traps and how to avoid them.
When laying out a multi-view drawing, the candidate places the top view above the front view but offsets it horizontally, or lets the side view drift so projection lines no longer align with the front view's height.
They treat each view as an independent sketch rather than as one projection system. Without construction lines tying views together, alignment is judged by eye and small offsets go unnoticed until the drawing is finished.
What to do instead
Before drawing any object edges, draw a horizontal construction line and a 45-degree miter line (or vertical and horizontal projection lines) from the front view, and build every other view off those. Verify alignment by checking that the top view and front view share the same width and that the side view and front view share the same height.
The candidate draws hidden features as solid lines, or omits hidden lines entirely because 'the section view shows it anyway'.
They confuse the purpose of line types: solid lines describe visible edges, dashed lines describe obscured edges. In a hurry, the difference is not checked, and the drawing looks cleaner without dashes.
What to do instead
Apply the line-type convention systematically: visible edges solid, hidden edges dashed with evenly spaced dashes, center lines as long-short-long chains. After finishing, scan each view feature by feature and ask whether it is visible from that viewing direction; if not, it must be dashed.
When dimensioning, the candidate chains dimensions end to end from one feature to the next so that the overall length is also given, creating a closed loop of dimensions.
Chaining feels natural because it follows the order in which features were sketched, and adding the overall dimension seems helpful. The candidate does not realize the loop over-constrains the part and is a rejectable drafting error.
What to do instead
Dimension from a common datum or baseline, and omit one dimension in any closed chain so the loop is broken. Keep the overall dimension only when it is functionally needed, and check every dimension string for a closed loop before submitting.
The candidate writes a fit as 'H7/g6' but then assigns tolerance values that do not correspond to the ISO grade, or mixes unilateral and bilateral tolerances inconsistently on the same drawing.
They memorize the fit symbol without understanding that the letter sets the fundamental deviation and the number sets the tolerance grade. Without a tolerance table, they guess values that look plausible.
What to do instead
Look up the actual deviations for the hole and shaft in an ISO tolerance table, compute maximum and minimum clearance or interference, and state the fit basis in the drawing notes. Use one tolerance convention throughout the drawing and verify the resulting fit type against the intended function (clearance, transition, interference).
In a section view, the candidate hatches the entire cut area as one continuous pattern, including separate parts in an assembly, and leaves the section line without arrows or letters.
They focus on showing the interior and forget that hatching communicates material identity and that the cutting plane must be defined so the reader knows where the section came from.
What to do instead
Hatch adjacent parts in an assembly with different patterns or angles, and never hatch standard parts such as shafts, bolts, nuts, or pins cut longitudinally. Label the cutting plane with two letters at the ends, add arrows showing viewing direction, and title the resulting view 'SECTION A-A'.
The candidate creates an auxiliary view by rotating the whole model or by drawing it freehand at an approximate angle, so the true shape of an inclined face is not actually shown.
They know an auxiliary view is needed for an inclined surface but do not construct the viewing direction perpendicular to that surface. Eyeballing the angle produces a foreshortened view that looks correct at a glance.
What to do instead
Establish the auxiliary viewing plane perpendicular to the inclined edge, project the feature's depth from the adjacent view using construction lines parallel to the inclined edge, and measure the true length along the projection. Verify by checking that the inclined edge appears at true length in the auxiliary view.
The candidate draws a tangent arc between two circles by picking an arbitrary radius or center, so the arc meets the circles at a corner rather than blending smoothly.
They treat tangency as a visual approximation instead of a geometric constraint. Without locating the tangent point, the join looks almost right but fails a smoothness check.
What to do instead
Construct tangency properly: for an external tangent arc of radius R, locate the center at the intersection of arcs of radius R+r1 and R+r2 from the two circle centers; for internal tangency use R-r. Mark the tangent points on the line joining centers, then trim the circles and arc to those points.
The candidate saves work only as the native CAD file and submits it, without exporting a PDF or plot, and the plotted output shows wrong line weights, a clipped border, or a scale that does not match the title block.
They regard the native file as the deliverable and treat plotting as an afterthought. Plot settings such as paper size, scale, and line-weight mapping are left at defaults and never previewed.
What to do instead
Before finishing, configure the page setup with the correct sheet size and a standard scale, map line weights by layer or pen, preview the plot, and check that the border and title block are fully inside the printable area. Export the PDF and open it to confirm scale, line weights, and text legibility.
The candidate leaves the title block incomplete or inconsistent: missing scale, revision, material, or units, or a scale stated as 'NTS' while the drawing is actually plotted to a standard scale.
The title block is filled in last under time pressure, and the candidate assumes the reader will infer missing data. Units especially are left implicit, so a dimension of 25 could be millimetres or inches.
What to do instead
Treat the title block as a checklist: part name, drawing number, revision, scale, units, material, projection symbol (first- or third-angle), date, and author. Confirm the stated scale against the actual plot scale, and set the drawing units explicitly in the CAD file before dimensioning.
The candidate produces an assembly drawing that is just the individual part drawings overlaid, with no item numbers, no parts list, and no indication of how the parts locate relative to one another.
They conflate a detailed part drawing with an assembly drawing and do not plan the assembly views around function and fit. Clearance and fit relationships are left to the reader to infer.
What to do instead
Draw the assembly in a view that shows the functional relationships, add balloons with item numbers keyed to a parts list (item, description, quantity, material), and note critical fits and clearances. Show fasteners and standard parts in their assembled positions and include a brief assembly note where the fit matters.