Performing Assembly, Fitting, and Bench Work
Module Introduction
This chapter covers all the practical and theoretical skills related to assembly, fitting, and bench work, as required for the Red Seal exam for the Tool and Die Maker trade. You must master not only the technical procedures, but also the calculations, tolerances, safety standards, and quality control procedures. This chapter is structured to follow the logical progression of work: preparation, execution, verification, and correction.
1. Fundamental Principles of Fitting
1.1 Key Definitions
Fitting: a manual or mechanical operation aimed at bringing two parts to a precise state of contact or clearance, often less than 0.01 mm.
Assembly: the operation of permanently or removably joining multiple components (screws, pins, rivets, welding, bonding).
Bench work: all operations performed on a bench or reference surface: layout, sawing, filing, drilling, tapping, reaming, polishing, lapping, and mounting.
1.2 Tolerances and Standardized Fits
The ISO tolerance system (standard ISO 286) is the reference. You must know the common fit classes:
| Fit Type | Symbol | Application Example |
|---|
| Clearance | H7/g6 | Sliding guides, rotating shafts |
| Transition | H7/js6 | Light press fitting, centering |
| Interference | H7/p6 | Bushings, rings, pins |
Formula for calculating maximum and minimum clearance or interference:
Max clearance = max hole − min shaft
Min clearance = min hole − max shaft
Max interference = max shaft − min hole
Min interference = min shaft − max hole
Example: Hole Ø25 H7 (tolerance +0.021 / 0), shaft Ø25 g6 (tolerance −0.007 / −0.020).
Max clearance = 25.021 − 24.980 = 0.041 mm
Min clearance = 25.000 − 24.993 = 0.007 mm
1.3 Surface Finishes
Roughness is expressed as Ra (arithmetic mean) or Rz (maximum height). For precision fitting:
Lapping: Ra 0.05 to 0.2 µm
Grinding: Ra 0.2 to 0.8 µm
Fine filing: Ra 0.8 to 1.6 µm
Rule of thumb: the tighter the fit, the finer the surface finish must be. A poor surface finish can distort measurements and cause seizing.
2. Bench Tools and Equipment
2.1 Layout Tools
Surface gauge: for scribing lines parallel to a reference surface.
Center punch: for marking drilling centers. Point angle: 90° for general use, 60° for precision work.
Layout dye (Prussian blue or copper sulfate solution): to improve the visibility of scribed lines.
Precision square: for checking perpendicularity, accuracy class 0 or 1.
Common error: failing to account for the thickness of the scribed line (approximately 0.05 mm). Always scribe from the reference surface, never from a raw edge.
2.2 Files and Filing Techniques
| File Type | Use |
|---|
| Rough (bastard cut) | Rapid material removal |
| Second cut | General finishing |
| Smooth | Fine finishing, fitting |
| Round (rat-tail) | Enlarging holes, concave shapes |
Filing principles:
Never use a file without a handle.
Apply pressure on the forward stroke, release on the return.
For flat filing, use the cross-filing technique (crossing the strokes) to avoid hollows.
Check flatness with a straightedge or control ruler: light should not pass between the ruler and the workpiece.
2.3 Drilling and Reaming Tools
Twist drill: standard point angle 118° for steel, 135° for hard steels.
Reamer: to obtain a precise hole (H7 tolerance). Reaming is done at low speed, with abundant lubrication, and in a single pass.
Tap: for internal threading. Three taps: taper, plug, bottoming. Reduced cutting speed, frequent reversal to break the chip.
Tap drill calculation:
Drill diameter = nominal diameter − pitch
Example: M10 × 1.5 thread → drill = 10 − 1.5 = 8.5 mm.
2.4 Measuring Tools
Caliper: accuracy 0.02 mm (vernier) or 0.01 mm (digital).
Micrometer: accuracy 0.001 mm. Reading: main scale (0.5 mm per division) + thimble (0.01 mm).
Dial indicator: for measuring variations, runout, flatness.
Surface plate: flat reference surface, class 0 or 1.
Micrometer reading: 1 turn of the thimble = 0.5 mm. Each thimble division = 0.01 mm. Add the readings from all three scales.
3. Assembly Procedures
3.1 Screw and Bolt Assembly
CSA B149.1 (Canadian Gas Installation Code) applies to gas-pressure assemblies, but for tooling, standard ISO 898-1 defines screw property classes (8.8, 10.9, 12.9).
Tightening torque: torque (T) is given by:
T = K × F × d
Where:
T = torque (N·m)
K = friction coefficient (0.2 for dry surfaces, 0.15 for lubricated)
F = clamping force (N)
d = nominal diameter (m)
Example: M12 screw, class 10.9, clamping force 50 kN, K = 0.2.
T = 0.2 × 50,000 × 0.012 = 120 N·m
Rule: always use a torque wrench for critical assemblies. Never exceed the elastic limit of the screw.
3.2 Pins and Dowels
Straight pin: for centering and positioning. Fit tolerance: pin m6 in H7 hole.
Taper pin: for removable assemblies, self-locking. Standard angle: 1:50.
Spring pin: for vibration absorption, press-fit installation.
Taper pin length calculation:
Length = (D − d) × 50
Where D = large diameter, d = small diameter.
3.3 Shaft Fitting (Press Fit)
Two methods:
78.Cold press fitting: the shaft is pushed into the bore using a press. Requires an entry chamfer (15° to 30°).
79.Shrink fitting: the female part is heated (furnace, induction) to increase its diameter. Thermal expansion:
ΔL = α × L × ΔT
Where:
α = coefficient of linear expansion (steel: 11 × 10⁻⁶ /°C)
L = initial length (m)
ΔT = temperature change (°C)
Example: Ø50 mm bore, 0.05 mm interference. Required heating:
ΔT = 0.05 / (50 × 11 × 10⁻⁶) = 90.9 °C
Therefore, heat the part to approximately 120 °C (ambient temperature 20 °C + 91 °C + margin).
Caution: never heat a hardened part above 150 °C to avoid unwanted tempering.
3.4 Riveting
Solid rivet: head deformation by hammering or pressing.
Tubular rivet: for thin sheets.
Blind rivet (pop): for one-sided access.
Required rivet length:
L = total thickness of parts + 1.5 × rivet diameter
Example: 2 sheets of 3 mm, Ø5 mm rivet → L = 6 + 7.5 = 13.5 mm.
4. Precision Fitting and Lapping
4.1 Lapping
Lapping is a machining operation using loose abrasives, used to obtain very flat surfaces and tight seals.
Procedure:
101.Apply an abrasive (diamond paste or silicon carbide) on a lapping plate.
102.Move the workpiece in a figure-eight motion to distribute wear evenly.
103.Check regularly with a surface plate and dial indicator.
Parameters:
Pressure: 0.1 to 0.5 MPa
Speed: 10 to 30 m/min
Abrasive: 5 to 20 µm for roughing, 0.5 to 2 µm for finishing
4.2 Scraping
Scraping is a manual finishing technique for guide surfaces (ways, machine tables). It involves removing high spots with a scraper, after applying Prussian blue on a reference surface.
Quality criterion: number of contact points per square inch (25.4 mm²). For a precision guide: 20 to 30 points.
Procedure:
112.Apply a thin layer of blue on the surface plate.
113.Place the workpiece on the plate and rub lightly.
114.Identify contact areas (blue).
115.Scrape high spots with a triangular or flat scraper.
116.Repeat until a uniform distribution is achieved.
4.3 Guideway Fitting
Machine tool guideways must have minimal clearance (0.01 to 0.03 mm) while remaining movable. Inspection is done:
With a dial indicator: measuring clearance by pushing/pulling the part.
With a feeler gauge: inserting a 0.02 mm feeler should be possible with slight resistance.
Adjustment: dovetail guideways are adjusted using friction strips or pressure screws.
5. Quality Control and Inspection
5.1 Inspection Methods
| Method | Instrument | Accuracy |
|---|
| Direct measurement | Caliper, micrometer | 0.01 mm |
| Comparison | Dial indicator, gauge blocks | 0.001 mm |
| Form inspection | Surface plate, straightedge, square | 0.005 mm |
| Position inspection | Height gauge, CMM | 0.001 mm |
5.2 Concentricity and Runout Inspection
Runout: variation in the radial position of a surface relative to an axis of rotation. Measured with a dial indicator on a rotating machine.
Concentricity: coincidence of the axes of two cylindrical surfaces.
Procedure:
130.Mount the workpiece between centers or on a mandrel.
131.Place the indicator stylus on the surface to be checked.
132.Rotate the workpiece 360° and record the total variation.
Typical tolerance: 0.01 mm for precision tooling.
5.3 Thread Inspection
Pitch: checked with a thread pitch gauge or caliper (measurement of 10 pitches divided by 10).
Pitch diameter: measured with a thread micrometer or measuring wires.
Class: checked with limit gauges (GO / NO-GO).
Rule: the GO gauge must thread by hand for the full length; the NO-GO gauge must not exceed 2 turns.
6. Safety and Applicable Standards
6.1 General Workshop Safety
Wear safety glasses at all times during cutting, drilling, or grinding operations.
Never wear gloves near rotating machinery.
Use a face shield for grinding and lapping.
Keep the workbench clean and clear.
6.2 Relevant Canadian Standards
CSA B149.1: Canadian Gas Installation Code (for gas connection tooling).
CSA Z432: Safeguarding of Machinery (defines safety requirements for machine tools).
Canadian Electrical Code, Part I (CE Code) (C22.1): applicable to machine electrical equipment, particularly Rule 8-200 for grounding conductors.
Rule 8-200: requires that all machine grounding conductors be sized according to Table 16 of the Code, based on the circuit breaker rating.
6.3 Material Handling and Lifting
Verify the load capacity of slings and overhead cranes.
Use lifting points compliant with CSA Z150 (safety of overhead cranes).
Never stand under a suspended load.
7. Step-by-Step Procedures for the Exam
7.1 Fitting a Bushing into a Bore
158.Measure the bore with a bore gauge (0.001 mm accuracy).
159.Measure the bushing with an outside micrometer.
160.Calculate the interference: interference = bushing diameter − bore diameter.
161.If interference > 0.05 mm: heat the part or cool the bushing (liquid nitrogen, −196 °C).
162.Press the bushing with a hydraulic press, using a protective pad.
163.Check alignment with a dial indicator.
7.2 Mounting a Punch into a Die
165.Lay out the punch position on the die.
166.Drill a pilot hole of Ø3 mm.
167.Ream to final diameter (H7 tolerance).
168.Check perpendicularity with a precision square.
169.Insert the punch with a light interference fit (p6).
170.Secure with a cross pin or locking screw.
171.Verify operation: the punch must enter the die with a clearance of 0.01 to 0.02 mm.
7.3 Grinding a Guide Surface
173.Mount the workpiece on the magnetic chuck of the surface grinder.
174.Check initial parallelism with a dial indicator.
175.Grind in passes of 0.01 mm, with lubrication.
176.Check flatness on the surface plate: the workpiece must not rock.
177.Scrape high spots if necessary.
178.Final inspection: 20 contact points per square inch.
8. Essential Calculations and Formulas
8.1 Tolerances and Fits
IT (tolerance grade): IT7 = 10 × i, where i = 0.45 × ∛D + 0.001 × D (D in mm, i in µm).
Tolerance position: uppercase letters for holes (H, G, P), lowercase for shafts (h, g, p).
8.2 Thermal Expansion
ΔL = α × L × ΔT
For steel: α = 11 × 10⁻⁶ /°C
For aluminum: α = 23 × 10⁻⁶ /°C
8.3 Tightening Torque
T = K × F × d
Typical K values:
Dry surfaces: 0.20
Lubricated surfaces: 0.15
Zinc-plated surfaces: 0.25
8.4 Cutting Speed for Drilling
N (rpm) = (V × 1000) / (π × D)
Where V = cutting speed (m/min), D = drill diameter (mm).
Example: Ø10 mm drill, V = 25 m/min (mild steel).
N = (25 × 1000) / (3.1416 × 10) = 795 rpm
9. Pitfalls to Avoid
201.Confusing clearance and interference: always calculate both values (max and min) and verify they correspond to the desired fit.
202.Forgetting surface finish: an H7/p6 fit with Ra 3.2 µm roughness will not work; you need Ra 0.8 µm or better.
203.Heating a hardened part: never exceed 150 °C for a heat-treated part.
204.Using a tap without lubrication: risk of breakage. Use cutting oil or specific tapping fluid.
205.Neglecting the entry chamfer: a bore without a chamfer makes press fitting impossible or damages the surfaces.
206.Measuring with an uncalibrated instrument: always check the zero of the micrometer and caliper before use.
207.Over-tightening screws: exceeding the calculated torque can shear the screw or deform the part.
208.Confusing screw property classes: an 8.8 screw cannot replace a 12.9 without recalculating the torque.
209.Ignoring safety standards: wearing safety glasses is mandatory, even for simple filing.
210.Scribing from a raw edge: always use a machined reference surface.
10. Summary
Fitting and assembly require mastery of ISO tolerances (H7/g6, H7/p6, etc.) and clearance/interference calculations.
Measuring tools (micrometer, dial indicator, surface plate) must be used methodically and checked before each use.
Press fitting requires precise thermal expansion calculations (ΔL = α × L × ΔT) and compliance with maximum temperatures for heat-treated parts.
Lapping and scraping are precision finishing techniques that require patience and regular inspection.
Canadian standards (CSA B149.1, CSA Z432, Canadian Electrical Code, Part I) apply depending on the tooling context.
Safety is paramount: safety glasses, face shield, file handles, and respect for lifting capacities.
For the exam, memorize the formulas for torque, expansion, cutting speed, and tap drilling.
Practice reading micrometers and calipers until it becomes automatic.
Always double-check your tolerance calculations before machining or assembling.
Precision is the hallmark of the tool and die maker. Every operation, from layout to final inspection, must be executed with rigor and verified. The Red Seal exam evaluates your ability to apply these principles in real-world situations — practice solving tolerance and assembly problems within time limits.
