Will the shaft fit if the order only says “20 mm hole”?
Compare the limits for both parts, their finished condition and the way the assembly needs to work.
Imagine ordering a locating pin and its mating part. Maintenance staff need to remove the pin periodically. Both quotations say “20 mm diameter”, but only the hole drawing carries a tolerance; the pin is described by nominal size alone. One delivered pair may go together, yet that says little about the next batch. This is a hypothetical purchasing example. A useful quotation connects the limits for both parts to the movement and retention the assembly actually needs.
A hole tolerance does not specify the whole fit
The nominal diameter supplies a reference size. The minimum and maximum permitted hole sizes, together with the corresponding shaft limits, describe the possible size relationships between the parts. Paying for a more accurately machined hole does not define the intended assembly if the mating shaft remains unspecified. A quotation needs to answer for both mating parts rather than treating “precision” as a complete requirement.
If the drawing uses classes such as H7 and g6, read the pair with the nominal size and applicable standard. The manufacturer’s reference tables include size-range notes.[1] Before reusing a pairing from another drawing, establish which dimensional range and function apply here. The class designation is a way to express a requirement, not a substitute for deciding what the assembly must do.
Combine the limits before relying on a trial pair
For arithmetic only, suppose the hole is 20.000–20.020 mm and the pin is 19.990–20.010 mm. These are invented limits, not values assigned to a standard fit class. The smallest hole minus the largest pin gives −0.010 mm. The largest hole minus the smallest pin gives +0.030 mm. These two calculations reveal the range that can be hidden behind the same nominal diameter.
Under this hypothetical specification, some combinations have interference and others have clearance. A sample pair that slides together by hand cannot demonstrate the behaviour of all combinations within those limits. For a removable maintenance pin, the next decision is the required removal and location behaviour. The responsible designer can then set paired limits that address that function, rather than accepting whichever sample happens to fit.
- 1The part with the hole and its size limits
- 2The mating pin and its size limits
- 3Compare deviations from nominal size
Schematic parts and invented tolerance zones. Values on the right are deviations in μm from a 20 mm nominal diameter. Shapes and gaps are exaggerated; these are not recommended tolerances or scale drawings.
Size limits and assembly behaviour are different questions
Even with agreed diameters, hole alignment over its length, shaft straightness, burrs and surface condition can affect assembly. A locating feature also has a position relative to other features and datum surfaces. MISUMI’s pin example discusses the mating diameters, screw fastening and geometric relationships separately.[2] A diameter comparison alone therefore leaves part of the practical question unanswered.
Mark the removable interface and the base attachment separately on the drawing. State whether the specified dimensions apply before or after any plating or other finishing operation. Where operating temperature, dissimilar materials, lubrication or contamination matter, include those conditions in the design assessment. This gives the supplier a defined manufacturing task while allowing the machine-side engineer to evaluate how the parts will work in service.
Compare what the quotations actually cover
“Precision machining” does not tell the buyer which characteristics will be checked or at what production stage. Compare responses to the same drawing revision: paired size limits, finished condition and inspection scope. The table illustrates possible replies to this purchasing example and the questions they leave open. It is not a record of real supplier quotations or prices.
A matched pair and freely interchangeable replacement parts create different acceptance questions. If an assembly trial is required, agree which pieces or batches it covers and how a pass will be judged. “The supplied pair assembled” should remain evidence about that pair. Future replacement combinations need an acceptance plan that addresses the intended interchangeability rather than assuming it from one successful trial.
| Item | Reply received | Question before ordering |
|---|---|---|
| Diameter | Reply received20 mm hole and 20 mm pin | Question before orderingWhat are the lower and upper limits for each? |
| Inspection stage | Reply receivedDimensions checked after machining | Question before orderingDoes this cover the final surface-treated condition? |
| Assembly | Reply receivedOne sample pair could be inserted | Question before orderingWhat applies to other pieces and the next batch? |
| Inspection scope | Reply receivedA report will be included | Question before orderingWhich characteristics, methods, pieces and batches? |
Send the mating-part context with the RFQ
Include both drawing numbers and revisions, quantities, the feature’s function and the reason removal is required. If the mating part must remain on the machine, separate its original drawing from present measurements and wear observations. A worn diameter is evidence of current condition; turning it into the new nominal size could lose the original design intent. Photographs can help locate the feature but do not replace its dimensional definition.
Ask the supplier to identify the manufacturing and inspection scope they can offer against the design and acceptance requirements, with deviations and unknowns made explicit. Where a prototype assembly is needed, agree the checks and the person authorised to approve changes before moving into production. This is the information that helps a Japanese supplier assess the inquiry and makes the eventual quotations more useful to compare.
Questions to adapt for a quotation
Open supplier questions
We are considering [pin and mating part] to drawings [numbers/revisions], quantity [quantity], for [location/removal function]. The attached drawing separates the removable interface from the fixed attachment. For the hole and pin separately, please confirm manufacturing capability against the specified lower/upper limits, the finished condition and surface requirements, and the inspection/report scope. For an existing mating part we attach the original drawing separately from current measurements. Identify deviations and unresolved questions, prototype/production scope and lead-time assumptions. Proposed changes remain subject to approval by our responsible designer.
Sources and scope
- MISUMI — Dimensional tolerances and fits (technical data)
Manufacturer reference tables, including dimensional-range notes. The full current normative standard was not reviewed.
- MISUMI — Commonly used combinations of fits
A specific locating-pin example, used to distinguish fastening and geometric relationships from mating sizes.