Understanding Engineering Drawing Symbols: Key Reference for Custom MIM Parts

Symbols used in engineering drawings refer to a standardized mark employed in engineering drawing. For example, the geometrics of a part, whether counterbore or diameter.

The post Engineering Drawing Symbols appeared first on The Engineering Projects. You need to take sintering shrinkage, gate locations, as-sintered Vs secondary-machined tolerances into account when interpreting these symbols in custom MIM (Metal Injection Moulding) parts to avoid expensive mis-interpretation.

A feature control frame is a precise symbol but the symbol guides treats every manufacturing process as same. A CNC drawing uses a tolerance that may be unrealistic or measurably expensive in a MIM drawing because MIM parts shrink substantially during sintering. This manual translates the fundamental symbol grades’ types, as well as how MIM’s shrinkaging, gating, and secondary – operation behaviours change how you go about calling them out.

Key Takeaways

● The engineering drawing symbols are used to communicate the geometry, tolerances, surface finish, and material requirements of your part in an efficient way, without a lot of text. If the symbols on your engineering drawing are misunderstood, scrap, rework, and disputes with your supplier may result.

● GD&T symbols fall under five categories: Form, Profile, Orientation, Location, and Runout. Each sits inside a feature control frame that defines the allowable zone of tolerance.

● MIM-specific standards (MPIF Standard 35-MIM, ASTM B883, ISO 22068) control material and dimensional acceptance whilst universal drafting standards (ASME Y14.5, ISO 128, ISO 1302) control how symbols are drawn.

● MIM parts shrink significantly during sintering, so the drawing must define whether a dimension can be achieved after sintering or requires a secondary machining, sizing, or grinding operation.

● The current ASME Y14.5 standards recommend against using symmetry and concentricity callouts, as they are hard to inspect. Instead, you should use position or runout controls.

● On MIM drawings, sharp internal corners should be avoided since they concentrate stress both during molding and sintering; fillets and radii should be called out directly.

● Verifying the title block, projection system, and datum scheme prior to reading GD&T callouts avoids the most frequent and expensive drawing misreads.

What Are Engineering Drawing Symbols?

Symbols used in engineering drawing are graphical marks which are used to present geometry, tolerances, surface finish and material. The symbolic system in GD&T (Geometric Dimensioning and Tolerancing) which defines the allowable geometric variation of a part using a feature control frame. According to most reference guides, they only cite ASME Y14.5-2018 as being the only standard that oversees this system. While this is true, the actual depth of a drawing and how effective it is, is based on a dual standard that the MIM platinum system uses. This is because it is the combination of ASME Y14.5-2018, ISO 128, and ISO 1302 that describe the universal rules of drafting. Conversely, MIM specific overlay standards govern the material acceptance and dimensions’ acceptance. The drawing reviews related to custom MIM parts often fail due to not treating these as a combined system instead of two separate layered systems.

Why Engineering Drawing Symbols Matter for Custom MIM Parts

It is crucial to understand engineering drawing symbols. Misinterpretation can lead to scrap or rework by designer or shop floor. It can cause assembly failures and supplier disputes. The likelihood of occurrences is raised under MIM. 60-80% of dimensional variation in MIM parts occurs in the injection molding not sintering. (Source: MIM Tolerances: Design Best Practices & Control Methods, 2026) What this means in practice is that it is the tolerance and datum callouts of your drawing that dictate explicitly which of the manufacturing stages, molding or sintering, that a supplier must actually control. Furthermore, vague callouts push cost and risk into the wrong stage completely.

Core Basic Feature Symbols

Diameter, Radius, and Counterbore

A basic feature symbol of a diameter, depth, radius, counterbore, spotface, or countersink marks shall be placed adjacent to a dimension. These symbols are used for basic annotation purposes across processes.

The MIM Corner Caveat

Corners that are sharp and have internal angles as shown by these basic symbols create a major problem in MIM as they increase stress during molding and sintering leading to cracking. In the design of parts for MIM, it is recommended to use fillets and radii of 0.4 to 0.8 mm to decrease stress during the moulding and sintering process and prevent premature failure of the parts (source: Cadmore, Designing for Metal Injection Moulding, 2025). We have found that when this radius requirement is explicitly flagged on the drawing (as opposed to being left implied), then that is the single quickest way to prevent RFQ revision cycles with a MIM supplier.

GD&T Symbols: Form, Orientation, Location, and Runout

The GD&T symbols are placed inside a feature control frame. Furthermore, these are separated into five major categories i.e. Form, Profile, Orientation, Location, and Runout. Different aspects of geometric variation are controlled by each category. Form controls flatness and straightness while Location controls position and concentricity. Many reference guides leave out the fact that, according to the current ASME Y14.5, symmetry and concentricity are not to be used anymore since they are hard to inspect (VMT CNC, Engineering Drawing Symbols Guide, 2026). The substitute option is MIM-able and more inspectable which is run-out controls and position controls.

Surface Finish and Datum Symbols

Surface finish symbols specify the quantifiable requirements for surface texture (e.g. Ra or Rz values) and the basis of measurement, and not cosmetics. The difference is more critical in MIM, as as-sintered surfaces do not have the same texture as secondary-ground or machined surfaces. Datum schemes for MIM components must meet the same level of scrutiny. Flatness, perpendicularity, concentricity, true position or tight alignment requirements should be reviewed against the functional surfaces and inspection method before the tooling is cut, since some may need machining or sizing or dedicated fixture to hold.

How MIM-Specific Factors Change Symbol Interpretation

MIM-specific factors affect symbol interpretation because the physical changes of sintering shrinkage, gate location, and parting lines affect which size and shape tolerances are achievable as-sintered. The feedstock enters the cavity through the gate and there are numerous gate types such as tab, tunnel, jump, drop, etc. Each of these types will leave a small vestige that the drawing should locate explicitly, away from any critical functional surfaces. All MIM parts have a witness line showing where the two halves of the mould come together. A creative design can hide this, but the drawing should still indicate where it is expected to be. Drawing callouts of gate and parting-line tolerance zones up front eliminates one of the most common RFQ back-and-forth questions we typically see.

Symbol Quick-Reference for MIM Parts

Which Standards Govern MIM Drawing Tolerances?

The dimensional tolerances of MIM drawings are under the jurisdiction of MPIF Standard 35, ASTM B883 and ISO 22068, via which testing, inspection and acceptance criteria are defined on a comprehensive basis with reference to specifications on dimensional tolerances, surface finish and mechanical properties (Source: PatSnap Eureka MIM Sintering Report, 2025). MPIF is encouraging readers to check their edition. 35-MIM Materials Standards for Metal Injection Molded Parts, 2025 Edition adds new standards for titanium alloys and updates for corrosion resistant 17-4 PH stainless steel (Source: MPIF, 2025). In our supplier RFQ review experience, we have seen quoting delays due to a small detail, an outdated edition cited on a drawing.

Tools, Checklists, and Practical Application

To avoid misreads on an engineering drawing, a quick checklist will take you a long way. First, check the title block to confirm a drawing number, revision, material, and governing standard. Then check the projection system first-angle or third-angle. After that, identification of the primary, secondary, and tertiary datums is imperative before reading any GD&T callout. In case of MIM parts, the fifth step to the quoting process is to confirm which dimensions are as-sintered versus secondary-machined. This single flag will change both quoted price and achievable tolerance. The most efficient way to reduce RFQ cycles for customized metal parts is with this five-step habit.

What's Next: Preparing Your MIM Drawing for Quoting

Prior to providing a drawing to a MIM supplier, ensure to check five things; the governing drafting standard indicated in the title block, the datum scheme and functional surfaces, which tolerances are as-sintered versus secondary-op, surface finish callouts by zone, and the material standard reference (i.e., MPIF 35-MIM edition, ASTM B883, or ISO 22068). In all the cases we’re aware of, running through this list before sending off the submission, rather than after a first-pass quote comes back with questions, will save at least one full RFQ cycle.

Conclusion

MIM is not simply a geometry. You need to layer in how MIM’s shrinkage, gating and sintering behaviour inform what’s possible, to engineering drawing symbols. As the global market for MIM parts matures and quality expectations rise in medical, automotive, and electronics, adding an extra review pass to ensure the drawing is right the first time is worth it. Use the checklist above on your next drawing and contact us with your specifications for a first-pass custom MIM part quote.

FAQS

1. What are engineering drawing symbols used for in MIM parts?
 Engineering drawing symbols communicate important information such as geometry, dimensions, tolerances, surface finish, and material requirements. For custom MIM parts, they also help suppliers understand which features can be produced as-sintered and which may require secondary machining.

2. Why are GD&T symbols important for custom MIM components?
 GD&T symbols define the allowable geometric variation of a component. They help control features such as flatness, straightness, position, orientation, and runout, which is especially important in MIM because shrinkage during sintering can affect final dimensions.

3. How does sintering shrinkage affect engineering drawing tolerances in MIM?
 Sintering causes dimensional changes in MIM parts, so a tolerance that is easily achievable through CNC machining may be difficult or expensive to achieve directly through MIM. Drawings should clearly identify whether critical dimensions are required as-sintered or after machining, sizing, or grinding.

4. Which standards are commonly used for MIM engineering drawings?
 General drafting and GD&T requirements may use standards such as ASME Y14.5, ISO 128, and ISO 1302, while MIM-specific dimensional and material requirements can reference MPIF Standard 35-MIM, ASTM B883, and ISO 22068.

5. What should be checked before sending a MIM drawing to a supplier?
 Before requesting a quote, check the governing drafting standard, datum scheme, functional surfaces, as-sintered versus secondary-machined tolerances, surface finish requirements, and the applicable material standard. Doing this can reduce RFQ revisions and drawing misunderstandings.