In the manufacturing industry, material prices, availability of the machine, or even labor rates are commonly cited as causing cost overruns. Nonetheless, poor drafting is one of the least taken into consideration factors that have led to increased manufacturing costs. Decisions in the engineering drawings might be adding hours of machining time, more inspective effort, and expensive rework even before a piece reaches the shop floor.
Even the most developed CAD models produced in SolidWorks, Inventor, or such programs are not able to cover the unclear, incomplete, or poorly designed engineering drawings. The failure of drafting to effectively convey design intent makes manufacturers go through the process of guessing; and it is quite costly.
This blog describes why the cost of manufacturing is raised when a design is poorly created, the number of mistakes that designers usually make, and what are the pragmatic options to correct such problems.
Why Drafting Has a Direct Impact on Manufacturing Cost
Engineering drawings are not just a documentation, they are a guideline to manufacturing. Each dimension, each tolerance, each note, each symbol has a direct impact on the production of a part, its inspection and assembly, and therefore clarity and precision is a necessity in order to perform it correctly on the shop floor.
Loss of clarity in drawings makes manufacturers waste more time in deriving the vague dimensions, seeking clarifications, changing machining plans and redoing mis-produced parts. These inefficiencies translate into higher manufacturing costs, resource wastage and long lead times in manufacturing.
Poor Drafting Issue #1: Missing or Ambiguous Dimensions
Missing or vague dimensions are one of the most widespread drafting issues. The most common assumptions made by designers include providing the manufacturer the chance to guess it out based on the 3D model but assumptions are very dangerous in the production setting.
Dimensions that are ambiguous compel machinists to:
- Make their own interpretation.
- Stop production in order to ask questions.
- Include additional set-up and verification procedures.
Any interruption costs time and time is money. Full dimensioning puts an end to uncertainty and ensures the flow of production.
Poor Drafting Issue #2: Over-Dimensioning and Conflicting Information
Although missing dimensions are troublesome, over-functioning can also be destructive. When the same feature is dimensioned many times or the dimensions do not agree, the manufacturers are left in a dilemma as to which dimensions to believe.
The dimensions that are conflicting tend to cause:
- There will be production delays as clarifications are sought.
- Wrong machining on the basis of the wrong reference.
- More inspection effort.
Well-written drafting can give only the required dimensions, which are clear and not redundant.
Poor Drafting Issue #3: Unrealistic or Excessive Tolerances
One of the largest cost drivers in the manufacturing process is tolerances but they are poorly understood and misapplied. Designers often impose very tight tolerances in a complete drawing without considering whether those limits are actually necessary or not, and it is rarely thought through how this complexity will be added to the manufacturing process.
Manufacturing wise, unnecessary tight tolerances result in slower machine speeds, extra finishing, and extra inspections all of which add more time and expense to the production process. They also increase the possibility of rejection and rework of part. The use of realistic, functional based tolerances assists in sustaining performance and at the same time, manufacturing costs are lowered and the overall efficiency is enhanced significantly.
Poor Drafting Issue #4: Ignoring Tolerance Stack-Up
Tolerance stack-up is a phenomenon whereby a series of individual tolerances is added together resulting in either assembly or functional issues. Weaknesses in drafting practice usually do not address the issue of how part-level tolerances combine at assembly level to create designs that are hard to make assembly.
Parts might not fit properly during assembly when tolerance stack-up is not considered, leading to either manual correction in assembly or rework becoming inevitable. In such cases, manufacturers have to reimburse the design aspects and this means that it takes more labor time, scrap rate and the total cost of production.
Poor Drafting Issue #5: Misuse or Overuse of GD&T
Geometric Dimensioning and Tolerance (GD&T) is an effective tool, errors in its application may disorient manufacturers instead of benefiting them.
Common GD&T mistakes include:
- Incorrect datum selection
- Delivering GD&T in cases where simple dimensions are applicable.
- Unnecessarily over-constraining features.
The bad usage of GD&T makes the inspection more complicated and tend to be misinterpreted. Good GD&T must not make design intent difficult but rather make it clear.
Poor Drafting Issue #6: Vague Material and Finish Specifications
The specifications of material and surface finish have a direct influence on tooling, machining plan, and cost. Callouts like steele or smooth finish are too vague and can be understood in too many ways.
The specification of material not being clear may result in:
- Wrong material selection
- Stalling as suppliers ask questions.
- Sudden performance problems.
The accurate material grades and finish specifications enable manufacturers to plan and quote in a realistic way.
Poor Drafting Issue #7: Drawings That Are Hard to Read
An untidy and badly arranged drawing is a drag to all the processes of production. Superimposition of the dimensions, varying text sizes, and crowded views would tend to complicate the learning of machinists and inspectors to extract data in a short period.
Poor readability leads to:
- Greater interpretation time.
- Increased probability of errors.
- Lower productivity at the work station.
Neat drawing with large spacing enhances understanding and minimizes errors.
How to Fix Poor Drafting and Reduce Manufacturing Cost
- Write the Process with Manufacturing in Mind.
Always think of the way in which the part is going to be produced. The datum dimensions used in machining and reference features of a functional nature as opposed to cosmetic features.
- Strategic use of Tolerances.
Tight tolerances should be used only when necessary by the functionality. General tolerances should be defined in the title block and critical features should be reserved specific tolerances.
- Use GD&T Purposefully
Use GD&T when it is clarifying. Make sure datum structures are realistic manufacturing and inspection arrangements.
- Streamline and Elaborate Drawings.
Filter out junk, make sure to eliminate unnecessary dimensions and set views in order. An easy-to-read sketch will save time in all the production stages.
- Bureaucratize Drafting.
Apply the same templates, title blocks and notes on all drawings. Standardization enhances communication and minimizes error.
Long-Term Benefits of Good Drafting
Qualified drafting is worth the time it takes:
- Reduced costs of manufacturing and inspection.
- Faster production cycles
- Fewer design revisions
- Better supplier relationship.
Good drafting is not an overhead- it is a cost saving measure
Conclusion: Drafting Decisions Have Financial Consequences
Ineffective writing quiets down the cost of manufacturing at each production phase. Unclear dimensions to over-tolerances, minor drafting errors will have an enormous financial effect.
The manufacturing-oriented drafting best practices will allow the designers to minimize the cost, enhance efficiency, and develop drawings that the manufacturers will trust and respect.
In manufacturing, transparency is productivity–and productive writing is profitable writing.