A well-prepared CAD model is the cornerstone of an efficient CNC machining quotation process. It is on this basis that the production engineer assesses the feasibility of the design, selects the production strategy, plans the tools and estimates machine running times. This means that the more complete and well-thought-out the model provided to the contractor, the quicker it is possible to prepare a reliable quotation and commence production.
In this article, you will learn:
- which CAD files are preferred by CNC companies and why,
- what technical information must be included in the documentation,
- how to design technology-friendly geometry,
- what additional data affects the quotation,
- what mistakes to avoid before sending a request for quotation.
CAD file requirements for CNC companies
Companies providing comprehensive CNC milling services from design stage onwards, as well as CNC turning with technical consultancy, analyse dozens of models sent for quotation every day. The quality of these files has a huge impact on the speed and accuracy of the quotation preparation. The CAD model is the primary source of technical information. It is used to determine, amongst other things, possible machining strategies, the number of operations, tool selection and potential production risks.
Inconsistencies in the documentation necessitate additional consultations, which in turn prolong the quotation process and increase the risk of misinterpretation. For this reason, most CNC companies have clearly defined requirements regarding file formats and the scope of documentation.
Preferred formats: STEP, STP, X_T, X_B
The most important rule is to submit files in neutral CAD formats, which allow the model to be opened without any issues in various CAM systems:
- STEP / STP – the most commonly used standard for 3D data exchange. It preserves the solid, surfaces and structure of the model, allowing machining preparation to begin quickly;
- X_T (text-based Parasolid) – a highly precise format that works particularly well with industry-standard CAM systems. It allows models to be transferred between different engineering applications;
- X_B (binary Parasolid) – the binary equivalent of X_T, typically with a smaller file size and faster import times.
3D model vs 2D technical drawing
A 3D model forms the basis for preparing quotations and production processes. It enables the determination of a part’s geometry, tool access, the need for special tooling, and the estimated machining time. Without a 3D model, preparing a reliable quotation is much more difficult, and often simply impossible.
However, a 2D drawing remains a key supplement to the documentation. It is on this drawing that tolerances, quality requirements and technical information are specified. Professional preparation of a technical drawing for CNC machining significantly reduces consultation time and minimises the risk of misunderstandings.
Tolerances and technical annotations
In addition to geometry, quality requirements are of paramount importance. These have the greatest impact on machining time, tool selection and production costs. Vaguely defined tolerances are one of the most common causes of discrepancies between the customer’s expectations and the final quotation.
How to dimension a model correctly
Dimensioning should clearly indicate the measurement bases and critical surfaces. An excessive number of dimensions can make interpretation difficult, whilst a lack of them necessitates making technological assumptions.
It is advisable to apply the principle of functional dimensioning – primarily marking those dimensions that have a real impact on the assembly and operation of the component.
Geometric tolerance designations
GD&T (Geometric Dimensioning and Tolerancing) allows the requirements for parallelism, coaxiality and flatness to be precisely defined. This enables the manufacturing engineer to select an appropriate measurement and inspection strategy. Well-defined geometric tolerances in the CAD model minimise the risk of quality disputes after production.
Required information on surface roughness
Surface roughness is defined by the following parameters:
- Ra (arithmetic mean of deviations of the profile from the mean line);
- Rz (maximum profile height).
The Ra and Rz parameters directly influence the selection of tools and the duration of the finishing operation. The lower the roughness, the more finishing passes are required and the higher the cost.
Optimising geometry for CNC machining
Optimising the geometry of a workpiece for machining is one of the most important factors in reducing production time and lowering unit costs. A design created solely from a functional perspective may be difficult or very costly to manufacture. Even minor design changes can reduce machining time by as much as several tens of per cent, minimise tool wear and reduce the risk of manufacturing errors.
As Krzysztof Piwowarczyk, an expert at RADMOT, emphasises:
“The most cost-effective designs are those created with technology in mind from the outset. Collaboration between the designer and the process engineer at the CAD stage helps to avoid many costly changes later on.”
Avoiding sharp internal corners
Sharp internal corners are one of the most common manufacturing challenges. Standard milling cutters have a cylindrical geometry, so achieving a perfectly sharp angle would require specialised tools or additional EDM operations. And every such operation means a longer process and increased costs.
The use of small internal radii significantly simplifies production. This allows standard tools to be used, reduces CAM programming time and extends the service life of the milling cutters. Additionally, it improves the stress distribution within the material, which in turn increases the service life of the workpiece.
Radii adapted to the tools
Radii matched to standard milling cutter diameters enable stable machining with higher cutting parameters. Radii that are too small necessitate the use of small-diameter tools, which are less rigid and more prone to wear.
Increasing the radius by even a few tenths of a millimetre often allows the use of a significantly larger milling cutter, which reduces machining time and improves process stability. In mass production, this is of enormous importance for unit costs.
Minimum wall thickness
Walls that are too thin result in a higher risk of vibration, deformation and measurement issues. During machining, cutting forces act on the workpiece, which can cause thin components to deflect. Consequently, achieving the required tolerances becomes more difficult and time-consuming.
Increasing wall thickness, even by a small amount, can significantly improve process stability. In many cases, this change does not affect the functionality of the component, whilst significantly reducing production costs.
Additional information affecting the quotation
A quotation for CNC machining cannot be based solely on an analysis of the geometry. Logistical and production information is also of great importance. It determines the degree of process automation, production organisation and opportunities for cost optimisation.
The prospect of long-term cooperation is also an important aspect. Series production projects allow for investment in tooling and automation, which translates into lower unit prices.
Material specification
The type of material influences almost every aspect of the production process – from cutting parameters right through to the selection of tools and cooling strategies. Aluminium, tool steels and titanium alloys require completely different technological approaches.
A lack of information about the material makes it impossible to prepare an accurate quotation. Differences in the production cost of the same component can be very significant depending on the raw material.
Batch size and order frequency
Bear in mind that single-unit and mass production are two entirely different technological scenarios. In mass production, it is possible to prepare dedicated tool holders; process automation and toolpath optimisation are also viable options.
Furthermore, regular orders also make it possible to maintain a stock of materials and reduce lead times.
Required surface treatments
Anodising, hardening or passivation require additional logistical operations and quality control. Taking these into account at the quotation stage allows the entire production chain to be planned.
The most common errors in CAD models
Errors in the documentation prolong the quotation process and necessitate additional consultations. Eliminating them speeds up the preparation of the quotation and reduces the risk of misunderstandings. Above all, attention should be paid to:
- missing critical tolerances – the lack of information on tolerances forces the production engineer to make assumptions. This can lead to costs being underestimated or overestimated,
- radii that do not match the available tools – radii that cannot be machined with standard cutters increase production costs and extend lead times,
- overly complex geometry – excessive detail increases the number of operations, the risk of errors and production costs.
A well-prepared CAD model is the first step towards efficient and predictable production. Check out our range, request a free quote and get in touch with us.
FAQ – frequently asked questions about CAD preparation
Can I just send an STL file for a quotation?
You can, but the STEP or Parasolid format is definitely better. STL makes it more difficult to prepare the manufacturing process.
What information, apart from the 3D model, is required?
A 2D drawing, tolerances, material and batch size.
How long does it take to prepare a quote?
Usually from a few hours to a few days. It depends on the complexity of the project.
Can a CNC company help optimise the design?
Yes, this is part of professional technical support.
What should I do if I don’t have a 3D model, only a sketch?
Many companies offer design support. Often, a free quote and technical consultation are also available.
