Design Tips for Low-Volume CNC Machining Production Runs

Low-volume CNC machining production can be an effective solution for businesses that require a reduced number of custom-manufactured items. Contingent upon the complexity of the parts, this technique can provide cost-effective fabrication without the necessity for expensive tooling. Conversely, extensive capital outlays for smaller, more labor-intensive manufacturing runs limit overall profitability.

This holds true for sizable production runs with a significant inventory carrying cost. If capital allocations are written off, and non-recurring charges are, the per unit charge is smaller, allowing for greater profitability.

New products have non-recurring developmental costs, including product design, product-specific manufacturing and quality operations development, programming, and tooling. Once the product is developed, these costs are then amortized. 

The unit costs include material, labor, machine time, and other part-specific costs such as tooling, inspection, and post-processing operations, including packaging. Overhead is also applied to the per-unit cost. If the non-recurring costs have been realized, then shorter runs can still be profitable. There are numerous considerations to optimize your costs to optimize the operation for low-volume machining.

 This article explores the design tips for low-volume CNC machining, the use of standardized and straightforward designs, material selection, and additive manufacturing as an option.

In order to minimize costs for low-volume production, it is essential to keep designs as basic and uncomplicated as possible. New, complex designs typically require additional tooling and machining time and more intricate designs invite quality concerns.

Standardized designs should be pursued for manufacture and it is essential to avoid custom tooling purchases with standardized material sizes, hole sizes, and generous tolerances. Simple product designs will reduce machining time and the likelihood of errors. Additional considerations include the design for manufacturability (DFM) by designing efficiently machined parts, and equipment capabilities should be included in the decision. Material waste is a consideration that not only impacts the product cost but environmental sustainability. 

Other suggestions include the following:

• Standard Holes Sizes – Non-standard sizes require supplemental machining time and increase the cost.
• Specify Tolerances Only when Necessary – If no specific tolerance is listed, parts are machined using the standard tolerance (± 0.125mm or better)
• Radius rather than Vertical Edges – Vertical edges are difficult to achieve since milling tools are cylindrical and create a radius. Reduce additional machining time by:
  • Using the same radius for all internal edges.
  • Making the corner radius at least one-third (1/3) of the cavity depth.
  • Use a small radius or no radius at all on the cavity floor.

• Limit Cavity Depth – Deeper cavities require more machining and waste more material. Minimize costs by:
  • Limit cavity depth to four times their length.
• Limit Thread Length – Keep thread lengths at minimum acceptable depths. Threads longer than 1.5 times the hole diameter do not provide greater strength. Designs should consider the following elements:
  • Use standard hole sizes.
  • Design threads a maximum length of 3 times the hole diameter.
• Minimize Machine Set-ups – One set-up is ideal. 
• Avoid Multiple Surface Finishes – Multiple surface finishes on the same part drives up the cost. To reduce the cost impact:
  • Choose the standard surface finish.
  • Only request multiple surface finishes if necessary.

Machining thin or tall product walls, deep cavities, or narrow pockets is a time-consuming and complex process. The slightest mistake or variation in material quality can lead to product damage and additional scrap costs. Thick solid parts are less expensive to machine, waste less material, and are more stable. To minimize machining costs, consider the following:

• Metal parts – Develop walls that are 0.8mm or thicker.
• Plastic parts – Maintain a minimum wall thickness of 1.5mm or greater.

While machining product serial numbers, descriptions, and client logos can be helpful, using text and additional finishing should be limited. These customized operations increase product costs and further limit valuable machining time. Pursue alternative processes that produce the same effect, such as laser engraving, embossing, silk screen, and paint operations, all of which are quicker and less expensive.

It is essential to minimize costs:

• Remove all text and lettering.
• Choose engraved over embossed lettering, if required.
• Use a minimum size-20 sans serif font (e.g., Arial or Verdana).

It is advised to maintain a standardized material size and selection. Some materials are more challenging to machine, equating to longer machining time and added costs, while softer, less expensive materials are preferred as they are easier to manufacture. Steer away from harder and more costly materials such as titanium.

To minimize costs:

• Consider less expensive material alternatives with similar specifications. Less costly materials are readily available, reducing lead times and inventory carrying costs.

The additive manufacturing arena provides superior solutions to traditional CNC machining operations. Additive manufactured (AM) parts are created layer by layer rather than removing significant amounts of material, and a considerable variety of materials are available for AM processes.

In addition to prototyping, 3D printing and other AM technologies are preferred for low to medium production and products with reasonable product tolerance requirements.

A professional manufacturing partner specializing in contracted services can help other manufacturing organizations by contracting their more difficult products and lower production items. Many are local experts in their specific field or included in a network of vetted manufacturers. The preferred partner should analyze your design, deliver a quotation, provide frequent updates upon selection, inspect the quality of your products, supply quality certifications, and ship the material to your client.

The advantages of prototyping and production with a knowledgeable partner include:

1. Speed

The quick, quality prototyping and production from vetted manufacturing partners reduce lead times and time to market. Production on-demand allows you to respond quickly to the market without turning your facility upside down. Smaller lot sizes don’t require massive internal changeovers for short production runs. 

2. Cost

Risks are reduced due to fluctuating customer orders being outsourced that do not impact your production, and equipment change-overs and global disruptions have far less impact on your facility. Your manufacturing partner receives the raw materials and ships the finished product to the customer, so there is no inventory carrying costs. 

3. Quality

By utilizing experienced manufacturing experts, you gain access to the best at their business. As such, their defect rates are lower thanks to the superior expertise of the expert manufacturing partner.