How to calculate CNC manufacturing costs and increase productivity

To understand performance, productivity, and costs in CNC manufacturing, special methods adapted to technologies and data management are required. Finding optimal machining variables requires advanced knowledge in terms of working times and their associated costs.

Mathematical relationships currently exist that allow for the determination of working times and associated costs in the machining of simple components. However, for complex mechanical parts, these variables are difficult to calculate manually, and therefore must be used Computer-aided manufacturing (CAM) software.

Currently, the use of CAM (Computer-Aided Manufacturing) software has become a basic necessity for all CNC machining teams. It is essential to keep them updated and invest in them. It has been proven that The return on investment of machine tools actualizadas is really high.

These software programmes allow for the simulation of manufacturing processes, where the initial design can be verified and corrections to the project can be made before its implementation. One of the most important advantages of computer-aided design in machining technologies is that it allows for the simulation of the working environment. The simulation of an operation or the entire machining process generally consists of:

• Determine the tool path and prevent collisions between the tool and the workpiece, avoiding impacts on the machine or its devices during manufacturing.

• Determine the machining time, both for each operation and for the complete part. These times include gross machining time, tool change times and machine stop-start times.

The simulation process provides important data regarding CNC machine capabilities, total machining time, and cost; to establish optimal working conditions and variables. Ultimately, all of this provides real criteria that allow for the optimisation of costs in CNC manufacturing. Here we will be reviewing how to calculate these costs, to optimise them and increase productivity.

CNC manufacturing costs to consider

There are various models for calculating CNC machining costs, and a multitude of factors are involved. CNC machining costs depend on a wide range of elements such as:

• The costs of CNC equipment.
• Costs of the tools to be used in the process.
• System machining monitoring costs.
• Quantity and costs of consumables.
• Costs related to the CNC machine personnel or operator.
• Equipment configuration and adjustment, downtime costs.
• Overheads incurred by the manufacturing process.
• Energy cost.

These latter can be subdivided into the direct cost of energy that has been consumed during the manufacturing process, and the indirect costs of supplementary energy consumed during the work process, such as general lighting, heating, etc.

The most important thing for optimisation is to analyse each of these costs in CNC manufacturing and minimise wherever possible. The greater the number of factors involved in simulation and optimisation, the more realistic the savings and productivity increases will be.

Example of cost calculation in CNC manufacturing

Let's look at a simple model that can be applied to the manufacturing of a component, and will likely be of great help in increasing productivity and optimising costs in CNC manufacturing. This model divides the total machining cost into 4 factors: Costs related to the raw material of the part, costs related to the CNC equipment, the cost related to the operator, and the costs of the tools used. Let's look at the following relationship:

Manufacturing = Material Part + CNC Machine + Operation + Tool

The cost of the CNC machine is linked to the write-off of electricity consumed during the manufacture of a single part, as follows, where P is the price and t is the time:

CNC Machine = Purchase Price - CNC Machine Amortisation * Time + Labour * Time + Programming + Electricity Cost * Programming * kW

The operating cost is represented by the operator's remuneration and the manual adjustment that must be carried out on the machine, where f, represents the fraction of programming time, necessary for adjustments and activity programming:

COperación=tProgramación+f*tPrProgramming* Hourly wage

The last cost component is represented by the tools used to machine the part and results from a sum of the cost related to the tool's cutting heads, with respect to the manufacturing time for each tool, edge durability and the number of cutting edges it has, where T_economic represents the economic lifespan of the tool.

Cutting headsTool Economic N° tool edge

As can be seen, this simple cost calculation algorithm can be of great help in optimising CNC manufacturing costs and increasing the productivity of a machine shop. However, the variables in each of these equations must be determined as accurately as possible, hence the importance of simulating an operation or the entire machining process, to feed the algorithm with reliable data. For each machining operation, CAM simulation can provide parameters related to manufacturing regimes, tools and work times.

For example, the cost generated by cutting tools can be determined by knowing the tool price, its lifespan, the number of cutting element replacements, and the price per minute of each tool. Based on the total machining time, the cost incurred by the machine, as well as by the operator and electricity consumption, can be determined. All of this is a result of the appropriate interpretation of the simulation results, which allows for proposing strategies for optimising costs in CNC manufacturing and managing machining operation variables to increase productivity.

Other models presented for the calculation of total machining costs have included energy costs as an additional and independent factor, but it has been shown that these energy costs are relatively low, potentially around 8%. However, for processes where a large volume of parts is manufactured, these energy costs may increase and would be a determining factor when optimising costs in CNC manufacturing.

The costs related to the part material and the CNC machine represent 40% of the total machining cost

It is important to remember that each workshop has different characteristics and the distribution of its costs and expenses can vary significantly from one to another; however, there is a trend in many workshops that shows that the weight of costs related to part material and the CNC machine represent 40%of the total machining cost, tool costs also approach 40%, 12% is estimated for operating costs and 8% for energy costs. From this, it can be observed that reducing work times, decreasing the number of operators, and increasing tool life are strategies that are constantly proposed to reduce costs in CNC manufacturing.

The above is a general idea of the strategies that many metalworking shops with CNC machines consider, but it is very important to carry out a thorough study of the costs associated with each company and to target those that can be reduced, in order to increase profitability.

We can finally conclude that the key to increasing productivity and optimising costs in CNC manufacturing focuses, firstly, on the implementation of CAM software that allows for the simulation of CNC machining processes, in order to obtain reliable data that feeds the mathematical model or algorithm suitable for the workshop's procedures and characteristics, with the objective of facilitating the calculation of the total machining cost. Once the process has been simulated and the total machining cost quantified, the optimisation of costs in CNC manufacturing can be effectively achieved.

It should not be forgotten that the Real-time monitoring of CNC machine production, will be the most efficient mechanism for the evaluation and control of such optimisation, ensuring that changes in the operational variables of CNC machines meet the proposed expectations and generate an increase in productivity.