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Debugging NC programs is costly

The preparation and debugging of programs for NC machine-tools are tasks that offer tremendous scope for increased productivity. Manufacturers have realised that that they do not make enough use of their installed machine-tools, and do not get enough effective uptime.

There are several reasons for this :

  • The environment : tool and tooling preparation, planning, deadlines, modes of use, etc.
  • NC programming and program modifications : CAM (Computer-Aided Manufacturing) or manual programming, WOP (Workshop Oriented Programming, i.e. programming in the workshop directly on the machine-tool), modification of ISO programs using one of these methods (NB: ISO is an encoding standard for NC machine-tool instructions).
  • Technology : cutting and clamping conditions, special machining, materials, machines and options, etc.

To take into account the most important aspects of your manufacturing process, and consequently your constraints on the shop floor, we shall take a detailed look here at the most common causes of loss of productivity in the programming of NC machine-tools, to provide you with arguments for formally justifying the adoption of NC machining simulation in your company.
This method is intended as an acceptable compromise between the reality of using NC machine-tools and a global approach encompassing the need to use a software product for machining simulation.
This method also demonstrates the potential for gains that are not necessarily visible day-to-day, given the way that machine-tools are used without a simulation implementation.
The information displayed on this page can then provide a basis for producing a formal justification in cost terms.

Cost factors

Here we shall analyse the items and related costs for the different tasks accomplished by users of NC machine-tools when problems occur in the debugging of NC programs.

Though other parameters such as the stress experienced by NC programmers and operators when proving a new machining program are not included as direct cost factors, it is very important not to neglect them when justifying the adoption of machining simulation.

Number of programs

The number of programs used provides an indication of the extent of a company's programming activity.
To simplify calculations, we suggest that program modifications also be included in this section. This is because when substantial changes are made to existing programs, these can be considered as new programs requiring the same test cycle.

Tests on NC machine-tools

The time required to test a program on the NC machine-tool is the difference between the machining time needed for a real part and the time the test takes. This difference varies according to the test methods that are used :

  • prior machining on a model made of resin, foam, wood, etc. then a real part using the "block by block" method,
  • machining of the real part, but using the "block by block” method,
  • machining of the real part at reduced speeds.

It is clear that if a problem occurs immediate action will have to be taken to modify the program, thereby causing the machine and the operator to become idle while this is done.

Breakage

This is the risk that all NC programmers and operators fear the most. The ability to avoid it can alone justify the adoption of machining simulation.
The breakage of equipment or machinery of course generates the cost of sometimes extensive repairs, but can also be a source of risk for personnel. Everything should therefore be done to eliminate it.
Though technological progress has reduced levels of breakage of tools, clamps and other fixtures, this is still frequent enough to be built into a cost analysis.
Downtime for repairs is included in this estimation to simplify the cost summary table at the end of this document.

Validation by the programmer

A programmer generally checks the ISO program before sending it to the NC machine-tool operator. This checking step essentially consists of reading the ISO program by “simulating” the behaviour of the NC machine-tool.

This time often goes unaccounted as it takes place at the manual or automatic programming station and is therefore included in the programming time.

Depending on the type of machining, the rereading step can vary in terms of time and efficiency relative to the complexity of the programs. Two main aspects determine the complexity of NC programs :

  • The programming strategy for the rough and finishing passes, the approach paths, tool change sequences, and calls to specific technological functions on the machine-tool...
  • The values of the addresses produced by the calculations performed by the CAM programs, notably in 3- and 5-axes for the motion and feed values, as well as by the computations by post-processors for the addresses of the rotation axes and the acceptance of tool compensations relative to the programmed toolpaths.
It is easy to see that the interactions between the NC programmer, the CAD/CAM system and the post-processor are such that in many cases programs must be validated by rereading.

Scrap

This is the cost of the rough stock to which the cost of already performed work must be added :

  • prior machining on a model made of resin, foam, wood, etc. then a real part using the "block by block" method,
  • machining of the real part, but using the "block by block" method,
  • machining of the real part at reduced speeds.

In some cases, the part can be repaired . This repair work does however engender costs that must be factored in to the cost of scrap.

Program correction

The time taken to correct programs is always longer when programmers have to dip back into work that has been filed away and become familiar with the context again.

The cost and time accounted for here is additional time that can be explained by the fact that program modification takes place at the same time as the program is being used or checked, and not at the actual time of programming as would be the case when using a simulation package.

This time generally breaks down as follows :

  • checking the program back out of the file system and checking it in again,
  • understanding and analysing the problem encountered,
  • defining and implementing the modification,
  • updating and saving the program.

Reworking

The time and the average cost of repeating a machining operation can be imputed to that of reworking a part when the modified program can be executed on the same physical part.
To simplify processing, we shall also use this time in the event of a new machining operation when a part has been scrapped.

Desynchronization

When a problem occurs during a machining operation, it is not unusual for the whole manufacturing process to be disrupted. This disruption, caused essentially by things going out of sync in the work schedule generates significant knock-on costs that are not always easy to identify and assess.

Undoubtedly in this type of situation, solving the problem takes priority over analysing the resulting costs. However, these costs do generate idle times at other workstations and can require tool setups to be dismantled and reassembled.

We should also look here at the repercussions for the company's external customers in terms of shipment times.

Dismantling and refitting

In contrast with the case discussed above, “long modifications” cause users to stop production and move on to some other temporary manufacturing task until the program has been modified.

This involves costs incurred by removing the unfinished part and refitting a new part for machining. This cost may appear negligible when the setup is simple, but can be very prejudicial in the case of complex assemblies involving, for example, swivelling heads, probing cycles, etc.

Return On Investment (ROI)

  • Exemple 1 : Machine shop Company
    In this case, the company is working on multiple customers programs, with low cost material (ie. Aluminum, steel…), with several setup on different machines due to complex geometries the operators have to realize.
  • Exemple 2 : Aeronautic Company
    In this case, the company is working on long programs on high cost rough stocks (ie. subcontractor for an aeronautic company with 5 axes and mill-turn machines).


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