June 2006

CAD/CAM - Cutting metal efficiently

Metal cutting industries have long realized that productivity cannot be guaranteed just by using expensive CNC machines. To be profitable in CNC machining, metal must be cut efficiently. And the three important links that lead to profitability are CAD/CAM, DNC and Machine Monitoring.

Let us focus on the first link in the productivity chain: CAD/CAM. Since most of the metal cut worldwide on CNC machines is in milling and turning, this article restricts itself to these two machining applications.

Programming for CNC machines has traditionally been done manually by a specialist programmer. The programmer is typically a machinist who is also familiar with the programming codes of CNC machines.

The process of writing the program is actually tedious and error-prone, and involves little creativity. The creative part of the process is what any good machinist already knows:

Studying the drawing
Process planning
Tool selection
Deciding work holding

These creative skills can only be acquired through actual experience on the shop floor. When CNC machines started seeing widespread use in shop floors in the early eighties, the general idea was that programming required more computer software skills and barely adequate machining skills. It was thought of as too exotic a skill to be managed by ‘ordinary’ machinists. CNC machines were still an experiment in a lot of shop floors. There was the phenomenon of hordes of fresh engineering graduates or post-graduates being appointed as CNC programmers, and they had little or no knowledge of the creative ‘engineering’ part of the programming process. Shop owners soon realized that their expensive CNC machines were not really productive. Their failure can probably be attributed to fact that they thought programming was everything.

With computers becoming popular and cheaper, it made eminent sense to allow computer software to generate the actual CNC program. A computer aided CNC programming software offered 2 distinct benefits:

There is no need to know CNC programming codes for any CNC system
The programmer spends time on the creative work to improve productivity, instead of being bogged down in routine programming.


CAD/CAM applications

CAD/CAM software comes in all sizes and applications, and there is no one size that fits all. It is best to carefully select a software that lets you program your parts easily. From the point of view of programming, metal-cutting applications can be classified according to the tool motions involved and as a corollary CAD/CAM software too gets classified this way.

2D

The tool moves in a single plane. There is no programmed tool motion in the third axis. Example : Turning

2.5D milling

The tool moves in a single plane at a time, and along the third axis for depth. It generates flat surfaces and holes this way. Example : Production milling

3D milling

The tool moves in 3 axes simultaneously, forming complex surfaces.
Example : Die milling

5 axes milling

The tool moves in 3 axes simultaneously with its axis keeps changing continuously, forming complex surfaces. Example : Aerospace parts milling

What it can do for you

Apart from generating NC programs, CAD/CAM offers a host of other benefits. Here are some examples how CAD/CAM can help the user to become more efficient and productive.

Reduced machine downtime resulting from checking programs on the machine in dry run or single block mode. Down time is drastically reduced since all or most of the dry run checking can be done on the PC.

Reduced machining times. A good CAD/CAM software can generate programs with efficient cutting logic and optimal tool paths, and hence reduce the machining time.

Scientifically determined cutting parameters are used from the database and not dependent on thumb rules of individual programmers which invariably lead to under-utilization of cutting tools. Low cutting parameters are the biggest cause of inefficiency on most shop floors.

Standardised cutting parameters. A common problem on large shop floors with multiple programmers or programming done by operators is that different parameters are used at different times for cutting the same material with the same tool. This is eliminated by a standardized cutting parameters database.

Interchangeability of programmers is easy. In the absence of the original programmer of a part, or even when reviewing a program for re-engineering after a few months, it is impossible to understand the program. CAD/CAM offers complete documentation of the programming process to avoid this problem. The better CAD/CAM systems automatically generate shopfloor documentation like the process sheet, tools list and accurate cycle time sheet.

Reduces operator errors resulting from improper understanding of programs. NC programs generated by CAD/CAM are fully documented with tool names, operation names, program date, part number, etc. There are no misunderstandings caused by verbal instructions.

Interchangeability of programs between machines. Once a CNC program has been made, the program for any other machine can be quickly generated in seconds. In manual programming, if a program is written for a particular machine but the machine is unavailable because of a breakdown, it must be written all over again for a different machine.

Benefits at a glance

No knowledge of CNC programming required.
A good machinist can become a good programmer overnight.
Avoids error prone and time consuming mathematical calculations.
Drawing data can be directly used to generate NC programs.
Automatically generates fast and efficient part programs.
Prompts standardized cutting parameters from database.
Reduces machine downtime for first part checking.
Reduces first part rejection / rework.
Simplifies tool selection from tool database.
Quickly check alternate machining plans, tools and cutting parameters to reduce cycle time.
Generate cycle time for use in job quotes and scheduling.
Interchangeability of programs for similar machines having different CNC systems.
Expensive options on the CNC control system can be avoided.

Software tool replaces cutting tool

This example shows how CAD/CAM software can help you cut tooling costs dramatically. Cutting ordinary threads is a simple process on a CNC lathe, using tools that have the same profile as the threads. For example, a 60 degree metric thread is cut with a tool that has a 60 degree metric thread profile. Cutting profile threads is however a complex operation. Such threads are used on ball screws, rope drums, screw conveyors, etc. Special form tools with the same profile as the thread are usually used. The process is simple but the cost of tooling is high.

Use of CAD/CAM in such cases can be far more cost effective, using standard ISO turning tools.

In an actual example, the initial cost of the tooling for form tools was proposed at Rs. 20 Lakhs. By investing in a CAD/CAM software which costs 10% of this amount, the part can be machined using standard ISO tools.

Using form tools

The tool path is the same as in cutting normal threads. Threading passes are performed at increasing depths till the final depth is reached. Each pass results in a thread form with partial depth. Demerits of this method:
Very expensive special tool.
The tool requires expensive regrinding or repetitive investment on new tool.
The accuracy of the thread profile depends on the accuracy of the tool.
Accuracy and surface finish of the thread worsen as the tool wears out.
Low repeatability.
Logistics problems – periodic reordering, minimum order quantity, etc.

Expensive tooling inventory

Using standard ISO tools with CAD/CAM

The tool moves along the profile in small increments. The whole cutting process is generative and similar to normal thread cutting on a CNC lathe.
Low tool cost – standard ISO tool with indexable insert.
Insert can be changed when it wears out.
The accuracy of the thread profile depends on the program.
Accuracy and surface finish can be controlled by replacing the insert in time.
High repeatability because of indexable insert.
No logistics or inventory problems.

Not just programming

Programming is just ONE of the things that need to be done before cutting a part. More money and time is spent on various other downstream preparatory tasks.

To program a part, the typical programmer quickly makes a mental process plan, writes the NC program, cuts the part and goes on to the next part because of production pressures. He has no time to pause and think whether the machining sequence that he first thought of is the best, whether he is using the machine’s spindle power to its capacity, or whether he can improve upon the cycle time. He has no time to document the program, prepare a process sheet for future reference, or prepare a detailed tools list for the operator.

As a result, the program that is going to be used for machining is most probably not going to result in the shortest possible cycle time. Because he did not make and file away the process sheet, when he needs to make a small change in the program six months down the line, he is going to spend a lot of time wading through it trying to figure it out. Because he did not give the operator a proper tools list, there is a high chance that the operator will make a mistake in setting the tool that will result in wasted time or a collision.
A good CAD/CAM system will do a lot more than programming, generating documents like these automatically:

Cycle time sheet with accurate operation-wise cutting and idle times. This can be used to analyze machining strategies and quickly try out different tools, processes and cutting parameters to arrive at shortest cycle time. The cycle time can also be used for making job quotes or scheduling. Process sheet with details of operations and cutting tools can be used as future reference, in case changes need to be made in the program at a later date.

Spindle power requirement for each operation, to determine if the spindle is being used to its full capacity and whether it is being overloaded.


Tools list with a list of tool numbers and tool offset numbers which can be given to the machine operator for setting tools.


Do you need it at all?

Use of CAD/CAM software is justified if these conditions apply:

High variety of parts
Frequent programming
Low batch sizes
Necessity of getting parts first time right
High cost of first-part rejection or rework
Using lower skilled persons to program


Deciding on the right software

CAD/CAM originated in the aerospace industry, and till very recently most of the biggest vendors of such software thought that only 3D and 5 axes milling required it for programming. Over the past 5 years many of them have realized that it can be of great help in the general engineering industry in milling, turning and other applications.

Most CAD/CAM vendors now have modified versions of the original ‘high end’ software for what they call ‘low-end’ applications. The logic here is that if the software can do 3D and 5 axes machining, it should be able to do 2.5 and 2D machining! This is like a truck manufacturer redesigning the front panel and seats of the truck to look like a car and saying “We too make cars”. The truth is that 2D machining is NOT a subset of 3D machining. Each application is genetically different, and the software for it needs to be designed specially from the bottom-up. Using a wrong software tool will impact hugely on the productivity. This is the biggest pitfall when trying to decide what CAD/CAM software to buy.

Questions you must ask about the software:

Is it made for your type of application?
Does it have special tool paths to reduce your cutting times?
Is it easy and fast to use?
Does it have specialized machining operations with unique and efficient tool paths for each?
Does it have specialized CAD functions for your application?
Does it have specialized postprocessors for your CNC machines?
Does it have a good library of tools, inserts and cutting parameters?
Does it only do programming, or can it generate other shop floor documentation too?
Can it calculate the cycle time accurately?
Does the vendor know machining to offer support or is he just a box-pushing reseller?


What you must expect

The shop floors of the world are littered with CAD/CAM software that is lying unused, bought with inadequate thought or with too high expectations. Keep your expectations realistic, and you will not be disappointed. What you must expect:

No software will solve all your programming problems. There may be parts with unusual operations or tool path requirements that you may have to program through special methods.
The software cannot replace the programmer – it can only assist him.
The creative part of the process – decisions on process planning, work holding, tooling and FS - must still be done by the programmer.
Only well designed software can reduce cutting times.
Programs generated may not exactly match manually written programs, and may be longer.
The right software can reduce your programming and documentation time, so that you have more time to do the creative work that improves productivity.

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Author
M.Hidayath
Country Manager
Cadem Technologies Pvt.Ltd.

About the author
Mohammed Hidayath is the Country Manager at the Bangalore based Cadem Technologies Pvt. Ltd. He is a Mechanical engineer with MBA in marketing and has a keen interest in being part of the manufacturing industry. He is responsible for marketing Cadem’s range of CNC software in India. Having been in the industry for the past 12 years, he has a wealth of knowledge in assessing needs and implementing CAD/CAM in a variety of shop floors, from small job shops to large corporates. He has been invited several times to speak at CNC user group seminars on CAD/CAM, including IIT, Mumbai.

About Cadem
Cadem specializes in developing software products for CNC applications. Its range of uniquely designed CAD/CAM software products is helping manufacturers around the globe to cut metal intelligently and remain profitable in the highly competitive metal cutting industry. Apart from CAD/CAM software, Cadem’s DNC and Wireless CNC machine productivity monitoring systems are industry standard. It has an installed base of about 10000 systems. Cadem is recommended by almost every CNC machines builder in India and sold as OE by a few. Developed in India with pride and used worldwide. For more information on Cadem see www.cadem.com .