All Shop Floor Operations References

Machine Qualification - Alternatives in Evaluating Machine Tool Accuracy

Friday November 07, 1997
INTRODUCTION

A PMPA member called and related this story. See if it sounds familiar.

"About three years ago, we were running a part on an older multi-spindle machine and it couldn't consistently hold the tolerances required by our customer. We sent RFQ's to a number of machine tool builders and selected one. Money was placed down on the machine and it was delivered. However, it couldn't hold the tolerance specified in the RFQ. The builder had as many as five people in the plant at one point trying to get it right. They never could get it to hold the required tolerances, so we got our money back plus expenses and sent the machine back. It was a frustrating and time consuming experience."

In the precision machined parts industry, one of the worst situations is to buy the wrong machine. It is a waste of time and money for both the buyer and builder. In addition, there is the chance of disappointing a customer that may have been depending upon parts from the new machine. When it is discovered that the machine cannot hold the required tolerances as planned, it may be too late to meet the customer's production schedule.

The purpose of this paper is to discuss some of the alternatives in evaluating the accuracy of new and rebuilt machine tools. The emphasis will be on the types of machines used by the precision machined parts industry, such as multiple spindle automatics.

A great deal of material has been written on this subject, much of it rather complex. In order to keep this paper to an easily readable length, the specific technicalities have been kept to a minimum. Hopefully, this paper will inform readers of considerations that they may not have been aware of, and which they can explore on their own if interested. For those wishing to study this topic in more detail, the PMPA has compiled a loan file entitled "Machine Qualification," which would be a good initial source of information.

A number of machine tool builders were contacted in the research for this paper, and their assistance is greatly appreciated. Special thanks go to Index Corporation and Tornos Technologies for the detailed and thorough information they provided. It is available in the loan material noted above.

PART-SPECIFIC MACHINE CAPABILITY STUDIES .

With the advent of SPC, this method has become very popular. It basically starts with the machine buyer showing the builder the part which is to be produced. The dimensions that are to be held are specified, along with any SPC requirements, such as Cp, Cpk, Pp, and Ppk. The buyer specifies that the machine purchase is dependent upon the builder verifying the machine's capability on a specified production run (called a "runoff"). The first runoff is usually done on the builder's premises. During the runoff, a specified number of parts are produced, and SPC data is collected. If this runoff passes, a second runoff is done after the machine is delivered to the buyer's plant. In both cases, these runoffs are supposed to be done in a way that simulates production conditions. For example, the parts per hour, efficiency rates, and tool change intervals may be specified.

This method, when done properly, is the best at proving a machine's capability on a specific part, and should be considered if the machine is going to be dedicated to one job. This is why the method is commonly used by large corporations that may have minimal machining experience, but want to make their own product.

A number of PMPA member companies have written their own procedures for this type of qualification. Some of these are on file with the PMPA for the interested reader to borrow. Another good resource is the TE-9000 Supplement. This booklet is part of the QS-9000 series published by the Automotive Industry Action Group (AIAG). Intended for tooling and equipment suppliers to Chrysler, Ford and GM, it contains a section called "Machinery Qualification Runoff Requirements". This booklet is very inexpensive, and a company may want to consider modeling their machine qualification procedure after the one in it.

In evaluating a new or rebuilt machine, a part-specific runoff is one of the most common methods. However, it does have its drawbacks. The method is very time consuming if done correctly. To be done properly, the runoff has to be done in a way that replicates production conditions. A machine builder may have good intentions, but they may not have the same appreciation for what "production conditions" and "down time" mean that a production job shop does.

Another problem is that a part-specific capability study requires the machine builder to be involved in engineering the job. In the case of a "turn key" solution, the builder will be doing all of the engineering. The first question is whether the builder is capable of doing this. The builder may make an excellent machine, but this does not necessarily mean that they know as much about tooling and machining as the job shop buyer does. The buyer may find themselves spending more time helping the builder get the job running than they bargained for!

Confidentiality is another issue. Does the job shop buyer want to show the part to be made to the builder? One way to get around this is for the buyer to design a fictitious part, perhaps one that contains features from a number of jobs that are to be run on the machine.

A part-specific machine capability study can be very useful in evaluating a new or rebuilt machine. However, one must be aware of this method's disadvantages, as noted above. Particularly when a faster evaluation method is required, for example to allow a quick comparison between different makes of machines, the use of accuracy data should be considered.

MACHINE TOOL ACCURACY DATA.

A company considering the purchase of a new or rebuilt machine should ask what type of machine accuracy data is available. For screw machines, this may include published data such as the following:

 

  • Spindle Runout
  • Axial play of Spindle Bearings
  • Radial Position of Spindles in Carrier
  • Angular Position of Spindles in Carrier
  • Parallelism of End Tool Slide to Axis of Spindles
  • Perpendicularity of Cross Slides to Spindles

There are a number of advantages to looking at this data. For a company considering a purchase of a new type of machine, it will allow a comparison with existing equipment. Certain machine makes may have a reputation for superior accuracy; are there numbers to back this up? In addition, evaluating the accuracy data allows the buyer another way of comparing various makes of machines.

Keep in mind, however, that accuracy data does not directly correlate to what tolerances the machine is capable of holding on a specific part. For example, this data does not allow for an evaluation of some very important machine quality features such as:

 

  • thermal behavior and thermal stability of the machine
  • rigidity
  • workpiece roundness
  • workpiece surface finish

The best way to account for these items is by performing a part-specific capability study. However, looking at the machine accuracy data first should allow the buyer to have a greater confidence that the chosen machine will pass the capability study. With the time and effort required to do a good capability study, it is certainly worth doing a little extra homework up front evaluating the accuracy data. This will minimize the risk of going through all the work of a part-specific capability study, only to discover that the machine is not capable.

The ease with which this data can be obtained varies greatly from builder to builder. Just as the machined parts industry faced a big change when customers started demanding numerical SPC data, some machine builders do not appear to be prepared to supply other than minimal accuracy data. This appears to be especially the case for machine rebuilders, who are usually smaller companies and less technically sophisticated.

In some cases, machine builders are hesitant to give out their accuracy data. There appeared to be an element of secrecy surrounding the accuracy specifications of some builders. When one machine builder was contacted about accuracy data, they said that they do not give out accuracy specifications for fear that companies rebuilding their machines may advertise tighter specs.

On the other hand, there are a large number of machinery builders that did supply very good information. Builders of CNC lathes and machining centers usually had some type of accuracy data available. In most cases, they would state their accuracy data in terms of a published standard.

There are four major accuracy standards in use throughout the world. This includes the following:

 

  • ISO, which is an international standard
  • JIS, which is a Japanese standard
  • NMTBA, the American standard
  • VDI, which has a German origin

In most cases they are similar. However, there are also some notable differences which will cause a machine to have accuracy numbers that look better under one standard than they do under another. The following is an example of this from the accuracy information for a vertical machining center:

Accuracy: .0003" positioning; +-.00015" repeatability (ISO standard); equal to .00008"/.00004" based on the JIS standard

This example shows the difference in accuracy figures depending upon the standard used. For this reason, it is important to know what standard is being used when comparing the accuracy data of different machines.

While there was some difficulty in getting accuracy data from builders of machines such as multiple spindle automatics, there were also cases in which the data provided was very impressive. In one case, a builder of multiple spindle automatics provides a book called the "Protocol of Geometrical Measurements". This book includes the various accuracy measurements performed upon their machines. The accuracy tolerance for each parameter is given, as well as the actual readings for the particular machine being measured. Each machine produced by this maker is supplied with its own protocol book.

Another interesting feature of the protocol book for this builder is that it includes the machining results of a short "production" run on a brass test part. The machining results, such as the variation between spindle positions, are documented.

While a machine without this documented accuracy data may be just as accurate, having a machine supplied with a protocol book should give the buyer greater confidence in its abilities before going into the time and expense of a part-specific capability study.

When considering the purchase of a new machine, the potential buyer should ask if there is any documented accuracy data available. This will help in the selection process, whether or not a part-specific capability study is to be done. While many machine builders and rebuilders who do a fine job do not have this data, one should keep in mind that documenting performance is still a relatively new idea for many companies. Just as most job shops would have never thought of statistically documenting the performance of a production run 20 years ago, the idea of documenting accuracy may still be a new concept to some builders. However, the general trend in business today is towards the numerical documentation of performance. ISO/QS-9000, SPC and TQM are examples of this. Buyers of precision machined parts are asking for and receiving documented performance data. If buyers of machine tools ask for accuracy data, more builders will supply the type of comprehensive data that will allow for better informed machine tool purchases.

 

 

 

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