Frequently Asked Questions

What is SPC?
What is a Multiplexer?
What are variables?
What are attributes?
Why do I need SPC?
Why should I consider purchasing data collectors?
Why can't I just use my PC?
Are data collectors networkable?
Is there a point at which it does not make sense to do SPC?

> What is SPC?

Traditional manufacturing relies upon production to make the product and the quality department to inspect it. After-the-event inspection is expensive and wasteful because:

• The product has already been made
• Costly re-work is not always possible

It is much more cost effective to avoid waste by monitoring and analysing the process during manufacture - this is the basis of Statistical Process Control.

Controlling Your Process

For a product to be made without scrap, it must be manufactured within specified limits. But factors can prevent this from happening:

Natural Variation: inherent in the machining process and cannot be changed without using a different process or machine.

Assignable Variation: outside influences that are controllable - temperature, sharpness of the blade, speed of manufacturing, skill of machinist etc.

An Example of Variation

A machine cutting straws to length will give an error from straw-to-straw. This is because of the inherent tolerances of the machine - Natural Variation. BUT this error is less significant than an individual cutting straws to length using a ruler - Assignable Variation.

This raises the question - Is my manufacturing process able to manufacture within specification?

Testing your Process Capability

Continuing our example, cut a number of straws to the required length (usually 50). Accurately measure the straw lengths. Plot the lengths on a graph to identify the variation.

Histogram and Capability charts can be used for this purpose. Once you have determined that you are process capable, you can now monitor your process over time.

Monitoring Your Process Over Time

In an ideal world, you would measure each and every product that is being made...… In the real world, there is not enough time or resource to do this so we measure a sample group of product on a regular basis. These groups are known as subgroups.

The subgroups of data are plotted onto a graph - in chronological order.

The average value of each subgroup is then used to generate the Process Control Chart - building up your actual manufacturing process over time - this is known as the Xbar Chart.

Setting Control Limits In order to prevent scrap, a set of “early warning limits” known as Control Limits are established. These warning limits are set inside the upper and lower specification limits.

Datastat+ provides the means to display your production process and highlights when your process violates these control limits. Datas tat+ can also predict future violations.

> What is a Multiplexer (Gage Interface)?

When taking measurements, you will often collect the data using an electronic measuring instrument to allow you to analyse your data further. For example micrometers, scales, thickness gages, etc are used to allow you to establish the consistency of a production line.

Traditional methods of writing the readings down and manually calculating the results have been superceded by the introduction of electronic measurement devices and the computer. gages are now simply connected up to the PC into a spreadsheet or production management software program or SPC package.

If you are simply using one gage to take all your measurements, linking it directly to your PC is relatively straight forward. It is often the case, however, that you are unable to use one gage to record all your measurements - you may, for example wish to measure length and weight.

In these cases, it is often harder to connect the gages to a PC as there are either not enough input ports (RS232) available on your computer, or the connection type is not compatible (eg Digimatic or analogue gages).

This is where a multiplexer can be used to solve your problem. A multiplexer is a device that connects multiple gages into a single PC input port.

> What are Variables?

Variable data is data that can be assigned a measurable value as in length, weight, acidity, etc.

> What are Attributes?

Attribute data is data that cannot be assigned a measurable value as in tear, scratch, dent, nick, etc.

> Why do I need SPC?

• Reduction in scrap - SPC detects processes which have problems before scrap parts are produced.
• Reduction in rework - Even though some scrap parts can be reworked, the additional labor to do so is costly, and detrimental to the bottom line.
• Remain competitive - Excessive scrap and rework could raise the cost of the final product to a point in which your product is no longer cost competitive.
• Satisfaction of current customers - Most companies today require SPC data with the parts, or prior to shipping. As more companies are performing SPC, the ones that do not do it will be left behind.
• Prospect of new customers - As more companies are left behind, many new doors will be opened to those companies that have successfully implemented SPC. You must spend money, to make money.
• Enhanced decision-making tools - Split second decisions are sometimes necessary in manufacturing. SPC tools are helpful in giving management the information necessary to make the right assessment.

> Why should I consider purchasing data collectors?

Time is money ... mistakes are money ... SPC data collectors reduce the time to measure, log, chart, and store the valuable data being gathered. Without automated data collectors, the operator must take their measurements, remember the value until they write it down, chart it, and eventually download it into a database for long term storage. With simply a few buttons, these arduous tasks are all performed automatically. There are countless testimonials proving the quick ROI on data collectors.

> Why can't I just use my PC?

• Environment - Many manufacturing environments are not conducive to PC's. Oil, dirt, water, shock, vibration, electromagnetic / radio frequency interference, and many others will quickly destroy a PC.
• User interface - Not all operators are PC literate, and become very intimidated by the PC's user interface. In a day when more and more non-English speaking people are working in America's manufacturing plants, the lack of simplicity in the PC will become an ever-increasing problem.
• Portability - You can't carry a PC everywhere you go. Even a notebook computer is not conducive to crawling out on an airplane wing, for roving auditors, or for vendor analysis.
• Maintenance - A PC has many moving parts. Moving parts are more susceptible to failure. Many data collectors have no moving parts, and are built to withstand the harshest of elements.

> Are data collectors networkable?

Many data collectors are networkable using either Ethernet or RS-485. In this day of increasing data sharing, any data collector, which cannot be networked, would be a great hindrance indeed.

> Is there a point at which it does not make sense to do SPC?

When the cost of your SPC system cannot be realized in scrap, rework and labor savings over a specified period, you have spent too much for your SPC system. The most expensive system is not always the wisest choice, due to the risk of not recuperating your investment in the time frame dictated by management. When it comes to SPC, all the costly bells and whistles just don't add up.