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Science

Precision Engineering and Accuracy

precision targets example

Precision engineering operations require both accuracy and precision.

•  Accuracy refers to the closeness of a measured value to a known value, such a
    specific linear measurement.

•  Precision refers to the closeness of two or more measurements to each other
    —often called repeatability.

•  The movement caused by thermal changes affects both the accuracy and precision
    in any engineering operation. 

Largest Source of Error

Thermal effects in a precision system represents the largest single source of systematic error and non-repeatability for nearly all ultra-precision manufacturing processes. As a result, minimizing and controlling thermal influences also offers the largest single source of improvement in a precision system and most often at a fraction of the cost of the overall system.

How Much Temperature Control Do I Need?

Most precision systems show dramatic performance improvement with temperature control 100 times better than ambient. However, the more error that you eliminate, the more accurate your manufacturing processes will be. So, in general, you want to eliminate as much error as possible in as many places as possible, and providing as much temperature control as possible is generally the easiest and most economical way to achieve improved accuracies.

Errors in a manufacturing process are cumulative. Therefore, error reduction in one area of your process will allow you to accommodate greater errors in other areas. For example, if you create greater accuracies by using precision temperature control, you may have more latitutde in part and machine setup.

Error Budgets

Error budgets are useful tools to categorize and predict the errors in precision manufacturing operations. Error budgeting is a methodology that allocate errors to components and processes of an instrument, and predicts the total error of the instrument's action.

When creating an error budget, precision determinism states that all error values—both systematic (straightness, squareness, positioning error, etc.) and non-systematic (thermal errors) are cumulative and additive. Therefore, eliminating as much error in any area of the process will contribute to the overall accuracy of the endeavor.

What Accuracy Improvements Can I Expect from Precision Temperature Control?

A Præcis Environment system can improve the accuracy of a manufacturing or metrology process by a factor of two up to ten. For example, if you have a well-designed machine operating in a poor environment (e.g., +/- 1.0 °C), you can expect a factor of ten accuracy improvement (+/- 0.01 °C).

The example below shows an interferometer measurement of a 200-mm long aluminum gauge block as it changed length in typical room temperature variation. A one-degree temperature change caused a 0.0046 mm change in length.

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What is Thermal Expansion?

Everything Changes Size When the Temperature Changes.

Bridge Divider

One of the most noticeable examples of thermal expansion is the spreading of joints in bridges. These joints allow for seasonal variations in the length of the bridge span as temperature vary throughout the year. In precision engineering, optics, metrology, and other ultra-precise engineering applications, small differences in size are absolutely critical.

When a material is heated the distance between individual atoms will change. For most materials the atoms get further apart and the total length change depends on how many atoms are in the length. This makes the temperature change proportional to length. For example, if a one-meter piece of metal changes length by some small amount, a two meter piece would be expected to change by twice the amount.

Changes in length is proportional to the temperature change. The constant of proportionality is called the coefficient of thermal expansion (CTE), denoted by the Greek letter alpha (a).

Predicting Thermal Expansion

Precision instrumentation is built from components of a variety of materials—which, with different CTE’s, expand and contract by different amounts with temperature changes. To further complicate the problem, different materials have different rates of thermal conductivity, making the individual instrument parts and the work piece fluctuate in size at different rates.

Because of the complexity of the system and time to execute the activity, the combined thermal changes in a precision manufacturing process are impossible to predict.

To prevent the effects of thermal expansion from harming ultra-precision measurements, Praecis Inc. has developed an ultra-precision air temperature control system. By stabilizing the air to +/- 0.003 °C from a set temperature, Praecis ATCU (Ultra Precision Air Temperature Control Unit) can virtually eliminate any thermal instabilities and makes even the slightest precision temperature control and thermal management easier. 

Learn more about stabilizing the temperature of your instruments and measured parts with precision temperature control.

The Science of Accuracy vs. Precision

Some companies focus on either accuracy or precision, but not usually both. But what is the difference and why does it matter?

Well there are 4 stages here…

-       Neither Precise nor Accurate

-       Precise, but not Accurate

-       Accurate, but not Precise

-       Both Accurate and Precise

 

Neither Precise nor Accurate

In the beautiful image above, the paintbrushes are scattered randomly on the table rather than having been placed precisely and accurately placed into the glass of rinsing water patiently waiting on the table.

Essentially, any instance of random outcomes would be both, neither precise nor accurate. Depending on the objective of the operation, this is usually not good.

 

Precise, but not Accurate

In the image below, a consistent 19.5 is held, but the objective was to maintain a consistent 20.

In this event, there is a consistent and repeatable demonstration of precision. But because the result is off by .5, it cannot be considered accurate.

 

Accurate, but not Precise

While accuracy without precision does not always equate to a wrong answer, it can be seen still negatively impact results.

In the image above, the goal is for the blue line to maintain 20. The blue line is seen oscillating around 20. Overall, this will in fact, average to 20 but because the blue line is oscillating and never has actually maintained 20 it is not precise.

 

Both Accurate and Precise

Getting both accurate and precise results can be difficult. It takes knowledge and experience in a field to do it, but when it is done and demonstrated as repeatable, it leaves behind impressive results.

Accurate and precise can be compared to hitting the bulls eye in a game of darts over and over again without ever missing.

Every operation is different and has different needs to achieve their goals. At Praecis, we strive to give our customers the precision necessary for them to achieve their desired level of accuracy. It is our duty to make sure our customers have consistent, repeatable results, time and time again.

In the image above, the blue line (Critical Point Temperature) uses a precision scale on the right and the orange line (Room Temperature) uses a less precise scale on the left. The two lines are superimposed over each other for reference purposes.

The average maintained on the blue line is 20.000. In this 24-hour test, it can be seen that the fluctuations rarely exceeds ±0.007 (7 millidegrees).

Learn more about Praecis Products.

Learn how Praecis ultra-precision temperature control technology works. 

Aww Man(ual)!

All technical writers know what it is like to cope with the frequent changes that occur in technical writing. From updating and reverting to rewriting and finding missing files.

Praecis Environment Ultra Precision Temperature Control Manuals

If the full printing process is done in-house then you may be familiar with some of the issues that can occur after writing Revision 4.2.3-1.4x7b.

Technical Writing Error 1
Technical Writing Error 2

Correct! The images above show a binding issue that occurred on the very last step! Binding twenty sheets of 36lb paper when the binding unit is designed for twenty sheets of 20lb paper. Mistakes happen, and so the saga continues… Reprint, reorganize, rebind… These steps are far easier written than done.

Once done creating a manual, all the hard work pays off from revision after revision and the end result is a beautiful book built from top to bottom by you (and maybe your team too).

Praecis Ultra Precision Climate Control Manual

The moral of the story here is that technical writing and user guides and manuals are an incredibly important part of technology. While you may not necessarily need a manual for your latest cellphone gadget, imagine if you boarded a SpaceX Dragon rocket that didn’t have a manual. It needs a manual, and so does just about everything else in the technology world. Never underestimate the manual, it may look simple but it too has it’s own nightmare and success stories.

Whoops! How Much Can We Mess Up?

As we all know from our cracked cellphone screens, technology isn’t perfect. It takes a lot of work for any company to get a product to you in one piece. This is especially true for technology companies whereas the parts and pieces to one product can be in the hundreds or thousands.

 

But unlike this sphere, nothing is perfect.

 

In fact, every company makes mistakes. It is often known as ‘dead-on-arrival’ or DOA. Quite frankly, it cannot be completely prevented.  Companies must plan and decide on how much error is accepted and what can it handle. Many larger companies use six sigma as a tool for controlling production errors. In fact the target is less than 0.00034%. This sounds extreme at first but it is important to keep in mind errors can destroy a company’s operation. It also keeps the product from getting to the hands of the customer.

When it comes to manufacturing, engineers utilize an error budget. An error budget is a tool to assist in design. In most operations it is used to predict part accuracy. 

In six sigma the end result depends on the companies operation and the complexity of the product. If there are too many errors, the company may need to shift to a new manufacturing process. While in error budgeting engineers must decide on which errors they can quantify and correct.

In the end, either way, nothing is perfect. But we can get very close.

Innovation Is Not Always a Product

The Præcis Blog – Damage During Freight

Believe it or not, accidents, miscalculations, misinterpretations, and unpredicted situations happen frequently in the technology and manufacturing realm, even if you never see them.

 Damage From Imperfect Planning

Damage From Imperfect Planning

In the image above there is damage that occurred during the freight shipment of this device. It weights nearly half a ton. As a result, during transportation, too much weight was concentrated in too few areas, resulting in damage.

 

The pinnacle of technology is innovation. Coming up with new ideas is the best way to generate solutions. In this case, an innovative idea was passed to an engineer who then designed a newer pallet that can support over a ton of weight.

 New Pallet Design

New Pallet Design

Without creative thinking and skillful execution, our technology world would be missing countless critical devices.. If any process between conception and delivery goes wrong in technology manufacturing, then the end product cannot be sold and made use of to better the tech industry.

Precision Engineering is Everywhere

Technology; it’s omnipresent. We use complex tools daily, but inherently forget how much time, energy, and design goes into each new high-technology product, which changes the way we live.

There are many people, and stories, behind every part of your phone, laptop, or car. Engineers, visionaries, programmers, artists, technicians, metal workers, and more spend countless hours designing, optimizing, and building each part, subsystem, and product.

We never think about certain parts, such as a cellphone camera lens, as a challenge to create.

In reality, however, it took a person to design the lens size, another person to replicate this design in CAD software, an ultra-precision engineer to mill the mold for the lens, and ultra-precision temperature control system to get a precise cut to the angstrom level for all eight million reincarnations of the lens.

This lens is but one small part of the cell phone.