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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.


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.

Trouble Maker Moon

This is a graphical representation of the moon taking a bite out of diamond turning accuracy.

This is a graphical representation of the moon taking a bite out of diamond turning accuracy.

When the Large Optics Diamond Turning Machine (LODTM, the most accurate “instrument” ever constructed) was being developed at LLNL during the mid-1980s, a team of engineers had to work meticulously to decide on many hundreds of possible error sources that could decrease the accuracy of the unit during use. Many machines designed by precision engineers have an error budget (more on this topic in a later blog). Depending on the amplitude of the error it would either require some sort of compensating or have some solution for correction. Of course, temperature was one of the issues, but this was much before the birth of Praecis. 

This is what happens when the moon goes on vacation... -___-

This is what happens when the moon goes on vacation... -___-

One morning one of the engineers working on the project had an epiphany, the moon might have an effect on the LODTM’s results! This wasn’t considered by any of the engineers yet but it made perfect sense. Think of how the moon has a direct correlation with the high and low tides. An engineer was then assigned with quantifying the resultant error.


The error from the moon’s gravitational precession about the earth was both indeed quantifiable and measureable. This meant that an algorithm would need to be created and implemented to compensate for this error and added the the many hundreds of other calculations. Fortunately, the amplitude of error was below the threshold for consideration of the error budget. Whew! Time to wipe off the sweat! Writing those equations and algorithms would be incredibly complex and time-consuming.

Temperature, on the other hand, does not play as nice or consistent as the moon and its effects are much greater and cannot be ignored. In fact, temperature is constantly and rapidly changing. This makes temperature control a nightmare for anything outside of a highly sophisticated HVAC system with may often cost several hundred thousand dollars to purchase and install.


For best results in diamond turning, use Praecis Air Temperature Control systems.

For best results in diamond turning, use Praecis Air Temperature Control systems.

Thankfully, there is a cost effective way to satisfactorily reduce error caused by thermal variation. Praecis has introduced state-of-the-art ultra precision temperature controlled enclosure systems 5 years ago and can stabilize temperature as precisely as ±0.002 ºC or a factor of at least 50. By using such an extreme degree of temperature control any heat generated from cutting a part would quickly return to its designated ‘set-point’ and thermal expansion on a given part would have no effect on the cut (as the temperature variation is too little and brief to allow for any changes whatsoever). Praecis designs the best known temperature management systems and offers the most comprehensive extended warranty packages.

Enjoy this blog article? Submit a request for quotation on a Praecis temperature control system by September 1st 2016 and mention 'Praecis Blog number 9' in your message to receive a quotation discount.

You can also request a brochure for Praecis products.


Temperature Variation Has Teeth

Temperature variation is bad. It cracks the pavement, breaks bridges, and it ruins results in precision manufacturing and engineering. 

Thanks Temperature Variation... -_-'

Thanks Temperature Variation... -_-'


In precision engineering, temperature variation is the single greatest source of error. This includes and is not limited to diamond turning machines, interferometers, atomic force microscopes, and profilers as well as many other types of metrology equipment. The good news is that error caused by thermal variation can be significantly decreased. Simply use a Praecis Air Temperature Control Unit to tame the variations in temperature.

Take a look at the figure below. It is an interferometric analysis of the flatness of a part cut over the course of 5 hours while the room temperature cycled by +/- 1 ºC with a period of 18 minutes. As you can see, the resulting part profile reflects the influence of temperature changes with an amplitude of 0.327 micrometers p-v.

To eliminate the waviness, temperature control was used:

The next figure displays the result of cutting the same part under identical conditions -- but with +/- 0.1 ºC temperature control.

The waviness is gone, and the part flatness was improved by a factor of 7 to 0.046 micrometers PV.  

Precision temperature control is the most cost effective way to improve machine performance and reduce errors caused by thermal effects. In the chart above basic temperature control was improved from ±1.0 ºC to +/- 0.1 ºC for the same part cut. The resulting cut showed error reduction by a factor greater than 7.


Don’t let thermal variation bite you. Bite back with high precision temperature control. By using products like the Praecis Environment you can essentially eliminate error by up to a factor of 10 or greater.

For best results in precision engineering, use Praecis temperature control systems.

For best results in precision engineering, use Praecis temperature control systems.


Ways to Improve Your Environment

Ways to Improve Your Environment


Here are five simple steps to make sure your high precision manufacturing and engineering machines can produce consistent results.

To do list for precision engineers

I.               Separate the machines! – Keep all machines at least 10 feet apart from each other. Some machines can generate heat, and light which can negatively impact precision results to another machine in use.

II.              Leave the lights on! – Lights, even the bland florescent lights used in your engineering lab, produce energy. This energy is transformed into heat when it hits a surface. If you are turning a part overnight or over the course of several days shutting off the lights at the end of a workday will change the temperature inside your enclosure resulting in potential errors. Imagine having a bunch of microscopic errors on a 72-hour cut that could have been prevented by leaving the lights on.

III.            Don’t stand so close! – If you stand too close to an enclosure, even just to see how cool diamond turning in action is, keep in mind that your body produces infrared heat and this heat can transfer into the diamond turning unit. Remember to stand clear especially when the door is open. Your IR energy may go directly to the part and spindle. Remember, changing the energy and temperature just a bit = changing the result just a bit… Nobody wants that…

IV.            Use temperature control! – If you’re going to use temperature control only go with the best. Investing in any high precision temperature controller is almost always a must, but do not let machines that can only maintain 0.1 ºC gimmick you into thinking that is all you need. Praecis Inc. can control temperature down to ±0.002 ºC. This will effectively eliminate any errors produced from temperature variation during long cuts.

For best results in temperature control use Praecis.

For best results in temperature control use Praecis.

cold as ice

V.             Set the HVAC system to one temperature! – When I mentioned temperature control above it did not include the HVAC system. Allowing the HVAC system to ‘shutdown’ after work hours or ‘lower it’s temperature’ can adversely effect the results from your precision manufacturing device. Even with a temperature control unit connected to your machine you want to keep the room temperature as stable as possible. Major temperature changes due to the HVAC system shutting down automatically can cause the temperature control system to work harder. “Work smarter, not harder!”  During long cuts simply remember to, “Set it and forget it!” on your HVAC system.

Don't subject your DTM part cuts to cold temperatures.

Don't subject your DTM part cuts to cold temperatures.

Temperature Control in the Precision Manufacturing Industry – Part II

What if temperature never changed inside of a diamond turning machine / DTM?

What if the temperature could be perfectly stable to a millidegree?


These are great questions because they are only a few ways to eradicate thermal errors inside of a diamond turning machine enclosure.

Controlled Temperature Vent inside of a Precitech Diamond Turning Machine

Controlled Temperature Vent inside of a Precitech Diamond Turning Machine

By adding temperature controlled ventilation to the enclosure of a diamond turning machine, internal DTM temperature can be stabilized and errors caused by temperature variations can be proportionally decreased. 

Freeform Mirror Inside a Precitech DTM

Freeform Mirror Inside a Precitech DTM

With precision air temperature control results are more accurate and repeatability increases.

There are very few products today that can eliminate error and stabilize temperature variation inside an enclosure. Many temperature control systems that exist today can control down to ±0.1 ºC. Holding ±0.1 ºC is great for some applications but it is possible to stabilize temperature to a much greater degree at a low cost.


Praecis Inc. is capable of producing a product that can stabilize temperature accurate to the millidegree at an affordable cost. Results can be as precise as ±0.002 ºC, minimizing changes to a parts dimensions due to thermal expansion. This effectively improves diamond turned parts by a factor up to 10 or greater.

Praecis Air Temperature Control Unit 4 on Precitech Freeform TL

Praecis Air Temperature Control Unit 4 on Precitech Freeform TL

Please visit the library at to learn more about how thermal effects can cause errors in diamond turning part cuts.

Click here to see the reference article

Temperature Control in the Precision Manufacturing Industry - Part I

Diamond turning machines are fairly complex pieces of machinery. Each one has been designed by engineers to produce repeatable results. Repeatability, is exactly what it sounds like. It describes a diamond turning machine’s (or DTM) ability to repeat the same exact cut more than once. This is not limited to simply ‘looking’ like the same cut. When looking for repeatable results DTM users are looking for the same exact cuts with the same exact errors down to the nanometer. To accomplish this, engineers must have a solid understanding of what influences temperature variation can have on a turned part.

Let's pretend these delicious strawberries represent repeatability...

Let's pretend these delicious strawberries represent repeatability...

Temperature variations are the single greatest source of error in diamond turning industry. In fact, thermal error can be found all over the precision manufacturing and engineering industry. Temperature sneaks its way in from various heat sources, many of which are outside of the DTM. This includes sunlight, room lighting, humans (yes, humans standing next to a DTM can have an effect on the outcome), and HVAC. 

Here is the catch about temperature variation… It’s complex… Even though changes in temperature are deterministic and measurable, correcting each change throughout an enclosure can be challenging. 

Errors are similar to a needle in the haystack at times...

Errors are similar to a needle in the haystack at times...


During a cut, thermal variation errors may be taken into account by certain diamond turning manufacturers, but generally if temperature has varied at all, the damage is already done… Thanks temperature change…



Let’s not be mistaken, thermal variation is not inevitable in precision engineering. There are things that can be done to control it accurately.

Want the solution? Come back for Part II next week!

Please visit the library at to learn more about how thermal effects can cause errors in diamond turning part cuts.

Click here to see the reference article


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.