A key contribution to precision machine tools was the understanding of the importance of flatness, and the development of processes that are used to make surfaces very flat. Henry Maudslay invented the first machine that could cut standard screw threads and contributed to the invention of the now-common surface plate, which serves an important role in metrology today. A classic book called Foundations of Mechanical Accuracy described this and other concepts in detail. It&#;s available as a PDF here and in print here.
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Typically made of granite, surface plates act as a datum, or the basis upon which precise measurements and movements can be made. They can be finished to a variety of grades of flatness, based on their intended use, and can even be used to build up precise structures (that whole video is a must-watch, btw). When dealing with such flat surfaces, tiny imperfections or gradual wear can have drastic effects: using a gauge over the same spot on a surface plate, or leaving it in a space where the temperature varies by more than a few degrees, can negatively affect their flatness. Due to this, calibrating and conditioning your plate is important. The process is really neat, partially because it requires an autocollimator and an incredibly precise repeat-o-meter. Watch a great video from Tom Lipton about the process here.
The neat thing about surface plates is that they do not require precision tools to create. By using the &#;three plate method&#;, developed by Joseph Whitworth, flat surfaces can be created by using gravity and a simple hand-scraping tool, or by lapping the plates against each other. By starting with three plates of relative flatness, rubbing the plates against each other in alternating pairs to remove the high spots can yield fantastic results.
The process can be completed in six steps, and then repeated until the desired level of flatness is achieved. In this visual explanation, the surface finishes of the three plates are exaggerated. Before beginning this process, the three plates (Red, Green and Blue) should be machined or ground to as flat a surface as possible, to remove all unnecessary lapping work. In addition, a fine abrasive compound is often used between plates to assist in material removal. Tom also has a great video series about this, which includes some compound and technique recommendations.
To begin, the Red and Green plates are lapped against each other in an alternating manner. That is, one plate remains stationary while the other is lapped against it, and then the opposite is performed:
The surface plate has stood the test of time and enables precise engineering measurements.
In engineering, it&#;s normally the machines that are the heroes &#; the mechanical robots that impress us all with advanced programming and swift movement.
When do we take time to appreciate the simple materials that have remained throughout modern developments? We can assume, as machines and tools advance, that there&#;s little to be learned from the past.
Now, we should appreciate a tool that has stood the test of time and teaches us the importance of accuracy in engineering &#; the surface plate.
A surface plate is also known as a true plane surface. It&#;s a standardised piece of equipment that enables precise engineering measurements to be taken.
To ensure that all machined parts produced are the same quality, dimensions need to be checked and verified. As the only variable should be the part being measured, the surface plate is just as important as the machine and software used to record the results.
Many testing facilities and laboratories rely on surface plates as the basis for accurate measurement. Every linear measurement relies on a precise point of reference when the final dimensions are recorded.
Surface plates feature in a wide range of measuring machinery and are considered the most reliable base for height gauges.
Historically cast iron was the material used for the production of surface plates, but granite is now also a common material. Surface plates are manufactured to a wide range of sizes. Depending on the accuracy level required, different grades are also available.
To comply with BS 817 (), surface plates have four different grades:
At the turn of the 19th Century, the era of mass production began. Henry Maudsley is credited as the one who leads the way in machine tool accuracy.
Maudsley started out as a blacksmith and was talented in producing light and complex metalwork. As his reputation grew, he was employed by the lock maker Joseph Bramah.
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Together, Maudsley and Bramah developed a famous unpickable lock that no one managed to break for 47 years. During the production process, Maudsley created specialised tools and machinery that enabled the lock to be sold for a reasonable price.
In his career of producing specialist tools and machine parts, Maudsley began to notice a problem with standardisation for mass production. As parts were having to be individually engineered, nuts and bolts had to be fitted, as a pair and were not interchangeable.
To ensure all parts could be produced identically, Maudsley introduced the requirement of a true plane surface for measurement. By ensuring standardisation was possible, parts and tools could be mass-produced, and an important part of the industrial revolution was founded.
In , the Windley Brothers founded Crown Windley, which is now owned by Eley Metrology, based in Derby. Crown Windley established an industry standard for surface plates, which enabled the development of accurate engineering.
As cast iron became a valuable commodity in the war effort, different materials were explored for creating surface plates. In America, a monument and metal shop owner, Wallace Herman, discovered that granite made an ideal alternative.
The properties of granite as a hardwearing, vibration damping, and non-magnetic material made it the perfect replacement for cast iron. Today, the majority of surface plates are produced from granite.
To ensure a surface plate complies with BS 817 (), new and resurfaced plates need to be inspected and calibrated on a regular, scheduled basis.
BS 817 () was developed as a specification for the construction and standardisation of both cast iron and granite surface plates.
Surface plates produced from cast iron or granite are manufactured to be extremely hardwearing, but over time, damage can still occur. As a flat surface, it&#;s common for them to be misused as a table or a seat!
Because surface plates are so durable, it can be a mistake to think that damage is not possible. Over time, repeated use for repetitive tasks can wear away some sections more than others.
When in use, different areas of the plates should be used if possible. This avoids the risk of some regions of the plate being worn out more than others. To check for any dints, cuts, or bumps, the surface plate should be visually inspected after each use.
As a surface plate is often overlooked as a tool and not considered part of the machinery, it can be forgotten when care and maintenance are required.
Dust, dirt, and grease from the parts being measured or the surrounding area can land on the surface, causing damage and affecting results. Caring for a surface plate requires simple procedures and regular inspections. When not in use, a surface plate should always have a protective cover. Ideally, after every use, the surface plate would be cleaned with a suitable cleaning solution.
Accurate measurement of parts is a crucial component for reliable machines. A correctly calibrated surface plate is required alongside other measuring equipment. Metrology equipment plays an important role in mechanical engineering.
As with machine parts, everything is important. All components need to work well together to create a successful outcome. This is the same for the surface plate.
Surface plates are an important tool but no more so than all the other machinery and software used to make sure the parts produced are of the correct standard.
Contact us to discuss your requirements of Precision Surface Plate. Our experienced sales team can help you identify the options that best suit your needs.