What is the best way of determining the diameter of a seal ‘energising’ spring after manufacture, while it is still an irregular loop of coil (with its ends welded together) and not yet formed into a circle by the seal it will eventually strengthen?

The standard options are traditional circular go/no-go gauges for a specific size of large spring or a conical step gauge rising in 1mm increments for a family of smaller sizes.

However, Redditch-based Clifford Springs (www.cliffordsprings.com) has established a more-accurate process using a Nikon Metrology (www.nikonmetrology.com) iNexiv VMA-4540 CNC video measuring machine.

Installed in May this year, the optical measuring equipment automatically determines the length of a free-form loop of coiled spring to determine what its circumference will be when it is circular, after which dividing by pi produces a precise result for the diameter — to a repeatability measured in single-figure microns.

Clifford Springs now plans to use the iNexiv VMA-4540 to automatically count the number of coils per unit length of spring — looped or unwelded — to predict its load-deflection characteristics in use. Compared with the present method of manually counting the number of coils in a given length and using a formula to extrapolate the total number for the whole spring, the optical method will be more precise.

Furthermore, measuring the angle between successive coils in a spring and their distance apart will be more accurate — as well as much faster — than current manual procedures using an optical profile projector. Another important benefit is that an automated system eliminates the risk of human error.

Managing director John Clifford says: “Customers are increasingly asking for reports with proof of the numerical values from our inspection of the springs we manufacture, but only transcription of manually measured results was previously possible. Since May this year, the Nikon optical measuring machine has allowed us to print off actual read-outs of spring diameter; coil count and geometry will soon be included.

This capability has enhanced the perception of our metrology expertise among our customers — and increased the confidence they have in our products and reports.”

Company background

Established in 1946, Clifford Springs is one of only a handful of companies in the world that produce such a wide range of canted, helical, garter and meander springs for ‘energising’ seals.

Springs range in diameter from a few mm to around 300mm and are mainly of stainless steel, although carbon steel, phosphor bronze and PEEK are also used in the manufacture of some varieties.

Half of the Redditch factory’s ouput is supplied as coil for the customer to process, the remainder being cut and welded into individual springs (adding a spring to a seal provides extra resistance to the outward radial forces imparted by a fluid or gas being conveyed under pressure).

With 60% of turnover coming from selling springs into the global oil and gas sector, which is currently suffering a downturn, one might suppose that the company is currently going through a difficult period.

This is not the case, however, as market share has been won due to the adoption of industry-leading production processes, from coil winding to welding — and now product inspection. Other industries in which ‘energising’ seals are widely used include the aerospace and automotive sectors.

Paul Thomas, development engineer at Clifford Springs, said: “We considered several alternative systems for automating the inspection of our springs. They included blue- and white-light scanners, but these are better for comparative rather than absolute measurements; also traditional optical profile projectors, which are too reliant on manual intervention.

“Tactile metrology on a co-ordinate measuring machine — CMM — was discounted, as touch-probing would move the spring during inspection. Trials were also carried out using a laser scanner on a CMM, but the resulting point cloud gave profile data that was difficult to translate into a length measurement.

“An optical video measuring system was really the only option and we looked at a number of potential suppliers. The Nikon Metrology iNexiv solution was the only one that worked successfully, and it has since proved very satisfactory in operation.”

Novel measurement method

Mr Thomas says that Nikon Metrology and Clifford Springs co-developed a novel method for measuring a spring’s length. The iNexiv’s AutoMeasure software instructs the 2-D measuring head to automatically follow the contour of the loop, repeatedly re-orientating itself and taking coordinates of points at the same height at intervals around the outside — or inside — of the spring. The operator simply has to enter the start point, end point and measurement pitch.

Every measurement is taken at the peak of an individual coil of wire (this is recognised automatically) rather than in the adjacent troughs, thereby ensuring that the true outside or inside diameter of the spring is obtained. Depending on the size of the spring, between 500 and 1,000 sets of co-ordinates are captured.

Successive points around the entire loop are linked to create a series of very small, straight lines which are added together to determine the length — or circumference — of the spring. To allow for the fact that it is the arcs between the points that should really be totalled, AutoMeasure introduces a small correction factor that increases the measured length to arrive at the true circumference.

Each result is not only very accurate but also highly repeatable. If the shape of the same loop is altered on the iNexiv table and the measurements are taken again, the lengths recorded have consistently been shown to be within a few µm of each other.

The iNexiv VMA-4540 at Clifford Springs is currently used exclusively for two-dimensional inspection. That said, the instrument does have optical auto focus; this has a 73.5mm working distance and allows the measurement of depth.

The facility is not used at the moment, as coil width is easily determined manually using a vernier caliper, but it remains a possibility if the manufacturer decides to increase the level of automation in the inspection of its products.

Furthermore, it is possible to fit a touch probe for capturing point data if, for example, sloping faces or undercuts need to be inspected. The probe is offset from the optical axis but works in the same 450 x 400 x 200mm co-ordinate space, with only a slightly reduced operating range; and because a stylus changer is provided, it is possible to mix optical and tactile measurements in the same cycle.

The instrument’s versatility is further enhanced by a variety of illumination options based on episcopic, diascopic and eight-segment ring LEDs. Combining these allows for the accurate detection of low-contrast edges.