Deep hole measurement on a CMM

CMM users have in the past been restricted to measurement by the length of standard extension used (generally only up to 300mm). This has always proved problematic for companies who need their CMM to pick up features down very deep bores.

Darron Oil Tools a manufacturing company in Rotherham in the North East of England had a similar problem when searching for a new CMM. Malcolm Harper their Quality Director explains, "We manufacture oil tubes for the oil drilling industry which are up to 7 metres long and 300mm in diameter. These parts have a lot of internal features, which can be up to 2 metres down inside the bore. These features have quite tight tolerances and we have traditionally never been too successful in measuring them prior to purchasing our new CMM."

We were looking for a solution to be able to measure externally around these tubes but also to get as far down the bore as possible to pick up internal features. We purchased a Merlin CMM from IMS, which had a measuring range of 750mm X 5000mm X 750mm with a special 1850mm carbon fibre probe extension along with the latest Virtual DMIS measurement software and PH10M/TP20 Renishaw Probing".

The 1850mm carbon fibre probe has a TP20 touch trigger probe attached to the end, which allows Darron Oil Tools to navigate easily down the deep bores and probe the required features. Malcolm said, "The probe means that we can now measure internal features on the oil tubes which was difficult before. This means a higher customer satisfaction and confidence in the product, which in turn led to an increase in orders. The carbon fibre probe is made up of 2 pieces so we can decrease the length down to 1500mm if required".

Recent trials proved that the probe was very accurate over the 1850mm (between the specified 10 to 15 microns). The probe locates directly onto the quill of the machine above the PH10M and can easily be removed when not required and stored safely in the box.

Malcolm continued "Driving the probe down the bore to a small feature that may only be a few millimetres long can be quite tricky. In these circumstances we utilise the full CAD linking capability of the software and use the CAD to guide us to the correct feature. We do this by stripping away the top layers of the CAD within Virtual DMIS to allow us to see the feature to be measured. The feature is then selected and the machine will drive to this and automatically measure it. The use of "smart measure" allowing the CMM to intuitively determine the feature being measured without operator input is invaluable".

The probe was designed using the latest Finite Element Analysis software proving its capability prior to manufacture.

Darron Oil Tools are now thinking of offering a sub-contract measurement service to try and capitalise on this different type of CMM. The 1850mm probe can even be retrofitted to any type of CMM already out in the field (subject to site survey) therefore giving more flexibility to the CMM.