Achieving a fine finish at Westland

Dorset company now regards the application of vibratory bowls as a “precise machining process”

Posted on 11 Nov 2017 and read 431 times
Achieving a fine finish at WestlandGalvanometers for laser beam steering and scanning in surgical, analytical and other applications include a precision-machined housing in which a stator moves.

At the Poole factory of Westwind Air Bearings, which manufactures galvanometer components for its US parent group Novanta, these coil housings are CNC-turned from mild-steel bar to within grinding tolerances.

l accuracy is down to 5µm, while the surface roughnesses of the bore and outside diameter are Ra0.4 and 0.8µm respectively.

Some might be bewildered by the fact that these precise components are then rumbled in batches up to 400 in a pair of vibratory bowls supplied by Bletchley-based PDJ Vibro Ltd (www.pdjvibro.co.uk), considering the process a possible source of damage — and not sufficiently precise for finishing the high-precision housings.

However, by developing a viable production route that incorporates vibratory finishing, Westwind has been able to save a lot of time and money.

Moreover, the uniformity of finish is better using the automated procedure, as each component is processed consistently rather than being subjected to the inconsistencies of hand deburring.

Westwind started manufacturing the coil housings in-house three years ago, saving the previous cost of putting the work out to a sub-contractor in the USA.

Four families of parts — ranging from 13 to 33mm in diameter and from 28 to 61mm long — are machined on two German-built Index C100 lathes. There are 12 ‘part numbers’ in total, two thirds of which are required in relatively high volumes of 3,000 per week.

John Bradley, senior manufacturing engineer, said: “A lot of deburring and radiusing on the parts is carried out in-cycle by the CNC lathes.

"However, a few whiskers often remain, which had to be removed manually using knives and scrapers; otherwise, there was a risk of fine wires being broken as the galvanometer stator is inserted.

"Additionally, fine fettling of the housing by hand with a nylon mopping wheel was required to prepare it for the stator.

“Together with washing cycles before and after finishing, the whole exercise took three people five hours — 15 operator-hours to complete a batch of 100 housings.

"Often, this was not quick enough to keep up with throughput, and sometimes there could be as many as 15,000 pieces queuing for finishing, with all the attendant costs associated with work-in-process.”

A further drawback was the need to take different groups of three staff away from other duties on the shopfloor to undertake the hand finishing operations, thereby ensuring that this tedious and unpopular job was shared around.

Furthermore, hand finishing resulted in variances in deburring and mopping, and hence a lack of consistency in the finished parts; and a slip with a deburring knife could result in a component being scrapped — plus there was the chance of burrs being missed and parts returned for re-work.

Optimised finishing


These problems no longer occur with automated vibratory finishing; there have been no rejects or returns to date, despite thousands of housings having been delivered to Novanta.

It is normally team leader Martin Graham who processes the components in the PDJ Vibro vibratory bowls in a 2hr cycle; and there is no need to wash the parts, as they go straight to plating (after a quick air blast to remove any media resting in the bore).

Overall, there is a 7.5-fold saving in labour costs compared with hand processing — and a 60% reduction in the finishing lead time.

Prior to initiating this project towards the end of 2016, Mr Bradley and quality engineer Jordan Tuxford thought that placing dozens of housings loose in a recirculating mass of stones would cause ‘impingement damage’.

Indeed, they were so concerned that they drew up plans to fixture the coil housings in jigs before processing in the bowls.

However, PDJ Vibro’s engineers said component fixturing would not be necessary, provided that batches of the correct number of parts — according to their size — are finished in the 400-litre bowls.

Westwind sent samples to the supplier’s Bletchley technical centre and the results returned to Poole were very encouraging, as was the speed of turn-round.

Sensing that they were close to a robust finishing solution, Mr Bradley and Mr Tuxford visited PDJ Vibro to carry out further trials on the full range of component sizes.

It was established that from 400 of the smallest coil housings to 80 of the largest could be processed at a time without damage.

In many finishing applications involving less accurate components, and when surface roughness is not so critical, labour and time savings can result from reversing the toroidal motion of the media in the bowl and lowering a flap into the recirculating mass.

Components emerge and are separated automatically from the stones, which fall back into the bowl through a screen.

However, PDJ Vibro advised against using that technique in this instance, as direct impingement of component on component while on the screen could cause scratches and dents and render the housings unusable.

To ensure that all parts are extracted manually from the bowls after processing, they are counted in and out.

Two different sizes of porcelain stone were initially supplied by PDJ Vibro, to suit the range of component sizes and to avoid the media becoming stuck in the bores.

PDJ Vibro Westwind 3The company has since provided a third size of stone to prevent media lodging in smaller bores and to achieve optimum results across all housing sizes.

A further recommendation was to employ automatic dosing of water containing a mix of detergent and rust inhibitor.

Whisker removal is successfully achieved by the vibratory bowls, which also attain the high level of cosmetic appearance required without removing too much material and altering the tight dimensional tolerances.

Moreover, in a single day shift the two bowls can finish all of the output from the two lathes working round the clock, so there is no longer a backlog of coil housings waiting for plating.

Mr Bradley says: “PDJ Vibro took responsibility for providing a bespoke finishing package that was ideal for our application.

"They were very accommodating, and the whole exercise amounted to a really good customer experience that is all too rare these days.

"We had no hesitation in buying the two EVP-400 bowls after visiting the supplier for a second time to tweak our process further.

“At the start of this project, it was not clear to any of us that high-accuracy housings could be placed loose in a vibratory machine and automatically mass-finished to a very high standard without damage.

"The apparent incongruity of machining parts to within microns and then tipping them loose into a vibratory bowl made us nervous, but PDJ Vibro made it work well.”

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