LMT increases productivity for valve guide reaming

LMT

Valve guides

LMT UK has improved the critical inlet and exhaust valve guide production for the 126bhp Daytona 675 Triple engine.

LMT UK demonstrated tool-life improvements of 43 per cent over the nearest tooling competitor.

So successful were the finish reamer trials that, once integrated into production, the tools led to a 15 per cent reduction in direct costs per valve guide for the 12-valve cylinder head, produced at Triumph Motorcycles' Hinckley (Leicestershire) factory.

As a result of the problems resolved and the savings achieved, Triumph's engineers then initiated further cutting trials for a pilot reamer application that pre-sizes the valve guide ready for final reaming.

The trial led to further savings - including the doubling of tool-life expectancy - and enabled the production line to reduce reamer consumption by 74 tools a year.

Rob Doyle, responsible for co-ordinating the extended trials, said: 'The 12-valve, three-cylinder inline Daytona 675 is our highest revving standard engine, at 12,600rev/min, which demands high levels of compliance to design specification of the head and valve assembly.

'We also have the greatest volume demand for this engine, meaning it is produced at a rate of 600 units a week at Hinckley.' He said that unlike other Triumph engines, which have larger diameter valve guides, they are able to achieve good tool life through the use of polycrystalline diamond (PCD) tools.

This engine has a valve-guide diameter of less than 5mm, which is too small for PCD, so the bore has to be held through carbide reaming to a tolerance of 0.015mm on both inlet and exhaust valves.

To improve the life expectancy of the valve guide, the design office changed both the inlet and exhaust valve design, which led to the original cast-iron valve guides being replaced with sintered guides.

So successful was this change that gains of 300 per cent were achieved in the running life of the guides.

With guide materials sintered in the cylinder head prior to assembly, the bore of the guide is machined, leaving 0.5mm of stock with a 0.05mm tolerance ready for final machining with the valve seat as an assembly in the head.

All 12 valve guides and their respective valve seats are produced on an Enshu JE 80 horizontal machining centre in a single operation.

The machine is fitted with an indexing pallet and A and B tombstone fixturing, which also enables the 'fire' face and cam bore to be produced on the same machine as the guides and seats.

To progressively machine the guides and corresponding seat, a combination tool with a short pilot reamer pre-sizes the bore and an insert generates the three angles that comprise the valve seat.

The pilot reamer enables the bore of the guide to be pre-sized, leaving 0.03mm of stock for final reaming while the turning insert cuts the top leading and bottom trailing angles of the exhaust seat.

A finishing reamer is then used to finally size the valve guide bore to a depth of 35mm while maintaining a tolerance of 0.015mm.

Using the same toolholder, the critical 90-degree sealing angle section of the valve seat, set between the leading and trailing angles, is finish machined so that it is totally concentric with the bore of the valve guide.

Due to the abrasive nature of the new valve guide-insert material, the in-cut time and tool life of the previous generation of carbide reamers became totally uneconomic.

Frequent experiments with adjustments to the feeds and speeds didn't lead to any improvements.

This led Doyle to call in tooling suppliers to attempt to eradicate the problem and reduce the growing cost of the operation.

In order for trials to be directly compared, only the finish reamer was initially selected.

LMT UK then produced a series of test reamers for the final sizing element of valve-guide production.

In developing the trial cutters, another member of the group, LMT Bohlerit, created a sintered slug of material incorporating radial-coolant feed holes in preference to the normal method of production, which uses the EDM process.

The tool designer also created a front centre hole with a cone to ensure an accurate and consistent grind when finishing the tool, especially at the leading edges of each blade.

The development trials covered many months so that trends, cutting life and performances could be logged and compared.

The finishing reamer was run at 50m/min using a feed rate of 500mm/min for the first portion of the 35mm-deep valve guide.

The speed was then increased to 100m/min and 2m/min feed rate, followed by the slowing of the spindle to 10m/min, and the feed was reduced to 54mm/min feed.

For the final 0.3mm of tool stroke, the speed was again halved to 5m/min and the feed rate set at 34mm/min, allowing the valve seat to be finished turned with a final dwell of 0.5 seconds.

In the final trials, LMT UK almost doubled the speeds against those used on the previous generation of cast-iron guides and tool life was extended by 43 per cent.

Against the two nearest competitors, LMT UK reduced the number of reamers required in a year from 186 and 137, respectively, to just 96.

Further savings were also logged due to fewer interruptions to production.

These demonstrated that by extending the frequency of tool changes due to increased cutting life and the related inspection and first-off check, this improved the overall utilisation of the machine.

Following the success of the finish reamer trials, LMT UK was then requested to perform trials on the pre-size pilot reamer.

The pilot reaming process is likewise subjected to a series of changes to speeds and feeds, with the initial pre-sizing carried out at 75m/min and a faster 1m/min feed rate due to the improvements in rigidity from the more 'stubby' tool length.

The spindle is then slowed to 9m/min and 74mm/min feed, and slowed again to 5m/min with 30mm/min feed prior to dwelling for 0.5 sec at the bottom of the guide bore.

This pause allows both the top and bottom angles of the valve seat to be finished by the turning insert.

As Doyle pointed out, this operation is the last of seven previous machining cycles for the production of the head, so any cost due to tool breakage (most reamers fail due to their small diameter and tend to snap and jam in the head) would lead to a scrapped head, further production delays and added cost.

'The LMT tools have a predictable life and quality, which is monitored by each valve guide in the head being checked with a plug gauge for size.

'This check is followed by a one in 10 audit using air-gauging.' The tools are changed as bottom limit is approached, then discarded.

Doyle concluded: 'This ensures that the quality of the original tool grind is maintained, giving us predictable and consistent performance from tool to tool.

'It also enables us to meet the high demands put on the engine by our enthusiastic customers.'

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