Breaking the speed limit

Toolmakers are questioning the received wisdom that running machines at slower rates to prolong their life is the most economical approach to production. As Douglas Friedli discovers, other costs can be incurred when manufacturers fail to get up to speed

When times are hard, companies in metal cutting businesses will often try to extend tool life by running machinery more slowly. But this approach introduces other costs, as cutting tool makers at this month’s Tooling 99 show at the Birmingham NEC will be trying to demonstrate.

The conventional view of tooling economics is governed by Taylor’s equation. This states that an increase in cutting speed will mean a disproportionate decrease in tool life. As the speed of the cutting surface increases, so does its temperature. At higher temperatures, strength and abrasion resistance tend to decrease. In terms of tool life, therefore, it makes more sense to run cutting machines at slower speeds.

But this could prove something of a false economy. Carbide tooling typically accounts for just 2-4% of production costs, increasing to 6-8% for diamond-based cutting. At lower speeds, other costs, such as labour and the cost of plant, will tend to increase for each part produced. Andy Smith, engineering manager at tool maker Sandvik, says: `Producing more components in a given time wears out tools faster, but it also starts eating away at fixed costs. For example, a 50% increase in tool life leads to a reduction of 1% in the cost of making a component, whereas a 20% increase in the speed of machining leads to a 15% cut in costs.’

Under normal circumstances, at some point either the rate of wear or the time needed to replace tooling will mean that increasing speed becomes uneconomic. There is an optimum level for this operation, just like any other. But Smith believes most companies set the ideal speed too low, increasing the number of machine tools they have to buy and accommodate.

`There has always been overcapacity in the UK,’ he says. `When demand increases, companies buy more plant in order to gain capacity. It’s the easy option: just buy a new machine. But it becomes a burden when there is a downturn.’

If metal cutting tool suppliers are finding it hard to make their case, it could be because of the lack of a strong lobbying body. The Hard Metals Association is the trade association for the sector, but it tends to concentrate on providing services directly to members rather than marketing or lobbying. `There isn’t really an organisation to put the message across,’ says Smith. `Cutting tool manufacturers have to take a lead in this themselves, and we will be doing so even more next year.’

Another reason could be that tooling is not regarded as an important part of company strategy. According to Ernst Wagner, sales director of round shank toolmaker Unimerco, tooling choices are usually made too far down the chain of command. `The problem is that tooling is often left to middle management – to people who have come up from the shopfloor. We have to explain to them that it is more cost effective to use more productive, if more expensive tooling.’

Wagner will present his case at a seminar on tooling economics at this month’s show. The UK spends less on tooling than Unimerco’s other markets in the US and Scandinavia, according to Wagner.

`We researched the UK market before setting up production here two years ago. The tooling level in the UK is lower compared to other countries where we operate, but at the same time opportunities here are greater and are bound to grow. Companies have started investing a lot in CNC machining, which is the first step,’ he says.

The obvious advantage for toolmakers is that if more machines are run at higher speeds, there will be more tooling purchased for the same number of goods produced. This holds true for conventional machining speeds. But it has been discovered that at ultra-high speeds a new phenomenon comes into play which can reverse this effect.

Graham Smith, principal lecturer at Southampton Institute of Higher Education’s mechanical engineering faculty, says: `Up to a certain point, higher speed does mean greater wear. But over around 1,600m/min of peripheral speed, you can jump the “valley of death”.

`At that speed, the material is plastically deformed at the chip/tool interface. The speed of the tool thermosoftens the material. You can have all the benefits of long tool life and machine a higher number of parts in the same amount of time.’

Peripheral speed can be increased in two ways. First, a larger diameter tool will have a greater peripheral speed at the same rotational speed. But where smaller diameter cutters are needed, the only solution is to rotate the tool faster. In practice, it is difficult to achieve ultra-high speeds on a machine which has not been designed for this purpose.

Simply increasing speed can bring problems such as `servo-droop’, in which corners tend to be bent rather than cut, so that a curve rather than a sharp 90 degrees angle results. Graham Smith says: `You need a controller with a faster processing speed, or you choke the controller. It will be trying to act on the other axis before the first is finished. So you need a machine with the latest fast spindle block processing controller, something which can overcome the problem with a “look-ahead” facility.’

Running a conventional machine with an accelerator or a new spindle could make it faster, but the structure may not be able to cope. It may lose its register, the bearings may malfunction, or acceleration times may mean that it never reaches its maximum potential speed.

`You really have to get a purpose-built machine. You can buy a great spindle, but if the rest of the machine isn’t up to it then you will always be compromised,’ Graham Smith adds.

Even a purpose-built machine tool may have trouble reaching its maximum potential speed, he says: `These sorts of machine tools are generating peripheral acceleration of 1g – that is seriously going it some. It can be a problem for the machines, as they tend to shake themselves around a bit.’

The need to buy new equipment brings the argument back to the balance between spending on machine tools and spending on tooling. The debate about machining at high and ultra-high speeds will continue.

Sandvik’s Andy Smith says: `It may sound like we’re trying to feather our own nests with this initiative, but the mathematics are straightforward.’