High pressure productivity
Today, the technology to release High Pressure Coolant (HPC) systems huge productivity gains and cost savings is both affordable and available in the form of modular systems that can be specified as original equipment with machine tool purchases or retrofitted to modern CNC lathes, milling machines and grinders. To address this, the US-developed CoolJet system is now available in the UK from 1st Machine Tool Accessories Ltd (1st MTA).
Cutting metal generates heat, and with the trend to aggressive machining strategies which maximise productivity and throughput, correct coolant selection and delivery have never been more important.
To understand how latest coolant system developments can dramatically increase cutting rates, extend tool life and improve piece part quality, the functions performed by coolant must first be recognised. Traditional flood coolant serves a number of purposes, including cooling the cutting tool in order to limit heat-related damage; lubricating the chip-tool interface to reduce heat from friction; and flushing away chips from the cutting tool. HPC systems, using properly designed coolant nozzles and tool sealing systems, perform these functions to a degree unachievable by normal flood coolant methods.
Studies have shown that the temperatures generated by the cutting speeds of today’s advanced tooling can prevent low pressure flood coolant from reaching the cutting zone, simply by vaporising it before it has a chance to approach the tool-workpiece interface. As a result, most of its cooling and lubricating properties are lost. By contrast, HPC systems produce coolant streams travelling at several hundred miles per hour, which easily penetrate the vapour barrier to provide effective lubrication and cooling of the tool. Indeed, when users apply high pressure coolant to long-standing processes that have previously produced dark blue chips, they are often surprised that even the use of higher speeds and feeds will still produce shiny silver chips that are cool to the touch.
Furthermore, the results of using HPC technology are equally impressive whether you are roughing mild steel components in high volume general engineering applications, or finishing exotic aerospace alloys. High pressure coolant systems enable maximum productivity to be squeezed from all tool cutting operations, from turning and milling to tapping and grinding.
Traditionally, gun drilling machines have employed HPC for efficient chip removal, in order to meet surface finish requirements in deep hole applications. However, in the early 1980s, several companies began experimenting with high pressure jet streams, often accompanied by compressed carbon dioxide or nitrogen, as a means of breaking up difficult, stringy chips. These systems were often very effective, but only won limited acceptance due to their specialist needs and relatively high cost.
The chip control achieved by HPC is impressive. At pressures of 1000-2000 psi, the coolant stream blasting over the tip of an insert or down the flutes of an end mill will virtually eliminate the risk of workpiece marking by chips. Its powerful flushing action minimises random tool failures from chip damage and results in greatly improved consistency of the machining process. Most HPC systems are not specifically marketed as chip breaking devices. However, a common by-product of their application is shorter, more manageable chips. This is attributed to two principal factors. Firstly, the superior cooling and lubrication associated with HPC allows higher feed rates without the rapid decline in tool life that would be expected with flood coolant methods. In addition, inserts with more aggressive chip-breaking geometries can be employed without giving rise to mechanical or heat damage. And while some chips may not be broken up completely, they can often still be controlled through effective nozzling.
The result is that when applied to drilling operations, systems such as CoolJet offer up to a six-fold increase in productivity; single pass deep hole drilling, a 50-100 per cent uplift in surface speed through the use of solid carbide tooling; and between a two and four-fold boost in feed rates - all with improved surface finish and dimensional accuracy.
Typically, the most significant gains are achieved in high speed machining operations, where elevated temperatures are incurred or exotic materials are being worked. Nevertheless, other candidates for effective HPC use include customised or high value tooling applications, where tool life is significant; multi-spindle and transfer machine applications limited by drilling penetration rates; or operations susceptible to surface finish problems or scrap due to chip packing or wrapping.
A typical production drilling example is given below:
Application: Drilling 0.5” diameter x 1.5” deep holes in grade 316 stainless steel:
| Original Method | CoolJet Method | |
| Tool used | Coolant Drill - 200 psi | Coolant Drill - 1500 psi |
| Type | Cobalt twist | Solid carbide |
| Depth / length of cut | 1.5 in, with 0.6 in peck | 1.5 in , no peck |
| Feed rate | 0.005 in / rev | 0.012 in / rev |
| Cutting speed | 610 rev / min | 1150 rev / min |
| Cutting time per part | 38 sec | 7 sec |
| Drill life | 200 components | 600 components |
In financial terms, the more than five fold increase in throughput achieved by the CoolJet system represents a significant cost saving. In addition, the trebling of tool life will minimise replacement / setting downtime and tool refurbishment costs, providing substantial savings to more than offset the increased cost of solid carbide tooling.
In this second example, the only change to the original process is the replacement of standard flood coolant by a CoolJet high pressure ball delivery nozzle:
| Original Method | CoolJet Method | |
| Tool used | Sandvik 80o Diamond | Sandvik 80o Diamond |
| Insert and grade used | CNMG 431 - 45psi | CNMG 431 - 1000psi |
| Depth and length of cut | .012in deep x .50 long | .012in deep x .50 long |
| Feed rate | 0.003 in / rev | 0.005 in / rev |
| Cutting speed | 1361 rev / min | 2722 rev / min |
| Cutting time per part | 8 sec | 2.5 sec |
| Drill life | 500 components | 1000 components |
In this case, cutting time is reduced by a factor of more than three and tool life is doubled.
1st MTA maintains that results like these are entirely typical. It backs its claims with an extensive library of examples that includes everything from machining mild steel components to high work hardening aerospace alloys - as well as cases where no other method could be found of achieving the desired results.
Novel and creative applications of HPC technology include its use to provide chip control and achieve a fine finish on a turning operation in dead soft copper. Another user eliminated secondary handling and cleaning operations, simply by using an HPC nozzle alongside his machining centre’s spindle to remove chips from the ribs of freshly finished parts. Grinding operations too, can benefit from improved finished part quality, reduced cycles and increased wheel life by directing high pressure coolant jets onto the wheel.
Fast Payback
In today’s competitive manufacturing environment, adoption of any new technology must be justified by proven impact on the bottom line, combined with a fast payback. HPC’s numbers make impressive reading for engineers and accountants alike, yet several considerations ought to be taken into account by first time users.
In particular, pressure and flow rate requirements will need to be determined. Tooling types, tool holding, sealing and coolant nozzling will all contribute to the success of an HPC implementation. Similarly, machine tools must be able to operate at high pressure - or at least be suitable for upgrading. Not only will the cooling circuit be required to accommodate the elevated pressures, but sheet metal guards and enclosures must also be able to contain the splashing coolant and associated mist. Cooljet’s systems are claimed to be the only ones on the market that are suitable for both oil and water based coolants as standard. In addition, companies like 1st MTA can recommend and supply high performance oil, mist and smoke elimination units that will combine with the HPC installation to provide a truly integrated coolant solution.
Additional implementation issues include the potential requirement for cutting oils and coolants to be changed to avoid foaming at elevated pressures, as well as the integration of HPC with the CNC controller for maximum safety and flexibility. Yet these are all relatively minor when compared to the impressive benefits on offer. What’s more, current technology has developed to the point that an increasing number of new machine tools are being supplied either ‘HPC compatible’ or complete with their own high pressure coolant systems. Equally, owners of existing equipment will find that systems like CoolJet offer OEM-approved retrofit solutions for machines originally built to operate at 20-100 psi.
A further factor worth bearing in mind is that the use of more aggressive machining strategies and - to a lesser extent, the use of high pressure pumps - brings the likelihood of generating even more heat. As well as promoting extra coolant evaporation, with its attendant cost and Health and Safety implications, this is likely to result in thermal creep of components and fixturing, triggering dimensional stability and quality control considerations on close tolerance work. To overcome the problem, CoolJet has introduced a series of chillers that are designed to control coolant temperatures to within + 1deg C.
Other innovations include the development of variable flow systems to complement the capabilities of fixed delivery types. In fixed flow systems, the delivery pressure is effectively modulated by the size of the tool. A regulating valve controls the maximum system pressure and enables excess coolant to bypass the cutting interface when small tools are in use. In contrast, variable systems automatically adjust the flow to maintain a constant pressure, regardless of the size or number of tools in operation.
Variable flow systems enable the optimum coolant pressure to be maintained throughout the complete cutting cycle, while also offering the benefit of lower power consumption. The reverse is often the case for fixed flow systems, with cheaper initial installation costs offset by higher power consumption and greater susceptibility to heat generation.
The all-important installation and running costs will necessarily vary with the particular application. However, a typical CoolJet lathe system (including replacement coolant joints in its turret, hoses, high pressure coolant nozzles, high pressure pump, coolant filtration and integrated electrical control) will cost in the region of £6,000-£9,000.
This level of investment is likely to represent a payback period of perhaps less than six months, based on reduced tooling consumption and improved productivity. In extreme cases, where previously un-manageable chip control problems and tool damage issues are resolved through the HPC investment, this period can potentially be reduced to weeks or even shifts, rather than months. And in batch manufacturing or highly competitive jobbing shop environments, utilising high pressure coolant may ultimately represent the vital difference between making parts profitably and not being able to produce them at all.
