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Industrial Analytical Instrumentation

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Machine tools and general engineering

What are BBT toolholders and the benefits?

When it comes to machine and toolholder selection, many engineers either specify spindles to suit existing toolholders or they rely on suppliers to provide guidance. After all, do we really have time to work out whether it's best to select a CAT, Capto, HSK, BT, BMT, VDI or BBT system? The rise of toolholders marketed as 'face and taper' contact systems, has led Industrial Tooling Corporation's (ITC) toolholding expert, Mr Neil Vine to explain why all toolholders are 'NOT' the sameā€¦

We've heard all the noise and marketing spiel about 'face and taper' toolholding systems and the added benefits of improved rigidity, precision and performance. But did you know that BBT is an abbreviation of BIG BT or what is  commonly known as the BIG PLUS system.  Did you also know that the BIG PLUS face and taper spindles from the BIG Daishowa Group, which is now more commonly known in Europe as BIG KAISER are the only genuine, face and taper toolholders on the market? Everything else is a pale imitation.

At present, the BIG PLUS system is available and readily installed on machines from over 200 different licensees. This list includes names such as FANUC, Makino, Matsuura, Mazak, FPT, Correa, DMG, Doosan, Dah-lih and many more household names from around the machine tool industry. The right choice of BIG PLUS face and taper toolholder interface for many of these machines is available exclusively in the UK from Tamworth based ITC.

A missed opportunity
Many manufacturers will believe that they have a standard BT spindle configuration when in-fact; they have a BIG PLUS system that readily accepts BT toolholders as well as BIG PLUS toolholders. In the chase for the 'holy grail' of precision, rigidity and machine performance, engineers will invest in shrink fit toolholders, balanced high-end tools and even a 'face and taper' contact toolholding system. However, the end-user is often duped by an inaccurate marketing message. Without a genuine BIG PLUS face and taper contact system, all the benefits of high-end tooling and shrink fit holders will only deliver limited gains.

This is truly a missed opportunity as a genuine BIG PLUS system massively increases tool life. In many instances the tool life of particularly costly tools has more than doubled by simply changing the tool holder to a BIG PLUS system. With the high cost of some cutting tools the payback period of a BIG PLUS system through tool life improvements can be just a few weeks.

Image 1 (L)  Genuine BIG PLUS BBT50 toolholder with no fretting corrosion. Image 2 (R) Standard toolholder with corrosion caused through fretting vibration.

The original and only 'Face & Taper' system
More companies are jumping on the marketing bandwagon of 'face and taper' contact toolholding systems. Unfortunately, none of this marketing admits the underlying truth. The truth that only the BIG PLUS from BIG KAISER provides 100% face and taper contact.

For example, many leading toolholders use a collapsible taper system. Of course, this system is in contact with the face and taper, but its rigidity, precision and overall performance still falls far short of the genuine BIG-PLUS system. For other less innovative systems, the taper will have very limited contact due to the taper angle of the toolholder being different to that of the machine spindle. The result is limited contact between spindle and toolholder, on both the face and the taper surfaces. As a manufacturer, you've just made a poor investment that won't yield the desired gains.

And the benefit of the BIG-PLUS BBT system?
Not only does the BIG PLUS toolholding system work on the elastic deformation principal, it also generates significantly more contact with the spindle interface. The more contact; the greater the rigidity, stability and precision. For example, a standard BT30, BT40 and BT50 toolholder will have the respective face contact of 31.75, 44.45 and 69.85mm. In contrast, the BIG PLUS averages 40% more contact on the face with a contact area of 46, 63 and 100mm for the BBT30, BBT40 and BBT50 toolholders. These comparative figures are also applicable to MAS 403 (BT) and DIN6987 (DV) systems where the BIG PLUS face and taper system is applied. Without perfect contact on the face, the standard BT spindle has poor taper contact that translates to an average contact area 4.5 times lower than the BIG PLUS system - a huge difference.

Precision is the Key
The huge difference in contact between the face and taper on the BIG PLUS and the standard BT configuration is the result of precision grinding of the BIG PLUS toolholders and spindle. This precision is also evident with the drive key inside the spindle. The gap between the ground toolholder keyway and the spindle drive key is 0.075mm, as close as comfortably possible without creating an interference fit. In contrast, a standard BT system will have a maximum gap up to 0.33mm, more than 5 times the gap of the BBT system.

Deflecting from The Benefits 
All this precision grinding and tight fitting serves a purpose. Tool deflection is a core factor in disrupting surface finishes, tool life and precision. The vibration causes considerable stress on the tools, the spindle and the subsequent performance and longevity of both spindle and tools. With a standard BT40 toolholder using a tool overhang of 70mm, the deflection is in the region of 0.1mm when cutting at a load of 500Kg/F. In comparison, a BBT40 will deflect at less than 0.05mm, 50% less.

Unfortunately, deflection extends exponentially with tool overhang. On a VMC with a 200mm tool overhang, a BT50 will deflect at over 0.6mm when loaded with a cutting force of 1,000Kg/F. The deflection on a BIG PLUS toolholder is over 30% less at a figure below 0.35mm. For a more conventional example, a 16mm diameter end mill with a distance of 100mm from tip to spindle face, cutting steel at a depth of 3mm will have a deflection of 0.1mm with a BBT40 toolholder. This will be 60% greater with a BT40 running at a deflection of 0.3mm.

This huge variation also has an impact upon repeatability. When conducting 50 tool changes with the 200mm tool holder, a BT shank has a dispersion rate of 3.8microns. This deviation in repeatability is reduced to 0.64 microns with the BIG PLUS system. What does this mean in practical terms? Repeatedly drilling holes at 20,000rpm, the BBT30 consistently maintains a precision repeatability of less than 4 microns. A standard BT30 has a deviation from 10 to 20 microns. The misfortune for the majority of workshops is that they cannot achieve the precision or repeatability they require.  The result could be easily obtained by just changing to a BIG PLUS toolholder, providing the machine tool is compatible.

Seeing The Signs..
Unfortunately many machine shops will never realise the benefits of a high quality BBT BIG PLUS toolholding system. Why? It takes a paradigm shift in thinking. Engineers only genuinely look at their toolholding system when the extremely tight tolerance job comes through the door, or the particularly troublesome material that burns through cutting tools arrives. Only when such a scenario arises, manufacturers take steps to investigate high end toolholding. Even then, many look at costly shrink-fit systems. The logical step for many should be to check the spindle configuration with the machine manufacturer and install a BIG PLUS face and taper contact toolholder system wherever they are compatible.

One simple step to identify excessive toolholder vibration is to check the toolholder taper for corrosion. If corrosion appears on the taper, it is most likely 'fretting corrosion'. This is a direct result of the tool holder vibrating excessively inside the spindle. If you're not pushing your machine and tools, then fretting corrosion may be the only sign of inadequate tool holding.

For manufacturers pushing their machines closer to the limit, other visible signs will be poor surface finishes, excessive tool noise and consequently poor or inconsistent tool life and precision.  It's only when end-users make the change to the BIG-Plus system that they genuinely realise the benefits and regret they didn't make the changeover sooner. The BIG KAISER toolholding system is recognised to improve tool life by beyond 30% over conventional toolholders. The reduced vibration also allows the machine to be operated at speeds and feeds up to 40% faster, giving huge productivity gains. As well as these immediate benefits, the allocation of robust toolholding extends the service life of the machine tool.

By reducing vibration and stress on the spindle and machine axes, the service life of a machine tool will extend immeasurably. The power consumption will fall and overall efficiency will significantly improve. Of course, you could obtain some of these benefits from more costly alternate systems - or you could look to ITC, the UK's exclusive dealer for the BIG KAISER BIG PLUS toolholding systems. Using anything less than the genuine BIG-PLUS system is a false economy that ITC's engineers would be more than happy to demonstrate.

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