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Force Multiplier: How Dual Contact Improves Tool Rigidity

A conventional toolholder is supported only by the connection between toolholder taper and spindle taper. Adding a second connection – between the spindle face and toolholder’s larger-diameter flange face – resists deflection up to 40% in long toolholders.

BIG KAISER’s BIG-PLUS Spindle System provides dual contact between both the spindle face and the flange face. A conventional steep taper toolholder is supported by one connection: the toolholder taper to spindle taper.
With BIG-PLUS simultaneous contact, machining rigidity is greatly enhanced due to the larger contact diameter of the toolholder flange face. This larger face contact, combined with the taper contact, works together to resist deflection. With less deflection, greater machining accuracy and superior finish can be achieved.

By Jack Kerlin

Spindles and toolholders are in a constant battle with the forces of nature, with this battle becoming more and more difficult with heavier cuts and longer projections. Chattering and deflection have always been the bane of machinists’ existence, so much so that the sight of a long and slender toolholder immediately raises  goosebumps.

If you understand why a long toolholder behaves the way it does, you’ll know how to fight bending.

Every machinist knows short and stubby holders are more resistant to deflection than long and slender holders. You’ve also probably heard that, if possible, you want most of your cutting forces to be axial rather than radial. Not only does this fight chatter in operations like boring, but your spindle also is better equipped to handle loads in this axis. However, these options aren’t always going to be on the table, especially in unavoidable long-reach situations and many milling operations.

In the constant battle against tool deflection, much time and effort has been spent designing shorter holders, stiffer tools, and clever anti-vibration geometry and materials. However, holder body diameter(s) is often overlooked as a way to  increase rigidity, especially in situations where that’s all you have to work with. This is a serious shame, as you’ll soon discover.

So What’s ‘Dual Contact’?

The concept of dual-contact technology has been around for years, existing in many forms but always with the goal of capitalizing on the untapped potential of rigidity.

For those who don’t know, “dual contact” refers to the shank contacting the spindle taper and the spindle face simultaneously. Often, the solution involved ex post facto alterations to the spindle or toolholder, such as using ground spacers or shims to close the gap. In other words, there was no standard solution. If you wanted dual contact, you had to spend time and money buying modified toolholders or modifying them yourself to adapt them to your spindle.

BIG KAISER Precision Tooling’s BIG-PLUS dual-contact spindle system resolves this issue. The spindle and toolholder are ground to precise specifications so they close the gap between spindle face and flange in unison (while depending on very small elastic deformation in the spindle). As a result, operators can confidently switch BIG-PLUS tooling in and out of a BIG-PLUS spindle and achieve guaranteed dual contact. Standard tooling can also be used in a BIG-PLUS spindle if necessary, and vice versa.

Though not technically an international standard, the system’s been adopted by many machine tool builders because of performance improvements and simplicity. In fact, BIG-PLUS spindles come standard on more machines than you’d think. We often come across operators who don’t realize their machine has a BIG-PLUS spindle.

How Dual Contact Improves Tool Rigidity

The torque (or moment) exerted by cutting forces is maximized at the point where the holder and spindle meet: the toolholder’s base. With standard CAT40 toolholders, this is the gage line diameter. When the holder contacts the spindle face via BIG-PLUS, however, the effective diameter is the larger diameter of the v-flange, since this is the anchoring point of the holder and spindle. You’re beefing up the diameter at the point where the reactionary force is greatest.

It’s not much of a leap to conclude that a larger effective diameter increases  rigidity. That said, you may still wonder if such a small increase in diameter makes much difference. Yes, it does. To understand why, you must understand the physics behind BIG-PLUS.

Imagine a toolholder is represented by a cylindrical bar that’s fixed at one end and free-floating at the other – in other words, a cantilever beam. If you think about it, this is essentially what a toolholder becomes once it’s secure in the spindle.

Now, let’s introduce a radial force (F) that acts downward at the suspended end of the bar. This represents a cutting force you’d encounter when milling or boring. The bar wants to bend downward. It’s similar to how a diving board bends when someone stands at the end, though less exaggerated.






It’s possible to predict the amount of deflection (or, inversely, bending stiffness) at the end of this hypothetical bar if you know its length, diameter, and material. The equation below represents the stiffness (k) at the end of the bar where d=diameter, L=Length, and E=Modulus of Elasticity (this depends on the bar material). The greater the k value, the stiffer (or more rigid) the bar will be. I’m not asking you to do any math here; I just want you to look at the equation.

You can see that increasing d increases the k value, while increasing L decreases the k value because it’s in the denominator of the equation. This makes sense if you think about it: A short and squat bar (large d, small L) will be more rigid than a long and slender bar (small d, large L).

Note that d is raised to the fourth power, while L is only raised to the third power. Diameter affects rigidity an entire order of magnitude more than length does. This is where the power of dual contact comes from and why a small diameter increase so powerfully affects performance.

Let’s Run Some Numbers

For a CAT40 toolholder, the gage line diameter is Ø44.45 mm and the flange diameter is Ø63.5 mm.

Let’s imagine two bars of identical length and material, so L and E remain unchanged. One bar has a diameter of Ø44.45 mm (standard CAT40) and the other has Ø63.5 mm (BIG-PLUS CAT40). If you plug these values into the equation above, the BIG-PLUS k value is around four times greater than the standard bar’s k value. You could say a BIG-PLUS holder is four times as rigid as an identical standard CAT40 holder because it’s four times more resistant to deflection.

Think of the tool life and surface finish improvements you’d see with a tool that rigid, not to mention the reduction in fretting and potential for reduced cycle time. You’ll get similar results if you make the same comparison for CAT50, BT40, BT30, etc., by the way.

Still not convinced? Let’s compare rigidity another way. Let’s say there’s a Ø63.5-mm BIG-PLUS CAT40 bar of some arbitrary length. One of our common gage lengths is just over 4 inches (105 mm), so let’s use that.

At what length would a comparable standard CAT40 holder be equally stiff? If we set the stiffness expression equal to itself (one side representing BIG-PLUS; the other side, non-BIG-PLUS), we can plug in the BIG-PLUS holder length and our known diameters to find the unknown non-BIG PLUS length, like so:







This means a BIG-PLUS around 4 inches (105 mm) long is as rigid as a standard CAT40 around 2.5 inches (65 mm) long. Thus, implementing BIG-PLUS is equivalent to a 40% reduction in length in terms of rigidity. Theoretically, the toolholder behaves like a standard toolholder nearly half its length.

Real-World Results

I’ve used simple and idealized cases to illustrate complicated and dynamic metal cutting principles. Toolholders don’t have uniform body diameters or materials and cutting forces usually aren’t acting in one direction in a constant and predictable way. If our holder necks up and down to different body diameters along its length, which is what actually happens, each section is its own microcosm of “beam” that influences overall behavior – and at that point, finite element analysis on a computer is the only practical way to predict behavior.

The advantage of BIG-PLUS probably won’t be as dramatic as our hand-calculated beam theory suggests, but it depends on the toolholder/tool. Most cases will follow our simple model quite closely in practice; others, not so much. If nothing else, this exercise demonstrates how dramatically the flange contact of BIG-PLUS influences rigidity.

In addition to increased rigidity, BIG-PLUS eliminates Z-axis movement at high speeds, improves automatic tool change (ATC) repeatability, and decreases fretting. This means you’ll take heavier cuts, scrap less parts, and increase tool and spindle life.

BIG-PLUS isn’t new, but it does have a proven track record of tackling tough jobs. It’s hard to imagine working in a modern machine shop and not taking advantage of what it has to offer.


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