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Toolholder connections: what to use and when

BT, SK and HSK refer to different types of tool holder connections used in CNC milling. 

Each toolholder has its own design that can affect the rigidity and performance of the tool setup.

Firstly, let’s have a look at the difference between each, when they are used, and why…

 

BT vs SK

The most common backends are BT40’s, BT50’s and BT30’s along with SK’s.

The ‘40’(Number) applies to the size of the taper (the back end) and the ‘BT’ or ‘SK’ refers to the flange.

The important thing to be aware of with BT and SK holders is the flange thickness. When you come to performing tool change, not having the right toolholder will risk smashing your tool changer.

If you change by hand, you’re not going to encounter too many problems. But if you’ve got machines with auto tool changing, it’s important to get it right.

It is obvious if you know what you are looking for… but if you aren’t aware of the differences the thing is, they’re not obvious!

MSC’s Applications Engineers have seen instances where companies have purchased £000’s worth of BT toolholders and then found out they needed SK.

Even worse, we’ve seen tool changers destroyed because of the wrong selection.

In newer machines, the arm would lock onto the spindle and stop. You’ll notice that it will still connect to the spindle but won’t perform the tool change. This is because the arm won’t locate the flange. You’d then have to do a tool change recovery. 

Older machines will try to pull every tool out of the spindle and destroy the changer in the process!

When buying a new machine always check to see what the taper is on the machine. If you have BT40 machines on your shop floor it’s wise to keep to the same spindle type to prevent wrong toolholders being used.

 

BT

·        BT doesn't stand for a specific phrase but is a standard designation for tool holders with a MAS-BT form, which is a Japanese standard. 

·        BT holders have a steep taper and are typically clamped into the machine via a drawbar that clamps the pull stud on the back of the taper. 

·        BT holders are robust and reliable, they may lose accuracy at higher speeds due to centrifugal forces.

SK 

·        SK stand for "Steil Kegel" in German, which is "Steep Taper" in English 

·        Same shape as BT, the SK holder also has a steep taper and is used in various machining centres and is one of the more popular options.

·        SK tool holders are similar to BT in terms of rigidity but also have limitations at higher speed which leads you to HSK holders

 

And…HSK

HSK are a newer type of tool holder that offer improved rigidity and accuracy. They are designed to maximise surface contact between the tool holder and the spindle, enhancing rigidity and damping characteristics, helping with surface finishes and improving tool life. There are different stlyes of HSK holders E.g HSK-A, HSK-F. Each letter refers to a slightly different design change on the machine connection. 

 

HSK

·        HSK stands for "Hohl Shaft Kegel" in German, which in English is “Hollow Shank Taper"

·        It has a dual-contact design, contacting both the spindle face and the taper, which increases rigidity and performance, especially at high speeds.

·        A very rigid connection due to its fit between the short taper and the face of the spindle, resisting axial movement at high speeds. 

 

So, how do you choose between HSK, BT and SK?

Most commonly, when you go to higher RPMs, you go to HSK.

As a rule of thumb, machines running up to 12,000 rpm and doing general machining are great contenders for the BT and SK holders. 

BT and SK tool holders have more traditional designs, which make them sturdy and reliable for general use, but they may not perform as well at very high speeds. 

If you want rigidity and more accuracy, you would choose a BT face and taper contact – providing a more accurate grind that will locate completely straight onto the spindle face. 

If you are running over 12,000rpm and machining high-precision components, HSK is for you. HSK tool holders offer improved rigidity and accuracy at high speeds because of their dual-contact design and balancing.