Choosing the best shrink fit toolholders does not have to be left up to guesswork; in fact, if you make the wrong choice you could be asking for huge problems with your CNC tooling later on.
Two popular choices for high speed machining are the shrink fit tool holders BT30 and BT40. These work particularly well when the application calls for high torque or high speed cutting. Most injection mold making and tool-and-die making is done with solid carbide or carbide inserts which perform extremely well with these toolholders, but HHS may be used as well.
The principles are quite simple: heat up the toolholder, insert the cutter, and let them both cool. In reality there is just a bit more to the picture.
How do shrink fit toolholders work
The cooling allows for the metal to contract, which creates a metal to metal fitting condition, very much as if they were all one piece. The holding pressure ensures a precise fit with little or no runout.
It also eliminates the possibility of tool slippage and the consequent damage to the workpiece. In fact, the holding power for the BT30 and BT40 is up to 10,000 lbs of force.
A special fixture is required to heat up the holder, and the time required varies from a few seconds to a few minutes, depending on the set-up.
One complaint precision machinists, CNC operators, toolmakers and injection mold makers have is the time it takes to change the tool. Taking time to heat up the toolholder, insert the cutter and let them cool does take time, therefore some planning ahead is in order.
Planning ahead saves time
Because the tools can be set up offline, it makes it possible to work ahead and thus eliminate the time factor. However, this is easier said than done in a real time precision machine shop.
Benefits Of Shrink Fit Toolholders
|Less than .0002" TIR||Solid spindle to cutter holding||Reduced vibration||Better surface finishes||Extended cutter life|
|Use carbide or HHS||Faster speeds and feeds||Spindle life is increased||Great for long tooling||Good for D2 and Inconel|
Often the lead-times and workload make it very difficult to plan ahead and be tooled up in advance. When you have so many things going on at once, it isn’t always easy to execute the plan.
Characteristics of shrink fit toolholders
Shrink fit tool holders are especially good for high speed and high torque machining. With the virtual one-piece, highly rigid tooling, more difficult to machine steels and details can be accurately cut. The spindle will last longer, the end mills will perform better, and accuracies will be more consistent.
Any precision shop should take a serious look at this technology to see if it applies. The increased production, with the increased accuracy will more than offset the initial cost of the set-up. The cost of industrial supplies in the typical CNC machine shop is high, but it is an investment that certainly pays off in the long run.
Used in combination with hydraulic holders, collets and HSK tooling, nearly any type of machining can be accomplished at increased speeds and precision.