by Preben Hansen
Consider your options if working with driven tools
Live tooling is driven by the CNC control and the turret of various spindle and powered sub-spindle configurations on CNC lathes to perform various operations while the workpiece remains in orientation to the main spindle. These devices, whether BMT or VDI, are also called driven tools, as opposed to the static tools used during turning operations and are usually customized for the particular machine tool builder’s turret assembly.
A common error is often made by accepting the standard tooling packages provided by the builder. This is not a criticism of the standard packages from builders, but this article is meant to give you a set of parameters to consider when evaluating the tooling and toolholding devices to use in your shop or production department. Do as much evaluation of your process, when determining the proper tooling to be used, as you did when you evaluated the various machines available
Tool life is the product of cutting intensity, materials processed, machine stability and, of course, piece parts produced. Two seemingly identical job shops can have vastly different tooling needs because one is automotive and one is medical, or one specializes in the one-offs and low-volume work, while the other has a greater occurrence of longer run jobs. The totality of your operation determines the best tooling for the machines being purchased.
Bearing construction and the resulting spindle concentricity drive the life of any tool and you might find a 10-15 per cent greater investment in a better design can yield longer lasting cutters and consistently superior finish on your products. Of course, the stability and rigidity of the machine tool base are also critical factors, especially on large or deep pocket workpieces, where the distance from the tool base to the cutter tip is greater. Bevel and spur gears that are hardened, ground and lapped in sets are best for smooth transition and minimal runout. Roller bearings are consistently superior to spindle bearings in live tooling applications, so look for a combination system to get the highest precision possible. Also look for an internal vs. external collet nut, so the tool seats more deeply in the tool, as superior rigidity will result.
Likewise, coolant high pressure might be desirable. Look for 2000 psi in 90º and 1000 psi minimum in straight tools.
You need to ask another question: is the turret rpm sufficient to handle the work to be done? It’s possible a speed increaser on the tool would be helpful.
Would it be beneficial to move secondary operations to your lathe? Gear hobbing can be accomplished or producing squares or flats through the use of polygon machining.
Standard live tooling is best suited to production work, where the finish, tolerances and cutter life are critical, while quick-change systems may be better suited to the shop producing families of products and other instances where the tool presetting offline is a key factor in keeping the shop at maximum productivity.
This opens the discussion of long-term flexibility, and it’s the most often overlooked consideration in buying live tooling. What work do you have in the shop, what work will be coming in the future and the overall economies of a changeable adapter system on your tooling may be considerations not often made when the focus is centered on the machine being purchased. Dedicated tools for large families of product may be desirable, but consider a changeable adapter system and talk to your supplier before making that determination. Likewise, if the future work you’re bidding involves more families of products, think ahead when buying the initial tooling on the machine.
If standard ER tooling is suitable for the work, there are many good suppliers, but consider the construction aspects noted above. For a quick-change or changeable adapter system, there are fewer suppliers in the market, so seek them out and be sure they can supply the product styles you need for all your lathe brands. Adjustable angle head systems can be costly, but worthwhile, owing to the stability and rigidity of their construction, when producing families of parts with only slight differences in the dimensions.
In the end, you may not need a +135º/-30 universal adjustable tool or a multi-spindle live holder or even a quick-change adapter system, but do consider all the options. Talk to your machine builder and several tool suppliers, plus the most important people in this equation, your shop personnel, as their input is invaluable. SMT
Preben Hansen is president of Heimatec Inc.