Exploring Advanced Manufacturing Technologies. Steve Krar
toolholding systems consisting of spindle, toolholder, and cutting tool, the toolholder is the most important link because it has the greatest effect on the overall concentricity and balance, Fig. 2-12-13. As spindle speed increases, the choice of toolholder has the greatest effect on the effectiveness of the machining process.
Two-Face Contact and HSK
Centrifugal force from fast spindle speeds can cause the toolholder to retract when the spindle and holder touch only along the taper. The HSK toolholder that provides two-face contact at the spindle interface can solve this problem. Fig. 2-1-14. Two-face toolholder contact is worth considering for any machining center run at 12,000 r/min. or faster.
Two-Face With Conical Taper
Standard toolholders leave a space between the flange and spindle nose. Some toolholder systems close this gap to achieve two-face contact with a conventional taper, Fig. 2-1-15A and B. Both systems use toolholders with a special or modified spindle interface. Both designs permit the use of regular toolholders when two-face contact isn’t required.
Fig. 2-1-12 An independent high-speed spindle attached alongside the main spindle. (Precise Corp.)
Fig. 2-1-13 The toolholder is the link that has the most effect on concentricity and balance. (Command Tooling, Inc.)
Fig. 2-1-14 Two-face contact prevents the toolholder from retracting due to centrifugal force. (Modern Machine Shop)
CONCENTRICITY & BALANCE
Concentricity and balance are very important in high-speed machining, Fig. 2-1-16. Concentricity measures how closely the toolholder aligns the tool to the centerline of the spindle. A concentric grip helps ensure that all cutting edges will take the same depth of cut. Balance measures the distribution of weight of the tool and toolholder together. A balanced toolholder is critical for producing high-quality surface finishes, extending spindle life, and reducing or eliminating vibration that can affect the metal-removal process. A perfectly balanced tool and toolholder combination would not generate any centrifugal force as it spins.
Fig. 2-1-15 Two types of toolholders with two-face conical tapers. A (Stanley Sheppard Co.) B (HPI/Nikken)
Importance of Concentricity
High-speed milling is generally low-depth-of-cut milling. The cutting load is lighter than in conventional milling. Therefore, the potential variation in load introduced by cutting tool runout becomes more significant by comparison.
Concentricity is also important because of the cutting tool materials used in HSM. These must offer high wear resistance and heat hardness, but they often achieve this at the price of low toughness. Carbide tooling, for example, will chip or fracture more easily than steel tools. More exotic materials, such as diamond, are more brittle still. At high speeds, evenly distributed loads resulting from low runout can be essential to achieving acceptable tool life with these materials.
HYDRAULIC TOOLHOLDERS
A hydraulic toolholder uses a reservoir of oil to equalize clamping pressure around the tool. Turning a screw increases the pressure on this oil, causing an expanding sleeve to grip the tool shank. This type of toolholder is used in many shops because it centers the tool well and is more versatile.
Fig. 2-1-16 Concentricity and balance are important to a successful high-speed machining. (Modern Machine Shop)
Researchers sampled a variety of collet, side-lock, and hydraulic toolholders, Fig. 2-1-17. The horizontal arrows show the range of tool runout errors measured on these toolholders. The curves show the effect of this runout on the average life of tools (in this case, drills) made from conventional carbide and from a carbide grade engineered for higher toughness.
SHRINK FIT TOOLHOLDERS
A shrink fit toolholder is the best toolholder because gripping force is higher, there are no moving parts, and the balance is near to perfect. It works in conjunction with a specialized heater and takes advantage of thermal expansion and contraction to clamp the tool. At normal shop temperature, the bore in which the tool locates is slightly undersize compared to the tool shank size. Heating the toolholder enlarges its bore to allow the tool shank to be inserted. As the toolholder cools, the bore shrinks around the tool shank to create a concentric and rigid clamp.
TOOLHOLDERS FOR BALANCE
Balance is important to an efficient machining operation. Some toolholders, such as shrink-fit holders, have good balance while others such as collet holders can be balanced. Most of these holders provide enough balance for many HSM processes. Balancing should be performed at the speed the tool-holder is to be operated.
BALANCEABLE HOLDERS
A balanceable toolholder is used in conjunction with a balancing machine, Fig. 2-1-18. The machine measures the unbalance in the tool/toolholder system that contains counterweights that can be adjusted to compensate for the unbalance.
Fig. 2-1-17 A comparison of toolholder performance. (Modern Machine Shop)
Fig. 2-1-18 A toolholder balancing machine. (American Hoffman & Lyndex)
Importance of Balance
Not all HSM processes demand the best achievable balance. Unbalance will cause vibration, but the cut also causes vibration. And the force from unbalance may be insignificant compared to the force from the cut itself. The important question is whether or not the unbalance affects the process.
Possible clues that better balance is needed include unacceptable surface finish or tool life, or problems meeting tolerance for characteristics such as hole roundness or trueness.
CUTTING TOOLS
In high-speed machining, the cutting tool may set the speed limit, Fig. 2-1-19. Many machining centers today run at speeds beyond what today’s tooling can put to use without premature failure or excessive wear. In a process optimized for high-speed machining, the tool will probably determine just how fast the cut can be taken.
Fig. 2-1-19 In high-speed machining, the cutting tool usually sets the limit. (Modern Machine Shop)
Tool Rigidity
High spindle speeds increase the severity of vibration at the tool tip. To protect tool life and surface quality, favor more rigid tools. Many times it is more efficient to rough with a smaller tool to get close to finish size and then finish with the final tool. For the best rigidity when using end mills: