::. FOR REFERENCE

MACHINE ACCESSORIES FOR TURNING

Workpiece turning is performed at machine lathes. It is difficult to categorize lathes definitely, for some designs represent intermediate types. In general, lathe machines can be divided into following major types:

-all-purpose lathe machines
-turret lathe machines
-automatic lathe machines
-copying lathe machines
-vertical boring machines
-front-loaded lathe machines
-single-purpose lathe machines

In lathe machines of above listed types cutting movement is implemented by the workpiece and feed-motion is performed by the tool. These machines are designed for processing parts like shafts and discs, which are secured in center points, chucks or in faceplates. Cutting tools (lathe tools) are installed into tool clamps. Today fast-change tool clamps with mandrels are widely used.

Turning chucks with two, three, four and even six cams are used for radial securing of workpieces. Besides self-centering multi-cam lathe chucks, faceplates also belong to radial securing devices. The advantage of faceplates is their wide range of clamping capabilities.
In gripping collets the holding force is applied to the workpiece through conic segments. Gripping collets are designed for securing workpieces of common dimensions or type, they are characterized by retaining precision for long operational life terms and show rotational accuracy of less than 0.02mm. Collets do not allow holding force to be reduced by changing friction force. Gripping collets are designed as splined bushings. To grip, their segments travel in radial direction by means of conic coupling. Due to uniform distribution of gripping force, collets are especially useful for holding tubes and thin-walled parts.

Gripping mandrels are designed for centering and positioning of workpieces along their axles, when they are installed across the aperture. Gripping mandrel body allows to install workpieces. The reference part of mandrel has conic or cylinder design, which allows to center workpieces with a cylindrical hole. Workpiece is centered in stiff cylindrical flanged mandrels by the cylinder part of mandrel, fixation in axle direction is performed by a nut or a screw. Cylindric mandrels with conic flanges are designed for securing workpieces with a deep and precise aperture. The cylindrical part of mandrel is used for centering the workpiece, the conic part applies rotational motion.

Expanding mandrels offer good centering and holding of workpieces by means of press seating and are loosened when workpiece must be removed. When a workpiece is being secured, the shell-shaped part of mandrel performs recoverable deformation (expansion). Under the effect of axial load the inner diameter of shell narrows, and the outer diameter extends, until a secure force closure is established between the mandrel and the workpiece. The advantage of expanding mandrels is their centering precision, which is the highest among all other clamp mechanisms.

Collet holders are used for securing sized rods of various profiles in lathe machines and automatic machines, and for securing parts in turret lathe machines. Collet holders perform centering and securing by means of a holder drive rod.

Parts like shafts and some others are matched to center holes by front and rear centers of lathe machine. The front center is installed in female cone of machine’s main spindle and rotates synchronously with the part. Rear centers are installed into a tailstock ram and are motionless throughout processing. Fixed rear centers (pointed members and half centers) undergo high pressure and suffer strong heat and wear. Therefore, pointed members and half centers cannot be used for processing at high speeds. In this case replaceable spur centers are used. Such centers allow to work at high speeds and provide sufficient precision. Standard spur centers fall into two types: type 1 – for securing workpieces with centering holes, type 2 – for securing hollow shafts and tube-shaped workpieces. For rotating spur centers of normal grade of accuracy, rotational accuracy of center roller must not exceed 0.015mm in normal series and 0.020mm in reinforced series. Spur centers for soft jobs allow radial load within 120…220kg. Medium jobs demand spur centers with load range within 250…600kg. Heavy loads require spur centers installed into tailstock ram, instead of insert ones. Such centers have reinforced design and short outreach.

Most clamp mechanisms that secure workpieces radially, can interact with the workpiece from the inside as well as from the outside.

Mandrels are used for positioning workpieces with center aperture in axle and radial directions. There are three types of mandrels: stiff mandrels (the workpiece is installed directly into their cylindrical body); mechanical and hydraulic mandrels (the workpiece is secured by means of recoverable deformation of thin-walled bushing); and expanding mandrels (a cone expands the body inside the workpiece).