The umbrella rotary top, also known as the large-head top or pipe top, is primarily used to support sleeve components. It can withstand higher cutting loads, enhance the stiffness of workpiece support, and reduce deformation. According to the "National Standard for Positioning, Clamping, and Device Integrated Labeling - JB/Z174-82," there is an annotation regarding the "umbrella-shaped top with positioning and lever clamping" within the circular outline of the tube blank. Similar plum blossom-style tops are also commonly used in machining, especially for positioning and clamping.
The cone angles of these umbrella-shaped or plum blossom-shaped tips are typically 60° or 75°, with a few being 90°. The main advantage of the large cone angle umbrella rotary top is its versatility—allowing one tip to process workpieces with internal hole sizes ranging from tens of millimeters up to 100–120 mm. However, it has lower positioning accuracy and a larger error in the end face position.
To address these limitations, the author designed and manufactured a series of small-taper rotary tops. After several years of practical testing in production, the results were very promising. Here's an introduction to the small-taper rotary top structure:
The figure above shows the small-taper rotary top. Solid lines represent the structure where the small end of the sleeve is smaller than Ø30 mm, while double-dotted lines show the structure for sleeves larger than Ø30 mm. The standard cone angle is generally 30°, but it can be reduced to 20° or even 15° for improved positioning accuracy. The cone material is 9Mn2V, heat-treated to 60–64 HRC, ensuring high hardness and wear resistance, thus extending its service life. The tapered shank is made of 40Cr, heat-treated to 40–45 HRC.
A gap seal is used between the tapered shank and the screw cap, with a clearance not exceeding 0.2 mm (for small-taper rotary tops with d0 greater than Ø100 mm, a felt seal is preferred). Two single-precision single-row radial ball bearings ensure high positioning accuracy and the ability to withstand radial forces. E-class precision one-way thrust ball bearings are used to handle axial loads. The tolerance zone for the bore of the bearing sleeve and the bearing is M7, while the shaft tolerance for the tapered shank and radial ball bearing is h6, and for the thrust ball bearing surface is j6.
During assembly, grease is filled into the rotating top’s bearings and the screw cap hole. After adjusting the bearing clearance, the screw cap is tightened with a screw. Finally, the Morse taper shank is inserted into the taper hole of the universal cylindrical grinding machine, and the cone sleeve is rotated by a dial to finely grind the cone surface, ensuring that all dimensional, shape, position, and surface roughness requirements are met. This method is also suitable for repairing worn or deformed tapers.
In our factory, this small-taper rotary top is used for grinding the outer circumference of hardened long bushings in tubular pumping products. These bushings are 300 mm long, with five different inner diameters: 32, 38, 44, 56, and 70 mm, and wall thicknesses ranging from 5.5 to 8 mm. The process involves installing two small-taper rotary tops in the taper holes of the cylindrical grinding machine spindle and tailstock (the lever on the main shaft must be added). The friction force between the workpiece hole periphery and the tapered sleeve drives the workpiece to rotate, while the compression mechanism of the tailstock forces the workpiece to be ground.
The workpiece achieves a surface roughness of Ra 0.4 μm, roundness of 0.005 mm, and coaxiality of the outer circle relative to the inner hole within 0.01 mm (excluding shape errors of the hole). The loading and unloading of the workpiece is fast and convenient. Compared to previous methods using ball spindles or cylindrical mandrels, this approach significantly reduces worker fatigue and increases efficiency several times, earning high praise from the workers.