Development of High-Precision Power Chuck that eliminates the need for Jaw Reforming


In order to grip the work with a power chuck and process it with high accuracy, it is necessary to form the gripping surface of the jaw (Machine forming). The gripping accuracy of the conventional technology is about 10 to 20 μm T.I.R. If the preformed jaw is fitted or removed, the gripping accuracy deteriorates, therefore requires re-forming. The technology consists of BR chuck (Fig. 1) and Tnut-Plus (Fig. 2). The gripping accuracy of the BR chuck is between 0 to 5 μm T.I.R. When used along with Tnut-Plus, the gripping accuracy is maintained even if the jaw is fitted or removed.


Fig.1. BR Chuck          Fig.2. Tnut-Plus

2.Contents of Technology

2・1 BR Chuck 【Patents No. 6453321, No. 6411619】

When gripping the work, the internal parts of the chuck are subjected to dozen tons of load. By devising the shape of these parts, the BR chuck suppresses elastic deformation and stabilizes the position of the internal parts, thereby improving the gripping accuracy, suppressing the work lift, and suppressing the gripping force loss due to rotation. Fig. 3 shows the conventional type, and Fig. 4 shows the BR type master jaw. Since the BR type has a wide wedge, elastic deformation due to load becomes small.

Fig. 5 shows the cross-section of the conventional type and Fig. 6 shows the cross-section of the BR type chuck . In the conventional type, elastic deformation of a shape similar to the rice ball shape is observed in the chuck body. This is due to the load generated from the plunger towards the outer circumference.  While in BR type, the load is dispersed, resulting in considerably small elastic deformation.

Fig. 3. Conventional type master jaw     Fig.4. BR type master jaw

Fig.5. Cross-section of conventional chuck  Fig.6. Cross-section of  BR chuck.

2・2 Tnut-Plus(Unique T-nut) 【Patent No. 6345375】

The jaw is fitted to the chuck (master jaw), by tightening the bolt to the Tnut after engaging the serrations. The clearance caused by the “gap① between the master jaw groove and the Tnut ” and the “gap② between the jaw groove and the Tnut “, changes due to the forming, resulting in deterioration of the gripping accuracy. (Fig. 7)

In T-nut-Plus, both gap ① and gap ② become zero. This is because the width of the Tnut-Plus is slightly larger than the width of the two grooves. Since Tnut-Plus is in the same position as Tnut without the gaps, the gripping accuracy does not deteriorate.

Fig.7. Cross-section of jaw mounting

The side surface of Tnut-Plus contains anomalous recesses. The width of these recesses is smaller than the groove width. Also, the contact part with the groove has a misaligned positional relationship (Fig. 8) that allows the Tnut-Plus to be inserted into both grooves with a slant. (Fig. 9) When tightening the bolt in this state, the jaw is forcibly straightened by the meshed serrations. The two grooves and Tnut-Plus interfere slightly, creating zero gaps. (Fig. 10)

Fig.8.Anomalaous recesses  Fig.9. Insertion with slant  Fig.10. Bolts tightened



This technology is an innovation that breaks the common sense of power chuck over half a century. We are confident that we will immensely improve the quality and productivity of lathe machining and support the foundation of manufacturing.

Yuji Kitagawa,
Corporate member, Kitagawa Corporation. (77-1 Motomachi, Fuchu City, Hiroshima Prefecture 726-8610)

Kazunori Kitagawa,
Kitagawa Corporation, KGh Company (same as above)

Tamio Nishimiya,
Member, Kitagawa Corporation, KGh Company, Technical Department, R&D Section (same as above)