The present invention relates to a tool transfer arm assembly for automatic milling machines, and more particularly relates to a pneumatically operated tool transfer arm assembly for automatically replacing a tool on the spindle head of a milling machine with another tool placed at a stand-by position within the tool magazine of the machine following a prescribed programme.
In the case of a milling machine provided with a spindle head in which a tool is attached to the spindle head by holding the pull stud arranged at the tapered end of a tool holder by means of a collect arranged within a main spindle, the tool holder is reserved within a tool magazine simply by holding the above-described pull stud by means of an elastic gripper. For transfer of the tool from the tool magazine to the spindle head, it is only required for a tool transfer assembly to clamp a flange formed on the body of the tool holder by means of a pair of cooperating holder jaws and pull out the tool hold forcibly from the grip by the elastic gripper. When the tool holder is to be returned to the gripper, the tool holder is forcibly pushed into the gripper by the holder jaws.
In contrast to this, in the case of a milling machine provided with a spindle head in which a tool is attached to the spindle head by fastening a drawing bolt arranged within a main spindle, the tool holder is provided with no pull stud. Therefore, it is necessary to provide the tool magazine with any jaws or hooks for holding the flange of the tool holder. In addition, at transfer of the tool from the tool magazine to the transfer arm assembly, the flange of the tool holder has to be clamped by holder jaws of both elements. In order to enable such concurrent double clampng of the tool holder flange by the holder jaws at transfer of the tool, it is required to for a specially designed flange on the tool holder or to use a specially designed tool magagine of a complicated construction.
A solution to this problem is found in the invention of the U.S. Pat. No. 3,949,462, in which a tool magazine of a milling machine includes a horizontally arranged indexing table having a plurality of holes formed along its periphery at prescribed intervals in order to receive the tapered ends of tool holders therein in an inverted disposition. A transfer arm assembly of this invention includes a pair of holder jaws adapted for clamping and pulling a tool holder off the main spindle at the spindle head of the milling machine. After the clamping, the transfer arm assembly turns towards the indexing table in the tool magazine whilst turning the holder jaws over 180.degree. in order to invert the tool holder upside down. Next, the transfer arm assembly moves downwards in order to insert the tapered end of the tool holder into one of the holes in the indexing table located at the stand-by position, and opens its holder jaws in order to release the tool holder taken from the spindle head. By subsequent rotation of the indexing table, a new tool holder is registered at the stand-by position in order to be caught by the holder jaws of the transfer arm assembly. Thereafter, the transfer arm assembly turns back towards the spindle head whilst carrying the new tool holder in order to attach it to the main spindle. After the attachment, the transfer arm assembly waits in the close proximity of the spindle head with its open holder jaws facing the main spindle.
With this construction, however, the driving force derived from a single pneumatic cylinder is divided into three components by a group of cams arranged in the power mechanism transmission, the first component being for the vertical movement of the transfer arm assembly, the second for the 180.degree. turning of the assembly, and the third for 180.degree. inversion of the holding jaws. Consequently, pressure angles between cooperating gears and slipping contact between cams and their associated cam followers have seriously mallignant influences on the power transmission, thereby requiring unnecessarily large power consumption and causing greatly variating loads on the arms used for the power transmission. These disadvantages disables high speed operation of the transfer arm assembly and, consequently, reduction in transfer operation time.