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Make it yourself very easy. From a sheet of plywood 10 mm thick cut out three squares of size SOOH X300 mm. In the middle of one drill hole ø 8 mm and two 25 mm. d. of the workpiece, as shown in the drawing, will receive a fixture base. In the Central hole and secure with nut bolt M8.
The center column carved from brass rod d 12 mm and length 80 mm. Cut at one end of the external thread M12, end in the hole on the other side, internal M8.
The rotary sleeve has an axial bore d 12 mm and a perpendicular threaded M10.
Carrier is brass rod d 10 mm. At its one end tapped M10 with length 10 mm, and the radial M3 hole at the other end is made after the subassembly carrier with rotary sleeve: it must be parallel to its bore d 12 mm.
Slide in a corner is cut from dural bar. In its vertical wall are two holes d 10 mm — one under the carrier, and the second, perpendicular to it and shifted down by 5 mm, is to lock the threaded pin with a semicircular groove. In a horizontal wall propisyvaetsya groove cross section 14X3 mm — under the knife. Material for its manufacture is a chip of the saw blade. In the vertical groove of the slider knife is fixed by screw MB.
The retaining plate is cut on a lathe from metal plates with a thickness of 2 mm or thicker plastic sheet. The Central hole should be ø 8 mm and outer diameter is slightly less cut.
Lock nut sleeve with a Central M12 external thread has a 0 25 mm and height 15 mm. Saw in it a transverse groove on polumetra in this part make a longitudinal hole. Then the bottom half cut the M4 thread, and at the top — drill the full diameter up to ø 5 mm. Now pull the halves of the nut screw, thus you will be able to lock it in any position on the centre pillar.
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Device for cutting large diameter holes:
1 — pen, 2 — led, 3 — M6 screw, 4 — slide, 5 — knife 6 — Central rack, 7 — lock nut 8 to the rotary sleeve 9 — spring 10 — bolt M8, 11 — retaining plate, 12 — blank,13 — base, 14 — threaded pin-locking.