FILENAMES: Worm01.jpg Worm02.jpg Worm03.jpg Worm03.jpg Worm04.jpg Worm05.jpg Worm06.jpg Worm07.jpg Worm08.jpg Worm09.jpg Worm10.jpg Worm11.jpg DESCRIPTION: These are pictures illustrating making a work gear set. Posted by John Kasunich . The following description was provided: ================================================================= These are photos of a recent project. I was asked to design and build a piece of lab equipment that needed to apply up to 1000 lbs of well controlled force to a slide with one inch of travel. The user is a small woman, and she wanted something that required very little effort. I used a 5 tpi Acme screw, driven by a 28:1 worm gear. The operating crank turns 140 times for one inch of travel, and only requires a few pounds of force. (Normal travel is only 1/8" or so, less than 20 turns.) I needed to make three of the test jigs. I couldn't find commercial worm gears, so I made my own, including the gashing and hobbing tools. I also made an Acme tap to tap the three Acme nuts. WORM01.JPG is a picture of the nearly assembled gearbox. Both the worm and the load screw are made from 5/8-5 Acme threaded rod from McMasterCarr. The rod is 4140 steel, and it comes with a smooth black oxide finish. The worm runs on two ball bearings. The one on the left is a press fit on the worm shaft and a slip fit in the housing. The one on the right is clamped to the end of the worm shaft by a screw, and retained in the housing by the aluminum cover. This bearing handles the worm thrust. WORM02.JPG is the gearbox with the screw and wormgear removed. There is a needle bearing in the bore that handles the side load on the screw, and one needle thrust bearing on each side of the gear to handle the thrust load (1000 lbs) from the screw. WORM03.JPG is a closeup of the wormgear and screw assembly, and the matching Acme nut. Both the wormgear and the nut are bronze. Both were tapped with an Acme tap made from a length of the same 4140 steel threaded rod. The nut is screwcut on the OD to mount into the slide. The gear is threaded onto the screw and pinned in place. Thin (0.030) hardened washers provide a running surface for the needle thrust bearings - both the bearings and washers are from McMasterCarr, and are quite cheap. A complete set (bearing & two washers) costs less than $4 and handles 3000+ lbs. WORM04.JPG shows all of the gearbox parts. The two gearbox halves are aligned by dowel pins, visible in the top half (right side of picture). WORM05.JPG is a shot of the tooling that I made for the project. The top item is the Acme tap. It is made from a of length 5/8-5 Acme threaded rod. The first section of the tap was re-threaded with a 60 deg triangular thread form, to reduce cutting forces when roughing the thread. The remainder of the tap widens the triangular form out to an Acme thread form. Below the tap are the gashing and hobbing cutters used to make the worm gears. Both cutters have 3/8 shanks to fit a collet in the mill spindle. WORM06.JPG is and end view of the gashing and hobbing cutters. The gashing cutter is made from drill rod, and the hobbing cutter is made from threaded rod. All three cutters were hardened - the 4140 threaded rod might not be hard enough to cut steel, but it worked fine on the bronze gears and nuts. This view shows how the cutters were fluted, using a 60 degree vee cutter on the horizontal mill. After using the tap, I found that I should have fluted it deeper. Chips tended to pack into the flutes, especially near the beginning of each tapered section. WORM07.JPG is the setup for gashing the worm gears on my Van Norman vertical/horizontal mill. The gashing cutter is mounted in a collet and supported by a center in the overarm. The gear blank is mounted in a chuck on the dividing head, and the head is tilted to the helix angle of the worm. WORM08.JPG shows the gashing process. Four teeth have been cut, and the mill is set up for the fifth tooth. The table is moved to the left to cut the tooth. Since the material is bronze, all cutting is done dry. I ran the cutters at a low speed to minimize wear. WORM09.JPG is the setup for hobbing the worm gears. After gashing, the gears have teeth, but the tooth forms are wrong. Hobbing refines the tooth form by allowing the gear to turn while the hob drives it and finishes cutting the teeth. The right end of the screw/gear is supported by a reamed hole in a plate attached to the angle plate. The left end is supported by a live center mounted in the small vise. WORM10.JPG shows the hobbing process. The hob turns the gear, and the table is fed upward very slowly to make the cut. WORM11.JPG is a closeup of a worm gear. The left half of the picture is after gashing but before hobbing. The right half is after hobbing. John Kasunich