In my head I have a kind of picture how a rotary table might be made.
You have a base, with a central pivot (could be bored for a taper). Around the pivot is a gear & the table, driven by a worm.
Some of these tables are advertised to have a method by which the table can be decoupled from the gear, to allow quick rotation without turning the worm. I have been turning this picture around and around in my head and cannot figure out how they do this.
The worm is mounted on a shaft which is cammed so that rotating the shaft causes the worm's center to move away from the gear's center, effectively disengaging it. This is a very desirable feature on RTs and in my opinion a must have.
Any method of shifting the motor/shaft such that the worm unmeshes from the wheel would work - Here's a few I came up with "on the fly": Maybe the whole motor/worm assembly pivots on an axis 90 degrees to the center line of the shaft the worm is mounted on, turning it off of the wheel. Maybe the motor/worm assembly pivots toward/away from the wheel, allowing it to be "lifted off" the wheel. Maybe the wheel "floats" on the pivot point the same way the clutch-disk in a car with a manual transmission does on the tranny's input shaft - Shaft is splined, inside of clutch disk/wheel is splined to match, but loose enough to slide, and something similar to a car's throwout bearing (or perhaps shift-fork...) rises and/or falls to boost and/or drop the wheel above/below the point where it can mesh with the fixed-location worm to be driven. Maybe the worm and wheel are permanently meshed, but something like a pair of crown rings mesh to connect the table to the wheel - separating those rings would accomplish the task.
The list of possibilities is probably a close relative to the Energizer Bunny, since they both go on and on and on and...
My several have the worm coming through a bushing a smidge off center, and when I release the bushing lock and turn it..it actually pulls the worm away from the gear inside the base. Very quick.
Gunner
"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."
Do that have just one bushing at one end of the worm, or is the bush really a pair, one each end, connected along the back side of the worm (so they'll turn together)?
I don't have a reference drawing to point you to, although someone else may know of one.
The RT worms that I've seen are mounted in an eccentric sleeve, so that both ends of the worm shaft are moved the same amount of distance, simultaneously. The worm shaft is mounted within the sleeve with provisions for eliminating/adjusting-to-minimum the axial movement of the worm shaft. The eccentric sleeve better describes the visualization of the two eccentric bushings question.
The eccentric engage-disengage sleeve/worm assembly rotates to move the worm on/off the table gear. The rotational movement of the eccentric sleeve may have a stop adjustment for the engage position, so that the sleeve is set to the same position easily each time it is engaged. The stop would be set to a point where most or all of the worm-gear backlash is eliminated, but stops short of the point where the worm and gear might bind. A stop for the disengaged point isn't really required.
With the table disengaged, the table is moved freely to any point, quickly. The degree scale around the edge of the table could be used for low accuracy positioning without needing to dial in accurate positions. In theory, the table locks would prevent the table from shifting while the machining operation is taking place, but engaging the worm before machining would give the operator the assurance that the table will remain locked in position.
The precision fit of parts in quality RTs allows them to perform well with power applied to the worm shaft, from either a motor mounted to the RT, or from a power feed shaft driven by the machine the RT is used on. A low quality RT will most likely present problems for the user. The first one I bought was a cheap import that was difficult to operate. Some other types of RTs don't utilize a worm shaft. They might be actuated to index a certain number of stops, sometimes with pneumatic power.
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