Most rotary tables and dividing heads have the worm rotating within an eccentric bushing. So technically your correct Timo, but moving the worm on that arc by rotating the bushing around allows the worm thread flanks to tooth clearances to be adjusted.
Vevor rotary table troubles
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If the addendum and dedendum are not right you can end up with rubbing in the inside of the worm wheel, (outside diameter of worm) and you might still have backlash on the tooth flank at the same time. Because the worm is touching the root diameter of the wormwheel the center distance cannot be adjusted any better. Then the only way to avoid high friction is to back off the excenter and accept the extra backlash.
If you have gaps on both tooth tips wormwheel and worm, then the eccentric bushing can be used to adjust until the tooth flanks touch, without rubbing.
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I don't know enough about any form of gearing to be absolutely sure of my facts Timo. But both the worm and wheel use an ACME profile. That eccentric bushing is used to reduce the clearances to "almost" nothing while still allowing a non binding rotation. Yes there's always going to be a slight amount of backlash between any worm and wheel, the eccentric just reduces it to the bare minimum. But I'm more than happy to learn where my gear knowledge is wrong.
Yes, I've done that before, and have considered that as a possible way to proceed. Will give it more thought when this rises back to the top of the pile.
It's been a while since this first happened, but as I recall, there were no parts available; only recourse is to buy a complete new RT.
Well ... let's clarify a bit. What we call an ACME screw thread is a thread with straight flanks, angled at 14.5° per side (29° total). By sheer coincidence, one of common standards for spur gears is based on a 14.5° pressure angle, which means that when the gear form is produced as a rack, it has teeth with straight flanks, angled at 14.5° per side (29° total). Obviously a coincidence ... surely ... ?
Of course, when this gear form is put on a circle, the straight sides need to be curved ... producing the familiar involute shape. Essentially, the involute shape is simply the shape the teeth need to be to roll smoothly with a rack having straight sided teeth. Of course, pitch has to match, but that is a separate issue from the pressure angle / shape of the teeth.
I must be missing something Andy. Yes obviously a rack can be cut using the typical B & S design of gear cutters, and yes there also made with that 14.5 degree pressure angle. But to use the standard involute gear cutter as an example, a rack won't be cut having those straight 14.5 degree angled sides since that type of multi tooth gear cutter still has the usual involute gear tooth profile.
Keep in mind that the B&S cutters are a compromise. Yes, you can cut a variety of gears with each cutter, but only one size will be absolutely correct; the rest will be close enough, but not exactly the right shape for the size. For a rack, the correct shape is straight-sided teeth.
How can one cut a perfectly shaped gear? By hobbing. What is a hob? Essentially, it is a rack with straight sided teeth. As the gear and the rack / cutter move in synchronization, the teeth of the gear are cut in the characteristic involute shape.
How can one cut a perfectly shaped gear? By hobbing. What is a hob? Essentially, it is a rack with straight sided teeth. As the gear and the rack / cutter move in synchronization, the teeth of the gear are cut in the characteristic involute shape.
Very true. You also or at least preferably need a way to positively drive the gear either electonicaly or mechanically (your synchronization) to get the best accuracy with the hob as the gear is cut. From everything I've read about it, free hobbing a gear may or may not produce equal results. But afaik, the gear still has to be pre gashed to allow the hob to then engage and help rotate the gear using the non positive drive method while cutting the teeth to depth.
Exactly. I've done the free-hobbing, many years ago, to make a fine down-feed for my pitiful "Big Red" mill-drill. It worked fairly well, but I did not / would not trust the results to provide a high degree of accuracy. Building a gear-hobbing system (something akin to this: ) is also on my long-term to-do list, so it may be that these will be linked projects ... "one of these days."
I've had the drawings and castings for the Jacobs gear hobber for quite awhile Andy. I guess it's on my "list" as well.
Mechanically it was designed to use Myford change gears which are probably quite accurate. I have doubts about the actual level of accuracy for the off shore change gears I have. So machine a full set of gears before I can actually use it after the hobber is complete?
Mechanically it was designed to use Myford change gears which are probably quite accurate. I have doubts about the actual level of accuracy for the off shore change gears I have. So machine a full set of gears before I can actually use it after the hobber is complete?
Richard Hed
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I would try it. Doesn't take a lot of time to make the threaded part.
Agreed, the addendum of the cutter does have to be larger, I was just making a suggestion and leaving the details to the OP.
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I doubt that the accuracy of the change gears will decide about usability of a "jacobs hobber"
I guess there are more important things, like runout of cutter and gear blank. Stiffness of setup etc.
For the change gears of your hobber you will probably get away using anything with the right tooth counts. It is all about ratios.
If you disklike electronics, have a look at this channel as well.
https://www.youtube.com/@thomasstover6272
p.s. I would be curious about the jacobs hobber built.
An interesting YT channel Timo, thanks for the link. There's one or two builds of this Jacobs gear hobber on YT. But I don't recall seeing one there that's been fully completed yet.
I'm seriously lacking in knowledge about electronics and computers. Not my thing at all. Mechanical I can figure it out one way or another. But given the cost and increase in tooth form accuracy with a single hob verses a full set of the B & S type gear cutters. It seems a bit counter productive to me to knowingly introduce lead / lag errors in the synchronization between the hob and gear blank with cheap and obviously poorly produced gearing. If that wasn't extremely important, then industrial level gear hobbers wouldn't be using expensive and properly ground, possibly lapped gearing within them.
Today and with industrial cnc gear hobbers, any known mechanical deviations are I would guess taken care of within the programing the same as with any highly accurate cnc. I'd agree that for most of what we'd need gearing for, what I have could be good enough, until your require that better accuracy. Worm gears used for division as a prime example.
I'm seriously lacking in knowledge about electronics and computers. Not my thing at all. Mechanical I can figure it out one way or another. But given the cost and increase in tooth form accuracy with a single hob verses a full set of the B & S type gear cutters. It seems a bit counter productive to me to knowingly introduce lead / lag errors in the synchronization between the hob and gear blank with cheap and obviously poorly produced gearing. If that wasn't extremely important, then industrial level gear hobbers wouldn't be using expensive and properly ground, possibly lapped gearing within them.
Today and with industrial cnc gear hobbers, any known mechanical deviations are I would guess taken care of within the programing the same as with any highly accurate cnc. I'd agree that for most of what we'd need gearing for, what I have could be good enough, until your require that better accuracy. Worm gears used for division as a prime example.
