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Latest issue - thread indicators

Is there an easier way?

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Tim Stevens09/08/2018 19:50:46
983 forum posts

I note the care with which the writer has made his indicators with skew teeth on each gear to fit with the relevant lead screw. I just wonder if it would be easier to make 'straight' gears and then connect the device with an angled bracket so that the mesh is perfect?

Unless (cynical old me) the object is to show how clever it is to cut skew gears?

Cheers, Tim

Neil Wyatt09/08/2018 20:05:42
15581 forum posts
652 photos
73 articles

If' he'd used straight gears, someone would be complaining he should have used skew gears...


XD 35109/08/2018 20:18:05
1174 forum posts
83 photos
Posted by Neil Wyatt on 09/08/2018 20:05:42:

If' he'd used straight gears, someone would be complaining he should have used skew gears...


And supplied some obscure theory written two centuries ago to back it up !

Bazyle09/08/2018 21:14:13
4391 forum posts
184 photos

The easy way is to make very thin plate gears so the skew/straight is both unnecessary and imperceptible. then the rivet counter can't tell whether it is or isn't what he wants it to be.

Andrew Johnston09/08/2018 21:18:33
4449 forum posts
516 photos

It's not clear from the article if the gears are proper helical gears, or if the teeth are simply hobbed in a straight line at an angle to the axis. For a TDI the latter would be fine, and in both cases the mesh will be a point contact, so it depends on what calls perfect mesh. For me it would be much easier to machine gear teeth at an angle, or make proper helical gears, than it would be faffing around trying to mount the TDI at the helix angle of the thread.


Mark Rand09/08/2018 22:36:49
657 forum posts

Nah, just cut some slots in the far end of the leadscrew and use it to free hob a worm wheel. cheeky

Hopper10/08/2018 00:49:01
3289 forum posts
58 photos

Straight cut gears can be meshed with a leadscrew or similar thread without offsetting to the helix angle. You just make either the gear teeth or the profile of the thread thinner to accommodate.

I did this with great success on my fabricated Versatile Dividing Head. I used a straight-cut Myford 60T change gear as the main "worm wheel" and turned the worm teeth to a thinner than normal profile so the worm shaft could be twisted from its natural angled position to one straight in line with the Myford gear. Yes, it gives point contact, which is sufficient for positioning only purposes like dividing heads and thread indicators.

I can't claim the idea as my own. Martin Cleeve in his book Screwcutting in the Lathe outlines the thinning of teeth/thread to achieve straight alignment between worm and and straight-cut gear.

The pic below shows the straight-cut main gear with the worm mounted square to its axis, using the above method. The smaller "worm-wheel" was made at home with the teeth straight but skewed at the helix angle of the worm, just for the sake of it. When cutting the gear it was no extra bother to kick the vertical slide around to the helix angle and cut the teeth skewed.


So for a thread chaser dial, proper helical gears would be overkill in most cases. Skewed would work perfectly and straight could be made to work perfectly also, if you cut your own gear with thinner than normal teeth.

Neil has a nice article on file on this very topic so I expect it will appear in MEW one day with all the gory details.


Edited By Hopper on 10/08/2018 00:56:13

Edited By Hopper on 10/08/2018 00:59:41

Neil Wyatt10/08/2018 10:16:44
15581 forum posts
652 photos
73 articles
Posted by Hopper on 10/08/2018 00:49:01:

Neil has a nice article on file on this very topic so I expect it will appear in MEW one day with all the gory details.

Indeed, but still a few other longer articles ahead of it in the queue.

Howard Lewis10/08/2018 13:47:06
1762 forum posts
2 photos

Lots know the theoretically correct way to mesh the gear with the Leadscrew.

But bear in mind that the gear on a T D I is only required to transmit enough torque to overcome the friction in the bearings of its shaft.

So, I would imagine that a theoretically incorrect spur gear, probably only making point contact, would be quite adequate for the task, as long as the tooth form conformed fairly closely to that of the thread on the Leadscrew..

"C'est brusque, mais ca marche"; as someone said of the "crash" gearbox.


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