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Richard P Wilson
04-24-2015, 03:05 AM
I know that the universal advice is not to run gears of 14.5 deg and 20 deg pressure angles together. I've got some 20 degree gears, 18DP and need to cut a 127T/135T compound gear to make a gear set to cut imperial threads on a metric lathe, but I've only got a 14.5 degree cutter. In all honesty, given the speeds involved (low), and the relatively light duty, and occasional use, am I going to have a problem running these gears together?

janvanruth
04-24-2015, 03:28 AM
i had the same problem and gave it a lot of thought
the teeth will get a sliding component in the contact and therefore some wear eventually
the gears i had did mesh but did not run very smoothly

shouldnt the combination be 127/100 tooth?
a 47/37 combination will give you 127.027027

Richard P Wilson
04-24-2015, 04:33 AM
No, for the lathe in question (A UK made Denford 280) it is definitely a 127T/135T in accordance with the original manufacturers details.

John Stevenson
04-24-2015, 04:42 AM
Richard,
Not a problem in this case.
I have seen special for hobbing machines flycut with a 60 degree triangular insert so the hobbing machine can make a proper one.

Richard P Wilson
04-24-2015, 05:24 AM
Thank you John. I was hoping you would reply, not many people with as much experience in gearing as you have.

Baz
04-24-2015, 01:02 PM
I have a set of Britannia changewheels that have a peglike profile, as cast, not machined. Seem to work ok for that lathe. Involute gears even of the same angle slide, that's their thing, and why you need oil in you gearbox plus are constant velocity so your car doesn't judder along the road. Clock gears are cycloidal and roll with low friction despite no oil but are not constant velocity which suits a clock just fine.

Paul Alciatore
04-24-2015, 03:15 PM
Generally speaking, when cutting metric threads with an English (inch) lead screw, you use a compound gear which has 127 teeth on one side and 50, 100, 135, or some round number of teeth on the other. These two gears are combined into a single compound gear which sits on a single hub.

But this is not the only way to do it. Notice that one of those gears on the compound is a driver gear and the other is a driven gear. Thus, using an example from my South Bend 9, to cut a 1mm thread, I would use:

A Stud gear: 16 . . . driver
B Compound: 127 . . . driven
C Compound: 100 . . . driver
D Screw gear: 40 . . . driven

The Stud gear (A) meshes with the 127 tooth side of the compound (B). And the 100 tooth side of the compound (C) meshes with the Screw gear (D). The sequence is A, B, C, and D. In this scheme both the 100 tooth and the 127 tooth must mesh with the existing change gears.

BUT, this gear train can be rearranged in any way that preserves the two driver gears and the two driven gears and the overall gear ratio will be exactly the same. Thus any of the following sequences would work:

A, D, C, B

C, B, A, D

C, D, A, B

All four of these sequences would produce the exact same, 1mm thread on my lathe. Keeping the 100 and 127 tooth gears as the compound is only for convenience. Notice that the first two of them have the 100 and 127 tooth gears (B and C) meshing with each other.

SO, you could make your 127 and 135 tooth gears using ANY pitch and ANY pressure angle as long as you use the same specs for both of them. Then arrange them to mesh with each other in your gear train instead of combining them as a compound. One of these odd pitch gears would be on either the driving stud position or on the screw gear position. The other one would be combined with one of your existing gears in a compound in the middle position. You do need to preserve the roles of each of these gears as driver or driven. I would assume that your gear train, like mine, has the 127 tooth gear as a driven gear. So the 135 tooth would have to be in a driver position in the gear chain.

In my example above, an additional, idler gear was needed to allow all the gears to mesh. You may also have to add an idler to your chain, but I suspect that it would be between the stock gears for your lathe and not on the 127/135 pair unless you use a really small pitch for them. Idler gears do not change the overall ratio so they can have any tooth count. That brings up an additional advantage of doing it this way; since these two gears only need to mesh with themselves, you can use a smaller pitch for these large tooth count gears and they will have a smaller diameter. This may help in the use of the limited space in the change gear mounting area.

And this would run a lot smoother than if you mix pressure angles.

Richard P Wilson
04-24-2015, 06:25 PM
All very true Paul, but I'm copying the OM's set up, and this uses a 127/135 compound gear, so thats what I'm stuck with I'm afraid.

Video Man
04-24-2015, 09:42 PM
Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....

Don Young
04-24-2015, 09:54 PM
More like threading a 55 degree Whitworth nut on a 60 degree UN bolt, which certainly can be done. I have seen pictures of working change gears made from wooden circles with nails driven in for teeth. Also a wooden circle with teeth made from a metal strip corrugated by running it between gears and epoxied to the circle.

Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....

Paul Alciatore
04-25-2015, 04:15 AM
?????

But you are NOT stuck with it. That was my whole point.




All very true Paul, but I'm copying the OM's set up, and this uses a 127/135 compound gear, so thats what I'm stuck with I'm afraid.

John Stevenson
04-25-2015, 05:41 AM
Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....

Bollocks.

Richard P Wilson
04-25-2015, 07:04 AM
?????

But you are NOT stuck with it. That was my whole point.

Yes I am. I'm following the OM's original system, and anyway there isn't room to mesh the 127 with the 135.

Richard P Wilson
04-25-2015, 07:06 AM
Bollocks.

Thats what I thought John, but I thought I'd let you say it first, you do it so much better.

Lew Hartswick
04-25-2015, 09:57 AM
Thats what I thought John, but I thought I'd let you say it first, you do it so much better.
Thanks. I was hoping someone else would take that one. :-)
...lew...

michigan doug
04-25-2015, 10:27 AM
IF...you really must run gears in a sequence as you suggest, use a good grease to minimize wear.

For a short run and very occasional use, it will work...

Richard P Wilson
04-25-2015, 10:33 AM
IF...you really must run gears in a sequence as you suggest, use a good grease to minimize wear.

For a short run and very occasional use, it will work...

Yes, it will only be when I'm cutting imperial threads, which isn't very often these days. I'll be using a spray chain grease which is what I always use on change gears. Speeds will be low as well.

I did try 'rolling' together a couple of 16DP gears, one with 14 1/2 and the other with 20 degree pressure angles, and it seemed reasonably smooth, so I'm hopeful my scheme will work.

Paul Alciatore
04-25-2015, 02:54 PM
Well, I guess you are going to do what you are going to do. Good luck.

But they would be room if you use a smaller pitch, as I suggested. I am will aware of the problems in getting the gears to fit.

And get some ear plugs so the sound of grating gears does not harm your ears.




Yes I am. I'm following the OM's original system, and anyway there isn't room to mesh the 127 with the 135.

Paul Alciatore
04-25-2015, 02:56 PM
No, not grease. Use a gear oil or way oil.




IF...you really must run gears in a sequence as you suggest, use a good grease to minimize wear.

For a short run and very occasional use, it will work...

Mark Rand
04-25-2015, 04:30 PM
Actually, grease will work adequately for change wheels, it does tend to get pushed out, but so does oil. The Hardinge HLV gearboxes are open and run with grease on the gears, so change wheels should be less of a problem.

Gear oil is good in an enclosed gearbox, but falls off open gears rapidly unless it's a heavy grade (got some ISO 320 for a project, but haven't used it), Way oil works very well at the low loads and speeds that change wheels normally get used at.