Thread: Change Gears Again

Originally Posted by Jekyll and Hyde

Agreed, I was triple guessing myself trying to see if I'd missed something really obvious and was just making an idiot out of myself.

I'm good at that . I think I'll leave this one to the experts and go out and work on the race car - it's gotta be easier.

Originally Posted by Gavin Newman

I'm good at that . I think I'll leave this one to the experts and go out and work on the race car - it's gotta be easier.

Good idea that, I've got a whole front end to straighten out and weld back on. Panel work is painful, especially when you're dealing with the sins of a previous owner (inch thick bog and bad lap joints anyone?)

So it now sounds like we have agreement that the ratio between spindle and gearbox should in theory be 1:1. Then how can my observation be explained that the gearbox shaft turns less than once for each spindle turn? Both those things can't be true. (I will be triple checking that tomorrow.)

Originally Posted by Bryan

So it now sounds like we have agreement that the ratio between spindle and gearbox should in theory be 1:1. Then how can my observation be explained that the gearbox shaft turns less than once for each spindle turn? Both those things can't be true. (I will be triple checking that tomorrow.)

That's what I don't understand - can we have more photos of the gear train. There must be something in the gear train we are missing????

Originally Posted by Bryan

So it now sounds like we have agreement that the ratio between spindle and gearbox should in theory be 1:1. Then how can my observation be explained that the gearbox shaft turns less than once for each spindle turn? Both those things can't be true. (I will be triple checking that tomorrow.)

Looking at the photo of your gearbox, I think you may not have given all the info we needed.

When you move the lever at the top up or down, does the drive go through both 'tumblers' to get to the compound gear? As far as I can tell, when you select one position, one tumbler will engage to the spindle and the other to the compound gear. If you select the other, the gear previously engaged to the spindle will engage the compound gear, and the one engaged to the compound gear will now engage the spindle? If this is correct, you need to count the teeth on the two tumblers as well, because they are changing the ratio between the spindle and the compound gear....

Scratch that. I just counted the teeth on your tumblers, and did some numbers, still works out to be 1:1 between spindle and compound. Seems that it doesn't matter how many idlers you have, they still don't do anything. Also worked out I'm wrong about how they operate anyway, since that's the FWD/REV lever I'm on about...

Maybe recount the teeth on the spindle gear? It seems bigger than the 38 tooth on the compound, but so do the teeth....

*edit* If you had a 40 tooth spindle gear, everything else the same, you'd get a 0.95 reduction over the whole geartrain. And this is driving me nuts, I want answers!

Hello Bryan,
I checked the chart details (metric) on my lathe by setting my digital vernier calipers up in the tool holder so that the "rod" came up against the headstock. I then turned the chuck ten turns for each combination of gear setting. Perhaps you could do this and see how the results compare to your chart.. I suspect that you may need more change gears.
Regards,
Russell

Originally Posted by Bryan

Then how can my observation be explained that the gearbox shaft turns less than once for each spindle turn?

Nothing that I can think of. Either your teeth counts are out or maybe there was some slop in the gears on the first turn?
Or we are still missing something

Stuart

I have an explanation for the anomoly. I was expecting to find a difference so when I thought I found one I didn't look any harder. Must have been a backlash error. I should have repeated the test several times. That's what I just did and it's definitely 1:1. My apologies for the red herring.

Attached are some solutions, including the default gearset(!).
Highlighted are the pitches I expect to find useful.

I think there is something wrong with your table.
For e.g.
In the top table
A1 = 6.35 = 4 tpi Ok
B1 = 3.175 = 8tpi Ok
A6 = 9.525 = 2.66666666666666667tpi ????????

or am I missing something yet again?

Stuart

Stuart you are correct. Collect $200. The numbers should get smaller as they go across, not bigger. Because per inch is a division. I remembered to correct going down, but not across. Thank you. I will start again.

Bryan,
The spreadsheets you posted with combinations of gears to discover 'near enough' metric threads is the method I have used in the past with great success.
You just need to record your own tables of gear sets versus metric threads.
(once you and Stuart get them correct)

The original question, I believe, was to understand the table affixed to the lathe.
I determined earlier that the only single gear that could reasonably produce the metric table was N/60 with N being selected from 25, 26, 43, 46, 47.

The set would hypothetically be a reduction:
- 38 spindle mesh via one or more idlers to 38 on layshaft on reverser yoke,
- yoke layshaft keyed to a 40,
- 40 mesh with 60 on layshaft on the banjo,
- banjo layshaft keyed to N,
- N mesh with 40 on QC input.
But, I am not at all sure you could fit a 25/60 compound on the banjo and have physical space to mesh the 25 with the 40 on the QC input.

Are you in a position to judge the fit as suggested above ?
What is the bush diameter that is used on the QC input shaft to space the gear outwards one thickness ?
What gear specification has been used, 20 DP 14.5 degrees, or other ?
With some help from Pipeclay on gear OD sizes, we should be able to determine if the set proposed above will work.

John.

Originally Posted by electrosteam

The original question, I believe, was to understand the table affixed to the lathe.

John, yes I was interested in solving the original table. But it's dawned on me that a lot of threads in that table I would never use. So it's an academic exercise rather than a practical one. The tangible outcome I need is to be able to cut common metric threads, ie 1, 1.25, 1.5, 1.75, 2, 2.5, 3 mm. The ones that are hard to find are 1.25 and 1.75.

The set would hypothetically be a reduction:
- 38 spindle mesh via one or more idlers to 38 on layshaft on reverser yoke,
- yoke layshaft keyed to a 40,
- 40 mesh with 60 on layshaft on the banjo,
- banjo layshaft keyed to N,
- N mesh with 40 on QC input.

I've, um 'XL'd' your suggestion as I understand it. Please see if I've done it right. It gets closer with some ranges than others. How close is close enough? I really don't know, but for some reason I have my sights set on 1% as a maximum error. If that means swapping two gears instead of one I think that's worthwhile. So again this is tending away from the concept of the original table as I understand it.

But, I am not at all sure you could fit a 25/60 compound on the banjo and have physical space to mesh the 25 with the 40 on the QC input.

Yes that does sound tight. At the moment I'm working on the idea of leaving the idler alone and just swapping stud & screw gears. If I can get the pitches I want that way in realistic sizes it will save some hassle. At least it seems simpler to my mind. If that doesn't work out I'll revisit the idea of adding a compound gear. Thanks, I appreciate your efforts.

What gear specification has been used, 20 DP 14.5 degrees, or other ?

16DP 20 PA. At least they're standard, unlike other gears on this machine.