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Help with change gears

pman92

Plastic
Joined
Aug 1, 2021
Hey guys. I have a vintage Australian made bench top lathe, see photos from my other thread here: https://www.practicalmachinist.com/...vintage-benchtop-lathe-identification-393201/

I'm trying to get my head around the change gears. I was supplied 8 in total: 25, 30, 35, 40, 45, 50, 55, 60 and 63 teeth. I believe I'm missing at least 1 or 2, probably a small one or very big one. The reason I think that is I've created an excel spreadsheet and crunched the numbers, and unless I've completely stuffed up I cannot find any possible way to cut any thread finer than 21 TPI. I'm also missing a few TPI values that I thought would of been possible (10 TPI, 13 TPI, 16 TPI). I've calculated with and without back gear engaged, and I'm not even sure half of those gear arrangements would physically fit either.

Here's what I've worked out:
-Engaging the back gear makes the chuck turn at 0.16x the spindle RPM (28T driving a 70T - 2 times)
-Spindle RPM is maintained through the forward/reverse selector mechanism (its all 20T gears)
-The 25 tooth gear is currently on the output of the reverse mechanism. It's possible to change this gear but nothing bigger will physically fit, but a smaller gear would.
-The amount of sliding adjustment on the idler gear stud indicates there probably a much bigger gear missing.
-The leadscrew is 8 TPI

I know I'm missing gears, but what gears would I likely be missing? Is this a common set of gears?

Also, what gears would I need to cut metric threads? Is that at all possible with this lathe?

Cheers
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
Back gears have nothing to do with lead screw drive - they have zero effect on thread pitches

Blew up your photo and cropped same

Note the two lower gear assemblies can be COMPOUNDED - are you taking this into account in your calculating?

thumbnail_20210801_160222.jpg

Hera are some scans from an old South Bend pub on direct (or simple) and compound end gearing

End Gear Stuff.jpgCompound End Gearing.jpg
 
Last edited:

Joe in NH

Diamond
Joined
Jul 28, 2007
Location
Stratham, Cow Hampshire
Most lathes come with a "gear chart." My Ca. 1866 Lathe & Morse Maker lathe shows 11 different gears on the chart for about 20 different TPIs.

I have 10 of the 11 gears. I need to make or find a gear with 87 teeth. This on a gear that is DP12.

Now if you don't have the gear chart, the adventure gets more challenging. Your best plan would be to work from what you have and (as you say) and try to fill in the gaps.

You're actually not that far away from what you need. Using a programmable calculator can help as you set up your "ratios." An automated spread sheet can do the same thing.

You will need some information from the lathe. There will be a ratio between the spindle shaft and your spindle gear position. It can be 1-1 (direct connected) but on the L&M the ratio is 1-1.5.(i.e. there are two non-identical intermediate gears as permanent part of the lathe as the shaft passes through the bearing bulkhead.)

The stud gear (intermediate between the spindle gear and the lead screw gear) can be ANY gear from your set which fits the spot and allows transmission between the two driver/driven gears. The stud gear is mounted on a "banjo stud" which can be moved to bring the assembly together - or allow compound gearing. The stud gear is almost a non-entity for calculation - UNTIL you get into compound gearing. For most TPIs a compound gear is not required. (Barnes lathes in the higher TPI ranges excepted. 36 and 40 TPI require compound gear.)

The lead screw gear mounts on the lead screw end. Some lathes (Flather) have an additional gearset between the lead screw gear and the actual lead screw - but these are on quick-change norton/south bend type gearboxes. The gear changeset is built into these lathes, as is the thread selection plate. The other "loose gear" Flathers and my L&M are "direct drive" at this location.

The range of threads for the L&M is 5 to 35, this includes the usual "normal" (i.e. ASME standard) threads. I would consider this common for the usual 14 inch swing lathe.

Knowing the lead screw TPI, any spindle/spindle gear ratio, any lead screw gear/lead screw ratio should be all you need to specify your complete gear train based on the gears you have.

Lay it all out on a spreadsheet so you can catch the "ratio" which is probably consistent based on the lead screw TPI.

Hope this brings some clarity.

Joe in NH
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
The simplest gearing for that machine will probably be for 8 tpi. That only requires a 1:1 ratio of spindle to leadscrew. So that is a great place to start.

As mentioned, it all depends on the lathe either having the "stud gear" rotate at spindle speed, or at some other speed.


If it is the same, then figuring gears may be simpler. If different, then that ratio has to be figured in for every gear set needed, making the gears a little harder to figure.

What is that ratio?

Also The set of gears you have looks more like a set for metric. For imperial, with imperial leadscrew, there are usually more even number gears and gears based on "2", such as 16, 24, 32, 36, 48, 56, etc. It's odd that the leadscrew would be 8 tpi, and the gears all on increments of 5.

The gears might not be intended for that machine as set up.
 

johnoder

Diamond
Joined
Jul 16, 2004
Location
Houston, TX USA
The gears might not be intended for that machine as set up.

Indeed...concentrate on Tooth Count, not physical size. Those are fairly COARSE PITCH compared to size of lathe (you DO have to have any two running together be the same pitch)

thumbnail_20210801_160222.jpg

Here is a chart for the SB 9C which has a 8 pitch lead screw

SB 9C Chart.jpg
 

macsonboy

Aluminum
Joined
Dec 27, 2020
how are you mate,,i also live in vic,,aust saw your post and thought good luck,,sounds like you know a lot more of this gear stuff than me so i will be watching.hopefully i will learn something cheers
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Sb9 change gears are 18dp.

The idler gears are 20dp.

The back gear is 16dp

The rack is 14dp.


Ah, thanks for clearing that up. I knew they were not 16 DP.

That one I mentioned was the early type that had the swap-in idler gear for reversing feeds. Not a standard 9", does that change it?
 

Greenwud

Hot Rolled
Joined
Apr 14, 2012
Location
New Zealand
Looks about right
If the leadscrew is 8TPI then 16 TPI will be a 20:40 train or equivalent, 10 TPI will be a 20:50 and 13 would be a 20:65.
19TPI uses a 38T gear in a compound train
The metric conversion gear is the 63, Drummond used a 73 and a 46 for metric pitches so I'm not familiar with the setup for the 63.
IIRC the Drummond was supplied with 2 of the 20T gears

[email protected] | ML7 change gears on M type
 

pman92

Plastic
Joined
Aug 1, 2021
Sorry for the late reply (I've only just got back to this).
I've redone the spreadsheet with a better understanding of the gearing.

With my current gears I now believe I can do 8, 12, 14, 16, 18, 20, 22, 24 and 32 TPI.

I'm pretty sure I'm missing a 20T to replace the 25T currently fitted. That would allow 12, 14, 16, 18, 20, 22 and 24 TPI without any compounding required. With compounding it also theoretically gives the option of 6, 7, 9, 10, 11, 15, 21, 25, 27, 28, 30, 33, 36, 40, 42, 44 and 48 TPI. Again I'm not sure if these would all physically fit but the numbers work.
 

franco

Hot Rolled
Joined
Jul 8, 2003
Location
Cairns, Queensland, Australia
It's odd that the leadscrew would be 8 tpi, and the gears all on increments of 5.

FWIW my Australian built Brackenbury and Austin lathe has an 8 tpi leadscrew, and the gears go up in 5 tooth increments. If anyone is interested I could post a copy of the thread chart, which is on another computer. It has the same gears as the OP's except that it has 2x20T gears and no 25T gear. It cuts all the common whole number imperial pitches within its size range except 19TPI.

As a matter of interest, a friend of mine has a small, old lathe with a 7 TPI leadscrew. Its gears go up in three tooth increments. I worked out a thread chart for him. Once again, it would cut most of the common imperial threads in its size range.

Frank.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
The Machinist hand book has instructions to determine the needed gears to cut any thread pitch. No need for a chart, simply calculate the ratio needed. For common threads a chart can be made. Both inch and metric pitch can be calculated no matter if the lead screw is metric or inch. But clock will only work with threads of the same kind as the lead screw or pitches that are within the division of the clock. I think that it's also available on the web.
 

franco

Hot Rolled
Joined
Jul 8, 2003
Location
Cairns, Queensland, Australia
Hey Frank,

it would be awesome if you could post up a copy of that thread chart! :D

Cheers, Daniel

Hi Daniel,

Sorry, the forum won't let me post this - too big, though it is only 20.5 kb, the limit for this format is only 20 kilobytes. If you private message me with your email address I will attach a copy.

I don't have a 63T gear. This can be used as a metric conversion gear in conjunction with the 40T gear, or ideally an 80T gear. On my lathe a 127T gear won't fit, so I use a 47/37T combination gear. The 40/63 gear combination is still small and slightly more accurate than mine. Both are accurate enough for most practical purposes.

40/63=0.63492, which is half 1.26984. The exact figure is 127/100=1.27 so 40/63 is pretty close. Of course you have to compensate for the difference between the 40T and 80T gears elsewnere in the gear train. If you do want to cut metric threads, you will need a few extra gears.

Regards,

Frank.
 








 
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