Auto-Feed Gearing: How to slow? (11" Standard Change Gear)

SB27A · May 29, 2020

Hi guys,

I'm the new owner of a 1921 South Bend 27-A, 11" lathe. Among all of the things I'm trying to figure out, to get it going, is how to get the auto-feed rate slow enough for longitudinal cutting. It has a 110T gear on the 8TPI lead screw, and a 32T on the spindle stud, giving me 27.5TPI, or 0.036"/rev, which is way too fast for auto-feed with this 1/3hp motor. 16T spindle gear, (smallest I've seen), gets me to 0.018"/rev, which is still too fast.

Is there some way to compound these, or am I missing something? The only way I see to get to the 0.004"/rev with simple gearing is with a 16T spindle and 500T lead screw gear, and I doubt that was ever a thing. I found mention of compound gearing with some of the bigger and later lathes, but never these 11".

Any suggestions?

Thank you,
SB27A

Conrad Hoffman · May 29, 2020

I don't know anything about the SB but my reaction is, "of course you'd have to compound." My Logan feeds off the leadscrew, which is a lousy way to do it, so I never power feed. I've developed a very steady hand over the years. :drool5:

SB27A · May 29, 2020

--> So I was missing something obvious... I looked at that photo 30s after posting and realized it's already compounded with a 53T:106T, giving me 1/2 inch/rev, meaning I'm already at 0.018"/rev. With a 16T stud gear, it would be 0.009"/rev. Maybe there's a bit more room to reduce the compound ratio.

Thank you for any suggestions, especially from someone who has the assortment of standard change gears.

johnoder · May 29, 2020

You have a compound there, just not enough - maybe 2 to 1. Smaller and larger compound gears would make quite a difference, but may require another idler to reach to the screw gear.

A good way to look at this is that 40 and 80 gears for a compound make a 2 to 1 version - but a 16 and and 80 make a 5 to 1 version - you just have to be practical enough to get the gears to mesh with each other from stud gear to screw gear - and this often will mean at least one idler along the way to do the "connecting"

Enough room on quadrant for a second compound?

As a start, it looks like about 40 teeth on the stud gear (closest to spindle) - the one meshing with the larger gear on the compound - if this was 20 you would be lots closer to where you would like to be feed rate wise

Here is an example on a later 9"

SB27A · May 29, 2020

Hey johnoder,
Yeah, it's a 32T on the stud gear. Then 106T:53T (2:1) compound, then 110T screw gear. From the thread chart, I know 16T was a standard stud gear. If I can find a 16T stud gear, it'd get me down to 0.009"/rev, which is in the ballpark of what this small motor can handle. Might need to figure out an idler setup, like you mentioned, to get a bigger compound ratio, which might be the ticket. I sure wish we could find a bucket of gears, somewhere in that garage. Thank you for the suggestion!

johnoder · May 29, 2020

And a way to calculate that is 110/32 X 106/53 = 6.875 to 1

lead screw pitch of 8 (do not know if your 11 has this) of .125" / 6.875 = .0182" for a feed rate

You can change this to about .009" with a 16 replacing the 32

(The 106/53 is a most unhandy combination)

Dobermann · May 29, 2020

Depending upon how adventurous you feel, you might consider driving the lead screw with a stepper motor instead of through the lathe gearing. You may find there's enough room at the right side support bearing to squeeze in a cog belt pulley or you might consider driving the gear train at the left end of the lead screw either with a gear on the stepper motor or a cog belt and pulley system driving the end of the lead screw.

I just finished up such a system on my 10L in order to get away from the gear train racket. The 10L is a little different from your situation because it has the drive for longitudinal and cross feeds in the apron but you can still make the lead screw turn pretty slowly with a stepper motor and cog belt system.

Some folks think it's a bad idea to drive the carriage for turning with the half-nuts (wear) but it should be OK if you keep it lubed and don't over-do it.

SB27A · May 29, 2020

Hi Dobermann,

I understand what you're saying about the stepper motor, but that may be out of my current technical capacity, and budget...

This one does have gearing for cross feed in the apron, (though it's currently missing the most critical and special clutching gear), but longitudinal is off the lead screw, via the half-nuts.

Do you mean to say that the original intent was for all longitudinal cutting to be done manually, and only threading off the lead screw? Or was wear just not a concern, due to available and ease of serviceability of the half-nuts?

SB27A · May 29, 2020

johnoder said:
And a way to calculate that is 110/32 X 106/53 = 6.875 to 1

lead screw pitch of 8 (do not know if your 11 has this) of .125" / 6.875 = .0182" for a feed rate

You can change this to about .009" with a 16 replacing the 32

(The 106/53 is a most unhandy combination)

Yeah, I REALLY hope we find a bucket of gears, somewhere....

hitandmiss · May 29, 2020

Replacing the 53T with a 24T will give you .008823'
replacing the 53 with a 16 = .00548"
Then changing the 32 to a 16 gives you .0027"

16/106 X 16/110 = .0219 X .125 = .0027"/turn

So those 4 gears is all you have?
So, take that 32 tooth gear and give us some info on it.
OD =?
ID =?
Width =?
Keyway =?

Maybe someone has loose gears that one could make fit.

Bill

SBLatheman · May 29, 2020

SB27A said:
Hi Dobermann,

I understand what you're saying about the stepper motor, but that may be out of my current technical capacity, and budget...

This one does have gearing for cross feed in the apron, (though it's currently missing the most critical and special clutching gear), but longitudinal is off the lead screw, via the half-nuts.

Do you mean to say that the original intent was for all longitudinal cutting to be done manually, and only threading off the lead screw? Or was wear just not a concern, due to available and ease of serviceability of the half-nuts?

If you were to fix your worm gear, not only would you have power crossfeed, you would also use the clutch for longitudinal feed reducing the feed rate by approximately 1/3 vs. the half-nuts.
Ted

johnoder · May 29, 2020

Tell us which you have

we are interested in Diametral Pitch

14 DP will be 2.429 O.D. on the 32
16 DP will be 2.125 O.D. " " "
18 DP will be 1.889 O.D. " " "

SB27A · May 29, 2020

SBLatheman said:
If you were to fix your worm gear, not only would you have power crossfeed, you would also use the clutch for longitudinal feed reducing the feed rate by approximately 1/3 vs. the half-nuts.
Ted

Hi Ted,

I guess I was confused, because there wasn't much information in the "How to run a lathe" about the auto feed. I assumed since I had 2-position, one was neutral and the other was for the cross-feed, though I did notice that there is a second set of gears that it meshes with, and didn't fully understand why.

Is the correct function: top selection of star knob is auto longitudinal feed, bottom position of star knob is auto cross-feed, and the other knob controls clutch engage vs disengaged? Then the half-nuts are only for accomplishing direct gear ratios for threading? Or what is the other purpose of the half-nuts?

Thank you for the info!

SB27A · May 29, 2020

johnoder said:
Tell us which you have

we are interested in Diametral Pitch

14 DP will be 2.429 O.D. on the 32
16 DP will be 2.125 O.D. " " "
18 DP will be 1.889 O.D. " " "

Hi johnoder,
My 32T is ~2.10" diameter. (Width of all are ~0.623".) The 53T is 3.42" diameter. I had to measure the rad of the 106T and 110T, but they are ~6.76" and ~7.02" diameters, respectively. The 32T and 110T have 0.625" shaft diameter, with 0.1875" keyways, though the 110T, on the lead screw, is worn to closer to 0.21". The 53T:106T has a shaft diameter of 0.75", and the two appear pinned/ground together, though I have not whacked it with a hammer, to see if it's just 99 years of fusion.

(All of those diameters are OD of the teeth tips, not the roots.)

johnoder · May 29, 2020

Sounds like they are all 16DP since they run with each other - so now you know about that fact as far as gear hunting

Example is the 53

We add TWO to that, making 55

55 divided by 3.42 = 16.08 DP

SB27A said:
Hi johnoder,
My 32T is ~2.10" diameter. (Width of all are ~0.623".) The 53T is 3.42" diameter. I had to measure the rad of the 106T and 110T, but they are ~6.76" and ~7.02" diameters, respectively. The 32T and 110T have 0.625" shaft diameter, with 0.1875" keyways, though the 110T, on the lead screw, is worn to closer to 0.21". The 53T:106T has a shaft diameter of 0.75", and the two appear pinned/ground together, though I have not whacked it with a hammer, to see if it's just 99 years of fusion.

(All of those diameters are OD of the teeth tips, not the roots.)

SB27A · May 29, 2020

It doesn't help that all of these have 99 years of wear that I'm measuring, and I should probably be calculating the real pitch diameter, instead of the tip diameter for diametral pitch, but yes, I 100% agree that these look to be the 16DP, for at least the change gears. I believe the gears under the apron, like Ted mentioned, changes pitches a few times, but for the change gears, they just all have to mesh together. I checked, though, and there won't enough room to get an idler in between that 110T and 106T, for a different gear reduction. That, and the 53T:106T being fused together, means I'd have to do some trial and error, or some math, on what reductions could pissibly fit.

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