# 12" Commercial Reduction Gearing



## mattygee (Feb 26, 2017)

Has anyone had any luck with a gearing setup on their 12" commercial to achieve a slowest feed rate of .0021?
I was able to do a double reduction gear on my 10F no sweat, but it seems like the banjo on the commercial is a little different and no matter what gear combos I try nothing will fit.   I'm curious if anyone has tried the same thing.


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## wa5cab (Feb 26, 2017)

Is your Commercial equipped with a QCGB or is it Change Gear?


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## mattygee (Feb 26, 2017)

Underdrive with QC box.


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## wa5cab (Feb 26, 2017)

OK.  The same quadrant or banjo was used on all three versions of the 12" QCGB made from 1950 through 1981.  So nothing changed there.  However, the angle or distance between the AB slot and the C and D slots on the QC banjo is a little less than it is on the L3-58M banjo used on the Change Gear models from 1938 through mid 1962.  Unfortunately, I don't have but two extra gears, or three if you count one of the two 48T gears normally on the quadrant.  And none are small enough to be half the size of any of the others.  So I can't experiment.  But the sliding gear obviously needs to remain in the OUT position.  It needs to drive the large gear of a 2:1 Idler.  And the gear driving the gearbox must be an Idler, either driven by the small gear of the compound set, or by another idler driven by the small gear.


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## mattygee (Feb 27, 2017)

Yeah, I know what you're saying; I've tried any number of gear combinations to no avail.  What I need to do is see if a 24/96 compound gear driven by the tumbler gear will fit.  I think the larger tumbler gears of the commercial lathe must restrict the room just enough to prevent doing a double reduction gear setup.


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## wa5cab (Feb 27, 2017)

OK.  You may be right.  But if I'm reading it right,  look at Table II in a Version 8 MOLO at either 69 or 70 TPI, but with the levers in positions E 9.  If you change the 46T gear on the line for 69 TPI to a 48 and the 48 to a 52, wouldn't that do it?




END


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## mattygee (Feb 27, 2017)

I did just find a YT video   



  where the gentleman uses a 64/32 compound gear with a 54 tooth idler.
I have the 64/32 combo which I'll try with a 56 tooth idler since that's what I have on hand.


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## roadie33 (Feb 27, 2017)

You only need to purchase one gear to make it work.
Find a 24 tooth gear.
Use the outside 48 tooth gear that runs the QCGB, with the 24 on the inside in compound configuration to run the other 48 tooth gear and you cut the feed in half.


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## mattygee (Feb 27, 2017)

Well, I tried the 64/32 compound gear; which to me, as long as the ratio is 2:1, it  shouldn't make any difference as to the tooth count.. Anyway, same deal.  Still getting .0042 per rev on the carriage.  I don't know why this thing is defying me, but I'm stumped.


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## roadie33 (Feb 27, 2017)

You set them up as the 32 towards the lathe and 64 to outside, right?
The 64 should contact the outside 48 tooth that runs the QCGB.


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## wa5cab (Feb 28, 2017)

Right.  It would seem likely that you are driving from the 32T tumbler gear instead of the 16T.  When you do that, your first 2:1 reduction ratio is lost.  So instead of a 4:1 reduction, you still have only 2:1 over all.


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## mattygee (Feb 28, 2017)

Definitely driving off 16 tooth gear of tumbler.  Setup up exactly like the YT video I posted. Like I said, I'm stumped!


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## roadie33 (Feb 28, 2017)

Yes, the 32 tooth gear, closest to banjo, should be the one driving the 16 tooth, the 64 tooth should be driving the outside gear that runs the QCGB.
Make sure QCGB is in E and  9

Watch this video and you'll understand better.


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## wa5cab (Feb 28, 2017)

Well, I don't know what to tell you.  Both videos show doing essentially the same thing, just with different tooth count gears.  If you have just the basic QC lathe and no extra gears, the second version is the most economical, requiring only a new 24 tooth gear.  However, the second guy did fail to mention one other part that you will have to have.  This is what the threading charts call a Steel Spacer.  It replaces the 48T that was removed from the front right position where it and the one in the back right position drove the long gear in the gearbox.  I am trying to determine the part number of that spacer.  In place of the 40T sliding gear, you put first the 24T in the back position and then the relocated 48T in front.

The tumbler gear (stud gear on a 10" change gear lathe) is a 32T/16T compound gear.  The 32T part is driven by either the forward or reverse gear on the tumbler.  The 16T part drives the relocated 48T gear.  That is your first (normal) 2:1 reduction.  The relocated 48T drives the new 24T through the 9-70A bushing that they are both mounted on.  The new 24T drives the remaining original 48T in the Back position.(its original position).  That is your second 2:1 reduction.  Finally, the original rear 48T drives the gearbox as it always has.  That looks like it worked.  I am going to order a 24T gear this afternoon, and if we can determine the part number, a spacer.  When they arrive, I'll try it.

I'll add that the slowest cross feed rate has always seemed to me to be too fast for power feeding for milling.  This will help that as well.


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## roadie33 (Feb 28, 2017)

I have done it that way and it works great.
I was lucky and had a 24 tooth gear. 
I just made the spacer myself on the lathe and broached the key slots to fit the bushing.
It makes it easy to switch back to normal operation.
I never tried to thread with it setup like that, so I don't know if that works like they say in the vids.


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## mattygee (Mar 1, 2017)

Soooo... Today I checked my gear setup and instead of measuring linear travel with a DI, I set it up for threading and did a scratch pass, with the results being I have doubled the thread count per inch vs the QC placard... After that I rechecked linear travel with a different measuring method and I do indeed have the desired .0021 slow feed rate.  Don't know why my DI doesn't seem to agree but that's another issue.  All's well that ends well... Thanks all for the input.  M>


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## songbird (Mar 1, 2017)

I am trying to do the same set up, having trouble locating a 24 tooth gear. Did you find one on e-bay, or did you order one from Boston Gear? If so, what exactly did you order? Thanks, Jim.


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## roadie33 (Mar 1, 2017)

Got one off Ebay with a set I bought for doing Metric threading.
Here is one now on Ebay.

http://www.ebay.com/itm/Original-At...18161d2&pid=100010&rk=6&rkt=6&sd=262857836121


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## songbird (Mar 1, 2017)

roadie33 said:


> Got one off Ebay with a set I bought for doing Metric threading.
> Here is one now on Ebay.
> 
> http://www.ebay.com/itm/Original-Atlas-Craftsman-10-12-Metal-Lathe-24-Tooth-Threading-Stud-Change-Gear/272568166636?_trksid=p2047675.c100010.m2109&_trkparms=aid=555012&algo=PW.MBE&ao=2&asc=41402&meid=1a01a56dc7914479bd05ce2ce18161d2&pid=100010&rk=6&rkt=6&sd=262857836121


Thanks roadie33, I will look it up, Jim.


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## wa5cab (Mar 1, 2017)

I was talking to Clausing about some drawings for restoring a 900B Collet Attachment, and ordered one from them.  Price was $42.92, which compares favorably with the cost of a plain gear plus cost to bore, face. and double broach.


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## roadie33 (Mar 1, 2017)

You can also go this route if you want a Boston steel gear from Amazon. Comparable price but better gear.

https://www.amazon.com/Boston-Gear-...1&keywords=Boston+Gear+GB24+Plain+Change+Gear


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## wa5cab (Mar 1, 2017)

Hmmm.  They don't give either the face width or the bore length.  But at least, you wouldn't have to bore and broach it.


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## Rob (Mar 1, 2017)

The Boston Gear GB series have 1/2 face and bore length.  Here is the link to there catalog and the GB series are on page 29.  http://www.bostongear.com/products/open-gearing/stock-gears/spur-gears/change-gears Amazon carries most of the gears and they are made in steel or cast iron depending on size.


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## 38Bill (Mar 1, 2017)

My 12" uses a 40T so I just added a 20T to the back of it and then removed the outside gear of the double 48T set that drives the QC. The compound gear drives the 40T and the 20T (mated to the back of the 40T)  then drives the remaining 48T.  NOTE: The outside gear of the double set needs to be removed as the other gear above it will hit it due to the difference with meshing the smaller gear to the 48T.


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## wa5cab (Mar 2, 2017)

Yes, that will also work.  However, in my experience the 40T gear wears more than the 48's for three reasons.  (1) it is loose on the bushing so the ID wears more, (2) it is engaging two gears while not being as constrained at the hub, and (3) it is running 20% faster.


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## 38Bill (Mar 2, 2017)

wa5cab said:


> Yes, that will also work.  However, in my experience the 40T gear wears more than the 48's for three reasons.  (1) it is loose on the bushing so the ID wears more, (2) it is engaging two gears while not being as constrained at the hub, and (3) it is running 20% faster.



As the 40T is the standard gear on my 12" it isn't loose or turning any faster than than before. It has a spacer behind (on the bushing) it that is simply replaced with a 20T gear. The lower, outside 48T gear needs to be removed and replaced with a spacer. If you don't have a spacer you can use any gear with a small enough dia that will clear the 40T. Here is a sketch I made when I did mine for the first time.


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## wa5cab (Mar 2, 2017)

Well, either way will work.  But if your machine had a spacer behind it, some owner added it after the lathe left the factory.  The 40T is the sliding gear, and is held in either front or back position by the metal disk that is between the 16T and 32T gears on the tumbler compound gear.  That's the only reason for that disk.  I have a 3996 purchased new which is that way, plus all of the manuals including the MOLO's show it that way.

My comment about wear rate was about the relative speeds or number of teeth engaged per revolution,  or tooth wear rate, not the gear RPM.  But the difference is only 20% and someone installed a spacer on your machine, so disregard.


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## songbird (Mar 5, 2017)

songbird said:


> I am trying to do the same set up, having trouble locating a 24 tooth gear. Did you find one on e-bay, or did you order one from Boston Gear? If so, what exactly did you order? Thanks, Jim.


I took the tip and got the one on e-bay, still waiting to receive it.


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## Gary Mundy (Apr 3, 2017)

Has anyone tried threading with the single 24 gear setup?  Also, has anyone found a source for the spacer?


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## wa5cab (Oct 23, 2019)

Whichever small gear you use (20T with the second 48T not used as shown in the sketch in 38Bill's last post about four posts up the thread or 24T with the second 48T replacing the 40T), the finest thread that you can cut will be 16T with the gearbox set up for 8T.  And all finer threads will be cut with the gearbox set for half of the desired TPI.  And no odd thread pitches.  Odd pitches and pitches coarser than 16 TPI will require setting the machine back to stock.


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## miken (Jul 1, 2022)

Hi, I know this is a very old thread but I'm hoping someone is still around and can help me understand this. I have an Atlas 3990 undermount with QCGB. If the QCGB is driven by a 40 I can understand how adding the 20 will directly cut the feed rate in half. I also have read that any combination (64/32, 48/24, 40/20) that will physically fit together will also cut the feed rate in half. What I cant seem to understand is why the additional number of teeth doesn't matter? With all other gears remaining the same, input 32/16 and 48 at the QCGB, why does switching the 40 to a 48 not change the original rate. Those additional 8 teeth don't matter? Seems like only a 40/20 should work, but I know the other options work as numerous people have verified. Thanks for any help, have a great July 4th. Mike -


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## Rob (Jul 3, 2022)

when it is a single gear on a shaft it is 1 to 1 ratio no mater how many teeth.  when you have a compond gear ie 2 different tooth counts the change is the ratio of the 2 gears.


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## wa5cab (Jul 4, 2022)

I sorta struggle too with understanding why changing the tooth count of any of the single gears causes no change to the overall ratio, but it doesn't.  
Consider only three gears - suppose that in the gear train you had a 32 tooth gear on the spindle driving a 32 tooth gear driving a 32 tooth gear on the lead screw.  It is easy to be able to say that the three gears will all turn at the same RPM as will the spindle and the lead screw.  Now change the 2nd gear to 16 teeth.  It is intuitively  obvious that the second gear will turn faster than it had before the change  but that the 1st and 3rd gears and the spindle and lead screw will still turn at the same speed.


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## miken (Jul 5, 2022)

wa5cab said:


> I sorta struggle too with understanding why changing the tooth count of any of the single gears causes no change to the overall ratio, but it doesn't.


Thanks Rob and wa5cab, I appreciate your replies. I am going to accept it and move on.


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