# Question on gear change to slow feed rate.



## GreggC (Mar 8, 2018)

Hey guys,
So, my feed rate is too fast, its .0047 and if I want a really nice finish I have been hand feeding.
I got to thinking,
Gear "B" drives the gear box that drives the lead screw.
Gear "B" is a 60 tooth gear,
My question,
If I increased the size of gear "B" wouldn't that slow down the lead screw in turn slow down the feed rate ??


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## rgray (Mar 8, 2018)

Most certainly. That or A gear smaller.


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## GreggC (Mar 8, 2018)

rgray said:


> Most certainly. That or A gear smaller.



Way easier to change the "B" gear,
In fact the parts list shows my lathe came with a stack of gears and one is a 80 tooth
Item 123, Im gonna try it when I get off work.


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## British Steel (Mar 8, 2018)

If the compound gear (120/127?) shown as intermediate is two separate gears, you could swap the B gear and its mate around and get a pretty substantial reduction - they may be permanently attached, though (if you could do it, you wouldn't even have to fiddle with the banjo...)?
That does suppose that they're on identically sized shafts, with identical keys...

Dave H. (the other one)


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## GreggC (Mar 8, 2018)

British Steel said:


> If the compound gear (120/127?) shown as intermediate is two separate gears, you could swap the B gear and its mate around and get a pretty substantial reduction - they may be permanently attached, though (if you could do it, you wouldn't even have to fiddle with the banjo...)?
> That does suppose that they're on identically sized shafts, with identical keys...
> 
> Dave H. (the other one)



Yeah, fitment might be an issue.
But, yes, big time reduction.


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## RJSakowski (Mar 8, 2018)

On my G0602, I run a 27 tooth gear in the a position, replace the 120 tooth gear with a 36 tooth, and put the 120 tooth gear in the b position. My gear box is run in position II C.  This gives me the slowest possible feed rate using stock gears, .00139"/rev.


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## GreggC (Mar 8, 2018)

RJSakowski said:


> On my G0602, I run a 27 tooth gear in the a position, replace the 120 tooth gear with a 36 tooth, and put the 120 tooth gear in the b position. My gear box is run in position II C.  This gives me the slowest possible feed rate using stock gears, .00139"/rev.



I tried Dave's idea trading the 120 and the "B" gear,
Didn't work, the 120 was too big and hit my bench.
I ordered a 80 tooth gear for the "B" spot.


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## savarin (Mar 8, 2018)

On my 9x20 if I want slow feed I just do as you have suggested.
Swap the 120 with the gearbox gear and the banjo does not need re-adjusting, if I want slower then I use the smallest change gear in place of the 120 with the 120 on the gearbox, this usually needs a banjo adjustment.
I cut a lot of M# x 1mm threads so I usually leave those combinations set up and swap to the lower feed as required.


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## GreggC (Mar 8, 2018)

savarin said:


> On my 9x20 if I want slow feed I just do as you have suggested.
> Swap the 120 with the gearbox gear and the banjo does not need re-adjusting, if I want slower then I use the smallest change gear in place of the 120 with the 120 on the gearbox, this usually needs a banjo adjustment.
> I cut a lot of M# x 1mm threads so I usually leave those combinations set up and swap to the lower feed as required.



Does the 120 hang below the lathe and hit the bench ??


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## RJSakowski (Mar 8, 2018)

I'm guessing that you have a G4000 or clone.  If you run the 60 tooth gear in place of the 120 tooth gear and the 80 tooth gear in the b position, you should get down to .00176"/rev.  You may be close to hitting the bushing behind the 80 tooth gear with the 127 tooth gear.  If you have interference, swap the 120 tooth gear for the 127tooth gear and you should be good. That would increase the feed rate to .00186/rev.


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## GreggC (Mar 8, 2018)

RJSakowski said:


> I'm guessing that you have a G4000 or clone.  If you run the 60 tooth gear in place of the 120 tooth gear and the 80 tooth gear in the b position, you should get down to .00176"/rev.  You may be close to hitting the bushing behind the 80 tooth gear with the 127 tooth gear.  If you have interference, swap the 120 tooth gear for the 127tooth gear and you should be good. That would increase the feed rate to .00186/rev.



Excellent,
Thank you !!


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## savarin (Mar 8, 2018)

GreggC said:


> Does the 120 hang below the lathe and hit the bench ??



Not any more, I raised my lathe onto blocks for a better height for me.


I also cut out a section of the cover for clearance.


I removed the hinge bolt and replaced it with a ring pin for easy removal.


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## GreggC (Mar 8, 2018)

savarin said:


> Not any more, I raised my lathe onto blocks for a better height for me.
> View attachment 261221
> 
> I also cut out a section of the cover for clearance.
> ...



I was just thinking about having to cutout the cover,
Whether I swap the 120 and the 60 or take RJ's suggestion at  least I now know Ill be able to substantially
reduce the feed rate from the original .0047 to  somewhere around .0018.
Looking forward to working with that.
Thanks guys !!


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## Tozguy (Mar 9, 2018)

GreggC said:


> Way easier to change the "B" gear,
> In fact the parts list shows may lathe came with a stack of gears and one is a 80 tooth
> Item 123, Im gonna try it when I get off work.
> 
> View attachment 261140



Just curious about why it is easier for you to change gear B than change gear A. On my lathe the spare change gears will fit equally well on shaft A or B.


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## GreggC (Mar 9, 2018)

Tozguy said:


> Just curious about why it is easier for you to change gear B than change gear A. On my lathe the spare change gears will fit equally well on shaft A or B.



That comment was in reference to post #2,
Before any suggestions were made on swapping gears,
rgray commented " That or A gear smaller"
The "A" gear is a 28 tooth gear, going smaller 
would mean having a custom gear made.
So in my mind its was easier to purchase the 80 tooth.


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## Tozguy (Mar 9, 2018)

OK, I see, thanks.
If the spare change gears start at 28T, is the 'standard' gear that was already installed even smaller? On my lathe (12x36) the 'OEM' gear in position A is a 24T and feed can be set as low as .002''. Are you sure that a 28T gear is the smallest available to purchase for your lathe?
It seems a shame to have to start cutting covers or blocking up the lathe to fit a larger than intended gear on B if it can be avoided.


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## GreggC (Mar 9, 2018)

Tozguy said:


> OK, I see, thanks.
> If the spare change gears start at 28T, is the 'standard' gear that was already installed even smaller? On my lathe (12x36) the 'OEM' gear in position A is a 24T and feed can be set as low as .002''. Are you sure that a 28T gear is the smallest available to purchase for your lathe?
> It seems a shame to have to start cutting covers or blocking up the lathe to fit a larger than intended gear on B if it can be avoided.



I just had a chance to check the included gears with my lathe,
28t installed in "A"
Two 30T's
36T
42T
45t
60T installed in "B"
I just ordered a 80T

Im going to go with RJ's suggestion,
I will run the 60 tooth gear in place of the 120 tooth gear and the just now ordered 80 tooth gear in the b position,
As per RJ that will get down to .00176"/rev, 
Going from .00476 to .00176, that should be perfect.


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## GreggC (Mar 9, 2018)

RJSakowski said:


> I'm guessing that you have a G4000 or clone.  If you run the 60 tooth gear in place of the 120 tooth gear and the 80 tooth gear in the b position, you should get down to .00176"/rev.  You may be close to hitting the bushing behind the 80 tooth gear with the 127 tooth gear.  If you have interference, swap the 120 tooth gear for the 127tooth gear and you should be good. That would increase the feed rate to .00186/rev.



RJ,
How do you calculate the feed rate with gear selection ??


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## RJSakowski (Mar 9, 2018)

I kind of cheated.  I used the published feed rate for the lathe and just did ratios.  The 60 tooth gear replaced the 120 tooth gear which reduced the feed rate by 60/120 or .5.  The 80 tooth b gear was replaced by the 120 tooth gear which reduced the feed rate further by a factor of 80/120 or .75.  The overall reduction is .5 x .75 or .375. that reduction applied to your slowest stock feed rate would take it from .00476 to .00176.

The calculation can be done from scratch but to do so, You would have to know what the gearing gear box and in the carriage is.
edit: As pointed out by Tozguy, 80/120 = .6667. That ratio times .5 equals .3333 and the feed rate would be .0159"/rev.  Thanks Toz!


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## Tozguy (Mar 10, 2018)

The overall ratio through the 28T-127T/120T-80T drive is .33.
The overall ratio through the 28T-127T/60T-120T drive is .11.
So the change of ratio from one configuration to the other will cut the feed rate TO one third of whatever you start with (.11/.33)
Basically I agree with RJ's approach but there might be a math correction to consider.
(RJ please note that 80/120 = .6667 and not .75)


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## GreggC (Mar 10, 2018)

Tozguy said:


> The overall ratio through the 28T-127T/120T-80T drive is .33.
> The overall ratio through the 28T-127T/60T-120T drive is .11.
> So the change of ratio from one configuration to the other will cut the feed rate TO one third of whatever you start with (.11/.33)
> Basically I agree with RJ's approach but there might be a math correction to consider.
> (RJ please note that 80/120 = .6667 and not .75)



Im going to use a dial indicator to confirm feed rate with stock gears,
Then Ill check with the 120 and the 60 swapped,
Then Ill check with the 60 in the 120 spot and the new 80 in the "B" position.
Ill post my results,
Waiting on the delivery of the 80T.
Like I said, this lathe is at .00476,
If I can reduce the feed by approx two thirds that'll be great.
I wonder why this feed rate is so fast by design ??
They gotta know as lathe designers and builders a slower feed rate is more desirable ??


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## Bob Korves (Mar 10, 2018)

Gregg, on my 13x40 the feed range chart goes from .0007 to .0400"/rev.  That is with the standard feed gears, many other combinations, smaller and larger, are possible by changing out the gears.  I find that for most ordinary work I tend to settle in on around .005"/rev.  A problem that I had, and I think others have, is using tools with profiles that leave something looking like a thread, and so they try to minimize the roughness by decreasing the feed.  Having a tool that follows the cutting edge from right to left (RH turning) with a small relief angle leaves a nice looking cut along with a much faster cut.  On many materials, it also gets well underneath the previous cut, making work hardening less of an issue.  On smaller lathes, sometimes the cut is limited by power and rigidity.  You do not state what kind of lathe you are using, the pics look like one with decent power and rigidity.


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## higgite (Mar 10, 2018)

RJSakowski said:


> I kind of cheated.  I used the published feed rate for the lathe and just did ratios.  The 60 tooth gear replaced the 120 tooth gear which reduced the feed rate by 60/120 or .5.  The 80 tooth b gear was replaced by the 120 tooth gear which reduced the feed rate further by a factor of 80/120 or .75.  The overall reduction is .5 x .75 or .375. that reduction applied to your slowest stock feed rate would take it from .00476 to .00176.
> 
> The calculation can be done from scratch but to do so, You would have to know what the gearing gear box and in the carriage is.
> edit: As pointed out by Tozguy, 80/120 = .6667. That ratio times .5 equals .3333 and the feed rate would be .0159"/rev.  Thanks Toz!


There is no 80T b gear, yet. That one is on order. The original b gear is 60T.

Swapping the original 60T and 120T gears would give an overall ratio of .110. The original ratio is .441 for .0047"/rev feed rate.
The new feed rate would be .110/.441 x .0047 = .117"

When OP gets his new 80T gear and uses it for b gear and replaces the original 120T with the original 60T, the overall ratio will be 28/127 x 60/80 = .165.
The new feed rate will be .165/.441 x .0047 = .00176"/rev.

Tom


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## RJSakowski (Mar 10, 2018)

GreggC said:


> Im going to use a dial indicator to confirm feed rate with stock gears,
> Then Ill check with the 120 and the 60 swapped,
> Then Ill check with the 60 in the 120 spot and the new 80 in the "B" position.
> Ill post my results,
> ...



The manufacturers are somewhat limited by design constraints.  As you have noticed, physical conditions  limit the overall speed reduction possible with a dual reduction gear set.  Having the possibility of adding a third set would allow a further reduction.  My Atlas/Craftsman has that capability.  However, that adds complexity and manufacturing cost.
In the case of your lathe, final gearing is determined by the worm and gear in the carrriage. The worm must be sturdy enough to deal with the forces needed to drive the carriage and the gear ratio is determined the number of teeth in the gear engaging the worm and is constrained by the maximum size that will fit the carriage .
As Bob said, changing the profile of the cutting tool will make a big difference in your surface finish.  Every tool creates a scalloped finish.  A radius on the leading edge will reduce the scallop heights.


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## GreggC (Mar 10, 2018)

About the cutting tool,
Agreed, 
I need to do some experimenting with tooling also.
However with the gear swap to reduce feed rate I will have
more flexibility. 
I need to order that tool set Mile suggested on a previous thread.


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## Tozguy (Mar 12, 2018)

GreggC said:


> I wonder why this feed rate is so fast by design ??
> They gotta know as lathe designers and builders a slower feed rate is more desirable ??



In my experience, lathe designers are usually not at their first rodeo. Also, hobbyists have been known to head in the wrong direction to solve a problem. As RJ explained, lathes are usually configured to provide the most common features. When these features need to be stretched, and sometimes they do, it is always a flag for me to review my assumptions to be sure the problem is not in my way of looking at things.

I am not questioning your need for a sub .002'' feed rate but to me it reflects your special needs rather a weakness in your lathe design.
There are other maybe more important factors involved in getting a good finish.


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## GreggC (Mar 18, 2018)

Hey guys,
A quick update.
RJ's suggestion didn't work,
"run the 60 tooth gear in place of the 120 tooth gear and the 80 tooth gear in the b position, you should get down to .00176"/rev. You may be close to hitting the bushing behind the 80 tooth gear with the 127 tooth gear. If you have interference, swap the 120 tooth gear for the 127 tooth gear"
The 120t and the 127t made contact with the bushing behind the "B" gear.
Dave and Savarin both suggested I swap the 120t and the "B" gear, so I did.
This setup gave me .0012 feed rate



Because I purchased a 80t gear I tried it, this gave me .0016



Both these feeds rated are with the gear box on #9 which is the slowest,
If I want faster I can select the gear box toward #1.
IMO I now have considerable flexability with feed rate.
Many thanks to all who took the time to help,
Your time and effort is much appreciated.


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