Question on gear change to slow feed rate.

[GreggC]

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 ??

 

[Bob Korves]

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.

 

[higgite]

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

 

[RJSakowski]

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 ??

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.

 

[GreggC]

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.

 

[Tozguy]

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.

 

[GreggC]

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.