Topics

TPI discrepancy


Michael R
 

I'm threading a backplate for the Jacobs collet chuck. The spindle is 8 tpi. I've checked that the gearbox levers are in the correct position and
did the obligatory scratch pass. The gage shows it closer to a 7 tpi than 8. Any thought as to what would cause this discrepancy?

This is the first time single pointing with this lathe.
Thanks,
Michael


comstock_friend
 
Edited

Correct end gears in the gear train? Using half nuts and not the feed knob/lever?

John


eddie.draper@btinternet.com
 

Oh dear!

Has whoever sold you the lathe kindly supplied a metric transposing gear (127 teeth) in the train under the cover on the left hand side?  Check the tooth counts for all the gears from spindle to gearbox input and see if they match what is in the catalogue.  There might be a listing somewhere in the files section.

I take it your leadscrew has an Imperial rather than metric pitch?  Worth a check.

Eddie

On Sunday, 26 July 2020, 19:46:42 BST, Michael R via groups.io <reflexermr@...> wrote:


I'm threading a backplate for the Jacobs collet chuck. The spindle is 8 tpi. I've checked that the gearbox levers are in the correct position and
did the obligatory scratch pass. The gage shows it closer to a 7 tpi than 8. Any thought as to what would cause this discrepancy?

This is the first time single pointing with this lathe.
Thanks,
Michael


glenn brooks
 

Hmmm,  you can always cut a test thread and measure the actual pitch with a known gauge.


On Jul 26, 2020, at 12:07 PM, eddie.draper@... via groups.io <eddie.draper@...> wrote:


Oh dear!

Has whoever sold you the lathe kindly supplied a metric transposing gear (127 teeth) in the train under the cover on the left hand side?  Check the tooth counts for all the gears from spindle to gearbox input and see if they match what is in the catalogue.  There might be a listing somewhere in the files section.

I take it your leadscrew has an Imperial rather than metric pitch?  Worth a check.

Eddie

On Sunday, 26 July 2020, 19:46:42 BST, Michael R via groups.io <reflexermr@...> wrote:


I'm threading a backplate for the Jacobs collet chuck. The spindle is 8 tpi. I've checked that the gearbox levers are in the correct position and
did the obligatory scratch pass. The gage shows it closer to a 7 tpi than 8. Any thought as to what would cause this discrepancy?

This is the first time single pointing with this lathe.
Thanks,
Michael


Michael R
 

Yep, using the half nuts for threading.

The gear configuration is as follows:
Top stud gear - 40t (according to my parts manual it should be 20t)
Idler gear - 80t
Stud gear to lead screw - 40t (this is correct as it's indicated on the gearbox thread chart)
Lead screw - 9p

Another issue with this machine is that the longitudinal power feed spins way too fast, not delivering a good finish.

Wondering if the top stud gear could be the culprit for both problems?


John Dammeyer
 

Threading isn't rocket science.  To me the 9 TPI leadscrew doesn't sound right but I don't know.  Maybe South Bends did come with that.

 

I'd put a dial indicator on the ways and then turn the lead screw by hand one turn with the half nut engaged and the backlash removed by first turning the leadscrew a few turns in the same direction.  If it's 8 TPI it would 0.125". If it's 9 TPI it's 0.11111111111111111111111…" which is not really logical.  Where if 10 TPI it's 0.1  Generally leadscrews are set up to not have infinite repeating decimals.

 

Next forget about the pitch you want and if the leadscrew is really 9 TPI then set up to cut 9 TPI.  Now the lead screw will turn exactly one turn for each turn of the spindle.  If that doesn't work again take a look at the gears.

 

John Dammeyer

 

"ELS! Nothing else works as well for your Lathe"

Automation Artisans Inc.

www dot autoartisans dot com

 

 

 

 

 

From: SouthBendLathe@groups.io [mailto:SouthBendLathe@groups.io] On Behalf Of Michael R via groups.io
Sent: July-26-20 2:31 PM
To: SouthBendLathe@groups.io
Subject: Re: [SouthBendLathe] TPI discrepancy

 

Yep, using the half nuts for threading.

The gear configuration is as follows:
Top stud gear - 40t (according to my parts manual it should be 20t)
Idler gear - 80t
Stud gear to lead screw - 40t (this is correct as it's indicated on the gearbox thread chart)
Lead screw - 9p

Another issue with this machine is that the longitudinal power feed spins way too fast, not delivering a good finish.

Wondering if the top stud gear could be the culprit for both problems?


m. allan noah
 

The leadscrew is 8 too, not 9. The gearbox input gear is 56 tooth, not 40.


On Sun, Jul 26, 2020, 5:42 PM John Dammeyer <johnd@...> wrote:

Threading isn't rocket science.  To me the 9 TPI leadscrew doesn't sound right but I don't know.  Maybe South Bends did come with that.

 

I'd put a dial indicator on the ways and then turn the lead screw by hand one turn with the half nut engaged and the backlash removed by first turning the leadscrew a few turns in the same direction.  If it's 8 TPI it would 0.125". If it's 9 TPI it's 0.11111111111111111111111…" which is not really logical.  Where if 10 TPI it's 0.1  Generally leadscrews are set up to not have infinite repeating decimals.

 

Next forget about the pitch you want and if the leadscrew is really 9 TPI then set up to cut 9 TPI.  Now the lead screw will turn exactly one turn for each turn of the spindle.  If that doesn't work again take a look at the gears.

 

John Dammeyer

 

"ELS! Nothing else works as well for your Lathe"

Automation Artisans Inc.

www dot autoartisans dot com

 

 

 

 

 

From: SouthBendLathe@groups.io [mailto:SouthBendLathe@groups.io] On Behalf Of Michael R via groups.io
Sent: July-26-20 2:31 PM
To: SouthBendLathe@groups.io
Subject: Re: [SouthBendLathe] TPI discrepancy

 

Yep, using the half nuts for threading.

The gear configuration is as follows:
Top stud gear - 40t (according to my parts manual it should be 20t)
Idler gear - 80t
Stud gear to lead screw - 40t (this is correct as it's indicated on the gearbox thread chart)
Lead screw - 9p

Another issue with this machine is that the longitudinal power feed spins way too fast, not delivering a good finish.

Wondering if the top stud gear could be the culprit for both problems?


Phillip Rankin
 

Michael;
It would be helpful to know what model lathe you are using. The gear train on a model 9 inch isn't the same as the gear train on a 13 inch model.

P. Rankin


ken campbell
 

if the upper spindle gear is supposed to be 20 instead of 40 ..... the thread pitch would be off by 2 - 1 , not 13 per cent.

and a 9-pitch lead screw could only cut a few useful threads.

interesting.


Ondrej Krejci
 

Howdy,

Either someone cannot determine inch pitch or the gear box is metric.  I am betting on the former.

Good Luck,


OK

On Monday, July 27, 2020, 10:39:28 PM EDT, ken campbell <deltainc@...> wrote:


if the upper spindle gear is supposed to be 20 instead of 40 ..... the
thread pitch would be off by 2 - 1 , not 13 per cent.

and a 9-pitch lead screw could only cut a few useful threads.

interesting.




Davis Johnson
 

I suspect that we are looking at two possible errors that stirred together produce more confusion than either would produce alone.

First, as Ondrej points out, an easy and common error when trying to count TPI with a rule results in a count one too high. If you lay a rule along a thread with a crest aligned with one inch mark and count crests until you get to the crest aligned with the next inch mark you will get a thread count one too high unless if you count both crests aligned with inch marks. I rather suspect this is what has happened to get 9tpi for the lead screw.

The other problem I rather suspect is that the end gears are set up wrong. If the gear chart specifies a stud gear than I think that implies a double tumbler gear box. The stud gear listed on the gear box goes on the output shaft of the reverse tumbler (on Southbend lathes that have a reverse tumbler). The gear on the input shaft of the gear box isn't usually, in my limited experience, called a stud gear. It is usually a 56 tooth gear.

If a 40 tooth gear is set up on the gearbox input replacing a 56 tooth gear you would get 8*56/40 or 11.2tpi. That wouldn't quite explain it.

On 7/28/20 3:45 AM, Ondrej Krejci via groups.io wrote:
Howdy,

Either someone cannot determine inch pitch or the gear box is metric.  I am betting on the former.

Good Luck,


OK

On Monday, July 27, 2020, 10:39:28 PM EDT, ken campbell <deltainc@...> wrote:


if the upper spindle gear is supposed to be 20 instead of 40 ..... the
thread pitch would be off by 2 - 1 , not 13 per cent.

and a 9-pitch lead screw could only cut a few useful threads.

interesting.




Davis Johnson
 

Correcting myself:

If you lay a rule along a thread with a crest aligned with one inch mark and count crests until you get to the crest aligned with the next inch mark you will get a thread count one too high if you count both crests aligned with inch marks.

On 7/28/20 8:02 AM, Davis Johnson wrote:

I suspect that we are looking at two possible errors that stirred together produce more confusion than either would produce alone.

First, as Ondrej points out, an easy and common error when trying to count TPI with a rule results in a count one too high. If you lay a rule along a thread with a crest aligned with one inch mark and count crests until you get to the crest aligned with the next inch mark you will get a thread count one too high if you count both crests aligned with inch marks. I rather suspect this is what has happened to get 9tpi for the lead screw.

The other problem I rather suspect is that the end gears are set up wrong. If the gear chart specifies a stud gear than I think that implies a double tumbler gear box. The stud gear listed on the gear box goes on the output shaft of the reverse tumbler (on Southbend lathes that have a reverse tumbler). The gear on the input shaft of the gear box isn't usually, in my limited experience, called a stud gear. It is usually a 56 tooth gear.

If a 40 tooth gear is set up on the gearbox input replacing a 56 tooth gear you would get 8*56/40 or 11.2tpi. That wouldn't quite explain it.

On 7/28/20 3:45 AM, Ondrej Krejci via groups.io wrote:
Howdy,

Either someone cannot determine inch pitch or the gear box is metric.  I am betting on the former.

Good Luck,


OK

On Monday, July 27, 2020, 10:39:28 PM EDT, ken campbell <deltainc@...> wrote:


if the upper spindle gear is supposed to be 20 instead of 40 ..... the
thread pitch would be off by 2 - 1 , not 13 per cent.

and a 9-pitch lead screw could only cut a few useful threads.

interesting.




Michael R
 

Thanks Davis, that's exactly what happened, I miscounted the crests.
Changing  to a 56t gear did the trick.
My confusion is that the threading chart on the gear box notes the stud gear (gear box gear?) as 40t.