Metric


Phillip Rankin
 

Is this what you are looking for? You can find it in the files section. Files,Techinfo, metric threading, metricgearchart.jpg


On Tue, Apr 17, 2018, 2:17 PM 913fred@... [SOUTHBENDLATHE] <SOUTHBENDLATHE@...> wrote:
 

So if I had ( for example ) a 100/127 tooth transposing gear set installed and a 36T stud gear, what would the QC box settings be?   To get .25mm pitch, I would set the QC box to what TPI setting?  How accurate would that be?

Would you please provide a table of settings for 0.25 to 1.25MM pitches.

Thank you for your information.

 

 

On 04/17/2018 01:58 PM, vtsblogan@... [SOUTHBENDLATHE] wrote:

[Attachment(s) from vtsblogan@... included below]

I think this is pretty well known. It pays to learn gear ratio formulas of course- Martin Cleeve's book explains this well, and though he is talking about change gear lathes, as long as you allow for the QC it works.

There are several metric pitches that can get pretty darn close with standard English overlaps.

Once you insert the transposing gears- I use 37/47 because it's more convenient and I don't cut metric much- it really is close enough for virtually all work, but of course you need the 100/127 for perfect match.

I'm using gears from my Logan here, because I have it right in front of me, but the same holds true for the SB lathes (stud gear may change though) With transposing gears in place, and a 36 stud gear I can cut pretty much any .25 metric multiple- .25, .50. .75, 1, 1.25 etc just by changing the QC lever- because a lot of the QC halves itself (or other ratios as well) as in 20tpi setting and 40 tpi setting etc are just like adding in an extra set of compound gearing into the mix. I.E. the QC at 8 TPI is 1:1, QC at 16TPI is 1:2

Compare that to the change gear lathes  I think you would see that the QC is hardly "extremely limited"

Hope I'm making that clear.

On Tue, 17 Apr 2018 13:04:12 -0500, "913fred@... [SOUTHBENDLATHE]" wrote:

 

>>In addition, you can cut Metric threads very, very closely with but one gear change

Please explain further....Thanks

 

 

On 04/17/2018 10:49 AM, vtsblogan@... [SOUTHBENDLATHE] wrote:



palciatore@... not to sidetrack your discussion, or open a can of worms, but to call a QC gearbox "Extremely Limited" is silly. In additionto all but therarestEnglish threads, you havethe ability to instantly change your feed rate- something that isfar more valuable. In addition, you can cut Metric threads very, very closely with but one gear change, and you can cut all of the same Metric threadsas the Model C since you can also use the Metric Conversion gears on the Model A as well. And you can still instantly change feed rates with the Metric gears in place if you need to do a non- threading operation on a part.
 


 

 



 


fwhite913
 

Thank you for your help, but the other individual has stated you can cut threads that are close with an inch gearbox and leadscrew.  I was inquiring what settings for the "inch" gearbox would provide these results. He has declined to provide the info.

 

 

On 04/17/2018 05:26 PM, Phillip Rankin phillip.rankin1964@... [SOUTHBENDLATHE] wrote:



Is this what you are looking for? You can find it in the files section. Files,Techinfo, metric threading, metricgearchart.jpg

On Tue, Apr 17, 2018, 2:17 PM 913fred@... [SOUTHBENDLATHE] <SOUTHBENDLATHE@...> wrote:
 

 

So if I had ( for example ) a 100/127 tooth transposing gear set installed and a 36T stud gear, what would the QC box settings be?   To get .25mm pitch, I would set the QC box to what TPI setting?  How accurate would that be?

Would you please provide a table of settings for 0.25 to 1.25MM pitches.

Thank you for your information.

 

 

On 04/17/2018 01:58 PM, vtsblogan@... [SOUTHBENDLATHE] wrote:

[Attachment(s) from vtsblogan@... included below]

I think this is pretty well known. It pays to learn gear ratio formulas of course- Martin Cleeve's book explains this well, and though he is talking about change gear lathes, as long as you allow for the QC it works.

There are several metric pitches that can get pretty darn close with standard English overlaps.

Once you insert the transposing gears- I use 37/47 because it's more convenient and I don't cut metric much- it really is close enough for virtually all work, but of course you need the 100/127 for perfect match.

I'm using gears from my Logan here, because I have it right in front of me, but the same holds true for the SB lathes (stud gear may change though) With transposing gears in place, and a 36 stud gear I can cut pretty much any .25 metric multiple- .25, .50. .75, 1, 1.25 etc just by changing the QC lever- because a lot of the QC halves itself (or other ratios as well) as in 20tpi setting and 40 tpi setting etc are just like adding in an extra set of compound gearing into the mix. I.E. the QC at 8 TPI is 1:1, QC at 16TPI is 1:2

Compare that to the change gear lathes  I think you would see that the QC is hardly "extremely limited"

Hope I'm making that clear.

On Tue, 17 Apr 2018 13:04:12 -0500, "913fred@... [SOUTHBENDLATHE]" wrote:

 

>>In addition, you can cut Metric threads very, very closely with but one gear change

Please explain further....Thanks

 

 

On 04/17/2018 10:49 AM, vtsblogan@... [SOUTHBENDLATHE] wrote:



palciatore@... not to sidetrack your discussion, or open a can of worms, but to call a QC gearbox "Extremely Limited" is silly. In additionto all but therarestEnglish threads, you havethe ability to instantly change your feed rate- something that isfar more valuable. In addition, you can cut Metric threads very, very closely with but one gear change, and you can cut all of the same Metric threadsas the Model C since you can also use the Metric Conversion gears on the Model A as well. And you can still instantly change feed rates with the Metric gears in place if you need to do a non- threading operation on a part.
 


 

 



 
 


 


Phillip Rankin
 

If the 100/127 transposing gear is used along with the proper change gear on the stud the end result will be 100%. In other words if you set up to cut a 2mm pitch thread using the 100/127 transposing gear you will cut a 2mm pitch thread. Not 1.9993mm, or 2.0002mm.

In the files section of this group there is a photo of a metric threading tag from a South Bend model A lathe. It list a full set of metric threads from 6mm down to 0.02mm. I attempted to post the photo earlier, but I don't know if it is visible to the group.

Nobody needs to create a chart or spreadsheet for you. South Bend Lathe did it for you many years ago. See my previous post to navigate to the image of this tag.


fwhite913
 

The information requested can be used on other brands of lathes- not just SB.

FWIW, I have previously made these calculations and was trying to see what criteria had been used for his statement of "close".  

 

 

 

On 04/17/2018 08:56 PM, phillip.rankin1964@... [SOUTHBENDLATHE] wrote:

If  the 100/127 transposing gear is used along with the proper change gear on the stud the end result will be 100%. In other words if you set up to cut a 2mm pitch thread using the 100/127 transposing gear you will cut a 2mm pitch thread. Not 1.9993mm, or 2.0002mm.

In the files section of this group there is a photo of a metric threading tag from a South Bend model A lathe. It list a full set of metric threads from 6mm down to 0.02mm. I attempted to post the photo earlier, but I don't know if it is visible to the group.

Nobody needs to create a chart or spreadsheet for you. South Bend Lathe did it for you many years ago. See my previous post to navigate to the image of this tag.


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Phillip Rankin
 


Phillip Rankin
 

Gotcha. He may have been referring to the use of a 37/47 transposing gear. That combination will render very close approximations that are good enough for the girls I date.


fwhite913
 

Results provided by Mr Beck indicate he appears satisfied with up to 3% error for short engagement threading.

Using a inch Clausing lathe with no conversion gearing, I can cut external threads that accept a commercial nut with a calculated thread error of up to 6% for .7 to 3 mm pitch ( shaft diameter of about 6mm ( 0.236" ) to 24mm ( 0.945")).  The error for the majority of these threads is below 2%.

Accuracy required is a function of intended use and length of thread engagement.

Also, investment in conversion gearing and back and forth changeover time for the lathe should also be considered and evaluated in the context of frequency of metric threading.

Overall, it appears approximating metric threads with inch threads ( either standard change or quick change lathe ) can, in certain cases, be a viable option.

 

 

 

On 04/17/2018 09:08 PM, phillip.rankin1964@... [SOUTHBENDLATHE] wrote:

Open the third item in the list https://groups.yahoo.com/neo/groups/SOUTHBENDLATHE/files/Techinfo/Metric%20Threading/


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David Beierl
 

 I can cut external threads that accept a commercial nut with a calculated thread error of up to 6% for .7 to 3 mm pitch

I would think that "will accept the nut" is too loose a criterion, unless the application could be served by a single-turn nut.  That would imply that you are willing to allow deformation of up to one whole thread-pitch under load before each of the few threads in the nut make contact.

Yrs,


fwhite913
 

I think I covered that...

>>Accuracy required is a function of intended use and length of thread engagement.

 

 

On 04/17/2018 11:33 PM, David Beierl dbeierl@... [SOUTHBENDLATHE] wrote:



 I can cut external threads that accept a commercial nut with a calculated thread error of up to 6% for .7 to 3 mm pitch
 
I would think that "will accept the nut" is too loose a criterion, unless the application could be served by a single-turn nut.  That would imply that you are willing to allow deformation of up to one whole thread-pitch under load before each of the few threads in the nut make contact.
 
Yrs,


 


Paul Alciatore
 

RE: ecodiesel14's statement about the accuracy of a metric thread:

While it may be true that, due to the present definition of the inch as EXACTLY 25.4 mm, the conversion ratio from English measure to metric measure is performed "exactly" by the 127:100 compound gear, that alone does not guarantee the accuracy of a metric thread. There are many other factors that come into play, the most important of which is the accuracy of the English lead screw that is in use.

Any mechanical component, including SB lead screws, is made to a certain level of accuracy. And both the English threads and the metric threads made with any particular lead screw can not be any more accurate than that lead screw itself. So, if the lead screw is off by 0.5%, then the threads cut with it will also be off by that amount or more. So the very high level of accuracy for metric threads cited in his post (1.9993mm, or 2.0002mm) is very unlikely to be achievable in practice. That implies about a 0.01% accuracy. I do not know South Bend's actual specifications, but I highly doubt that they were as tight as 0.01% (0.0012" per foot) on a lead screw that is 24 to 48 inches long. The tolerance on the actual lead screws was probably at least twice that amount and more likely three times as much. Perhaps even more.

In addition to errors in the lead screw, there can be many other errors of various types in any thread being cut. Some of these errors are linear or they will be the same for the entire length of the thread being cut. Others can have a changing nature; for instance if one of the gears in the gear chain has it's center hole off center, then there will be a periodic error due to that gear.

Wear on the flanks of the lead screw's threads will introduce another error factor. As I said, there are many factors, not just the one (compound ratio).

The study of errors in threads and of the various ways of correcting or minimizing them is a complicated matter.


Phillip Rankin
 

All true. Just bear in mind that none of the figures I used were based on anything I have actually measured. In reality most lathes probably have far more error at the tool post than what I stated. My figures we're hypothetical, and my intention was to show that in theroy (a perfect word) the use of a 100/127 transposing gear would produce a thread with 0% error. In reality we all know that a perfect world does not exist. The original question was more about the amount of error of a 100/127 transposing gear, not the end result at the work piece.


Phillip Rankin
 

By the way, thanks for shedding light on the difference between my theoretical perfect world, and practical application. There is a big difference.


Paul Alciatore
 

If you are only worried about the error due to the transposing gear (100:127) then:

1. The 100:127 gear ratio is, by the international definition of the inch, completely exact. You can go to billions of decimal places and there will be no deviation from that ratio because the inch is DEFINED as EXACTLY 25.4mm.

2. But there can be errors due to that gear. For instance, any errors in the location of the flanks of the gear teeth will be reflected in errors in the pitch. These errors would occur whenever a faulty tooth is in contact with the meshing gear.

3. Also, if there is any deviation of the form of the gear teeth from a perfect involute, then that will cause errors. These will be of very short duration and probably not detectable without expensive lab equipment.

4. If the center hole in the transposing gear is not perfectly located, then there will be a periodic error for every revolution of that gear.

I am sure there must be others. But, as the poster was probably trying to say, the gear ratio is mathematically, completely correct.