Lathe Shopping. Help!

[hman]

The approximation part comes in when an imperial thread is cut from a metric lead screw. This entire approximation is handled by the change gears. When you buy a lathe with an imperial lead screw, it can be made to cut "approximate" metric threads the same way the metric machine cuts approximate imperial threads.

I'll beg to differ with you, Joshua. Effective July 1, 1959, the National Bureau of Standards defined the yard as being 0.9144 meter exactly.
http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

Dividing it out, one inch comes out to 25.4mm exactly. 254 can be divided by 2, to get 127. That number is a prime, so it can't be divided further. Thus the use of a 127 tooth gear (generally paired with a 120 tooth gear) when you're making metric threads on an inch-based lathe. I've not looked at any lathes with a metric lead screws, but I wouldn't be surprised to see a 127 tooth gear somewhere in the mix (to be used when making inch threads).

So unless you're dealing with parts older than 1959, using a 127 tooth gear in an inch lathe will yield threads that match the part exactly.

That said, I've seen lathes that don't use the 127/120 tooth gear pair (which gives the exact ratio). There's 91/86 (giving 126.9767, 0.018% low), 80/63 (giving 126.9841, 0.012% low) and 47/37 (giving 127.0270, 0.021% high). These other ratios, which make use of smaller sized gears, are indeed approximations. But the errors are small enough that they'd be absolutely no problem for threaded fastener. The only time the error would affect anything would be if you were trying to make something like a leadscrew or a measuring instrument.

 

[SuedePflow]

I decided to increase my budget a little bit. I'm going to go with the grizzly G4003G instead. It makes sense to spend a little more and get something I shouldn't ever outgrow.

 

[janvanruth]

You will hear vicious rumors like this on some of the "pro" forums, it is simply not true. Sometimes machinery salesmen will feed you this as a lie to sell you a machine, sometimes they actually believe it. You should be able to download the manual of any machine you are considering, the manual will tell you the truth.
If it says 8TPI it is 8TPI.
TBK, there is no machine with an "approximated" lead screw. Seriously, why would any factory tool up to make an approximate thread when they can just make a real thread. Chinese factories may be have all sorts of issues, but there is nothing wrong the Chinese engineer who designs the equipment.
The approximation part comes in when an imperial thread is cut from a metric lead screw. This entire approximation is handled by the change gears. When you buy a lathe with an imperial lead screw, it can be made to cut "approximate" metric threads the same way the metric machine cuts approximate imperial threads.

Pretty much all of these machines do have metric screws in the cross and compound, with an imperial dial added. You can spot these machines easily - the dial will register 0.040" per turn. The type of lead screw can be spotted by the number of teeth on the change gears, this is a separate topic. You should be able to find what gears go in an imperial or metric machine for cutting imperial threads and go from there. The gear ratios are pretty standard across all machines.

You can blame the French for this by the way. When the meter was arbitrarily set to 1/0,000,000 of the distance from the north pole to the equator measured through Paris, they lost any chance for convergence with the imperial system. The meter has basically gone down hill since then and is now some distance of how far light travels in some bizarre tiny fraction of a second and is still almost as arbitrary as before since the actual conditions for measurement only exist in a lab. Wouldn't it have been nice if the meter was 2 or 4 feet long and converting from metric to imperial was a simple matter of dividing by 2 or 10 depending on the direction of conversion?

i would rather blame the USA as it fixed the inch to 25.4 mm long after the meter was a standard
it would have been so much easier if it would have set it at 25 mm......

 

[4GSR]

i would rather blame the USA as it fixed the inch to 25.4 mm long after the meter was a standard
it would have been so much easier if it would have set it at 25 mm......

Hey, wait a minute here!!! I thought the INCH came first??? That what I was taught....

 

[janvanruth]

Since 1893 the National Bureau of Standards and its predecessor agency, the Office of Standard Weights and Measures of the Treasury Department, have
derived the yard and the pound and the multiples and submultiples of these units from metric standards, namely, the international meter and the
international kilogram.
So the yard etc. have derived from the meter therefore the meter must have been prior

 

[higgite]

i would rather blame the USA .....

Most foreigners would.

I believe the yard and the meter were developed independently, the yard a few centuries before the meter. The NBS may define one in terms of the other, but didn't derive one from the other.

Besides, who ever heard of drinking a meter of beer?

Tom

 

[John Hasler]

i would rather blame the USA as it fixed the inch to 25.4 mm long after the meter was a standard
it would have been so much easier if it would have set it at 25 mm......

No, it would have been much more difficult because of all the records that would have had to be changed and all the instruments that would have been obsoleted. The previous inch was so close to 2.54cm that the only measurements affected enough to matter were those used in surveying and even there only over distances of hundreds of miles (thus the survey foot).

 

[janvanruth]

Most foreigners would.

I believe the yard and the meter were developed independently, the yard a few centuries before the meter. The NBS may define one in terms of the other, but didn't derive one from the other.

Besides, who ever heard of drinking a meter of beer?

Tom

as it happens to be it is quite customary in The Netherlands
google: meter bier

"Since 1893 the National Bureau of Standards and its predecessor agency, the Office of Standard Weights and Measures of the Treasury Department, have
derived the yard and the pound and the multiples and submultiples of these units from metric standards, namely, the international meter and the
international kilogram."
This is the offical text of the NBS.
The length of the original USA yard was derived from the meter.

 

[janvanruth]

I'll beg to differ with you, Joshua. Effective July 1, 1959, the National Bureau of Standards defined the yard as being 0.9144 meter exactly.
http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

Dividing it out, one inch comes out to 25.4mm exactly. 254 can be divided by 2, to get 127. That number is a prime, so it can't be divided further. Thus the use of a 127 tooth gear (generally paired with a 120 tooth gear) when you're making metric threads on an inch-based lathe. I've not looked at any lathes with a metric lead screws, but I wouldn't be surprised to see a 127 tooth gear somewhere in the mix (to be used when making inch threads).

So unless you're dealing with parts older than 1959, using a 127 tooth gear in an inch lathe will yield threads that match the part exactly.

That said, I've seen lathes that don't use the 127/120 tooth gear pair (which gives the exact ratio). There's 91/86 (giving 126.9767, 0.018% low), 80/63 (giving 126.9841, 0.012% low) and 47/37 (giving 127.0270, 0.021% high). These other ratios, which make use of smaller sized gears, are indeed approximations. But the errors are small enough that they'd be absolutely no problem for threaded fastener. The only time the error would affect anything would be if you were trying to make something like a leadscrew or a measuring instrument.

i believe the standard is 100/127

 

[wrljet]

That looks very nice.

I decided to increase my budget a little bit. I'm going to go with the grizzly G4003G instead. It makes sense to spend a little more and get something I shouldn't ever outgrow.