# Thread: Metric vs Inch - arguements pro & con

1. ## Metric vs Inch - arguements pro & con

I'm looking into buying a benchtop lathe and I understand that the hand wheels may be graduated in either inch or metric. I also understand that some machines can cut both inch or metric threads. I live in Canada where we are officially metric but functionally, many of us still use inches. I think that metric is by far the easier system to use after having used both.

The lathe is strictly for hobby use (so far) and I may end up buying some plans for projects to build. I want to buy a good quality lathe.

Assuming that the lathe can be ordered in either metric or inch graduations, what are the arguments for and against metric? Any other considerations ?

Does any of this make sense ?

2. Use what ever system you want. If you are looking at doing threading it may matter what system the lathe is setup for. To get both systems, just get a DRO that can display both.

3. If you've got metric micrometers, then it would make sense to have metric dials on the machine. You'd want to check that the dials are not "dual scaled" because in one system or the other (inch or metric), you're going to have an odd amount to contend with (in whole turns of the dial) that will plague you forever.

Either way, you're going to have to work with common conversions of common numbers between both systems, so you can interpret your micrometer measurements correctly. It would seem that it takes quite a bit of practice to become thoroughly familiar with either system, until you "just know" what size is being referred to in an instant.

4. ## metric vs. imperial thoughts

I faced the same dilemma when purchasing my Sherline mill. I grew up using the metric system in school but the imperial system elsewhere. Combine that with the fact that U.S. building products and tape measures are typically imperial somewhat forced the matter to imperial "wins", a decision that always left me a bit uncomfortable.

When it came time to actually order the mill I considered the situation more and came to the conclusion that I didn't really "know" the imperial system and more than the metric system; it was simply more familiar. Further, if necessary I could translate any measurements from one system to the other. That last fact didn't seem like much of an issue: it seemed more like a method to make "doubly" certain of a measurement.

Based on those considerations I ordered the metric mill and have not regretted the decision. Because I didn't have any measuring devices more sophisticated than a tape measure there wasn't a big worry about having already made a big investment in one measuring system.

Instead, to help myself make the transition, I just made sure that I snapped up as many metric or combination metric/imperial measuring tools as necessary from eBay.

Did that mean *not* purchasing nice imperial tools too? Not a bit. I've acquired quite a nice set of "tenths" gages, micrometers, calipers, and the like. It's just that as a novice mill hobbyist I am more interested in perfecting my setup and milling techniques to ensure meeting >repeatable< tolerances than I am in using a particular system. As I develop these skills I start reducing the tolerances to a level appropriate for the task at hand. Whether those tolerances are metric or imperial has become immaterial as long as I can meet them.

• Jamill, it sounds like you were in the same situation that I'm in. I don't have a big investment in either metric or imperial tools. All of my motorcycles are metric ( can you tell I'm not a Harley guy ?). I think your reasoning is very well thought out and it really confirms where my logic is taking me.

Here's a piece of trivia that might help other fence sitters like me:

Q: Which countries are still officially non-metric

A: Liberia, Myanmar, and the USA

• most mini lathes are actually metric, If using it in inches, the extra 3 tenths is nothing(I think thats what it is).

Jon

• Maybe you should to change this threads title to Catholic vrs Protestant, as you will then have less heated opinions amongst machinists

I grew up in NZ (Metric) but am an Aircraft Engineer (Imperial). You say you are more comfortable in Metric, then buy a metric mini-lathe. Most of the imperial mini-lathe have the same 1mm pitch leadsrews as the metric version. This means you have 0.3937" per rev, although they are graduated in 40 0.001" divisions.

To make imperial threads on a metric lathe, you will need 57 tooth gear somewhere in the change gears. (25.4:1 --> 254 is divisable by 57). I think you can buy these from www.littlemachineshop.com (Can't access the website today).

• Originally Posted by RotarySMP
To make imperial threads on a metric lathe, you will need 57 tooth gear somewhere in the change gears. (25.4:1 --> 254 is divisable by 57). I think you can buy these from www.littlemachineshop.com (Can't access the website today).
Could you explain that? 254 is divisable by 57, as my calculator does not blow up.
My small lathes have a 127 tooth gear to convert metric -> imperial. As far as I know this is the smallest wheel that will do the conversion with no error. There are other combinations (using 2 wheels) that give a very small error.

My big one have a 90 tooth wheel, but it's a strange beast having a metric gearbox driving an imperial leadscrew. And that's on a Belgian lathe! Need I say I never used the thread dial? Good thing it reverses by just throwing a handle.

This is a thing you might consider in choosing system. If you do much threading in one system, either buy one with that system in the leadscrew, or make sure it can reverse using clutches. The clasp nut cannot be used when threading the "foreign" system. And the motor really don't like the frequent start/stop reversing. And with no clutch at all, combined with no use of clasp nut, you can not run threads up to a greater diameter.

• 254 is not divisable by 57 (254/57 = 4.4...).

254 is divisable only by 2 and 127.

edit: and 1 and 254
edit2: I had written 254/54 above...

Arvid

• Woops, should check what I write before sowing seeds of disinformation.

Of course it is a 127 tooth that is required.

• ... and you will have to combine the 127 tooth with a 50 tooth (2.54:1).

The greatest common divisor of 127 and 50 is 1 (since 127 is prime), and so it's impossible to get this particular gearing with any smaller combination of gears .

(Hey, this is great, I actually get to use what I learned in Discrete Mathematics! )

Arvid