What makes a 'metric lathe' a Metric Lathe?

[calgaryguy]

Neighbour has a Craftex CX709 lathe with Metric dials on compound and x-slide.

Does anyone know if there is anything else specific on this lathe that makes it a 'Metric lathe'? Is there something specific about the lead screw threads or gearbox **on the CX709** that makes it uniquely metric, or is it just the presence of the metric dials on the x-slide and compound?

I'm asking because working with his lathe is a real PITA as all either of us have are imperial measuring tools. The thought occurred to me that I'he might be able to simply change the metric dials for imperial ones.

 

[PaulL]

Yes. The lead screw is different - it has a metric pitch. That sounds minor, but the interaction with the threading dial is completely different. If you have an imperial lead screw the threading dial lets you fall back into imperial screw threads by watching that dial. If you have a metric lead screw you can drop back into metric screw threads using the equivalent. But if you have the wrong screw you *cannot* disengage the half nut while making a thread of the other system because you can't reliably re-engage in the correct thread of the lead screw. That makes the work a quite bit more tedious.

 

[calgaryguy]

So the 'Metric' CX709 has a metric lead screw?

I know some more expensive Metric lathes have metric lead screws, but Im curious if Craftex actually went that far in their build.

 

[PaulL]

Oh, what an ugly compromise they made: No threading dial? From their manual:

I'm not sure I'd want to cut a lot of threads on this lathe.

 

[jcdammeyer]

The issue is exactly the same in the other direction. If I want to create metric threads on my 1942 imperial 10TPI lead screwed lathe I'd have to have a set of gears (large ones) to translate the ratios so the carriage moves at metric ratios relative to the spindle. And you have to leave the half nut engaged since the threading dial isn't calibrated for metric.

That means turning off the spindle and letting it coast to a stop which means requiring a much longer over run area past the thread. ie: threading up to a shoulder is a bitch. You then retract the X to clear the thread, turn the lathe on in reverse and wait till the tool is back at the starting point.

On mine I select metric threads on my ELS instead of TPI or Pitch.

Now if it were a metric lathe with the lead screw config set at a metric pitch

I'd then cut imperial threads like this.

And since the lead screw can stop on a dime so to speak you can thread up to a shoulder and the half nut is not disengaged and the spindle doesn't have to stop.

 

[calgaryguy]

I'll have to go over and look, but I'm m fairly sure his hs a threading dial. Apparently it was a option?

 

[jcdammeyer]

I'll have to go over and look, but I'm m fairly sure his hs a threading dial. Apparently it was a option?

On some lathes it might have been an option.
Why is one needed when using the half nut? (Here comes one of my long explanations).
If you are turning a thread the same pitch as your lead screw, say 10 TPI, the spindle turns once for each turn of the lead screw. So it doesn't matter where you engage the half nut.
Now say it's 20 TPI. For each turn of the spindle the lead screw turns 1/2 as much since the threading pitch is twice as much.
So here is what I just tried on my South Bend. I set up with gears (shudder!!!!) 10 TPI. I made a mark on a piece in the chuck to represent the starting point of the thread. I engaged the half nut and adjusted the cross slide and compound slide so once the backlash was out (half nut engaged) and turning the spindle by hand the tool bit lined up with the mark on the work.
I then turned the spindle one turn and used the calipers to measure the distance the carriage had moved. Pretty close to 0.1" which is 10 TPI.
Next I disengaged the half nut and moved the carriage to the right. Re-engaged half nut and turned spindle by hand until the tool bit lined up with the mark. It did.
Then I set the thread pitch to 20 TPI. Now the spindle turns twice for every single turn of the lead screw because the pitch is 0.050".
If you're with me this far you'll understand that if I turn the spindle one turn without the half nut engaged the carriage is now out of position by 0.050". If I try and engage the half nut it won't. I need to turn the spindle and one more turn which turns the lead screw another half a turn. Now the half nut engages.

And to make the really long story short. The lines on the top of the threading indicator are used to signal when it's time to engage the half nut. For some threads that are multiples of the lead screw pitch it's every number or every even number or line. For the odd threads like 16 TPI it may well be only #1 or whatever specific line/number you used the very first time for this thread.

And there's the problem with metric on an imperial lathe or the other way around. There's no easy way to look at the threading dial and engage the half nut since you may be at a pitch point that is a non-repeating fraction.

So to ensure that everything lines up properly the half nut is left engaged and the spindle reversed with the tool out (backlash can snap a carbide tip in a heartbeat) until you are reading to take a run at the next pass. Go forward and the spindle to carriage lead screw relationship remains the same. The threading dial is useless.

 

[Susquatch]

@jcdammeyer @PaulL @calgaryguy

What a mess eh!

This is exactly why @Brent H has a Miss Metric and a Mr Imperial.

FWIW, my lathe is imperial and I thread using a 127 gear.

I am not afraid of using my imperial threading dial to cut metric threads and I am not afraid of disengaging the half nut. There is a cool trick for doing it. I'll see if I can find where I described this before and post a link here.

Edit - Found it

Post in thread 'CX701 gearbox questions' https://canadianhobbymetalworkers.com/threads/cx701-gearbox-questions.4960/post-71454

 

[jcdammeyer]

@jcdammeyer @PaulL @calgaryguy

What a mess eh!

This is exactly why @Brent H has a Miss Metric and a Mr Imperial.

FWIW, my lathe is imperial and I thread using a 127 gear.

I am not afraid of using my imperial threading dial to cut metric threads and I am not afraid of disengaging the half nut. There is a cool trick for doing it. I'll see if I can find where I described this before and post a link here.

Like everything, it will depend on which pitch you are cutting, how long and how loose a thread you can be happy with.

 

[Susquatch]

Like everything, it will depend on which pitch you are cutting, how long and how loose a thread you can be happy with.

Na, I think you misunderstand what I was getting at. Read the linked thread. It's cool and makes perfect threads in both metric and imperial.

Of course, your ELS is superior!

 

[PaulL]

I like the trick, but it's risky - you miss once, you blow the part.
It reminds me of when I was putting the gearbox in forward or reverse *instead of the motor*. All worked well for a while, but then suddenly it failed and cut entirely wrong. In the end it was because the gearbox didn't guarantee the shaft couldn't spin one geartooth while shifting. Live and learn.

 

[jcdammeyer]

Na, I think you misunderstand what I was getting at. Read the linked thread. It's cool and makes perfect threads in both metric and imperial.

Of course, your ELS is superior!

Ah. I get it. If you are using the 127T gear to create the metric threads your approach will also allow the use of the much shorter run-out area. As @PaulL says though, does require a lot of concentration.

Thank you.
It's one of the reasons cutting with my ELS always starts at X == 0.00 for the cross slide if using it manually. I have forgotten sometimes, (actually more than sometimes), after backing it out, where I had it for cutting. Now on my South Bend I either use the X axis mechanical stop or just set it at 0.00. My ELS tells me how far from 0 to index the compound.

So set compound to 0.00. Move X in until it just scratches the surface of the part to thread. Set X dial to 0.00. (or the DRO if you have one).

Now on each pass the Z stops at the end of the thread. Bring the X out a turn or more to clear the thread. Press start to return to the begin position. Bring X back to 0.00. ELS tells how far to move the compound from 0.00. Say it's 0.005". Press start and away it goes on the next pass. And so on.

My goal over the next while is to add a remove ramp at the end of threading if you have a powered X axis. The idea is that there is no run-out area at the end of the thread. The thread just slopes outwards to the surface.

Apparently that's easy to do with LinuxCNC on a lathe. Not sure MACH3 could ever do that. Not sure if MACH4 can do it. No idea if the bundled CNC for Lathe systems can do that. Perhaps others can let us know about that.

 

[VicHobbyGuy]

I know some more expensive Metric lathes have metric lead screws, but Im curious if Craftex actually went that far in their build.

Not a matter of expense or quality, I think.....
The Vevor 7x lathes made by RealBull mostly have metric carriage leadscrews, and Sieg 7x lathes sold outside N. America do also. The 7x lathes mostly have metric screws on the cross slide and compound, with dials graduated in inch-thousandths. MicroMark sells 'Tru-Inch' 7x lathes with all inch-pitch leadscrews. The good news for 7x lathe owners is that a new set of leadscrews and dials can be bought for not much money.

 

[jcdammeyer]

Not a matter of expense or quality, I think.....

Exactly. The worst machines were the mills with metric lead screws but dials in inches. If you have a metric lathe then eventually it's easiest to just start working in mm. For my South Bend the dials are on depth of cut in inches.

Now the mill, although imperial, has a DRO. So at that point it was not turns of the handle that I counted but the numbers on the display. I tend to use imperial on the mill because all the different touch off items I own, including even the 1-2-3 blocks are all imperial. If the diameter of the touch probe is 0.2" it's kind of a pain to touch off and then to move to the center of where it touched move 2.54mm. Not that it can't be done but a pain.

The ball screws I intend to install will be metric so then we'll see.

 

[VicHobbyGuy]

Exactly. The worst machines were the mills with metric lead screws but dials in inches.

Exactly the reason my BB Craftex Sieg X2 mill has DRO now, and I've only owned it a few weeks.

 

[jcdammeyer]

Exactly the reason my BB Craftex Sieg X2 mill has DRO now, and I've only owned it a few weeks.

Yeah my Shumatech DRO-350 was installed in January 2007. Never looked back. I also have a DRO-450 but can't find any pictures and don't have any scales for it.

 

[Former Member]

The way I see it, on the lathe it can be an issue for thread and methodology you use to do said threading.

Beyond that your dials or or DRO will do just about everything else. Get into CNC the same applies.

All that really matters is the level of accuracy you wish to achieve and how you go about it. Second is truly what you machine is really capable of and what you do to maximize it vs what you think you are reading on your scales or DRO.

 

[calgaryguy]

So the CX709 apparently doesnt have any sort of thread dial indicator available for it, and some of my reading leads me to believe that thread dials on a metric lathe are somewhat of a rare thing?

So I guess I wont ever be threading on this lathe lol. And maybe I'll keep an eye out for metric micrometers to make life easier when using the neighbours lathe.

 

[Susquatch]

some of my reading leads me to believe that thread dials on a metric lathe are somewhat of a rare thing?

I think this is probably a North American thing that relates to metricated imperial lathes. As far as I know, my method of using an imperial thread dial for metric threads is not well known or commonly used.

But don't give up so easily! I posted a link to my method in an earlier post of mine on this thread. It really isn't that hard. It's well worth learning. Please don't hesitate to ask questions about it.

Many metric lathes made for the rest of the world have metric thread dials. And for those that don't, it's not hard to make one. Just be aware that you usually need a range of them - not just one like imperial lathes.

I'd be most interested to know what our members with non-american roots think about this.

@gerritv ?

 

[gerritv]

I left Holland at age 10 so no direct experience in NL. However the EU-based machinists I have seen do threading all seem to just leave the halfnut engaged. I personally tried using the threading dial a few times on imperial threads and found it unrewarding.
Now I just use the ELS

Flemish, mostly Dutch thankfully.