Metric threads on an Imperial lathe

[verbotenwhisky]

Tom Lipton, OxTools, did a video on cutting metric threads on an imperial lathe. The problem with not being able disengage the half nuts due to losing synchronization is that you have to be able to stop the lathe quickly enough to avoid running past the gutter. For lathes with a VFD and dynamic braking,it isn't that big an issue but for a lathe with an induction motor, stopping when the you hit the gutter but avoiding running into a shoulder can be tense.

However, if you disengage the half nuts and shut the lathe down, then reverse the lathe and reengage the half nuts when the same number on the thread dial hits the index mark, you will not lose synchronization. You still have to run the lathe in reverse to the starting point but it eliminates the butt clenching for metric threading.

My PM1236 has a break that halts everything all at once...

 

[Jake M]

It depends.....

The 127 tooth compound gear is driven directly or indirectly by the spindle. Driving the 127 tooth side of the gear imparts the 127 number into the transposing gear's ratio to the spindle, and every gear and rotating thing beyond it. Unless you installed an untransposing gear somewhere to cancel it, that transposing gear and everything downstream is still driven by some ratio that includes that 127 number or some fractional portion of it.

 

[rwm]

I believe my Grizzly 12 x 36 uses a 91/86 transposing gear. I've used it a half dozen times without a problem. Of course I never release the half nuts.

My SB uses a 127/120 transposing gear. I can't make sense of that??? Thoughts?



You can see in the diagram on the left, F, the 127T gear is just an idler and the transposing gear is not in play. In the other three diagrams the transposing gear is in use to yield the specified metric threads. In J you would have two ratios in play 127/120 and 30/64.
I believe it has an 8 TPI lead screw.

 

[woodchucker]

My SB uses a 127/120 transposing gear. I can't make sense of that??? Thoughts?

View attachment 499969

You can see in the diagram on the left, F, the 127T gear is just an idler and the transposing gear is not in play. In the other three diagrams the transposing gear is in use to yield the specified metric threads. In J you would have two ratios in play 127/120 and 30/64.
I believe it has an 8 TPI lead screw.

makes sense to me.
your threading for mm is h i j
your feed is the right F

so if you need a 0.5mm pitch you would use hole C and 6 .

I do not understand the bottom right, the 1/1 and 1/3 seems like turning and facing (facing being 1/3 the speed)

 

[rwm]

I do not understand the bottom right, the 1/1 and 1/3 seems like turning and facing (facing being 1/3 the speed

^^^ This is correct. Sort of. The speed is 3:1, 3 times the speed shown.
Why is my transposing gear not 127/100? My gears introduce a ratio of .94488 into the gear train?

 

[Beckerkumm]

I think SB changed their gear train in later models to make metric threading easier. There are some threads over on PM discussing change gears on various SB models. I've noticed when rehabbing old lathes that gear train changes were fairly common over the years within models. Dave

 

[rwm]

I just realized something important about my machine. It has a compound gear right after the tumblers. The spindle gear is 24 tooth but the compound gear translates that to 30 tooth. This compound gear has two different diametral pitches so the OD of both gears is similar. This is not obvious and I missed it originally. The driven gear on the gearbox is 60t so the ratio is a perfect 1:2. The A1 position of the gearbox gives 2:1, so in the A1 position you get a 1:1 spindle/ lead screw ratio. I originally missed the compound gear and I could not understand the ratios! This makes perfect sense! Are the older 10Ks made this way as well?

 

[SLK001]

This makes perfect sense! Are the older 10Ks made this way as well?

No. The SB1002 is made by Grizzly after they bought the SB brand. It is entirely a different lathe from an original SB 10k lathe.

 

[rwm]

Yes of course. I was under the impression that they tried to keep the specs in line with the originals. I thought the gear train was similar until I found this compound gear in the mix! There are other major differences though. It is made in Taiwan and I am pretty happy with mine for its size/weight.

I still don't understand the transposing gear situation even after doing some reading on this. My transposing gear gives a weird ratio. If any one cares to elaborate, I am all ears.

 

[rwm]

OK, I think I have a basic mathematical explanation.
If you use the 120 tooth transposing gear, the drive ratio is actually 120/127 or .9449...
The lead screw pitch is 3.175mm (8TPI in mm)
3.175 x (120/127)= 3mm.
So once you get that whole number, the QTGB can divide that up into any fraction of 3. All the rest is fluff.
The 100/127 gear gets you to a 2.5mm pitch which is arguably more useful and more common.

Another thought: The reason for the compound gear with a change in DP (on my machine) is so the transposing gear does not have to be ginormous. Otherwise, the 120 tooth gear might be too large to fit in the housing.