Will this thread chasing dial idea work?

[HcraftsmanM]

Hi all, new forum member today.

I have a Hercus (Australian made version of a Southbend 9") metric lathe with metric quick change gearbox and 3mm pitch leadscrew. Also have a complete set of change gears to facilitate imperial thread cutting which I use occasionally. Fortunately, this includes the old style 120/127 tooth compound idler which to accurately produces imperial treads without the minor pitch error normally encountered when using a 63/64 compound idler for the same purpose.

This got me thinking if I could use an imperial thread chasing dial by replacing its drive gear with a 32 tooth gear (3mm pitch). Would the modified imperial thread dial then work as it would on an imperial gear/lead screw lathe? or is my reasoning flawed?

FYI - haven't tried this yet but the metric thread chasing dial I have has 3 gears - one of which is 24 tooth. I'm guessing some of the marks on the dial will synch with some imperial pitches if using this gear as well.

There's heaps of info regarding imperial to metric thread cutting but not much about meteric lathe to imperial.

Any comments would be appreciated.

Cheers - Fred (Tasmania) - beer o'clock time

 

[john.k]

The thread dial is immaterial cutting threads with a conversion gear.....you need to leave the half nuts engaged and revrse the lathe .......thread dial is not used ..if you disengage the half nuts,youre stuffed..............well ,not really.....,you can pick up the thread again using the compound .

 

[HcraftsmanM]

Thanks for your reply John. I understand that a conventional thread dial (esp metric one on converted imperial) can't be used and what you describe is how I cut conversion threads now - without the dial. What I was wondering is if an imperial dial with different gear (not my metric one) could be modified to work as expected.

Thanks for taking the time to reply.

 

[EPAIII]

In words of one syllable,

NO!

Just like converting the gear train for cutting the English threads, the thread dial also must have the magic number, 127, in it's gear train if it is going to convert distances along the metric lead screw into English measure.

BTW, you do know that thread dials function by representing linear measure along the lead screw? Most English thread dials have the numbers 1 to 4. And those numbers are for a DISTANCE along the lead screw of FOUR INCHES. All common English threads will synchronize in either 1, 2, or 4 inches. That's how thread dials work. And since you are using the 127 tooth gear for the exact conversion, you need the same, exact conversion in a thread dial.

But, even if you do add the correct gearing in the thread dial, you may spend a lot of time waiting for that lathe to properly synchronize. Worst case could be 127 revolutions of the work or the lead screw, I'm not sure which. If you use a slow spindle speed for threading (I do) then that could be one or even several minutes. And if you miss it, then the wait starts over again.

Some time ago I was selling the 100/127 conversion gears for SB lathes. And I looked into the idea of an electronic threading dial that would be universal. But I figured that the possibly long wait times were a real hindrance to that idea. And there is no way around them.

I recommend just keeping the half nuts closed and reversing the spindle motor to back up. It's faster.

 

[john.k]

Even cutting metric threads with a metric leadscrew isnt as straightforward ,as you generally need three thread dials to cover the possible pitches .

 

[Limy Sami]

For small lathe screw cutting, this is probably the definitive work on the subject, with just about every permutation going ???, - well worth reading and keeping as a reference as Martin Cleeve goes in to very fine detail .

screwcutting in the lathe by martin cleeve - Google Search

 

[HcraftsmanM]

In words of one syllable,

NO!

Just like converting the gear train for cutting the English threads, the thread dial also must have the magic number, 127, in it's gear train if it is going to convert distances along the metric lead screw into English measure.

BTW, you do know that thread dials function by representing linear measure along the lead screw? Most English thread dials have the numbers 1 to 4. And those numbers are for a DISTANCE along the lead screw of FOUR INCHES. All common English threads will synchronize in either 1, 2, or 4 inches. That's how thread dials work. And since you are using the 127 tooth gear for the exact conversion, you need the same, exact conversion in a thread dial.

But, even if you do add the correct gearing in the thread dial, you may spend a lot of time waiting for that lathe to properly synchronize. Worst case could be 127 revolutions of the work or the lead screw, I'm not sure which. If you use a slow spindle speed for threading (I do) then that could be one or even several minutes. And if you miss it, then the wait starts over again.

Some time ago I was selling the 100/127 conversion gears for SB lathes. And I looked into the idea of an electronic threading dial that would be universal. But I figured that the possibly long wait times were a real hindrance to that idea. And there is no way around them.

I recommend just keeping the half nuts closed and reversing the spindle motor to back up. It's faster.

Thanks for the explanation, Paul. I'll give up on that that idea then. Have seen another technique (yet to try) where a mark is chosen to pick up on the dial, and the half nut disengaged at the end of the pass, tool withdrawn and spindle stopped before the dial makes a complete revolution. Reverse lathe, pickup same mark in reverse (as long as a revolution of the dial hasn't occurred) and return to make second pass without disengaging half nuts. Not that confident having the spindle stop in time with half nuts engaged - no brake.

 

[HcraftsmanM]

Even cutting metric threads with a metric leadscrew isnt as straightforward ,as you generally need three thread dials to cover the possible pitches .

Sure is. My metric chasing dial has 3 gears on the shaft and is raised lowered on the carriage to engage the different gears for respective thread pitches. Dial face is busy with numbers relating to gear combo

 

[sfriedberg]

What I was wondering is if an imperial dial with different gear (not my metric one) could be modified to work as expected.

No. English threads are in turns-per-inch but metric threads are in mm-per-turn. The major problem is not the inch-mm conversion, but the reciprocal conversion. Thread dials with a single choice of gearing simply don't work for actual metric threads.

 

[HcraftsmanM]

Thanks all for your replies - Solid No advice on dial modification understood and taken.

I'm going to give the following method of disengaging half nuts a go, albeit threading imperial on a metric lathe with metric dial.

Regards - Fred

 

[HcraftsmanM]

In words of one syllable,

NO!

Just like converting the gear train for cutting the English threads, the thread dial also must have the magic number, 127, in it's gear train if it is going to convert distances along the metric lead screw into English measure.

BTW, you do know that thread dials function by representing linear measure along the lead screw? Most English thread dials have the numbers 1 to 4. And those numbers are for a DISTANCE along the lead screw of FOUR INCHES. All common English threads will synchronize in either 1, 2, or 4 inches. That's how thread dials work. And since you are using the 127 tooth gear for the exact conversion, you need the same, exact conversion in a thread dial.

But, even if you do add the correct gearing in the thread dial, you may spend a lot of time waiting for that lathe to properly synchronize. Worst case could be 127 revolutions of the work or the lead screw, I'm not sure which. If you use a slow spindle speed for threading (I do) then that could be one or even several minutes. And if you miss it, then the wait starts over again.

Some time ago I was selling the 100/127 conversion gears for SB lathes. And I looked into the idea of an electronic threading dial that would be universal. But I figured that the possibly long wait times were a real hindrance to that idea. And there is no way around them.

I recommend just keeping the half nuts closed and reversing the spindle motor to back up. It's faster.

Thanks for the explanation, Paul. I'll give up on that that idea then. Have seen another technique (yet to try) where a mark is chosen to pick up on the dial, and the half nut disengaged at the end of the pass, tool withdrawn and spindle stopped before the dial makes a complete revolution. Reverse lathe, pickup same mark in reverse (as long as a revolution of the dial hasn't occurred) and return to make second pass without disengaging half nuts.

Not that confident having the spindle stop in time with half nuts engaged - no brake or speed controller on my lathe.

 

[EPAIII]

I have seen videos on one or more of those methods. They can work if you are careful. They amount to the same idea as keeping the half nuts closed, but they allow opening them and then stopping the spindle motor. The spindle then stops and the method allows you to make up for the revolutions it made between disconnecting the half nuts and when the spindle actually stops.

They can work, but you must understand them and follow them carefully. I would practice on a scrap part first.

Thanks for the explanation, Paul. I'll give up on that that idea then. Have seen another technique (yet to try) where a mark is chosen to pick up on the dial, and the half nut disengaged at the end of the pass, tool withdrawn and spindle stopped before the dial makes a complete revolution. Reverse lathe, pickup same mark in reverse (as long as a revolution of the dial hasn't occurred) and return to make second pass without disengaging half nuts. Not that confident having the spindle stop in time with half nuts engaged - no brake.

 

[EPAIII]

Bingo! I have said this about the reciprocal relationship many times.

You can say all you want about how metric is superior to English measure. I will not argue. I like metric.

But the common thread pitches that those two worlds have chosen are one example of where the English measure guys got it so much better than the metric guys that it is hard to even describe. And it is because threads per inch is a much better way of measuring pitch than just millimeters.

Much of the difficulty in going back and forth between English and metric threads is simply due to that reciprocal relationship.

No. English threads are in turns-per-inch but metric threads are in mm-per-turn. The major problem is not the inch-mm conversion, but the reciprocal conversion. Thread dials with a single choice of gearing simply don't work for actual metric threads.

 

[EPAIII]

I am not sure if it will work or not if you reverse English and metric, but good luck.

And please let us know how it turns out. I seriously want to know. And to know any details that you pick up along the way. This is a great experiment.

Thanks all for your replies - Solid No advice on dial modification understood and taken.

I'm going to give the following method of disengaging half nuts a go, albeit threading imperial on a metric lathe with metric dial.

Regards - Fred

 

[sfriedberg]

I just spent some time with a spreadsheet. If metric coarse and fine threads from 0.20 to 6.00 mm pitch had been defined as so-many turns/centimeter instead, about half the pitches have an exact match, all but one of the others are within 5% of the official defined pitch, the exception is 3mm pitch for which 3 turns/cm differs by 10%. You do need a 12.5 t/cm for 0.8mm, and once you get larger than 3mm pitch, you do need things like 2.75, 2.5, 2.2 t/cm. But the same thing is true for inch threads, where you need an 11.5 t/inch. Because inches are larger than centimeters, large inch threads usually top out around 4 t/inch, which is comparable to the metric 6mm pitch.

So I think it's fair to say the metric thread standardization committee could have achieved their functional goals with a turns/cm definition, instead of a mm definition. If they had, we could be using simpler threading indicators on metric lead screws.

Here's my data. BBCode tables are incredibly stupid about column widths, sorry about how this looks.

mm pitch​	turns/cm​	percent discrepancy​
0.20​	50​	0​
0.25​	40​	0​
0.30​	32​	-4​
0.35​	28​	-2​
0.40​	25​	0​
0.45​	22​	-1​
0.50​	20​	0​
0.60​	16​	-4​
0.70​	14​	-2​
0.75​	13​	-2.5​
0.80​	12.5​	0​
1.00​	10​	0​
1.25​	8​	0​
1.50​	7​	5​
1.75​	6​	5​
2.00​	5​	0​
2.50​	4​	0​
3.00​	3​	-10​
3.50​	2.75​	-3.75​
4.00​	2.5​	0​
4.50​	2.2​	-1​
5.00​	2​	0​
5.50​	1.8​	-1​
6.00​	1.6​	-4​

 

[john.k]

In one of the 1980s Model Engineer magazines ,a contributor had worked out hundreds of thread approximations with metric, imperial,and in a more difficult area...the BA thread .......BA is a true metric thread that is pitched as a descending mathematical series ,where each next pitch is a power of the next higher (0.9).........so there is 0BA that is 6mmx 1mm ,next is 1BA that is 5.3mm x 0.9mm, next is 2BA which is 4.7mmx 0.81 mm..(0.9x0.9 = 0.81)....and so on down to 12 and 14 BA that are miniature watch sizes .

 

[eKretz]

Have a look at this: