cutting threads

[grimmeParticipant @grimme]

[grimmeParticipant @grimme]

i have always cut threads with the half nut engaged as i dont understand why the chaser dial on my student has a choice of gears? can some explain the theory or reasons why there is multiple following gears?

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[Anonymous]

It must be a metric lathe? Metric thread pitches are not necessarily simple multiples of the leadscrew pitch. So a metric thread dial indicator has a choice of gears, five on the metric version of my lathe (Harrison M300), to simplify the maths. The chart on the indicator should list the gear to be used for a given pitch as well as the numbers at which the half nuts should be engaged.

Andrew

[Thor 🇳🇴Participant @thor]

Hi Geoff

There is an explanation of how metric thread dial indicator works here, it is for the Mini-Lathe with 1.5mm pitch leadscrew. Here is a Thread Dial Indicator calculator, it allows you to select different (metric) lathes.

Thor

[DC31kParticipant @dc31k]

As a corollary to Andrew's post, and something which will simplify your life considerably, any thread which _is_ a factor of the leadscrew pitch does not need a thread dial at all. You can engage the half nut wherever it drops in.

I am guessing that the Student has a 6mm pitch leadscrew, so this means that the following pitches do not need the indicator dial:

0.2 ,0.25, 0.3, 0.4, 0.5, 0.6, 0.75, 1, 1.5, 2, 3, 6.

[Martin ConnellyParticipant @martinconnelly55370]

DC31k, 6mm would be quite a coarse thread, for most of us a leadscrew pitch of 3mm (close in size to an 8tpi thread) seems more likely so the pitches you gave would work at any point on the leadscrew apart from 0.4, 2 and 6 for which a thread dial indicator would be needed.

Martin C

[DC31kParticipant @dc31k]

Posted by Martin Connelly on 11/09/2021 09:20:06:

6mm would be quite a coarse thread, for most of us a leadscrew pitch of 3mm

He did say in the post that his lathe is a Student. I am assuming he means a Colchester Student.

The principle holds for any pitch leadscrew, imperial or metric and the calculation is not difficult (far easier than working out change gear combinations, for example). Divide your leadcrew pitch by the thread pitch and if the answer is a whole number (integer), the dial is not needed.

With a typical imperial leadscrew of 8tpi, the indicator is not needed for 8, 16, 24, 32 or 40 tpi (I wonder if this is why the ME threads were chosen with these pitches).

If you were to be doing batch work, Martin Cleeve was very big on choosing a leadscrew pitch to suit the thread being cut. If you had a load of M12 stuff to make, there would be a good case for a 3.5mm pitch leadscrew.

[Anonymous]

The OPs lathe is stated to be a Student, so I'd agree with DC31k, most likely a 6mm pitch leadscrew. My imperial lathe (Harrison M300) has a 4tpi leadscrew, the metric version is 6mm pitch.

Andrew

[Clive FosterParticipant @clivefoster55965]

geoff

When you have the thread dial operation data figured out I suggest you make up a data plate giving the threading dial drive gear and number of divisions between engagements for the various pitches. Either attach it to the lathe or somewhere else where it can easily be found and read when using the machine.

My metric Smart & Brown 1024 has a factory made one screwed directly to the apron. Frankly I'd be lost without it.

The S&B has a 4 mm pitch screw with 12 division dial so, unfortunately, you can't just copy mine. The relevant data for your machine will be in a manual and so should be findable via the internet if you don't have the right book.

I find the cheap laminators, mine is from LiDL, work well for making decently durable machine data labels from simple printouts. Stick or screw to the machine as needed. I've yet to replace one in over a decade of making them. Roller type paper cutter, LiDL cheapy again, makes a neat job of cutting to size.

Clive

[Howard LewisParticipant @howardlewis46836]

If you are unsure of what to use, or don't have the alternative gears, the safe method is to keep the half nuts engaged.

Withdraw the tool and reverse the motor to take the tool back, well past the start of the job.

Having reset the tool, the lathe can be restarted in the forward mode, and by the time that the tool contacts the work, the distance travelled before cutting, should have taken out the backlash.

Howard

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