Turning a L.H.external thread without an undercut

[Alf Scotting]

I need to cut a L.H. external thread on a set of brake rods - all the videos on youtube are cheating by using an undercut to start the thread - the material is 10mm. dia. and I wish to cut a 10 x1.5 thread about 50 lg. - Any advice would be appreciated.- when I cut a thread I usually leave the half nut engaged and reverse the headstock rotation - I offset the compound rest but in this instance the CR would be angled towards the headstock but do I set it at 29 or 31 degrees?
Alf Scotting (Taree NSW)

[simonl]

I've yet to do a LH thread so maybe of little help. Do you have to angle the compound towards the chuck for a LH thread? Can't you just set the compound the same way, as in (in your case) approx. 29 deg from perpendicular? I can't see a reason that it needs to be angled towards the chuck just because it's feeding from left to right. It's a symetrical V form tool so should cut anyway, just means it's cutting mostly on the RHS of the form and not the left.

With a 10mm rod, deflection would be the biggest issue. I dare say you would need to use the support of the TS at the other end and use very light passes with lubricant. Make sure your tool is sharp and nicely honed. If you have a metric leadscrew then why not take advantage and disengage the half nuts to make reversing easier? Especially if you don't want to start in a runout groove or undercut.

If you are going to angle the compound towards the chuck then that 29 deg (or thereabouts) is a deviation from perpendicular so going the other way would mean 29 degrees on the other side.***

** Only if your compound shows 0 degrees when it runs perpendicular to the ways. Some compounds, such as mine show 90 degrees when perpendicular to the ways.

Simon

[bob ward]

One way around this is to use your internal threading tool on the back edge of the 10mm rod. Start from the RH end and pull out at the 50mm mark every pass.

To achieve what you want if you are threading from the front, ie cutting L to R, your only option is to force the cutter into the job at the start of each pass.

[Michael G]

Not hard to do "normally"
The method I used for Dale's LH bolts
P1020991 (Medium).JPG
(https://www.woodworkforums.com/showth...56#post1865956)
was to put a mark on the stock where I wanted the thread to start and then position the tool a bit to the left of the start point, almost but not quite on the job. Start the lathe, engage the feed and as soon as the tool gets to the mark position, wind it in the required amount.
For the length of thread you are doing I'd suggest supporting it with a centre in the tailstock but other than that, not hard.

Michael