Thread cutting

This may seem to be a silly question, When the cross slide is engaged with the lead screw will the cutting tool be at the same position on the workpiece each time the cutting tool is reversed to make a deeper cut ? or is there a different tecnique for getting the cutting tool to start a deeper cut on the second & third passes at the original start cut.?

You have to withdraw the tool at the end of each cut before reversing the saddle to the starting position, then before taking the next cut wind the tool in further to take a deeper cut, carrying on until you have cut the full depth. There must be a video on screwcutting somewhere on YouTube.

Have I understood your question?

Depends on tpi being cut and tpi of leadscrew

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As John says there are many good instuctional videos on youtube. These are far more informative and easier to follow watching somebody actually do it than digesting a text method.

I like thread cutting. If I have a few mins spare I will often pop a bit of scrap into the lathe, turn a diameter and cut a thread just for the hell of it. .......... Probably sad I know to many people, but if there is some girly crap on TV that Steph is watching I would much prefer to do that. I find it quite therapeutic.!

Nick

Strangely enough, I spent yesterday afternoon trying to get the grey matter around all this.

I watched two videos;

1/ Toms techniques (screwcutting n°4)

2/ mrpete222 screwcutting.

After this I went out to the workshop and cut my first recognisable thread

Would recommend watching these

All the best

Daniel

You can keep the half nuts engaged as you wind the saddle back to start the new cut so the synchronisation will stay in step but do not get the idea that the tool will follow the same thread groove in reverse. Owing to backlash (looseness in the halfnut/leadscrew) it will not be aligned and will destroy your thread if the tool is not withdrawn.
With metric treads it is simplest to keep the halfnuts engaged and reverse the lathe to get back to the start, With an imperial leadscrew cutting an imperial thread there is a device called a Thread Dial Indicator that allows yo to disengage the halfnuts and re-engage in step. Maybe leave that for a later lesson.

If you use the topslide for applying more depth by having it set over at an angle you can use the cross slide for just getting the clearance for the return and return it to a fixed setting ("0" for the next cut. It keeps the two elements, clearance and increasing depth as separate actions. Some lathes (eg Boxford) have a cross slide stop option to make returning to zero easier.

Long travel is engaged for screw threading. The cross slide should be set at the initial 'touching point', and moved back or forth to clear the already cut thread before the cutter is returned for the next pass. The cross slide is always returned to that initial 'touching point' setting (usually the dial would be set at zero and the cross slide returned to that point in the same direction clockwise or anticlockwise (to avoid backlash)). The cutter advancement is by the top slide only, the new cuts being linear but proportional to the sine of the thread angle. That should mean that the cutter only cuts on one side as it is advanced into the work (pushed or pulled). The other side traditionally (for ease of setting up with setting guages) will simply rub on the already cut surface.

If using a lathe with thread dial, all backlash is removed by the machine before engaing the halfnuts, but if the machine needs to be reversed, with the lead screw engaged, allowance for leadscrew backlash effects, when starting subsequent cuts, must be taken into account as the cutter will momentarily travel in the wrong direction!

Best to look at the innumerable U-toob vids on the subject, but remember that there are some videos that

are rubbish! Doubleboost explains it well (at times), at least on his later offerings.  

Edited to add that the threading dial (on both imperial and metric lathes) allows half nut engagement for multiple start threads as well as single start threading (which you will, no doubt, be mastering first). 

Edited By not done it yet on 23/10/2016 12:03:36

Edited By Daniel on 23/10/2016 22:45:48

Roger, first always a good idea to say where you reside, however a bit more info on your machine would probably help, make, model and does it have a gearbox or just change wheels, is there a chasing dial fitted? Is it imperial or metric.
My guess is from what you have said is its change wheels and no chasing dial ?
JohnF

When possible I like to thread cut with the compound slide set at the angle of one side of the thread being formed. In the case of a 60deg UN or metric thread that is 60 deg from horizontal or 30deg from inline with the cross slide screw. I set the compound slide dial to 0 and then use the cross slide for the initial touch off on the stock diameter. Some trig is required to get the thread depth but that changes with the nose radius of the tool. I find with the full form threading inserts, if I leave the threaded area a little larger on diameter, I can keep cutting until I have cut to the marker pen. Then measure the diameter of the stock and then make the thread untill the stock diameter is about 0.05mm smaller than the nominal thread. Ie on a M10 thread, I leave the material at 10.1 or 10.2 mm diameter and colour it with a red or Blue marker pen. At the end of the thread length I just wind out the cross slide. Reverse the lathe spindle direction and as it goes back, I put the next 0.05mm or 0.1mm cut on. Re position the crosslide back to the original zero point and keep cutting. Now that I have the VFD and 3 phase motor, thread cutting is even easier as I have a limit switch that stops the motor at the same distance point every time. Normally better than 0.02mm on the main leadscrew dial of my Myford Super7. Where ever possible I have a run out area that is undercut to the root diameter of the thread for about 1 to 2 pitch lengths long. So on the M10 I would have an a length that is relieved about 2 to 3 mm long. On internal threads I usually use 2 pitches to give myself more lee way as to the outside threads which are more easy to see where the tool is. Most of the time I leave the leadscrew always engaged. Only when doing multi start threads , I leave the compound slide horizontal and then do all the thread depth on the cross slide dial. I then feed on the compound slide the thread pitch for the second or however many start threads it may be.

Neil L