Threading myth .... busted!

Joe Piecynski in his latest YouTube video has busted the myth of needing to cant over the compound slide half the thread angle, to carry out single point threading. Although I think he has missed a trick! He advances the cross slide, set at zero degrees to full depth, but without moving the top slide.

Over the years of machining and during my apprenticeship I have never moved the compound to half the thread angle, and have always set it at zero degrees, the cross slide is advanced an appropriate amount, and the top slide is advanced a little bit every so often, maybe a third of the depth of cut, or a thou or two. A couple of spring cuts will finish off the thread nicely, by cutting on both sides of the tool.

Depth of cut is easier to gauge too, because the cross slide is advanced into the job to final depth.

I'm glad someone has at last attempted to call out what I think is the over complicated method of adjusting the compound to half the thread angle!

IanJ

I also never turn the compound slide for it and also do more or less coordinated action with the two handles. Saves a lot of time on my mini lathe. My thread cutting dies also just plunge in

Hi there,

I can't remember (without logging off and going to consult the relevant books) whether it was Tubal Cain or G.H.Thomas who pointed out that the usual way of angling the tip-slide optimises the finish of the wrong flank of the thread!

Best regards,

Swarf, Mostly!

I hope this Joe chap is not claiming to be some kind of guru who has just found this out. If so he has had his head in a bucket for 50 years. Must have been known to the ancient Egyptians. We have been discussing it on this forum since the internet was invented.

However I suspect the improved tool geometry when angled is a help if you are treadling the lathe.

I like to set the top-slide over so that I don't have to remember what the last cross-slide setting was - particularly useful if, as happened to me today, you get interrupted by the DPD man.

I've never really bought the "cutting on one side" thing though and just set it to half the thread angle.

I leave the topslide parallel, then for every 10 thou of radial feed move the topslide 5 thou (or pro rata). This effectively advances the tool at an angle of 28.57 deg to the radial, so with Whit thread form you get a slight scrape on the left hand edge. If you set the dials to zero at the start, the topslide dial keeps track of where you are.

The effective thread angle is then 53 degrees, I've always wondered whether this is where Mr Whitworth got his 55 from. It works with 60 degree threads as well, just has a bigger 'scrape'

I set at 0 degrees normally as it is easier to manage the thread depth using the tables. The only time I moved the top slide was when matching the wear of a long leadscrew so that antibacklash nuts did not become locked up at the extremes of the travel. In that case, the wear in the centre was not too bad, and the thread was a large ACME which was very easy to pick up when working on only 25% of the length at one time.

Maybe I'm just watching too many YouToobers using this convoluted method, it does get on my nerves when they make a point of adjusting the compound to 29 degrees for a UNC thread, as if it's the only way to single point cut a thread!

IanJ

Typo, for 28.5 read 26.5

Aways thought the business of setting the top-slide at half thread angle as normal procedure was a wind up on the same level as sending new apprentices to the stores for a tin of elbow grease. What sane person would voluntarily get involved in all the trigonometry needed to recalculate thread depths. I do, however, get the impression it might be popular in America. Which could explain a lot.

Putting the top-slide parallel to the bed and tweaking every so often to clean up the sides appears to add difficulty for no sensible return. Accurate job needs the top-slide set dead parallel. A pain in itself. Need to keep track of the top-slide tweaks as well as the infeeds to get the thread depth and root radius right. So twice the work on things its easy to mis-step on. For me that technique is strictly reserved for when I want an acme or square thread cut dead on for minimal backlash. I know exactly where Duncan is coming from but its a method that has never felt right to me for normal threading.

Doesn't help that on many small lathes you can't leave the top-slide parallel to the bed for all normal work because it tends to argue with the tailstock unless tool stick-out is way too big. Life is to short to keep swinging the topslide around.

Why do folk have to make stuff so difficult!

Easy way was taught to me as the zero-to-zero (often zero-2-zero in online postings) which takes all the tricky bits out. Geo. H. Thomas described it as being the technique preferred by one of his best lathe operators.

It is an angular infeed method but the top-slide can be at any sensible angle a bit under half the thread angle. The lathe does all the trick trigonometry calculations for you.

My top-slides live at 25° off angle which suits both 55° and 60° threads and keeps them nicely out of the way of both tailstock casting and saddle controls. Important considerations with the S&B 1024 which has a properly man sized top-slide that wouldn't be out of place on a lathe of twice the swing and a hefty tailstock too. I frequently go months between changing topslide angle.

Basic zero-to-zero method as described for an external thread, starts with preparing the blank to size and set the tool tip perpendicular in the usual way. Its best to cut a relief groove at the end of the thread.

Then bring the tool tip up to touch the work and set both cross-slide and top-slide dials to zero.

Pull the top-slide back a touch so the tip doesn't scrape the work and move the saddle to bring it into clear air.

Feed the cross slide forward from zero by the desired thread depth and re-set the dial to zero at the new position.

Pull the top-slide back to gain clearance then move in enough for the first cut. I just shift along and touch the stationary job to find top-slide reading that corresponds to zero cut and go from there.

Threading passes are made with the cross-slide dial reading zero. Pull back half a turn, or whatever seems comfortable, for the return then move back to zero for the next pass.

Feed goes on the top-slide.

When you have worked out the spring with both dials on zero you have cut the thread you set-up.

Assuming adequately accurate feedscrews, if your tool tip is to book radius and your infeed is to book it will fit first time.

Generally hand ground tools come out too pointed so you will need to feed in a touch more. Cross-slide or top-slide, you choose, both work and both let you keep track of the little extra being added on. I stick with the top-slide because I use "finish on zero" a lot when setting up general cuts (no DRO in the lathes). Once you have the first part size right re-set the dials to get zero where it ought to be and any repeats will be on size.

Faster to do than to explain. All the set-up is done first using book figures so no mental maths or thinking wasted keeping track of where you are so you can concentrate on how the cut is going and material is behaving.

Internal threads need a bit more attention but the method works just fine.

Clive

Although I'm a straight in feed type it's interesting to see what the insert manufacturers have to say on the subject as it looks like they suggest the equivalent to setting over the tailstock though the CNC takes care of the angle.

Funny, I didn't get that from the video. The point I thought he was making was that both sides of the tool cut when using a 29-degree compound.

Surely it depends on the size of the thread? If cutting 4tpi then the ability to put some top rake on the tool will help it cut, hence a better thread? Going straight in on a 1/4" cut is going some on a small lathe?

Hi Jason,

I suspect this was a typo error?

Regards

Gray,

While the plunge in technique has been advocated for as many years as I have been doing engineering. The plain fact of the matter is, it is down to personal choice.

Having served my apprenticeship with the Dowty Group, (Propellers and Undercarriage, mainly). We apprentices were taught the set-over topslide method, (also advocated by South Bend). Some of the threads were quite coarse pitch, and the materials involved would be problematical to machine using the plunge in method.

Time in my workshop is not a problem so I stick to the tried and tested method I was taught. It is far too easy for me to make a mistake with moving the topslide and cross-slide in unison.

Regards

Gray,

Ahh, but what you're forgetting Gray is that most on this forum work to NASA standards and have limited time for what used to be a HOBBY. Also from an angled luddite.

Regards Ian

That's the problem with us woodworking types we are used to pointed screws

I like the Hardinge style of setting the top slide at half the thread angle. You move the cam, the tool retracts. While it is going back to the start, you can wind on the next cut with the cross slide or just use the topslide infeed dial until you get to full depth. On my Myford I like the 1/2 thread on the top slide method. It allows for more room when a piece needs to be supported by the tail stock for example.

I'm not sure where the need to use a set over topslide to give more tailstock clearance means you actually have to use it to feed in the tool. Quite possible to set the topslide at any angle and just use the cross slide to feed straight in.

Sine of thirty degrees is 0.5. Depth of cut is (almost) half the distance the compound is advanced. Close enough for me as I am a hobbyist and check the fit of the components, rather than make assumptions on fit.

I do go straight in at times - and carbide thread cutters are a simple cut until the cutter flanks rub. Cross slide DROs make things easier for both metric or imperial. I just do as I please, but definitely lean towards setting over for large diameter coarse threads. There is no backlash problem arising, if the cut is predominantly in one direction, so those with fairly worn lathes might like to consider that, too.

Depth of cut v depth of cut might also be a factor with low-powered machines - those with low power lathes may need to take almost ‘rubbing’ cuts as the depth of the thread increases if cutting on both sides rsther than one side only. That could be quite important for those with DC motors running at low speeds - particularly on those machines that have been prone to popping the control circuit boards.

I'm with AdrianR on this one, the message I got from it was that you should be angling the compound so that it takes a light shaving with the left flank of the tool (if cutting in the traditional right to left direction).

Rob