[Newbie] When Is It Good/safe To Engage The Power Feed Lever

Then it looks like you're feeding in at the wrong angle. When people cite 29.5 degrees for the compound they mean that much from perpendicular. This almost looks like you're set 29.5 from the axis.

A spring cut will help clean up the surface of the leading edge.

Some steel just doesn't like to be cut at slow speeds. You might want to try other steels. Don't get discouraged; look at the progress you've made so far!


Steve Shannon, P.E.
 
Spindle speed? When threading to a shoulder or ramping out a thread (no relief groove), I'll change down to my lowest gear (30 rpm) - with this machine I can speed change on the fly. I get a very nice thread finish.

There are machines with threading stops, but that is very different from a carriage stop. Any Force lathe I saw was not the type of machine that had a kick out for threading.

As Steve pointed out, keep practicing. I have pretty normal backlash on my carriage hand wheel, in my half nut, on my cross slide and on my compound slide. Backlash is a fact of life with this style of machine. The presence of backlash is not the problem with your threading result.

Quick retract tooling? I have never used it. It looks cool. I don't really understand the benefit. You can retract the tool with the cross slide in about a quarter second. I'm not clear how flipping a lever is much faster. .??

Leaving the half nut close and reversing for another pass? There is nothing wrong with that, however the lathe has been designed so that you can open the half nut (which is a very nice feature on a long thread with a single speed machine) - I encourage you to learn to use it.

Threading looks routine? Like everything else in life, once you know how, it is routine. Stick with it, and you will find it is pretty straight forward.
 
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IMG_0499.jpg

This is roughly the setup I had when doing it [the cutting tool in the picture is likely at the wrong angle because it's not clamped down anymore because I was looking at it afterwards]. I'm pretty sure this is the angle for the compound slide [which is 60 degrees on the markings], as 30 degree's would make it move more parallel to the work. This angle should make the compound slide move parallel to the trailing edge of the cutting tool.

I would do the half-nut/thread dial thing, except
-it's more likely I will remember how to do a given operation one way vs two different ways
-the thread dial doesn't work properly for metric threads, which I will be using (at least, this is my understanding of it)
-the dial [just a thin plate] with the markings has come off the part that rotates on the saddle

I think I've handled the backlash on both the cross and compound slides [by screwing them inwards], but I didn't do anything about the backlash for the saddle when I start another pass. But this should only be a problem for the very initial part of the thread at worst.

And it shouldn't be that the rod is flexing, as, though I am working with it extended out from the headstock so I don't run the saddle into the chuck, I am using the tailstock to support it so it doesn't move.
 
Even with support from the tail stock long stock will flex. Any flex can lead to a rough surface and turning long screw stock is one of the most difficult operations; just see posts from others where they have tried to turn lead screws! Your picture shows a very long unsupported length. I wouldn't try to turn a thread that way.

Something that occurred to me is that perhaps you don't have a lot of experience with turning, let alone threading. It might be helpful to just turn a whole bunch of stock and get a feel for finishes and bits before embarking on threading. There's nothing so satisfying as the first time you get a part to not seem "fuzzy". Until you can do that on a plain surface you won't be able to do it on a thread. Speed certainly can have much to do with that, but the geometry and sharpness of your cutting tool does too, as well as just practice. The type of metal being turned is extremely important. You might wonder about the stiffness of your lathe, but people have been getting very smooth surfaces from Unimats and Sherlines for years.
I prefer to use the thread dial and half-nut myself, but I am aware that sometimes cutting metric threads on a lathe with an English unit lead screw isn't supported by the dial. I would use the half-nut wherever I could though for threading.


Steve Shannon, P.E.
 
The stock will want to bend more with each successive pass, as the width of engagement is getting bigger.

You could practice some on a more rigid(bigger) piece of stock to get the hang of the repetitive process.

Stuart
 
Hi Dave, thank you for the picture of your set up. That is a very, very flexible set up (even though you are only threading a couple inches from the tailstock support).


There is usually more than one way to set up a job. I would normally have the theading tool on the left side of the toolpost, which would allow you to work in much closer to the chuck. With that size of stock, you will need tailstock support if you are out from the chuck more than about 1”. True, this means you will be threading in close to a spinning chuck – that is called machining.


Backlash in the carriage hand wheel will not effect your thread. The leadscrew/half nut works like a bolt & nut to pull the carriage along. The carriage feed and hand wheel have nothing to do with threading.


The issue with threading (and a number of other types of operations) is that the width of cut will get wide (the full width of the flank of the thread). One of the things which took me a long time to appreciate is the concept of rigidity (or flexibility).


Nothing in machining is truly solid. You need to do the best you can to make your set up rigid, then compensate for the flexibility.


The reason for the 29° (or 29.5° or 30° - depending on who you talk to, there really is a reason for each of those. I go with 29°) is so that you are mainly cutting on just one flank. Of course you can simply plunge cut with the cross slide – that is fine if your set up is stiff enough, but then the cut is twice the width.


I recently completed a lead screw (15.75” of ¾” NC thread) – see my posts elsewhere. A ¾” NC thread has quite a wide flank as it approaches full depth. With this long thread, there is quite a bit of deflection (~0.003”at the mid-point) of the work piece and guaranteed chatter even with a very small depth of cut.


I acknowledge that you would like to sort out one good method for a certain procedure, such as threading by fwd/rev your machine and leave the half nuts closed. That approach does not work very well for machining when every job will be different and present new challenges.


In fact, even on an imperial machine, when cutting a metric thread you can still open/close the half nut (described in an old MW magazine – I can dig it up and post the reference). I don’t cut many metric threads so for that occasional job I use the procedure of keeping the half nuts closed and reversing the machine. The point being that as you gain expertise you will learn lots of strategies for getting great results.
 
The rod was supported by the tailstock [it wasn't in the picture because I had cut off the end that I had worked on] and I worked only within the 1.5" right next to the tailstock.

I will try a larger bar, that should also be a stronger/stiffer type of steel [unfortunately, I can no longer ask my dad, who would know all this stuff, about it]. This 3/8 rod was just the fairly soft, bendable stuff you get at homedepot, that I was using just to test/try out doing threading.
 
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A long piece like that will deflect away from and try to ride up onto the bit. As it deflects the end pulls away slightly from the center in the tail stock, allowing it to deflect even more. I would run almost all of the stock through the headstock and just leave the portion that you want to thread hanging out of the chuck or support it with a steady rest. The tail stock only supports the right end of the stock, none of the length between the headstock and tail stock. When I was learning I had stock climb up on top of bits.


Steve Shannon, P.E.
 
I don't know the specs for that rod. It's from home depot, just says 3/8 steel rod, cuts pretty easy with a hacksaw and I could likely bend it around my knee [if it were still 3+ feet long]. Looking at the thread up close, the leading edge of the threads look better than my first try, but it still looks in a bunch of places that the metal was tore off rather than cleanly cut off. I've been using a carbide tipped tool, which I understand likes a higher speed vs a HSS tool, and the spindle speed was fairly low, which likely aggravated the problem.

I've got a larger 5/8" rod [also don't know the specs, but it's not from home depot, and harder to cut with a hacksaw], I'll give that a go with a shorter distance between the head and tailstock.
 
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