Lathe threading just cuts the diameter down for every pass.

Hi guys. Long time reader, first time poster. I feel real stupid needing to ask but i can’t wrap my head around this. I’ve just started playing with a small lathe, making threads. My gearing is good, tpi is correct (and checked), compound at 29, tip of the tool (carbide insert) is in center, but the two times i have tried the same thing has happened. This is aluminium, so just running slow and steady, but after the first 2-3 passes, taking very shallow cuts, the carbide insert (i thought and still think is a threading insert (60 degree)) starts eathing into the shoulders of the threads, so i end up with a shallow thread i can check tpi in, but the more passes i take, the thinner/smaller the diameter gets. Have run a 25mm down to 23-23.5, should be deep enought to get some real threads.

What type and diameter thread or threads are you trying to make?

How about a picture of your threading insert? You are cutting in aluminium and not steel but I still have a hard time imagining what you are doing wrong.

http://f-m-s.dk/1.09.pdf

Are you dropping the half nut at the end of each threading pass or simply leaving the half nut engaged and running back under power? If you are dropping the half nut make sure you are reading the dial correctly. If running back under power you must withdraw the tool clear of the work before returning or the backlash in the system will offset things causing a cut on the return path.

In my experience the best method for a neophyte is what was taught to me as the Zero-to-Zero technique. The great advantage is that everything is set-up before you start, there are no funny numbers involved and if things do come out wrong its possible to figure out what the heck happened. Almost invariably not cutting deep enough is what happened, especially with home ground tools, which is easily fixed.

I've posted the method several times before but can't get the search function to work to find an older link so here we go again.

Start with the topside set to slightly under the thread angle. I use 25° which works fine for both Whitworth 55° threads and US/Metric 60° ones. Exact angle doesn't matter as the lathe sorts the feed calculation out for you. Set the threading tool exactly perpendicular the work.

Now use the cross slide to bring the tool up so it just touches the workpiece and set both top and cross slide dials to zero. Using the carriage move the tool clear of the end of the workpiece and feed forwards using the cross-slide by the depth to which the thread is to be cut. Then re-set the cross slide dial to zero.

Use the topside to pull the tool clear of the work ready to start, don't alter the dial.

Cut the thread with the cross slide reading zero.

Use the cross slide to pull the tool clear of them work for the return.

Apply feed using the topside.

Job is finished when both cross and top slides read zero for the final threading pass. Generally advised that you make coupe of spring passes at the same setting to work out any flex in the system. Most likely error is that the thread is too shallow. If so adjust the cross slide position past zero as appropriate for subsequent threading passes unit its right. When you have more than one to do set the cross slide dial back to zero at the new, correct, position and all the rest will come out right. If you regularly use partial profile inserts its worth making note of the correction factor needed for the different threads covered by the insert and adjust the initial indeed to depth of thread accordingly before setting the cross slide dial to zero.

Using this method the lathe calculates the angular in-feed for you correctly for the actual angle at which the top slide is set. Hence no errors if the setting is a bit out, no trig to calculate feed depth and you can get the feed by simply looking up the thread depth in standard tables.

Clive

Is your compound 29 degree from the main ways or the cross slide? The compound should be 29 degrees between the cross-slide and the compound.


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I think you should tell;
the diameter and length of stock
How you are holding it
the kind of insert you are using
and your process(you have given some)
add what hand wheel you are in-feeding with, amount of in-feed,what halfnut line you drop the half nut, how you go back to start position,what the thread pitch(number).

Otherwise the PM guys are thinking of all/every possible error you might make.

Lathe threading is easy-as-pie when all your ducks are in a row (doing the drill).

As said above, Are you cutting a metric thread? Are you keeping the half nuts engaged or are you opening them up at the end of the cut? Are you sure you're not using the feed knob instead of the half nut? What model lathe is this?
Lots of questions need to be addressed before anyone can really help!
Dan

Here is the old Navy/Lathe Op. manual. is is short sighted because not much on carbide tooling or rake attitude...
but is still good to read.

Read pages 114 through about 130 to get basic threading.
https://archive.hnsa.org/doc/pdf/lathe.pdf

And you might comment on WHAT are you using to make the carriage "go" - half nuts or carriage feed?

I hopefully managed to add two photos, one of the tool and one of the work.
I'll try to answer and add as much info as i know.
the 29 degree of the compound is from the cross slide yes. I run with the half nut connected, when i hit the end, i move the cross slide out, return the cross slide, set it up on 0 again, and turn the compound. I turned it roughly 0.1mm on each run, if i remember correctly. than repeat.
The first try was turned down to 19.92mm. Yesterday i tried without turning it run first, at 24.98mm.
Im trying to cut a imperial thread though, 20 tpi. The piece is around 100mm long, hold it in a 3 jaw chuck, and also with the tail stock to avoid any out of balance.
The only think i have thought about being the issue is the insert/tool itself.

Kinda looks like you threaded thru the middle of your good thread.
Did you engage the half-nuts at the right spot, not half a mark off?
On my lathe, the lead screw has a shear pin. It broke once and sometimes it would slip, other times not.
I messed up a lot of threads til I figured that one out.

this is a small 8x16, completely new, so should not be an issue with wearing (fyi), but how do i know if it is half mark or not? I don't have a counter or anything like that, just a handle that is on or off. But your statement that it seems it run over the good threads sound right actually. weird.

Is there a thread dial and is it engaged with the leadscrew? "Just a handle" isn't going to be enough because it can engage at the wrong point on the leadscrew. That's what the thread dial tells you.

The thread dial that people are referring to is typically on the right side of the apron. It’s a dial that usually has the numbers 1-4 (depends on lead screw pitch)on it with half marks in between. It has a gear on the bottom that mates with the lead screw. It’s the only way that the work in the chuck stays properly synchronized with the tool when you open the half-nuts. There is usually a chart that tells you what marks on the dial that you can use for each thread.
The dial is often rotated away from the lead screw when not in use.


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The point folks are making is that for each pass, the saddle has to be engaged into the correct thread of the lead screw. What this does is it makes sure that your threading insert is cutting in the same groove as the last pass. You can't just disengage the lead screw and move the saddle back to any thread and reengage because if you guess wrong, you'll be cutting the thread crest down, which is what your picture shows.

A thread dial looks like this (the best pic was from a Sears Atlas lathe, but they look similar on all lathes). There are methods of not using a counter (you can cut the length of thread, stop the lathe, keeping the saddle engaged on the lead screw, note the cross slide position, retract the cross slide, run the lathe in reverse to get the saddle back over to the right, then put the lathe back in forward and position the crossslide back to its last position (minus depth of cut). This keeps the saddle engaged on the same lead screw thread. But the easy way is to have the thread counter, and to use it to reengage the saddle on the correct lead screw thread.

i do not have a thread dial, but when you say that, i though of something that occured earlier when i was testing it before i got the tools, just running it, and i could get it to go back and forth with the leadscrew, but without the handle thoroughly pushed into place, just half way. Did actually not know that that could make the thread cutting wrong. Going to try that when i get home.

EDIT: i'm aware what a dial is, but since i don't have one, haven't paid much attention to what is does/how it works.
But i have off course not disengaged the leadscrew, but i think the issue is related to it not being fully engaged even though it moved back and forwards.

Here's how one guy does it without a dial:
YouTube

the video just makes me more confused, i only have 1 handle, the same one for threading and if i just want autofeed.

Gjermund said:

...the video just makes me more confused, i only have 1 handle, the same one for threading and if i just want autofeed...

Click to expand...

I wouldn't worry too much about that video. It shows one way of making threads but it's not what the majority of
people will do. Most threads are cut using a thread dial or leaving the carriage engaged. Since you have no
thread dial you can only cut threads by leaving the carriage engaged...

New machine. 8x16.. Probably from a company we're not supposed to mention here..

It should have a thread dial.. Go look at the pics and the parts breakout...

Years ago I bought an unmentionable 9x20, and it didn't come with a thread dial.
After a bit of hooping and hollering I got my thread dial.. I still have it, I just
don't have the lathe...

Bobw said:

New machine. 8x16.. Probably from a company we're not supposed to mention here..

It should have a thread dial.. Go look at the pics and the parts breakout....

Click to expand...

The lathe specs say it can cut both inch and metric threads. The leadscrew is one or the other, but not stated in the specs, so the change gear set includes a 127. Bottom line is that a thread dial will only work for cutting the inch or metric threads, but not both. So the lathe maker simply did not supply a thread dial, and the half nuts must be left engaged until the thread is finished, whether the thread is inch or metric. All the online pictures of the lathe show no thread dial.

Larry