Change Gear Conundrum

Hi all,

I recently started trying to cut threads on my Warco WM250V lathe, but I have encountered a problem. I was trying to cut a 1.25 pitch thread, but after assembling the change gears to suit, the thread which was cut was far too fine. After checking with a thread gauge, it appeared to be 0.6. I then double checked the gears and they seemed to be correct, a 30, an 80, a 75, and a 60 tooth, with the gear selector in the A position. I then moved the gear selector to the B position and cut a thread, which seemed to be very slightly finer than a 1.25 pitch. My thread gauge doesn't measure 1.2 pitch, but I suspect that is what it was. I then moved the gear selector to the C position and tried to cut a thread. What I got could hardly be called a thread, but it was definitely very fine, much finer than the 0.6 earlier. Looking at the gear chart on the lathe (I have attached a photo) the results I'm getting would indicate I have a 60, a 50, a 30, and an 80 tooth gear train, but I definitely don't. What's confusing me is that I've done the calculations as best as I can, and from what I can tell, the gear train I have fitted should have given me a 1.25 pitch originally.

Can anyone shed any light on what I'm missing here?

(FYI the number of teeth on the spindle gear is 40, and the lead screw is 2.0 thread pitch if you want to double check my calculations)

Maths for the gear train shown looks to be right

40 drives 30 = 1.3333333

30 drives 80 = 0.375

75 drives 60 = 1.25

1.333333 x 0.375 x 1.25 =0.625

0.625 x 2mm pitch = 1.25 thread pitch when selector in A which is 1:1

Make sure you have the right gears meshing, the vertical line between 30-80 and 75-60 is where they mesh

Better way to measure is put a indicator on the carriage and measure over several revolutions

Also position A is with the lever to one side not the mid position which the chart may have you think.

Edited By JasonB on 13/07/2023 07:30:23

Thanks Jason.

They are meshed correctly in the right sequence.

I believe when making the calculations you can simplify it further and ignore the 30 tooth gear as it's simply acting as an idler. (40/80)x(75/60) gives the same result.

I will try with an indicator, although I'm definitely not getting the thread pitches which the chart says.

I have had the lever in all three positions, and none seemed to give the desired thread.

I guess all that is left, bar something stupid on my part, is to count the teeth on the change gears in case they have been stamped with the wrong tooth count.

Edited By John McCulla on 13/07/2023 22:37:43

Have to ask: You are using the half-nut engagment lever and not the feed lever to move the carraige?

Using the 30, 80 75 60 set-up in the feed position suggests to me that the feed rate would be 0.595 (per rev).

Edited By Hairy Pete on 14/07/2023 00:19:23

Hairy Pete, I think you have it!

Looking at it now, I can't believe I have never noticed that lever, but... I haven't!

Ok, so please explain to me what the difference is between the feed lever and the half nut engagement lever.

Presumably they both drive the carriage using the lead screw, but does the feed lever use additional gears or something? Because the change gears still changed the speed when I was using the feed lever, just not to what I wanted.

When engaging the feed lever that drives gears within the apron to either turn the cross slide leadscrew of in your case it is driving a gear that engages with the rack that runs along the bed. These gears don't give the same ratio of movement as using just the half nuts on the lead screw.

The 250 size lathes are a bit of a half way house between a small lathe that uses the leadscrew for both feed & screwcutting and a larger lathe that has a separate feed shaft which saves wear on the leadscrew, look at a 280 size and above to see this