Lathe change gears noise

[PeterT]

I posted this on another subject post, but I think worth initiating here as its own theme, so copy-pasted.

I don't do a lot of metric threading but the few times I have, I have been greeted with an unnatural chug-A-chug sound, coming from the gears. I chocked this up as introducing new gears to the train from my normal IMP setup in order to do metric threads and/or additional gears to the train, 4 surfaces vs 3. I had the same tooth spacing about right with the paper trick & they are always well lubricated. Well this time, it sounded its typical chug-A-chug growl, then it improved a bit. Then after an hour of threading a new semi clackety noise started which I did not like. I looked at the gears & observed they had slipped out of position just a bit, yet all the bolts were tight.
Adjusting the gears on the swing arm has never been smooth. After some investigation I could now see what was going on. The curved slot is not machined, its just cast so lots of flashing & irregular width. At the position I was adjusting into, one side of the slot was hanging up on the bolt shaft. Not only making positioning difficult, I think it was actually putting some undesirable bend into the big gear plane. So I just ground & filed some clearance in the curved slot until the arm moved smoothly & unencumbered over the bolt. Instant happiness. The gears immediately quieted down. Knock on wood. So sometimes its these QC headaches that cause secondary problems.

So after posting ^this^ I came to realize my noise improvement was real but rather confined to the lower speeds. When I progressed to higher speeds I could hear still hear the intermittent noise in the background. I think what going on is when I set the gear train to 40-120-40 or 40-127-40 life is good, very quiet. Its when I go 40-127 on one plane & 120-40 on the other plane (regardless of flipping the big gear plate around 40-120 & 127-40) its this combination when I hear the chug-a-chug. This makes me think the 2 gears on the 127/120 fixed cluster are not exactly concentric. I haven't pulled out the measuring stuff yet, so more to come.

But it seems to me I hear about this same 'metric threading noise' issue from folks with similar Asian lathes. Now maybe what is actually behind that is 4 gear surfaces meshing together (while metric threading) vs only 3 (while imperial threading). That would explain 'more' noise, but not 'intermittent' noise. Before I go too far down this rabbit hole, does anyone else share this same noise issue?

 

[ChazzC]

Don’t share the noise, but use plastic gears, and only engage them when I need power feed. I did clean up the slot in the banjo, but only to improve the adjustment.

However, from your last photo you need to add a washer between the bolt & banjo: as is, the banjo (and attached gears) can flop around - even if you didn’t tighten the bolt, a washer will spread the load on the banjo. If the banjo needs to be in that location in order for the gears to properly engage, you will need washers on both sides so you can tighten the bolt.

 

[PeterT]

Thanks. I do have washers on both sides of banjo. They were just removed from pics to convey the first issue, the shank interfering with curved slot. That's been resolved.

I haven't seen many plastic gears on 14x40 2+HP machines though? I didn't look at my existing metal one too close yet. Initially I thought it was a cast as a double blank & the teeth then cut. But I'm not sure that could be so, the 120 rack cutting would interfering with 127 rack. So either broached or somehow sandwich joined? They are both on a common bearing race. Now it the counterbores were slightly off that would explain it.

 

[PeterT]

and only engage them when I need power feed

I think these gears are always engaged. Power feed & threading are via 2 separate shafts coming out of the headstock. PF lever engages slotted plain shaft. Threading engaged with clamshell lever on leadscrew shaft.

 

[ChazzC]

I apologize for my assumptions, shouldn’t have jumped to conclusions before asking for lathe details.

 

[Susquatch]

Initially I thought it was a cast as a double blank & the teeth then cut. But I'm not sure that could be so, the 120 rack cutting would interfering with 127 rack. So either broached or somehow sandwich joined?

Guessing you already know that it's the norm to cut the two gears separately and then fix them to each other on a common axis in some way - through bolts, pins, common keyway, etc. Input power goes to one gear and output comes from the other. In normal mode, both input and output are on the same gear - doesn't matter which one cuz it's just an idler.