Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Hello! Welcome to Mart's Garage. This one won't be for everyone, It's my lathe. It wasn't doing the metric conversion correctly for screwcutting. Guess what! I managed to fix it! As a bonus it was a zero cost fix! Always nice.
Back to the usual content on the next one. Mart.

Please click the youtube icon to watch in youtube. The like and subscribe buttons become available then. Because, obviously you're going to need them.

https://youtu.be/0AVVQarvqTA

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

That's great, a Eureka moment.
Wish I had a lathe like that.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Ever learning. I like stuff like that.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

My old lathe only does thread per inch but it works for all the stuff that I generally work on.

Folks will take things apart for repairs or cleaning but they don't always get them back together as they should be.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

I’ve always found the best way to thread a metric screw is with a standard die.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Another informative video from Mart's Garage(Lathe Division)😉

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

I'm confused how the imperial side still worked unless the ratio was dictated entirely by the distance between gears rather than the teeth. Which would mean there was a tiny difference in distance when switched to metric. Which there apparently isn't.
Curious.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

In imperial the 127T gear simply acts as an idler. It's a 1:1 between the headstock gear and the input to the gearbox. It could be any number of teeth. That's why it still worked on imperial when it was the wrong way round and using the 125T gear as the idler.

For metric it needs to be geared down in a ratio of 127/125 so the drive from the headstock gets transferred to the 127T gear, which turns the 125T gear at 1:1 as it is on the same hub, and then the drive goes from the 125T gear to the gearbox. Hence going from 1:1 to 1:0.984 (which is 125/127).

It was when I converted the imperial pitches to metric that I realised that it needs to be geared down to cut the metric pitch. (20TPI = 1.27mm pitch) On my lathe the same setting but in metric should give 1.25mm pitch so I realised what the gears were supposed to be doing.

I was getting 1.29mm pitch so I realised it was gearing up instead of down by the same amount of difference.

When I looked at the gear arrangement and saw how they were arranged I realised what was up. Realising that the right hand gear was an idler was also important. It explained why the imperial threads were coming out ok. As I said the tooth count isn't relevant for a simple idler. it transfers at 1:1 between input and output.

It's nothing to do with clearances, you need clearance for the gears to run correctly.

Hope that helps.

Mart.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Mart I enjoy your videos, you define the hands on type of do it yourselfer that is becoming a dying breed.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Ah ok, i didn't realize the 1:1 ratio.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Just take the nameplate off and turn it around.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

I think that is a neat lathe. None of my lathes cut metric without removing and replacing gears. To be honest I have needed metric capabilities only once and took the part to a friend for that work. I have been able to get by with taps and dies. I think my newest lathe is from the 70's.

Re: Tuesday tech. My lathe. Why is metric screwcutting screwy???
 

Sure, then you have to stand on your head to read it :confused: