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8 x16 chinese lathe screw cutting question

Brian H

Brian H

Super User
Does anyone have an 8 x 16 chinese lathe that can help me with a couple questions about screw cutting.
I have had the lathe since Feb of this year and have finally ventured into trying to do some threading. To my surprise the charts supplied didn't work (gear set up isn't tall enough to reach the spindle drive gear). So I did a little checking and according to the manual and sticker on the lathe I'm supposed to have 56 teeth on the spindle drive gear but only have a 40 tooth gear.

I searched and found the gear set ups for a lathe with 40 teeth and I don't have the correct gears to make the appropriate set ups. I e-mailed the supplier, but, they have not responded (I read don't care, we've got your money).

Any one with any insight or suggestions would be wonderful
Lathe.jpg
 
On my 7 X 12 none of the gears I need to change interface to the spindle gear directly. Also, they are on swing arms that need to be adjusted to make the gears mesh. Perhaps a pic of your drive train and the appropriate pages of the owners manual would help here?
 
Belt drive spindle.... that's interesting. There is no gear on your spindle to engage or am I missing something?
 
Your lathe looks similar to this one from Grizzly. https://cdn0.grizzly.com/manuals/g0768_m.pdf
The Grizzly one has a 40 tooth gear on the spindle but yours appears to be 56.
Between the Grizzly manual and your thread chart there should be enough to figure it out.
My guess is that to cut a .5mm thread you need a 84/30 compound gear. The 84 meshes with the spindle gear and the 30 meshes with gear D - 60 teeth In this case it looks like there is no gear C or E so gear D drives gear F (80 teeth) and the leadscrew directly.
 
Johnwa said:
Your lathe looks similar to this one from Grizzly. https://cdn0.grizzly.com/manuals/g0768_m.pdf
The Grizzly one has a 40 tooth gear on the spindle but yours appears to be 56.
Between the Grizzly manual and your thread chart there should be enough to figure it out.
My guess is that to cut a .5mm thread you need a 84/30 compound gear. The 84 meshes with the spindle gear and the 30 meshes with gear D - 60 teeth In this case it looks like there is no gear C or E so gear D drives gear F (80 teeth) and the leadscrew directly.
Click to expand...

Yes John, that's exactly my problem. I'm supposed to have a 56 but I have a 40 tooth.

I did look at the grizzly manual on the website. It has different "recipes"for metric thread so I think they have an imperial lead screw where mine is metric. I also got a chart from a youtube video from "Ades workshop" to compare to the grizzly one and I know Ade's machine is virtually identical to mine (read has a metric lead screw).

The biggest issue I have is that once I get the appropriate series of gears for a particular thread together they don't reach to the drive gear on the spindle. There is about 3/8" gap. The other issue I've run into is that after I compare the gears I need for most of the chart are missing (20, 24, 33, 35,and 80 tooth).

It is a bit frustrating since I haven't been able to find a set of gears that I know will fit. There are gear sets but they don't give dimensions for shafts and key ways.

I was trying to find a chart to determine if I could use a different series to obtain the same pitch using the gears I have but I have't found that. (and I'm not smart enough to figure that out on my own...LOL)
 
Brian H said:
Yes

The biggest issue I have is that once I get the appropriate series of gears for a particular thread together they don't reach to the drive gear on the spindle. There is about 3/8" gap. The other issue

It is a bit )
Click to expand...

Can you post a picture with the gears set up that shows the gap?
 
The whole drive train and/or arm holding the gears should adjust to slide up to the drive gear???
 
Last edited:
I take it you haven't received a reply from the machine supplier yet?

Try contacting LMS and see what they have to say.
 
That doesn’t look like the right arrangement for 1.5mm. The 60 tooth gear should mesh with the spindle. The smaller gear keyed to the 60 should be 40 teeth. It should then mesh with an 80 tooth gear which in turn meshes with the 50 tooth gear on the lead screw
 
Johnwa said:
That doesn’t look like the right arrangement for 1.5mm. The 60 tooth gear should mesh with the spindle. The smaller gear keyed to the 60 should be 40 teeth. It should then mesh with an 80 tooth gear which in turn meshes with the 50 tooth gear on the lead screw
Click to expand...

according to the chart, the lower gear is 80 t -second row 40t outer, 60 t inner (contacting the 80t) and the 52t on top contacting the 40t and theoretically the spindle. (this is from the chart with the 40t spindle gear)
 
I was looking at the chart you posted with a 56 tooth spindle gear. Calculating back that setup would require a 2mm pitch leadscrew. I haven’t calculated the Grizzly one yet
 
Johnwa said:
I was looking at the chart you posted with a 56 tooth spindle gear. Calculating back that setup would require a 2mm pitch leadscrew. I haven’t calculated the Grizzly one yet
Click to expand...

I just checked and yes, I have a 2.0mm lead screw. (dam your smart)
Now, purely for my curiosity, and hoping I may understand, how did you figure that out. I have been looking for some way of figuring that out. I cannot wrap my brain around this
 
The Grizzly leadscrew works out to 12tpi (about 2.12mm), I’d double check the leadscrew, close the half nuts and then use a dial indicator to see how much the carriage moves when you manually turn the leadscrew 12 times.

The calculation is as follows
For 1mm pitch and spindle 56 teeth
Gear A (84 teeth) and gear B turn 56/80 turns for 1 spindle revolution
Gear D is just an idler and has no impact.
Gear F[80 teeth) effectively meshes with gear B(60 teeth) and turns 60/80 for each revolution of B or 56/80x60/80=0.5 for each revolution of the spindle. To move 1mm in 0.5 turns of the leadscrew requires a 2mm leadscrew.
The Grizzly gear configuration is different, gear A is the idler so the calculation has to be modified to take that into account.
 
Brian H said:
according to the chart, the lower gear is 80 t -second row 40t outer, 60 t inner (contacting the 80t) and the 52t on top contacting the 40t and theoretically the spindle. (this is from the chart with the 40t spindle gear)
Click to expand...

Which chart is that?, it isn’t the same as the Grizzly one for 1.5mm
 
Based on the chart posted in message #3 Gear D is an idler for all the metric threads, you can replace it with a larger gear to get the spindle gear to mesh. Likewise for the arrangement in message #9, gear A is an idler so you can use a larger gear.
Of course if you have a 40 tooth spindle and a 2mm leadscrew then the feed won’t be correct.
 
Johnwa said:
Which chart is that?, it isn’t the same as the Grizzly one for 1.5mm
Click to expand...
Johnwa said:
Based on the chart posted in message #3 Gear D is an idler for all the metric threads, you can replace it with a larger gear to get the spindle gear to mesh. Likewise for the arrangement in message #9, gear A is an idler so you can use a larger gear.
Of course if you have a 40 tooth spindle and a 2mm leadscrew then the feed won’t be correct.
Click to expand...

I think I'm starting to get a handle on this.
Yes, I agree, The chart for the 56 tooth spindle gear don't work with the 40 tooth spindle gear.
What I am trying to sort out (and you have helped understand a bit of the thought process) is what gear do I substitute and how do I determine the size to get the end result I need.
I am going to spend some time going through the process using the math you showed to see if i can get this sorted. I will likely need to make a new chart

Thanks for your patience Johnwa, you have helped me start to get a handle on this
 
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