Metric Thread Pitch on PM1340 Lathe

[davidpbest]

Chevy, The PM1340 comes with 40, 30 and 32 tooth small gears, and 127 and 120 large gears. So the possible combinations are 24 gear arrangements. Your spreadsheet doesn't cover all the possible combinations - looks to me like you're assuming there is only one large gear tooth configuration. Like I said originally, this makes my head spin so I could be all wet. Working with mksj's spreadsheet, I found a path to the 0.8mm pitch for M5 coarse threading that I need just now, but I haven't tried all possible gear combinations yet, so have been unable to solve for 1.75, 0.2 and 0.35 (all of which are found in standard metric coarse or fine threads). Here's where I am so far:

 

[[X]Outlaw]

Hey David,

If you change the fields Gear A and Gear B in my spreadsheet, the column Pitch MM will be automatically recalculated based on the change gear combination you input.

Chevy

 

[davidpbest]

Sorry, I still don't see all possible combinations because the lower small gears can mesh with either 120 or 127 tooth large gears. Maybe I don't understand your spreadsheet, but I don't see how to specify which of the two large gears are meshing with the lower set of small gears. Seems to me there are three variables, not just two. These are the possible combinations.

 

[[X]Outlaw]

Ah, I understand what you are saying David.

When I made they sheet I did it with the assumption that the lower change gear will always me meshed with the 120T gear and the upper gear will always be meshed with the 127T gear as this is what was shown in the manual for cutting metric threads.

I've modified the spreadsheet. There is a new field called transposing gear. This only applies to what gear the lower change gear is meshed with. The formula still assumes that the top change gear will always be meshed with the 127T gear.

http://s000.tinyupload.com/index.php?file_id=36905162139584955074

Chevy

 

[jbolt]

Question, how do you go about figuring out the gears / gear ratios in the gearbox? I would like to do this for my PM-1440GT

 

[mksj]

David, I recalculated the spread sheet flipping the 120/127 gear and looking at the metric combinations. Based on the calculation you can cut every metric thread pitch up to 6.0, with the exception of the 5.5. The 0.200 pitch works out to 0.201, the 1.75 to 1.745. I have attached the worksheets, you can plug in your gear ratios and see other combinations.

Jbolt, let me look at the manual and gear chart and see what I can put together for you. Have a few other things so it will take a day or two.

 

[[X]Outlaw]

How I did it with the PM1340GT is like this.

The lead screw is 8TPI
When setup for imperial thread cutting the 127T gear is just an idler between the upper and lower 40T change gears.
This setup gives a 1:1 output to the lead screw (Before the QCGB comes into play).

So without the QCGB coming into play we are essentially setup to cut a 8 TPI thread.

We can calculate the ratios for the QCGB by taking 8 and dividing it by each QCGB setting on the imperial threading chart.

So for example in the 1340 imperial threading chart A1 will give 4TPI.
8/4=.5

So A1 has a ratio of 1 : .5

I'm sure there is someone that could explain this a lot clearer than me but this is the way I came up with the ratios.

Chevy

 

[jbolt]

David, I recalculated the spread sheet flipping the 120/127 gear and looking at the metric combinations. Based on the calculation you can cut every metric thread pitch up to 6.0, with the exception of the 5.5. The 0.200 pitch works out to 0.201, the 1.75 to 1.745. I have attached the worksheets, you can plug in your gear ratios and see other combinations.

Jbolt, let me look at the manual and gear chart and see what I can put together for you. Have a few other things so it will take a day or two.

Thanks Mark, no hurry.

Looking at you spreed sheet helps. I will take a crack at it myself.

 

[jbolt]

How I did it with the PM1340GT is like this.

The lead screw is 8TPI
When setup for imperial thread cutting the 127T gear is just an idler between the upper and lower 40T change gears.
This setup gives a 1:1 output to the lead screw (Before the QCGB comes into play).

So without the QCGB coming into play we are essentially setup to cut a 8 TPI thread.

We can calculate the ratios for the QCGB by taking 8 and dividing it by each QCGB setting on the imperial threading chart.

So for example in the 1340 imperial threading chart A1 will give 4TPI.
8/4=.5

So A1 has a ratio of 1 : .5

I'm sure there is someone that could explain this a lot clearer than me but this is the way I came up with the ratios.

Chevy

Thanks Chevy,

You explanation is clear enough.

 

[brino]

how do you go about figuring out the gears / gear ratios in the gearbox?

For my Southbend 9" since I did not find a manual that matched exactly, I saw two ways of doing it:
(from the manual the PM1340 looks similar)

1) using a known setting from the chart, the tooth counts of all the visible gears, and the lead-screw pitch attempt to reverse calculate all the QCGB ratios. The problem here was that the number of levers made it a pain to work out for all the different combinations.

2) using a mirror, a flashlight and standing on my head I peaked under the open bottom of the QCGB and carefully counted the teeth on each gear. I would wipe the lube off the side of a tooth, mark it with a dot of paint, then slowly spin the input shaft and count teeth until the dot came around again. I made several little sketches of how the various gear were slid in and out of mesh.

Using the tooth counts and sketches I put together a spreadsheet showing all combinations.
Some time and effort invested, but well worth the understanding.

-brino