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Metric Thread Pitch on PM1340 Lathe

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davidpbest

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#1
The common (coarse) thread pitch on M5 screws is 0.8mm. On M12, it's 1.75mm. For the life of me, I can't figure out how to configure the PM1340 for 0.8 and 1.75 mm thread pitch. Those commonly used pitches are not listed in the online manual, nor on the thread/feed faceplate on the machine. At least, if it is listed there, I can't find it. Maybe I'm going blind. I've cut other metric pitches, but these two elude me.

Has anyone here have a clue how to configure the PM1340 for those thread pitches?


If all else fails, I can probably figure a way to use adjustable split dies in a tailstock die holder, but on my current job I need to really dial in the thread depth for a very close fit, and turning the threads would be a lot easier.

Thanks
 

RJSakowski

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#2
I looked at the manual for the PM13450 and it looks like you can cut .8mm threads by replacing the 30 tooth gear in Fig. 3-18 with the 32 5tooth ger and the gearbox position C6.
 

davidpbest

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#3
I looked at the manual for the PM13450 and it looks like you can cut .8mm threads by replacing the 30 tooth gear in Fig. 3-18 with the 32 5tooth ger and the gearbox position C6.
What am I not seeing here? This is the Fig 3-18 from the PM1340 manual posted at QMT web site. No such configuration noted here. Scratching head - again and again. :)

Metric Thread Configuration.jpg
 

RJSakowski

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#4
In the last figure the ratio between the drive gear and the driven gear is .75 (30/40) . If you substitute the 32 tooth gear, the ratio becomes .80 (32/40). In the third chart, the C6 gearbox setting gives .75mm pitch with the 30t gear. With the 32t gear, it should give you .8mm pitch. It's just an undocumented combination.

Unfortunately, I am looking for something that will give you the 1.75mm pitch but haven't found anything yet. Still looking.

My Grizzly G0602 has hundreds of undocumented combinations. Many of them are useless or redundant but there are some useful combos.
 

davidpbest

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#5
Thank you RJ. This ratio figuring puzzle would be a nice spreadsheet, or iPhone app - put in the pitch and it spits out the configuration data. I did ponder the ratios behind the published combinations and speculated that the answer is in there somewhere. But I thought about puzzling it out for about 3 seconds based on the published ratios, and then reached for the gin bottle to dampen my head spinning. :) If you can give me the formulas, I can do the spreadsheet and post it for others to use.
 

RJSakowski

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#6
Dave, It looks like a 35t drive gear, 40t driven gear, gearbox B6 would get your 1.75mm pitch.

As to the spreadsheet, I've done one for the Grizzly G0602 and G4000. I haven't ventured into other lathes.

In order to understand all the ratios, you have to know what;s going on in the gearbox. The A, B, C, D, & E positions are in the ratio 1:2:4:8:16 (as you go from A to E, the pitch decreases and the tpi increases). It appears that the ratios of the numbered gears are 16:18:19:20:22:24:26:28 for positions 1- 8. More than likely, this is the actual number of teeth on the gears.

When the drive and driven gears are equal, position C6 gives 1 mm of travel for each revolution of the spindle. Increasing the ratio of drive to driven increases the travel by the ratio of the gears so changing the driven gear from 40t to 30t will increase the travel by the same ratio (40/30) or 1.333 mm per revolution.

My head spins as well but I find hitting the brandy bottle just makes it spin worse.:drink:
 

mksj

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#7
I have attached a gear chart with the different imperial and metric pitch for various gear ratios, these are generated for both TPI and metric pitch mm thread to thread. Several combinations will give a 0.800 mm pitch, one will give a 1.745mm pitch which should be very close. It is in a PDF file so you need to zoom in to see the fine print. If someone is wants the excel worksheet PM me. There may be other combinations by flipping the 120/127 gear, but I did not generate those numbers. Not to difficult to add if there is some other combinations you want me to generate.
 

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davidpbest

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#8
Thank you both, very helpful. I knew that place would have all the answers. Terrific resource.
 

jer

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#9
Somewhere I remember an article that included a spreadsheet that would give you all the possible thread combos using the available gears on a lathe. As stated many were duplicates and several weren't mentioned in the manual. Maybe an internet search would find it.
 

[X]Outlaw

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#10
When I was waiting for my lathe to arrive I notice some gaps in the metric threading chart. This is a link to a spreadsheet I developed to calculate all possible thread pitches from any combination of change gears not just the stock set. I have included a link to the spreadsheet below.

From my notes I don't think its possible to get 1.75mm pitch with the stock change gear set, you would need to place a 42 tooth gear in the top change gear position and the stock 32 tooth gear in the lower position, then set the QCBG to C2

Using the spreadsheet:

Gear A is the top change gear
Gear B is the lower change gear
B1 to E8 are lever positions on the QCGB
Ratio is the effective ratio of the QCBG at a particular setting
Pitch in MM is the metric pitch for a particular QCGB setting given the combination of change gears

Excel spreadsheet:

http://s000.tinyupload.com/index.php?file_id=41165643326520255331

Let me know if there are any problems with the file.

Chevy
 

davidpbest

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#11
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:

PM1340 Threading Gear Combinations That Work.jpg
 
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[X]Outlaw

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#12
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

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#13
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.

Change Gear Conbinations.jpg
 

[X]Outlaw

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#14
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

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#15
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

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#16
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.
 

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[X]Outlaw

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#17
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

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#18
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

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#19
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

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#20
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
 

jbolt

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#21
On the PM-1440GT gear box there are 2 knobs A-B & C-D. These are straight forward as they are doublers, (A/D =1x, B/D = 2x, A/C = 4x, B/C = 8x) but there are two levers, one with 5 positions and the other with 4 positions. And then there are the 4 change gears for TPI and 5 for metric.

I should be able to modify mksj's spread sheet and then reverse calculate from there.
 

jbolt

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#22
I was calculating the various ratios for the lever positions on the PM-1440GT and discovered an error on the thread chart.

On the TPI chart for the 5.75, 11.5, 23 & 46 threads using the 69T lower gear the shift lever setting should be R-X. It is currently listed as P-X.

When I first ordered the lathe they were still being made at the factory. Matt had sent me a thread chart for a different machine with the same gearbox. That thread chart is correct.

Thread-Feed-Chart.jpg pm1440gt thread chart.jpg
 
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