Metric Thread Pitch on PM1340 Lathe

[davidpbest]

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]

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]

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.

 

[RJSakowski]

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]

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]

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.

 

[mksj]

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.

 

[davidpbest]

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

 

[jer]

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]

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