Metric Thread Pitch on PM1340 Lathe

Hi @Ischgl99

I think my spread sheet must be working ok as we are in agreement.

Ischgl99 said:
No, I used the 32 gear at top to the 127 gear and the 40 gear on the 120. It’s not exact 1.75, it is 1.7454
Click to expand...
That IS the last entry of the table of gears that I generated via my spreadsheet. See:

B2 said:
I will paste a table below.
Click to expand...

I will re-paste the last line below, which agrees with your statement. See the last column:
ENGLISH:PM1340GTGEARPOSITIONSorTEETH:METRIC:
TPIFEEDXFEEDFEED_TPI1 to 8A to ELS-FeedLowExGearMid1GearMid2GearTopExGearmmPTh
14.552080.003230.00161309.61879FiveBLeadScrew40120127321.74545
 
B2 said:
Hi @Ischgl99

I think my spread sheet must be working ok as we are in agreement.


That IS the last entry of the table of gears that I generated via my spreadsheet. See:



I will re-paste the last line below, which agrees with your statement. See the last column:
ENGLISH:PM1340GTGEARPOSITIONSorTEETH:METRIC:
TPIFEEDXFEEDFEED_TPI1 to 8A to ELS-FeedLowExGearMid1GearMid2GearTopExGearmmPTh
14.552080.003230.00161309.61879FiveBLeadScrew40120127321.74545
Click to expand...
Sorry, I missed that one, I was looking at your chart with the gears the other way in mind with the first gear column as the top.
 
B2 said:
Anyway, I only have the 1340 manual to get a Power feed rate from the front panel label. It says in the gear position A-One it is 0.047"/turn. I can put that in to my spread sheet to generate all of the possible TPI you could get using the Power Feed rather than just the using the 1/2 nut. Not as accurate a cut and has no thread dial but it gives you a lot more possibilities of threads a might cover the metric mm/T you are looking for.

Dave L.
Click to expand...

The manual on PM website is wrong for feed rates on the PM-1340GT and even though wrong it has position A-8 as being .047".turn. The corrected feed rate chart is here. Position A-1 = .05896"/rev, A-8 = 0.03369"/rev. I confirmed those rates when setting up ELS. My crossslide is not 50% of feed rate but is closer to 60%. I can set ELS to .005" on feed rate and get .0501" in 10 turns of carriage movement or change to cross slide and get .0313" in 10 turns. Doesn't really matter unless your paying attention to a feeds and speeds book.
 
@xr650rRider

Thanks for your help. @Ischgl99 and I have figured it out completely/exactly. The PM1340GT and the PM1440GT have the same gear box, so as long as you reference the feed rate to the power bar turns and not to the spindle it all becomes clear. Then you can relate it to the spindle turns by knowing the gear box. I just posted what I think is our summary to the nth decimal place! The Ratio of the x-feed to the power feed is: 3.13540842.
The actual feed rates per power bar turn are also given. So perhaps you can calibrate your ELS even better if so inclined.

Check it out on the other thread we have been using:
https://www.hobby-machinist.com/threads/x-feed-rates-on-pm-lathes.98071/page-2#post-924729

Wishing for an ELS!!!

Dave L.
 
No calibration necessary as it's all based off of an encoder on the spindle. Previously I had DRO setup for diameter instead of radius mode. So quick test. Set the ELS feed rate to .040"/rev. Took out backlash and then turned spindle 20 times. Z reading was .8018"/20=.04009" of travel. (error is just where I was visually observing a turn). Flip lever to facing, took out backlash and turned 20 times. X reading was .25520"/20=.01276"/rev. Ratio X/Z=.3183 or Z/X=3.1418. That's all gear ratio between feed and face and will not change no matter where gearbox is set, mine is just driven by a servo. Only way to change is modify the gears in the apron.

The feedrate is actually a calculation using the ratio between the threading and feeding. Using the updated chart the ratio on the PM-1340GT is exactly 4.2405 for every gear change. So looks like the feed is calculated very accurately.
 
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@xr650rRider
Interesting. I came to the conclusion that one could not see the error in the 3rd digit with so few (20) turns even if one was very precise in coming back to the same position on the spindle turn. So you got Z/X=3.1418 but we found it to theoretically be be 3.13540842. I ran several experiments where I let the number of turns be much higher and found that as I went up the 3rd digit could easily change between a 3 and a 4. I understand that it is extremely anal and that no one really cares beyond the third digit, but if so then why do people insist on an integer TPI when threading. So an error of .005/3.1354 = 0.15%. One has to wonder if the teeth on the Rack are cut to that much precision anyway. I suspect that the gear teeth in the Apron at lease average to the correct value and the lead screw on the X-stage is probably dead nuts on if you go far enough in motion. Anyway, I just did a run where the spindle turns was 1000.0 for both feeds as counted with both my my electronic counter and my my mechanical counter. The distances were measured by my magnetic DROs after the backlash had been rotated a way. For that I found that the ratio was 3.13112. So my guess is that even with 1000.0 turns one does not necessarily get the 4th digit correct. (My electronic counter and my 10 magnet(sensor) yields 10 counts per revolution. That is it resolves a count every 36 degrees. I marked the spindle at a point just after one of the counts took place so that I knew how to get back to the exact spot. But even a couple of degrees shows up on the DRO. So, one needs more turns than 1000 to distinguish that 4th digit. ) Think I will just stay with the theoretical value for the spread sheet I almost have built.

By the way, thank you for the chart!

Dave
 
So you think it's a coincidence that changing from radial to linear movement that there is a constant so close to π ?? I believe the 3.1418 I got is backing out that constant.
 
@xr650rRider Sorry, I guess I do not really understand what you are saying. It is probably not critical that I understand as long as you are happy.

There is no coincidence, just the physics..... What I was trying to say, or thought I had said, is that the power feed rate has a factor of Pi() in it, which is due to the linear Rack threads. As @Ischgl99 pointed out, the Rack threads matches the M1.5 18T gear to yield a tooth spacing of 1.5mm*Pi() = 4.71239.....mm. (This was essentially the same value that I had physically measured on the Rack: 330mm/70T= 4.71429..., but is theoretically exact.) This Rack tooth spacing is used in the Power feed calculation. Since the X-feed drives a gear tied to its lead screw the x-feed rate does not have a factor of Pi() in it. . So when one takes the ratio there is a factor of Pi() involved.

Dave L.
 
I would love to have this spread sheet but for some reason I can't seem to download the file. I just get a "failed to ope the page" message.
Could you email it to me in excel format? daconpaul7@gmail.com Thanks.
 
Hi @Dpaul7 ,

I am not for sure if you are asking for my spread sheet tool or some other. Anyway, I am still polishing my tool up, but when I am finished I will be happy to sent it you. Yes, I am writing it in Excel. I am traveling a bit currently so am not really working on the spread sheet ... for a while. I will let you know. By the way, in the future I would suggest that you sent a private message to folks rather than posting your email address for all the world to see. ...unless you do not care.

Which model lathe do you have? Is it a PM1340GT ? Do you have the standard set of change gears for the PM1340GT or did you purchase a 35T gear in addition?

Perhaps what I can send you, at this time, is a pdf print out of a table of all of the threading options/possibilities for the standard set of gears plus the 35T for the PM1340GT. There will be some duplication in it as I have the spread sheet set up to put a gear at either top, bottom, or both of the external gear positions. I will see if I can get that generated now and if so I will attach it for you. If you cannot download it I will sent it to you via your email. The table has two types of threading: 1) via the conventional leadscrew and 1/2 nut. 2) by using the power feed which may not be as good. The later opens up some non-conventional TPI values. For each of these, it is sorted by increasing TPI. Anyway, maybe this is of some use to you for now. The titles describe the column content. Note TPI is at the far left with other distance per turns info following in a few columns. The gear settings are in the middle and immediately following that is the Metric info.

I make no guarantees about this being correct. Take a look and let me know what you think or how it could be improved. When the spread sheet is complete the table will be something that you can customize or improve upon..

In the spread sheet that I will be trying to provide, will have macros that allow one to sort one a column or search for any TPI value with wild cards. etc. There are other macros coming that will be provide additional features. Within a limited format one can change lathe properties to generate this sort of table for any lathe the user wants to describe into the spread sheet. So far I have input three different lathe models. Determining the feed rates accurately has turned out to be a challenge.

You maybe interested to follow this thread where we have been discussing X-Feed rates and Power feed rates. https://www.hobby-machinist.com/threads/x-feed-rates-on-pm-lathes.98071/

Dave L.
 

Attachments

  • PM1340-TPI-FEED Sorted Thread Table M216_1151.pdf
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