Pm1340gt Lathe Threading With A Proximity Stop

mksj

mksj

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Was asked to post a video of threading with using a proximity sensor stop. There is no need to disengage the half-nut, so there is no issue with engagement or cutting metric threads, but one can disengage the half-nut if one so desires to manually back up the carriage.

Threading on a lathe using a proximity stop which is integrated into the VFD, is used to control the stopping position of the lathe carriage. The proximity sensor position is set by the micrometer stop, and the stopping position repeatability is ~0.0004" or better even at high feed rates. In this case, every pass hit 0.5000" on the DRO. This is probably due to the use of the VFD 1 second braking and use of an inverter rated motor. My lathe is set up with a single belt speed using a 2.5" motor pulley and the low speed 5.5" stock pulley. Inverter motors can maintain full torque down to 0 RPM, and full Hp out past 2X their base speed. Braking is very accurate and quick. The default setting is 3 seconds, but the E-Stop and proximity sensor default it to 1 second automatically. Also because of the use of the low speed pulley ratio, the applied Hp to the spindle at maximum speed is doubled. The speed usable motor speed range is 20-120Hz, so something in the range of 40-1800 RPM with 6 mechanical gears. In practice I usually use 2 or 3 gear settings most of the time.

In this example I am cutting a 3/8-24 thread in oil hard drill rod with a Dorian thread cutting insert at 250 RPM. When treading a 0.1" relief grove is cut to approximately the depth of the final thread, the thread cutting insert is placed in the center of the relief and the cutter insert point advanced to the depth of the final thread cut. This is then used to set the lathe proximity carriage stop position, the tool post slide dial is set to 0 (the position of the cutter is expected to be in on the final cutting pass). The lathe cross slide is locked, and only the tool post slide is moved from this point forward.

When threading there is no need to disengage the half nut for each pass, at the end of each pass the cutter head is backed out with the tool post slide, the machined reversed by engaging a proximity bypass button, and then the tool post slide advanced to the next cutting depth and the next pass is taken. It is easy to cut a full thread in about 2 minutes with high RPMs and with no worries.

 
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That's awesome Mark!
I haven't had a chance to thread with mine yet but I'll definitely give this a try.
Thanks for the video.
 
Thanks Mark for the video. As one who always sweats crashing my thread tool into the work piece that is a great addition on any lathe.

What motor did you use on the lathe?
 
BOOM! there it is. Video was nicely done Mark. Of all the new features, the proximity carrige stop is my favorite.
 
I'm sold Mark!

Thanks so much for sharing your amazing setup.
 
Let me start out by saying the stock PM1340GT motor is very good, works well with a VFD and has been extremely reliable. Adding an inverter type motor gives a small but measurable performance difference, but the stock motor is a high quality unit and I have been very happy with it. An inverter rated motor can stop the lathe a bit quicker and with a higher repeatability (we are talking 2-3 ten thousandth of an inch), and slightly better speed regulation under load (inverter motor is usually +/- 1 RPM, vs. 2-3 RPM for the stock motor).

There are a few motor choices that should work as replacements for the PM1340GT, the motor space is very cramped so one can only use a 145 frame motor and it most likely needs to be a TENV type. A TEFC 145 framed motor would be too long, as the distance between the splash shield and the front belt tension support is 11". It is possible to shift the splash shield over by 1/2-1" with spacers, so that a TEFC might fit if the overall motor length is 11" or shorter. All the inverter/vector rated motors in the 145 frame size I have found are typically TENV type.

A inverter type motor are rated to provide full HP to at least 2X their base speed and often up to their rated maximum speed which is around 5400-6000 RPM for this frame size motor. A vector motor, is an inverter motor with an encoder attached to a stub shaft at the back of the motor. An encoder provides very precise speed regulation and shaft positional control at very low motor speeds or 0 motor speed. The other difference with these types of motors, is that they can provide almost double their rated torque from 0 RPM up to their base speed (60 Hz) and about 1.5x at 120Hz for about 1 minute. All the inverter motors I have seen have higher temperature rated insulation and thermostats in the wingdings for over temperature. The NC thermostats can be wired directly to the VFD or in series with the E-Stop to shut down the machine if it gets too hot. Thus far, I cannot even get my TENV motor to get past slightly warm. Speaking to motor manufactures, most quality 1800 RPM motors are capable of 2X their base speed, but their performance characteristics like Hp and torque can drop off more than predicted beyond 1.5X their base speed.

Typical 2Hp 1800 RPM base speed Inverter/Vector HP and Torque Ratings vs RPM
Inverter motor specs.jpg


The Marathon E467 is an induction/inverter rated motor that is the most compact TENV 2Hp model that I could find that would fit the PM1340GT as a replacement (requires redrilling the mounting holes) for the stock motor if needed, or you wanted to convert over to 3 phase. The rated constant torque ratio is 10:1, meaning the torque is flat from 6-60Hz and the rated maximum speed that it maintains constant Hp is 90Hz. The motor probably can easily spin to 120Hz, but the Hp and torque may not be so even in the 90-120Hz range. The Marathon Y551/Y526 (Black Max and Blue Max Inverter models) is slightly longer, has a small back stub shaft to attach an encoder and has a C face. The Y551/Y526 is an inverter only motor (designed to run off of a VFD only for the rated specs), has a constant torque ratio of 1000:1 (so has full torque essentially down to 0 RPM), and maintains full Hp to 120Hz (and probably beyond) with a maximum speed of 5400 RPM. So the Y551/Y526 (inverter style motor) has a wider operating performance envelope and is a more ideal motor if one wants to use a single belt ratio. The Y551/Y526 (inverter style motor) has a back cover plate and a 5/8" x 1.5" encoder shaft on the back of the motor that can be used to attach an encoder. The cover plate is permanently removed, and the back encoder shaft needs to be cut down from 1.5" to about 0.5" for the motor to fit the PM1340GT with no sheet metal mods. It will not be used. I used a hack saw to cut down the encoder shaft on my motor, otherwise a die grinder will work just fine. With these changes the Y551/Y526 will have an overall length of about 10.5" and the motor space is 11" from the front belt tension bar to the splash shield. The other dimensions are essentially identical to the E467, just the overall length and the C face are primary external differences (along with the encoder option). Blador, Leeson and Lincoln have inverter only models with similar specifications which are more expensive, I do not feel they add any improvements or better performance, but I do have a Blador on my lathe. So it is a matter of whichever is the least expensive/preference.

http://www.electricmotorwholesale.com/MARATHON-E467.html
http://www.electricmotorwholesale.com/MARATHON-Y551.html
http://www.electricmotorwholesale.com/MARATHON-Y526.html
http://www.electricmotorwholesale.com/LEESON-810546/
https://www.mrosupply.com/motors/ac-motors/inverter-vector-motors/24473_idnm3587t_baldor/

So I have recommended the E467 as a "replacement motor" for PM1340GT because it will work w/wo a VFD, is a relative easy fit, and is reasonably priced. On a single belt speed conversion my recommendation would be the Y551/Y526 as an optimal replacement because it can be belted with a smaller motor pulley and operated to a higher frequency. This will give one more Hp and torque (~30%) to the lathe spindle relative to a larger motor pulley, and the Y551/Y526 has a larger optimal speed range. The cost difference is about $150 between the motors, unless you can find a NOS on eBay for less.The Marathon E467, Y551, Y526 motors do come up on eBay, but the condition and price is all over the place. Other 1800 RPM inverter rated motors in a 145 TENV frame by Baldor, Leeson and Lincoln are available, but less frequently seen on eBay and often at inflated prices and questionable condition. As mentioned, there are a number of options, not having tried them all, I can't say what is the best choice so it is always important to check the dimensional drawing for the motors. They all require some modifications to the motor/motor mounting plate to fit and sometimes the belt tension support.

One other option for a single pulley speed machine is to use the Marathon E467 or the stock motor and change the pulley ratio, the PM1340GT low speed ratio is ~2.5:1 (2.2" to 5.5") the high speed is ~1:1 (3.5" to 3.5"). With the VFD you can push the stock motor or the E467 up to 90-100Hz and go down to ~25-30Hz, so a ratio of around 1.7:1 will give you a maxim spindle speed of around 1800 RPM. The stock motor shaft is 3/4", so a pulley in the 3.0-3.2" would work well in combination with the stock low speed 5.5" pulley. You probably will need a BX-26 or BX-27 belt. The Maska 3.1" MB31 or BK31 "B' Pulley, 3/4" bore for stock motor should work well. If using an inverter type motor capable of 120+Hz operation I would suggest anything in the 2.5-2.7" pulley size with a 7/8" shaft bore, so something like a BK25 or BK27. A double belt pulley is not needed and a single groove may be easier to position on the shaft, but a double groove pulley could also be used.
 
I talked to Mark about this, and while this may be a future mod, I find the current motor (as Mark says) to be a pretty good motor for what it is. I don't have/use a switched stop, and then I realized that I have a perfectly good, brand new stock motor (the motor fan and shroud was mangled during delivery of the lathe) that hasn't even been used yet sitting under my bench. By the time I wear out these two motors I could be to the point where I want to CNC this thing and step up to a BLDC motor for added spindle control. Who knows what I'm going to do in the future? ;)

EDIT: Mark, would a vector motor have the same angular resolution as a BLDC?
 
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jeff_g1137 said:
I cannot remember if i have asked you this before, old age LOL, is this motor, a good inverter rated motor .
Click to expand...
Yes this is a very good motor for use with a VFD, probably the usable frequency range would be ~20-100Hz.

BLDC vs. an inverter/vector motor, my basic understanding is that they are very similar, the BLDC being synchronous because it uses a position sensor to trigger the DC coil fields, usually 3 phase motors are considered asynchronous. But when adding an encoder to an inverter motor to get a vector motor, you essentially have close to a synchronous motor due to the feedback. I have seen both used, but I think a BLDC would not be worthwhile in larger lathes. VFDs interface with CNC controls, a vector motor gives you very tight motor control and spindle positioning like a BLDC. My mill uses a 2.2Kw BLDC, I do not see it performing any better than a good inverter motor of a similar size. But no expert here on BLDC.
 
Mark
I just came in from giving your threading procedure a try and all I can say is WOW!
I have some fine metric threads to do on a telescope spacer ring that I want to build and this should make it a breeze!
 
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