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Logan Model 200 Servo Threading

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Briney Eye

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#1
Just throwing this out there to see if anybody is interested. I see a fairly straightforward way to set up a Model 200 for servo threading that would be easier than retrofitting a quick-change gearbox while also being much more flexible.

Conceptually it's simple. Put an encoder on the spindle and a servo motor on the lead screw, with some digital intelligence in between and a user interface to select the ratios. Feed rate would become continuously variable, with simultaneous display of both axes. The noisy change gears would go away. Metric would become easy. The function of the forward/reverse lever, half nut lever, and crossfeed plunger would remain the same.

I would avoid "modes" and go with a full-time display of feed rate and pitch in inches and millimeters, with a "highlight" when you landed on a standard pitch. I see a clear-lidded box with a power switch, a display, and a knob, mounted where a gearbox normally sits. When the box was on, the lead screw would behave as if there were still gears coupling it to the forward/reverse lever. You would still use the thread dial. And of course the lathe could still be returned to its original state.

I possess the necessary skills (from nearly 40 years doing this sort of thing), and think it would make a fun (and functional) project.

What are your thoughts?
 

markba633csi

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#2
It's been proposed but I have not seen a working example. The trick of course is to make sure the leadscrew never becomes de-synchronized with
the spindle. The circuitry should be immune to electromagnetic interference and you would need a backup system for power failure
Mark S.
 
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T. J.

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#3
I have no personal experience, but I've heard this one mentioned on the Atlas 618 Yahoo group before:
http://www.autoartisans.com/ELS/

There is also a Yahoo group called "Electronic Lead Screw".
 

Briney Eye

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#4
I have no personal experience, but I've heard this one mentioned on the Atlas 618 Yahoo group before:
http://www.autoartisans.com/ELS/

There is also a Yahoo group called "Electronic Lead Screw".
There are quite a few projects like that floating around. I just think that it would be worthwhile to do something without all the bells and whistles. I'm not shooting for a CNC lathe. I want something very straightforward, without menus or modes, no programming, no tapering or jogging, just a "continuously variable" gearbox. If the spindle is turning and the forward/reverse lever is engaged, the lead screw turns at a proportional rate. What got me started was wishing that I could slow down the cross-feed. I even bought a DC gear motor and worked up some plans, but I eventually realized that this approach would be more worthwhile.

"All" I have to do is attach a suitable encoder to the stud gear and servo motor to the lead screw, make it all fit under the cover without modifying any of the existing parts, and write a program. Nothing to it :rolleyes:.
 

T. J.

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#5
I start out pretty much all of my projects thinking like that...;)

I don't have the electronics knowledge to contribute meaningfully to a project along those lines. But I do agree that it would be an improvement to these smaller Logan lathes. I have the quick change gear box, but still the finest feed available is 0.004" per rev.
 

markba633csi

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#6
My thoughts, If I had the coding skills to do it (which I don't, yet) would be to make it a closed loop rather than an open loop one.
In other words, instead of just one optical encoder on the spindle, have two; one on the spindle and another on the screw. Then your
microcomputer would act as a comparator in addition to a ratio-changer hence it would never get out of sync. I'm sure it's been done.
Mark
ps if you didn't care about cutting threads then an open loop system might suffice, or simpler still, just a geared-down motor drive; no micro.
 
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Briney Eye

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My thoughts, If I had the coding skills to do it (which I don't, yet) would be to make it a closed loop rather than an open loop one.
In other words, instead of just one optical encoder on the spindle, have two; one on the spindle and another on the screw. Then your
microcomputer would act as a comparator in addition to a ratio-changer hence it would never get out of sync. I'm sure it's been done.
Mark
ps if you didn't care about cutting threads then an open loop system might suffice, or simpler still, just a geared-down motor drive; no micro.
Thanks, I should have been more clear. Any servo motor that I used would certainly include an encoder. I'm enamored with the Teknic Clearpath motors. This project would be an excuse to try one. Their standard models have, I think, 800 counts per revolution, so I would want a spindle encoder with at least that resolution.

I'm pretty sure that one of the Clearpath motors will do pretty much all the hard work, in fact, by connecting the spindle encoder directly to it and programming the ratio. I just have to get one of the models that I can "talk" to and then the control box really is just a front end.
 
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Briney Eye

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My thoughts, If I had the coding skills to do it (which I don't, yet) would be to make it a closed loop rather than an open loop one.
In other words, instead of just one optical encoder on the spindle, have two; one on the spindle and another on the screw. Then your
microcomputer would act as a comparator in addition to a ratio-changer hence it would never get out of sync. I'm sure it's been done.
Mark
ps if you didn't care about cutting threads then an open loop system might suffice, or simpler still, just a geared-down motor drive; no micro.
I apologize for my initial misunderstanding of your comment. A true closed loop system with the controller monitoring encoders on both the spindle and screw might be ideal, but I think I'll have to leverage the Clearpath motor's built-in brain to tell me if it gets out of sync. But that's a "feature" of this approach, anyway. I will be able to program the Clearpath so that I have an "overload clutch" on the feed, something the Model 200 sorely lacks (I know this from personal experience).

Also, after reading the Clearpath documentation more closely it's become clear that I will still have to do most of the work. While they have quadrature input and scaling capability, it's limited to multiples of 800 encoder counts.

I'm going to start by trying to implement this on an Arduino with a small touch screen, and see if I can do quadrature decoding with interrupts. At 1440rpm a 100ppr encoder will be interrupting almost 10,000 times per second, which I'm sure is at the limits of what an Arduino can do. If it can't keep up I'm looking at a programmable logic chip, a PIC32 microcontroller setup with dedicated quadrature decoding hardware, or a Cypress "Programmable System On a Chip" micro which I know supports quadrature decoding logic blocks. Years ago I did a lot of quadrature decoding with programmable logic for robotics applications. Dedicated hardware is really the way to go, so I probably won't spend too much time trying to make the Arduino work before I punt.
 

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#9
Well, the first small step is a success. I've hooked up a 200ppr (pulse per revolution) encoder to my Arduino Uno. With quadrature encoding, that equates to 800 counts per revolution. Starting with the SpeedTest sketch from the Encoder library I kludged up something that seems to be working fine at over 1400rpm. First I tried SpeedTest straight, which pulses a pin that you can integrate with a DVM and see the average voltage decrease as the interrupts come faster, and it looks like it takes about 17% of the processor time at 1400rpm. Then I substituted a print statement for the PWM code. When I spin it up my oscilloscope says one phase is running at 4.7khz. If I run it for ten seconds, it comes very close to the 188,000 expected counts. I guess I'll have to hook up the 360ppr encoder and see how that works, and then try the 600ppr encoder (which ought to soak up about 50% of the CPU cycles).

The performance is much better than I was expecting. I'll have to study the code.

It's dead quiet, too.

This is fun!
 
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RandyM

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#10
Excellent! I am glad it is working out for you with the added bonus of FUN. You are doing something most of us would not even consider. If you could add pictures of your work we'd greatly appreciate it. Photos really add a lot to the story, particularly for those of us that are more than out of our element. Keep up the good work.
 
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Briney Eye

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Excellent! I am glad you is working out for you with the added bonus of FUN. You are doing something most of us would not even consider. If you could add pictures of your work we'd greatly appreciate it. Photos really add a lot to the story, particularly for those of us that are more than out of our element. Keep up the good work.
Thanks for the encouragement. When I have something picture-worthy I intend to post it.

I think I've worked out how to make the change-over from gears a clean swap. I'm waiting for a few more parts, then I'll be making brackets for the stepper motor and rotary encoder to mount them on the banjo in place of the change gears. Timing pulleys will replace the screw and spur gears. Belt tensioning will be accomplished with the banjo. That will keep everything looking neat, fit under the cover, and be worth taking pictures of. It will also make it easy to change everything back.

I'm going to start with a 5:1 reduction between the stepper and the lead screw. With the stepper spinning 1000rpm (half max) the carriage will move the full length of the bed in under a minute, which seems fast enough to me.

I picked up a 2.8" color display with a resistive touch screen that I think, along with a knob, will make a good user interface. After I got the display I read something that implied that it used up all the Arduino's pins, but after looking at the schematic I don't see a problem.

Initially I plan to make it function purely as a continuously-variable gearbox. I suppose it will have to have a fast jog function so that the half nuts can stay engaged for metric threading. I like the idea of prompts to remind you where to engage the threading dial for a given pitch. What else do you think would be useful?
 

Briney Eye

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#12
If you don't have pictures it didn't happen, right? Baby steps.

IMG_1600.JPG

The encoder is plugged directly into the Arduino and running from 5V. You can see that the Arduino internal pullups are pretty weak.

I highly recommend these little Rigol scopes, by the way. I see that they're going for $350 on Amazon Prime. I paid $399 for mine a couple of years ago. Great little scope.
 

Briney Eye

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#13
I thought I ought to post a progress report to pique the interest of the vanishingly small number of people who might actually be interested.

The Arduino Mega is going to get it done, I think. I have the prototype interrupt-driven low-level code working well, covering inch feed rates from 0.0005" to 0.250", from 0.01mm to 4.5mm, and the pitch of every tap available from McMaster-Carr.

Then I thought "why stop there?" So I'm adding diametral pitches from 112 to 12, and module pitches from 0.2 to 7. It's no harder than doing metric feeds, so why not? I got the tables from pictures of a Victor lathe that I looked at on eBay and thought it would just be cool to do.

I'm still waiting for slow boats from China, but should have all the pieces within the next week. I do have the leadscrew timing pulley machined, but have other pulleys to adapt and brackets to make.

I'm hoping to start work on the GUI this week. Pictures soon, I hope.

All the best, folks!
 

TomKro

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#14
Plenty of interest here.

Having a simple setup that cleanly attaches at the banjo is a really good idea. There are many, many change gear Logan's out there that would benefit tremendously from this setup. My problem is that I have no electronic experience, wouldn't know where to start, and really can't add much to the conversation.

Once you show a setup that can cut a variety of thread pitches with the push of a button, you'll have plenty of requests for hardware lists and code. Keep up the good work.
 

Briney Eye

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#15
I'm still making steady progress, if anyone is interested.

I'm driving a stepper motor and it's looking really good. I couldn't get the noise immunity that I wanted on the ATmega2560 with full quadrature decoding. I just couldn't make it behave with the interrupt latency and the slow, sometimes noisy rising edges, so I changed the logic to interrupt on falling edges of one phase and simply read the state of the other. I gave up resolution, but gained very quiet and stable tracking of the spindle right down to a dead stop. I started with a 200p/r rotary encoder because 800 counts per spindle revolution just scaled really nicely to the stepper controller, and now I'm waiting for an 800p/r encoder to get that back with the new decoding scheme. I may try full decoding again when I get the voltage-output 200p/r encoder that I also have headed my way.

There are practical limitations on how fast you can feed for a given spindle RPM, just like with gears, so it will tell you what that maximum RPM is, warn you if you're trying to go too fast, and simply refuse to go if you're exceeding the limit.

I have an encoder knob hooked up and working well (with a bit of debouncing) for changing parameters in the user interface. And I have a "smart" Nextion 3.2" display connected to the Mega's third serial port which is going to take some time to figure out. The extra ports are another reason to go with the Arduino Mega. I can be debugging over Port0 while controlling the display over Port2.

I hope it won't take too much longer to get all the pieces working together on the lathe. Stay tuned.
 

Briney Eye

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#16
Prototyping the display layout. This is just a dummy layout. I haven't got it talking to the Arduino yet.

IMG_1612.JPG
 

TomKro

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#17
Can't wait to see the drive motor mount and the spindle speed pickup.

As to other features, using the display as a spindle tach when you're not driving the screw would sure be nice.

Since you're counting turns, can you create a feature to set/reset an infeed limit to help avoid crashes?

Very interesting project.
 

Briney Eye

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#18
Several iterations later it looks like this. The icon in the lower right is the placeholder for feeding to a shoulder. I still have to write that code. Actually, everything is a placeholder since it's just a mock-up. But I am talking to the display at this point:

IMG_1614.JPG
 

TomKro

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#19
When you say "talking to the display", do you mean the stepper motion and direction? Just wondering what you start with on something like this.

From what I can find on-line, there's graphical interface programs to set up the display, and separate code for the Arduino. There appear to be various "starter kits" for both the display and the control board. Any thoughts on which kits might be a good starting place for a digital dinosaur? I got off the programming train at Fortran and punch cards.
 

Briney Eye

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Frankly, I think the display has a lot more raw processing power than the Arduino Mega that I'm using for motor control.

At this point I can send commands to it over the serial port. The Mega has four serial ports. Port 0 is used in the Arduino environment as the debugging port, and I have the Nextion set up on Port 2 (because that's what their examples use). All I've done so far is send text to it, but that's most of what I'll be doing anyway. The Nextion itself can handle most of the user interface, and only send the button press events to the Arduino to let it know when something is changing that the Arduino needs to know about. I've already set up a second page to control the display brightness, but the Arduino doesn't need to know that, though I might have the Nextion check the RPM and not switch pages unless the spindle is stopped, for instance.

Nextion provides an editor to set up the display, and I'm gradually figuring it out. The display itself keeps track of when a "button" is pressed and highlights it. For the left and right feeds, for example, I have it sending a button press event and a button release event to the Arduino. That way, holding the button pressed will allow me to traverse the carriage, and feed faster the longer the button is held. All I need for the inch, metric, diametral and module buttons is the press event, though.

On the Arduino side, you have to use the name of the display object that you defined in the Nextion editor to send commands. So if I want to display RPM in the appropriate text box I send "rpm.txt=" followed by the string. There's nothing to stop you changing any number of other attributes. You could conceivably leave nearly everything undefined and configure the entire display by sending commands to it.

Communication in the other direction is different. For instance, when I press the left feed arrow, the Nextion sends 0x65 0x00 0x0E 0x01 0xFF 0xFF 0xFF to the Arduino. The first byte is the "Touch Event", the second byte is the page number, the third byte is the component ID, and the fourth byte indicates a Press, followed by a three-byte terminator. When I release the button it sends almost the same thing, except that the fourth byte becomes 0x00 indicating a Release. So you need to keep track of component names and numeric ID's on the Arduino side to manage two-way communication. Clear as mud, I'm sure.

After you've designed the Nextion display look and behavior, it creates a file that you copy to a MicroSD card and plug into the display. When power is applied it copies the file to its on-board memory and runs it. It can also be done serially, but I haven't tried that.

The display was about $30, and the Arduino Mega about $20. I also got a "shield" for the Arduino with screw terminals and a prototyping area for about $10. Arduino and Nextion provide free downloads for their software. So just buy some stuff and give it a try. My first exposure to a computer was running an IBM 1230 Mark Scoring Reader with a card punch console attached, for a Work-Study job at the University of Northern Colorado back in the early 70's. The grading scheme was "programmed" with rotary switches on the front panel. Once I had to deliver 50,000 punch cards to the real Computing Center for analysis. I was a Music major, and didn't actually get into computers for real until a few years later.
 

TomKro

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#21
Now you got me hooked.

I have zero electronic hardware, so I ordered an Uno starter kit, motor shield and a display. I was trying to get a "mega" card, but couldn't find a true Arduino made unit, and have no idea as to the quality/reliability of the other card builders. If there are any particular brands that you've found to be goods units, can you please advise? Not trying to bad mouth anyone's hardware, but the range of pricing was crazy wide.

As to the infeed shoulder stop / crash stop, I wasn't considering the operating concept properly. I'm guessing it doesn't help much to count motor turns if the half nut isn't connected all the time. Maybe some sort of carriage travel sensor would work? Can the motor controls detect a "crash" into a bed mounted travel limit, and prevent continued infeed movement?
 

Briney Eye

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#22
Pardon me, I did say that I was using an Arduino Mega. Actually I'm using an Elegoo "Mega" clone that I picked up from Amazon Prime for $14.99. So far I have zero issues, other than blowing out the bootloader with an Atmel ICE and having to figure out how to restore it. It took me a couple of hours to figure out that the hex file existed, buried in the Arduino folder structure (and now I can't remember where!!).

--- Okay, I had to figure that out. The bootloader hex file for the Mega 2560 is buried deep. It's in "Program Files (x86)/Arduino/hardware/arduino/avr/bootloaders/stk500v2/stk500boot_v2_mega2560.hex". Now I'll be able to sleep tonight :).

I've got a DIN rail Mega adapter coming that will give me screw terminals for ALL the pins, unlike the Uno shield that I have plugged in now. Kind of painful to pay more than twice as much for it as I paid for the Mega.

Assuming that I get it written, the "feed to a limit" mode will require that the half nuts be engaged at all times. Anything else will just be too much of a pain. I'll probably put a big button overlay right in the middle that says "Engage Half Nuts NOW!" Hmm, I suppose I could add a Bluetooth sniffer and steal the data stream from the TouchDRO ......
 

TomKro

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#23
Mega it is. I saw the Uno kit with wire, instruction book, etc., and figured that would be a good starting place. The motor shield was pretty cheep, so simply a low cost opportunity to look into the code (but absolutely not a programmer). I thought best to wait for feedback on the various "clones" to see if they were OK. I did notice other small touch screens that mount directly to a shield. Not sure if they have any sort of user interface to help with setup though.

As to the half nut, are you working with a DRO to locate the carriage position?

Sorry about the issue with the lost file. At that point I would have been wondering what was wrong and ordering a new board.
 

rwm

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#24
This is a brilliant idea! I can't wait to see it in action.
I have a question regarding threading up to a shoulder. If you thread up to a shoulder it seems it would be easy to stop the lead screw to prevent a crash. Is there a way to re-synchronize the spindle and the lead screw after this even if the half nut stays engaged? The thread pitch would play in to this since the ratio of spindle to lead screw turns could be greater or less than 1.0? I suppose the software could do it somehow but I can't get my head around it. Have you figured this out?
I guess this device is actually a single axis CNC on the Z axis.
I'm thinking about putting this on the far end of my lead screw and leaving my gear box intact.
Robert
 
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Briney Eye

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#25
This is a brilliant idea! I can't wait to see it in action.
I have a question regarding threading up to a shoulder. If you thread up to a shoulder it seems it would be easy to stop the lead screw to prevent a crash. Is there a way to re-synchronize the spindle and the lead screw after this even if the half nut stays engaged? The thread pitch would play in to this since the ratio of spindle to lead screw turns could be greater or less than 1.0? I suppose the software could do it somehow but I can't get my head around it. Have you figured this out?
I guess this device is actually a single axis CNC on the Z axis.
I'm thinking about putting this on the far end of my lead screw and leaving my gear box intact.
Robert
I'm keeping running totals on the spindle and lead screw, so as long as both are kept engaged it knows where they are relative to one another. Disengage either one and all bets are off. I know how many counts per revolution the spindle encoder puts out, and I know how many steps per revolution the lead screw takes.

Perhaps I should elaborate. The spindle encoder is going to be driven by the stud gear, so it won't see spindle counts unless the feed lever is engaged. It's directly analogous to using the change gears. Engage the feed lever and the lead screw will turn, but of course you don't know where the carriage is unless the half nuts are engaged.

I actually have a drive gear mounted on the tail end of my lead screw that I haven't gotten around to taking back off. I was going to try a simple gear motor drive from that end before I decided to go for full programmability. I figured I would have to pull the screw gear to use it anyway, so why not just replace that gear with a timing pulley and drive from that end? And if I was replacing the screw gear with a timing pulley, why not drive it with a stepper? And if I was driving it with a stepper, why not ...
 

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#26
I've done a short video of my bench prototype and posted it to the 'Tube. All the basic functionality is done, but I still have some "features" to program, and of course get it all installed on the lathe.
 

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#27
This is rather interesting. Taking old iron and 'computerizing' it, adding functionality and 'ease of use' to it.
I haven't used my ol' Logan for threading at all since I got the Birmingham.
 

Briney Eye

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#28
Time for a progress report. There is just barely enough room to mount a NEMA23 stepper motor with clearance for the cover. I'm waiting for another timing belt for the encoder. The ones that I have are either too short or too long, and I finally had to go directly to Misumi for a complete selection of sizes. Still have to make the encoder bracket as well. I'm still adding things to the code, the final piece being the ability to set left and right feed limits to allow feeding/threading to a shoulder.

IMG_1633.JPG
 

Richard White (richardsrelics)

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#29
This is fascinating stuff to me. I have run CNC lathes previously, and currently have a 1947 Logan that I would love to convert to full CNC to cut tapers as well as threads.....this is some cool stuff
 
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Briney Eye

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#30
This is fascinating stuff to me. I have run CNC lathes previously, and currently have a 1947 Logan that I would love to convert to full CNC to cut tapers as well as threads.....this is some cool stuff
Well, I'm keeping this simple insofar as I started off just wanting a power feed, and then the feature creep started. I wanted it to be invisible, bolt-on, and reversible. Besides the feed and threading control, I'll have control of feed direction, carriage jogging, and left/right limits. Beyond that the lathe will operate completely normally in the default mode. Once it's up and running I may work on the half nuts to take out some backlash, and maybe build a taper attachment, but it's still going to be the same manual machine -- just a lot easier to use. When I'm gone and it's in the hands of a new owner, if the electronics fail in a few minutes they can still go Old School. I don't see why it can't keep on going for decades yet (even if I can't say the same for myself!).
 
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