# A different way cover



## homebrewed (Dec 7, 2020)

This project is an example of finding an application for an idea I had for using a DC motor as a constant torque device.  The concept:  when stalled, the torque a brushed DC motor exerts is proportional to the current flow through it.  Obviously, to avoid burning it up you don't apply full voltage to it.

In this case, the application is an unusual kind of way cover for my mini lathe:




The arrangement is a bit like a miniature roller blind for a window.  It's not easy to see, but the way cover is a sheet of mylar wrapped around the roller.  Not shown:  a cheap PWM motor controller to adjust the drive level to the motor, so it exerts enough torque so the mylar sheet is kept taut but not burn up.

I used ball bearings for the cylinder and motor pulley, but properly-made bushings would work.  The bearings I had on hand were pretty stiff due to the grease in them so I soaked them in mineral spirits and then used light lubricating oil -- they aren't under any kind of a load so light oil will be just fine for this.

The little 12V motor is barely up to the task so at some point I may have to get a heftier motor, or greatly increase the reduction ratio in the pulleys.  Gear reduction motors would be much stronger but I can't hand turn the shaft on the one I have (the motor has to have some "give" so the mylar can be unrolled when the carriage is moved to the right). 

Right now the plastic is just taped to the carriage.  The final version will use the steady rest mounting holes and an aluminum bar for a clamp.

I set the roller length long enough so the lead screw also is covered.  Hopefully that will help keep swarf off the LS, too.

The only items I needed to buy for this were the bearings (VXB), motor (Jameco), O-ring (local H/W store) and PWM controller (ebay).

Yes, mounting bellows on the lathe would be much simpler, but not near as much fun.


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## benmychree (Dec 7, 2020)

Using a cord wrapped around the roller with a weight attached and dangling down would be a lot simpler, but I guess not much fun either.


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## RJSakowski (Dec 7, 2020)

Another approach would be to use a window shade mechanism.  Defeat the pawl so there is always tension on the cover.


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## NC Rick (Dec 7, 2020)

homebrewed said:


> This project is an example of finding an application for an idea I had for using a DC motor as a constant torque device.  The concept:  when stalled, the torque a brushed DC motor exerts is proportional to the current flow through it.  Obviously, to avoid burning it up you don't apply full voltage to it.
> 
> In this case, the application is an unusual kind of way cover for my mini lathe:
> 
> ...


i Love the novel approach and the excellent execution.  I'm having fun imagining where the inspiration came from.


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## homebrewed (Dec 7, 2020)

benmychree said:


> Using a cord wrapped around the roller with a weight attached and dangling down would be a lot simpler, but I guess not much fun either.


Yep, and that was my first thought.  I eventually didn't like the idea of having a cable potentially interfering with other stuff on the bench so I went with this more compact design.


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## Shootymacshootface (Dec 7, 2020)

Your screen name should be Rube Goldberg.


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## homebrewed (Dec 7, 2020)

RJSakowski said:


> Another approach would be to use a window shade mechanism.  Defeat the pawl so there is always tension on the cover.


McMaster-Carr has something like that.....you too can own one for $393.33!  $$$

I looked around on the web to find info on how roller blinds work but didn't find anything regarding the "motor" part of the mechanism.  I bet some folks come up with the info now!


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## homebrewed (Dec 7, 2020)

Shootymacshootface said:


> Your screen name should be Rube Goldberg.


They called me McGyver at work


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## macardoso (Dec 7, 2020)

I agree with a clock spring mechanism to keep tension. It would accomplish the same thing without the need for external power or driver circuitry.

Cool application and nice work either way!


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## NC Rick (Dec 7, 2020)

macardoso said:


> I agree with a clock spring mechanism to keep tension. It would accomplish the same thing without the need for external power or driver circuitry.
> 
> Cool application and nice work either way!


A spring mechanism would not provide "constant torque".  I am fine with over thinking stuff.


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## RJSakowski (Dec 7, 2020)

NC Rick said:


> A spring mechanism would not provide "constant torque".  I am fine with over thinking stuff.











						McMaster-Carr
					

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.




					www.mcmaster.com


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## NC Rick (Dec 7, 2020)

Also, a counterweight would require a reduction mechanism or a fairly deep hole in the ground for my application.


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## NC Rick (Dec 7, 2020)

See, this kind of banter leads to learning stuff.  I may order one just to play with it!


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## Winegrower (Dec 7, 2020)

Well, an interesting idea.   Following your “concept”, how about choose a DC motor with the right torque constant, no gear train, no pwm controller, just a constant current source to maintain desired torque.   This is how some devices maintain tension on mag tapes, for example.

But really, I have never felt a need for way covers.   Or pocket protectors, car bras, etc.


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## benmychree (Dec 7, 2020)

Winegrower said:


> Well, an interesting idea.   Following your “concept”, how about choose a DC motor with the right torque constant, no gear train, no pwm controller, just a constant current source to maintain desired torque.   This is how some devices maintain tension on mag tapes, for example.
> 
> But really, I have never felt a need for way covers.   Or pocket protectors, car bras, etc.


My sentiments, exactly!


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## benmychree (Dec 7, 2020)

It seems to me that way covers always allow crud to accumulate underneath where it can't be seen, and cause damage unobserved.  When I worked at Kaiser Steel, we retrofitted a bunch of old planers with milling heads using 24" and smaller carbide insert face mills to mill various weld preps and to finish to width and with caulking grooves, the tunnel liner segments for BART in the SF bay area for the underground parts of the system, they made way covers that rolled up, made of cloth inserted rubber; what happened, when they rolled up, the stuff on top transferred to the underside, dropped off and got onto the way surfaces and caused a lot of damage which was not repaired, when the job was finished, most all the machinery was scrapped -----


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## homebrewed (Dec 8, 2020)

benmychree said:


> It seems to me that way covers always allow crud to accumulate underneath where it can't be seen, and cause damage unobserved.  When I worked at Kaiser Steel, we retrofitted a bunch of old planers with milling heads using 24" and smaller carbide insert face mills to mill various weld preps and to finish to width and with caulking grooves, the tunnel liner segments for BART in the SF bay area for the underground parts of the system, they made way covers that rolled up, made of cloth inserted rubber; what happened, when they rolled up, the stuff on top transferred to the underside, dropped off and got onto the way surfaces and caused a lot of damage which was not repaired, when the job was finished, most all the machinery was scrapped -----


Perhaps that's an unintended "benefit" of using mylar -- it's transparent!


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## homebrewed (Dec 8, 2020)

Winegrower said:


> Well, an interesting idea.   Following your “concept”, how about choose a DC motor with the right torque constant, no gear train, no pwm controller, just a constant current source to maintain desired torque.   This is how some devices maintain tension on mag tapes, for example.
> 
> But really, I have never felt a need for way covers.   Or pocket protectors, car bras, etc.


A constant current generator will generate heat, unlike the switch(es) in a PWM controller. I initially considered a constant current source but decided a cheap PWM controller would suffice, without the need to worry about power dissipation in the controller.

I agree that a big enough DC motor could be direct coupled.   If I find one for a reasonable price I'll give it a shot.  However, it will be hard to beat the $3 motor I've got in there right now.  BLDC motors can be very powerful for the money, but ESC's aren't suited for this kind of application.  Maybe a sensored BLDC motor + an Arduino?  Now it gets even more Rube-Goldberg-ish.  And sensored BLDC's cost more.


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## benmychree (Dec 8, 2020)

homebrewed said:


> Perhaps that's an unintended "benefit" of using mylar -- it's transparent!


Transparent for how long?


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## NC Rick (Dec 8, 2020)

homebrewed said:


> A constant current generator will generate heat, unlike the switch(es) in a PWM controller. I initially considered a constant current source but decided a cheap PWM controller would suffice, without the need to worry about power dissipation in the controller.
> 
> I agree that a big enough DC motor could be direct coupled.   If I find one for a reasonable price I'll give it a shot.  However, it will be hard to beat the $3 motor I've got in there right now.  BLDC motors can be very powerful for the money, but ESC's aren't suited for this kind of application.  Maybe a sensored BLDC motor + an Arduino?  Now it gets even more Rube-Goldberg-ish.  And sensored BLDC's cost more.


Go old school man, use a micro-switch with a lightweight arm extension mounted on the saddle that becomes activated by the mylar cover being under tension (using the N.C. contacts) to power the motor.


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## homebrewed (Dec 8, 2020)

benmychree said:


> Transparent for how long?


Good point.  I guess time will tell, if it's invited to stay on the lathe  . 

Mylar isn't real expensive if it comes to replacement.  I'm actually using Duralar, found in art supply stores.  A book of 25 11 x 14 sheets costs about $15 through Blick, not including shipping.


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## homebrewed (Dec 8, 2020)

NC Rick said:


> Go old school man, use a micro-switch with a lightweight arm extension mounted on the saddle that becomes activated by the mylar cover being under tension (using the N.C. contacts) to power the motor.


I like it!


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## NC Rick (Dec 8, 2020)

Or, when it goes slack, the marbles are released and...  
anything at all to avoid programming or computers!


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## GunsOfNavarone (Dec 8, 2020)

If complicating it, but making it cooler is welcomed, 1 encoder and one motor. The encoder is tied to the motor in a one to one ratio, if the encoder moves 1", the motor moves 1". Probably a bit less complicated than what you have but totally clean/reliable. I think a simple Arduino board aught to do the trick!


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