Sewing Machine Mods

Hey, did you know the throat on a sewing machine is usually more than 5 1/2 inches? This means you can sew booklets and signatures printed on
your computer printer. I did one last night.
Short of refitting a stepper motor, how could I make progress toward the goal of having the knee lever on my Sears Kenmore 1750 sewing machine (36 years old) control the speed of the motor more directly, rather than controlling the power input to the motor?
Sure I can oil the machine.
I was wondering if adding a cap bank to reduce the supply impedance during stalls would make a difference.
And it seems adding a bump of epoxy putty to the flywheel that would operate a leaf switch with adjustable dwell to put a variable extra resistance into the circuit at top dead center would also make a difference. This would tend to stall the machine at TDC then complete one stitch at a time.
Any ideas for sewing machine tweaks and mods?
Yours,
Doug Goncz Replikon Research Falls Church, VA 22044-0394
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The correct solution for speed control here is probably the electronic one, using electronic control of the motor to make the motor operate at a specific RPM.
One way to do this is to use a 3 phase motor and an electronic speed control, which can also do the 1 phase to 3 phase conversion. But then you would have to buy a new motor and a $200 speed control. It would be pretty bulletproof, however.
Another way to do this is with a DC motor speed controller controler that senses the motor back EMF. These are specifically used to have relatively constant motor RPM, regardless of load. That assumes that you have a motor that will run on DC, which you might. The standard controls, like dimmer switches for lights, don't have a feedback system, so just reduce power, rather than really controlling RPM.
Take a look at the motor data plate and tell us what is says. Is the motor AC/DC? How many amps? How many volts?
We can let some of the resident EE's give us some more details.
Richard
snipped-for-privacy@aol.com wrote:

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Is it possible that you would find that speed controller and that DC motor in an old exercise treadmill?
Vaughn
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On 28 Mar 2005 03:16:38 -0800, snipped-for-privacy@aol.com wrote:

Domestic sewing machines usually use universal motors, and rheostats to control speed. Rheostats are lousy speed controllers under load. You'll probably have much better results with a "router speed control" from Harbor Freight for about $20. Rig a knee lever or foot pedal to operate the potentiometer on the router speed control.
These are similar to lamp dimmers, but they use a circuit that claims some responsiveness to motor back EMF, hence speed. Details of how this works can be found in an old GE SCR manual.
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I take it the Dremel 221 foot speed control does not have the responsiveness to motor back EMF that a real router speed control would have?
Doug
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On 28 Mar 2005 17:43:52 -0800, snipped-for-privacy@aol.com wrote:

I don't know. It might be even better. The price would suggest that it should be!
It is possible to make a very stiff control, if it's "tuned" to a specific motor design. On a different motor, speed might droop under load, or might even increase with load. There used to be an IC for tachless speed control of universal motors, TDA1185, but it's been obsolete for years. I will eventually get around to making a foot control for my wife's Featherweight and 301 but it's low priority since she isn't complaining.
The router speed controls are pretty basic, but they definitely work better than a rheostat. Their response is about like that of a variable-speed corded drill.
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>>SNIP<<
| Domestic sewing machines usually use universal motors, and rheostats | to control speed. Rheostats are lousy speed controllers under load. | You'll probably have much better results with a "router speed control" | from Harbor Freight for about $20. Rig a knee lever or foot pedal | to operate the potentiometer on the router speed control. | | These are similar to lamp dimmers, but they use a circuit that claims | some responsiveness to motor back EMF, hence speed. Details of how | this works can be found in an old GE SCR manual.
I was kind curious if these were SCR or not. Good to know. Which leads me to my next question. MrTig sells a conversion kit that allows you to TIG on your buzzbox. As I understand it, they only run the welder on one AC leg rather than both, hence the 110V plug I saw in the picture. That being the case, what about using this particular router controller for that part of the conversion? Am I off base, or is this a idea worth using my OCD's (obsessive-compulsive disorder!) to consider some more?
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On Tue, 29 Mar 2005 01:58:05 GMT, "carl mciver"

A buzzbox doesn't need such a conversion to TIG. Running a 220 machine on 110 would just reduce the minimum amperage, which may be useful for TIG-welding thinner metal.
Triac-based controls don't, in general, work well with transformers because there can be some dissymetry (hence DC component) in the output waveform. This doesn't matter at all with universal motors, lights or heaters, but it can be a problem (smoke) with transformers.
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| > | > >>SNIP<< | > | >| Domestic sewing machines usually use universal motors, and rheostats | >| to control speed. Rheostats are lousy speed controllers under load. | >| You'll probably have much better results with a "router speed control" | >| from Harbor Freight for about $20. Rig a knee lever or foot pedal | >| to operate the potentiometer on the router speed control. | >| | >| These are similar to lamp dimmers, but they use a circuit that claims | >| some responsiveness to motor back EMF, hence speed. Details of how | >| this works can be found in an old GE SCR manual. | > | > I was kind curious if these were SCR or not. Good to know. Which leads | >me to my next question. MrTig sells a conversion kit that allows you to TIG | >on your buzzbox. As I understand it, they only run the welder on one AC leg | >rather than both, hence the 110V plug I saw in the picture. That being the | >case, what about using this particular router controller for that part of | >the conversion? | > Am I off base, or is this a idea worth using my OCD's | >(obsessive-compulsive disorder!) to consider some more? | | A buzzbox doesn't need such a conversion to TIG. Running a 220 | machine on 110 would just reduce the minimum amperage, which may be | useful for TIG-welding thinner metal. | | Triac-based controls don't, in general, work well with transformers | because there can be some dissymetry (hence DC component) in the | output waveform. This doesn't matter at all with universal motors, | lights or heaters, but it can be a problem (smoke) with | transformers.
Wonder if it's worth 20 bucks to figure it out. My Buzzbox is an old (!)AC 180 amp with selector. No fine control, and a variable control would make it nice, especially for someone who needs to learn.
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use a DC motor controller - I have a pile of them for $25 and up - you use a resistance to set the speed and it will use back emf from the coil for feedback. I don't have any up on e-bay right now, but I have a lot of suitable ones sitting here waiting for someone in need. I have a lot of MM23012D controllers, see my web site, www.wbnoble.com and look around - they are under "controllers"
bill n

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I have a table top speed controller for my Foredom. If I set the knob for a nice no load speed, the motor slows down under load. Will one of these speed controllers work on the foredom?
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Dear William,
You use a resistance, not a potentiometer, to set the speed? That would be ideal, because with a compatible controller, I could use the rheostat in the sewing machine as the resistance, maintaing the ergonomics.
The rheostat is rated 1A. The motor is rated 1.2 A, 110/120 V AC / DC, 25/60 Hz.
It's a universal motor.
Are you sure you use a resistance to set the speed?
I have to measure the rheostat's resistance. Although I know it goes from open to shorted, I wonder what value it has half way.
Doug
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snipped-for-privacy@aol.com wrote:

Potentiometer/rheostat - They're two words for the same thing: An adjustable resistance. A pot is often (not always, but...) "lighter duty" than a rheostat - A carbon-film or similar construction pot might cook off under half an amp of load, where the heavier-built rheostat might well be able to cope with tens, even hundreds or thousands, of amps without so much as "breaking a sweat" - but otherwise, they perform the exact same task.
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    [ ... ]

    Not quite. In theory, both are variable resistances, but the intended application (and thus the design) is different.
    A rheostat can be a two-terminal device with variable restance between the fixed terminal and the moving terminal (which can be a rotary wiper, or a sliding tap on a cylindrical wirewond resistor). Its function is to be an adjustable *series* resistor. (Granted, there is often a third terminal at the other end present, simply because it costs so little to add it, and it offers the choice of a rheostat whose resistance increased with CW motion, or one whose resistance decreases with CW motion.)
    Most rheostats are low enough in resistance value so they would consume far too much power wired as a voltage divider.
    A potentiometer is a an adjustable voltage divider. Three terminals -- two fixed ones at the ends of the resistance element, and one adjustable one -- usually (but not always) moved by rotating a shaft. You hook one end to your reference ground and the other to a known voltage, and obtain a variable voltage (at fairly low current capacity) from the third terminal.

    The foot controller for my Singer 221B is a stack of carbon blocks, whose total resistance is an inverse function of how hard you press of the foot control. The resistance at maximum (excluding wide open with no pressure) is too low to serve as a control element on a VFD at least -- which typically wants a 10K potentiometer to adjust a 10V or 5V reference signal down to the desired control voltage. I don't know about the controllers for DC motors,but I would expect something nearly the same.
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On 29 Mar 2005 12:31:12 -0800, snipped-for-privacy@aol.com wrote:

Your sewing machine rheostat will not work with these controllers. You'd need to devise an ergonomic design incorporating a pot compatible with the controller: 5W pot, 10K IIRC.
I intend to move forward with this if Mr. Noble responds to my inquiry re the Minarik controllers. Ergonomics is definitely an issue. I'm entertaining the notion of using a straingage interface because, while a pot would work, a straingage/spring offers much more latitude for ergonomic experimentation. I never make more than one of anything so I don't have to be "practical". This is for my wife the quilter.
I initially thought the straingage idea was whimsical, justly deserving of rockets from Jeff Wisnia about gilding turds, but the more I think about it the more I like it because it offers nearly total freedom in design of the footpedal package. The whole works could be contained in the space of the toe-control button of a Singer Featherweight foot control, and such a design could be shit-simple to adjust in terms of travel and force to the preference of her nibs.
Having some straingages and some instrumentation opamps in my goodiebox may color my thinking a bit...
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I was thinking of a regulator design similar to the alternator voltage regulator in my DJ-5A of 1969 vintage, the one I owned from 1983 to 1985.
Apropos the antique technology in place, a coil relay would accept the rheostat signal and PWM a low impedance switched contact supply to the motor, maybe using one or more incadescent bulbs as current regulator / constant current references. This is normally done with low currents, though, to obtain reasonable contact life.
With the rheostat, an increase in motor load increases voltage drop, decreasing supplied voltage.
In 1969, one might have had two light bulbs in a sheet metal box with a photoelectric cell balancing their brightness and hence current flow. Or would semiconductors have been available then?
I have disassembled the rheostat housing. A 16 gage or thicker steel box contains a single ceramic component with two electrical #8 screw terminals and an opposing stud. Knee pressure _releases_ pressure on the stud, and presumably a t-bar within reaching nearly all the way to the terminals compresses carbon buttons, similar to a carbon microphone in operation at "sound" frequencies well below one Hz.
I too have a maxim about polishing or gilding a turd. The soft surface makes the effort expended last only a short while.
I believe I owned a Minarik controller sold by surpluscenter.com some time ago. I no longer have it. I don't think surpluscenter.com sells them any more. Minarik does, and IIRC there was online documentation. I will go have a look.
Doug
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Minarik confirms in their MM23000 series manual,
http://www.minarik.com/PDFs/250-0091.pdf
the pwm is controlled by a voltage, not a resistance, so wiring the rheostat as a pot with parallel and series resistors would produce a controllable range. However, the ergonomics might not work out.
It takes a 5K pot, but that's just what is engineered into the current supply for the voltage divider. The actual control is done by a voltage, 0-1.4 VDC if I recall what I read correctly, for 0-90 VDC using a 110 VAC supply.
Their mm23011 controller looks suitable. I think that is what I owned. Now, where to buy one?
Doug
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Hi Group, From an old Sewing Machine Mechanic. Unless you want to have greatly variable speed, why don't you just add a smaller pulley on the motor and shift the belt over. Jack

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Why did I put a multi-hundred watt DC generator on my recumbent bicycle and go around the neighborhood with 24 watts of Christmas lights on New Year's Eve? Because I can.
Why do I want to put an electronic control on my sewing machine instead of adding a smaller pulley on the motor? Because I can.
Peace and Love,
Doug
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There's no room to stack the knee control rheostat cases face to face.
They have to go side by side. That means I have to saw off one face of each box and have them welded, or braze them, then add an insulating spacer between the rheostats. But the opposed rheostat has to opearate in reverse and the sping load isn't right. The one directly operated by the knee level is biased internall to short and biased by the lever to open. So there would need to be a spring or spring pin on the opposed rheostat to make it follow the other one, starting at short, and ending at open when the lever removes the external bias on the unmodified rheostat to short it.
Doug
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