D
DaveC
Guest
Can you get just thermal overloads without a contactor?
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Can you get just thermal overloads without a contactor?
Guest
>"Burnt bits of saw dust ..."
Bandsaws are also used to cut metal. No burning necessary.
Bandsaws are also used to cut metal. No burning necessary.
J
Jim Wilkins
Guest
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CEF75B1E01445600B01029BF@news.eternal-september.org...
The problem is that the contactor needs a bulky and expensive sealed
enclosure. I've wired my machinery with waterproof outdoor boxes and
flex conduit from big-box stores, but they don't carry large JIC boxes
with watertight doors.
http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
news:0001HW.CEF75B1E01445600B01029BF@news.eternal-september.org...
The reason to use a contactor instead of just an on / off switch is
that the
insides of the switch can get full of saw dust and turn on the saw
with the
switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed
switches with lower current ratings which is enough to engage the
contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be
an issue (actually the friend would appreciate the saw continuing
the cut
after power comes back!), but the "auto turn-on" feature )c: is one
I hadn't
thought of.
Thanks!
The problem is that the contactor needs a bulky and expensive sealed
enclosure. I've wired my machinery with waterproof outdoor boxes and
flex conduit from big-box stores, but they don't carry large JIC boxes
with watertight doors.
http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
S
SteveF
Guest
On 1/12/2014 8:01 AM, Jim Wilkins wrote:
A small IEC motor starter may be used with a cheap plastic enclosure. 4
screw JIC enclosures will suffice. 6x8x4 inch should be adequate. If
the motor has internal overload protection, smaller contactors/
enclosures may be used.
The problem is that the contactor needs a bulky and expensive sealed
enclosure. I've wired my machinery with waterproof outdoor boxes and
flex conduit from big-box stores, but they don't carry large JIC boxes
with watertight doors.
http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
A small IEC motor starter may be used with a cheap plastic enclosure. 4
screw JIC enclosures will suffice. 6x8x4 inch should be adequate. If
the motor has internal overload protection, smaller contactors/
enclosures may be used.
S
SteveF
Guest
On 1/12/2014 1:56 AM, DaveC wrote:
Yes.
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
i
cc
Can you get just thermal overloads without a contactor?
Yes.
S
SteveF
Guest
On 1/12/2014 8:38 AM, SteveF wrote:
something requiring a size 4 motor starter or something on that order.
Note: The above is in reference to smaller motors, in general. NotOn 1/12/2014 8:01 AM, Jim Wilkins wrote:
The problem is that the contactor needs a bulky and expensive sealed
enclosure. I've wired my machinery with waterproof outdoor boxes and
flex conduit from big-box stores, but they don't carry large JIC boxes
with watertight doors.
http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
A small IEC motor starter may be used with a cheap plastic enclosure. 4
screw JIC enclosures will suffice. 6x8x4 inch should be adequate. If
the motor has internal overload protection, smaller contactors/
enclosures may be used.
something requiring a size 4 motor starter or something on that order.
F
Fred Abse
Guest
On Sun, 12 Jan 2014 07:23:14 +0100, Sjouke Burry wrote:
There's a difference. European 240 (nominally now 230) volt, single
phase, supplies have one pole at or near ground, being derived from one
phase of a 400 volt, three phase, supply, and (grounded) star point
neutral. A single pole switch on the "hot", (or "live"), side is
sufficient.
North American 220 volt, single phase, supplies have no neutral pole,
being derived from a 220 volt, single phase, transformer winding, which is
center tapped to ground (neutral). Either pole is at 110 volts above
ground. Two pole switching (and fusing), is necessary.
--
"Design is the reverse of analysis"
(R.D. Middlebrook)
Thats just silly. 99 percent of switches in european houses work just
fine with 240 volts. Only the door bell and the thermostat runs at low
volts.....
There's a difference. European 240 (nominally now 230) volt, single
phase, supplies have one pole at or near ground, being derived from one
phase of a 400 volt, three phase, supply, and (grounded) star point
neutral. A single pole switch on the "hot", (or "live"), side is
sufficient.
North American 220 volt, single phase, supplies have no neutral pole,
being derived from a 220 volt, single phase, transformer winding, which is
center tapped to ground (neutral). Either pole is at 110 volts above
ground. Two pole switching (and fusing), is necessary.
--
"Design is the reverse of analysis"
(R.D. Middlebrook)
L
Leon
Guest
On 1/12/2014 12:45 AM, John Robertson wrote:
Regardless, there is still electricity in that switch, just a lower
voltage. Burnt bits of saw dust can burn at a lower voltage sparks as
well as a higher voltage spark.
I suspect that the previous poster witnessed a defective or broken switch.
FWIW this links to the purpose and or how a magnetic switch/contactor works.
http://www.ask.com/question/how-does-a-magnetic-contactor-work
On 01/11/2014 8:41 PM, Mike Marlow wrote:
DaveC wrote:
The reason to use a contactor instead of just an on / off switch is
that the insides of the switch can get full of saw dust and turn on
the saw with the switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed switches with lower current ratings which is enough to engage
the contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be an issue (actually the friend would appreciate the saw
continuing the cut after power comes back!), but the "auto turn-on"
feature )c: is one I hadn't thought of.
Thanks!
It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator. Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work. It would
probably be
hard to convince your friend their is a real risk here. For good
reason...
Wood may be an insulater but carbon certainly isn't. I suspect what the
provious poster was talking about was burnt bits of sawdust being
conductive. They would be burnt by the sparks present when the switch
was opened...leading to a potentially hazardous situation!
John :-#(#
Regardless, there is still electricity in that switch, just a lower
voltage. Burnt bits of saw dust can burn at a lower voltage sparks as
well as a higher voltage spark.
I suspect that the previous poster witnessed a defective or broken switch.
FWIW this links to the purpose and or how a magnetic switch/contactor works.
http://www.ask.com/question/how-does-a-magnetic-contactor-work
J
Jim Wilkins
Guest
"SteveF" <lost@found.net> wrote in message
news:lau9c4$mn6$1@dont-email.me...
That's up to you.
I designed industrial controls to GM's specs which required NEMA-rated
enclosures and conduit that gave reasonable protection from a fork
lift, and interconnecting wiring no smaller than 16 AWG for mechanical
strength.
Hobby machinery may not be subject to the same level of abuse, but it
wasn't initially designed with protected spaces for add-on controls
and wiring. I've milled connector openings in a lot of 'cheap plastic
enclosures', usually ABS Hammond boxes from Digikey, and seen how
easily they fracture. Personally I like metal weatherproof outlet
boxes on machines that throw wood or metal chips, or plastic for low
voltage and safer locations.
news:lau9c4$mn6$1@dont-email.me...
On 1/12/2014 8:01 AM, Jim Wilkins wrote:
The problem is that the contactor needs a bulky and expensive
sealed
enclosure. I've wired my machinery with waterproof outdoor boxes
and
flex conduit from big-box stores, but they don't carry large JIC
boxes
with watertight doors.
http://www.austinenclosures.com/products/view/JIC_Continuous_Hinge_Boxes/
A small IEC motor starter may be used with a cheap plastic
enclosure. 4
screw JIC enclosures will suffice. 6x8x4 inch should be adequate.
If
the motor has internal overload protection, smaller contactors/
enclosures may be used.
That's up to you.
I designed industrial controls to GM's specs which required NEMA-rated
enclosures and conduit that gave reasonable protection from a fork
lift, and interconnecting wiring no smaller than 16 AWG for mechanical
strength.
Hobby machinery may not be subject to the same level of abuse, but it
wasn't initially designed with protected spaces for add-on controls
and wiring. I've milled connector openings in a lot of 'cheap plastic
enclosures', usually ABS Hammond boxes from Digikey, and seen how
easily they fracture. Personally I like metal weatherproof outlet
boxes on machines that throw wood or metal chips, or plastic for low
voltage and safer locations.
Guest
On Sat, 11 Jan 2014 22:45:56 -0800, John Robertson <spam@flippers.com>
wrote:
wrote:
On 01/11/2014 8:41 PM, Mike Marlow wrote:
DaveC wrote:
The reason to use a contactor instead of just an on / off switch is
that the insides of the switch can get full of saw dust and turn on
the saw with the switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed switches with lower current ratings which is enough to engage
the contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be an issue (actually the friend would appreciate the saw
continuing the cut after power comes back!), but the "auto turn-on"
feature )c: is one I hadn't thought of.
Thanks!
It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator. Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work. It would probably be
hard to convince your friend their is a real risk here. For good reason...
Wood may be an insulater but carbon certainly isn't. I suspect what the
provious poster was talking about was burnt bits of sawdust being
conductive. They would be burnt by the sparks present when the switch
was opened...leading to a potentially hazardous situation!
John :-#(#
Or dust physically jamming the motion of the switch contacts - - -
G
Gunner Asch
Guest
On Sun, 12 Jan 2014 13:56:33 -0500, clare@snyder.on.ca wrote:
Indeed.
I do service work on the side for a So Cal tool buyer/seller and the
number of switches Ive pulled out of wood working machinery jammed
solid with sawdust is huge. Even so called "sealed" switches.
Gunner
__
"Anyone who things Obama is doing a good job
is either stupid or a perpetual societal leech"
---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
On Sat, 11 Jan 2014 22:45:56 -0800, John Robertson <spam@flippers.com
wrote:
On 01/11/2014 8:41 PM, Mike Marlow wrote:
DaveC wrote:
The reason to use a contactor instead of just an on / off switch is
that the insides of the switch can get full of saw dust and turn on
the saw with the switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed switches with lower current ratings which is enough to engage
the contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be an issue (actually the friend would appreciate the saw
continuing the cut after power comes back!), but the "auto turn-on"
feature )c: is one I hadn't thought of.
Thanks!
It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator. Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work. It would probably be
hard to convince your friend their is a real risk here. For good reason...
Wood may be an insulater but carbon certainly isn't. I suspect what the
provious poster was talking about was burnt bits of sawdust being
conductive. They would be burnt by the sparks present when the switch
was opened...leading to a potentially hazardous situation!
John :-#(#
Or dust physically jamming the motion of the switch contacts - - -
Indeed.
I do service work on the side for a So Cal tool buyer/seller and the
number of switches Ive pulled out of wood working machinery jammed
solid with sawdust is huge. Even so called "sealed" switches.
Gunner
__
"Anyone who things Obama is doing a good job
is either stupid or a perpetual societal leech"
---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
G
Gunner Asch
Guest
On Sat, 11 Jan 2014 23:56:48 -0800, DaveC <invalid@invalid.net> wrote:
Yes..but they are NOT..NOT fuses or circuit breakers. They are
designed to carry SMALL current loads..often times much smaller than
the motors they are protecting.
They may read the current..but the internal switches are too small to
handle repeated ons/offs
__
"Anyone who things Obama is doing a good job
is either stupid or a perpetual societal leech"
---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
i
cc
Can you get just thermal overloads without a contactor?
Yes..but they are NOT..NOT fuses or circuit breakers. They are
designed to carry SMALL current loads..often times much smaller than
the motors they are protecting.
They may read the current..but the internal switches are too small to
handle repeated ons/offs
__
"Anyone who things Obama is doing a good job
is either stupid or a perpetual societal leech"
---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com
D
DoN. Nichols
Guest
On 2014-01-12, DaveC <invalid@invalid.net> wrote:
Not really. The thermal overloads sense the current in each
winding, and generate heat to soften either a wax or a low temperature
solder to allow a wheel to rotate and a low-current contact to open.
All of the phases have their contacts in series, and in series with the
coil of the contactor. Any one of them opening drops the contactor out
to protect the load.
With single phase, a slow-blow fuse in series with each hot lead
should sufficient, but with three phases, you need to sense any
condition of over-current in any single phase, and to drop everything
once it is sensed. (Of course, if you are running the three-phase motor
from a VFD, that incorporates the functions of the starter (current
sensing and automatic shutdown) in the electronics of the VFD.
Enjoy,
DoN.n
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Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
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--- Black Holes are where God is dividing by zero ---
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
i
cc
Can you get just thermal overloads without a contactor?
Not really. The thermal overloads sense the current in each
winding, and generate heat to soften either a wax or a low temperature
solder to allow a wheel to rotate and a low-current contact to open.
All of the phases have their contacts in series, and in series with the
coil of the contactor. Any one of them opening drops the contactor out
to protect the load.
With single phase, a slow-blow fuse in series with each hot lead
should sufficient, but with three phases, you need to sense any
condition of over-current in any single phase, and to drop everything
once it is sensed. (Of course, if you are running the three-phase motor
from a VFD, that incorporates the functions of the starter (current
sensing and automatic shutdown) in the electronics of the VFD.
Enjoy,
DoN.n
--
Remove oil spill source from e-mail
Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
M
Mike Marlow
Guest
DoN. Nichols wrote:
You are absolutely correct Don - and thank you for that correction. I got
sidetracked by the one poster who mentioned that he had once seen sawdust
create a short. While I disagree that such is a common or even an
anticipated occurance, I do acknowkedge your clarification - you point is
well taken.
Yes... and the earth could have been flat before centrifigul force changed
all that. My point Don is that yes - things are possible - probable, or
even common in the real world is something different.
I will concede that anything is possible - even my wife deciding... well...
we won't go there... Just because something is possible does not put it in
the realm of probable, practical, or even worth thinking about. I am not
saying that your comments are not worth consideration with that statement,
I'm simply trying to make your statement that it is at least possible as
meaningless in its context as I can.
--
-Mike-
mmarlowREMOVE@windstream.net
And while wood may be an insulator, horizontal bandsaws (which I
remember from the earlier articles in this thread, and which I
use) are more commonly used to cut metal. And metal swarf is
certainly conductive.
You are absolutely correct Don - and thank you for that correction. I got
sidetracked by the one poster who mentioned that he had once seen sawdust
create a short. While I disagree that such is a common or even an
anticipated occurance, I do acknowkedge your clarification - you point is
well taken.
B) Toggle switches typically have a spring-loaded actuator which
bears on a metal plate which rocks back and forth to bridge
contacts or not bridge them.
Even a buildup of wood sawdust or plastic dust *could* trap the
metal rocker in a position to keep the bridging happening, even
when the toggle is in the "off" position.
Yes... and the earth could have been flat before centrifigul force changed
all that. My point Don is that yes - things are possible - probable, or
even common in the real world is something different.
It is at least a *possible* one.
I will concede that anything is possible - even my wife deciding... well...
we won't go there... Just because something is possible does not put it in
the realm of probable, practical, or even worth thinking about. I am not
saying that your comments are not worth consideration with that statement,
I'm simply trying to make your statement that it is at least possible as
meaningless in its context as I can.
--
-Mike-
mmarlowREMOVE@windstream.net
I
Ignoramus28861
Guest
On 2014-01-13, DoN. Nichols <BPdnicholsBP@d-and-d.com> wrote:
No, on bandsaw, the main problem is that it runs unattended and can
easily get stuck.
i
On 2014-01-12, Ignoramus13867 <ignoramus13867@NOSPAM.13867.invalid> wrote:
[ ... ]
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
This is more of a factor with three phase motors, where a motor
once started will continue to run if one phase of power fails, but the
current drawn in each remaining winding will be higher, and will be
likely to burn out the motor. (And he was going to add a 240 VAC single
phase motor to the bandsaw in question.)
No, on bandsaw, the main problem is that it runs unattended and can
easily get stuck.
i
D
DoN. Nichols
Guest
On 2014-01-12, Ignoramus13867 <ignoramus13867@NOSPAM.13867.invalid> wrote:
[ ... ]
[ ... ]
[ ... ]
This is more of a factor with three phase motors, where a motor
once started will continue to run if one phase of power fails, but the
current drawn in each remaining winding will be higher, and will be
likely to burn out the motor. (And he was going to add a 240 VAC single
phase motor to the bandsaw in question.)
A proper motor starter (of which the contactor is a part)
includes thermal sensors for the current through each winding, and if
any one of the three goes well above the normal full-load running
current (as will happen if one phase dies), it will interrupt the power
to the motor and stop it before the motor is damaged. And it also
protects the windings from burning out when trying to start with one
phase of three missing.
And actually, some starters only measure the current through two
of the three legs, because if one is out, one of the two monitored will
be over-current enough to trip it.
So -- with a single phase 240 VAC motor, the over-current from
loss of a phase would not apply.
But the other reasons for using a motor starter/contactor still
stand.
Enjoy,
DoN.
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Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
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--- Black Holes are where God is dividing by zero ---
[ ... ]
On 2014-01-12, DaveC <invalid@invalid.net> wrote:
A friend (in USA) with a 220v, 1-phase horizontal bandsaw in his home machine
shop wants to replace the 1/4 hp motor with 1/2 hp and rewire with an
auto-off switch (when the blade cuts through the metal).
I always default to using a 3-wire control with contactor, but he asks the
logical question: "Why?"
[ ... ]
What are the arguments for using a contactor as opposed to a simple on-off
switch of equivalent rating?
[ ... ]
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
i
This is more of a factor with three phase motors, where a motor
once started will continue to run if one phase of power fails, but the
current drawn in each remaining winding will be higher, and will be
likely to burn out the motor. (And he was going to add a 240 VAC single
phase motor to the bandsaw in question.)
A proper motor starter (of which the contactor is a part)
includes thermal sensors for the current through each winding, and if
any one of the three goes well above the normal full-load running
current (as will happen if one phase dies), it will interrupt the power
to the motor and stop it before the motor is damaged. And it also
protects the windings from burning out when trying to start with one
phase of three missing.
And actually, some starters only measure the current through two
of the three legs, because if one is out, one of the two monitored will
be over-current enough to trip it.
So -- with a single phase 240 VAC motor, the over-current from
loss of a phase would not apply.
But the other reasons for using a motor starter/contactor still
stand.
Enjoy,
DoN.
--
Remove oil spill source from e-mail
Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
D
DoN. Nichols
Guest
On 2014-01-12, Mike Marlow <mmarlowREMOVE@windstream.net> wrote:
A) Note the newsgroups to which this is cross-posted:
sci.electronics.misc
sci.electronics.repair
rec.crafts.metalworking
rec.woodworking
And while wood may be an insulator, horizontal bandsaws (which I
remember from the earlier articles in this thread, and which I
use) are more commonly used to cut metal. And metal swarf is
certainly conductive.
B) Toggle switches typically have a spring-loaded actuator which
bears on a metal plate which rocks back and forth to bridge
contacts or not bridge them.
Even a buildup of wood sawdust or plastic dust *could* trap the
metal rocker in a position to keep the bridging happening, even
when the toggle is in the "off" position.
And most of those switches are not located near the point of
generation of metal swarf as they are on the typical inexpensive
horizontal bandsaw. The airborne sawdust in a wood-working shop is not
likely to be a problem. However, metal chips dribbled from the blade
(after going almost a full turn around the path of the blade) are a
possibility, depending on the construction of the switch. (And, they
could also conduct from the switch terminals to the chassis of the saw,
if they got under the frame.) Certainly a sealed limit switch (more
commonly found on serious machine tools) would be free of that
particular problem. They are a small MicroSwitch module inside a metal
housing with O-rings to keep out oil as well as chips. They are
operated by a cam, also sealed, or an external plunger -- operating
through a sealed path. (Honeywell is one maker -- and they *are* quite
expensive, but the best thing where swarf will build up.) The wiring
comes in through a fitting which normally has a tapered gasket which
compresses around oil-proof wiring, so there is no path for the chips in
through there either. And yes, the are best used at low voltages and
low currents. I recently re-designed the limit switches in a conversion
of a CNC milling machine, and needed to get switches of the same
physical mounting but which had two separate circuits depending on which
direction they were operated in.)
It is at least a *possible* one. Now, if the power is fed to
this through a wall-mounted disconnect switch, so you could stop the
bandsaw without having to unplug it (which might be difficult to reach
with the saw still running), that would be less of a problem. It would
be nice to have a warning LED near where you would reach to change the
clamping of the workpiece, so you know the saw is still running,
especially of other noisy tools are being run at the same time in the
shop. A horizontal bandsaw is typically fairly quiet, unless it is
cutting relatively thin metal.
Enjoy,
DoN.
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Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
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--- Black Holes are where God is dividing by zero ---
DaveC wrote:
The reason to use a contactor instead of just an on / off switch is
that the insides of the switch can get full of saw dust and turn on
the saw with the switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed switches with lower current ratings which is enough to engage
the contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be an issue (actually the friend would appreciate the saw
continuing the cut after power comes back!), but the "auto turn-on"
feature )c: is one I hadn't thought of.
Thanks!
It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator.
A) Note the newsgroups to which this is cross-posted:
sci.electronics.misc
sci.electronics.repair
rec.crafts.metalworking
rec.woodworking
And while wood may be an insulator, horizontal bandsaws (which I
remember from the earlier articles in this thread, and which I
use) are more commonly used to cut metal. And metal swarf is
certainly conductive.
B) Toggle switches typically have a spring-loaded actuator which
bears on a metal plate which rocks back and forth to bridge
contacts or not bridge them.
Even a buildup of wood sawdust or plastic dust *could* trap the
metal rocker in a position to keep the bridging happening, even
when the toggle is in the "off" position.
Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work.
And most of those switches are not located near the point of
generation of metal swarf as they are on the typical inexpensive
horizontal bandsaw. The airborne sawdust in a wood-working shop is not
likely to be a problem. However, metal chips dribbled from the blade
(after going almost a full turn around the path of the blade) are a
possibility, depending on the construction of the switch. (And, they
could also conduct from the switch terminals to the chassis of the saw,
if they got under the frame.) Certainly a sealed limit switch (more
commonly found on serious machine tools) would be free of that
particular problem. They are a small MicroSwitch module inside a metal
housing with O-rings to keep out oil as well as chips. They are
operated by a cam, also sealed, or an external plunger -- operating
through a sealed path. (Honeywell is one maker -- and they *are* quite
expensive, but the best thing where swarf will build up.) The wiring
comes in through a fitting which normally has a tapered gasket which
compresses around oil-proof wiring, so there is no path for the chips in
through there either. And yes, the are best used at low voltages and
low currents. I recently re-designed the limit switches in a conversion
of a CNC milling machine, and needed to get switches of the same
physical mounting but which had two separate circuits depending on which
direction they were operated in.)
It would probably be
hard to convince your friend their is a real risk here. For good reason...
It is at least a *possible* one. Now, if the power is fed to
this through a wall-mounted disconnect switch, so you could stop the
bandsaw without having to unplug it (which might be difficult to reach
with the saw still running), that would be less of a problem. It would
be nice to have a warning LED near where you would reach to change the
clamping of the workpiece, so you know the saw is still running,
especially of other noisy tools are being run at the same time in the
shop. A horizontal bandsaw is typically fairly quiet, unless it is
cutting relatively thin metal.
Enjoy,
DoN.
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J
Jasen Betts
Guest
On 2014-01-12, Gunner Asch <gunnerasch@gmail.com> wrote:
http://www.sensata.com/klixon/thermostat-precision-7bt2.htm
they describe it as a "thermostat" and rate the contacts at 10A 240V.
probably enough for a 1/2 HP motor.
but if you need the cut-out to switch both terminals that's not going
to suit.
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On Sat, 11 Jan 2014 23:56:48 -0800, DaveC <invalid@invalid.net> wrote:
The best argument for using a contactor in a small bandsaw, is that
you can get proper thermal overloads to protect the motor.
i
cc
Can you get just thermal overloads without a contactor?
Yes..but they are NOT..NOT fuses or circuit breakers. They are
designed to carry SMALL current loads..often times much smaller than
the motors they are protecting.
They may read the current..but the internal switches are too small to
handle repeated ons/offs
http://www.sensata.com/klixon/thermostat-precision-7bt2.htm
they describe it as a "thermostat" and rate the contacts at 10A 240V.
probably enough for a 1/2 HP motor.
but if you need the cut-out to switch both terminals that's not going
to suit.
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L
Lloyd E. Sponenburgh
Guest
"DoN. Nichols" <BPdnicholsBP@d-and-d.com> fired this volley in
news:slrnld6mot.id6.BPdnicholsBP@Katana.d-and-d.com:
> It is at least a *possible* one.
Don, I'm not picking on your answer... I had to respond to someone's, and
yours got the prize...
This is the dumbest discussion I've ever heard (short of all the
political spew on here).
Even half-quality garbage plastic switches from China are NOT prone to
accidentally 'switching on' from accumulations of dust or swarf.
That it's a remote possibility, I won't deny. But such switches typically
serve for years to decades without a malfunction in the dusty, dirty,
swarf-filled environments in which we use them, and it's dumb to think of
the average home craftsman's going to the trouble to re-wire, retrofit,
and otherwise jigger-up his equipment with low voltage contactors and
safety circuits. That some would or even could is beside the question.
This discussion should be turned to "What's the best-quality switch I can
buy affordably that will suit the safety needs of the application." For
that, I recommend a good industrial-quality safety-style switch that
requires a simple swipe of the hand to turn off, and a positive 'de-
locking' action to turn on.
My old (1970s) Shopsmith came with one. When I finally wore it out in
the 1990s, I replaced it with the same-quality switch from a US maker
(IIRC it was a Square-D safety switch, specifically for table saws). You
could knock it off easily, but had to pull the bat out manually to turn
it back on -- heavy-duty thing. It lasted more than 20 years of nearly
daily use, and the replacement is still on the machine, still working.
LLoyd
news:slrnld6mot.id6.BPdnicholsBP@Katana.d-and-d.com:
> It is at least a *possible* one.
Don, I'm not picking on your answer... I had to respond to someone's, and
yours got the prize...
This is the dumbest discussion I've ever heard (short of all the
political spew on here).
Even half-quality garbage plastic switches from China are NOT prone to
accidentally 'switching on' from accumulations of dust or swarf.
That it's a remote possibility, I won't deny. But such switches typically
serve for years to decades without a malfunction in the dusty, dirty,
swarf-filled environments in which we use them, and it's dumb to think of
the average home craftsman's going to the trouble to re-wire, retrofit,
and otherwise jigger-up his equipment with low voltage contactors and
safety circuits. That some would or even could is beside the question.
This discussion should be turned to "What's the best-quality switch I can
buy affordably that will suit the safety needs of the application." For
that, I recommend a good industrial-quality safety-style switch that
requires a simple swipe of the hand to turn off, and a positive 'de-
locking' action to turn on.
My old (1970s) Shopsmith came with one. When I finally wore it out in
the 1990s, I replaced it with the same-quality switch from a US maker
(IIRC it was a Square-D safety switch, specifically for table saws). You
could knock it off easily, but had to pull the bat out manually to turn
it back on -- heavy-duty thing. It lasted more than 20 years of nearly
daily use, and the replacement is still on the machine, still working.
LLoyd
L
Leif Neland
Guest
DaveC sendte dette med sin computer:
Why on earth would frind want the saw to turn on after a power failure?
--
Husk křrelys bagpĺ, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.
Yes, we all know about "during a power failure" but many times this won't be
an issue (actually the friend would appreciate the saw continuing the cut
after power comes back!), but the "auto turn-on" feature )c: is one I hadn't
thought of.
Why on earth would frind want the saw to turn on after a power failure?
--
Husk křrelys bagpĺ, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.