Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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Old June 6th 05, 05:17 PM
SA Development
 
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Default Does polishing one or both surfaces reduce metal to metal friction?

Hi,

I have some Flitz metal polish that I am using to polish the internals of a
trigger and there seems to be two ideas of thought on which has less
friction, so I thought I'd post to the metalworking group and see if anyone
here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another
piece of metal, is it smoother to polish both metal surfaces, or just one
metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other
surface left alone is better because they think that two smooth surfaces
have a tendency to stick and catch on each other much more than one smooth,
one rough...

Thanks,

SA Dev



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Old June 6th 05, 05:44 PM
 
Posts: n/a
Default

Ok now, I don't know much about metal, but my common sense would say
that if you have one rough sde and 1 smooth side, the rough side would
just scratch it up and undo your polishing. Besides, if you polsh both
sides and use grease, the parts don't come into direct contact with
each other anyway. Sinc, Gothfox

  #3   Report Post  
Old June 6th 05, 06:12 PM
Tim Wescott
 
Posts: n/a
Default

SA Development wrote:

Hi,

I have some Flitz metal polish that I am using to polish the internals of a
trigger and there seems to be two ideas of thought on which has less
friction, so I thought I'd post to the metalworking group and see if anyone
here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another
piece of metal, is it smoother to polish both metal surfaces, or just one
metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other
surface left alone is better because they think that two smooth surfaces
have a tendency to stick and catch on each other much more than one smooth,
one rough...

Thanks,

SA Dev


Here is what little I know, from being an amatuer machinist and a
control system's engineer who has to push mechanical engineers around:

There are three major types of friction, and a gazillion different
materials, so both camps are probably right. You should choose a
solution that works best for your particular application.

Running friction is easy. Coulombic (dry) friction is the amount of
frictional force your parts will experience once they're moving. It's a
constant, and in the absense of liquid lubrication it's the only running
friction you'll see. Viscous drag is a frictional force that comes from
your lubricant, it goes up with speed and lubricant viscosity.

Stiction is the amount of force required to get the parts moving in the
first place, and can range from nonexistant to many times greater than
coulombic friction. Material scientists are still studying it, and they
still don't understand it very well. It's difficult to model and dang
hard to compensate for in a control system. It appears to be a function
of just about everything, and seems to involve real (albeit weak)
molecular bonds that develop between the two pieces.

If you want to reduce running friction (coulombic friction and viscous
drag) then you want two smooth pieces made out of dissimilar metals.
You also want to use some fairly low-viscocity lubricant consistent with
the use (in a gun that probably means fairly thick grease, by the way).

If you want to reduce stiction then two smooth pieces in intimate
contact may be the worst thing you could do -- this may be why some folk
want one piece rough, to reduce the amount of surface area in contact,
and hence the amount of adhesion. If I were to use the "one rough
piece" approach I'd do something like use the piece as cast (or as
knurled) with a single light pass to make the tops of all the little
knobbies flat with respect to each other.

You can do a lot with the right lubricant - lubricants with molybdenum,
graphite or (AFAIK) sulfer are good things to use to reduce stiction,
because they prevent the molecular bonding that it rises from. You can
also control stiction to a great extent by careful choice of materials
-- but be aware that many "low friction" materials only reduce running
friction, and it's often not the _amount_ of stiction that really kills
you but the _ratio_ between the stiction and the running friction. In
automatic control systems (and perhaps human-in-the-loop ones as well)
it's often best to use materials and lubricants that allow a higher
running friction if it means reducing stiction.

If you wanted to make the ultimate friction-free trigger mechanism you'd
eliminate all sliding contact, and either run hardened steel shafts in
bronze or if you're really nuts get a whole bunch of miniature ball
bearings, one for each pivot point.

Stiction can be a real bitch in a control system, whether its automatic
with some poor little microprocessor trying to make things work, or if
there's some human in the loop, squeezing a trigger with no tactile
feedback until the thing lets loose with a great big "BANG"!

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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Old June 6th 05, 08:31 PM
DeepDiver
 
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"Tim Wescott" wrote in message
...
SA Development wrote:

Hi,

I have some Flitz metal polish that I am using to polish the internals
of a trigger and there seems to be two ideas of thought on which has
less friction, so I thought I'd post to the metalworking group and see
if anyone here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another
piece of metal, is it smoother to polish both metal surfaces, or just one
metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other
surface left alone is better because they think that two smooth surfaces
have a tendency to stick and catch on each other much more than one
smooth, one rough...


There are three major types of friction, and a gazillion different
materials, so both camps are probably right. You should choose a solution
that works best for your particular application.

Running friction is easy. Coulombic (dry) friction is the amount of
frictional force your parts will experience once they're moving. It's a
constant, and in the absense of liquid lubrication it's the only running
friction you'll see. Viscous drag is a frictional force that comes from
your lubricant, it goes up with speed and lubricant viscosity.

Stiction is the amount of force required to get the parts moving in the
first place, and can range from nonexistant to many times greater than
coulombic friction. Material scientists are still studying it, and they
still don't understand it very well. It's difficult to model and dang
hard to compensate for in a control system. It appears to be a function
of just about everything, and seems to involve real (albeit weak)
molecular bonds that develop between the two pieces.

If you wanted to make the ultimate friction-free trigger mechanism you'd
eliminate all sliding contact, and either run hardened steel shafts in
bronze or if you're really nuts get a whole bunch of miniature ball
bearings, one for each pivot point.



Running hardened steel shafts in bronze (or even ball bearings, for that
matter) do not eliminate sliding contact, they just reduce it.

But I don't think the OP was concerned about the pivot points in his trigger
group so much as he was concerned about the lineal sliding of the sear.

Actually, in the sear, you want some static friction as that is part of the
"break" of the trigger. But once you overcome that, you want your dynamic
friction to be minimal.

Yes, both sliding surfaces should be polished. As for lubrication, do not
use grease or oil on the sear. Use a dry lubricant (like Kano Laboratories'
Molyfilm) that will not hold dirt or other contaminants.

- Michael


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Old June 7th 05, 12:45 AM
Gunner
 
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Default

On Mon, 06 Jun 2005 19:31:38 GMT, "DeepDiver"
wrote:

"Tim Wescott" wrote in message
...
SA Development wrote:

Hi,

I have some Flitz metal polish that I am using to polish the internals
of a trigger and there seems to be two ideas of thought on which has
less friction, so I thought I'd post to the metalworking group and see
if anyone here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another
piece of metal, is it smoother to polish both metal surfaces, or just one
metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other
surface left alone is better because they think that two smooth surfaces
have a tendency to stick and catch on each other much more than one
smooth, one rough...


There are three major types of friction, and a gazillion different
materials, so both camps are probably right. You should choose a solution
that works best for your particular application.

Running friction is easy. Coulombic (dry) friction is the amount of
frictional force your parts will experience once they're moving. It's a
constant, and in the absense of liquid lubrication it's the only running
friction you'll see. Viscous drag is a frictional force that comes from
your lubricant, it goes up with speed and lubricant viscosity.

Stiction is the amount of force required to get the parts moving in the
first place, and can range from nonexistant to many times greater than
coulombic friction. Material scientists are still studying it, and they
still don't understand it very well. It's difficult to model and dang
hard to compensate for in a control system. It appears to be a function
of just about everything, and seems to involve real (albeit weak)
molecular bonds that develop between the two pieces.

If you wanted to make the ultimate friction-free trigger mechanism you'd
eliminate all sliding contact, and either run hardened steel shafts in
bronze or if you're really nuts get a whole bunch of miniature ball
bearings, one for each pivot point.



Running hardened steel shafts in bronze (or even ball bearings, for that
matter) do not eliminate sliding contact, they just reduce it.

But I don't think the OP was concerned about the pivot points in his trigger
group so much as he was concerned about the lineal sliding of the sear.

Actually, in the sear, you want some static friction as that is part of the
"break" of the trigger. But once you overcome that, you want your dynamic
friction to be minimal.

Yes, both sliding surfaces should be polished. As for lubrication, do not
use grease or oil on the sear. Use a dry lubricant (like Kano Laboratories'
Molyfilm) that will not hold dirt or other contaminants.

- Michael

I agree. Though I tend to simply use BreakFree or TriFlow on the
sears. The only weapons I worry about having too light a trigger pull
are the small bore match guns and the iron monsters.

The street guns all have intentionally firmish trigger pulls, which is
a result of springs, proper sear geometry and so forth. They are glass
smooth, but firm.

Gunner

"Considering the events of recent years,
the world has a long way to go to regain
its credibility and reputation with the US."
unknown


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Old June 7th 05, 01:19 AM
Tom Gardner
 
Posts: n/a
Default


"SA Development" wrote in message
...
Hi,

I have some Flitz metal polish that I am using to polish the internals of
a trigger and there seems to be two ideas of thought on which has less
friction, so I thought I'd post to the metalworking group and see if
anyone here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another
piece of metal, is it smoother to polish both metal surfaces, or just one
metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other
surface left alone is better because they think that two smooth surfaces
have a tendency to stick and catch on each other much more than one
smooth, one rough...

Thanks,

SA Dev


Polish both but don't stone. Changing the geometry on your sear should not
be attempted no matter how tempting. Just a little polish and a dry lube
and keep it clean. For best results take it to a gunsmith. It depends on
what the gun is for and how you like your sears. Every gun I ever had I
examined the mechanics and at least removed all burrs and polished.




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