Reciprocating Saw Impact Force 1

[dreinert]

Hi,

I am try to figure out how to find the impact force that a Recip saw (Sawzall) can exhibit.

To better understand:

If you took the blade out of the saw and just used the reciprocating rod (.5" Dia) was held 1 inch off a block and then the trigger pulled so the shaft smashes into the block.

I had asked a friend of mine in materials and he thought you could use:

Power=Force*Velocity=Current*Voltage

And measure the peak current of the saw. Does this make sense?

Thanks for your help.

dreinert

 

[jte]

Sounds like a junior or senior level homework problem to me. Is it? Or is this really an on the job issue?

jt

 

[MikeHalloran]

Must be an electrician, using a saw to drive nails.



Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

lol, saw to drive nails. That cracked me up.
P.S. That doesn't make sense to me. You would need to know a bit more then that to figure out the impact force. You could reference mechanical engineering handbook for example.
Alternatively, a quick solution is to strap a force transducer to something and 'wack' it with your saw. However, this method is not the best, just the quickest.



Fe

 

[FeX32]

Why do you need to know the exact force?



Fe

 

[IRstuff]

The equation wouldn't work anyway. The peak current occurs at maximum acceleration, not maximum velocity. Besides, your force*velocity are both unknowns.

TTFN

FAQ731-376

 

[hydtools]

If you know the rpm of the crank and the eccentricity of the driver you can calculate the velocity. Weigh the parts.
Then you have what you need to calculate energy anywhere in the stroke.
Or you know the strokes per minute and stroke length. You can calculate velocity.

Ted

 

[ScottyUK]

Must be an electrician, using a saw to drive nails.

C'mon Mike, even a mechie should know that's what a hammer drill is for. Why carry two tools when one does both jobs.


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If we learn from our mistakes I'm getting a great education!

 

[MintJulep]

Point it at a scale and fire.

 

[dreinert]

Hi guys,

Thanks for the input.

The issue with the saw is that after it experiences several hard impacts gear teeth start breaking off.

I was thinking like TED and thought that with the motor specs and gear ratio we could calculate the velocity of the recip rod.

I also thought that upon the impact of the saw the amps would peak because the motor would stop "instantly" and still be appling power, operating higher than rated.


The problem with the transducer is that a force transducer setup is going to cost me $600+, unless anyone know of a cheaper place.

Thanks for the help.

dreinert

 

[hydtools]

Now you know the saw will not last as a nail gun.

There is no give in the drive to allow the shaft to impact while the drive follows through. Everything in the drive comes to an abrupt stop at impact. Eventually something breaks.

One method used to estimate impact is to drop a steel ball onto the reciprocating shaft and measure how high the ball is thrown. Using the ball weight and height reached you can calculate the energy trasferred to the ball. Be careful.

Ted

 

[drawoh]

dreinert,

Impact is not just a straight force calculation. You are developing kinetic energy in something, and then dissipating it into whatever it is you are hitting. The force is affected by the response of whatever it is you are hitting, and probably by the mass of whoever is holding your reciprocating saw.

The quick and dirty answer is that your impact is the abuse that voids the warranty of your saw. The long, complex answer requires a detailed study of what is holding your saw, and what you are hammering. It is probably a waste of time.

JHG

 

[SincoTC]

dreinert,

It seems obvious that the saw wasn't designed to put up with these "hard impacts", so what's causing them and can't they be eliminated?

If not, would it be possible to fit some protection for the gears in the form of a shear pin that will let go before the gears break or an adjustable slipping clutch that you find on the feedscrews of some lathes, you could adjust this until it transmitted enough power to cope with the worst cutting conditions but would let go on "impact".

 

[IRstuff]

You could use an accelerometer:

http://www.analog.com/en/mems-and-sensors/imems-accelerometers/ADXL180/products/product.html

$15 from Digikey.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ADXL180WCPZ-RLCT-ND

TTFN

FAQ731-376

 

[MintJulep]

The accelerometer may be cheap.

The power supply, amplifier, and FFT-capable recorder aren't.

 

[berkshire]

dreinert (Mechanical)
Since a Milwaukee Sawzall costs a little over one hundred dollars, Wouldn't it be cheaper just to give the operator another saw?
Then if the guy keeps breaking the saws, replace the operator.
B.E.

 

[IRstuff]

Actually, that particular accel is digital out, so all you need is some crazyglue and the serial interface.

TTFN

FAQ731-376

 

[Ron]

Make the gears stronger than the blade....after a couple dozen blade changes, the operator might decide to change his technique.

 

[MikeHalloran]

The saw is handheld, which is all that mitigates the impact.
Dreinert's problem is that indestructibility is a big part of the Sawzall brand.

If he's working on warranty issues for the real Sawzall, then he has to find and reverse the last cost-saving initiative on the gears.

If he's working on a faux sawzall, then he hasn't quite got it right yet.






Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

What about decreasing the spring rate and increasing the travel of the rest piece that is supposed to be flat against the cutting surface. This is what absorbs the impact force when the saw binds and is pulled towards the work piece. If the saw is forced away from the work piece the impact is likely less as it is absorbed by the operator.

One test would be to reduce the cross-section of the blades so that the blades start to fail when the saw binds/grabs. Then compare blade tension and/or Charpy tests to get an idea how much energy is involved.