Dewalt 788

[Troyz]

Howdy folks, New member here and looking for some help.

 

When I stop sawing with my Dewalt and go to raise the arm, most of the time it will not raise up.  I can turn on motor for a second or two and then turn it back off, then the arm will raise with no problem.  It has always done this occasionally, but now it does it almost all of the time.  Any suggestions?

 

Thanks in advance for your input

 

 

Troy

[Iguanadon]

Hi Troy, mine does that on occasion, only raising about 1/2 way and as you mentioned, I simply tap my foot pedal for a second and then it will raise up all the way.  Not sure what causes it though.

[Sparkey]

Mine also does it but not very often.

[hawkeye10]

This is strange. I also have a 788 and I hope someone comes up with an answer.

[hotshot]

Yep, same here, I just tap the pedal and accept that annoyance as a characteristic of the saw.  It's interesting that the behavior is happening more for you now.

[dgman]

They all do it. It is just one of those little things with the 788.

[Scrappile]

I have an Ex and it does that once and a while.  Something to do with where the motor stopped last time it ran.  Well not so much the motor as the running gear attached to it.  Why it is happening often, I so not have a clue.  Does it shut off quickly when you stop it or does it (the motor take a split second to stop?  Is it vibrating more than it use to? 

[amazingkevin]

Whoah! the hours i've logged in on DW788's,an now things like this start to show up.???No rhyme or reason i know of.But they do need to be re greased for longevity .

[woodduck]

My EX also does it very occasionally.

[KurtP]

I have the same issue occasionally with my Dewalt, haven't had the issue with my EX yet,

[ike]

All DW 788 do that. it don't seem to hurt anything just snowing. just tap the foot pedal or on button and you can set the arm up. Life is too short to worry about things like that..

IKE

[jerry1939]

Just a thought (I have very few, but I digress.) Perhaps it's (a lack of??) lubrication on the slide that tensions the blade.

 

Next time it "locks" for you, see if it helps to move the tension lever back and forth.

 

jerry

[Denny Knappen]

It happens infrequently on both DeWalt and EX21. I just tap the foot switch and no problem.

[CharleyL]

If the saw stops at it's full down stroke, the cam action of the mechanism at the motor will cause this. It is just at the point on the cam motion (bottom dead center) where lifting the arm manually won't allow the mechanism to coast forward or backward to move it to  it's full up position.

 

The next time that it does this, try turning the motor shaft about 15 degrees in either direction by using a flat blade screwdriver in the slot in the back end of the motor shaft and turning it a just bit either way. Then try to lift the upper arm. The motor shaft will turn about 1/2 revolution in the same direction that you turned it with the screwdriver, as the upper arm of the saw is lifted to it's full up position. Any machine that uses a rotating cam action to convert the motion into an alternating back and forth motion will have this problem, but if you understand what is happening, it's easy to get around it.

 

The piston arms that drive the wheels of a steam locomotive (now why did I think of this analogy?) are not positioned so both arms (one on each side of the engine) move forward and backward together, One of them is set to about 30 degrees of wheel rotation ahead of the other. This is so one of the arms can move the wheel if the other arm is at the full end of it's stroke and at this dead spot where it can't go either way, Since we don't have this "double rod power source" design in our scroll saws, we have to manually work around this "dead spot" by turning the motor (wheel) a few degrees either way, and it's easiest to just bump the power and hope the motor stops somewhere other than it's "full down position". (actually there is another dead spot like this at the full up position too, but we are always running the motor to get past this one as a normal part of using the saw, so we don't even realize that it exists).

 

Now that you know what causes this, you also know how to get around it by turning the motor shaft slightly, but it's easier and faster to just tap the pedal or power switch and hope the motor stops somewhere else to allow the upper arm to then be lifted all the way up. There's nothing wrong with your saw. Just move the motor a little bit manually or with power when it does this and everything will be fine.

 

Charley

Edited September 9, 2016 by CharleyL

[Scrappile]

Charley,  Troy  states that this happens almost every time on his saw.  Is there something worn or out of wack that would cause the motor to stop at bottom dead center more often than not?  Or, something else worn that make it difficult to raise the arm that may not be related to the motor stopping at BDC more often than not.

[DWSUDEKUM]

What causes this I believe is that the rotor ( the spinning part ) has a slight magnetic imbalance that over time gets worse.  When it occurs the motor tends to stop in a position where the arm mechanism is locked in the position of neither going up or going down.  This is why when you tap the foot switch and the saw moves a little the problem is generally fixed.  The only real solution that I know of is to replace the motor.  I have had this happen to me and I ignored it until the motor shorted out and had to be replaced.  This is not uncommon to Dewalt or any other brand that works like it.

 

DW

[CharleyL]

That's good speculation DW, but it could also be a slightly increased amount of friction at that point and the motor coasts very easily, but stops when the friction at this point increases. 

 

Check the motor brush length often on these variable speed motors and replace them when they begin wearing down to anywhere near the springs. If the spring touches the commutator (electrical contacts on the spinning part of the motor), it's almost instant death for the motor. As the brushes wear, the electrically conductive carbon wearing away from the brushes settles to the bottom of the motor it and can build up enough to short the motor windings, so blowing this carbon dust out of the motor every once in a while (at least when the brushes are replaced) is a good idea too, but make sure the brushes have been removed from the motor before you open the the motor case to do this. If you don't, you can damage both the brushes and the motor. If you can get the end cover on the opposite end of the motor from the brush location off, you will disturb less of the electrical wiring inside the motor. It's always best to disturb the wiring as little as possible to avoid breaking anything.

 

Charley

Edited September 10, 2016 by CharleyL

[jerry1939]

I have a Delta that is "about" 15 months old.  Today this happened to me for the first time.  While holding the arm up (to where it "locked"), I tapped the foot switch.  Took care of it in one second.

 

jerry

[Troyz]

Thanks for all the input.  Had to go out of town for a few days on family emergence.  Just got back home.  Will check lubrication and see if that helps.  If not, guess new motor may be next option.

 

Thanks again,

 

Troy

[CharleyL]

Why buy a new motor? Read the posts again.

 

We didn't say you needed a new motor, but lubrication may reduce the frequency of the problem. We said just tap the power switch or foot pedal on and back off quickly, and your problem will go away. It's not the motor. It's just a bit of a machine design quirk that's easy to get around. Any machine that has a cam type motion that gets converted into a linear back and forth motion (or up and down), has this problem. Don't buy a new motor, because the new motor will work the same way.

 

Charley

Edited September 18, 2016 by CharleyL

[Troyz]

My problem is not that it occasionally locks up, it is almost every time I stop.  It use to do it once in a while.  I know it is common for all motors of this type to do it on occasion, but mine has gotten a lot worse. 

[CharleyL]

Have you tried to quickly turn the power on and back off when it "locks up"? Your descriptions are a bit too short and confusing. Please explain your problem more completely. I still am not convinced that you need a new motor. 

 

Charley