DW707 SCMS running slow

[RogerS]

Gut feel suggested that my DW707 was running slow and sure enough, when I put the tachometer on it the speed is 5500rpm instead of 7000 rpm.

Does anyone have any suggestions? Could it be a winding on the armature gone?

TIA

 

[9fingers]

Someone here recently let their SCMS slow down without investigation and it turned out to be seized bearings. As most of these saws are plastic bodied, an over heating bearing soon melts the nearby plastic and writes off the whole machine.

If the problem is armature then there will be a firework display around the commutator, not just the usual little blue sparks along the edge but a ring of sparks running from one brush to the other.

My advice would be to investigate sooner rather than later.

Bob

 

[RogerS]

OK..thanks, Bob. Time for the spanners!

 

[RogerS]

Well stripped it as far as seemed reasonable. There is no ring of sparks around the commutator. Can one winding go o/c?

The blade drive shaft is geared into the motor and difficult to take apart any further. It seems to turn OK but not really sure what I'm looking for. I can fathom out if a bearing is totally seized but binding?

The good news was that I found the problem as to why the guard was reluctant to get out of the way as the saw was plunged. At some time in the past, some oak (most like) offcut had got caught up inside and badly scored/raised the plastic which caused the guard to foul. That's all good now but the speed reduction has got me foxed.

 

[devonwoody]

To slow the machine down, its either a bearing or a winding or belt, nothing else enters the equation? (unless you have low voltage)?

 

[9fingers]

OC windings either cause the machine to stop (field OC) or create sparks.
If the bearings are free then that sounds ok.
Does it have plenty of power to cut still?
Try bypassing the switch/controller temporarily as it might just be the soft starter playing up

Bob

PS I presume the blade is tight? Don't laugh - that did turn out to be the answer to a similar problem a while back!

 

[Eric The Viking]

Does the 707 have variable speed? It might be the speed setting pot.

I did some repairs on 701s (previous generation) - pretty robustly built. Don't despair - you can usually get spares from Miles Tools in Shepton Mallet (even plastics). If DW have gone over to gears as you say (the earlier ones were wide belts), there may be a problem in the gear train.

FWIW, I couldn't get the guard to return very nicely, even when cleaned-up!

E.

PS: forgot to say: on the older ones, the blade shaft is pretty much the same length as the motor. It's a good idea, as it keeps the saw accurate (far less flexing if it gets asymmetrically loaded), but it does make it awkward to strip down. Did you check the blade shaft bearing next to the commutator end of the motor? It's quite a bit smaller than the main one - I'd look there first. Also are any areas of the casing getting noticeably hot in use? E.

 

[RogerS]

Well I'm flummoxed.

I've measured all the windings just in case. 9fingers was spot-on...no arcing round the commutator means the windings are OK.

I ran it for a few minutes and felt the two bearings at the brush end (that's the motor itself and the end of the blade drive shaft)...neither anywhere near warm.

I can turn the end of the motor shaft with my fingers without any difficulty which also seems to suggest that the bearings are not seized or tight.

It doesn't have a variable speed.

Volts from the 110v transformer are OK (I knew they would be as the other mitre saw worked fine)

:? :? :?

 

[9fingers]

Did you bypass the switch/controller? What was the result

Bob

 

[OLD]

Some high resistance may be, check connections and by pass on /off switch or check out with meter.

 

[RogerS]

I didn't bypass the switch/controller as it is a simple on/off switch.

I've tracked down a parts diagram and it appears to be belt driven between motor and blade shaft. Loose belt maybe? Can't see how yo get to it though. If I gripped the end of the motor shaft with mole grips and then tried to turn the blade by hand, that should show if it's loose? Or does it 'bounce' as it rotates and is only loose then?

 

[9fingers]

Does it slow down further when under load. If it does and yet does not get hot (much) suggests belt slip

I would have expected it to have a toothed belt - but they don't slip just break.

Bob

 

[RogerS]

Will check that. The belt is ribbed according to the diagram.

 

[9fingers]

According to the diagram I've been looking at there is an electronic module in there but that is the DW707E

Is yours a DW707 or DW707E?

Bob

 

[RogerS]

According to the diagram I've been looking at there is an electronic module in there but that is the DW707E

Is yours a DW707 or DW707E?

Bob

Neither..it's an LX

 

[devonwoody]

blunt blade :mrgreen: Roger has been slicing cucumbers on the machine.

or bent blade?

Or have you been spraying with wd40 and it has got on the belt?

 

[RogerS]

Took it along to my local service centre to see if they had a belt/give it quick once over. It's a superb service centre (Pen Tools)...and the guys know what they are talking about...he fired up the saw and reckons that judging by the arcing on the brushes and the sound and the fact that a few of the solder connections on the armature look as if they have melted at some point plus one or two of the windings look a bit dark and suspect .....you can see where this is going.

Getting a quote for a new armature but since the saw doesn't trench guess it will be the big skip in the sky. Shame, been an old faithful for 6 years or so.

 

[doorframe]

Getting a quote for a new armature but since the saw doesn't trench guess it will be the big skip in the sky. Shame, been an old faithful for 6 years or so.

Roger... DON'T SKIP IT!!!! :x

I'd happily pay the postage + beer tokens for it. I have a cunning project waiting for such a machine. :wink:

Roy

 

[RogerS]

They recommend replacing both armature and stator (if that is the right word) ...£200 :shock:

It's going to have to last a bit longer.

 

[billybuntus]

Does anyone have instruction on how to take apart a DW707? my spindle lock has seized and theres an ali cog inside thats split in two :-(