Knock-Knock. What's There?

[Joe W.]

Happy New Year all!

Well, after much deliberation, I adopted the practice of OCToolguy and like-minded scroll sawing addicts and bought a used DW788 (2016 on the motor) for a fair price. 
It did (and still does) have some knocking sound, but appears to cut well.
Since it is used, I was OK with opening it up and seeing what I could see. 
Armed with camera/video, plastic bags, labels, tape, tools, every Bob Brokow video available (thanks Bob!), and a prayer, I disassembled the saw and examined most of the sleeves and bearings.  This being my first time, I labeled and bagged every part/screws and took numerous photos throughout the process. 
The good news – 1) The bearings had minimal grease in them but/and the sleeves showed on signs of abnormal wear.  2) No leftover parts after reassembly.
I cleaned out the bearings and sleeves and re-greased them using the suggested Valvoline synthetic grease.
Reassembly, like disassembly was slow, but without any issues. Whew!
 

Crossed my fingers, said another prayer, and hit the power button.
Dang!  Still have the knocking.
Went to Rick’s ScrollSaw workshop and tried the “adjust the tension rod” tip without success.  Stopped at only rotating it counter-clockwise one turn.  There didn’t seem to be any improvement with the one turn, so decided to stop there, say Uncle! and ask for help . . . HELP!

The bearings I greased:  Front and rear upper and lower link drive bearings/sleeves
                                        Rocker Assembly vertical bearing/sleeve
                                        Blade Chucks upper and lower bearings/sleeves

The knocking symptoms: 
Speed setting 1:  Knocking
                      2:  smooths out
                      3+ Knocking returns
                      4:  Knocking gets louder
                      5:  smooths out
                      6:  smooth
                      7:  Knocking returns
                      8:  Knocking

I have gone back over the forum postings to see if there was anything like this mentioned but came up empty.  And looked elsewhere with no success.

The knocking – smoothing out – knocking returning stumps me. 

A prize to the person who solves this mystery - an autographed photo copy of my very first scroll saw project.  But seriously, any and all comments, thoughts, ideas are welcome.
I could use it as is, but that’s just not me. 

[kmmcrafts]

Did the sound get better at all from before and after greasing it? Sound the same? Coming from the same area? 

Man it's really hard to chase these noises down when you have the saw in front of you.. let alone trying to help someone when you've never seen the saw run.  Also really hard to know what your view of a little wear on the sleeves is.. There are a lot of bearings and sleeves in these saws.. a small amount in one or two bearings that is hardly visible to the eye could possibly pass and be okay.. you start getting wear in every sleeve and that adds up to a lot of slop when you add all that slop from each bearing until it gets to the business end of the saw.. Remember, a little visible wear on the sleeve is also wear on the bearing that sleeve came from and it's hard to see wear on those tiny bearings. 

It also could be something totally different, Motor bolted up tight? 

Since you say it's noisy at different speeds and quite at others it kinda makes me wonder about "other things"  is it bolted to the stand good and tight? Is the stand sitting solid to the floor? Will the knocking change with more or less tension applied to the blade? With no blade in the saw grab the upper and lower blade holders and try moving them against one another sort of wiggling.. should feel a little slop or give but not a excessive amount this could be indication of really worn bearings / sleeves if there is a lot of slop. 

I have read a few instances where folks greased the saw and even put new bearings in etc. and the issue was the bearing in the connecting rod was good but it's supposed to be a pressed tight fit.. and under power it was slipping / spinning inside the connecting rod.. Sometimes you can get away with using a punch and sort of peening the metal around it in a couple places to make it a tighter fit.. But being made from a soft metal this might only be a temp fix depending how loose fit it was in there. Hope I made some sense with that. The connecting rod is what I call the part at the back of the saw directly bolted to the motor shaft.. has the bigger sealed bearing in it.  

My gut kind of telling me it's just worn enough in every sleeve / bearing that adding new grease isn't going to do the trick.. But I listed some things above off the top of my head to maybe give you some things to think about and possibly check.. I'm not sure how badly you really want to tear into the saw again.. or how much money you have invested or want to invest in. But I might just run the saw for a little while and see how you like it.. maybe order new bearings and sleeves and have at it again in a few months. The noise isn't going to really hurt the saw if it is just the bearings.. as they are already junk and damaged so running it and giving it more damage isn't really going to break the saw.. so long as it is running fairly smooth and able to use it..  

The cheapest and easiest thing to do is keep tinkering and see if you can find the source of the sound.. Check all your scews and bolts etc. tight but not too tight.. some of them are just snug up and others should be tight.. you don't want them so tight that they bind up the bearing.. but not loose to flop around either, LOL

Good luck, 

[Joe W.]

2 hours ago, kmmcrafts said:

 

Thanks for sharing your thoughts.  Here's some more information . . .

2 hours ago, kmmcrafts said:

Did the sound get better at all from before and after greasing it? Sound the same? Coming from the same area? 

Reply:  No the knocking sound didn't get better after greasing - sounded the same and from the same area.

hard to know what your view of a little wear on the sleeves is..

Reply:  Using Bob's videos as a reference, the sleeves looked shiny and new - no signs of discoloration or "photographing" from the bearings (Bob's term).  I was surprised as the saw is 6 years old.

Motor bolted up tight?

Reply:  Yep, good and tight. 

is it bolted to the stand good and tight? Is the stand sitting solid to the floor? Will the knocking change with more or less tension applied to the blade? With no blade in the saw grab the upper and lower blade holders and try moving them against one another sort of wiggling.. should feel a little slop or give but not a excessive amount this could be indication of really worn bearings / sleeves if there is a lot of slop. 

Reply:  bolted to stand and solid on floor.  No difference with more or less tension applied to the blade.  I did test upper and lower arms for slop and did not find any - good and tight.  

the issue was the bearing in the connecting rod was good but it's supposed to be a pressed tight fit.. and under power it was slipping / spinning inside the connecting rod.. Sometimes you can get away with using a punch and sort of peening the metal around it in a couple places to make it a tighter fit.. But being made from a soft metal this might only be a temp fix depending how loose fit it was in there. Hope I made some sense with that. The connecting rod is what I call the part at the back of the saw directly bolted to the motor shaft.. has the bigger sealed bearing in it.

Reply:  Yeah, I'm going to open it again and run it with the cover off (using Bob's technique in his videos to keep the frame together and see if I can spot where the Knocking is coming from.    

The noise isn't going to really hurt the saw if it is just the bearings.. as they are already junk and damaged so running it and giving it more damage isn't really going to break the saw.. so long as it is running fairly smooth and able to use it..  

The cheapest and easiest thing to do is keep tinkering and see if you can find the source of the sound.. Check all your screws and bolts etc. tight but not too tight.. some of them are just snug up and others should be tight.. you don't want them so tight that they bind up the bearing.. but not loose to flop around either

Reply:  Well, yeah, but I want it to run like it is supposed to run without the knocking - especially at that 3+ to 4 setting.  That is the worst setting for the knocking.
             And regarding tightening the screws - is there a service manual I can get that specifies the torque values for the DW788?  Everything I saw on Bob's videos suggested all the screws should be tight.  Is it possible too tight will damage the saw or cause it to knock?

 

 

[kmmcrafts]

9 minutes ago, Joe W. said:

Thanks for sharing your thoughts.  Here's some more information . . .

Is it possible too tight will damage the saw or cause it to knock?

 

It is possible to get the bolts too tight at the bearing pivot points on some saws.. It's been 8 years ago that I rebuilt my DW so I couldn't tell you which ones or even if that is true on the DW now as I don't really recall what I did etc.. I have rebuilt so many different brands and most was years back so now I can't say for sure.. You just don't want to create a bind or drag down onto the pivot points.. they should move freely but not be so loose that they slop around too.. The ones to make sure are tight are those that bolt to the motor.. Many times with you have a bearing that shouldn't be too tight they use a lock nut on it so it won't vibrate loose over time.. I know there are some lock nuts at the front end of the saw.. I just don't remember about what is inside. 

Based on what you're saying on the bearing sleeve condition etc.. I feel like maybe you should look at maybe turning that tension rod another turn or so.. If all seems fairly tight etc.. The tension rod could need more tension on it.. I don't know where the tension rod is positioned but when I adjusted mine my blade would be good and tight when moving that lever to apply tension to the blade.. blade would be tight on number 2-3.. most times 2 but sometimes depending on blade maybe 2.5 - 3.. If you're having to push that lever over quite a ways to get the tension applied.. it may need to adjust that rod in another few turns..  I think I would start with that since you don't really have to take the whole saw apart to make this adjustment..

[teachnlearn]

Here is one idea, a mechanics or auto stethoscope. It is direct contact for moving parts. The trick is going to be deciding what your listening too. RJF

https://www.amazon.com/s?k=auto+stethoscope&ref=nb_sb_noss

 

[WayneMahler]

59 minutes ago, teachnlearn said:

Here is one idea, a mechanics or auto stethoscope. It is direct contact for moving parts. The trick is going to be deciding what your listening too. RJF

https://www.amazon.com/s?k=auto+stethoscope&ref=nb_sb_noss

 

You can also use a well placed long screw driver, wooden dowel rod or the like to accomplish this. I fixed cars for over 45 years and used what I could get in my hand at the time. 

Another thought, is I had a similar issue with my EX-21. After talking with Ray @ Seyco, he suggested holding one end of a bearing connected part and trying to move the free end. If there was any play then more then likely the bearing and or sleeve was bad. Took a while but did find my issue and the amount of play was very small. Just a thought for you.

[OCtoolguy]

Did you see the mention on scroll saws.com of relieving the area of the aluminum cover over the tension rod? I had to do that to get mine quieted down. I had turned the rod 4 to 6 turns but to no avail. So I put it back to where I started and moved ahead with filing down that area of the cover that was in line with the rod. I had to take quite a bit off off it but that solved it. If I recall I did still turn the rod 2 turns to shorten it. I also gave the sliding wedge a heavy layer of grease. You are very close to having a sweet saw. 

[Joe W.]

7 hours ago, octoolguy said:

. . . relieving the area of the aluminum cover over the tension rod? . . . I had turned the rod 4 to 6 turns but to no avail . . . If I recall I did still turn the rod 2 turns to shorten it. I also gave the sliding wedge a heavy layer of grease. You are very close to having a sweet saw. 

Ray, yes, I did file down a goodly amount of the underside of the tension cover plate when I had the unit opened up.  Figured I might as well do that then just to eliminate that as a possibility.  It did not make any difference (dang it!)

I will give the tension rod another turn or two, but I don't think that it the cause as the knocking kicks in noticeably starting at 3 1/2 on the dial and get worse thru till 4 1/2 on the dial and then smooths back out the rest of the way to full stroke speed.  
I think that's telling me something, but I don't know what - it does remind me of "harmonic resonance" where troops marching over a bridge made it collapse.  
And I am awaiting a response from DeWalt for a service manual.  I figure an "authorized service center" must have something to go by when repairing them.   

[teachnlearn]

One line of conversation was the motor bolts were loose or the motor bearings were bad. Some one may remember more of the details. RJF

[OCtoolguy]

12 minutes ago, Joe W. said:

Ray, yes, I did file down a goodly amount of the underside of the tension cover plate when I had the unit opened up.  Figured I might as well do that then just to eliminate that as a possibility.  It did not make any difference (dang it!)

I will give the tension rod another turn or two, but I don't think that it the cause as the knocking kicks in noticeably starting at 3 1/2 on the dial and get worse thru till 4 1/2 on the dial and then smooths back out the rest of the way to full stroke speed.  
I think that's telling me something, but I don't know what - it does remind me of "harmonic resonance" where troops marching over a bridge made it collapse.  
And I am awaiting a response from DeWalt for a service manual.  I figure an "authorized service center" must have something to go by when repairing them.   

You can download a manual online. I do it for all of my used tools. I still think your knocking has something to do with the tension rod banging on something. You mentioned 3 1/2 and 4 1/2. Are you referring to the speed dial or the tension dial? Really it shouldn't matter about speed. Vibration will factor in with saw speed but the knocking shouldn't. Did you have this knock prior to your rebuild? When I did mine, I found a very loose bolt that held the arm mechanism to the motor drive link. Once I tightened that up, my noise went away. You may not have relieved that cover enough. It's easy to remove it and see if you find any marks on the under side of it. You only have to relived one end of it. Your tension rod may still be too loose too. It takes some fiddling with but I really think that is your sound. There nothing else to really knock unless you left something loose. I wish we were closer so I could visit and see for myself.

 

[OCtoolguy]

Here's a link to the manual download. 

https://www.manualslib.com/manual/556654/Dewalt-Dw788.html#product-DW788

[kmmcrafts]

Think maybe Ray is right.. If it's a bad bearing or mechanical it seems like it'd knock all the time.. but get louder and faster with higher speeds etc.. and may change with a load on it or different blade tension etc..  

[Joe W.]

23 hours ago, teachnlearn said:

Here is one idea, a mechanics or auto stethoscope. It is direct contact for moving parts. The trick is going to be deciding what your listening too. RJF

Yes, I tried that using my wife's stethoscope.  The loudest knocking sound is heard near where the upper connecting rod connects to the vertical rocker assembly.  I didn't find any sign of wear on any of the sleeves when I had it open, so I don't think that is the culprit.

[Joe W.]

14 hours ago, octoolguy said:

Here's a link to the manual download. 

https://www.manualslib.com/manual/556654/Dewalt-Dw788.html#product-DW788

Ray, I'm seeking - if one exists - the service manual for the DW788 - -   what the certified/authorized service centers  would use - I assume DeWalt would provide that to them.   I have an email request in to DeWalt and am awaiting their reply. 

[Joe W.]

14 hours ago, octoolguy said:

. . . tension rod - 3 1/2 and 4 1/2. Vibration will factor in with saw speed but the knocking shouldn't. Did you have this knock prior to your rebuild? When I did mine, I found a very loose bolt that held the arm mechanism to the motor drive link. Once I tightened that up, my noise went away. You may not have relieved that cover enough. It's easy to remove it and see if you find any marks on the under side of it. You only have to relived one end of it. Your tension rod may still be too loose too. It takes some fiddling with but I really think that is your sound. There nothing else to really knock unless you left something loose. I wish we were closer so I could visit and see for myself.

I adjusted - counter/clockwise (as per the suggestion on Rick's Scrollsaw website) another 3 turns and tried that - no change in knocking.  So . . .  I'm going to try going in the other direction with the tension rod adjustment and see what that does.  While I was at it, I looked for marks on the underside of the tension rod cover plate and did not see any marks, so I don't think that is the cause.  
The range of knocking is on the speed  setting I3 1/2 - 4 1/2) and not the tension setting.  The tension setting does not alter the knocking sound. 
Still a mystery,   

[OCtoolguy]

2 hours ago, Joe W. said:

Ray, I'm seeking - if one exists - the service manual for the DW788 - -   what the certified/authorized service centers  would use - I assume DeWalt would provide that to them.   I have an email request in to DeWalt and am awaiting their reply. 

Don't hold your breath.

[OCtoolguy]

2 hours ago, Joe W. said:

I adjusted - counter/clockwise (as per the suggestion on Rick's Scrollsaw website) another 3 turns and tried that - no change in knocking.  So . . .  I'm going to try going in the other direction with the tension rod adjustment and see what that does.  While I was at it, I looked for marks on the underside of the tension rod cover plate and did not see any marks, so I don't think that is the cause.  
The range of knocking is on the speed  setting I3 1/2 - 4 1/2) and not the tension setting.  The tension setting does not alter the knocking sound. 
Still a mystery,   

You want to take as much slack out of the tension mechanism as possible. It sounds like you might have gone the wrong direction. 

[kmmcrafts]

I think when I took my saw apart to rebuild it that when it came to that rod.. I screwed it in as far as I could and have it still reach into the notch / slot that it fits in on the tension lever mechanism.. I know it was much tighter after rebuilding it..  I never did have the nocking noise from it ever before or after the rebuild.. Mine was a type 2 from 2008 on the motor..

Before rebuilding it I'd have to turn the lever to number 3 - 3.5 ish for the blade to be tight.. after the rebuild usually 2 was tight.. sometimes 2.5 on the tension lever dial..

I don't think that rod even has to be in there if you install the pegas clamps on it for the Excalibur style.. If I were to have a DW now days with the Pegas upgraded clamps available.. I'd ditch that DW tension system.. as flipping the lever over is more convenient than turning that lever.. But the nice thing with turning the lever is you can adjust the tension to where you need it easier..  

[OCtoolguy]

41 minutes ago, kmmcrafts said:

I think when I took my saw apart to rebuild it that when it came to that rod.. I screwed it in as far as I could and have it still reach into the notch / slot that it fits in on the tension lever mechanism.. I know it was much tighter after rebuilding it..  I never did have the nocking noise from it ever before or after the rebuild.. Mine was a type 2 from 2008 on the motor..

Before rebuilding it I'd have to turn the lever to number 3 - 3.5 ish for the blade to be tight.. after the rebuild usually 2 was tight.. sometimes 2.5 on the tension lever dial..

I don't think that rod even has to be in there if you install the pegas clamps on it for the Excalibur style.. If I were to have a DW now days with the Pegas upgraded clamps available.. I'd ditch that DW tension system.. as flipping the lever over is more convenient than turning that lever.. But the nice thing with turning the lever is you can adjust the tension to where you need it easier..  

I'm not sure that would work Kevin. Without it, the only tension adjustment you'd have would be the flip lever. At least with the rod in place you have some adjustment as to how tight or loose you want it prior to flipping the lever. On the EX/Pegas that knob at the back gives you a way of tightening up the tension over what the lever gives you.

 

[new2woodwrk]

You know, I'm not saying this is the way it is but...

That's the 3rd saw you bought and you still hear a knocking

Are you mistaking the knocking for a normal sound?

My saw has what you might call a knocking, but its the normal movement of the machine - heck you should hear it on 9 LO(L

Why not record the sound and let us hear it.

[OCtoolguy]

Another thing comes to mind here. Do you have your saw firmly mounted to a base? I MUST be held down tight so that vibration does not enter into the equation. I have experienced this first hand when I brought home my used EX16. I had it sitting on top of my workbench and I started checking everything out. It worked great but if I ran it up on the speed dial it would start to vibrate and made a bit of noise. When I pushed down hard on the table with both of my hands it smoothed out and the vibration and sound went away. Just thought I'd add that.

 

[kmmcrafts]

Vibration and noises can go hand in hand and the noise can travel through the saw and come out at odd places.. My Hawk has adjustable feet on the legs.. if I bump the saw hard enough to move it slightly it'll either vibrate or make a sound that isn't normally there.. Sometimes I have to sort of kick outward at the stand legs to get it planted to the floor firmly.. Doesn't take much to throw it off balance.. My EX-21 is the same way.. and there is no visible wobble to the stand or anything like that.. it seems like it's solid to the floor when in fact it's not.. 

I think I mentioned this already and it was said you had it solid on the floor.. If factory stand the DW's have a three legged stand and there isn't as much of an issue with this.. that said though.. just for kicks.. run the saw and sort of tap the legs to just slightly move the saw and see if there is any different sound from the saw or if it makes any difference.. 

[Joe W.]

16 hours ago, new2woodwrk said:

You know, I'm not saying this is the way it is but...

That's the 3rd saw you bought and you still hear a knocking

Are you mistaking the knocking for a normal sound?

My saw has what you might call a knocking, but its the normal movement of the machine - heck you should hear it on 9 LO(L

Why not record the sound and let us hear it.

Great idea (sound recording) and thank you for it - I am attaching the MP3 file will upload (fingers crossed) for all to listen to.  The recording is a little over a minute long and starts at the lowest speed setting and goes to the highest and then back down again.  
And I appreciate your thought that I might be hearing a perfectly normal sound and just don't know it. 
I wish that were the case, but I don't think so. 
But as my mother used to say:  "You are never so right that you can't be wrong".
   

DW788 Sound.mp3

[kmmcrafts]

52 minutes ago, Joe W. said:

Great idea (sound recording) and thank you for it - I am attaching the MP3 file will upload (fingers crossed) for all to listen to.  The recording is a little over a minute long and starts at the lowest speed setting and goes to the highest and then back down again.  
And I appreciate your thought that I might be hearing a perfectly normal sound and just don't know it. 
I wish that were the case, but I don't think so. 
But as my mother used to say:  "You are never so right that you can't be wrong".
   

DW788 Sound.mp3 1.78 MB · 6 downloads

Sounds like a broken Harley Davidson LOL.. 

Does the saw seem to run smooth even though is sounds terrible? 

Is everything clear of the rocker arms at the front of the saw? I've seen where the lower arm guard thing got bent once and sounded similar to that.. Also had a saw where the Bellows rotted away and broke.. which made a similar sound.. Is the Bellows hooked up and fasten to the rocker arm correct? 

I know you mention it sounds like it's coming from the back of the saw but sound can travel through the saw in odd ways.. My gut tells me its the rocker assembly at the back of the saw.. either the connecting rod deal that connects to the motor or that long arm that it connects to that spans between the upper and lower arm rods.. 

Also wonder since you say the bearings and sleeves looked good etc.. I question if it's put together correctly.. I know you say you took lots of pictures etc along the way and maybe you have it put together the way you took it apart.. But what's the odds that maybe the previous owner took it apart and put it together wrong.. Might want to study rebuild videos that Bob put up and compare how it's put together.. Just some thoughts.. It's definitely a issue.. 

[Joe W.]

4 hours ago, kmmcrafts said:

Sounds like a broken Harley Davidson LOL.

Thanks Kevin,

Your appraisal makes me feel better, as in, it is the machine and not me (I'm not going crazy).

The person I bought it from said he was the original owner and that he never did any maintenance on it, so I assume it was assembled correctly at the factory.  
I am leaning in the same direction as your assessment of the cause being one (or both) of the bearings/sleeves in the connecting rod that attaches to the vertical rocker assembly. 
When it gets past the 4 1/2 speed setting on the dial, it sounds to me like it smooths out all the way up to full speed.  
I did - after I made the sound recording - readjusted the tension rod back, reversing the 4 counter clockwise turns I originally put on it, and giving it two more turns in the other direction (clockwise).  Tried it again and got the same knocking sound.
I'm going to order a connecting rod, with bearings and sleeves for it, and install it using the method Bob Brokaw did on his video.  Even if that doesn't eliminate the knocking, at least I'll know those items are new.