Troubleshooting DeWalt 788 Blade Movement Issue

[Rick Kr]

I have a new DeWalt 788 that I've been learning some basics on scroll sawing.  I'm exploring a blade movement issue, that of the blade moving forward and backward through the stroke.  I've determined that the blade moves forward about 1/32" from the top to near the center of the stroke and then back again towards the bottom of the stroke, but not as far as at the top.  Here is a video documenting that movement.  The blade is essentially traveling in an arc, which I would have thought would not be the case, given the complex mechanics and geometry of the machine design. 

In this setup, a piece of brass 1/32" wide is held in the blade clamps upon which the indicator needle is riding. 

I've read the DeWalt Tune-Up material on Rick Hutchenson's site (http://www.scrollsaws.com/) but what I am seeing does not seem to be the same thing.  I believe I read somewhere early on that the DeWalt has a design "feature" of the blade moving back away from the cut on the up stroke.  But as I'm learning by trying to make tight turns and from reading that such a blade movement is not conducive to making nice tight, sharp turns, such as at sharp points in patterns.  The blade is moving forward almost the distance of the blade width (#7 Olson PGT) and back again a little over half a blade width.  I can see how that is not good for making tight, crisp turns.

Has anyone observed this motion/behavior and is there anything that can be done about it? 

If there is no fix for this, I am not wed to this saw and wonder if there are other saws out there that do not have this problem. 

Rick

[Bill WIlson]

This is a condition shared to one degree or another by practically all scroll saws.  I believe the Eclipse saw was the only one that had a true vertical blade movement during the stroke.  That saw is no longer being manufactured.

The Dewalt (as well as Excalibur and other saws of similar design) does have some adjustment available to minimize the aggressiveness of the blade movement.  You can change the stroke slightly by loosening the bolts that mount the motor and turn the motor a bit.  It doesn't eliminate the forward/backward movement, but can reduce it somewhat.

[Rob]

I haven't tried this but you might find it interesting.

Rob

[Rick Kr]

Thanks. 

I saw that video, but I'm having trouble seeing how moving the lower mounts forward solves it.  I can try it by removing the screws and clamping the lower unit out a bit further, but...  I just don't see how that solves the arc travel path. 

Rick

[Rick Kr]

I tried it.  It worked to a certain extent. I moved the lower unit out 0.050".  Movement is now about 0.020" from the top to mid-stroke, rather than 0.030".  That isn't much improvement, but I'll take it.  The only downside seems to be about 0.005" rearward travel at the bottom of the stroke (beyond the zero point at the top of the stroke). 

Rick

[JTTHECLOCKMAN]

No saw on the market today has a true up and down motion. There are some old version saws that did this. It is the nature of the parallel arms to rock back and forth. The Dewalt as well as a few other saws have the close parallel arms and other saws such as the Hawk have the longer ones. neither one is better than the other just that they operate differently. What you want is the distance at the top of the stroke to be the same as the bottom of the stroke when measuring from the center of the stroke. Now you have to keep in mind that the blade must sit in the clamps the same at all times in order to be a true measurement. The video shown was one way this was accomplished. Another way was to shim the table. That was the first way developed by scrollers when Dewalt first came out with the 788. It was known for that problem in their Type #1 saws. What I did was took a machinist square and layed it flat against the back of the blade. I manually moved the arm up and down measuring the distance at top stroke and down stroke. I then figured out weather I needed to lift the back of the table or the front of the table to accommodate for the error. I then need to make a shim to correct error. It may have taken a few tries to get dead on but was able to do this. In this process you need to enlarge a couple screws holes also.  It worked for ma and many others back then. I guess it basically is doing the same thing either way. I included a photo of my fix. 

[OCtoolguy]

15 hours ago, JTTHECLOCKMAN said:

No saw on the market today has a true up and down motion. There are some old version saws that did this. It is the nature of the parallel arms to rock back and forth. The Dewalt as well as a few other saws have the close parallel arms and other saws such as the Hawk have the longer ones. neither one is better than the other just that they operate differently. What you want is the distance at the top of the stroke to be the same as the bottom of the stroke when measuring from the center of the stroke. Now you have to keep in mind that the blade must sit in the clamps the same at all times in order to be a true measurement. The video shown was one way this was accomplished. Another way was to shim the table. That was the first way developed by scrollers when Dewalt first came out with the 788. It was known for that problem in their Type #1 saws. What I did was took a machinist square and layed it flat against the back of the blade. I manually moved the arm up and down measuring the distance at top stroke and down stroke. I then figured out weather I needed to lift the back of the table or the front of the table to accommodate for the error. I then need to make a shim to correct error. It may have taken a few tries to get dead on but was able to do this. In this process you need to enlarge a couple screws holes also.  It worked for ma and many others back then. I guess it basically is doing the same thing either way. I included a photo of my fix. 

As always, JT is the man with the answer. Thanks JT. I'll have to check this out. So far, I haven't been troubled by and forward/backward movement. If I was to have it, how would it effect my cutting? Is it a real problem or just something that is part of the striving for perfection? 

 

[Scrappile]

If the little bit of back and forth is within the expected tolerances, isn't it easier to compensate for the aggressiveness by using less aggressive blades?  Like the man said all saw on the market today have back and forth movement, from the most expensive to the cheapest.  And it is helpful, without some you would be cutting slower and pushing harder.... that's my take and I am sticking to it!...  As Lilly would say.

[Bill WIlson]

1 hour ago, octoolguy said:

As always, JT is the man with the answer. Thanks JT. I'll have to check this out. So far, I haven't been troubled by and forward/backward movement. If I was to have it, how would it effect my cutting? Is it a real problem or just something that is part of the striving for perfection? 

 

As JT says, the important thing is that the forward/backward motion is as close to equal as possible, through the entire stroke.  If it wasn't, you would experience over or under cut.  Over cut would mean that the blade is cutting further on the top of the wood than the bottom.  Under cut is the opposite.  The problem would be that when you cut to a line on the top of a thick piece or stack of wood, the bottom cut isn't in the same place.  So either the cut isn't complete on the bottom and you have to cut past the line on the top or when you've cut to the line on the top, you have cut beyond that point on the bottom. 

You can check your saw by taking a thick piece of wood and gently feed it into a running blade, just so the blade kisses the edge enough to make a kerf.  Examine the kerf.  It should be readily apparent if it is deeper on the top or the bottom.  That indicates if your table would need shimmed, such as in JT's pictures.

[OCtoolguy]

15 minutes ago, Bill WIlson said:

As JT says, the important thing is that the forward/backward motion is as close to equal as possible, through the entire stroke.  If it wasn't, you would experience over or under cut.  Over cut would mean that the blade is cutting further on the top of the wood than the bottom.  Under cut is the opposite.  The problem would be that when you cut to a line on the top of a thick piece or stack of wood, the bottom cut isn't in the same place.  So either the cut isn't complete on the bottom and you have to cut past the line on the top or when you've cut to the line on the top, you have cut beyond that point on the bottom. 

You can check your saw by taking a thick piece of wood and gently feed it into a running blade, just so the blade kisses the edge enough to make a kerf.  Examine the kerf.  It should be readily apparent if it is deeper on the top or the bottom.  That indicates if your table would need shimmed, such as in JT's pictures.

Well, I guess I don't have enough of a problem for it to show up. Thanks for that great explanation. So far, all I've cut is a maximum of 3/4".

 

[Sycamore67]

I think that for some people and cases this is worth trying to fix.  However, for most of us, is it worth the effort?  As one new to scroll sawing, I think trying different blades and working on technique is more worthwhile.

In the case your saw is way out, then of course fix it.

[OCtoolguy]

1 hour ago, Sycamore67 said:

I think that for some people and cases this is worth trying to fix.  However, for most of us, is it worth the effort?  As one new to scroll sawing, I think trying different blades and working on technique is more worthwhile.

In the case your saw is way out, then of course fix it.

I agree.

 

[JTTHECLOCKMAN]

It is most definitely worth the effort to fix if needed. As I said back when the Type #1 saws first came out, that was one problem, another was the table was off center for the blade hole and some people did some redrilling and some just enlarged the hole. Also the knocking problem was a huge thing. That is why Rick Hutchenson has a huge write-up. Back then he was one of the first people to start playing around with fixes for this saw. When they moved out of Canada these problems got even worse. 

 

As Bill mentioned it can have a huge effect if stack cutting or doing puzzles because the pieces will not fall out both ways. But as i mentioned the blade must be fully put into blade clamps the same way all the time. With the Dewalt and sure other saws it is possible to not fully push the blade all the way back and still catch the thumb screw. 

[munzieb]

This forward backward aggressive blade motion has always frustrated me. When making tight turns or thin lines it can be hard to stay on line. I'm currently doing the Ford font script on my Mustang project and it has been a challenge to stay on track. Decided to try elevating the  back of my EX-16 by adding 6 each 1/4" washers along with rotating my motor. It's not perfect but it is a major improvement. Much less bounce on the blade. I did have to go to Lowes and buy 2 #6 1.0mm screws longer than the ones currently installed.

Great recommendation.