TS55 8ft plywood cut

[WeekendWoodWerker]

HI All,

New member to FOG, but I have been reading the posts for a long time. I ran into an issue today and just want to double check that I am not missing something simple. So here goes:

TS55 two 55 inch guides rails connected, confirm straightness as best I can using extruded piece that shows the back of each 55 rail is straight and is straight across the joint (back of rail as reference). Cutting 8ft length of half inch ply.

The cut is 7 inches wide. Using dewalt track clamps at both ends, and supporting the piece on rigid foam full 8ft length and width of workpiece all on top of a flat workbench.

I ripped a straight edge onto the factory ply removing about a kerf width from the edge. I then spin the guide rail around and reference the back of the rail to this straight edge at both ends using a digital caliper and double checking the splinter guard against measured marks at both ends.

I made the first cut and checked both ends were exactly 7 inches. I then made a second cut referencing off of the fresh edge of the previous cut using exactly the same process. Again, measurements at both ends exactly 7 inches. Great! However when I stacked the second cut on top of the first on the tack for storage I notice the boards are exactly the same width at the ends (feels perfectly flush to the fingertips), but they dont line up in the middle section. Indeed, after clamping one end to be perfectly flush, then adjusting the other end flush, it's an exact match at the ends, but as you move toward center they deviate by a few mm to either side. It's not just one board wider or narrower than the other in the center section (at least I dont think)

Several thoughts:
- belly in the guide rail, not straight?
- need to additionally clamp in the middle somehow, not just the ends?
- stress in the wood that released upon cut?
- somehow I torque the saw or rail or something in the middle of the cuts?

What could it be?

 

[nclemmons]

Welcome to FOG!

I'd put a true straightedge (4' level, piece of angle, even a piece of string) on the rails as you clamp them together as the first test.  Which rail connectors are you using?  They have been known to move on you particularly on a long rail like two 55's.

I would not expect to see spring in a piece of ply like you are talking about.  if you put the two pieces side by side, I assume the cuts will match up end to end, which would mean your rails are not totally straight.

Should not need an additional clamp in the middle.  One thing I do do is clamp one end on the line and then pick up the other end to move it rather than sliding it across the ply to make sure I'm not putting undue pressure on the connecting strips.

 

[waho6o9]

View attachment 1

I had similar results so I attached a center stop to reduce lateral movement.

YMMV

 

[WeekendWoodWerker]

Welcome to FOG!

I'd put a true straightedge (4' level, piece of angle, even a piece of string) on the rails as you clamp them together as the first test.  Which rail connectors are you using?  They have been known to move on you particularly on a long rail like two 55's.

I would not expect to see spring in a piece of ply like you are talking about.  if you put the two pieces side by side, I assume the cuts will match up end to end, which would mean your rails are not totally straight.

Should not need an additional clamp in the middle.  One thing I do do is clamp one end on the line and then pick up the other end to move it rather than sliding it across the ply to make sure I'm not putting undue pressure on the connecting strips.

Thanks for the reply and for the welcome! Glad to be here.

As for the straight edge, I suppose that could be the issue, but to be honest the piece I used was half inch thick extruded aluminum 2 inches wide and 48 inches long that registered as straight against the back of both the 55 inch rails independently. To me, that is probably as close to straight as anything I've got. I have a long level, but apparently not a very good one because it does not register as straight against the rails individually, or against the aluminum fence piece I am referring too.

I am using the Festool guide rail connectors. I can look up the part number, but it's the standard ones that are $20 each individually packaged. One on top, one on bottom. Tightened the bottom so it wouldn't move and then backed off the screws half turn or so, then flipped and registered against the straight edge and tightened top. Then flipped at snugged the bottom. BTW, according to my straight edge, the ends of the rails should be butted together; which assuming those are 90 (which I know is not guaranteed officially) also is more evidence to the straightness of the combined rail.

I also did not expect any spring in the plywood. Hence my confusion and question. Although, I do recognize it as a possibility, however remote.

As far as you comment about matching end-to-end - I will try them side-by-side and also flip one and try again and see what I come up with.

For the last comment about pickup up rather than sliding. I did that to some extent, but in the end I need to slide a little bit for final adjustments to get them indexed on the workpiece exactly at both ends. I am probably doing it wrong, but I don't think I can pick it up and set it down perfectly under any circumstance. Especially without the use of parallel guides, which unfortunately I do not have any that will index such a narrow width of 7 inches. But I do understand the comment and "felt" that sliding one end could add stress on the joint of the rails given the grippy-ness of the rails to the wood along such a length.

Upon further reflection, I did make at least one mistake in not verifying the indexing in the middle in addition to both ends.

I will try some more cuts tomorrow and check back in with the results.

Thanks again for the reply.

 

[WeekendWoodWerker]

View attachment 1

I had similar results so I attached a center stop to reduce lateral movement.

YMMV

Thanks for the reply. I may try this too, if for nothing else to confirm that the middle part of the rail is not moving relative to the stop block. I will first check the indexing at that point is consistent with the indexing at the ends (something I failed to do the first time around, thinking I did not need to if the rail was straight).

 

[waho6o9]

View attachment 1

I had similar results so I attached a center stop to reduce lateral movement.

YMMV

Thanks for the reply. I may try this too, if for nothing else to confirm that the middle part of the rail is not moving relative to the stop block. I will first check the indexing at that point is consistent with the indexing at the ends (something I failed to do the first time around, thinking I did not need to if the rail was straight).

You're welcome and checking the middle and both ends should yield better results for you.
Welcome to FOG!

 

[WeekendWoodWerker]

Curiosity got the better of me and I had to go check right away

So clamped side-by-side with zero gaps at the ends as you move toward the middle the gap opens from zero to about 1/16th of an inch, or maybe slightly less, in the middle. Hopefully you can make it out from the attached images.

 

[fshanno]

Joining Festool guides is a tricky thing.  Sometimes it's good, sometimes it's not so good.

That said there is plenty of spring in Asian plywood and even some in domestic.  I wouldn't be surprised at all to see as much 1/4" crook in a 7" x 96" rip.  It could start out as a 1/16th crook this afternoon and then it could be 1/8th in the morning. 

I've cut 3" and 4" wide strips and had them bow as much as 1/2" in the middle of an 8" rip.  Ain't no way that much curve is because of Festool guides not lined up.  That's the plywood.  I've gotten noticeable crook in 14" wide rips.  I've done some concrete countertops.  Did a lot of 2" x 96" wide rips in melamine particle board.  There was no telling what was going to happen.  Not just crooks but S curves.

And we aren't even going to talk about bow.  Bowed plywood is my way of life.  I've had 3/32 S curves in 23 1/4" wide pieces cut for the sides of base cabinets.  Wiggle wood I call it.  Mostly imported.  You have to special order domestic around here.  That Columbia stuff that Home Depot gets is just as bad as the Asian.

What I've never seen is a piece of plywood decide, after the fact, to make itself wider or narrower in the middle than it was when it was cut. 

So, you have this piece.  First you made a dust cut to get rid of the factory edge.  Then you flipped the guide around and made another cut.  So, if the piece is 7" wide in the middle then you are dealing with spring in the plywood.  It couldn't possibly be anything else.  Could it?  And that's not only possible, I my plywood world I call it likely.

No problem though if it is truly a crook.  Nobody depends on narrow plywood rips for straightness.  Bows and crooks get straightened out at assembly time.  With a little luck.  That's what a Domino is for.

 

[Jiggy Joiner]

Sounds almost certainly like there has been alignment or movement issues at the rail joints. It’s very easily done.
Another thing to consider, often a user will adjust the saw to remove any side to side slack or slop on a rail, with the adjusters on the saw base but, not check it on the second rail.

 

[Rob Z]

Hello

I have noticed the same effect as noted by fshanno.  I have been utility building cabinets for church and using the ~ $35/sheet plywood from HD.  Nothing super-high quality about this stuff  [eek] .  Whether ripped to size on my table saw , or my track saw, I come back the next day and start to assemble panels and say “Uhh did I really get that sloppy on my cuts ?”  [mad] This plywood does have a mind of its own.

But also as fshanno noted, with dominos/biscuits/glue, the pieces all come together and it’s not a problem.

 

[chrisrosenb]

Plywood has stress in it. It will bow when cutting strips off of a full length piece. Even cutting a 4' wide sheet into 2' x 8' pieces will bow. 
I learned a long time ago when cutting long pieces of plywood off of a full sheet, was to cut the pieces slightly over size.
Then straight line the pieces, then cut them to final size. 

 

[Michael Kellough]

To reiterate some good comments,
1. wood moves, you pretty much have to cut twice
2. Use a truly straight edge to align rails

What I’ll add is that guide rails are not the same width, nearly so but not exactly, so you have to align two rails at the point closest to the cutting edge, the splinter guide side of the middle spine. If you register from the side all the way opposite the splinter guard you’re guaranteed to get a less than straight cut. Finally, if you flipped the cut pieces on top of one another so that you compared concave to convex then the actual deviation from straight is only half of that ~1/16”.

 

[WeekendWoodWerker]

I am starting to get convinced that it's related to the plywood and not my setup.

Upon further inspection this morning I realize that one of the pieces is almost perfectly straight along both edges with minimal or no gap when held against the backside of the guide rail. The other piece clearly has a bow in it - along one edge it touches the backside of the rail on the ends with a gap in the middle, and the other edge touches the rail in the center and can rock back and forth. Also, the bowed piece is 7" in width measured anywhere along the piece. In other words, the size of the gap is an order of magnitude larger than any variation in width along the board's length. Also, the remaining edge on the larger sheet that I am ripping from appears to be straight as well relative to the backside of the guide rail. I could be thinking about it wrong, but I think if the guide rail connection was not straight, spinning the rail around and backing it up to the remaining edge should exaggerate the issue and be obvious.

I have lost track, but I believe the bowed strip is the first one I ripped and the straight strip is the second.

I will try to find a longer reference straight edge and do some more cuts paying more careful attention to the setup.

Thanks for the replies and suggestions.

 

[xedos]

Having read this, it seems to me the saw and rails are doing their job.  If both strips are 7" wide along their length this must be true. 

That the "pieces" don't mesh back together without gap isn't ideal in many cases.  In some it won't matter. I don't think it's an issue with the saw/rail at any rate.

If we were discussing maple staves just ripped on a Unisaw with a Forrest blade that had a gap like shown when place together , I doubt many of us would think twice about it. Especially if they were 8' long !  Joint em, glue em, and clamp em up.

Wood moves.  And not in a reliably, quantifiable predictable way. Is just part of the craft dealing with it.

 

[RJNeal]

Be careful as you cut. With both end clamped it has been reported here operators have a tendency to push the saw towards the back of the rail as they walk the saw down the rail. With both ends clamped it will bow the rail.
Rick

 

[sheperd80]

With all the variables here; two joined rails, imperfect plywood, long cut length, tracksaw instead of tablesaw... id say less than 1/16 cut deviation is a success story.

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[WeekendWoodWerker]

With all the variables here; two joined rails, imperfect plywood, long cut length, tracksaw instead of tablesaw... id say less than 1/16 cut deviation is a success story.

Sent from my SAMSUNG-SM-G890A using Tapatalk

I have a table saw but don't have the skill or means to feed a 4ftx8ft sheet of 1/2" ply through it by myself safely while keeping it dead straight. That's exactly the reason why I got the tracksaw

 

[chrisrosenb]

With all the variables here; two joined rails, imperfect plywood, long cut length, tracksaw instead of tablesaw... id say less than 1/16 cut deviation is a success story.

Sent from my SAMSUNG-SM-G890A using Tapatalk

I have a table saw but don't have the skill or means to feed a 4ftx8ft sheet of 1/2" ply through it by myself safely while keeping it dead straight. That's exactly the reason why I got the tracksaw

I cut all of my plywood on a table saw. The long pieces still bow. If I am going to be using pieces that are over 4', after rough cutting them on the table saw, I straight line them with the track saw & then cut them to finished size on the table saw.

 

[ChuckM]

With all the variables here; two joined rails, imperfect plywood, long cut length, tracksaw instead of tablesaw... id say less than 1/16 cut deviation is a success story.

Sent from my SAMSUNG-SM-G890A using Tapatalk

I have a table saw but don't have the skill or means to feed a 4ftx8ft sheet of 1/2" ply through it by myself safely while keeping it dead straight. That's exactly the reason why I got the tracksaw

If yours is a cabinet saw, the Jessem saw guides will handle a 1/2"  4×8 sheet with ease. Even a 3/4" sheet can be cut straight after a bit of experience.

 

[Laminator]

I rip a good bit of plywood, mdf, particle board (melamine) and very few pieces remain straight.  After ripping I slide the pieces together and the kerf gap will show how much tension was released. If the gap is small enough I will clamp the pieces, move the guide rail over slightly and re rip. After this the kerf closes tight from end to end. Worst case  I remember had almost 1\4" gap after ripping, and once had some mdf that the kerf closed so tight that it stopped the saw (pinched) and I had to drive a chisel into the kerf to free the saw.