Bowed cut with my 3000 Guide Rail

[phmade]

Hi all -
I feel like I've been chasing my MFT/3 all over the place to get a square cut... I have a jig that I use to square it up and I get it set just right and then in the middle of the project I start noticing parts that aren't square.  Tonight I finally found the culprit...  

I should note that I work primarily with sheet goods (usually PB core melamine).  I start by completing all of my rip cuts and then I move to the MFT for my cross cuts.  I noticed tonight that my rip cuts were bowed.  If I measure an equal distance at each end and line up the guide rail with those marks, the ripped piece is nearly 1/16" narrower in the middle of an 8'L cut.  The reason for the out-of-square cuts on the MFT stems from this problem.  When I place my rip cut against the fence on the MFT - it's actually indexing from the bowed portion of the cut - therefore not providing a square cut.  I suppose if I had an 8'L fence I could index along the whole edge - but that's not really convenient with the MFT.

I am attaching a photo to try to show exactly what's happening.

Does anyone have any suggestions for dealing with this?  Could it be a moisture content issue within the PB?  I'm in a climate controlled shop which I maintain at a minimum of 50 degrees F.  Could the Guide Rail actually be bowing from the torque of the spinning blade?  Any other ideas?

I'd appreciate any help you can offer.  I'm getting very frustrated that my accuracy is seriously affected by this!
Thanks in advance.

 

[Rick Christopherson]

The picture you show is not related to the straightness of the cut, but about the deflection of the blade during the cut. You have a faster feed rate in the middle of the cut than you do at the beginning or end of the cut.

This gap in the center is common with any ATB blade, but is at least minimized by the low angle of the Festool ATB blade. You may want to consider the Triple Chip Grind (TCG) blades.

 

[norwegian wood]

i use a sliding table saw to cut my melamine sheets and can still get bowed cuts. reason being sometimes the sheets have tension in them that releases after the cut and the rip will bow then. i sometimes have to rip them over sized first to release the tension then make my final rips for a straight rip. some materials are worse then others.

 

[junk731]

Don't how if this is the same problem but I've had PB core melamine bow from tension release as I'm cutting. Now in my case its on a 10' slider using a ATB Blade and scoring, as opposed to a Festool saw. My solution was to cut a little wide and make a second correcting cut. This doesn't happen all the time but in my case I bought 10 sheets and they all had the same problem.

John

 

[Laminator]

Norweigen and Junk describe exactly what happens to probably 95% of the melamine  I rip.  Tension in the sheet is released when the sheet is split and right after the cut if I slide the two halves back together, I have seen the ends of the sheets touch with almost 1/4" gap in the center.

Now I split the sheet and don't even check to see if it bowed but proceed to straightline the edge by recutting.  It is more work but yields perfectly straight edges.

MDF sometimes does the same thing but sometimes does not.  I usually don't check it either but just recut to be sure the edge is straight.

 

[phmade]

Thanks for all of your responses.  I do think that it's material related - as opposed to a tool issue.  FYI - I did switch to the TCG blade and I'm still having the same problem.  I used to work at a large production shop using a 10' slider and I don't recall ever having this issue, but maybe I'm forgetting...  For now, I'll trying cutting oversize and then making another straight cut after the tension is released.  Maybe I'll notify my supplier too and see if they can offer any suggestions. 

I sure wish I (or my customers) could afford to build everything out of nice plywood.  Then I could stop worrying about this and chip out... Although then it would probably be a new finishing problem - I guess there's always something....

Thanks again.

 

[tiralie]

i use a sliding table saw

Nice setup!
Tim

 

[Les]

Are you sure your problem isn't in the rip setup? What size guide rail are you using? Are sure the rail is straight? How much material are you removing? Is the saw tipping sideways because you are having to reach out? If you're reaching out the saw will tip and you will cut a bow and face won't be square. Try checking the bow with the back side of the rail, side opposite of splinter guard.

 

[norwegian wood]

the problem is the material, that plain and simple. nature of the beast sometimes thats all. all i can say is use one of your npt so sharp blades to do all your rough sizing then your your sharp blade for your final sizing cuts so you don't dull out your good blade making those extra cuts when rough sizing.

 

[phmade]

Thanks for your suggestion Les - I did check it against my guide rail and it's definitely the material that's bowed.  I have a 3000 Guide Rail and I've verified for straightness.  Additionally, I use a hydraulic table to cut on so that I can lower it to a comfortable height when I have to reach.

Also thanks for the tip on switching blades Norwegian - I didn't think of that before!  That should save me some sharpening cost.  Next thing I'll want a TS55 setup for my "rough cut" saw. haha - better get back to work so I can afford it!

 

[norwegian wood]

good luck!