Rip fence tolerance? hourglassing cause?

[Jay Yoder]

Tonight I was ripping some MDF and discovered that instead of a straight cut I ended up with a bit of an "hourglass" cut. I have a Jet deluxe with riving knife. I just checked the fence and discovered a couple of "dips" of .002". I know this is woodworking, but does this sound like alot? It doesn't to me but... one of the "dips" is about 3" in on the leading edge of the rip fence. Any other causes of "hourglassing"?

[Myk Rian]

Any other causes of "hourglassing"?
Arbor bearings?

[Jay Yoder]

Myk,
I sure hope not, it hasn't been used all that much!

[Dan Friedrichs]

Can you shim behind the fence face to fix it?

0.002" is really very good - you can spend a few hours trying to fix it if you like, but you're already flatter than most people will ever get.

[Dave Lehnert]

1) Are you sure the edge up against the fence (Factory edge?) was straight to begin with ?

2) Make sure your splitter is in-line with the blade.

3) Do you have a micro adjust fence? I do on my JET cabinet saw and if move the fence without un-clamping the micro adjust, it will give me problems.

[Chip Lindley]

I'm with Dave! Factory edges are not all created equal. You must have a perfectly straight edge against any fence, to achieve a straight cut. Also, double-check that your splitter is set up exactly, and not pulling the stock.

[Jay Yoder]

Dan,
Maybe I am just being too picky....002-.003 isn't too bad.

Dave,
I checked the edge and it was pretty straight. I never thought of the fence, I wonder if maybe when I had the blade sharpened that caused some issue with the splitter? The fence I have is the standard fence that came with the saw.

I am just frustrated and stumped. Is it possible that the MDF is bowing across the face after cutting?

[joe milana]

What material is the fence made of? Is it that formica covered plywood with that black plastic edge banding? If so, check the first and last inch or so of your fence. That push in edge banding can cause the perimeter of the fence to "swell". I would make note of your .002 "dips" and take the fence face off, pull that edge banding off, trim about 1/2" of the top and ends of the fence face, shim the "dips" with painters tape, and reinstall the face.

[Jay Yoder]

Joe,
My fence is a Jet deluxe xacto fence with UHMW faces. I am not sure how it even attaches to fence...the dip is on the inlet side...

[Chip Lindley]

.002-.003" dip in your rip fence is no cause for the results you seem to be getting. MDF rarely has *reaction wood* that would bow after ripping.

If you've had the blade sharpened, it thinner by a couple of *thou* now. If your fence is properly aligned parallel to the blade (wide by a couple of *thou* at the outfeed end) I am led to believe your problem is in the splitter. If misaligned, it can pull the stock one way or the other. If the splitter is wider than the blade, that will have an effect also. Remove it during a few cuts and see if things improve.

[Jay Yoder]

Chip,
Great suggestion, I will try that out in a bit and report back. I hope it is something simple like that!

[Jay Yoder]

Well all, I adjusted the riving knife and that seemed to do the trick. I need to tweak it a bit but all in all I just got a pretty straight cut! Woohoo! Thanks for the help everyone!

[Will W Brown]

I have that same fence and the same problem. If it's like mine, there are square holes holes in the bottom of the steel tube for access to lock nuts which hold the fence face in place. A socket wrench fits easily which makes loosening/removal easy. I first thought it was the fence face but it turned out to be a slight bend in the tube (forget exactly how much; around .002-.004). So shimming the the face is also easy but getting it right is much harder. I think the UHMW is so flexible that it's difficult to shim uniformly. Shim out in one spot, I get a dip in another when nuts are re-tightened.

Coke can works in some spots, paper in others. When I began using thick and thin paper, I decided "enough". Still it's annoying when use of a featherboard causes saw marks by pushing the work into a low spot on the fence.

Yes, the length of my fence is properly adjusted to the miter slot. I give Jet credit for the easy access detail and who ever saw a rolled steel tube absolutely straight?

Will

[Lee Schierer]

First off, a .002-.003" error in a cut piece in woodworking is not serious. We're talking the thickness of a piece of paper or less. Your wood will change that much just due to seasonal weather changes. I think your problem my be that your fence may be deflecting during the cut. Place a dial indicator against the free end of the fence and have some one watch while you make a cut. Most likely you will see .002-.003" deflection, more if it is an aluminum fence.