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Thread: Table saw fence flatness

  1. #1
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    Table saw fence flatness

    I just got a UNI-T fence from Peachtree woodworking. I set it up, tried to get It parallel and noticed ever time I fed a long length of wood (30" or so) that the back end (operater side) of the board would pull away just before the blade and through the cut. I repeated four more rips and same scenario (even with board buddies). I whipped out the digital calipers to measure test pieces and was getting a narrower measurement at the front of the board than rear (or maybe I have it backwards) either way I'm not getting parallel rips.

    I decided to take a 24" framing square and test the UHMW fence face for flatness. I put one edge of the framing square just before the beginning of the blade. I was able to see a gap at the front edge of the fence and also able to rock the framing square back and forth. I didn't measure the gap, but from aligning the fence to fence to miter slot with a dial indicator I was getting a hump in the middle of the fence from .008-.013 (8 to 13 lines on the dial indicator, if I didn't list the decimals correctly). When I moved the dial indicator to the very back of the fence (rear of table) it would right itself / zero out.

    Peachtree list that UHMW varies in thickness and that the UHMW may not be perfectly flat with the top part of the aluminum extrusion. But they do not list anything about the UHMW face not being flat along it's length.

    1) Is too much deviation in the middle of the fence? (The OEM unifence extrusion has some slight deviation along it's length but only like .002-.003 at most)

    2)if yes to above and Peachtree doesn't see it as a issue, how could I accurately shim behind the UHMW to get the fence flat along it's 43" length or should I make a new face out of hardwood?

    Thanks in advance
    Michael

  2. #2
    If there are no exposed fasteners, could you face joint the whole assembly?

  3. #3
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    Hmm, 13 thousands isn't a lot and the fact that the far end of the fence goes back to zero. Are you seeing any burning on the sides of the cut? The hump in the middle should also be causing some burning on the side of the cut next to the blade on the fence side. Have you checked your blade alignment to the miter slot recently? An angled blade will give you the same type of problem you have noticed with almost no burning.
    Lee Schierer
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  4. #4
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    UHMW and most other plastic sheet goods have a huge tolerance on thickness/flatness so I'm surprised you're only seeing .013". The industry standard for most plastic sheet is +/-10% of thickness, so a 1/2" thick piece of plastic facing can be "acceptable" if it measures .450 to .550" (.100" variation). Plastic is machinable if you can avoid melting issues, but I'd probably just make a hardwood face and move on.

  5. #5
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    Shimming it would be a good start. You already know how much you need, somewhere between 2 and 4 thicknesses of paper.

    I'd worry about hitting the extrusion if you tried to joint it. And I have no idea how it work going thru a planer. Some of these plastics are difficult to get a good finish on. Some you can just forget about sanding.

  6. #6
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    Quote Originally Posted by Bruce Wrenn View Post
    If there are no exposed fasteners, could you face joint the whole assembly?
    There are three removable Phillips in the face. It would have to be machined down removed from the extrusion

  7. #7
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    Quote Originally Posted by Lee Schierer View Post
    Hmm, 13 thousands isn't a lot and the fact that the far end of the fence goes back to zero. Are you seeing any burning on the sides of the cut? The hump in the middle should also be causing some burning on the side of the cut next to the blade on the fence side. Have you checked your blade alignment to the miter slot recently? An angled blade will give you the same type of problem you have noticed with almost no burning.
    Hi Lee
    No burning, but I was only cutting 1/2 spruce.i did noticed crosshatching / teeth marks on the back end of the board.
    I did just do the blade to miter slot and got it within .001, but I did that with a think kerf blade and have now switched to a full kerf.

    It may be worth double checking the blade alignment.

  8. #8
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    Quote Originally Posted by Lisa Starr View Post
    UHMW and most other plastic sheet goods have a huge tolerance on thickness/flatness so I'm surprised you're only seeing .013". The industry standard for most plastic sheet is +/-10% of thickness, so a 1/2" thick piece of plastic facing can be "acceptable" if it measures .450 to .550" (.100" variation). Plastic is machinable if you can avoid melting issues, but I'd probably just make a hardwood face and move on.
    Wow good to know.
    I think your right and I'll just make a hardwood face. I have some nice sapele and other tropical pieces that I could use.

  9. #9
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    Quote Originally Posted by Wes Grass View Post
    Shimming it would be a good start. You already know how much you need, somewhere between 2 and 4 thicknesses of paper.

    I'd worry about hitting the extrusion if you tried to joint it. And I have no idea how it work going thru a planer. Some of these plastics are difficult to get a good finish on. Some you can just forget about sanding.
    I think this will be my first plan of attack. I can remove the screws holding the UHMW on and try sticking sheets of paper behind it to see. Thanks

  10. #10
    Quote Originally Posted by Lee Schierer View Post
    Hmm, 13 thousands isn't a lot and the fact that the far end of the fence goes back to zero. Are you seeing any burning on the sides of the cut? The hump in the middle should also be causing some burning on the side of the cut next to the blade on the fence side. Have you checked your blade alignment to the miter slot recently? An angled blade will give you the same type of problem you have noticed with almost no burning.
    I would bet that some table saw blades deflect more than what you are seeing in your fence.

  11. #11
    You can joint the face.

  12. #12
    It does wear. I replaced it with a piece of ply with laminate.

  13. #13
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    I have used a hand plane to plane uhmw if you own a hannd plane.

  14. #14
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    I too have hand planed UHMW but, for your task I wold be more inclined to put sand paper on a flat surface and rub it back and forth a bit. If your reference surface is good you should be able to gt much closer than you are. BTW, the problem you describe is what kept me from getting a UNI-T fence many years ago. I would not hesitate today having gained some skill in making things "un-crooked". ;-)
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  15. #15
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    Michael, if this was mine, I’d first call Peachtree and report the issue. They may sort it out for you.

    If I set out to fix it myself, I would mark the high spots on the fence, and plane those down with a block plane.

    Only if this failed would I replace the face with something else. Probably not hardwood — this moves. Waxed MDF?

    Regards from Perth

    Derek

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