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  1. #1
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    Default Mitres on a disc sander or linisher

    Following some posts on another thread, I promised to write a thread about doing mitres on a disc sander or linisher, rather than on some sort of saw. Why would you want to do this? Well, it has a few advantages. If you hand-cut your mitres you'll need a shooting board to clean them up - a time consuming step, and it can be difficult to get the lengths exactly correct. Drop saws and table-saws need to be set up accurately and precisely - possibly time consuming, and sometimes they don't stay that way for long, especially with drop saws. On cheaper saws, the quality of the cut may not be very good, and still lneed to be cleaned up on a shooting board. Also, mitre gauges (unless you have something like the Incra) are notoriously inaccurate and sloppy in the track.

    On the sander, the fence has to be set every time the method is used, but it's quick to do, and you know that it's right. So let's get started.

    First, dress the timber to size and cut the pieces slightly longer than their finished length. (This can be done before or after the joints are marked out.) It is important that the top and bottom of the material are parallel, as the material will be inverted to make one of the mitres on each board.

    As with any other method, mark out the joints on all the sides using a sharp marking knife.The cuts should be distinct, but not too deep, especially the cuts on the outside of them box. The reason will be apparent later.

    Before making the joints the sander needs to be set up. First, square the table to the disc.
    Squaring sanding disc table 1.jpg

    Next, clamp the fence to the table at as close to 45 deg as possible. I have the angle marked on the table. Use two test pieces to check that the angle is really 45. If you put them together and they add up to 90 deg, they are correct.
    Setting sanding disc fence 1.jpg

    Next, check that the fence is perpendicular to the table. If it's not quite perpendicular, adjust it by slightly changing the position of the clamp head under the table.
    Squaring sanding disc fence 1.jpg

    Once all is set up, I always do another test of the angle, just to be sure nothing has moved. You are now ready to start.

    With the inside face of the wood against the fence, feed the material into the disc. Don't force it too hard against the fence, you don't want it to move. As the marked mitre gets close to the disc, take it easy. When the cut line reaches the disc, you will see a small feather of timber form. When this happens, the mitre is in the correct position. Now you can understand why the cut should be shallow. The deeper the cut, the bigger will be the microscopic flat on the thin end of the mitre.

    When the feather forms, it should be even from top to bottom. If it isn't, you can slightly adjust it by applying a little pressure to the top of the board if the feather is forming at the bottom, or easing the pressure off if it starts forming at the top.

    Okay, dress and mark a few pieces of scrap and give it a go.

    Any questions?
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  3. #2
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    Default

    +1 for Alexís approach. I have an MDF jig which sits on my 300mm sander:

    5C659F89-CA37-4D5D-9A6E-FD60DFD7F386.jpeg

  4. #3
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    I should get off my backside and make myself a proper fence like that.
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  5. #4
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    If using a disc sander, wouldn't the abrasive need to be stuck directly onto the metal disc to make sure everything is perfectly flat? Then the table/jig could referenced accurately off this.

    The usual velcro backed discs, combined with the "give" in the velcro backer attached to the sander plate surely wouldn't give very flat/accurate mitres?

  6. #5
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    Apr 2014
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    Iíve always used sticky-backed discs on my two sanders for exactly that reason. Iíd be interested if anyone has tried this with hook-and-loop discs.

  7. #6
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    A guillotine miter trimmer like this works great. Just keep your fingers away from the blades when using it.
    I never had luck disc sanding an accurate miter.
    I think I read somewhere. Remove adhesive back disc when the adhesive disc is still hot from use.
    Attached Images Attached Images

  8. #7
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    I use the same method for metal working like on this SS furnace trolley I made a couple of months back.
    The mitres are all TIG fusion welded (no filler rod used) so they need to be nice and snug.
    I use a 75 x75 x 5 mm piece of angle as a fence on teh docs sander.

    FInished.JPG

  9. #8
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    Quote Originally Posted by Mr Brush View Post
    If using a disc sander, wouldn't the abrasive need to be stuck directly onto the metal disc to make sure everything is perfectly flat? Then the table/jig could referenced accurately off this.

    The usual velcro backed discs, combined with the "give" in the velcro backer attached to the sander plate surely wouldn't give very flat/accurate mitres?
    I use velcro-backed discs, and they work fine.
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  10. #9
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    What grit do you recommend Alex?

  11. #10
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    Looks like 120 on mine, it's been there for years. I wouldn't go any coarser.
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  12. #11
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    Quote Originally Posted by Mr Brush View Post
    If using a disc sander, wouldn't the abrasive need to be stuck directly onto the metal disc to make sure everything is perfectly flat? Then the table/jig could referenced accurately off this.

    The usual velcro backed discs, combined with the "give" in the velcro backer attached to the sander plate surely wouldn't give very flat/accurate mitres?
    It's not so much that backed papers becomes inaccurate, as more that the edge leading in becomes rounded over in comparison to the trailing edge. I guess that the disc 'bunches up' before the cut when pressing the work into the paper.

    This can be remediated to a degree by using very light touches when approaching the final angle.

    I've also noticed that pieces greater than 6mm in thickness suffer less of this rounding compared to thinner pieces.

    Either way, I prefer other methods to make such mitres... IMHO it's one job where hand tools excel for both speed and accuracy.l
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc

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