Results 1 to 12 of 12

Thread: Geometry mystery

  1. #1
    Join Date
    Nov 2016
    Location
    Itapevi, SP - Brazil
    Posts
    391

    Question Geometry mystery

    Perhaps some of you can cast some light to one fact happened last week and I couldn't explain its reason.

    I used my planer sledge to get some tapered legs for a project I am doing - actually I have made tapered cuts in the past but always using my table saw, it was the first time using my planer and its sledge. Please see the attached schematics.

    Tappered legs.jpg

    I used two supports, one 5 mm height and another one 10 mm. It looked me perfect except the planned surface is around a couple of mm convex!

    I cannot figure why as the legs were attached to sledge with hot glue... perhaps the legs flexed at the middle?

    Personally I liked the slightly convex legs and I prefer them as a boring straight one but it is disturbing do not understand why I got that. I am used with Geometry but I missed that - your input to explain that will be very welcome!

    Thanks in advance for your help!
    Last edited by Osvaldo Cristo; 07-11-2019 at 8:36 AM.
    All the best.

    Osvaldo.

  2. #2
    Join Date
    Apr 2013
    Location
    Stone Mountain, GA
    Posts
    417
    Looks like there is no support in the middle of the span between the two points where it rests on the sled. So the center portion of the workpiece will flex away from the cutter, and less material will be removed there than at the ends. Result is a convex surface.

  3. #3
    Join Date
    May 2012
    Location
    Gatineau, Québec
    Posts
    116

    Cool

    Quote Originally Posted by Osvaldo Cristo View Post
    Tappered legs.jpg

    I used two supports, one 5 mm height and another one 10 mm. It looked me perfect except the planned surface is around a couple of mm convex!

    I cannot figure why as the legs were attached to sledge with hot glue... perhaps the legs flexed at the middle?

    !

    Osvaldo:

    I am pretty sure that your geometry is adequate . I suspect that your sled was not sufficiently rigid and that, as you suspect, the legs flexed in the middle section.

    The good thing is that you like the end result, even though it was not planned.

    Regards,

    Jacques

    PS: Looks like Robert answered the question while I was typing.

  4. #4
    Join Date
    Dec 2012
    Location
    Bedford, NH
    Posts
    1,210
    Robert is correct. Deflection of the wood while under the cutting forces will result in less material being removed, more at midspan. How long are the leg and what is the cross-sectional size? You could wedge under the legs to provide support. If you do, you could glue sandpaper to the wedges to prevent slippage.
    Thoughts entering one's mind need not exit one's mouth!
    As I age my memory fades .... and that's a load off my mind!

    "We Live In The Land Of The Free, Only Because Of The Brave"
    “The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
    "
    Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill

  5. #5
    Join Date
    Sep 2016
    Location
    Modesto, CA, USA
    Posts
    2,751
    Agree it is the flex caused by the infeed roller, pressure bar, chip breaker and outfeed rollers all pushing down with a strong force. I do not think the cutter really cases much deflection. I would guess 20-50 KG total force on a smaller planer.
    I have found that when I plane a piece like that I get it to the thickness that looks good to my eye then run it one more time removing about 1/16" more. this seems to make the piece a little lighter and better looking when it is all connected together.
    Bil lD
    Bil lD

  6. #6
    Join Date
    Dec 2005
    Location
    West Lafayette, IN
    Posts
    5,115
    Flex is my guess as well. Planers put a lot of pressure down.

    You can also taper on the jointer.
    https://youtu.be/nZTXvsrzbSQ

  7. #7
    Join Date
    Jan 2013
    Location
    Pittsburgh, PA
    Posts
    753
    Osvaldo,

    I think you may have discovered a new a new way to make cauls ...


    Actually I am serious!
    Too much to do...Not enough time...life is too short!

  8. #8
    I was just thinking the same thing

  9. #9
    Join Date
    Nov 2016
    Location
    Itapevi, SP - Brazil
    Posts
    391
    Quote Originally Posted by Bill Space View Post
    Osvaldo,

    I think you may have discovered a new a new way to make cauls ...


    Actually I am serious!
    You are correct!

    I will put that idea at work this weekend!
    All the best.

    Osvaldo.

  10. #10
    I suspect that the the sled is where the deflection occurred. Was it just a peice of plywood or some other sheet good?

  11. #11
    Join Date
    Apr 2017
    Location
    Michigan
    Posts
    688
    Do you have your feed arrow pointing the wrong way?

  12. #12
    Join Date
    Nov 2016
    Location
    Itapevi, SP - Brazil
    Posts
    391
    Quote Originally Posted by Tom Bender View Post
    Do you have your feed arrow pointing the wrong way?
    No. It is that...

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •