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Thread: slab flattening sled

  1. #1
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    slab flattening sled

    I am going to make a sled to flatten slabs plan to use linear rails (SBR series)

    There are SBR12, 16 and 20 (in mm's) and wondering if anyone has built a sled with these rails and what size did you go with.

    here is an example of a set that is 20mm (SBR20)

    https://www.amazon.com/gp/product/B0...A27FLKY64E6N9J

    Thanks in advance
    Distraction could lead to dismemberment!

  2. #2
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    I'm confused. In the usual router bridge slab flattening jig, there are fixed rails which are parallel to the sides of the slab, and there are moving rails which run cross-grain, which the router rides on. Which of those are you talking about?

  3. #3
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    If you are making a power driven sled those should work well. If you are making a manual sled they are a waste of money IMO. Wooden rails work just fine. And you don't need any rails at all if you have a flat bench/table for a base.



    John

  4. #4
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    I used a set of twin bed frame rails. Width set to match the router base. End rails can be adjusted for height.
    Joe

  5. #5
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    Quote Originally Posted by Jamie Buxton View Post
    I'm confused. In the usual router bridge slab flattening jig, there are fixed rails which are parallel to the sides of the slab, and there are moving rails which run cross-grain, which the router rides on. Which of those are you talking about?
    It's two pair, one longer pair that go on either side of the slab. The second pair holds the router and rides perpendicular on the first set of rails. The end result is much like Woodpecker's sled.
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  6. #6
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    If you decide to go this route, I'd definitely use the heavier 20mm diameter product for durability. The concern I have is that the longest length is "only" 2000mm (about 78") so that's going to be limiting if you intend to mill longer slabs without repositioning them...something that can be "fun" while trying to maintain registration.
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    The most expensive tool is the one you buy "cheaply" and often...

  7. #7
    I would look at the slab flattening sled that Lee Valley sells. If you didn’t want to buy, there design seems to be good and probably easy to duplicate the router holder, cause the rails are pipe you supply.

  8. #8
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    Quote Originally Posted by Jim Becker View Post
    If you decide to go this route, I'd definitely use the heavier 20mm diameter product for durability. The concern I have is that the longest length is "only" 2000mm (about 78") so that's going to be limiting if you intend to mill longer slabs without repositioning them...something that can be "fun" while trying to maintain registration.
    The rails are designed to extend by adding another length, just like railroad tracks.
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  9. #9
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    Quote Originally Posted by Justin Rapp View Post
    The rails are designed to extend by adding another length, just like railroad tracks.
    True. I overlooked that. My bad.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  10. #10
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    My flattening rig is all-wood, and manually operated. One issue is that the cross-grain rails can flex under the weight of the router and my hand pressure. Flexing and bouncing leaves divots in the workpiece surface. So my rails are 5" tall hardwood. If you use the linked rails - particularly the small-diameter ones - as the sole supports for your router, I think they'll flex. You can fix that with wood rails, so the wood supports the weight, and the linear bearing is the running surface for the router.

  11. #11
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    I made mine with a set of linear guide rails bought on eBay. Not very expensive and very smooth.

  12. #12
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    Quote Originally Posted by Jamie Buxton View Post
    My flattening rig is all-wood, and manually operated. One issue is that the cross-grain rails can flex under the weight of the router and my hand pressure. Flexing and bouncing leaves divots in the workpiece surface. So my rails are 5" tall hardwood. If you use the linked rails - particularly the small-diameter ones - as the sole supports for your router, I think they'll flex. You can fix that with wood rails, so the wood supports the weight, and the linear bearing is the running surface for the router.
    Well i'll definitely go with the 20mm so the rails are stronger. I don't think they will flex under the weight of a router. These thing are used for larger machine heads that are much heavier than a router. My concern is if my porter cable 1 3/4 hp will be able to swing the 2" flattening bit. I have a 3 1/4 in my router table which was a motor only purchase, not a plunge.
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  13. #13
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    Quote Originally Posted by Dan Gaylin View Post
    I made mine with a set of linear guide rails bought on eBay. Not very expensive and very smooth.
    Which ones did you go with?
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  14. #14
    I used 80/20 series 10 extrusions with the linear bearings mounted on top for the longer run and below for the cross piece. I used it on a slab that was ~4x11. You could expand the basic idea to any size really.

  15. #15
    Quote Originally Posted by Justin Rapp View Post
    My concern is if my porter cable 1 3/4 hp will be able to swing the 2" flattening bit.
    That is a large bit so you will need to observe the max rpms (probably around 15,000) and take light cuts. I have a PC 7518 motor in my cnc router and run a 2 1/4" spoilboard bit, typically removing .020" at 250-300 in./min on ultralight mdf. That is a comfortable rate on my machine - with a deeper cut the rpms slow down. I would recommend a smaller diameter bit as well as a more powerful router for what you are doing as you will want to knock down the high spots quickly and that will be a pain with the depth of cut you will be limited to. When you run a large bit into a high spot it can be an unwelcome surprise. With a 1 1/4" bit you will be able to flatten a good percentage of a typical piece with your first pass at a greater depth of cut.

    I have done a bit of slab flattening by hand and it is quite tedious and dusty. If you can rig up some dust collection it would be a good thing. If you have a regular need for it you may want to invest in cnc or sub it out.
    Last edited by Kevin Jenness; 12-21-2021 at 9:43 AM.

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