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Thread: blade size and cutting rpm

  1. #1
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    blade size and cutting rpm

    Hi all;
    This is kinda about the same piece of equipment I am probably going to buy, my 20" dewalt ras, but I have been alerted to a potential problem.

    I was laboring under the assumption that i could use a 12" blade any time I wanted.

    I have since been told that the saw may not have enough rpm for a 12" blade. the motor says rpm 2825/3425. it is set up for 20" but will it cut ok with a 12" if I need it too?

  2. #2
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    20 inches down to 12 is a big drop in diameter. If you were able to cut 4 inches with the 20 inch blade, you have just taken away 4 inches, so you are down to nothing. If your new RAS would cut 6 inches deep, you are now down to 2 inches depth of cut with the tiny 12 inch blade.

    Remember that a RAS likes a negative hook blade, or it will climb and attack you. Is your 12 inch blade negative hook?

    When you go to a large blade saw the motor of a RAS has to pass over the work, and can get in the way, and the arbor washer tends to be pretty large.
    Charlie Plesums, Austin Texas
    (Retired early to become a custom furnituremaker)
    Lots of my free advice at www.solowoodworker.com

  3. #3
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    Absolutely the negative hook blade! I just put a dado set on my RAS. I had one without the negative hook blades. That saw was lethal. What a difference the negative hook set made! Much, much, much easier to control. I cannot begin to imagine the power of a 20" blade. When the blade climbs the wood and races back at you, it is a very scary time. I don't know that the thing cannot break the stops and come off the rail.

    MY RAS is designed for a 9" blade. I had an 8" dado set on it that scared the soup out of me. The new negative hook dado set is 6". Much tamer.
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  4. #4
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    Yes. I have learned about the need for a negative hook angle on the teeth for a ras. I had a small dewalt ras and tried using my frued dado blade on it. On my 3rd dado the blades climbed on top of the wood and stalled the motor. IT was the last time I used it.

    My worry now is that 2800 rpm may not be enough speed to turn a 10" dado blade. from what i know about changing the drive sprocket on a motor cycle the diameter correlates with the speed it travels. I think the outside edge of a 20" blade would travel twice as fast as the out side edge of a 10" blade at the same rpm. Is that correct and do yo think 2800 rpm would be enough speed for smaller blades.

  5. #5
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    I don't know what the speed for wood is but that you looking at SFM or Surface Feet per Minute. You can calculate the circumference of the blade times the RPM and see how the distance/speed the blade is turning. But 10" vs 20" is going to be less than half if I remember right. Been a LONG time since I calculated these things and I don't know the recommended speeds for wood. But yes, you are a looking at big difference that could mean it would work, but you would have to feed the wood much slower.
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  6. #6
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    Quote Originally Posted by Charlie Plesums View Post
    Remember that a RAS likes a negative hook blade, or it will climb and attack you. Is your 12 inch blade negative hook?
    Lets not go overboard (pun intended) in explaining the tactics of a RAS. Even with a positive hooked blade, in order for the blade to ride over the work, would require room between the table and the arm...it just isn't there. The saw may feel like it's coming at you or lurch, but that's about it. If the saw can make it on top of the stock, your saw needs a lot of adjustment and shouldn't be used until it's brought into specification.The cause being feeding the saw into the work too fast, or a very dull blade. The RAS is no different than any other tool in that it has it's operating quirks, and good operating techniques makes the saw a very safe tool. You might attribute good practice to have a "feel" for the procedure.





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  7. #7
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    I don't think anyone is trying to say the saw will lift up and come running after you. But a RAS by definition cuts in the climb mode where the direction of the blade helps to feed the stock, opposite to the way you feed a tablesaw, router table, shaper without auto feeder, circular saw, planer, jointer... The point is just that there is much less tendency to self-feed when you have a negative rake blade, so it is easier to remain in control. Can you remain in control with a positive rake blade? Of course, but lurching and feeding too fast (AKA attacking you, and sometimes leading to the saw trying to bite off more than it can chew, stalling a small saw) are nice to avoid easily.

    I don't think there would be a problem using a smaller blade on the saw, as long as it has the depth of cut you need. At worst you might need to cut more slowly to get the same surface, but the decreased tooth speed comes with better leverage, so more force behind the tooth.
    Last edited by Alan Schwabacher; 03-03-2012 at 03:19 PM.

  8. #8
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    Now that i see how much quality large diameter blades are I sure hope there wont be a problem. I just got sticker shocked when I looked up the cost of a large dado set. even an 18" 90 tooth cross cut blade is pretty pricy. For cross cutting sheet good I really hope I can use a 12" cross cut blade with a 1" diameter arbor hole. One thing I never considered.....

    how much would it cost to sharpen a 20" blade.

  9. #9
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    You have moved into the league of professional equipment, not the high volume mass produced blades, etc.

    Sharpening is often based on the number of teeth - I figure about 25 cents per tooth ($10 for a 40 tooth blade) but most sharpening services now use a computer controlled sharpener, to get all the angles perfect. Be sure your sharpening service can handle a 20 inch blade. The sharpening machines I have seen probably would not.
    Charlie Plesums, Austin Texas
    (Retired early to become a custom furnituremaker)
    Lots of my free advice at www.solowoodworker.com

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