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Kickback on Miter Saw - How I did it and how it broke my saw

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  • Kickback on Miter Saw - How I did it and how it broke my saw

    Well, I guess I need to share my misadventure in which I killed my beloved Hitachi C12FDH dual bevel miter saw and its blade.
    I was trying to cut off the corners of a 3.5” square in preparation for making a circle out of it. Basically I had a miter saw kickback.
    In 20-20 hindsight, here’s what went wrong.
    • Tried cutting too short a piece. 3-1/2” square
    • Plastic HDPE, not wood so it was a little harder.
    • Trapped the piece by cutting on the left with the miter angle 45 degrees to the left and a stop block clamped to the saw behind the piece
    • Used a hold down stick with a rubber pad to hold the piece, in retrospect this rested on the center of the piece and afforded no resistance to rotation
    • Having a positive hook angle blade… it was a small positive hook angle but I incorrectly thought it was a negative hook angle, as a result there was a small force trying to lift the workpiece.
    • Failed to recognize the classic table saw kickback formula (nearly square piece about 3.5” square trapped between the rip fence and blade), some lifting forces.
    Anyway, here’s the sequence of events.
    1. Cut off the first corner OK.
    2. Rotated to cut the second corner. I did not realize at this time that the cutoff corner now rested against the fence and presented a much shorter contact area to resist rotation.
    3. When I pressed my holddown to the piece and turn on the saw, I lowered the blade and there was a huge bang just like a kick-back upon which I released the switch.
    4. What I figured happened afterwards is that the saw blade bit into the workpiece and put a rotational moment on it that the hold down, centered as it was, did not resist at all. The fact that I cut the outer edge gave it a large moment of force.
    5. The workpiece actually force itself to rotate 90 degrees as The third corner had a lot of scoring marks and I only was on the second corner.
    6. The force of the piece turning, but constrained by the blade and the stop block and the fence, put a huge amount of force on the fence and the blade
    7. The left fence casting upon examination was slightly bent from the force.
    8. The blade a 12” thin kerf, 96 tooth CMT deflected to the right.
    9. The blade hit several things, The right fence – made of two pieces, the top part hinged for dual bevel, snapped the casting into two pieces and the blade teeth ate a significant damage into the front edge of the fence.
    10. The blade also twisted and is between the sides of a steel chute, perhaps inch wide that forms the sawdust ejection guide.The edge of the blade caught the chute and cut into it about of an inch, destroying a lot of carbide teeth.The chute is cut and deformed.
    11. The sudden deceleration of the blade caused a lot of damage to the saw. Aside from the dust chute, the main overarm casting has a 3-4 inch crack that extends through the arbor bearing hole in the casting.
    12. The bearing thrust holder is cracked.
    13. The blade is twisted into a pretzel and embedded in the dust chute. Many of the teeth are missing. I was unable to pull the saw blade from the gash in the chute.
    I did a quick estimation from a parts site and the fence parts that were bent and broken added up to around a hundred dollars before I was through; this did not even include the cracked housing which was $185. Clearly with assembly issues and hidden damage, repair would be huge. I ordered a replacement reconditioned saw of the same model from CPO for $190. And a new miter blade.
    Moral of the story, think it through a little more on unusual cuts. This one cost me $270 and fortunately no one was hurt.

    Pictures below show some of the damage. Click on them for enlarged views.
    Attached Files
    Last edited by LCHIEN; 11-12-2019, 04:01 AM.
    Loring in Katy, TX USA
    If your only tool is a hammer, you tend to treat all problems as if they were nails.
    BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

  • #2
    Wow! Impressive how much energy was clearly directed into the saw structure! Very glad that you came through unscathed. Thank you for sharing the story - it helps remind us to be cautious and thoughtful when setting up for a cut.
    I'm curious about how you did, or are going to complete the type of cut you are trying to accomplish.

    Comment


    • #3
      I saw the first thumbnail and I was hoping you were using a hold down stick (which you were). That is an impressive amount of damage. I'm glad you weren't hurt. I would not have realized I was in a kickback situation.

      Comment


      • #4
        I'm glad you were not hurt! Thanks for posting that. I have had more kickbacks on the miter saw than on TS's. But they have always been small pieces and I just move to something else for small cuts.

        This accident points out a very specific area of danger of MS's that is often overlooked: Inherent MS design prevents good hold down of small pieces and blade rotation "cuts Upward."

        On the pen forum, there are a few there who use a MS to make their blanks. Scary!
        Hank Lee

        Experience is what you get when you don't get what you wanted!

        Comment


        • #5
          Originally posted by Slik Geek View Post
          I'm curious about how you did, or are going to complete the type of cut you are trying to accomplish.
          This was a couple of months ago.
          I still haven't finished the project.... gunshy,
          Thought about it:
          Going to use the bandsaw. I think.


          Loring in Katy, TX USA
          If your only tool is a hammer, you tend to treat all problems as if they were nails.
          BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

          Comment


          • #6
            About the only thing I could salvage was the belt. and a few screws and bolts. And the hold down clamp. The blade was firmly wedged into the steel shroud and I could not move or rotate it. Even after removing the arbor bolt.

            I put the whole mess out one morning with the garbage. Some jogger knocked on the door and asked if I was throwing the saw away and if he could have it and I said sure, but the main housing is cracked. He didn't take it.
            Last edited by LCHIEN; 11-12-2019, 12:04 PM.
            Loring in Katy, TX USA
            If your only tool is a hammer, you tend to treat all problems as if they were nails.
            BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

            Comment


            • #7
              Wow, I would not have imagined such damage from a kickback like that!

              I've read the warning of kickbacks on the CMS, but see such damage to the saw itself is pretty alarming. I'm certainly happy to know that you didn't get hurt, but I'm sure it scared the heck out of you... surely it would have more than done so to me as I have a real challenge whenever I use any kind of 'circular' saw.

              I greatly appreciate your post, as it will make me even more aware whenever I use my Ridgid 10" CMS.... something I am already in high respect for.

              Glad you are okay,

              CWS
              Think it Through Before You Do!

              Comment


              • #8
                Boy it did a number on that saw! A saw explosion like that will make you re-think your hold down techniques. After several kick backs on the CMS with small pieces I decided no more small pieces, if the stock wasn’t long enough to be clamped to the fence it wouldn’t be cut on this saw.... But I still got a couple of kick backs when the small piece cut off and got tangled up in the blade. No way to prevent it. That proves to me that the wrong saw was used. I use my small hand saw more now. My band saw gets more use now for a small cut off like this, but don’t think the band saw is immune to saw explosions though. Sounds like we need another thread just to discuss band saw boo-boo’s

                Comment


                • #9
                  Makes me want to re-think thin kerf blades on a CMS. That wasn't the root cause of the problem but it seems blade deflection did magnify the consequences. And thin kerf + large diameter makes it that much more flexible. I don't use my CMS all that often; most cross cuts are done on the BT3 or a radial arm saw as they are both more-or-less stored in ready to use positions... the CMS is on a Ridgid MSUV and the whole assembly is rather wedged into a corner. It's more trouble than it's worth to set up for one or two cuts. Maybe a good thing safety-wise? I too tend to use the bandsaw for small cuts, or a benchtop scroll saw. Neither makes as straight a cut as a CMS would... but it's generally easy to hand-plane or file to the line afterwords. Think I'll keep doing what I've been doing after seeing those pics. Glad you weren't hurt.

                  Another CMS factor, especially for non-sliders like mine: the blade is cutting into the work from above... typically many teeth are actively engaged at the same time so there is a lot of force going into the workpiece. Table saws, bandsaws, and a sliding CMS can cut into the thinner edge of the material so only a few teeth are engaged at any one time resulting in less force. Of course the table saw has the disadvantage of the back of the blade coming UP from the saw table, trying to throw the work back at you...

                  I wonder how much engineering goes into "when something goes wrong" or "what could go wrong" on some of these power tools. I.e. if the blade suddenly is stopped by getting jammed in the workpiece, all that kinetic energy plus the still-running motor energy (basically they become two flywheels) has to go somewhere or be resisted by structure. Are there designed-in weak points (mechanical fuses, drive belts that snap, etc.) for such events? Is the structure sized to absorb the energy without totally coming apart - yielding/bending/cracking are okay as long as the blade doesn't end up "off the saw" somewhere for example? The SawStop cartridge is designed with that type of thinking: a sacrificial part to absorb the energy in a controlled manner with the cartridge and trunion supports further designed with some sort of weak spot or catch that "gives" to allow the whole shebang to drop into the saw body, arcing away from errant fingers and hotdogs Modern cars with "crumple zones" to absorb crash energy - keeping as much of it away from the occupant cell - are another example. The resulting front-end damage looks horrendous to most folks but is engineering "doing what it's supposed to do" artwork at its finest to those who understand it. Somehow I doubt, given the rapid year-to-year tool redesigns, that such "energy absorption" is part of the overall design. Yet your Hitachi, as mangled as it ended up, might be an example of "failing in a controlled manner" to make sure the blade didn't "depart" (fly away from) the saw. Such considerations are a big part of commercial transport aircraft design which is one reason they are so safe. "If this breaks off or gets knocked off, what might it hit and what collateral damage can it cause? Design it so that a) it shouldn't break, and b) make sure it can't hit anything critical."

                  mpc

                  p.s. something I heard at a local woodworking retailer: SawStop provides a certain number of cartridges per store per year for the hotdog demos; the saw that is used though can never be sold to a customer after a certain number of demos because the repeated structural stresses may lead to small bends, deformations, or even cracks/damage in the saw guts that could adversely affect the saw performance and/or safety... I guess SawStop doesn't want such damaged goods sold. A couple of "trips" of cartridges by SawStop owners won't cause issues but accumulating lots of demo "trips" evidently can affect the fatigue life of the saw structure.

                  Comment


                  • LCHIEN
                    LCHIEN commented
                    Editing a comment
                    To me the miter saw is for cutting long boards. (Yeah what was I doing cutting 3.5") so putting it in a corner is not going to allow it to do its job. I don't like to crosscut long boards on the table saw because the miter bar support is only so long and the long moving board wants to twist when you are cutting anywhere but the middle. The miter saw is the moving saw tool and the table saw is the moving workpiece tool.
                    Last edited by LCHIEN; 11-13-2019, 05:35 AM.

                • #10
                  I agree with 'capncarl' about the only thing perhaps safe enough in my shop to cut small pieces would be my small 9-inch band saw. I'm not at all comfortable with either my CMS or my BT-table saw unless the stock is at least a foot long. My RAS is probably my most comfortable tool in that I've never had a single kickback or climb incident in the forty-plus years that I've had it; but, I don't cut anything that is smaller than four inches (I keep my fingers out of what I consider a danger zone, and even then, I use a push stick to hold the object.

                  Still I am quite surprised that the blade bent like that. Even with a thin-kerf blade, I thought they were much more rigid than that.

                  This is a great post and I'm glad that Loring enlightened us. It certainly was a lesson to me and I'll surely approach my CMS with even more caution.

                  CWS
                  Think it Through Before You Do!

                  Comment


                  • #11
                    CMS’s (or the ones that I have personally used) are fitted from the factory with a different blade that is used on a table saw, and replacements are available at the big box stores. I didn’t notice them being thin kerf but their angle is almost reverse. A few years after I purchased a Harbor Freight saw blade sharpener I went on a mission to sharpen every dull blade I had in the shop. When I got to the blade on my Hitachi CMS I found it to be totally different from all the other table saw blades. Not to be one that looks for extra work I figured it wouldn’t be much difference if I sharpened the blade with whatever I already had the sharpener set up for. I’m glad that Hitachi blade had large pieces of carbide because I had to grind a lot to get it near that the others were ground to. Boy what a mistake! This blade on this saw is a death trap. I done ruint me a good saw blade! It was so aggressive that it would pull across the wood....about 400 mph. The pucker factor was maxed out. Don’t do this at home!

                    Comment


                    • #12
                      Wow, that sucks. I had a couple of minor kickbacks recently, that made me remember that the CMS isn't quite as totally safe as it usually seems. It was warped stuff with lots of interesting defects in the grain. Very beautiful, but lots of internal stresses. In a couple cases, it "grabbed" the blade after being cut. I started to realize which direction to face up in order to minimize the grab.

                      I also learned that either my Hitachi 12" SCMS has some kind of kick/grab detection that stopped the motor, or it kicked hard enough to remove my finger from the trigger. I don't want to find out how reliable that is in the future.

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