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How does a hammer drill work?

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  • How does a hammer drill work?

    I have a garage sale hammer drill, I think the type is called 'SDS', it takes a special type of drill (I got a couple with it and a chisel).

    The machine has a vertical motor axis and a simple pinion drive to rotate the chuck and there is also another drive that operates a piston which looks like it would be hitting the back of the chuck shaft.

    There is no hammer action although the 'piston' operates and the drill rotates according to the settings of the two levers.

    So how do these things work? Surely the piston does not just hammer on the end of the chuck shaft? It seems much too lightly made for that mode of operation.

    Brand name is 'Cobra', generic Chinese I assume.

    Maybe there is some trick I dont know about?


  • #2

    Here is a simple graphic.


    • #3
      thats the hammer & yes it does hit the inside/top of the chuck...some have a seperate striker pin through the chuck onto the top of the drill


      • #4
        They ( at leats the ones I used) don't hammer until you put pressure on the drill bit. I hated drilling the holes for my vehicle hoist, 12 3/4" holes in fibermesh super hard concrete. Each hole took about 15 minutes to drill. After awhile I had a buddy stand on the drill while I pulled the trigger. I was so happy when that final hole was drilled!


        • #5
          the piston that hits the back of the drill bit is called a flight piston. it is not connected to anything internally but skudes back and forth from the piston thats connected to a crankshaft thats pushing and pulling air in the cylinder. both pistons have orings on them.

          bosch uses a special grease, hilti uses special oil. . .

          the smaller tools use a cam action with return springs. . .


          • #6
            Not merely semantics, but a hammer drill is a totally different mechanism and concept from an SDS type rotary hammer.

            The SDS type rotary hammer has a much more effective hammer action than the typical hammer drill. Most are able to be used in rotary mode, hammer only, and of course the two actions can be combined.
            Check the link below to see what I believe the OP has, it's pretty well self explanatory.

            Home, down in the valley behind the Red Angus
            Bad Decisions Make Good Stories​


            • #7
              If I recall correctly a sds works by forcing a piston down a cylinder and never actually comes into contact with any part of the chuck or tool. contacting against the head of a chisel for example would soon splay out the contact surface and make the chisel impossible to remove from the drill.


              • #8
                Originally posted by Willy
                Most are able to be used in rotary mode, hammer only, and of course the two actions can be combined.
                Thats commonly known as the RTB switch.


                Just south of Sudspumpwater UK


                • #9
                  shipto is right. An SDS is more sophisticated, and uses a central piston.

                  Having said that, I do not enjoy using my SDS. I suppose it's good for really heavy hammer drilling, but I find it makes holes too large, and it almost carves out a cavern within the material.

                  In chisel mode I find it pretty weak.

                  I've been called out before for using a DeWalt product.

                  If I want fairly fast and very accurate hammer drilling, I revert to my ten year old Bosch.
                  Richard - SW London, UK, EU.


                  • #10
                    if you are having problems with hole size on an sds drill:- operator error, crap bits or crap drill....its the only way to drill hard building materials, I have had one for getting on 20 years (when they were seriously expensive and bits were only to order), and would not even concider using anything else these days


                    • #11
                      Thanks everyone....

                      DavidH, there was a pot of grease came with the drill so I put a dollop in front of the piston and it works, at least for now! Thanks.

                      Willy, that is a good diagram, this drill is quite similar, thanks.
                      Last edited by The Artful Bodger; 09-20-2011, 04:50 PM.


                      • #12
                        The grease is used up ..

                        You put it in the little chamber where the crank is ..and it gets sucked up by the piston ..and makes its way to the the chuck.

                        the grease is used as a piston sealer whilst its there .

                        There are a few things to look out for ..the ball bearings that hold the drill shank in ..can fall out .

                        they go in the holes ..the holes get worn out .

                        There are also balls inside the gearbox ..same things happen to them ..keeper holes get elongated and eye shaped ..soft metal ...and balls wonder ..then and don't keep it in hammer mode ..

                        if you get to that stage before the motors burn out can count yourself a lucky man ..
                        basically they are crap ..probably a copy of a hilti ,..but in soft hardening ..with a crap motor .

                        all the best.markj


                        • #13
                          I have 4.. one 20 years old the rest 10,4,&2,, none have chuck problems... but all are decent makes and always decent quality bits.... the old one is having a few issues, bearings mainly, but god only knows how many holes it has drilled.

                          IIRC the system was a Bosch invention

                          is that a yellow crap & decker i see before me?


                          • #14
                            i was talking about the chinese ones ..what you see before you is one of them ..

                            probably the NUtool Ufool brand

                            not that in calling you a fool ..its just a dirt cheap brand sold in the uk .

                            All the best.markj


                            • #15
                              Fortunately I do not expect to have much work for this cheap machine, but it does work and drilled the two holes I needed without any more problems.