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  • Drive dogs, what's their intended purpose?

    I saw a video recently from Stefan Gotteswinter where he mentioned manual mills don't really need the drive dogs, because the taper in combination with a threaded drawbar creates enough holding power on it's own. But on CNC machines they are supposedly important because the systems there do not develop as much pressure.

    That seems logical to me, but then I was told by someone else who claims to have worked 30 years in the machining business that the drive dogs on CNC machines are just for "locating" and if the taper lets go those drive dogs will be ruined and they are no good for holding the tool so it does not slip. Whereas Gotteswinter seems to say they are needed to prevent slippage.

    So now I am wondering if that's true. I am always sceptical of opinions from a single person on the net, I try and find what the aggregate opinion is.

  • #2
    Hmmmm...... the 30 taper tool holders on my CNC mill don't have drive dogs, after many years of use they show no signs of slipping it their taper. From memory, the 40 taper tool holders on my formerly owned non-CNC mill did have drive dogs. The 200 series Universal Kwik-switch holders on my non-tool changer CNC mill have drive dogs, but those may be more for the tool change fixture than for the spindle of the mill..


    I've read the tang on Morse tapers is not to prevent slippage, only to provide a contact for removal wedges.

    FWIW, it seems in theory,at least, that shallow tapers are less likely to slip than more severe tapers like mill spindles have.

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    • #3
      Well. If the taper spins, you potentially ruin the spindle bore. I'd say that's reason enough for a drive dog. If the spin ruins the drive dog, you have other problems and should probably sell your equipment. Only partially kidding- anybody can have an accident and wreck something, but you're supposed to pay attention and not do things like that.

      Some of my tooling uses a pair of dogs that fit into slots in the spindle- other things use a simple pin that nests into one of the slots. I use music wire for the pins, or drill rod. Never had a problem with it not being able to take the torque. The only time I don't use a dog of some kind is if there's nowhere to put it.

      Another way to look at this is if you have to tighten a taper so much that it won't spin under load, then you'll probably have a hell of a time removing it. I don't know if it's ever been suggested to draw a taper up that tightly. I've never found the need myself. But then I don't own anything CNC, and I don't know if there's any built-in 'spin prevention'- that would probably make it hard to use tooling with drive dogs that would have to be oriented before the tool is 'homed'. But the other possibility that I'm not aware of is whether the tooling can hold tight, yet still be easily removable. All my experience has been with MT3. I've never seen any machine with a tool changer use this taper, so the best advice I can give is if you are using MT3, then do what you can to prevent spin, short of over-torqueing the draw bar.

      On manual machines, some use the R-8 taper. I'm sure someone will chime in and give us the lowdown on that.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Read

        https://en.wikipedia.org/wiki/Machine_taper


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        • #5
          The general rule of thumb is: self-releasing tapers need drive dogs while self-holding tapers do not. The little key in an R8 spindle is for locating / holding the collet in place while tightening the drawbar and would absolutely be trashed if you spin the collet. Just because your friend has 30 years experience in the machine industry with CNC machines doesn't mean he ever graduated from R8 spindles.

          I've never seen a CAT40 or CAT50 spindle that didn't have nose keys / drive dogs. Self-releasing tapers can't handle as much torque as a binding taper can so I'm with Gotteswinter on this one. In fact, it's covered in Machinery's Handbook. I remember designing a cone clutch for a go-kart way back when and they had a bunch of formulas for torque vs angle and at what angle the tapers become self-holding.

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          • #6
            Also remember - mills used to/might still have have gear boxes. The K&T's low gear is 32:1... So with our measly 5hp spindle motor at 15 ft-lbs at 1800rpm - that is 480 ft-lbs at 56rpm.

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            • #7
              Originally posted by Fasttrack View Post
              The general rule of thumb is: self-releasing tapers need drive dogs while self-holding tapers do not. The little key in an R8 spindle is for locating / holding the collet in place while tightening the drawbar and would absolutely be trashed if you spin the collet. Just because your friend has 30 years experience in the machine industry with CNC machines doesn't mean he ever graduated from R8 spindles.

              I've never seen a CAT40 or CAT50 spindle that didn't have nose keys / drive dogs. Self-releasing tapers can't handle as much torque as a binding taper can so I'm with Gotteswinter on this one. In fact, it's covered in Machinery's Handbook. I remember designing a cone clutch for a go-kart way back when and they had a bunch of formulas for torque vs angle and at what angle the tapers become self-holding.
              Gotteswinter was talking about his self-releasing ISO30 taper in this instance though when he said they did not need drive dogs, here's the video, 44 minutes in or so:
              https://www.youtube.com/watch?v=OllQ7FdBmP8

              His claim was only machines that don't use threaded drawbars need drive dogs.

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              • #8
                Drive dogs on 30-40-50 taper can verge on belt & suspenders, but the folks at K&T and Cincinnati didn’t put them there for decoration either. 1/2” end mill at 10k rpm is night and day from 8” slab mill at 50 rpm.

                And no, Morse Taper tangs ARE NOT there to transmit torque.

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                • #9
                  Like DR said, the universal engineering kwik switch has the dogs but not for holding. They are there to use with the bench mounted nut tightening fixture. The nut is what keeps things from slippin and sliding. And if setting up on the mill itself, say with not a collet holder (bench mounted) but an end mill holder it keeps the holder from slipping in the spindle as you use the spanner to tighten the nut. The nut along with the taper is plenty to keep it from spinning. JR

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                  • #10
                    I’ve seen a few 40&50 Spindles on Mills that they only run one key or dog,one fellow said that’s for protection in case of a crash.

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                    • #11
                      I've read the tang on Morse tapers is not to prevent slippage, only to provide a contact for removal wedges.
                      Correct. The tang on those Morse taper tools that actually do have a tang only "engages" with the removal wedge, nothing else. I have cut off the tang on an MT3 drill chuck adapter so that the lathe tail stock could read "0" rather than starting at about 5/8in. All the holding on MT is provided by its shallow taper, and in fact in many cases you could shorten the taper by a fair bit and still have plenty of holding power.
                      "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                      • #12
                        None of the NMTB, ISO, BT or CAT etc. spindle tapers are self-holding tapers. All of them should always be used with drive keys. R-8 should really be used with a drive key also but they are notorious for breaking and causing all sorts of damage.

                        While any taper can be run without drive keys if they're pulled in tight enough, they have to be cranked down pretty tight to hold well without them and have no worries of slippage. I sure as hell would never run my 30 taper machine without drive keys. I'm not sure why anyone would want to do that. And all bets are off if you run a large flycutter or something that takes a hit with a large interrupted cut or something. Absolutely foolish not to use the drive keys if you've got them IMO unless it's a very very light duty application.

                        FWIW, I've run machines with spindles up to 7" in diameter and pushing 100 HP+. The drive keys can and do indeed transmit torque. The spindle taper on a non-self-holding taper machine is there to positively align and locate the tool/holder, not to drive it. Those keys are stout and usually hardened, set in keyways both on the face of the spindle and the holder. If you shear even one of those keys, you have a serious problem somewhere, that may require a spindle rebuild afterwards. These machines are not meant to have a "safety valve" like a slipping taper - if those types of thing are present they are designed in such that they don't cause damage if they fail. A spring loaded dog clutch for instance.
                        Last edited by eKretz; 06-22-2022, 03:11 PM.

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                        • #13
                          Well that's a different take than what I've hard so far that's for sure. I googled on PM and Forest Addy there said the R8 keys where useless too. It's clear this is an area of differing opinions.

                          For what it's worth, I don't use drive dogs with my deckel which has a 40 taper. And I notice the deckel collets for it, which interface directly to the taper, cannot take a drive dog.

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                          • #14
                            Originally posted by DennisCA View Post
                            Drive dogs, what's their intended purpose? .............
                            I would say that it is in the name ! Thats what they are for !
                            The argument about whether they are needed or not is rediculas,

                            The question is what do you want to do with the tool ?
                            You want 50,000 RPM and can pull the drawbar up with 2 tons of force on a 1/8 endmill fine , you will never slip
                            You want a face mill at 400 RPM and don't know the drawbar pull .hold on to your pants

                            Rich

                            It's like a machine with a "E" stop.. never use it But there may be a time !
                            Green Bay, WI

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                            • #15
                              probably want something more than a taper...

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