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  • Practical coefficient of friction question.

    Dumb question.

    Whenever I move a milling machine with my pallet jack, I put some 1/8" masonite sheets between the forks and the base of the milling machine, to increase friction. Coefficient of friction for steel on steel being what it is, I'm fearful that it would be too easy for the milling machine to slip off the forks, particularly on a slope.

    Never had any slippage doing this.

    Now I'm about to affix a new 8" chuck to a 10" rotary table. The chuck is called "front mount", meaning it has 3 through holes to bolt the chuck directly to the t-slots of the rotary table. The bolts are 3/8".

    Should I consider some added friction between the two, or will the 3 bolts provide enough force to keep the chuck from slipping, steel on steel.

    I think I read somewhere that even newsprint can increase friction dramatically in such a scenario. Sort of like a fine sand paper.


  • #2
    Not a dumb question as when i first started machining i thought exactly on those lines your talking,

    you really should have no problem with slippage even with lubricating the surface first for rust protection, bolts tightened good will create metal to metal under extreme pressure and should be good enough unless you crash the machine,

    you have to keep in mind your nibbling metal away with a tool bit in a controlled fashion - if you hit the chuck on the side with a hammer it will most likely move but that's not what your doing (or should be doing) when machining...

    Edit; I will add, I have a self aligning taper mount that keeps my chuck centered on the R. table so my concern is lessened,

    my biggest problem that happens with material sticking out too far or taking a cut a little too aggressively with that set up is that the R.Table will start to "float chatter" so I drag both locking handles a little and that seems to take care of things...
    Last edited by A.K. Boomer; 02-21-2019, 05:19 PM.

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    • #3
      I wouldn't worry about the lack of friction between them. The 3 bolts will be more than sufficient to hold it in place and not move, unless you crash, or are really hogging something.

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      • #4
        It's a bolted joint. Steel on steel is much preferable to anything else added in the joint, including paper, paint, adhesive, etc. Just wipe the surfaces clean and bolt it together.

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        • #5
          Originally posted by Yondering View Post
          It's a bolted joint. Steel on steel is much preferable to anything else added in the joint, including paper, paint, adhesive, etc. Just wipe the surfaces clean and bolt it together.
          This. Clamping pressure is what's important - make sure it is clean and flat.
          www.thecogwheel.net

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          • #6
            Paper works , but , there can be a problem if you use coolants as the paper absorbs water or corrosive material.
            I have to ask , why not use a double end center plug. One end fits the center hole of the RT and the other sized to the chuck.
            Doing so eliminates any possible movement .

            Rich

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            • #7
              That is exactly how I mount my 6", four jaw on my 10" RT. Even the same three 3/8" bolts into standard Tee nuts. I have not had any problems.

              Physics Fact: The standard equations for frictional forces between two surfaces use the force that is normal (perpendicular) to their mutual surfaces and completely ignore the area of contact. They assume that the frictional force is proportional to that normal force and to a coefficient that depends on the materials. That is a fact. And that, at least to a first approximation, is how frictional forces are calculated in the real world.

              BUT, when you examine this on a microscopic level you find out that the normal force determines just how big each microscopic area of contact between the surfaces is. This is because of the surface roughness which all (well, most) materials have. So the frictional force is actually dependent on the actual area of real contact. And the greater the normal force is, the greater those small areas are, and the greater the frictional force is.

              I guess that Jo blocks in intimate contact would be a confirmation of this because they cling to each other with a large amount of frictional force even with zero normal force acting on them.
              Paul A.

              Make it fit.
              You can't win and there is a penalty for trying!

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              • #8
                You could make up some custom tee nuts which are longer than standard, and a close fit in the table top to enable a little extra tightening. Special long tee nuts could have a lip at the outer end to hook onto the outside of the table.
                That said, I don't think there is anything to worry about with just a standard setup and friction.
                Have a close look at your chuck, it might be possible to drill a second set of front mounting holes on the same pcd so all the tee slots are used. I have three jaw chucks that have been successfully modified this way, it depends on the individual chuck.
                It is not a dumb question to be concerned about a matter of safety.
                Last edited by old mart; 02-22-2019, 09:47 AM.

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                • #9
                  Originally posted by Rich Carlstedt View Post
                  ... why not use a double end center plug. One end fits the center hole of the RT and the other sized to the chuck.
                  Doing so eliminates any possible movement .

                  Rich
                  That was my original intention, but the bore in the table is MT3, and the bore in the chuck is like 2.5" diameter. Making such a "double end center plug" would be a challenge for this homey.

                  Even then, the run out on my lathe and any offset in the bore of the chuck would negate the whole purpose, wouldn't it?

                  But any tips would be gratefully received.

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                  • #10
                    Not adding to the friction, but helping align the chuck with the RT axis, I put a MT alignment bar in the table, and gently close the chuck jaws on it before tightening the chuck down. This gets the chuck lined up within 0.003",which is good enough for most work, or within easy tweaking with a dti. The alignment bar is not seated down fully, as it then is a problem to remove without drifting it from underneath the table.

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                    • #11
                      A slight drift but you should work to the finest tolerance you're able to measure. My mill head tram was within 1.0-1.5 Thousandths like all the major YT machinists claim to be sufficient. That may be fine for end mills under a half inch but spinning a fly cutter or face mill of significant diameter will create Elliptical Moon Craters in your work with that tolerance. I now have on order a couple of indicators to measure to .0001" and will spend the extra time to get my mill tram spot-on Zero! I will also use one to get my "indexable" carbides even on my face mills. A variance of just .001" on face mill carbides will make a huge difference in finish.

                      Back on topic:
                      I'll be mounting a chuck to my RT soon and planned to make an 8" back plate for the 6" 3-jaw chuck I just bought. I actually bought the chuck for use on my Dividing Head and since the 1 1/2-8 spindle on the DH is the same as my lathe, I just got a new lathe chuck too. I'm thinking I may use the old lathe chuck for the RT exclusively so I won't have to remove the chuck from it's RT backing plate and reinstalling the lathe backing plate. I may have to remove the arbor for chuck storage purposes but it will be threaded to receive the MT3 arbor. I just have to sync the plate mounting hole to the arbor height above the face of the RT.

                      I thought about using a MT3 to 1/2-13 arbor as the arbor for turning the backing plate for the chuck in the lathe. The lathe also has a 3MT headstock. After the chuck is mounted to the plate, the plate could be lowered onto the RT using the MT arbor to center. The plate would still be bolted down to the RT. (8" table, 8" plate, 6" chuck)

                      Another option would be to use the arbor in the lathe headstock to simply turn a step in the back of the chuck plate to fit the RT. This would solve the storage problem and risk loss of tolerance by having to remove the arbor.

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                      • #12
                        The more I think about it, the less enthused I am about mounting a chuck to the RT. I have a chuck that will fit the spindle of the Dividing Head and the DH will rotate to a 90* vertical. I'm thinking I'll stick with that plan if I need to mount something in a chuck.

                        These discussions always get me thinking needlessly.

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                        • #13
                          Its not much bother to mount chucks directly to a rotary table, I can mount 5" three jaw, 6" four jaw and er25 collet on the 6" table at the museum. They can be exchanged in minutes, and all of them can go on the lathe as well. I can bore soft jaws in the three jaw using the lathe or the mill if necessary.
                          Mounting a 6" chuck on an 8" backlpate has an added advantage, it can then be clamped directly to the mill bed if the need arises.
                          Last edited by old mart; 02-22-2019, 05:35 PM.

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                          • #14
                            I see your chuck on the R/T is leaving roughly an inch of T-slot visible. You could use more T-nuts to bolt on some short bars that touch against the OD of the chuck. That should help keep it centered.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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