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Depth of threaded hole ?

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  • Depth of threaded hole ?

    I am building a steam engine from some old drawings, lots of holes to drill and tap (in cast iron). I have always thought that the maximum strength of a thread was the depth equal to the diameter. Well on these drawings the depth of threads are shown to be about three times the diameter of the screws. Are the drawings going over the top or am I wrong in my assumptions ? David

  • #2
    No, that might be correct for the application. Cast is rather unique and at that time in history cast was not as stable as current production material.


    • #3
      Thread Depth

      Regardless of material, you want the thread depth to be more than the minimum amount the fastener will need. For a thread of the same material as the fastener, 1 to 1 might be OK, but, for example, cast iron will be a lot softer than a grade 8 bolt. I like to go for at least 1-1/2 depth to diameter, although sometimes that isn't possible. I would be fine with making the hole 3 x diameter and thread as deep as a plug tap will readily go. Best of all is to drill and tap through any time you can.


      • #4
        IIRC, that figure is for steel bolts in aluminum. Three times the bolt diameter gives equal strength in the threads and the bolt. I could be wrong-

        I would be tempted to at least go twice the width of the bolt in cast iron, just from a gut feeling. The figure of three times diameter for hole depth might be correct if using grade 8 bolts with a maximum of rip-out strength required. Look at head bolts in engines- never seen one that lacked lots of thread length in the cast. You probably will be using strong bolts- yeah, I'd say go with the callout on your plan.

        The other thing is that maybe the length of the threaded hole is to accommodate the variations in bolt length, and not knowing exactly where the end of the bolt will come to when properly tightened. Maybe the depth of the hole is to allow for that.
        Last edited by darryl; 12-04-2010, 05:07 PM.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


        • #5
          When drilling a hole to tap you have to consider chip clearance in the bottom of the hole and the tapered tip of the tap. Unless your using a bottom tap there in an length at the end of the tap that makes no thread.
          It's only ink and paper


          • #6
            Originally posted by Carld
            When drilling a hole to tap you have to consider chip clearance in the bottom of the hole and the tapered tip of the tap. Unless your using a bottom tap there in an length at the end of the tap that makes no thread.
            My thoughts exactly. Except I would add that even a bottoming tap will have 1-2 partially formed or unformed threads at the bottom. If possible, many would prefer to use the taper tap only and not have a second or third operation with the plug and bottoming taps. Add chip clearance and even a small extra factor for safety or to avoid cutting a standard length screw and you can easily get a drilling depth of three diameters or more. Oh, and you do chamfer the holes, don't you. That will easily wipe out a thread, more if you are generous here.

            In most cases it does no harm to go deeper but if there are too few threads holding you can easily have problems. I would not try to minimize the holding power of the screws in an engine. And drilling is perhaps the easiest shop operation.
            Last edited by Paul Alciatore; 12-04-2010, 08:19 PM.
            Paul A.
            SE Texas

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


            • #7
              As stated by toolguy, a 1 to 1 ratio ( diameter to depth) is fine for like materials, but
              Cast Iron is probably 20 K (gray) to 40,000 (ductal) pounds tensile.
              If you use a mild steel screw (60 K), you would need 1.5 to 3 times diameter for depth to gain max joint strength.
              For Aluminum at 10 K tensil , you need 6 times. This you can see is impractical in such cases, but it illustrates the point !
              If you are worried about stripping the hole, go to whatever you can get.
              Most machinery building designs, like 1.5 as a number to use, unless it is a critical application, then you look at tensile.
              You should be quite safe in a model with 1.5

              Tap drill depth is one diameter deeper than tap depth (look at a standard tap and you see 1 diameter to full thread).
              So design engineers will spec drilled hole depth at 3 times(tap) diameter if they want a 1.5 thread depth engagement, as it gives them 2 times threaded surface ( the "extra" half diameter is safety for longer screws)
              Remember on drilling, the cutting lip angle is not counted, only full diameter length is considered

              Last edited by Rich Carlstedt; 12-04-2010, 11:16 PM.


              • #8
                The depth is probably almost irrelevant, as soon as you get to maybe 3x diameter.

                The real determinant SHOULD be the relative Young's modulus, because long depth really may have NO EFFECT.

                The screw stretches under load, by a predictable amount.

                A perfectly rigid screw, or one which is vastly less in elasticity than the material it is threaded into, could hold approximately equal force on every incremental area of thread contact.

                But a real screw is elastic.

                The very last thread, at the "bottom" of the screw, carries a certain load. Each thread "above" it in the hole carries a load also. At each succeeding thread, the stress in the screw shank is the sum of all the thread loads "below" it.

                So at the "top" thread, the stress in the screw shank is more than at the "bottom".

                Therefore the stretch in the shank is also more at the top than teh "bottom".

                if any "lower" thread carries MORE load, the stress and stretch will increase at each point "above" that thread.

                It might be possible for the top threads to move enough due to stretch to actually fracture less elastic material, transferring the load to lower threads.

                And it might be possible for the "bottom" end of the screw to actually carry nearly ZERO load, because the stretch in the "upper" parts of the screw has put almost the full load onto some number of the "upper" threads. That would be the case if the hole is very rigid, and the screw is elastic.
                Last edited by J Tiers; 12-05-2010, 11:35 AM.

                Keep eye on ball.
                Hashim Khan


                • #9
                  don't forget

                  Not only do you have to consider length of engagement, you also have to consider depth of thread engagement. It is possible that the call out for length took into account for say 50% engagement.


                  • #10
                    With a taper tap you have to drill deep to get the correct thread length but you can alway tap it again with a plug tap then a bottom tap to get more thread. Thread length should be 1.5 x the diameter of the bolt for steel. I like for things to be strong and safe and what if the bolt becomes loose and unscrews a few threads on its own will it be strong enough? If your taping aluminum you need more thread. For mechanical punch press dies I would go 2 x bolt diameter.
                    Last edited by gary350; 12-05-2010, 11:19 PM.