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  • #16
    What do mold makers use Silly Putty for?
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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    • #17
      To check the cavity for texture, appearance, engraving and such, if it shows up in silly putty it will show up in the plastic part. Silly putty is basically silicone based. Silicone flashes in a mold at around .0002-.0003 air gap.

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      • #18
        Cool -- thanks McRuff!
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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        • #19
          3-wire measurement

          There might be a re-think needed here.

          If I had the need for a 3-wire set (I've got one), I'd look for other options first - such a nut with a hole tapped with a good HSS tap. I'd make nuts and cut a radial slot (hack-saw) through one side and about half-way through the other (sort of like a common button die) on two of them and leave the other/third "as is". I'd spread those nuts with a screw-driver/wedge in the slot so that I had three nuts to be used progressively as regards "fit" as I was working my way through the thread cutting in this order:
          - "like a dick in a shirt-sleeve" (loosest fit);
          - "getting close"; and
          - "that's it".

          Using the 3-wire method, keeping the 3 wires under control is not all that is needed. Measuring accurately to the order of accuracy suggested by the need for the 3-wire method is pretty well an art in itself.

          The number of points that must remain in contact simultaneously is very large.

          Each of the three wires will have three simultaneous points of contact: the two flanks of the thread and the face of either the spindle or anvil of the micrometer.

          The micrometer axis must not only be normal to the thread axis but pass through it while keeping all 3 wires in place.

          The order of accuracy is to be better than 0.001" and usually much better.

          Use of the 3-wire method pre-supposes and requires a thread form (and tool) that is very accurate indeed - which is or may be very difficult in a HSM environment.

          I'd practice pretty extensively first on a known good (read: precision ground) thread first to get consistent good measurements as there is quite some practice and skill needed.

          There is quite a market for tools/"gadgets"? to hold the 3 wires.

          Frankly, I'd opt for a thread micrometer with a set of changeable anvils if the job was that important and there was no other alternative.

          http://www.machineryhouse.com.au/Pro...stockCode=Q138

          http://www.google.com.au/search?hl=e...e+Search&meta=

          The "3-wire method" has been "talked-up" as a sort of highly necessary/essential skill that is absolutely needed as a "right of passage" and a "holy grail" for someone to be called a "Machinist". I have three responses to that:
          - maybe so;
          - maybe no; and
          - BS.

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          • #20
            Originally posted by oldtiffie
            Frankly, I'd opt for a thread micrometer with a set of changeable anvils if the job was that important and there was no other alternative.
            I bought a thread micrometer from CDCO for $50. Works great -- way quicker than thread wires. It's limited to 0 - 1", so in the rare cases where I'm doing threads bigger than that, I use thread wires.
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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            • #21
              I'm not so sure that Oldtiffie's concerns are that important. It is true that there are a lot of points of contact but if the mike's jaw faces are parallel and the mike is approximately centered on the thread AND the wires are FREE TO ASSUME ANY POSITION, then the method is self-aligning.

              In practice, the mike jaw faces will be parallel beyond most shops ability to measure and with a reasonable amount of care the mike will be centered well enough (+/- 1/4 jaw diameter). That leaves the wire's freedom to move into the desired position. Frankly, I do not like the foam or clay ideas as these may restrict the movement of the wires as the mike jaws are closed. The wires will need to be fairly accurately positioned (parallel and properly spaced) by hand in these holding devices before the mike is applied. A wire that is at an angle will rise up (or down) in the thread and give an inaccurate reading. Clay or foam may not allow the wire to assume the proper position. One possible alteration to these methods would be to remove the foam or clay as the mike is closed, thereby allowing the wires complete freedom of motion. It could be restored after the reading is taken and before opening the mike.

              I would use the grease. It will easily squeeze out of the points or lines of contact as the mike is closed and will not affect the measurement by more than +/-0.0001". And it will not stop the wires from assuming the correct position. Of course, you will need to clean up the wires and mike afterwards.

              I have seen the pictures of rubber bands used for this. It would probably work for larger threads and wires. But again, they may hold the wires at an angle.

              Another important consideration is the point nature of the contact between the wires and the thread. It will not take much pressure to push the wires into the thread distorting it several tenths and causing an inaccurate reading. A light touch on the mike is definitely called for. The softer the material being threaded, the lighter the touch needed. This is also in opposition to the need to accurately position the wires as with clay or foam you may need excessive pressure from the mike to get a good position.

              This could also be a consideration with Forrest's tape method. If I were using tape, I would EQUALLY space the three wires about 1.5 to 2 diameters apart along the edge of the tape. The single (bottom) wire would be the MIDDLE one and the two end wires would be the two (top) ones. The tape would be in roughly a circle in use. And I would use "Scotch" tape, not duct tape as it is a lot more flexible to allow the wires to self position as needed.

              All of the above may sound excessive. But for most threads, I do not even think of using the three-wire method. IMHO, it is only for the most accurate ones and there you need to take the best precautions. Oldtiffie's comments here are dead on and even not sufficient. Any roughness in the cut will also add to inaccuracy in the three-wire method. You really need to be doing superior work for it to be appropriate. For regular threads I just use a nut or tapped hole (gauge) and the feel of the fit and I bet most machinists do likewise.
              Last edited by Paul Alciatore; 06-14-2008, 02:26 PM.
              Paul A.
              SE Texas

              And if you look REAL close at an analog signal,
              You will find that it has discrete steps.

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              • #22
                Inside too

                Good response Paul.

                But there is far too much emphasis on the "external" thread for which "3-wire", "thread micrometer" et al can all be used.

                Fine - but there is the small matter of the "internal" thread - "female"/"nut" as none of those methods apply. The most likely scenario in the average HSM shop is that the "nuts" will be either tapped or "made to fit" (the "screw"). That is no bad thing as if those two parts are only ever likely to be used together, a "fit" that is "good enough" will suffice more often than not. This particularly applies to things such as "backing plates" for chucks etc. that may only ever go on the screwed spindle of a (particular) lathe.

                "Class of fit" - and its various names/guises - was only ever made to ensure inter-changeability within specified limits for required parts when in a "production" environment or where parts are made in different shops etc.

                A major problem with the "3-wire" - and to some extent the "thread micrometer" approach is that the most difficult thing to get right is the symmetry of the (assuming it is accurate) threading tool. Usually - or all too often this "alignment" is done with the side of a tool-holder against the face of a chuck of with a "thread form tool" (aka "fish").

                This can be quite difficult with a small threading tool (for "finer" threads) in a "cramped" environment. You only have to be "off" by a fraction of a degree and all of the "super accuracy" tools and settings can be for naught.

                I suggest that any who doubt it, draw it all up in a CAD environment and see for themselves.

                Cutting that "last thou" can be a problem too if the tool is not very sharp and well oiled (cutting oil) as it may either "slip/rub" and/or "dig in". This is where a DRO or dial indicator on the cross-slide is essential - "setting" that "thou" (or so) on the dial is one thing but making sure that it is repeated on the cross-slide may well be another.

                So, unless there are compelling reasons for the "super accurate" and/or "high tech" approaches, use the "tried and true" methods that most of us can and do.

                There is nothing wrong with thread-cutting on the lathe to almost finish the thread and then "following up" with a tap, die or "mating part" as the thread will be "true" before they are applied to "finish it off".

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                • #23
                  Originally posted by oldtiffie
                  "Class of fit" - and its various names/guises - was only ever made to ensure inter-changeability within specified limits for required parts when in a "production" environment or where parts are made in different shops etc.
                  Class of fit comes into play in the Home Shop when you're replacing a leadscrew. The 2G Acme taps that Enco and MSC sell, for example, are a Class 2 fit which is really too sloppy for a cross-feed or compound leadscrew.

                  By the way Paul: I also do the "match the nut" class of fit But with a thread micrometer, you can measure the thread depth as fast as you can find and fit the nut...
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                  • #24
                    I also like thread mics.

                    I have thread wires, thread triangles, and several thread mics.

                    The mics are handiest, but you need a bunch of them. Each one is good over a range, and even within the range they are not perfect everywhere. In fact they are never perfect.

                    The wires are the most hassle, but are the real deal for measuring at the pitch line. Of course, they can totally MISS other errors that might be caught by the triangles or mics.......

                    The triangles are intermediate in hassle.

                    The triangles sorta check pitch diameter and form in one fell swoop, but can be fooled by certain errors. Ditto for mics. Both depend on fitting the correct form and seating across the area of the pitch line.

                    If you really want the truth, you have to use the wires AND check thread form separately. Almost certainly, your thread wires are not of that caliber (mine aren't, they are a set, each size covering a range) , and so any of the three methods will work about as well for you.

                    To be correct, each pitch must use just ONE perfect size of wires. The "one size fits many" sets are really not a lot better than mics or triangles, and are not made as precisely as they "should be" anyway.

                    hey, if you use ANY of the methods to actually verify the thread pitch diameter, you are way ahead of most all "practical home shop guys" for whom any nut of generally that pitch is all that is needed as a gage.
                    CNC machines only go through the motions

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                    • #25
                      Check the reference

                      Originally Posted by oldtiffie
                      "Class of fit" - and its various names/guises - was only ever made to ensure inter-changeability within specified limits for required parts when in a "production" environment or where parts are made in different shops etc.
                      Originally Posted by lazlo
                      Class of fit comes into play in the Home Shop when you're replacing a leadscrew. The 2G Acme taps that Enco and MSC sell, for example, are a Class 2 fit which is really too sloppy for a cross-feed or compound leadscrew.

                      By the way Paul: I also do the "match the nut" class of fit But with a thread micrometer, you can measure the thread depth as fast as you can find and fit the nut...
                      The acme tap mentioned may well be over-sized, but equally the "original" cross/top-slide lead-screw may be either over or under size and worn (as well). The screw pitch micrometer should pick that up.

                      There are many machines yet that are turning out good work despite back-lash in the order of 1/8 to 1/4 turn of "back-lash" which can be any combination of wear in the nut and/or lead-screw plus end-play at the dial position.

                      Minimmal/zero combined back-lash, wear and end-play are nowhere near as critical in a manual machine (if at all) as they are in a CNC(-ed) machine.

                      The mention of buying a tap and measuring a lead-screw suggests that it is the nut that needs to be replaced or manufactured.

                      This being the case there is no way of using "3-wires" or a thread micrometer to measure the female threads within the nut. This seems to necessitate or pre-supposed the need to grind an acme screw-thread tool to shape and screw the nut to suit the lead-screw anyway.

                      Despite all the varying opinions, I think I'd strongly advise reading"Measuring Screw Threads" at page 1,893 in my 27th. Edition of "Machinery's Handbook".

                      It is heavy going but it will make you more aware in detail of what the facts and situation really are.

                      Selecting the right sized precision hardened wire pairs is critical as is the pressure exerted on the micrometer.

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                      • #26
                        Originally posted by oldtiffie
                        The acme tap mentioned may well be over-sized, but equally the "original" cross/top-slide lead-screw may be either over or under size and worn (as well). The screw pitch micrometer should pick that up.

                        There are many machines yet that are turning out good work despite back-lash in the order of 1/8 to 1/4 turn
                        True, you can get by with 1/4 turn of backlash, but the goal of replacing a leadscrew is to improve the backlash, not replicate it.

                        What a lot of folks do is tap the nut with a 2G (Class 2) acme tap, and then turn the leadscrew to a tight fit. So you end up with Class 3 nut/screw pair, with a non-standard thread geometry (which doesn't matter because you're not interchanging the screw).
                        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                        • #27
                          I think I just found "the" answer to 3-wire measuring of threads on Tony's Lathe website. The photo tells it all:

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                          • #28
                            Screw options

                            Originally Posted by oldtiffie
                            The acme tap mentioned may well be over-sized, but equally the "original" cross/top-slide lead-screw may be either over or under size and worn (as well). The screw pitch micrometer should pick that up.

                            There are many machines yet that are turning out good work despite back-lash in the order of 1/8 to 1/4 turn
                            Originally posted by lazlo
                            True, you can get by with 1/4 turn of backlash, but the goal of replacing a leadscrew is to improve the backlash, not replicate it.

                            What a lot of folks do is tap the nut with a 2G (Class 2) acme tap, and then turn the leadscrew to a tight fit. So you end up with Class 3 nut/screw pair, with a non-standard thread geometry (which doesn't matter because you're not interchanging the screw).
                            Exactly so.

                            There is no way that I am aware of that both the nut and screw can be checked to a standard that even suggests let alone approaches a Metrology standard in the average HSM shop or most small to medium production or jobbing shops.

                            So, in most/many cases at least one has to be made "to fit" to suit the other.

                            So, the options are really to check the screw using "3-wires" or a thread micrometer - which can do very well indeed in the right hands - and check for wear and "size".

                            The options for the screw are:
                            - buy a new screw and use it "as is" and make the nut "to suit";
                            - if the screw is unevenly worn, (re)machine it for a consistent reading using the "3-wires" or "thread micrometer" and either accept the "clearance" or lesser quality fit and/or make a new nut "to suit"; or
                            - leave it and use it "as is".

                            The options for the "nut" are generally:
                            - buy it and use it "as is"; and/or
                            - make one and get a "fit that fits" manually and using experience by using the "screw" as a "master" or "guage".

                            Frankly, if i were that concerned about the quality of the lead-screw, I'd be concerned about the quality of the cross-slide and top/compound slide assemblies as well.

                            If that were a problem, I'd be using either a DRO or a good dial indicator on the cross-slide.

                            I cannot and will not speak for others as every case and every person is different and if they use different paths to achieve a satisfactory result then all methods are correct even if different.

                            Many machines - mills in particular - have "back-lash eliminator/reducer" mechanisms on the "nut/s". This even applies on the better ground ball-screws as used in CNC(ed) machines. They effectively "adjust" the "back-lash" in the "nut" by either open/closing the "gap" or moving one part relative to another in a circular motion (typically in good "ball-nuts").

                            CNC(ed) machines by their nature require very little or no combined lead-screw "back-lash" and "end-play" where-as "manual" machines do not.

                            While I agree that the least practical end-play and back-lash in a machine nut/screw combination is desirable in a "manual" (ie "non-CNCN-ed" machine), I doubt that it is pratically achievable nor really necessary in most cases. If it were so there would be an awful lot of other wise perfectly useful machines on the scrap-heap.

                            It is odd (well it is to me) that all the discussion on lead-screws and nuts centres on everything else but and to the exclusion of the lathe lead-screw and half nuts (including "end-play/float") which despite all the frequent concern for "quality" turned threads hardly - if ever - gets any consideration or mention. I would guess that many lathes have more effective "back-lash" and "end play" than any other screw/nut combination on the lathe. It never seems to bother any-body and very few seem to be concerned about "repairing" or making new main lead-screws and half nuts. I wonder how many even think of let alone measure the "wear" in the lead-screw and half-nuts. The reason I mention this is that the lead -screw will be "making" any other lead-screws or nuts that are required on the lathe - unless the owner/operator has access to another lathe - and the "accuracy" of the lead-screw may be crucial - but is it ever checked? Of not - why not. It will be a good excerise in use of "three-wires" unless you have a (usually) "Acme" thread micrometer!!

                            It will all boil down to the requirements of the machinist/owner to determine for himself.

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                            • #29
                              Agree Mick. In Foundation of Mechanical Accuracy, the Moore boys would lap a precision screw, measure the OD, and then use a set of set of custom taps to tap a prototype nut. They would intentionally tap the bronze nut oversize, and then force-fit it onto the Nitrided (hardened) screw, and lap the pair until the screw ran smoothly.

                              At that point, the non-standard screw and nut were mated for life.

                              Originally posted by oldtiffie
                              There is no way that I am aware of that both the nut and screw can be checked to a standard that even suggests let alone approaches a Metrology standard in the average HSM shop or most small to medium production or jobbing shops.
                              Way Off-Topic, but I've actually read/heard of two ways that precision shops measure inside threads: one is with the metrology-grade Repro-Rubber, which supposedly matches features to within some Un-Godly tolerance. The Old-School version of Repro-Rubber is a Cerro-Bend (Wood's Metal) variant that Steve Acker mentioned in the gunsmith columns of Machinist's Workshop a couple of times: Cerro-Safe. It has a melting point of 165؛ F, and shrinks very slightly for the first 30 minutes after its poured, and then returns to exactly the original dimensions. Gunsmiths use it to measure gun chambers.

                              http://www.hitechalloys.com/hitechalloys_002.htm
                              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                              • #30
                                It is most accurate metod for measuring the diameter of the screw thread.But the measurment is a difficult task.The thread check measuring system provides an easy and accurate way of measuring diameter.The thread measuring wires are held in predetermined positon by light pressure clips.
                                Questions? Ask it here!
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