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Threaded spindle nose/ backplate fit

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  • Threaded spindle nose/ backplate fit

    I have a new 6” 4-jaw that I need to mount on my 11” Logan with a 2-1/4” - 8 threaded spindle nose. I have a semi-finished back plate that I will be using. The threads are complete but I need to bore the register for the correct fit on the spindle nose register diameter. I haven’t been able to find any specific callout or recommendation for this fit (Lathes.co says “Finely-finished “register” to match spindle) but I would assume a slip fit, maybe .0005-.001” My mics don’t go that large so it’s going to have to be a digital caliper job. I will also be boring it while on my actual spindle so I only get one shot at it.
    Any advice, recommendations, suggestions or warnings would be appreciated.

    SM


  • #2
    Some will say a little clearance won’t effect performance, others will suggest what you did, a very close slip fit. My experience was that both chucks that came with my lathe were a close slip fit and it’s certainly not harming anything to make it as close as you can get it, so that’s what I would do.

    Digital calipers on the OD will give an acceptable measurement. The bigger challenge will be accurately measuring the ID as you machine it. Do you have telescoping gages? If not I’d make a plug from scrap that matches your spindle register as close as you can make it and use that for a test gauge as you machine.

    In the event you go a little small and the backplate doesn’t fit you can use the plug as a lap to open it up.
    Last edited by JCByrd24; 03-18-2021, 05:38 PM.

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    • #3
      with a 4-jaw, it doesn't matter a whole lot since you will indicate all your work anyway.
      But, if it a collet chuck, then you need to creep up slowly on the register dia. to make the two parts fit each other.
      Unless you got a cal lab, metrology lab, for both your ID and OD tools, you just have to creep up until you make 2 parts match fit

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      • #4
        Eh, I have chucks that are kinda close, and chucks that are very loose as to what you called the "register". They all repeat fine.

        That part is ONLY there to get the chuck started on the threads..... kinda the same idea as a "dog point screw", in reverse. It is NOT there to set the concentricity of the chuck, and I don't care WHO claims it is.

        There are lathes that have that part of the spindle actually cut away, relieved, and they are fine. Sherline is one of those.

        Think about it, if the unthreaded part were supposed to control alignment, you'd have the threads (a conical surface) and that unthreaded part, fighting one another. Would not work very well, which is why it is not done.

        Anyone who would use a parallel fit as an alignment means for removable parts, in a design, has a few things to learn.... like why there is such a thing as a "taper mount".

        Just make that section reasonably close, so it helps align the chuck to the threads. That's what it is for. The whole "register" thing is an urban legend among hobbyists.
        CNC machines only go through the motions

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        • #5
          In the process of machining the backing plate, you can unscrew the chuck/faceplate it is held by, check the fit, then screw the assembly back on the spindle. That's how I did mine.
          It's all mind over matter.
          If you don't mind, it don't matter.

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          • #6
            Make a copy of the spindle nose and use that as a GO gage to test for fit. It'll make things go really smoothly with the backplate plus it'll give you some nice turning and threading practice if you need it.

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            • #7
              I spend an evening looking at the need for a close fit between the un-threaded portions of the spindle and back plate/chuck ("register"). I started a discussion over at practical machinist.com describing my measurements and results that generated a lot of good discussion. Here's a link:

              https://www.practicalmachinist.com/v...tering-373761/

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              • #8
                My Myford has a register which is a very close fit for all of its backplates.My hands are old and arthritic, changing chucks is a chore which I try to avoid.. starting the chucks on the threads HURTS !
                My Southbend uses a variety of chucks with backplates from various sources, none of which in any way closely fit the register. Some have 20 thous clearance.
                Chuck changing is easy.
                The only times I have ever had poor results in fitting on Southbend chucks have been when I have not thoroughly cleaned the threads.

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                • #9
                  Originally posted by MarshSt View Post
                  .......................................... The threads are complete but I need to bore the register for the correct fit on the spindle nose register diameter. I haven’t been able to find any specific callout or recommendation for this fit ..................................................Any advice, recommendations, suggestions or warnings would be appreciated. SM
                  SM You will not find specs, because non exist* ! That is harsh reality
                  The "Register" concept is a old wives tale . Handed down by those who do not understand Helical Thread interfaces ,
                  To understand this, think of a nut on a threaded shaft and a load being placed on it....It will always center itself on the helix
                  You can't move it off
                  What is critical is that the rear face on the chuck plate be "Faced" at the same time as threads are cut ! That is imperative !
                  The Threads control 4 degrees of Freedom and the Face does the other two for full control of the 6 degrees

                  Rich

                  * There is a Metric DIN standard, but its a rule without basis
                  Green Bay, WI

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                  • #10
                    I gave my registers about .010 on the diameter, and the threads are about Class 3. Works perfectly. I used the calipers to do it. So, don't be afraid. As long as you face off the part at the same setting that you do the threading, you'll be OK. In other words, make sure the face that butts onto the spindle is at 90 deg to the axis of the threads.

                    You could make up a thick "spacer washer" to temporarily thread the back plate onto the spindle "backwards" without galling, and then take a clean-up facing cut across the spigot. Then, just bore it a bit bigger than the spindle register. Usually enough to just clean up the threads down to the root.
                    Last edited by nickel-city-fab; 03-18-2021, 10:40 PM. Reason: clarity
                    25 miles north of Buffalo NY, USA

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                    • #11
                      sorry, I read that incorrect. I thought you were on the register for the chuck itself.
                      I re-read, you are one the spindle thread. my bad

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                      • #12
                        Don't.

                        The threads are the resister. The "register" is just a place to jam chips up and cause problems.

                        I cut a backplate for a chuck a short while ago, a set tru. Dialed it to 3 tenths. Take it off and put it back on, still runs at 3 tenths. That is with the register some 10-20 thou over. All the chucks with that lathe have oversized registers, except for a homemade one, which is difficult to screw on.

                        Rich has it right. I threaded and faced my backplate in one setup. As he says, that is the critical operation.

                        Originally posted by Rich Carlstedt View Post

                        SM You will not find specs, because non exist* ! That is harsh reality
                        The "Register" concept is a old wives tale . Handed down by those who do not understand Helical Thread interfaces ,
                        To understand this, think of a nut on a threaded shaft and a load being placed on it....It will always center itself on the helix
                        You can't move it off
                        What is critical is that the rear face on the chuck plate be "Faced" at the same time as threads are cut ! That is imperative !
                        The Threads control 4 degrees of Freedom and the Face does the other two for full control of the 6 degrees

                        Rich

                        * There is a Metric DIN standard, but its a rule without basis
                        Originally posted by mattthemuppet View Post
                        Make a copy of the spindle nose and use that as a GO gage to test for fit. It'll make things go really smoothly with the backplate plus it'll give you some nice turning and threading practice if you need it.
                        This is what I did, and is the best way to go. You can gauge your go gauge to an existing chuck, or to PD wires (preferred).
                        Last edited by The Metal Butcher; 03-18-2021, 11:46 PM.
                        21" Royersford Excelsior CamelBack Drillpress Restoration
                        1943 Sidney 16x54 Lathe Restoration

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                        • #13
                          Originally posted by The Metal Butcher View Post
                          Don't.

                          The threads are the resister. The "register" is just a place to jam chips up and cause problems.

                          I cut a backplate for a chuck a short while ago, a set tru. Dialed it to 3 tenths. Take it off and put it back on, still runs at 3 tenths. That is with the register some 10-20 thou over. All the chucks with that lathe have oversized registers, except for a homemade one, which is difficult to screw on.

                          Rich has it right. I threaded and faced my backplate in one setup. As he says, that is the critical operation.
                          Problem is, his is already threaded, and I'll bet they left the rear surface of the spigot as cast, or rough.
                          On mine, there is a 1/4" length of spindle after the threads, which is at the same major diameter as the threads. Therefore you *need* to counter-bore it, in order to get the back plate to set square against the "face" of the spindle. I imagine he's running into the same thing, different brand. If no counterbore, then no repeatability, and massive galling. Like a long bolt, the threads do not go all the way up to the head. But the backplate still needs to contact the head. Like Rich said, it gives the last 2 degrees of constraint, and the threads do everything else.
                          Last edited by nickel-city-fab; 03-19-2021, 12:31 AM.
                          25 miles north of Buffalo NY, USA

                          Comment


                          • #14
                            Originally posted by nickel-city-fab View Post

                            Problem is, his is already threaded, and I'll bet they left the rear surface of the spigot as cast, or rough.
                            On mine, there is a 1/4" length of spindle after the threads, which is at the same major diameter as the threads. Therefore you *need* to counter-bore it, in order to get the back plate to set square against the "face" of the spindle. I imagine he's running into the same thing, different brand. If no counterbore, then no repeatability, and massive galling. Like a long bolt, the threads do not go all the way up to the head. But the backplate still needs to contact the head. Like Rich said, it gives the last 2 degrees of constraint, and the threads do everything else.
                            If the back is faced true to the threads, then no problem. He can chuck it up and bore the "register" oversize. If it's not, then it's a ****ty backplate.

                            Ok... I get what you're saying. It's gotta be bored. Yeah, just make it oversize then, call it a day.
                            21" Royersford Excelsior CamelBack Drillpress Restoration
                            1943 Sidney 16x54 Lathe Restoration

                            Comment


                            • #15
                              Originally posted by The Metal Butcher View Post

                              If the back is faced true to the threads, then no problem. He can chuck it up and bore the "register" oversize. If it's not, then it's a ****ty backplate.

                              Ok... I get what you're saying. It's gotta be bored. Yeah, just make it oversize then, call it a day.
                              Personally, I don't trust the partially machined ones. Even if they already finished the rear face, you don't know how good they did. They did it on their machine, not yours.
                              So yeah he has to face that rear face while having it mounted on the spindle. Then counter bore it to the major dia.

                              On my own stuff, I do just like Rich says. I don't even worry about how close it fits, except for the threads.
                              25 miles north of Buffalo NY, USA

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