I screwed up a press fit part. How do I fix it?
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  1. #1
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    Default I screwed up a press fit part. How do I fix it?

    This is a part from a motion picture film scanner I'm rebuilding. The black part is new: aluminum that we cut on a CNC router and just got back from powder coating. The steel spring-loaded clip mechanism is from the original machine. I'm trying to re-use these more complex parts where possible. This is the clip for a door on a housing that contains a mirror. it needs to be opened and closed fairly frequently, for cleaning.

    img_1419b.jpg

    In any case, in my excitement to assemble everything, I put the damned latch on the wrong side of the black part. Knocked it out with a punch and hammer and when I went to install it on the other side, it's loose. Well, not loose but once it's in I can spin it more or less freely by hand - it's a snug fit, but it rotates and it shouldn't. The issue is that the position (rotationally speaking) of the post that holds the spring is critical, as that sets the spring's tension, and holds the latch in the closed position. The spring will want to rotate the shaft so that there's no tension and because it spins relatively freely, this is what's happening.

    So the question is - what do I do here to get this part to fit properly. I know Loctite is an option but i'm also not sure what the optimal position for the spring is, so I'll need to play with it a bit. And will it hold up with this being a part that will see frequent handling, as well as the spring putting constant tension on it?

    I managed to do so much on this project with basically no machining experience, and wouldn't you know I make a major screw up after spending a small fortune on powder coating a bunch of stuff!

    Any advice is welcome.

    Thanks!
    Attached Thumbnails Attached Thumbnails img_1419.jpg  

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    Just peen it, should hold. If not and the back side is flush/flat you could drill and tap the pin for a small flat head screw so that the screw head acts as a key (part of the c'sink in the black part and part in the pin face).

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  4. #3
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    You might try putting a large ball bearing on the hole and giving it a good tap a couple times, should tighten it right up

    Sent from my SM-G973U using Tapatalk

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    Make a small V-block out of a piece of hardwood or aluminum and have someone carefully hold the pin
    end in the block while you raise some linear ridges in the pin with a sharp cold chisel. Three little grooves
    or ridges should be more than enough to tighten up the fit...

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    I would go with Loctite. It does work really well, and there are various grades so you could experiment with a weak one, mark the location, then use red for final fit.

    The various methods of physically tightening up a fit work by making a dent in one spot, which is kinda okay sometimes but longevity is not as good as filling the whole space with an epoxy. I might even run some coarse emery cloth around your male part a couple times to give it more of a bite. Loctite is good stuff, honest.

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    The final position needs to be determined by how the spring tension works. If possible, mount an additional block/bracket that also has a screw that pushes on the existing component to rotate it into the position that works for you. This will allow a wide range of adjustment to the spring tension and secured by a lock-nut on the screw when you're satisfied with the tension. This way the hole doesn't have to be altered, "glued", or staked in a location that hasn't been determined yet. It will only require one more component to be made, will make spring adjustment easier, will lock position more securely, and look like you intended it that way from the beginning. Lots of mechanisms use this arrangement for adjusting/locking under spring load with a moving/rotating component.

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    I'm with AD. You will need to know the location before putting the latch on. Once you have that, you can use some Loctite 609 or 680 depending on how tight or loose the fit is(don't forget the primer T). You will also want the bore and shaft to have a rough surface finish(something like a 120Ra), so the loctite has something to adhere to.

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    thanks for the suggestions!

    I thought about it last night and may have come up with a solution that solves both the spring position and lock-down problems without fitting this in permanently. The pin sits proud of the back side of the 3/8" black aluminum plate by about 1/8". On the front side, under the latch, is a thick washer (an almost 1/8" thick stainless ring) that sets the height of the latch mechanism relative to the surface of the plate. I'm thinking if I thread the pin as far as I can go, I can replace that ring with a nut and a small washer or two to set the height, and with 1/8" of pin sticking out on the other side of the plate, I should be able to get another nut on there. Because the whole pin will free-spin in the hole, I can position it where I need it, then tighten the nuts. This will allow me to disassemble in the future if need be, as well.

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    An 1/8" on the pin isn't much more than 2 threads for the nut and that's really not enough to hold anything in place IMO. The adjusting screw in a bracket/block is a lot more secure, it can even be used as opposing screws to lock in place. The pin can be free to rotate until the screws are set/locked. If the base plate is 3/8 thick there's plenty of material for a couple of 8-32 mounting screws nobody will ever see. It will also make for easier dis-assembly if a different spring is used. It's your job so do it however you wish.

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    Quote Originally Posted by friolator View Post
    thanks for the suggestions!

    I thought about it last night and may have come up with a solution that solves both the spring position and lock-down problems without fitting this in permanently. The pin sits proud of the back side of the 3/8" black aluminum plate by about 1/8". On the front side, under the latch, is a thick washer (an almost 1/8" thick stainless ring) that sets the height of the latch mechanism relative to the surface of the plate. I'm thinking if I thread the pin as far as I can go, I can replace that ring with a nut and a small washer or two to set the height, and with 1/8" of pin sticking out on the other side of the plate, I should be able to get another nut on there. Because the whole pin will free-spin in the hole, I can position it where I need it, then tighten the nuts. This will allow me to disassemble in the future if need be, as well.

    Dont do this

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    Quote Originally Posted by APD View Post
    Dont do this
    Care to elaborate?

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    Quote Originally Posted by AD Design View Post
    An 1/8" on the pin isn't much more than 2 threads for the nut and that's really not enough to hold anything in place IMO. The adjusting screw in a bracket/block is a lot more secure, it can even be used as opposing screws to lock in place. The pin can be free to rotate until the screws are set/locked. If the base plate is 3/8 thick there's plenty of material for a couple of 8-32 mounting screws nobody will ever see. It will also make for easier dis-assembly if a different spring is used. It's your job so do it however you wish.
    It's metric, so if threaded it would be 4-5 threads exposed, I think if fine-thread. I like the idea of an additional block on the inside, but there isn't really room for that as anything big enough would obscure the mirror on the inside. Really, I don't have more than about 1/8" to work with inside the part.

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    Quote Originally Posted by friolator View Post
    ...So the question is - what do I do here to get this part to fit properly. I know Loctite is an option but i'm also not sure what the optimal position for the spring is, so I'll need to play with it a bit. And will it hold up with this being a part that will see frequent handling, as well as the spring putting constant tension on it?
    I would assemble the pieces and rotate the pin against the spring until you get the desired tension. Then scribe a mark so you know where is should be.

    Take it apart, put a straight knurl on the pin, and reassemble aligning it to the mark. Put it back together with a dab of loctite.

    If you can't put a knurl on it, you can take a center punch and dimple the pivot pin around the area you want to be a press fit.

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    Quote Originally Posted by jancollc View Post
    I would assemble the pieces and rotate the pin against the spring until you get the desired tension. Then scribe a mark so you know where is should be.

    Take it apart, put a straight knurl on the pin, and reassemble aligning it to the mark. Put it back together with a dab of loctite.

    If you can't put a knurl on it, you can take a center punch and dimple the pivot pin around the area you want to be a press fit.
    I'm curious about how this works: With the knurl, is it just creating more surface area for the loctite to sit in, or is the knurling process actually increasing the diameter of the pin slightly? Several people have suggested similar solutions here and in other places I've asked, but I'm not clear on how this would work. It seems like a decent short-term solution if its slightly expanding the pin, but wouldn't it require additional work if I have to disassemble this?

    I do have a knurling tool for my lathe, but haven't used it much yet,

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    Knurling or striking it with a cold chisel or punch will deform the metal. some goes down, directly under the knurl or punch and this pushes the surrounding area up a little. Something like this you want straight knurl so it goes in smoothly, standard diamond knurls will file the hole even larger as you press it in, better to not use diamond knurl.
    Fastest and easiest is to use v block and cold chisel as mentioned above. you want small lines chiseled into the length of the pin.

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    Quote Originally Posted by friolator View Post
    I'm curious about how this works: With the knurl, is it just creating more surface area for the loctite to sit in, or is the knurling process actually increasing the diameter of the pin slightly? Several people have suggested similar solutions here and in other places I've asked, but I'm not clear on how this would work. It seems like a decent short-term solution if its slightly expanding the pin, but wouldn't it require additional work if I have to disassemble this?

    I do have a knurling tool for my lathe, but haven't used it much yet,
    There are 2 basic kinds of knurling- cut and form. Cut knurling does not change the OD. Unlikely you have cut knurling.

    Form knurling displaces the material, so it expands the OD. If you have a knurling tool it's probably a bump knurl (single knurl) or a scissor knurl (2 knurls). Either one will expand the OD of your pin. You don't need much, you're just trying to create a little interference.

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    Peening, knurling, ball bearing indenting....all methods that create a tighter fit through the use of displacement. All of them are half-ass fixes IMO because they all:

    1) Rely on a displacement that can/will undo itself over time and use

    2) Provide less metal-metal surface contact than the original fit, and in different places.


    I'm in the Loc-Tite camp for this one....

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    Quote Originally Posted by GregSY View Post
    1) Rely on a displacement that can/will undo itself over time and use

    2) Provide less metal-metal surface contact than the original fit, and in different places.


    I'm in the Loc-Tite camp for this one....
    So these are my concerns with this as well. I'm fairly certain I'm going to need to disassemble this at least one more time, and I don't want to mangle the hell out of the pin. I suppose I could always remake the pin on the lathe with a slightly larger diameter, but I haven't turned any stainless on it, just delrin and a little aluminum, so I'm probably out of my depth on that front.

    If I go the loctite route and need to eventually undo this, will I be able to tap the pin out or is pretty permanent?

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    Loctite it.

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    Loctite is easily removable with a little heat, like a propane torch to get it to 300º? of so.

    The studs on your car to hold on the wheels are probably straight knurled. This is to prevent them from spinning in the hole. Nothing wrong with straight knurl parts that press in.

    I would do loctite and maybe chisel in some straight knurling if needed.


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