A new challenge...shortening screws
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  1. #1
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    Default A new challenge...shortening screws

    Good morning All:
    So I just got a new project this morning...I have to shorten some screws...some LITTLE screws...120 of them!
    I have to take about 0.5 mm off them and they all have to be the same within about 0.05 mm (0.002")
    I'm thinking a U shaped steel bar with 40 holes tapped into them and a quick whirl on the surface grinder.
    I want the heads of the screws to touch the mag chuck so they'd go between the legs of the U, and if this turns out to be the best way, I need to make the feet of the U big enough so they'll stick to the chuck.
    I want the screwheads to touch the chuck so they can't back up when the wheel hits them.
    I am free to stone the heads if I have to in order to get them all to touch the chuck.

    Problem is, it's all so skinny I'm afraid the heat from the grinding is going to suck the jig plate up into the grinding wheel.
    So I thought maybe 40 counterbores into a thicker plate, but now the jig is getting to be more work than the screws.

    Any brilliant ideas out there??
    I do have a sinker and a wire EDM but I can't see a way to avoid having to make the fixture, even if I use one of those machines to eliminate the spinning screw problem that comes with the grinder.

    Sorry about the blurry pic...the screw is so damn tiny it's hard to keep it from rolling off my finger when I put it under the camera lens, so I wasn't all that steady when I tripped the shutter.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Attached Thumbnails Attached Thumbnails dscn5106.jpg  

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  3. #2
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    You are on the right track with c'bored holes in thicker plate. I would do it 10 at a time since I hate tapping small holes and would rather spend the time changing screws and grinding. That looks like a whole lotta no fun!

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    What are those things going to cost when you're done??

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    Hi ChipSplitter:
    You wrote "What are those things going to cost when you're done??"

    I don't know and I don't care.
    It's a cost plus job and the customer is working with a multi million dollar grant from the Canadian Government, so I'm really lucky to have been able to attract these guys as my client.
    Let's just say they're spending freely at every high end shop in Vancouver...I'm a small player with what they call "Special Skills" so I get called for the weird shit nobody else wants to tackle.
    'Tis a good gravy train...I'm fortunate to be on it for now, so I'm gonna deliver the best value I can for as long as I can.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    What about a tapped and c'bored block? Screw the part in from the side and as the threads come out the far side they go up against the side of your grinding wheel.

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    Quote Originally Posted by implmex View Post
    Hi ChipSplitter:
    You wrote "What are those things going to cost when you're done??"

    I don't know and I don't care.
    It's a cost plus job and the customer is working with a multi million dollar grant from the Canadian Government, so I'm really lucky to have been able to attract these guys as my client.
    Let's just say they're spending freely at every high end shop in Vancouver...I'm a small player with what they call "Special Skills" so I get called for the weird shit nobody else wants to tackle.
    'Tis a good gravy train...I'm fortunate to be on it for now, so I'm gonna deliver the best value I can for as long as I can.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Use a lathe type of machine. Put screw in collet,bring forward tailstock to steady screw and face to length.
    ailstock

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    Quote Originally Posted by implmex View Post
    I don't know and I don't care.
    It's a cost plus job and the customer is working with a multi million dollar grant from the Canadian Government
    Those are the best kinds of customers, eh? Fortune favors the prepared.
    I looked at your pic and almost screamed. Better you than me.
    I guess "small" means different things to different people.
    I think you're on the right track with a thick plate, drilled, counterbored, and tapped, then skim the whole enchilada on the surface grinder in one go.

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    Quote Originally Posted by pavt View Post
    I looked at your pic and almost screamed. Better you than me.
    That's what I thought, too.

    Ah well, nothing like goobermint money.......

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    Make sure you sweep the floor in the area you are working. Although I know you won't drop any on the floor, right?

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    Thirty-five years ago, I had to shorten a couple hundred 0-80 x 1/8 socket head cap screws to about 1/16". The length tolerance was not stated; they just had to screw into a 1/16" thick gun sight and not stick out the far side. The screw head was a stop for the sliding part. No need for a chamfer on the threads, of course.

    I drilled several .060 holes in a piece of 1/16 steel. A screw was inserted in each hole and a piece of leather was used to hold the about to be hot screw heads tight against the steel holder with a finger. Then I held the assembly against my 1 x 42 belt sander for a few seconds, all it took to grind off the excess length of the tiny screws. Got the job done in less time than tapping a bunch of tiny holes in a fancy fixture for my surface grinder or using the Levin lathe with lever cross slide.

    That pan head screw in the OP's picture does not grip nicely in a watch lathe collet, so I don't see using a lathe to shorten them. Watchmakers do have special tools for holding a screw while shortening the threaded end, but they are not suited to doing large quantities.

    Larry

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    The edge break issue is still present. Here's a really nuts idea - take a carbon electrode and mill a blunt-ended cone in it that matches the screws minor diameter at the small end, and flares a little bigger than the thread OD. You can do this to the side or bottom of the electrode, depending on how you want to orient the other part.

    make a plate with a hole(s) that matches the OD of the screw thread into a vise standing vertically or horizontally to suit the electrode. Have a thin stainless shim on a screw pivot such that it can slide over the head of the screw, capturing it.

    Now you can plunge or orbit your electrode over the screw thread, removing the excess length and leaving a chamfer behind. Adjust number of holes in electrode/plate to whatever makes sense.

    Bingo (or whatever you Canucks say) - shortened screw, chamfered thread, job milked, invoice massive.

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    I have done the same on 250 pcs of a 3mm machine screw.
    Had to shorten them to a tightly controlled length. (about the same tolerance of the OP)
    I went with 2 fixture plates, with 50 screw locations each.
    That way I could machine a set, while loading the next. (loading the fixtures took much longer than the 1 minute machine time.)

    As stated above, screw the screws in from the bottom of a (machined on all sides) fixture plate.
    When you back them out, it will repair the starting thread.
    Fortunately, we did not have to put a chamfer back on the starting thread.

    Could you use a face mill, followed by a 45° chamfer mill to chamfer each screw in the plate?
    The ends of the screws would either have to be proud of the plate, or a c-bore with enough clearance to allow for the end mill to chamfer the parts.

    Just my $0.02

    Doug.

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    Personally, I would make a block with counterbores and rather then tap the hole, just make the hole pretty damm close fit on the screw so that after the heads are stoned, you can pull them back to a mag chuck well. As if they are threaded, it’ll be a pain to screw them back to the mag chuck.

    If your taking 0.5mm from nominal make the jig 0.6mm under nominal.

    I would also mount the mag chuck on the WEDM and wire the front off. That way you will make the smallest burr compared to grinding. Do you have to put a chamfer/lead back on after? Or just leave able to thread?

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    I had one screw shortening job years ago. It was a small screw, maybe a 4-40,
    HUGE compared to your screw, but still small.

    Instead of tapping my fixture for the small screw, I drilled through at the major
    diameter, then drilled the backside bigger and tapped it, and used a 3/8 set screw
    to hold the tiny little screw in. It was just easier to handle a 3/8 screw than
    trying to hold and start the little screw. And I could crank it down a lot harder,
    to make sure it didn't move.

    That looks like a miserable little job.. No pun intended.

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    Quote Originally Posted by Milland View Post
    Bingo (or whatever you Canucks say) - shortened screw, chamfered thread, job milked, invoice massive.
    I think it's pronounced "Molson!" (hat tip to Bob and Doug McKenzie)

    I've seen clockmakers tools that put a chamfer on the end of screws, kinda like an inside-out countersink. Just a piece of HSS round, with a couple of 90deg vees milled in the end at 90deg to each other. Stick it on the screw end, give it a twist, and instant chamfer!

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    I once did a fussy length screw job but not that small and they were Allen heads so that made the job easier with my jig...A block that set on the magnet with having a vertical portion just a tad less wider than the screw length spec.
    With the fixture block I put each screw through the hole it being the major screw diameter +.001.

    Turning the screw while side wheel grinding so to take away the burr and make size I held .002 easily ..then taking the screw out I brushed each to a rag wheel to make a small bevel, with taking from the grinder to the rag wheel in one motion...so an hour to make the jig and an hour to run the job.

    Agree screws that small can be a problem..to take .019 0r so.

    Yes I could have locked the table and run the screws to the wheel OD on the right side with the wheel .o20 above part center so to get de-burr, and almost flat.

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    Hi All:
    Some great alternatives here, and I'm going to shamelessly steal from every one of them to get to my goal.
    So here's my new thinking:
    1) I'll make 10 spots instead of 40 per EOLSON's suggestion in post #2
    2) I'll drill for the major diameter and not tap per L Vanice, Luke.kerbey, Bobw, and michiganbuck.
    3) I'll threadmill the counterbores for 8:32 setscrews to touch the screw heads and I'll get nylon tipped setscrews also per Bobw.
    4) I'll kiss them on the wire EDM per Luke.kerbey to avoid the burr I'd get from grinding them (They won't need them chamfered, Milland, but that's a GREAT idea if they did!)

    So now to get my fat ass in gear and go make a noise !

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    I do some but not this small.
    Screw them into plates and grind.
    Before removing I hit them with a silicon carbide nylon filament brush wheel which leaves an nice .002/.005 or so radius on the lead thread.
    For me screwing them out leaves an upset burr which makes them not start nicely in their new home.
    Bob

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    I do those once in a while. Not in big batches.

    Make a thick coin with a threaded hole in the center.
    Insert your screw with the end sticking out the other side.
    Add a back-stop to a coin sized collet and insert coin.
    Face screw with parting tool or tool bit.
    Dress emd of thread with file at 30-45 degrees.
    Repeat with next screw.

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    Looks like we have all shortened commercial length screws. I have never seen the need for many tapped hole fixtures. You still have to screw each one in and out. Done several of that I used a 2 fixture set up, while one is running screw remove and replace the other. The ones in the photo were 1/2 long button heads shortened to .135 long. The socket was clamped to the bench next to the machine. My secretary did more than I did and she could and got so she had no lost time, ready to load the next part as soon as the part came out of the collet. Smaller ones were done the same.

    shortscrews.jpg


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