Bridgeport knee crank retensiion

Discussion in 'Machine Modifications' started by GailInNM, Feb 12, 2019.

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  1. Feb 12, 2019 #1

    GailInNM

    GailInNM

    GailInNM

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    This mod won't improve the precision of a Bridgeport milling machine, but it does eliminate two sources of irritation of operation. Over the last 35 years that I have had my BP I have probably knocked the crank off its axle al least 500 times necessitating picking it up from the floor. The second irritant has been the crank disengaging from the dogs on the axle when making large moves on the knee. I crank the knee down quite often to make an air cut to check a program (CNC) and then back up to make the part. As I count turns, when ig get a "whoops" moment as the crank disengages it usually distracts me so I forget my count.

    The solution to all of this has been to add a screw on knob to retain the crank. I made the knob heavy so by just loosening it I can flick it with a finger and it unscrews about 3 turns so the crank can be disengaged and repositoned. If desired a second flick will frop the knob into my hand so I can remove or reverse the crank to get it out of the way.

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    This should have been an easy job, but we all know that it never works out that way. The first part of the mod is to drill the crank axel and putt trreads in it. I made up a drill jig so I could drill the axle square. Don't even think of trying to drill the hole with oout a jig. Then the first problem. The crank axle was made of the toughest piece of steel that I have ever drilled. Took me half an hour to get the hole 5/8 inch deep using a #6 drill. This is a little larger than the normal #7 frill used for a 1/4-20 thread but I wanted a fairly loose thread so te screw would turn easily. Of course lots of cutting oil was used. Then the hole in the drill jig was opened up to take a 1/4-20 tap and the hole was tapped full depth. Tapping was as tough as the drilling operation. A lot of pressure was required to get the tap started for the first few threads. If I were to do this again I would drill the axel to 5/16 diameter and Loctite a tapped bushing in the hole. This would get rid of the danger of snapping a tep in the tough steel.


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    Gail in NM
     
  2. Feb 12, 2019 #2

    GailInNM

    GailInNM

    GailInNM

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    A knob was made up out of 1-1/8 diameter steel 12 inch thick. The knob wants to be made of a heavy material so it will rotate about 3 turns when flicked with a finger after being loosened. The recess around the screw positions the knob so there is about 1/32 inch of play in the crank position when the knob is seated on the axle. About 1/18 inch deep in my case. If it bottoms on the crank it will loosen with crank operation.

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    The nob was knurled to make griping it easy and the corners wer e chamfered and smoothed to make it easy on the fingers.

    I inserted a 5/8 long 1/4-20 socket head cap screw in the knob. and it was ready to go.

    Gail in NM
     
  3. Feb 12, 2019 #3

    DJP

    DJP

    DJP

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    Thanks for sharing. I did the same for the handle of the vice on one of my milling machines which was constantly at risk of falling off from vibration. The knee handle on my Bridgeport is not used often for precision movement as I have DRO on the Z axis. I only position the table once when starting. I'll keep your idea as I'll probably have the same issue one day. My handle does disengage but it doesn't fall to the floor. There is a set screw on the side of the handle to provide some friction on the shaft. As long as I leave the handle pointed downward it's not in my way.
     
  4. Feb 13, 2019 #4

    petertha

    petertha

    petertha

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    After recently trading in my RF-45 for a Taiwan mini BP clone, I am new to a knee style myself. Its taken me a while to get used to levers & handles in different spots than I'm used to. I had to retrain my brain to walk around a bit wider so as not to bump the handle, avoid leaving it in detente at an angle or detach the cogs so handle hangs free. Knee raising actually doesn't take a lot of cranking torque (probably because I don't have a heavy vise or heavy parts). So the handle seems like a heavy duty hunk of iron for the job. I was thinking of adapting a ~8" wheel for fine setting lift, which would always be in balance svs the Model-T starter handle. But since I never saw one on a mill, I figured it must just be me & I should get used to it LOL
     
  5. Feb 13, 2019 #5

    DJP

    DJP

    DJP

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    I was told by Bridgeport expert users that hanging the work over the front of the table for big pieces and repositioning the head is normal to get full use of the machine. With a wheel for the knee you may have interference. That's the only reason to avoid it that I can think of. On a full sized Bridgeport the cranking effort is substantial so the Model T crank works well. I wouldn't change.
     

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