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  1. #1
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    Default Long reach boring,FP4NC

    Bit slow here and folks seem a bit tense so though it might be nice to share a few shots of a job i am finishing up.
    Some time back i posted photos of the parts of a job i was given to straighten the main housing bores on a Porsche 917K.

    Crankcase is mag. and long enough (35") to be a size issue.
    Lots of work has gone into this project. Some tooling construction and some false starts , and finally i am down to the short straws.

    Engine is built with a lower shaft running in the case that delivers the output power from a spur gear at the center of the crank.
    Last portion of the job was to over bore the housings that carry the shaft support bearings....
    This was necessary to assure that the crank and output shafts were running dead parallel and at the correct center to center distance to give the proper gear lash.

    Machine is setup to run horizontal. Alignment is done using the actual rotation of the crank and all adjustments to tram done via the universal table (that thing might be the best accessory ever)

    Boring tooling is Sandvik "Capto" fine boring head coupled with a base adapter (CAT 40) a 100 mm spacer sleeve and two 80mm spacer sleeves. (needed to buy an additional 80 mm spacer for this)

    Here is a shot of the boring bar fitted so you can see the relative length (20" from the tool face to "CAT" flange.



    Fine boring head....uses insert tooling, in this case high positive. Adjustable in ..01mm increments on the diameter....



    Setup is too long to to fully exit the case on retraction, so the technique is to set the bar into the case then make the connection between the machine and the bar.
    I used the Capto joint at the spindle adapter being it was shorter and easier to control when making up the joint....






    More fun to follow.

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    One of the issues here is that because i can't get the bar to retract fully, i can't indicate off the full length of the bar.
    So to get things in the correct position, the first order is to indicate the spindle with the housing bore (this bore is in the correct position relative to the crank...already established in earlier work.
    First i indicate the reference bore with my indicator as close to the spindle fae as possible (sorry no photo)

    Next i fit the bar into the bore and make the connection. An indicator is placed with the "Y" slide almost fully retracted. The machine is moved to get the bore true to the spindle.
    The length of the indicator point is measured to the same point where the bore was first indicated. (short reach)


    Note tenths reading indicator......



    Once i have the spindle aligned in this position i note the change in position from the first indication....this is the bar sag or droop....

    Using the above info i calculate the proportional change that will be present at the tool (further along the bar)
    I make the correction to the bar position based on my calculations and set all registers to zero....

    Use very sharp tool with almost zero radius to control the chatter (insert tool sharpened using diamond "Accu-Finish" setup...Fine feed and slow RPM
    View of the tool working as seen through one of the cylinder bores....



    Bore end view.....You can just make out the inner housing bores (2) down the bore....




    More to follow.....

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    Need some long reach measuring tools for this sort of work.....



    Overall view of the layout after finishing bores and splitting the case half's....



    View of the housings with boring completed., case split...

    ...

    Dummy bushings fabricated to allow fitting the drive gear from the output shaft....testing gear lash and alignment...Actual lash at the drive gear and fan gear .002" on both...






    In all this worked out well...
    Cheers Ross

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    Ross,
    How much does the bar droop over that length (your measured value)?
    What is the size of the cut you are taking with the set-up?

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    Matt:
    Calculated droop just at .0032" in 20" of length.
    I was taking .001"/.0015" per pass.....
    Not going for a specific diameter, just a full contact clean plus several additional cuts to assure equal forces....
    Cheers Ross

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    Ross, I'm amazed that you can cut a smooth bore at that extension, and also that it's possible to line everything up with such poor access. The time spent setting up and making custom tooling must be much larger than the time you are actually cutting metal. Kudos!

    Question: is that a lump of clay stuck on the boring bar to help cut vibration?

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    Yes clay helps dampen the vibrations.....
    Tooling companies make "silent" bars....but they cost cubic money, and there is nothing this long in this diameter (2.5" or so)

    Might have noticed the shiny surfaces on the face of the main webs leading up to the actual bores....Had to open those surfaces up some to get clearance for the boring bar.
    to withdraw the bar, you have to orient the cutting tool away from those cleaned surfaces or it will hit.

    Would not wish to run this job if the case was steel or iron....Mag cuts pretty well....makes pretty much dust at these depths of cuts.
    To check size, have to pull the bar in order to measure...no way to get in there through the cylinder bores unless you have little kids hands..

    The bore gauge helped me know when everything was cleaned up (round) Pretty hard to see all of the bores.
    Cheers Ross

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    I'm trying to visualize the drooping and then spinning the drooping tool. The measured droop is the offset of part/spindle axis and axis where the tool is actually spinning? This means these two axis are not quite parallel but that doesn't matter because you are using the machine travel to make the cut (correct?).

    What is the RPM?

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    Matt:
    Believe you have the concept correct....The bar is an arc...not dead straight. But the center of rotation is still centered at the far end of the arc....
    Envision that the bar is made from rubber.... One end is being rotated,at a fixed height while the opposite end is holding the bar's center, but allowing it to rotate.
    The bar is being held below the height of the driven end...It still rotates just fine just lower than the driven end.....The axis of rotation is in this case curved, and the holding action at the non driven end
    Is gravity....In reality, the end rotation of the tool is not dead perpendicular to the slide movement, making a slightly oval cut,but the error is very small. Reading with the bore gauge..couldn't find more than a tenth
    of difference in the bore geometry top to bottom or. side to side...
    And yes the feeding is happening on the "Y" slide....

    As to spindle speed.....60 RPM.
    Thing about chatter...you want to keep it from starting from teh beginning of the job, even roughing cuts. Once started, its the devil to clear up on flimsy setups.

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    60 rpm... what’s the DOC?


    Sent from my iPhone using Tapatalk

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    Depth at .001-.0015" on the diameter.....
    Cheers Ross

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    Ross

    is mid-crank gear machinied from integral billet forging?

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    The crank and gear are one, unlike the 30's Alfa Romeo 8C engines which were built up with the center gear being bolted between the two half's of the crank...
    Tricky bit of work there....Got to work around the crank throws. Pretty sure this is an aftermarket crank...Done by "Arrows" if i am not mistaken. Gear is finish ground as one might expect, not just hobbed.
    All the oil for the rod throws comes in from each end of the crank, not through the mains as with most conventional setups....Said to reduce the required oil pressure lowering oil temp and the like....Involves some slick drilling and large diameter holes in the rod pins.

    Cheers Ross

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    Wow Ross, just Wow. Thanks for posting this.

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    thanks Ross, very cool.

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    Beautiful works per usual Ross... So many different ways to pooch that job.

    Cheers, Brandon

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    so I noticed the 917 frame was pressurized
    internet details are lacking as to what gas was used and pressure sets---but
    some years ago I was a contract team member performing diagnostic services on Bucyrus Erie draglines---1260 and 1370 series

    and these 6 million pound diggers have tube booms pressurized to 100 psi
    using nitrogen

    so--917 pilot Brian Redman comments on presence of analog gauge in instrument cluster--calibrated in kg/cm2 or atmospheric units
    my guess is mid gauge or 75 psi tube frame pressure setting is likely value of a no leak system

    and I suspect nitrogen gas was used in the 70 series aluminum alloy frame--
    hopefully the more knowledgeable will set record straight
    Attached Thumbnails Attached Thumbnails 1-2wek0-2-.jpg   34q.jpg   123d.jpg  

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    Very interesting ! Thank you Ross for those beautiful pics...

    The pressurized frame is also fascinating, but what I'd love to know more about is *how* the central gear on that crankshaft was machined then ground... 😬

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    J:
    The auto folklore about the frame being pressurized was in effect a crack detection system. Don't know any of the details, but my guess is that it was nitrogen.


    T:
    Pretty sure that the gear could be hobbed in the space avaliable Would need a machine large enough to carry the crank between centers, and the hob likely mounted on an extended spindle to allow the drive for the hob to clear the throws......Appears ground, but might have been lapped, which could be done with a similar setup to the hob.

    Cheers Ross

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    Thanks for showing Ross, very impressive!

    I am not sure exactly what happened with the lengths: you managed to machine all journals without moving the cases? Also, I would assume that you had to remove extensions at some point to reach the journals closer to the machine...

    BR,
    Thanos


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