Trepanning on a larger scale
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    Default Trepanning on a larger scale

    I doubt you'll ever have application for trepanning, boring, and reaming through solid stock to make sleeves, hollow shafting, pressure vessels, cylinder tubing etc but this guy in his videos demonstrates several techniques adaptilble for work on a smaller scale. The presenter doesn't talk much so a viewer has to look sharp for nuggets of usable information.

    The few times I've used packed bits for boring deep holes the wood bearings had to be turned to size separately. What goes unremarked is how he sizes the wood support bearing for his boring bar. It's quite a simple trick. He's left the stock face as-sawn and indented several facing teeth (like file teeth) with a cold chisel held at an angle. These teeth face off the slightly oversized end grain hardwood block as the bar feeds into the cut. This automatically sizes the support bearing, centering the bar in the cut and providing radial resistance opposing the cutting forces.

    This is much the way I learned the technique from Bill Polhamus and Dick Perkins when I was an apprentice in 1963 and the very last ever PSNS made fully machined propellor shafts were being trepanned, reamed, etc. This was on a much larger scale of course, the shafts were to transmit 60,000 HP at 230 RPM for the USS Sacremento AOE-1. The shafting was 23" OD with 44" dia flanged connections 8" thick hollow, with a 13" reamed bore. The longest shaft was 55 feet and had the propellor taper. What I can't recall is how the 7" dia x 7 ft long bore through the propellor taper bottle necks out to merge with the reamed bore.

    YouTube

    This fellow has many videos all pretty much the same but scattered among them is much information including how to DIY trepanning tooling. Enjoy.

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    Nice to hear from you, Mr. Addy.

    You mentioning your experiences in the shipyard brings back great memories of mine at Savannah Machine and Foundry a smaller shipyard located in Georgia. The people who worked there -- including the ones who passed through -- had a huge store of knowledge.

    I hope you are well,

    --david

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    Quote Originally Posted by Forrest Addy View Post

    The few times I've used packed bits for boring deep holes the wood bearings had to be turned to size separately. What goes unremarked is how he sizes the wood support bearing for his boring bar. It's quite a simple trick. He's left the stock face as-sawn and indented several facing teeth (like file teeth) with a cold chisel held at an angle. These teeth face off the slightly oversized end grain hardwood block as the bar feeds into the cut. This automatically sizes the support bearing, centering the bar in the cut and providing radial resistance opposing the cutting forces..
    "SImple, Elegant,Cool, WOW" are a few words that come to mind.
    Oh ya, "Thank you for posting." also comes to mind!

    Denis

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    Thank you for posting that Forrest. I have no experience using packed bits and I found it very interesting. I have bored my share of deep holes and usually revert to making a lot of "spring" cuts to bring things to size. I assume that not only would packing the bit reduce spring but also help in eliminating chatter. Thanks again.

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    Another good method for long bores is double ended tools. They need to be ground to the size you want the bore to cut and best if ground by machine to keep the cutter as symmetrical as possible. I have also used finishers with a very light fuzz cut and large radii on the double tool ends - very good finishes on very long bores are achievable this way but the finish cut must be very light.

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    Quote Originally Posted by eKretz View Post
    Another good method for long bores is double ended tools. They need to be ground to the size you want the bore to cut and best if ground by machine to keep the cutter as symmetrical as possible. I have also used finishers with a very light fuzz cut and large radii on the double tool ends - very good finishes on very long bores are achievable this way but the finish cut must be very light.
    Clark Tools if they're still in business sold a whole range of floating reamers. One set of seven or so bars cover the range frrom 5/8" to 8". They were once part of Giddings & Lewis but since the convulsions in heavy manufacturing over the last generation who knows who is affiliated with whom and where?

    The cool feature was their adjustability. The cutting tool was in two parts.that could be adjusted with a set screw and clamped with a couple button head cap screws. You could set them with a mike right at the machine and once set held bore size to small tenths consistancy.

    The downside was the design featured "floating" tools. They cut diameters accurate as hell but they followed the trend of the bore. The sequence was rough to hole, semi-finish bore to location leaving 0.015 to 0.030 stock and Clark ream to size. I bet I bored bearing fits in a zillion split transmission cases over the years with Clark reamers.

    I gotta research to see if Clark is still in business.

    Back to topic. Deep bore trepanning does not generate true cylindrical bores. Any small inconsistancy in the work piece material will cause the trepanning tool to deviate to the more easily machined side. Packed bit or floating bit reaming will not improve it. Most very high L/D bore application are tolerant of slight bore deviation, hydraulic cylinder tubing, down-hole special well tooling for example. However, very straight continous cylindrical bores having large L/D ratios pose a very difficult machining problem usually finessed: rifle and artillary barrels are commonly drilled, bored, and reamed leaving the OD rough then the rough turned blank was straightened to correct the bore. In the propellor shaft example in the OP, the OD was left with an inch or more stock. The OD was machined concentric with the bore after the trepanning and reaming operations.

    Quite a trade we serve. There's quirks and tricks enough to gratify anyone with a technical mind to appreciate its range and subtlety.

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    Forrest,
    The trepanning tools used have 2 cutters 180* apart. One establishes the ID of the bore, the other sets the OD of the core. Wouldn't the opposed cutters tend to limit deviation of the tool due to material variations? I can't remember if they use bearing blocks for centering top to bottom.

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    The few times I've used packed bits for boring deep holes the wood bearings had to be turned to size separately. What goes unremarked is how he sizes the wood support bearing for his boring bar. It's quite a simple trick. He's left the stock face as-sawn and indented several facing teeth (like file teeth) with a cold chisel held at an angle. These teeth face off the slightly oversized end grain hardwood block as the bar feeds into the cut. This automatically sizes the support bearing, centering the bar in the cut and providing radial resistance opposing the cutting forces.

    I have a couple of questions. Firstly, what is a "packed bit"? I've never heard this term used before! Secondly,are you able to expand on your description of the "wood support bearing"? I think I've got the idea, but I'm unsure enough to question my understanding. Can you make a napkin sketch of some sort? I really enjoy learning about these esoteric methodologies!

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    Quote Originally Posted by millwrong View Post

    I have a couple of questions. Firstly, what is a "packed bit"? I've never heard this term used before! Secondly,are you able to expand on your description of the "wood support bearing"? I think I've got the idea, but I'm unsure enough to question my understanding. Can you make a napkin sketch of some sort? I really enjoy learning about these esoteric methodologies!
    The video linked in the first post shows the process clear as day. You can watch as the wood is chewed away as the wood packing enters the bore.

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    I'm surprised how fast they are Turning. I'm a coward when it comes to Trepanning.

    R

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    I used to trepan stainless 316 valve seats 11inches diameter out of 14inch blanks.
    Generally OK but occasionally the tool would jam and snap.
    There are videos of Dutch guy trepanning bowls out of huge chunks of wood with a weird curving tool setup and the wood was held by a threaded hole about one inch dia and deep. He is braver than me.


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