Boring of a long, small tapered hole
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  1. #1
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    Default Boring of a long, small tapered hole

    Hi all,
    I am an undergraduate student at WPI needing to cutting a certain oddly-shaped hole as part of an independent study project. The material being cut is normalized 4340 chromoly steel. The hole starts off straight for about 0.916" which I can cut with a regular drill bit. The problem lies in having to cut the two tapered portions of the hole, the first being a 1° per side taper and the second a 3° per side taper. Seeing as how the starting and ending diameters of such holes are non-standard and lie past an already 0.259” diameter hole, I have not been able to find any tapered end mills which meet such requirements. I tried using some old boring bars found on Ideal Surplus from Bokum Tool (BSC5), but was unable to get the bar to bite without significant deflection, leaving the hole undersized. If needed I may resort to attempting to machine my own custom tapered D-Bit as a last resort. Any ideas on how best to machine this part?
    Hole Geometry:
    hole-geometry.jpg

    Thanks,
    Jack Ruddat
    WPI 2021

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    What kind of finish are you looking for? Not knowing anything else, I’d probably get a custom reamer ground.

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    As Eric U says you need a reamer. You will most likely be money and time ahead if you just order a custom one in the morning.

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    Boring bar seems like a fair option....

    R

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    The finish is not much of a concern. I've looked into custom made tapered end mills which tend to be very expensive. Would you expect a custom ground reamer be a cheaper option?

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    Quote Originally Posted by litlerob1 View Post
    Boring bar seems like a fair option....

    R
    Well it is a 10:1 length to diameter ratio which is quite high even for solid carbide boring bars.

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    How do you plan on measuring any of this?

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    Quote Originally Posted by Bendak View Post
    Well it is a 10:1 length to diameter ratio which is quite high even for solid carbide boring bars.
    I see that. I would try it straight up first, if it turned into a Bear, I'd do it in 2 Operations. Do the Straight bore from one side, then do the Tapered Bore from the other side. Obviously runout would be something to be careful about, but not a huge deal-especially if the Surface Finish is not a big deal. But I think Lathe not Mill for your drawing. What you are asking about (reamer and such) sounds like Mill work, not necessarily but sounds like it. If you are talking Mill than it'll be tough to get a Bar in there, but Lathe should a piece of cake.

    R

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    Looks that part is round and this can be a lathe job (even if the outside is not round). I make long tapered holes with a custom made tapered boring tool. The profile of the boring tool can be close the the taper hole, slightly smaller and with a cutting insert at the tip. Making the bar is relatively a simple and quick job. Similar tool can be made for a boring head in a mill. The bar shown was for somehow shallower high precision and finish bore, but I did make those longer as well.

    bar.jpg

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    One shop I worked at had a single lip cutter grinder and we used it to make taper drills (5 to 15 degrees) most here call them D bits. They give a good finish and if you have the right tools to make them they are easy.

    I would not know how to make both angles on the same tool but if you have tool positions open why not 2 tools?

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    Quote Originally Posted by [email protected] View Post
    How do you plan on measuring any of this?
    I have a part which is supposed to fit within this hole, acting like an extractor to retrieve a core sample cut-out by the finished product. In the past I have also dropped various sized dowel pins and measured their distance from the end with a depth gauge.

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    I previously tried boring the hole straight on but damaged three of my five boring bars (lost the tip), leaving the hole undersized. I used a 0.005" DOC, 0.001" IPR feed rate, and 300 SFM surface speed. Perhaps I should reduce the surface speed to something like 85 SFM? If it ends up not being possible, then using a back boring bar such as the SDM 668 492 (solid carbide) from GenSwiss may be necessary as I would only need 1" of stick-out instead of 2" (8:1 instead of 12.8:1 length to shank diameter ratio).

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    Quote Originally Posted by Bendak View Post
    I have a part which is supposed to fit within this hole, acting like an extractor to retrieve a core sample cut-out by the finished product. In the past I have also dropped various sized dowel pins and measured their distance from the end with a depth gauge.
    If you have a set of pin gauges it would be easier to measure with them. Use your calipers to check a gauge's length and set it to zero with the pin in the jaws. Read the caliper directly with the step measurement.

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    Boy - you sure have a Doosie to deal with as an undergrad!

    Is this your design - possibly showing you how NOT to design something?

    Or the teacher tasked you with this?


    Personally - I don't see the reamer thing working at all!
    WAY too much surface contact and especially in 4340!
    (It will break)

    We use'ta doo similar work in production in 1215 with modded up twist drills, but we were hitting maybe 1.5" or so long taper with a 9/16 minor - so basically using a 5/8 or 11/16 tool to start with.

    That was in oil lube too.

    I just don't see that working well in 4340 tho. But could be worth a try?
    Also - I would break up the two angles for sure!
    You may even be able to split up the first - long one.
    Essentially make 2 identical tools to make the whole thing, and then whack one off 1/2 way through and re-cut the lead chamfer, and then run that in to depth, and then come back and get the other tool and go in 'till you mate up with the previous surface...

    I think that the first thing that I might try is to make a boring bar like above.
    Prolly better material than he used for your app tho.
    Prolly 4340 there too and H/T'd.
    You may need to make two different sized ones for that too?

    Beyond that - you could for sure get it done in the Sinker EDM.


    ------------------------

    Think Snow Eh!
    Ox

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    Any option to redesign?

    If you could at least split the part at the bottom of the straight portion.
    Then you could bore it much easier.
    (not "easy")

    Make a "tongue and groove" and braze it together when done?


    --------------------

    Think Snow Eh!
    Ox

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    Having a decent OD grinder you might take a standard straight flute reamer and grind the tapers and diameters with using simple measuring tools. I used to back spin such reamers so wheel contact would start at the heal not at the face. Then back off heal to perhaps .010 / .015 circular land.. trying to bore it and measuring the taper lengths and diameters would be difficult. Making special gauges would be more work that grinding one special reamer. IMHO.
    Much like making a rifle breech reamer.

    I guess one could use the taper attachment following a special tapers multi feature gauge and a stiff boring bar.

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    Drill all the way through at your smallest diameter, helically bore the straight portion. Surface the rest of the fuckaroo with a long 3/16 EM. You can get remarkably good surface finishes with a square endmill if you take small enough steps. We are all used to having to make 10's of thousands of pain in the ass parts... you need to make one. Let the machine run for 2 hours and you'll have a very good part.

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    Quote Originally Posted by G00 Proto View Post
    Drill all the way through at your smallest diameter, helically bore the straight portion. Surface the rest of the fuckaroo with a long 3/16 EM. You can get remarkably good surface finishes with a square endmill if you take small enough steps. We are all used to having to make 10's of thousands of pain in the ass parts... you need to make one. Let the machine run for 2 hours and you'll have a very good part.

    I read that "3/16 mill" part and I was


    I went back and looked at (what I could read) of the print, and it does say that it is only 2" long....

    But he said that the major bore was something like .925 or whatnot. (can't read it)
    If so - then the pic is WAY outta proportion!

    ???


    It looks like it's 6" long at least in the drawing.


    ---------------------

    Think Snow Eh!
    Ox

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    Quote Originally Posted by Ox View Post
    I read that "3/16 mill" part and I was


    I went back and looked at (what I could read) of the print, and it does say that it is only 2" long....

    But he said that the major bore was something like .925 or whatnot. (can't read it)
    If so - then the pic is WAY outta proportion!

    ???


    It looks like it's 6" long at least in the drawing.


    ---------------------
    Think Snow Eh!
    Ox
    Sorry about the small print, I guess that's the largest the image will show in the browser. As for a verbal description, the largest ID at the far right is 0.259", staying straight for 0.926". The hole then tapers at 1° per side for 0.724" until the ID becomes 0.234". The final portion of the hole tapers at 3° per side for a distance 0.276" down to the final ID of 0.205". Overall, the hole is 1.925" long, 0.926" of it being straight, and 1" of it as two different tapers. The OD in the beginning is 0.375", leaving 0.058" of wall thickness.
    Eventually this part will be welded to a longer section of straight pipe with the same initial dimensions as in this drawing. It is essentially a hollow auger made for the purposes of retrieving core samples from trees wherein a serrated half-pipe extractor "spoon" slides in to remove the wood sample from the auger. The taper is there to compress the 0.205" diameter wood sample to the teeth on the extractor so that the end of the sample can latch onto the spoon and break from the rest of the tree beyond the end of the auger bit. I am essentially attempting to reverse engineer an already existing tool known as an increment borer. Is has been done all on my own time with the intention of eventually receiving credit on such a project given its unforeseen complexity and time spent research. Let's just say that I had no idea what I was getting into when I first set out to machine this (the external threads are an entirely different story finished beforehand) but ended up enjoying the machining process so much that I have decided to stick with it since October of 2017.
    I don't know if it would be wise to split the part as the difficulty in welding such a part at that point would add to expenses given its internal geometry, but perhaps worth a try if all ends up going sideways. I very much appreciate yours and everyone else's input.

    Jack Ruddat
    Attached Thumbnails Attached Thumbnails hole-geometry.jpg  

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    Quote Originally Posted by G00 Proto View Post
    Drill all the way through at your smallest diameter, helically bore the straight portion. Surface the rest of the fuckaroo with a long 3/16 EM. You can get remarkably good surface finishes with a square endmill if you take small enough steps. We are all used to having to make 10's of thousands of pain in the ass parts... you need to make one. Let the machine run for 2 hours and you'll have a very good part.
    Could you clarify what you mean by helically boring the straight portion? Would I not instead drill it straight (a 0.257" drill would be close enough, as they tend to drill 0.001" oversize anyway)? When it comes to surfacing the rest with an EM does that just involve helically boring down the inner circumference of the tapered portions? I would imagine that would be even more susceptible to tool deflection than the boring bar, but I've never had any experience doing that as of yet. Also, why a square EM and not a ball EM?

    Thanks for the suggestion,
    Jack


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