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Lathe hole drilling off center, Why?

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  • wombat2go
    replied
    A small boring bar could get 25/64 (0.390 inch) by 2.5 inch deep
    I have a boring bar here that is 0.312 inch dia ( 5/16 inch)
    Last edited by wombat2go; 07-31-2017, 04:57 PM.

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  • Forrest Addy
    replied
    My trick for deep holes is to give them a good start. Bore to drill size for the first couple diameters in depth then drill to full depth. This gives the drill a centered start to follow from which is is less likely to deviate. I've drilled 5/16" water passage holes 20" deep in cast copper nickel to intersect at an angle deep inside the casting using this trick, although not on a lathe.

    Your SB 9A is old right? I'm guessing your tailstock quill axis is worn low and pitched down a small angle at the headstock end. It's hard to drill a straight hole when the thrust axis is off at a shallow angle. My suggestion is to set aside some time to overhaul your tailstock. That is scrape the base to make the tailstock quill bore parallel to the spindle axis and place shim between the base and the upper casting to align the quill axis parallel to the spindle axis in the horizontal and vertical plane. This is skilled work but not out of reach for a home shop guy willing to invest in a little study and skill development. The tools are mostly what's on hand plus a scraper and a tube of blue.
    Last edited by Forrest Addy; 07-31-2017, 04:45 PM.

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  • old mart
    replied
    I agree with macona about using a spotting drill, centre drills are intended for producing centre mounting holes, the 60 degree angle is not ideal for starting drilling.
    For checking out the lathe setup, there is an Indian firm selling test bars on eBay called etoolinghub24x7. The bar that I bought from them has a morse taper one end and centres and was under 0.0001" tir, good enough for me.
    I turned a soft 60 degree centre in a collet and left it in place while I fine adjusted the tailstock for height and lateral alignment, and also checked whether the tailstock barrel was parallel to the lathe axis. You can also check the headstock alignment, some can be adjusted.

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  • macona
    replied
    Using a center drill can cause drills to start off center. Use a spotting drill instead and just make a small indentation.

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  • J Tiers
    replied
    When the OP said (IIRC) that the drill entered on center, it appeared less likely it was off. Usually they droop, and end up off center low.

    BUT, mine is to some extent, yet it drills on-center and does not drift much in even deep holes. Even with the droop, I have drilled a quarter inch hole 8 1/2 inches deep, and was off less than 10 thou in that distance (making a new spindle for a small lathe). I am not convinced that it is a really strong cause of drift if you get the drill started on-center.

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  • BCRider
    replied
    Originally posted by CCWKen View Post
    Maybe the tailstock isn't centered or the extension isn't parallel to the ways.
    I'm rather surprised that Ken is the first to suggest that the lathe should be checked to see that the tail stock is axial to the headstock in all respects. I'm thinking along the same line as well. It may be worthwhile doing some testing with a dial indicator and a slug in the chuck that is cut to a size close to the diameter of the tail stock ram to use as a reference.

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  • CCWKen
    replied
    Maybe the tailstock isn't centered or the extension isn't parallel to the ways.

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  • J Tiers
    replied
    Originally posted by darryl View Post
    .....

    JT added another insight here, that of taking off the rake angle at the very edge of the cutting lips. I do this for brass and plastic, and it allows for a more precise hole in addition to it's benefit in the other materials in that it doesn't grab.
    No such thing. Never mentioned it, did not intend to in this context.

    But it is a good plan for some brasses etc where the drill may dig in.

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  • darryl
    replied
    I do pretty much as Rich described. It works. In my experience you can never rely on a drilled hole being concentric as many factors contribute to the wander. But this method gives about the best result for me. Depending on the final size desired, if you can bore after drilling you'll be good. If the hole size doesn't allow for that, then you can always make (or buy, presumably) some reamers or D-bits.

    Getting the hole started perfectly on center is your best bet if you are going to drill only. The best way to do this is to use a tool in the tool post to actually machine the starting point. Using a stub drill bit or a center bit in the tailstock is no guarantee that you'll get perfectly centered, though it helps a lot.

    If you're going for a fairly large hole, you might start with a pilot hole, then bore that out some. It may be quicker to then follow with the final drill bit, but at least you have a full well-centered through hole to guide it. I do this quite often if the hole is less than an inch deep or through, using a boring bar that will start working in a 1/4 hole. I drill with a 1/4 inch bit, then use my solid carbide 1/4 inch boring bar to clean that up, which is making the hole about 5/16. A 1/2 inch bit will follow that nicely, and it's quicker than boring the rest out. Of course you can always drill undersized and finish by boring if you need the best finish and accuracy.

    JT added another insight here, that of taking off the rake angle at the very edge of the cutting lips. I do this for brass and plastic, and it allows for a more precise hole in addition to it's benefit in the other materials in that it doesn't grab.
    Last edited by darryl; 07-31-2017, 01:35 AM.

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  • J Tiers
    replied
    If you start the drill. but go slow for a bit as you come up to cutting full diameter, you will end up using the drill as a sort of "single point tool" if it is off-center. If you let it cut until it just stops cutting and is about to rub, then feed in a little, you will end up on-center after a few of those cycles, assuming it is fed on-center. Do that until you have it drilling at full diameter.

    It does not take too long, and it always ends up on-center for me. Does take a sharp drill. but dull drills tend to wander because of excessive pressure..

    For that technique, I prefer the cutting edges more vertical so the swarf falls away and cannot collect on one edge and possibly cause a force to make it go off center. That may not be necessary, but it does not seem to hurt.
    Last edited by J Tiers; 07-31-2017, 01:21 AM.

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  • RichR
    replied
    In high school the teacher showed me how to use the tool bit to create a centered dimple for starting a drill bit.

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  • Rich Carlstedt
    replied
    You don't say how much drift you had ?
    There are many reasons for the drift and some comments made by others are spot on !
    Excessive drift is considered to be more than .015" per inch. so if it was ( 2.75x .015) over .040" from center, then it is excessive.
    Center drills are for centers , not necessarily for starting drills, although everyone seems to use them that way
    A far better way for accuracy is to use a "old timers" technique that gives dead nuts centering , and eliminates the use of a centerdrill.
    I don't see guys doing this, but in the old days , job shops did not want to waste time changing out drill chucks and taught this method.
    It takes a bit of skill but gives great results.. It helps even more if you have any kind of quick change toolpost ( Aloris ie)
    Do this: ALWAYS mount the drill bit so the cutting flutes are horizontal ..Imperative , and a good practice.
    Enter the drill bit into the turning workpiece about 2/3'rds of diameter. When you are in about that far, stop feeding and raise your tool bit ( right, the lathe tool bit !) about 3/16" above it's normal position by loosening the post clamp and just lifting the tool block . Now , run the tool bit up against the drill bit and then give it ( the drill) a nudge off center ( visually seen. maybe .010 to .030 or more for bigger drills) .
    Now feed the drill bit in ( It will be cutting on one flute only, the rear flute) and as you approach full diameter, back out the tool bit with your other hand. The move together is all of maybe two seconds , and the skill needed is the coordination of feeding and backing out.

    Now what happens is this. the drill bit is started into the piece without a centerdrilled hole. It may or may not be on center - no matter !
    The tool bit forces the drill off center and thus it only cuts on one flute and creates a conical hole with a tit in the middle. At first it may vibrate if the starting point was not centered--no matter, wait for it to stabilize, and then fed in while relieving side forces. The angle of the flute will allow the drill to follow it's own centered hole form and the tit will be knock out as the conical shaped hole provides support for the drill bit . In other words, the cutting force for the tit is less than that required to remove stock on the larger diameter.

    Of course this is not a solution to all the drill ills, but is a darn faster way of getting the drill centered and in the proper location and is really quite simple when you get the hang of it.

    when you look at your drill bit , choose the following point for you lathe support. Drills have different helix angles so there is no magic number . Since the cutting edge is horizontal , chose the next horizontal point of the drill- which is back 180 degrees from the cutting flutes to place the tool bit support. This is the most even point that will not slip due to the helix angle. If you do not want to use a tool bit, any block/rod will work. If you use carbide insert tooling, just raise the tool holder high enough to not allow the carbide to touch the drill bit. When done, loosen the clamp and let the tool slide down and you are good to go.

    Rich
    Last edited by Rich Carlstedt; 07-31-2017, 12:59 AM.

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  • Mr Fixit
    replied
    I held the stock in a 3 Jaw chuck as that was the last setup. Yes the tip was in the same shape as started and showed no chip or wear on the cutting edges. The chips came out as flakes or small spirals from both sides equally. The drill is a USA drill with the extra relief at the land for a crankshaft or deep hole drill.
    Sounds like I didn't let the drill bit do its work, and probably pushed it too fast.

    TX
    Mr fixit for the family
    Chris
    Last edited by Mr Fixit; 07-31-2017, 12:33 AM. Reason: typo!

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  • JoeLee
    replied
    I would try piloting the hole first with a 1/8" drill as deep as the drill will go, then work your way up to the 25/64".
    Sometimes in "crappy material" like hot rolled trying to drill a hole with out piloting can lead to wander. I like to use the short screw machine drills because they wander less.
    Once you get a straight hole started the drill should follow since it has support around it.

    JL....................

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  • digr
    replied
    What did you have the piece held with, chuck, collet,??

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