Drilling and Tapping problems

Discussion in 'General Engine Discussion' started by JimM, Feb 28, 2010.

  1. Feb 28, 2010 #1

    JimM

    JimM

    JimM

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    Hi all

    I'm very new to this hobby and am having problems trying to get a straight thread when tapping. Today I was trying to make a part which had an 8mm male thread at one end and an 8mm female thread at the other. The male thread went OK but when I screwed some rod into the female end you could see that it was skew.

    When drilling for the female end I started with a centre drill, followed by 4mm then 6mm and finally 7mm. However when the drill was going in it looks like the tip moves up and to the right (ie away from the operator) slightly. I've checked headstock alignment using a centre in the head and tail stocks and they look to be pretty close but is there anything else likely to cause the tip to wander?

    If the tailstock is misaligned slightly is it possible to drill a hole at an angle on the lathe ? I would have thought if anything the hiole might be off centre but still straight ? The hole looked straight to me but my eye is still getting used to machining tolerances as apposed to 'close enough is good enough'

    To actually do the tapping I put a centre in the tailstock and pressed this to the end of the tap which I thought would almost guarantee the tap going in straight - am I missing something ?

    Any thoughts or suggestions on what else I can try to improve my skill base would be much appreciated.

    Thanks

    Jim
     
  2. Feb 28, 2010 #2

    kendo

    kendo

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    Hi Jim
    I know this might sound like a daft question, But when centering
    and drilling did you lock your tailstock, of not this could cause drill
    to wander a bit.

    Ken
     
  3. Feb 28, 2010 #3

    Ed T

    Ed T

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    Jim,
    As is often the case "it depends". IMO the most likely cause is that one of the drills you use was dull or at least had one flute duller than the other. The forces driving the drill into the metal cause a side force if the flute is unable to cut or is cutting less than the other one. In a perfect world, both flutes "screw" into the work removing a chip of natal as the go. In the real world, this is seldome the case and one flute is always working a bit more than the other. The one doing les work may be jst leaning on the metal just cut by the good flute. The side force generated wil tend to bend any drill some, but smaller ones are more prone to bend. Since you used a series of drills, it is likely that the first one ( the thinnest) drilled crooked for the above mentioned reason or some other and the other two folloed the crooked hole. So, what can you do about it?
    1) Use sharp drills
    2) Unless there is some limitation of your machine it's probably OK to go from the center drill to the 7mm in one shot. Multiple steps are tedious and probably not helping you out any. The 7mm drill is MUCH stiffer than the 4mm and less likely to wander on its own.
    3) Especially when using small dia. drills, "choke up" on the bit or use short bits. There's really no reason to have any more drill sticking out than the depth you are trying to drill. Shorter is stiffer and less likely to wander.
    4) Be sure that the chips are being ejected from the hole. If they pack up on one side, they will generate a side force driving the bit off course. Also packed up chips make a lot of heat and cause drills to break. So, if you don't see chips coming out, back the drill out and clear them.
    5) In general, I like to use cutting fluid. Others don't. Take your pick.
    6) Philosophical point. While it's nice to have everything come out nice and straight and it's nice to understand why when they don't; you should ask yourself whether the observed problem is really a problem for what you are doing, or is it just an observation. Don't get me wrong, it may be really important for some reason, but if not, move on and save your time for something that really does matter in your project.
     
  4. Feb 28, 2010 #4

    Metal Butcher

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    Hi Jim, If your drill creates an 'off center' hole it will not be strait by nature

    If your drill sharpening is off a bit (one flute produces more chip) it will create a 'strait hole' that will be over size.

    It could be a miss aligned tail stock, but you should have observed a serious wobbling of the drill bit.

    If the tap angles off center, than your 'tapping set up' may have too much play.

    After drilling I use the chuck itself to guide the tap in strait. I grip down on the tap and crack open the chuck just enough to allow the 'tap' to slip while I turn the 'tap' with a 'tapping disc'. The disc holds on the mid section of the taps shank with a set through the discs OD.

    I never had any luck with a center mounted 'tap guide' or 'tap wrench', to sloppy for my taste.

    Most all the holes I tap are on the small side and under a #8 screw size. I use three sizes of discs, 1", 1-1/2", and 2" on both my mill and lathe.

    With out being on site to check TS alignment and run of few tests, the best we can do is guess work in an attempt to offer you some help.

    -MB
     
  5. Feb 28, 2010 #5

    Longboy

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    ..........are you using a 2 flute tap, resharped drills? As MB said you would see the orbiting of the drill as you drive it into the stock if off center. How long a piece of drill rod did you use to make the observation and was it threaded a good length? The spiral of the threads hanging out from your tapped hole can give an "optical intrusion" to the eye and something that appears so just isn't there. I can't imagine a threaded hole in this size as being more than an inch deep. Do you have some shafting or series of rods screwed together? Dave.
     
  6. Mar 6, 2010 #6

    Antman

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    My 2 cents worth, you should drill 6.8mm to tap M8.
    Ant
     
  7. Mar 7, 2010 #7

    Blogwitch

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    Jim,

    The very first cutting tool to touch the metal sets up for everything that follows. If the first hole is off centre, then most likely everything after that will go the same way.

    I have only recently started to use spot drills again, instead of centre drills, only because I found a source in the UK at a reasonable price.

    These are what I use now, about a third of the price they used to be.

    http://www.engineeringsupplies.co.uk/drilling-c-160.html?9=172&10=&11=192&12=&13=

    A centre drill is only like using a normal small drill, but it has the advantage of being a lot more rigid. Even so, they can wander off centre sometimes, especially if there is a little 'pip' on the bar end. A spot drill on the other hand is ground on the end to very fine tolerances and will put a spot mark for drilling more accurately than anything else available.

    Just two or three of them at varying sizes is all that is needed for whatever you want to do.

    I am not saying that by using these would cure your problems, but by going back to the very initial start point and getting it spot on (excuse the pun), you are then eliminating any self imposed errors.


    Blogs
     
  8. Mar 7, 2010 #8

    bearcar1

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    Hello Jim,

    What has been stated previously is true, however what has not been addressed is the simple and sometimes overlooked errors created by the drill chuck itself. The chuck sometimes does not close evenly and will tighten up so the tool being used is not truly centered. I have encountered this problem on occasion and now inspect and clean out any errant chips that may have found their way into the body of the chuck and apply a light coating of oil to keep all of the internal parts moving freely. Also, if there is a small burr present on the bit shank, that too will cause the drill to be gripped off center and cause the conditions you described. Keep the shanks smooth by the application of a file to the offending area only to remove the burr followed up by a few strokes of a dressing stone. Use all three key holes around the periphery of the chuck when closing and tightening a tool in the chuck, this will also help the chuck to close evenly. If you notice a drill approaching its mark off center, try loosening the tool in the chuck and rotate it about a quarter of a turn and re-tighten. This sometimes releases uneven jaw pressures and reseats the tool, if this does not work attempt the process once more, if the tool then does not center up, it is more than likely a bit that is not straight. All sorts of little things to watch for and all sorts of "tricks of the trade", some work, some do not. Taking care of the tooling itself I think is one of the most overlooked parts of this hobby, it is usually never talked about and the novices are never made aware of the consequences. Even some of the experienced machinists that I know scoff at some of these ideas, but they continue to have problems and chalk it up to all sorts of other reasons, anything except that the tools are telling them that they require some attention.

    BC1
    Jim
     
  9. Mar 7, 2010 #9

    vlmarshall

    vlmarshall

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    So true, Jim! I don't know how many times I've pulled drills out of the drawers at work, to find chuck burrs on the shank. :mad:
     
  10. Mar 7, 2010 #10

    doc1955

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    I personally have found the biggest contributor to a hole that wanders off and isn't straight is being in a hurry to get it done.
    When you back off to clear the chips and re enter hole to continue drilling be very patient and don't bang the drill into the bottom of the hole. As stated before drills will have one flute that will do just a little more work then the other and the moment you bang into the bottom of the hole that is when you will get it traveling off center. Plus on deep holes clear you chips often a loaded up drill will also run the risk of pushing off center.
     
  11. Mar 7, 2010 #11

    Metal Butcher

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    I would like to add another problem to watch for, that can also cause a drilling misalignment on the lathe.

    I have a quirky cam lock on my tail stock. It's an on/off lock that allows the tail stock to lift up off the ways if I'm not paying strict attention. This lift can allow swarf to get under the tail stock while I'm bringing it up towards the work The result is an undetected misalignment.

    -MB
     
  12. Mar 7, 2010 #12

    JimM

    JimM

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    Hi all

    Apologies for the delay in responding and offering thanks for all the gret advice. I've been a bit manic for the last week or so and haven't managed to get much shop time and as yet haven't had a chance to try out the suggestions. Rest assured they're all logged away and next time it comes to drilling I'll be following the advice and will let you know how I get on.

    Many Thanks

    Jim
     
  13. Mar 8, 2010 #13

    Maryak

    Maryak

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    Hi Guys,

    When drilling vertically as in most normal drilling machines, I was taught that the drill can cut to a maximum depth of 3 x drill diameter before it is necessary to clear the flutes and the hole being drilled of swarf. It's a guide I try to adhere to and IMHO is of increasing importance the smaller the hole being drilled. It's also relative to the material, i.e. those that chip, (e.g. brass, cast iron) require more frequent backing off and clearing than those which form spiral swarf, (an aid to self clearing).

    Best Regards
    Bob
     

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