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Problem drilling holes on the centre line of items

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  • MattiJ
    replied
    For tiny holes and parts I would try spotting with PCB carbide drill. Very sharp, reasonably stiff and cheap.
    No scribing or punch marks, clean surface and just mill DRO.

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  • J Tiers
    replied
    If you are sure the spot drill is in the right place, then please tell us if you are moving the head when you change drills, or if you are just retracting the quill.

    On your mill, both the head and possibly the column might have a tram problem.
    Last edited by J Tiers; 04-08-2018, 11:20 AM.

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  • loose nut
    replied
    Originally posted by danlb View Post
    I trimmed out the stuff that does not matter. I won't bother addressing why.

    Back to Loose Nut's latest post:

    You are dealing with really small sizes. When you are off .010 on a 3/32 drill, that's a lot. It sounds like you are saying that the spot drill is always correct but that the drill is not making it's hole within the dimple left by the spotting drill.
    That is how it appears to me. Yesterday I drilled a couple of 3/32" holes and they where OK. .000 run out one way and 2 or 3 thou the other which is acceptable to me for a drilled hole. The next time it could be way off and there is no change in the machine tram or any other set up IE: the vise isn't moved etc. ???

    Originally posted by machinejack View Post
    When I want a hole on location I get the boring head out. Changing work heights will get you off if the mill is not true the spindle may be parrellel to the work, but if the post is not you will be off. If everything is trammed in you should be spot on.
    Boring a small hole (3/32 or 1/16) is a bit impossible for me.

    Me thinks the drills must be jumping around some of the time??
    Last edited by loose nut; 04-08-2018, 10:40 AM.

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  • David Powell
    replied
    Everything matters.

    I spent the last 7 years or so of my working career running a " First " mill ( and a Clausing lathe). A large proportion of my work was checked, ( BY a Chinese lady named Ying with a vastly expensive co_ ordinate measuring machine )and reports of my work were filed so that any discrepancies could be followed through. What we found was simply that everything mattered. I was in the land of small items, generally from 1/32 holes up to about 1/4". Certainly, if the maintenance gang used "MY" mill on the weekend or night shift I checked the tram before starting my day. We knew of, and tried spotting drills but found equally good results with tiny centre drills. We bought good quality drills, mainly from McMaster Carr, and once they lost their initial sharpness replaced them rather than even trying to sharpen them .Drilling fibreglass pallets generally meant drilling only 10 or so holes before the drills began to dull. We normally worked from an imaginary centre point on flat items, and when allowable I marked that with a small centre drilled hole so that if later modifications were needed we knew that was how it had been made. We began to be concerned if any hole was more than 2 thous off position. I hope this is encouraging and helpful. Regards David Powell.

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  • machinejack
    replied
    When I want a hole on location I get the boring head out. Changing work heights will get you off if the mill is not true the spindle may be parrellel to the work, but if the post is not you will be off. If everything is trammed in you should be spot on.

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  • J Tiers
    replied
    Originally posted by danlb View Post
    I trimmed out the stuff that does not matter. I won't bother addressing why.
    ....

    Dan
    Dan, it's a wonder your own spit does not burn your throat. You are the nastiest poster on the forum.

    oh, yeah.... go ahead and report this post.

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  • danlb
    replied
    Originally posted by J Tiers View Post
    I trimmed out the stuff that does not matter. I won't bother addressing why.

    Back to Loose Nut's latest post:

    You are dealing with really small sizes. When you are off .010 on a 3/32 drill, that's a lot. It sounds like you are saying that the spot drill is always correct but that the drill is not making it's hole within the dimple left by the spotting drill.

    The center drill is likely to have a 120 degree tip. Per https://www.destinytool.com/spot-drills.html you may need to change to a spot drill for a smaller angle or a twist drill with a 135 degree angle. Per the above link, if the spot drill angle is less than the twist drill will result it the flutes digging into the sides of the dimple.

    Dan

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  • J Tiers
    replied
    Just for grins, try doing things asif you had backlash..... coming back past center and then going forward to the setting.

    While I assume your DRO is reading the sliding motion,as any real one* will, if a bracket is too flexible, or screws are loose, or if the table gib is loose, etc, there can be wrong display that does not reflect the actual motion. Then reversing motion can "cross the effective backlash" due to the loosely-mounted DRO.


    Originally posted by danlb View Post
    While that is often true, you almost never have a situation where the drills are all the same size and don't require moving either the head or knee to change the tools or to get the depth that you need for the hole.


    Dan

    Spotting drill, followed by screw machine length drill. Just as in the part (of my post) you failed to quote
    Last edited by J Tiers; 04-07-2018, 01:20 PM. Reason: clsrification

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  • loose nut
    replied
    O, to allay any fear of "out of tram" it is 0-0 in the Y axis and out about .0005 in the X axis, to close to mess with. I understand how out of tram works against accurate work.

    I generally use the spotting drills for larger sizes but for the small ones (3/32 or less) I use center drills. Smallest spotting drill I have found is 1/8" and not much good for small drills.

    The quill seems OK and the machine (RF-45 bench mill) is not that old at least as far as use is considered.

    To explain how I pick up the center of a part. I pick up one side and then zero the DRO, pick up the other and then hit the divide by 2 function and dial back to center. Sometimes I use a wiggler, repeating 2 or 3 times with good repeatability and sometimes a 3D Taster (accurate to under a half thou), it doesn't seem to matter.The "spot" always seems to be in the right position. I can lay it out on the surface plate first and use that to verify the "spot" and it will line up OK. I use a small drill to start and then drill to size, sometimes I drill one size under and then finish to size.

    The weird thing is sometimes I will very close to position (a couple of thou, this isn't a jig borer) and sometimes way out using the same techniques. I guess it is something I'm doing wrong.

    P.S. I should add that I generally work with small parts, the current ones are 1/4" x 1/4" x 5/8" with a 3/32" hole through one of the 1/4" sides so off center holes stick out.
    Last edited by loose nut; 04-07-2018, 10:06 AM.

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  • Richard P Wilson
    replied
    Originally posted by Paul Alciatore View Post
    Gentlemen, TRAM your mills!
    This point was covered at some length recently in the CNC section in a thread called 'Albrecht chuck/CNC', and concluded that if the head is misaligned (and it need not be misaligned much) then changing from a short tooling set up, like a spotting drill, to a longer tooling set up can easily result in a misalignment of 30 thou or more.

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  • danlb
    replied
    Originally posted by J Tiers View Post
    The tram issue will only come up if you have to move the table between using the spotting drill, and using the actual drill. If you are just using the quill, there will be no issue unless it is really sloppy.
    While that is often true, you almost never have a situation where the drills are all the same size and don't require moving either the head or knee to change the tools or to get the depth that you need for the hole.

    It's bad practice to use your mill if you suspect it of being out of tram even one degree. As Paul said, Gentlemen, TRAM your mills!

    Dan

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  • J Tiers
    replied
    The tram issue will only come up if you have to move the table between using the spotting drill, and using the actual drill. If you are just using the quill, there will be no issue unless it is really sloppy.

    Following a spotting drill (usually short) with a screw machine length drill, will be ideal.

    Originally posted by CCWKen View Post
    Tom Lipton (Oxtool) starts the hole then rams the bit through with considerable pressure. He demonstrated this while making a mini pallet. Perhaps it just takes good drills and the right speed.
    Whatever works.... The key is, both for him and for the situation described by the OP, that you are STARTING the hole with light pressure. After that, you can do as you wish, although "ramming the drill through" is not the best... usually you want to keep enough pressure on for it to cut, but not so much that you begin to split drills, or the like. A sharp drill has a rate of cutting and advance that suits it, and "ramming" with extra pressure is counter-productive at worst, and not helpful at best.

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  • Paul Alciatore
    replied
    Yes, nothing says the initial prick punch will be on center.

    When I punch and drill I:

    1. Scribe the cross hairs
    2. Lightly prick punch USING A MAGNIFYING HOOD
    3. Use a 10X magnifier to check the location of the prick punch
    4. Lightly punch with a fine punch, tilting the punch to adjust the location of the punch
    5. AGAIN use a 10X magnifier to check the location of the punch
    6. Use a larger punch, again tilting it to adjust the location of the punch as needed
    7. Repeat #5 and #6 as needed to get the punch centered on the cross hairs.

    With that procedure I can usually guarantee +/- 0.002" or better accuracy for the punch mark.

    Now even a jobbers length drill can "find" that punch mark and start on it. But, if I want the best accuracy I start with EITHER a center drill or a spotting drill. I find that the results are much the same with either one of them. I mount the part in a drill press vise and allow the vise to float on the table. The short center or spotting drill will center the punch mark under the tip of the drill. I suspect that a center drill will make a better hole for the next drill to follow due to the small diameter hole it drills at the tip. That small hole, made by the tip of the center drill, will be around the web thickness of your next drill which should follow it almost perfectly. So, center drill, pilot drill, and finally full sized drill.

    Yes, it is a lot of steps, but it does work.

    When I drill in the mill-drill, first, I have a well trammed mill. No errors due to the length of the drill or the position of the head on the column or of the table on it's vertical slide. Without that, you are just whistling Dixie.

    Then I find two perpendicular edges with an edge finder. I find that technique is everything with an edge finder. First, I LOCK DOWN any slides that I am not presently using. Thus, while finding the X edge, the Y gibs are LOCKED and visa-versa. If you don't believe this is necessary, try this: put a DTI in a collet in the quill and bring it into contact with an edge. Now grab one end of the table and push it back and forth while watching the needle on the DTI. If it doesn't move 0.005" or more you have a very tight mill table. Locking the gibs on all unused slides will eliminate much of this movement.

    Then I would find the edges TWICE. Once to get in the ball park and the second time with a lot finer motion on the axis to allow the best possible accuracy in the location of that edge.

    I often scribe lines on parts that I am going to drill in the mill. This is a double check on the location of the holes, mostly to avoid the dreaded, "one turn" error (0.100"). When I do this, I often find that the holes are offset just a bit from the scribed positions, usually 0.005" or less. For many parts, this is acceptable as the relative locations of the internal features are more important than their relationship to the edges. Such errors are usually about the same for each and every hole or other feature. If not, then I will chase after it when I see that offset when I touch off the first hole. And I have found that this can have many different causes.



    Originally posted by JoeLee View Post
    I've seen and had prick punches slide off the scribe line when the point sinks into the steel.

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

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  • CCWKen
    replied
    Originally posted by J Tiers View Post
    heavier pressure does lead to more "walking" of the drill, and even more "wandering" after the drill is started. Some of that may be due to a bad grind, some just to random things like hard spots.
    Tom Lipton (Oxtool) starts the hole then rams the bit through with considerable pressure. He demonstrated this while making a mini pallet. Perhaps it just takes good drills and the right speed.

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  • tom_d
    replied
    What type, or style of mill is being used here?

    The way I read the OP's problem is that he has started off with using both the center drill and spotting drill, confirmed his spot position location, and then sees the problem when switching to a twist drill to complete the drilling operation. This tool change most likely requires a move of the quill to accommodate a change in tool length. If the machine used is along the order of a traditional Bridgeport type mill then it would most likely be the Z axis of the head not trammed in. If that's the case then the error would be consistent in the same direction.

    He says that sometimes the holes are off. The head tram could be the problem going from one set up to another set up. If the errors are from piece to piece with the same set up then it sounds like there is movement in the head, or table, that's causing a problem.

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