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Thread: Dowel pin press

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    Default Dowel pin press

    I am going ream for a 1/2" dowel pin with a .4995 reamer on a new Bridgeport. Holes are thru holes at 2.125" depth. The dowel pin is 3" long and centered in the block with equal portions above and below. What are my chances they will match a mating base part whether the block is upside down or right side up?

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    Quote Originally Posted by GZ3S36 View Post
    I am going ream for a 1/2" dowel pin with a .4995 reamer on a new Bridgeport. Holes are thru holes at 2.125" depth. The dowel pin is 3" long and centered in the block with equal portions above and below. What are my chances they will match a mating base part whether the block is upside down or right side up?
    .
    1) most spindles are out of tram or leaning at a angle
    .
    2)due to hardness variations most drill bits tend to drift off center the deeper they go
    .
    3) due to runout or reamer wobble the beginning of reamed hole is typically bigger, just saying .4995 might not be small enough

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    How close is "match"?

    I think you have 2 chances. Slim and none.

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    Drill & ream the holes first leaving material on all four sides to grind into the holes.

    When you spot your holes, don't move the table, that includes the knee.

    I would use a .499 reamer, and hone to size.

    Works every time it's tried.

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    it's going to depend on two things. How straight is the hole, and how straight can you insert the pin.

    To get the hole as straight as possible, make sure your head is trammed in dead square. Drill the hole and then bore it to around .492. Now ream it to size.

    Take your pin and leave it for a few hours soaking in dry ice and alcohol. Now it will slide in and you won't get it cocked trying to pound it in with a hammer.

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    drilled holes are often curved a bit, if you take a long length plug gage and insert in deep hole they often will tighten up the deeper the hole cause hole is curved.
    .
    get worse if reamer flute length is shorter than the drilled hole. and reamer will bend a bit if drill hole is curved a lot. i have seen random curved holes before. it helps if you got a spiral flute reamer. straight flute chucking reamers are more common. hand reamers often have longer flutes than chucking reamers

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    Quote Originally Posted by GZ3S36 View Post
    ...What are my chances they will match a mating base part whether the block is upside down or right side up?
    Just one pin? Assuming you have a slip fit hole in the mating part, and nothing else interfering, sure- it will go.

    If you have 2 pins that have to go at the same time, you grind one to a diamond to allow for location error.

    Example:
    Diamond Pins | Carr Lane

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    Quote Originally Posted by GZ3S36 View Post
    I am going ream for a 1/2" dowel pin with a .4995 reamer on a new Bridgeport. Holes are thru holes at 2.125" depth. The dowel pin is 3" long and centered in the block with equal portions above and below. What are my chances they will match a mating base part whether the block is upside down or right side up?
    Ream all the holes, through all the parts, as one assembly. Mark the parts so that they will always be assembled in the same order. That's being a practical machinist.

    The most tried and true method is to assemble the parts in final position, with the dowel holes already rough drilled. Use a taper pin reamer to ream the holes through all the parts in one go. If you need to make further slight adjustments in part positions, you can ream the holes a little more to correct. You'd use taper pins for alignment.

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    Thanks for all the great feedback. Typical engineer...over engineering and over tolerancing. Guess I can use (2) 1-1/2" long pins and machine from the top as well as the bottom and let them meet in the middle.

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    I would drill half way through from each side, then ream straight through. Make a matching test block to qc them. How many you making?

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    I have to make two of these blocks.

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    OK, two of these blocks. There is the first detail that was left out of the first post. But there are so many more.

    1. How many of the bases that the blocks must match are there or are going to be made? 1? 2? 100? 100,000? Are you going to also make those base(s) or are they an already existing part(s) who's dimensions you must work towards. If there are more than one of them, just how well do they match each other?

    2. Just how many of these pins will there be in each block? One pin will always match. Two pins are a sufficient number for determining a single position for that alignment so that would be the optimum solution (see note below). So, is it two? Or are you going to over constrain it with three or more pins?

    3. You say the pins must match holes in the base(s) when the block is right side up and when it is upside down. But you do not make any mention of what kind of fit that "match" entails. Is that also a press fit? Is it a close, sliding fit? Is it a loose fit? You really need to think about the numbers here. I mean if you have a 1/2" allowance here, I don't see much of a problem. But if you are splitting tenths, then that is an entirely different situation.

    4. You say two of these blocks. Do they need to be interchangeable? Or can you just make one to match one base and the other to match the other base? This may determine how the job should be done.

    5. Are you dead sure that this is the only time that you will be asked to make these? Or will the customer come back in a month, year, or several years and ask for additional, matching blocks and/or bases?

    Note: Actually two pins in matching, round holes are still an over constrainment in this situation. One pin in a round hole and a second pin in an oblong hole would be the minimal constrainment needed to orientate the block on the base. I have seen this used in many situations where two parts had to be assembled in precise alignment. It is the generally recognized, optimum and professional solution for this kind of alignment problem.

    Many questions need answers here.



    Quote Originally Posted by GZ3S36 View Post
    I have to make two of these blocks.

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    capture.jpg
    Quote Originally Posted by EPAIII View Post
    OK, two of these blocks. There is the first detail that was left out of the first post. But there are so many more.

    1. How many of the bases that the blocks must match are there or are going to be made? 1? 2? 100? 100,000? Are you going to also make those base(s) or are they an already existing part(s) who's dimensions you must work towards. If there are more than one of them, just how well do they match each other?

    2. Just how many of these pins will there be in each block? One pin will always match. Two pins are a sufficient number for determining a single position for that alignment so that would be the optimum solution (see note below). So, is it two? Or are you going to over constrain it with three or more pins?

    3. You say the pins must match holes in the base(s) when the block is right side up and when it is upside down. But you do not make any mention of what kind of fit that "match" entails. Is that also a press fit? Is it a close, sliding fit? Is it a loose fit? You really need to think about the numbers here. I mean if you have a 1/2" allowance here, I don't see much of a problem. But if you are splitting tenths, then that is an entirely different situation.

    4. You say two of these blocks. Do they need to be interchangeable? Or can you just make one to match one base and the other to match the other base? This may determine how the job should be done.

    5. Are you dead sure that this is the only time that you will be asked to make these? Or will the customer come back in a month, year, or several years and ask for additional, matching blocks and/or bases?

    Note: Actually two pins in matching, round holes are still an over constrainment in this situation. One pin in a round hole and a second pin in an oblong hole would be the minimal constrainment needed to orientate the block on the base. I have seen this used in many situations where two parts had to be assembled in precise alignment. It is the generally recognized, optimum and professional solution for this kind of alignment problem.

    Many questions need answers here.
    Yes, I am well aware of locating pins and there uses as opposed to dowel pins but unfortunately I didn't get to design this. I have used locating pins a lot. I agree with you 100%. My job however is to build to the print. I have to make two replacement blocks. There is a base and it has 2 sets of 2 dowel pin holes in a line (slip fit .5004 +.0004/-.0000). The two new blocks set on those pins. Then a tie bar with matching holes to the base sets on top of the 2 new blocks (slip fit .5004 +.0004/-.0000).

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    capture.jpgHere is an image of the 3 mating parts. Tie bar, new blocks then base.

    Sorry, Just checked image after post and it's not good at all.

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    1. How many of the bases that the blocks must match are there or are going to be made? 1? 2? 100? 100,000? Are you going to also make those base(s) or are they an already existing part(s) who's dimensions you must work towards. If there are more than one of them, just how well do they match each other? One base. Already made. Never make again. Made elsewhere.

    2. Just how many of these pins will there be in each block? One pin will always match. Two pins are a sufficient number for determining a single position for that alignment so that would be the optimum solution (see note below). So, is it two? Or are you going to over constrain it with three or more pins?
    There are 2 pins in each block. Only need to make 2 blocks. We are altering an existing machine but need to be able to return to current condition if mods don’t improve machine function. Otherwise it would be tapered pins for sure.

    3. You say the pins must match holes in the base(s) when the block is right side up and when it is upside down. But you do not make any mention of what kind of fit that "match" entails. Is that also a press fit? Is it a close, sliding fit? Is it a loose fit? You really need to think about the numbers here. I mean if you have a 1/2" allowance here, I don't see much of a problem. But if you are splitting tenths, then that is an entirely different situation. It is a slip fit.

    4. You say two of these blocks. Do they need to be interchangeable? Or can you just make one to match one base and the other to match the other base? This may determine how the job should be done. The blocks need to be interchangeable/same.

    5. Are you dead sure that this is the only time that you will be asked to make these? Or will the customer come back in a month, year, or several years and ask for additional, matching blocks and/or bases? One time job. I work for the customer/end user. Sad thing is I know the tolerances don't need to be that tight but I have to give them what they want.

    Block holes are press fit
    Mating top and bottom parts are slip fit: Ø0.5004 +.0004/-0.000


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