drilled hole matching

I will be drilling some plates with holes around 13/16 that have to match up. The bolt hole pattern is for eight holes on three different plates that have
to match. From what I understand there will be 26 sets of three. One set if the three will be done by a machine shop and the rest are my responsibility. I am thinking of an accurate scribed layout, pilot drilling and drilling one plate to use as a master before stack drilling. The plates are around 3/4 thick. Any suggestions on how to make locations more accurate??? I am limited to a radial arm drill press. No milling machine. Edge distance are not critical but hole pattern is. Randy
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How much time / money do you have to spend on this? I'd suggest asking the machine shop to make your holes undersized in the "template" and pressing in drill bushings for a pilot drill operation. Then follow through the pilot holes to make the larger finished holes.
How many parts total do you have to make? i.e. is this batch the entire job or are you eventually looking at hundreds, thousands, etc.
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Regards,
Joe Agro, Jr.
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There are 26 sets of three from what I understand... We have a bid price to keep under on this job. I am pretty sure the holes will be 1/16 oversize for structural connections. The idea of me drilling these is to save time on delivery plus cost. What concerns me is the three plates. Two plates with matching holes is not difficult. From past experience it seems to be a geometric increase in possible misalignments.

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Can the machine shop spot or center drill the top plates of each set? You could chuck up a 60 deg pointed rod to locate each hole. Can someone tack weld the plates together to prevent movement? Randy
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Good point about a rod. I just might machine up a centering rod before spotting my large drill bit on the radial arm. I always tack three or more bars across the sides of the plates. I hate people who tack directly. Listening to me rant about their ignorance while trying to break plates apart is not pleasant. My guess is that I will not see the plates drilled in the machine shop until long after I have drilled my plates. Randy

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It sounds to me like what you need to do is get or make your self a transfer punch. This is a center punch that fits snugly in the hole you are trying to match and allows you to mark the center on the second plate.
Have the machine shop turn some 13/16 plugs with a hole in the center that will match the diameter of a transfer punch.
Set the master plate over the other plate insert one of the plugs and center punch the hole location. Drill that hole to size and then reassemble with the master plate. Align the hole you drilled with the hole you marked it from and insert a plug through both the master and the copy. Mark your second hole and drill, reassemble and continue till all the plates are done. Doing one at a time will be slower than marking them all at once, but it should get you dead on that way.
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Roger Shoaf

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We'll assume that each set of three only has to match itself (this is *very* important.)
I would take one plate that the machine shop has done and use it as a master template. Make or buy a transfer punch of the appropriate size. Clamp and punch 26 blanks using the master and the transfer punch.
Now, simply clamp (I also like the idea of welding them) two unpunched blanks to each punched blank and drill together.
From my experience, the fixturing on the radial arm is going to make or break your profit margin. You need to be able to clamp/unclamp your part, and index it to the next location, very quickly. I wouldn't reposition the head on the press unless the plates' size prevents them from being moved between holes.
Also, make sure you don't have to spend a lot of time clearing chips/coolant as this will make the job much longer. Using posts or blocks to raise the part off the table will help this issue (perhaps 2"+). Make sure there is lots of support around the hole being drilled as you don't want anything to move.
I would also suggest splitting the point on the drill and popping the hole in one shot. You should be able to get a straight hole although you may be inclined to peck the drill as this will help the location.
HTH.
Regards,
Robin
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I am getting some good ideas. We have used our punching dies from the ironworker for larger transfer punches in some cases. In most of my situations the pieces I drill are heavy being over 100 pounds. Recently I was doing stacked drilled 3.75 thick plate. It took as much time to load and align as to drill the nine holes. We used a quarter inch plate masterpunched and transferred with punch dies. On this coming job I will try the transfer punch idea. I have not had much luck using a split point on large bits and starting on the centerpop. I have been using centerdrill first then changing bits when the arm is locked. Randy

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There's no doubt it is ideal to c-drill, pilot and then drill to full diameter (or even double drill), but that takes time. In your situation, it would take a lot of time. This is why it is critical to know if the sets must match only themselves, or all the other plates as well.
When drilling large-ish holes in steel with a hand drill, you really start to understand why they sell split-point drills. Correctly ground, these significantly reduce the power required to drill a hole. Naturally, if they are incorrectly ground, you're going to lose your edge prematurely, or the drill will wander, or, or, or...
This is the kind of job where a well thought out work plan and efficient fixturing will save lots of time.
Good luck, and of course let us know how you make out.
Regards,
Robin
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Have the machine shop drill just the first three holes with a 1/8" bit, then make a 1/8" punch to fit the drilled holes to mark the centers for all of them, then drill all using the marked centers. May be cheaper that having the shop make your transfere punches
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When I need a large or odd sized transfer punch, I use an accurate 1/4" punch I made and hardened guided by a brass or aluminum bushing. I face, drill and ream a piece of brass or aluminum then turn the outside to the required diameter. This has a number of advantages including the flat face that holds the guide square to the surface being punched.
Ted
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