Setup Spacing For Holes To Be Drilled

[Keith Foor]

OK, need to know what your preferred method of laying out holes in a plate or bar to be bolted to another fixture that has blind holes. Of course with the blind holes, using a transfer punch is out. I attempted to thread in bolts to each hole and measured the outside distance across and then deducted the diameter of one bolt but that wasn't correct and I was WAY off. So how is this done correctly?

 

[Chipper5783]

There are a few different options. In similar situations I have made up little transfer pegs (cut off bolt), pointed to make a mark on the mating component, with a couple flats filed in so it can be gripped enough to install/remove.

How accutate does the spacing need to be, what size holes, how many holes in the set? If you can drill 1/32 oversize - then it is pretty easy. If there are quite a few holes and they need to be size on - that is a bit more difficult.

 

[Wreck™Wreck]

Depending on the accuracy required.
If the part will fit in a mill put it on the table, find a 0, 0 data point on the part, insert a snug fitting gauge pin in the holes and indicate the center of the pin for each hole and write it down.
Place the mating part in the mill and indicate the appropriate 0,0 datum, move it to the coordinates recorded and drill away, the accuracy will be within the limits of the machine and your skill combined.

As an example, in the cartesian coordinate system the 0,0 data point is where the sharp corner used to be at the lower left. This is easily found by edge finding 2 sides. The 8 holes to be drilled are in absolute coordinates from 0,0. A DRO makes this easier as it eliminates having to reset the dials to account for any lash in the lead screws when changing direction 180° in either axis.

 

[Paul in OKC]

Done the same with making little pointy bolts. Turn a point, put a flat on either side enough to grab with some needle nose pliers, but leave the point and screw down, lay plate on and whap it a time or two. Presto!

 

[Keith Foor]

OK, I am gonna put a bit more info in here. Not sure that I put enough before. I am trying to make a set of jaws for an old 6 inch vice. The bolts that hold the old jaws in are 1/4 - 20 so I figured cap head screws would work well. The heads are a shade under 3/8 so I pocket milled the holes with a 3/8 4 flute after drilling the holes a 32th over 1/4. I measured from the outer edge of the vice to the outer edge of the first bolt and got 1.1 inch. I subtracted .125 (half of .250) from the 1.1 and drilled my first hole. I got 3.25 measured from outside to outside (measured with a caliper) of the two holes and got 3.25 inch so I subtracted .250 (thinking half of the bolt diameter again but for two bolts to get the center. I was off at least .250 inch with my second hole that I drilled at 3.00 inch center to center from the first hole. My logic said that taking away the .250 would be half the diameter times 2 because I was measuring the outside of both bolts. Now this was done on a mill with a DRO on it, so it SHOULD be right. I am obviously missing something here but I don't know what. Did I measure wrong, the math was simple enough and the first hole was right on for the first bolt. I rezeroed the DRO before moving to the seond bolt hole so I don't believe that was it. I honestly am a noob at this. So far I have used the mill to carve some .5 slots in a plate with a .5 3 flute cutter. Other than that I have used a fly cutter to clean up the same plate prior to milling the slot and it wasn't required to be exact as it's a slide reference for a tool holder that is yet to be built for a cutter grinder that is still taking shape.

 

[David VanNorman]

You can use blind hole spotter screws , or an attachment for your caliper that is a set of points that fit in the holes and give you the distance between them.

 

[Keith Foor]

But there should be a way of putting pins (bolts) in the holes since I know the pin diameter and calculate the center to center distance from hole to hole.

 

[RJSakowski]

But there should be a way of putting pins (bolts) in the holes since I know the pin diameter and calculate the center to center distance from hole to hole.

.250" is a lot to be off. The practice of measuring outside to outside of a pair of bolts and subtracting the diameter of the bolt should get you much closer. The error arises because on clearance between the bolt and threaded hole tilting the bolts slightly so the measured distance is slightly short (To compensate for this, I try to run short socket head cap screws instead and measure the head diameter and the outside to outside of the heads). But the error should only be a few thousandths.

 

[RJSakowski]

How did you actually measure the 3.25"?

 

[Keith Foor]

So, from what you are saying RJ, I did it right and either read the caliper wrong or botched the math and possibly both. But the logic of what I was thinking was sound. I will measure the drilled holes tomorrow and see how far off I am.