Measuring distance between holes - need advice from the pros

[Pepster]

I would like to learn a more accurate method of determining the distance between two holes. I'm building a 13xx breach and I want to determine the distance between the rear screw hole, the little 4-48 hole in the loading port and the hole for the transfer port. What I have been doing so far is using a caliper to measure center to center and while I have a pretty good eye, I keep thinking there must be a better way. Is there?
Also, I thought that once I compile all of the critical dimensions, I could post them in a sort of data base for other members. I was unable to find anything like that in these forums.
Thanks,
Pepster

[Pepster]

I tried that but with my caliper, I still have to eyeball the outside edge of one of the holes.  My eyes aren't as good as they used to be and I want that third space to the right of the decimal point. I can measure the hole diameters with good accuracy and add the radii to the distance of the inside edges between the holes but this becomes more difficult with small holes.  I was hoping someone had some sage advice or clever trick that they would be willing to share.
Pepster

[Roadworthy]

I've never done this before, but it occurred to me that if you took a screw of the appropriate diameter and pitch, ground it to a centered point on one end, and screwed it into a hole the point would be at the center.  Using two such screws in this manner would yield a center to center marking.  If you can precisely dimple the center of a couple of appropriate screws you could use that approach as well.  That was just a thought.

[rsterne]

Measure the ID of both holes, add them and divide by two.... Measure the distance between the INSIDE (closest) edges of the two holes, and add the sum of the half diameters.... This works for any size holes, even ones that are different diameters, and can be done easily with digital calipers....

Bob

[Motorhead]

Measure the ID of both holes, add them and divide by two.... Measure the distance between the INSIDE (closest) edges of the two holes, and add the sum of the half diameters.... This works for any size holes, even ones that are different diameters, and can be done easily with digital calipers....

Bob

Exactly .... Hole diameters added together divided by 1/2 ( keep this number ) Then measure outside to outside of the holes using calipers inside fingers.  Subtract the first number saved from the second measurement and you have a precision center to center measurement.

[SeanMP]

Using the inside jaws of your caliper measure the inside distance between your two holes
Use the outside jaws to measure the outside distance between the two holes.
Add the two together and divide by 2

For an edge to hole center stand the part on a flat surface for the outside measurement.

[willbird]

I have done the best with putting something in the hole, a screw or set screw if it is threaded, a twist drill shank if it is a smooth hole (I have gauge pins in .001 incrememts from .01 to .900 but not many folks do) then measure over the objects and subtract the radius of each if they are different, or the dia if they are both the same size hole.

Bill

[rsterne]

I like Sean's method, only 2 measurements required (instead of three) and take the average....

Bob

[chuckinohio]

I have done the best with putting something in the hole, a screw or set screw if it is threaded, a twist drill shank if it is a smooth hole (I have gauge pins in .001 incrememts from .01 to .900 but not many folks do) then measure over the objects and subtract the radius of each if they are different, or the dia if they are both the same size hole.

Bill

  This always worked for me, when unable to vise it and indicate the holes in.

[bil.brown1951]

Measuring from front edge of hole to front edge of second hole is the best way. However, to improve on that thought to find the exact measurement, is to simply put the screws into the holes and use the edge of the screws to measure from. That way the calipers have something they can press against to take the measurement.

[Oldgringo]

Measuring from front edge of hole to front edge of second hole is the best way. However, to improve on that thought to find the exact measurement, is to simply put the screws into the holes and use the edge of the screws to measure from. That way the calipers have something they can press against to take the measurement.

Agreed! 

I just finished making walnut grips and pump arm for my 1377.  The calipers will involve a few thousandths of 'eyeball'.  Had I measured with the pins in the holes of the original pump arm (template), I'd be more pleased with the end result.

[rangerman]

Measuring from front edge of hole to front edge of second hole is the best way. However, to improve on that thought to find the exact measurement, is to simply put the screws into the holes and use the edge of the screws to measure from. That way the calipers have something they can press against to take the measurement.

That only works if the two holes are of the same ( equal) diameter.
The correct way, as many have already stated, is to get the average diameter of the two holes and add that average to the measured distance between the facing edges of the two holes.

To make it visually clear why your method won't work for unequal diameter holes is to picture that when you measure from one front face of one hole to the front face of the second bigger hole and draw a straight line between these two end points, one end of the line would start and include the whole diameter and center of one of the holes while the other end would terminate and touch only at a point on the outside edge of the other hole.
Now, if you move/shift one end of that  line to the center of the particular hole where it is at it would only move a distance equal to the radius of that hole.
Since the two holes are of unequal diameters it would not be possible for the other end of the line to also move/shift to the exact center of the other hole.
If the two holes were of exact diameter the ends of the line would of course terminate on the centers of each hole and the length of the line would be equal to the center distances of the two holes.

[Bent]

I have done the best with putting something in the hole, a screw or set screw if it is threaded, a twist drill shank if it is a smooth hole (I have gauge pins in .001 incrememts from .01 to .900 but not many folks do) then measure over the objects and subtract the radius of each if they are different, or the dia if they are both the same size hole.

Bill

Or measure ID of the screws/pins and OD of the screws/pins. Add them, and divide by 2. Then you also take in to account if there's a little backlash in the holes.

[ZiaBeam]

Using the inside jaws of your caliper measure the inside distance between your two holes
Use the outside jaws to measure the outside distance between the two holes.
Add the two together and divide by 2

On smaller holes your caliper jaw edge "flats" won't actually measure the true span between each hole's true arc insider or outside edges.
 Using Sean's method will average OUT these variables, negating each bias, resulting in a much better figure.

[Frank in Fairfield]

I bought a sliding caliper from Brownell's that takes the guess work out of it.
You measure outside of the group, look on the caliber for caliber on the printed list select your measurement and voila.
Group size.
Too easy.

[Bill L.]

Got this off the CharlieDaTuna web site.It's for a 22XX but should be the same.
http://www.charliedatuna.com/images%20co2/22xx%20breech.jpg

Tube--
http://www.charliedatuna.com/images%20co2/22xx%20tube.jpg

[Slavia]

I use Sean's method, unless I can't easily reach the holes with the caliper.  In that case I insert screws or pins and use the same method to measure the protrusions.

It sounds like RETATCDUDE is talking about measuring target holes for group size.  In that case I scan the group (with a ruler for reference) and import it into MS Publisher.  The next step is to insert a 1" box to see if it matches 1" on the ruler for scale.  Then I insert caliber-sized circles and center them on the target holes in the photo.  Publisher gives X and Y coordinates for the hole positions and point of aim, which go into a spreadsheet.  That calculates group center, group diameter (group size), mean radius (average distance from the group center), figure of merit (average of vertical and horizontal dispersion), and group center translation from the point of aim.

I know it sounds complicated, but once you get the spreadsheet set up it's pretty easy.  For more information about those methods of analysis see a book called "Hatcher's Notebook", available for free online. 

[shu2kill]

i have always just measured from the right edge of one hole to the right edge of the other one.


....... Ol                  Ol

from one l to the other l

or from top to top

is this less accurate than the other methods described??

[chet]

Seans method will work regargless if the holes are the same dia, or not, i  bore some holes in large cast iron frames for printing cylinders, and the only way to keep the hole locations sym on both l & r frames were to measure the inside of the 2 holes and the outside, add them up divide by 2, even if one hole was 7" in dia and the other is 17" in dia, you still get CTC dims which is close what i did. this stuff measuring the dia of one and the dia of the other, then take 1/2 of each dia and then measuring the space between the 2 will not be 100 % effective, especially if you are depending on the gaps between the cylinders when done to be within .003 within 60 inches.

   Chet

[rrdstarr]

Measure the ID of both holes, add them and divide by two.... Measure the distance between the INSIDE (closest) edges of the two holes, and add the sum of the half diameters.... This works for any size holes, even ones that are different diameters, and can be done easily with digital calipers....

Bob

X2!