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Ways to measure holes center to center?

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Uglydog

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#1
In a separate thread Brockwood inquired how I intended to measure large holes center to center on a VN overarm support.
Well, I'm finally getting around to answering how I might do it.
But, more importantly while I'm presenting some options.
I suspect that there are many more ways to accurately measure.
I'm very interested in how many different ways we might get the job done.

Please submit your solution to the problem.

Daryl
MN
 

Uglydog

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#3
IMG_1523.JPG IMG_1525.JPG

This Sorenson Center Mike is my go-to center to center mic. (Yes, they call it a "Mike").
It somehow calculates the actual centers of the holes.
Note the directions.

Daryl
MN
 
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Uglydog

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#5
IMG_1527.JPG

This device uses a dial tail indicator.
It's great for accurately comparing holes.
I've found it awkward for this application as lining up the gage blocks is a clumsy process for me .
Likely a better way to do this.

Daryl
MN
 

Bob Korves

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#7
Daryl, I would first measure the overarms, outside to outside dimension, then inside to inside. Then do the math to determine the center distance. Then make a gauge the exact length you need and use it as a spacer gage for the stop dogs on the mill table, or just measure the distances between the stop dogs as you set them. Set up the first hole while against the first stop, then bore away! Repeat at the other stop.

Edit: for best results, use your jig borer... ;)
 

Tony Wells

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#8
For lack of a Cordax (CMM), If my mill had a DRO I would sweep the holes and get the c-c dimension through a little trig if I didn't want to bother lining up one axis. That's one advantage to a CMM. It will establish an artificial axis independent of alignment with the machine axis and give a direct reading. One assumption is either required, or averaged into the calculations by the CMM, and that is the roundness of the holes. Holes, similar to planes, can be defined by any 3 points in a plane, and for every 3 points, there is only one circle that includes these three points. If the hole is not round, wherever the 3 points are taken is where the center of that circle is derived. Any points beyond the three simply make the infinitely thin theoretical line defining that circle "thicker" so as to include those additional points. Generally, the more points taken in an out of round hole, the better representation of the actual base circle will be developed and the center of that sort-of-round shape can be determined. Obvious defects such as gouges and intentional sawed expansion slots should be avoided when taking the series of points measurements.
 

Rustrp

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#10
Just a bit of quick input based on my sheetmetal layout experience. The first thing I see is a part with three holes, so my first thought or question is center to center or centers. Do the holes have the same axis? For center to center (if the holes are the same dia. and accurate) I measure outside to inside. If the holes are different diameters then I do the same and add a little math. Of course when the holes do not have the same axis there's just more math because we have center to center and centers to measure. When the part we are measuring is worn the challenge is determining what the hole diameters should be and then their location and if possible we would measure the shaft diameters and centers before the part is removed for repair. As Tony stated the DRO goes a long way in making this easier (not simple) to determine.
 

Bob Korves

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#11
My understanding is that you were just looking to bore the two overarm holes. If there are three, that complicates things a bit, but the approach is the same.
 

Reeltor

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#12
Perhaps I don't fully understand the issue. Problem: making overarm support needing holes to be correctly located respective to each other. How to measure the distance between the center of each hole to match the mill in question.

quick and dirty: the round overarm has a center drilled into it from the factory. Place cutter head in horizontal position, using a small collet or something similar chuck up a small pointy center in the cutter head. Slide the over arm to be on the same plane as the end of the pointy thing in the cutter head. Measure between the center of the overarm and the center of the cutter head. Will this provide the level of accuracy needed to layout the center of each hole and then drill then bore the large hole on the overarm support for the overarm side? Mount the overarm support on the overarm and then drill/bore the arbor/cutter head side in place.

I know that I am missing something, please enlighten me 'cause I'm confused, not an unusual situation
 

Uglydog

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#13
My intent on this entire thread is who to measure the center of holes. This might be a daunting task for some of us. It wasn't that long ago when I would have been overwhelmed by the problem.
I did not intend this thread to be a conversation about VN overarms. However, if we find a VN which this might fit. Then GREAT!!

The third hole is the broken site glass for the integral overarm oil reservoir.

Tony Wells, That an interesting idea!
I've been refusing to "upgrade" to DRO. I'm also using a cast surface plate. etc... This is my eccentricity. I don't necessarily recommend it...
I'm guessing I could do the same thing with handwheels. Would have to watch for backlash.

Reeltor,
Perhaps I communicated poorly. As I understand the process you are exactly correct.
This overarm is only valuable as a model on which to base a pattern prior to pouring an aluminum or cast iron copy.
It is my thought that we can bore the support arm and the arbor needs to be marked and bored on the machine for which it will be used.

Again, if we find accidentally find someone who might have an overarm with a center-center distance of 9.150 then a happy dance might result.

Brockwood,
I got 9.150 with the Sorenson. I only took one measurement.
I want to do multiple measurements on both sides of both the A & B and watch for repeatability.
Then it's my intent to build a pattern of these A and B overarms.

Daryl
MN
 
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DaveD

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#14
I might be inclined to machine plugs out of some scrap material and leave a 1" diameter stub on the plugs. Put plugs in bores, measure outside to outside of the stubs and subtract 1".
Result is your center to center distance.
 

Bob Korves

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#15
For making castings, just leave enough room for the overarm and arbor rough holes in your pattern, and the usual over sizing. Don't forget the draft, on both holes and outside dimensions. Then machine the casting to fit. There are plenty of foundries around for doing good iron castings. If you mistakenly cut the holes oversize or in the wrong places, well, that is why bushings were invented!
 

Technical Ted

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#16
This uses a vernier caliper and some telescoping gages.
Gotta do a little math. Not difficult.
The vernier tips do not have a not radius therefore I'm not sure that it is as accurate as I'd like.

Daryl
MN

View attachment 235572
If you don't like the square external measuring edges on the vernier, use the ID side and measure the distance between the far sides of the holes (I believe those sides have a radius on them anyways). Using this technique, is most likely the method I would use to determine the center to center distance.

Ted
 

BGHansen

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#17
View attachment 235569 View attachment 235570

This Sorenson Center Mike is my go-to center to center mic. (Yes, they call it a "Mike").
It somehow calculates the actual centers of the holes.
Note the directions.

Daryl
MN
Daryl,

Thanks for the tip. Just bought a 13" Sorenson off eBay for $50. Will be much nicer than using those slotted adapters that screw to vernier caliper arms.

Bruce
 

Uglydog

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#19
I might be inclined to machine plugs out of some scrap material and leave a 1" diameter stub on the plugs. Put plugs in bores, measure outside to outside of the stubs and subtract 1".
Result is your center to center distance.
Or any size dowel pin or drill rod etc you might have.
However, fitting the plugs might be problematic as it could introduce error (free running, slip fit, press fit).
Options are great!!

Daryl
MN
 

BGHansen

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#20
BGHansen,
I've been facinated that it does the calculations without electronics!!

Daryl
MN
Daryl,

There is a "quaintness" to old school. Though I did just break out the surface plate and a digital height gauge to scribe some lines on the horizontal center. Have to admit to finding the center of a 1.375" round mounted in a V-block was easier "going digital"; zero out on the top of the round, then come down 0.6875". Looking forward to trying out the Sorenson.

Bruce
 

Silverbullet

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#22
Ball trammels , used with a good steel rule, that's how I learned eons ago. The ball centers the hole on one end the other use inside or outside leg. Once you know hole size , easy peasy. It works easy once you do it.
 

Uglydog

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#24
Ball trammels , used with a good steel rule, that's how I learned eons ago. The ball centers the hole on one end the other use inside or outside leg. Once you know hole size , easy peasy. It works easy once you do it.
Like many things, I'd not heard of ball trammels.
http://www.starrett.com/metrology/product-detail/59F
While I've only done a very preliminary search, and may yet find an answer.
Could someone please explain how a ball trammel is used?
Please.
Thank you,
Daryl
MN
 

RJSakowski

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#25
How to do it depends greatly on the final intent and required accuracy. For center to center distances, I usually measure inside to inside distance or outside to outside distance. For same diameter holes, I will zero out the caliper on the hole diameter and then measure the distance. Zeroing out will compensate for any errors due to the lack of a sharp edge, important on small holes. Measuring outside to outside, I use tight fitting pins and again zero the caliper on the pin diameter and then read the center to center distance directly. The two formulae used to calculate the c/c distance are: inside; c/c = i.d + (d1/2 + d2/2) and outside; c/c = 0.d. - (d1/2 + d2/2).

In the case of your part and your final intent, I would approach the measurement differently. Given the repairs and assumed prior use, I would expect egg-shaped holes, in which case, the above procedure gets flushed down the toilet. Having a microscope on my mill, along with a DRO, I would record coordinates at a number of points around each hole and plot them out in CAD. Then I would do a best-fit circle for each of the holes and measure the center/center distance in CAD. Kind of the poor man's version of the CMM in post # 21. If the mating pins/shafts are available, measuring diameters at an unworn area would be helpful.

Reverse engineering a part is always problematic. Even on pristine parts, you have no idea as to where in the tolerance band the part lies. A good example of this was fitting a back plate to a Chinese chuck. In theory, the three mounting holes are equally spaced. However, they in fact were not. They were probably close enough so the hole clearance would allow assembly and may have been within their tolerance but had I made the new back plate to fit the measurements, I would most likely only be able to install the back plate in one position. Add to that measurement errors and you can end up with parts the just don't mate.
 

Silverbullet

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#26
Like many things, I'd not heard of ball trammels.
http://www.starrett.com/metrology/product-detail/59F
While I've only done a very preliminary search, and may yet find an answer.
Could someone please explain how a ball trammel is used?
Please.
Thank you,
Daryl
MN
The trammels attach to the rule line the trammel to say the inch line , second trammel will attach close to the second hole . One will have a ball on the attached rod ,set it in hole one the other end will have an inside or outside leg to feel the other hole either side once you have that measurement add or subtract half the diameter of the hole size . That will give the distance between the holes . It can be very precise . Old timer taught me that back in 75
 

BGHansen

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#27
Daryl,

Got my 13" Sorenson in the mail yesterday. Works great though I need a magnifying glass to read the vernier! Thanks for the tip!

Bruce
 

Billh50

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#28
Make a plug that just fits each hole. Then mike the outside of the plugs and subtract 1/2 of each plug. The tighter each plug is the closer the reading.
 

Uglydog

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#29
Got my 13" Sorenson in the mail yesterday. Works great though I need a magnifying glass to read the vernier! ...
While the vernier doesn't read to .0001 the standard does.
I believe this might be helpful in reading light or heavy on the .001 vernier lines.
Do you find this to be an accurate statement?

Regardless, a fun inspection tool.

Daryl
MN
 

BGHansen

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#30
While the vernier doesn't read to .0001 the standard does.
I believe this might be helpful in reading light or heavy on the .001 vernier lines.
Do you find this to be an accurate statement?

Regardless, a fun inspection tool.

Daryl
MN
Hi Daryl,

I do find your statement to be accurate. In the grand scheme of things, taking an existing part and reverse engineering it to better than 0.001" is creeping into "the emperor's new clothes" area. I would think the designs on old iron were done in nice and easy 0.250" or 0.500", etc. dimensions. If my Sorenson told me 6.001" on the spacing of two threaded holes, I'd assume 6.000". It is a great measuring tool, thanks again for the addition to my layout arsenal!

Bruce
 
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