Ways to measure holes center to center?

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.
 
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
 
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.
 
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
 
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
 
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.
 
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
 
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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