Let's talk measurement devices

I have an upcoming project where I will need to get semi accurate measurements (hand fabrication, not machining, although I may go to 3D printing for repeatability if I find it as useful as I think.)  Problem is that these measurements are too deep inside a hole for me to get accurate readings with my Mitutoyos.  

I could get some of those bolt-on caliper extension sets, but I was also looking at a set of inside/outside measurement transfer calipers, or even a bore gauge set.

Anyone have suggestions? Tolerance i am looking for is probably going to be in the range of 1/32nd to 1/16th, but I'm open to future proofing. 

I use a set of bore gauges for deep measurements. You can pick up a Chinese set for reasonable money. 

How deep is the hole? 

Are you tying to measure depth or are you trying to get a diameter behind a step? Can you add blocks or pins to the area of interest and then measure to the block then add the size of the block to the measurement?

914Driver said:

How deep is the hole? 

4-5 inches, roughly 3" diameter inside 

New York Nick said:

Are you tying to measure depth or are you trying to get a diameter behind a step? Can you add blocks or pins to the area of interest and then measure to the block then add the size of the block to the measurement?

Diameter.  There's not a step, but its flared semi irregularly.  Its a plastic piece so it has two separate draft angles on it to allow it to come out of the mold.

Use two popsicle sticks and hold them together touching the diameter you want. Mark the overlap location. Remove, align the overlap and measure with calipers/tape measure/etc.

Usually draft angles are consistent, can you measure as deep as you can and as shallow as you can and then figure the angle to extrapolate the diameter at the max depth?

In reply to New York Nick :

There are two angles on this, though.  Both act as draft angles, but one appears more cosmetic.  The point I can reach at the second one is just at the transition from first to second and doesn't give me a good reading for figuring the angles.

Laser measurer? Mine goes to 1/64.

From out of left field: Play Doh? Fill up one side of the hole, let dry. This gives you depth and profile.

I'll see myself out.

I have a set of telescope gages with 6" long handles.  Because they're not a precision instrument, buy cheapie ones.  With some practice you can get measurements W/I .0003   If you have to get closer than .0002 see if NAPA et al wil rent a dial bore gage or inside mic.

Cut the part in half and take your measurements if that is an option.

Since mold w/ draft, apply resin release agent, paint on fiberglass resin, several coats if necessary for rigidity, allow to cure and remove. Shrinkage should be minimal. Measure part outside the mold. 

Got pics?