Indicating a Horizontal Hole with Mag Base and Indicator

I had a You Tuber email me today thanking me for getting the ball rolling on this. I had a small part in it. I have to give credit to my old friend Forrest Addy for showing me this 40 years ago when I was teaching scraping classes at Puget Sound Naval Shipyard where he was the machine shop apprentice instructor. It is worth sharing to all new and old to our trade.

Machineshop Physics: Indicator sag - Durchhang von Messstativen - YouTube

A couple of related eye-openers:

1. Set a couple of 1-2-3 blocks a foot apart, on a rigid base. Bridge them with a 1 inch x 2 inch x 12 inch parallel, and set up a dial gage in the middle of the parallel's top surface.

Use thumb pressure to deflect the parallel, and note how little force it takes to induce significant deflection.

Repeat the exercise after rotating the parallel 90 degrees around its long axis.

2. Set up a millwright level (such as a Starrett Model 98) on a stable surface,. so that the bubble is approximately centered in the vial. Lightly hold your finger on the vial, at one end of the opening in the vial carrier for a few moments, and watch the bubble run.

Or, if you would rather use a Master Precision Level (think Starrett model 199z), except instead of touching the vial, shine a flashlight on one end of the vial.

John I di the flashlight trick too a Old one with a bulb, the new LED's don't heat up so much. Last week I was helping scrape a 48" Brown and Sharp camelback and it hinged or pivoted in the middle. We estimated it was about .0002" high in the middle. I wrapped my bare hands around the top center and held them for 1minute and that hand heat changed the hinge out 12".

I also use the test you told us on straight-edges and see how much the bend. It is freaky how some bend. One has do it several times just to make sure there is no crud between the plate and blocks. (wipe with your hands first to feel the dirt)

Straight edges still fall under the sliding or running fit allowances, Machinerys handbook has the fit allowances, the way oil fills the gaps and the machine moves accurately under the " Systematic distribution of error rule. "
See Moore's book.