Never estimate

[epanzella]

I just made a front sight for my smokeless muzzle loader. Found a nice little steel block that was bound to be high enough so I didn't measure. . 1/8 inch short. Had to use a TIG steel extender. Being gunshy now, I welded on an extra .100" that I had to mill off again. If I wrote a book the title would be "How to make any Job Harder".

 

[NortonDommi]

With welded stuff it is amazing how much something can move no matter how careful with the welding sequence. Bottle jacks and sledgehammers are useful tools.

 

[hman]

My Daddy told me the only way to feel confidently sure about measuring anything was to measure only once!
"Measure twice, and you are doomed! You will surely the have to measure a third time to explain why the second measure was different from the first".
Unless the third measure was close enough to one of the previous to confirm it, you could be into a fourth!

Famous Swiss saying ... "A man who wears two watches never knows what time it is."

 

[graham-xrf]

With welded stuff it is amazing how much something can move no matter how careful with the welding sequence. Bottle jacks and sledgehammers are useful tools.

When molten steel cools, it takes up it's new size with all the force the steel strength can deliver, basically as much as it would take to stretch it back to where it started. I have encountered a guy with exceptional skills in welding up stuff that would end up how he wanted. He would set up angles looking that bit "wrong", and watch it bend itself into shape as it cooled. The trick was only good at the beginning. As more bits get added, there comes a point when you have to force things!

 

[Bi11Hudson]

So basically make it with extras poking out everywhere, and hack off the excess when it's all together.
I get it as in the carpentry context, though maybe the same idea can apply to welded stuff, scaled down as seems sensible.

Somehow, I think this philosophy would not sit well with folk who spend lots on thick very flat level weld tables, and would clamp stuff down using expensive squares, and measure to 4/10 mm (about a sixty fourth inch).

The "quote" was intended to be facetious. One of the problems I face doing carpentry work is using metal working techniques. Working to 1/64th is usually reserved for fine cabinetry. When welding, I try to work to 1/8. The difference can be made up with rod. Doing framing work, it's a little looser. 1/4 inch for some things can be taken up. For finish work, 1/32 is usually a good fit, a "tube of caulk and a coat of paint" will make anybody a fine carpenter.

1/32 inch is (roughly) 30 thou, a 64th is half that at at 16 thou. Try building an engine to a tolerance of half that again, 8 thou, and you'd be thrown out of the shop. Not escorted out, literally thrown out. I work with mechanical models to 1/87 scale. Model trains. . . My tolerances are to a thou. If I was a real machinist, I would work to much less. My scale is 140 thou to the foot, roughly. An inch is 0.0115. When a wheel is only concentric within 0.001", it still wobbles so bad I can't use it

When I go from model trains to working on a 120 year old house, I try to carry my model work tolerances with me. The house isn't square, probably wasn't true square when it was built. Nor plumb. Every cut is custom for a house built in 1887. A stud might be 1/8 different over 16 inches. Trying to work with machinist tolerances on such a structure is an exercise in futility. It gets quite expensive when a board cut 1/64 too short is scrapped out. 1/64th too long can usually be beat into place.

There are different tolerances for different jobs. Framing work is not finish work is not cabinetry. The same applies to working with metal. I have been a maintenance man my entire life(>50 yrs). My machine work won't get you to the moon, but it will get the broken machine running. The expression by Big Jay is how things get done in a steel mill. A welder for American Bridge states that if he can spit across a gap, he can weld it closed. Again, there are different tolerances for different jobs. That's why the 310(ton) crane rides on equalized trucks where a quarter inch is on the good side.

.

 

[kb58]

I "self-jig" parts as much as possible. Once a project is partially complete, the existing structure is used to dictate where to cut and drill.

 

[Ianagos]

My Daddy told me the only way to feel confidently sure about measuring anything was to measure only once!
"Measure twice, and you are doomed! You will surely the have to measure a third time to explain why the second measure was different from the first".
Unless the third measure was close enough to one of the previous to confirm it, you could be into a fourth!

I like your way of thinking. I end up have to measure 3-4 times too often as the first too didn’t line up.

 

[epanzella]

I measure everything twice and how close they are to each other becomes my "report card". I've found thru the years they have gradually converged.