An exercise in frustration, being a hobby machinist

[akjeff]

I know I’m supposed to make mistakes, I do, all the time.
I’m making some clamps like the Kant-Twist variety.
I am very careful, this is a simple part but it needs to be accurate to +0 -.002”.
I need two pivots, the first one is spot on, OK, piece of cake. I measure one final time, dial in .004” and I turn .010”.
One thing about this project, I’ve never had to turn to a shoulder with this accuracy.
I have to walk away for the rest of the day…….. maybe.
I need to remind myself, it’s OK, you don’t know what you are doing and look what you made. = scrap?

Here's a good video on hitting your target diameter. Like most all of Stephan's videos, there's some excellent info to be had. I especially like his explanation on why we shouldn't "sneak up" on a final dimension. Better to divide your cuts so they consistently load your machine the same way, each cut. Made sense to me after he explained it.

 

[BGHansen]

One of the machinist at work was known for the saying, "The sign of a good machinist is how well he can hide his F-ups". Could you use 0.002" steel shim stock punched like a washer to make up the difference if the shoulder was cut too deeply?

Bruce

 

[Dabbler]

@BGHansen the first line of the saying is "Every machinist makes mistakes. The sign of a good..."

 

[Weldingrod1]

In my professional life, if the machine shop -ever- comes back with a problem with tolerance, I always prod the responsible engineer to look at what's ACTUALLY required for tolerance. And then i ask pointed questions ;-)
It's too easy to use the defaults and not your brain!

Sent from my SM-G892A using Tapatalk

 

[Tipton1965]

"The sign of a good machinist is how well he can hide his F-ups".

Years ago when I worked as a machinist I was talking to an older co-worker. He said he once worked at a shop that had a pond out back. He said anytime someone scrapped a part they would just throw it in the pond to hide the evidence. He said they had a really dry hot summer one year and the pond dried up. Years worth of scrapped parts were sitting at the bottom. He said there was even large scrapped castings in there.

 

[westerner]

why we shouldn't "sneak up" on a final dimension

I recognize that you are working on OD stuff. Still, Mikey's advice on boring applies here. If you are attempting a cut at less depth than the nose radius of your tool, your results will not be consistent. I have seen it enough times in my hobby shop to make me a believer.

Rest assured, my scrap/repurpose pile is not small. The lessons learned are absolutely delicious, and my mistakes far outnumber the numbers actually hit.

 

[akjeff]

Sorry for the foulup guys! I edited my post to include the link to the video. CRS strikes again!

 

[Doug Gray]

The lessons learned are absolutely delicious

Love this sentiment and couldn't agree more!

 

[westerner]

Sorry for the foulup guys

No worries. I only quoted your post to illustrate the concept of nose radius/repeatable results. Gazillions of other factors are at play.
Me too.....