8” level scraping advice

I have my first scraping project going.

It’s a 8” level that I purchased the casting and level from Denis Foster.

The casting was very nice and free of voids, and Denis’ machining was top notch. This is the first of three, I bought the 18” and 36” from him also.

All 3 are very well machined, the man does nice work.

Here are some pics. The close up and the 1” square are from the same end.

The part hinges at 1/3 from both ends.

I’m using a Sandvick scraper with a 60mm radius insert that’s finished on a 1200 grit Accufinish wheel, and then lapped with 1 micron diamond spray on a ceramic wheel.
I tried a 20mm radius after the rough scraping, but went back to the 60.

I could use some advice on how to proceed from here, I’d like to get to 40ppi.

Thoughts?

Thanks for any help






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I’d do one pass, just the middle third, and re hinge and print.

L7

(Nice work so far!)

Thanks!, should I just hit the blue, or just scrape the whole section?


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I think you are pretty close to flat and maybe ten ppi, maybe a little more. The highest points are in your middle third and show up as halos. I’d make sure to scrape all the halos and the bigger blue marks in middle third only at this stage. Guessing you’ll get halos over the rest of base with one or at most a few more passes. Only then would I start to go for more ppi.

L7

If you think you will ever scrape the upper surface, so it now.

I would scrape it even if I did not plan on using it. Why? (other than overcautious)

Machining a surface imparts a stress to it. Scraping removes that stressed surface layer. As that stressed surface "relaxes" over time, the shape will change. Scraping the surface removes the stressed surface, leading to the shape change occurring now, when they will be corrected by further scraping. Its the same idea as doing all the rough machining on a part before the finishing passes.

All this is very unlikely to be a factor with something that short for the height, which is why it would be "overcautious" to do it.

A better reason to scrape it is that it will be ready when you do need it. It will also look "done".

One more aspect of Lucky's good advice to scrape the middle, is that there's a potential for misreading the markup if the surface is slightly convex. That is, one can inadvertently rock it slightly from end to end making the markup look like a good flat surface when it's not in fact straight. If the center is slightly concave you'll see a true contact showing more spots on the end than the center.

So scraping the center, getting it to show slightly fewer spots than then ends and then evening everything up is a good strategy for avoiding one potential problem.

One other bit of random advice -- paint it now, before finishing the scraping, rather than after. Scraping should always be the final step.

Lots of good advice, I appreciate it.

The casting hinges properly and I put a .0001 indicator on the top ends and pushed down and there is no movement, so I’m fairly confident that the bottom is fairly flat


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