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Scraping L shaped straight edge - balance issues

learning80

Plastic
Joined
May 10, 2018
Hi,

I'm new to scraping with only about 30-40 self taught hours under my belt and have a straight edge with an L cross section to scrape in.
The issue I'm having is it spots heavily on the left side of the "L" because that section has more weight over it.
Is there a solution to this sort of thing - to counterweight it?

Also, I've noticed when scraping something flat I introduced a bow. Is this sometimes done by learners? If so, how do you avoid it?
I'm especially curious how you avoid it on things you can't hinge - e.g., ways on machines. I guess you could hinge a straight edge or parallel on the way but what if it was hard to get to or on an angle - a V way?
I believe I introduced a bow tonight by applying some downward pressure while bluing unintentionally, causing a rock. I would then scrape the whole part, including the low area on the end which received blue and gave a false reading. When I was careful about bluing and avoiding downward pressure it spotted up correctly and the low point on the side revealed itself to be low by the blue (it had very little).

Thanks.
 
Hi,

I'm new to scraping with only about 30-40 self taught hours under my belt and have a straight edge with an L cross section to scrape in.
The issue I'm having is it spots heavily on the left side of the "L" because that section has more weight over it.
Is there a solution to this sort of thing - to counterweight it?

Also, I've noticed when scraping something flat I introduced a bow. Is this sometimes done by learners? If so, how do you avoid it?
I'm especially curious how you avoid it on things you can't hinge - e.g., ways on machines. I guess you could hinge a straight edge or parallel on the way but what if it was hard to get to or on an angle - a V way?
I believe I introduced a bow tonight by applying some downward pressure while bluing unintentionally, causing a rock. I would then scrape the whole part, including the low area on the end which received blue and gave a false reading. When I was careful about bluing and avoiding downward pressure it spotted up correctly and the low point on the side revealed itself to be low by the blue (it had very little).

Thanks.
There's a downunder forum .

METALWORK GENERAL

Ask for Grahame Collins. He's the resident expert.
 
Hi,

I'm new to scraping with only about 30-40 self-taught hours under my belt and have a straight edge with an L cross section to scrape in.
The issue I'm having is it spots heavily on the left side of the "L" because that section has more weight over it.
Is there a solution to this sort of thing - to counterweight it?

Also, I've noticed when scraping something flat I introduced a bow. Is this sometimes done by learners? If so, how do you avoid it?
I'm especially curious how you avoid it on things you can't hinge - e.g., ways on machines. I guess you could hinge a straight edge or parallel on the way but what if it was hard to get to or on an angle - a V way?
I believe I introduced a bow tonight by applying some downward pressure while bluing unintentionally, causing a rock. I would then scrape the whole part, including the low area on the end which received blue and gave a false reading. When I was careful about bluing and avoiding downward pressure it spotted up correctly and the low point on the side revealed itself to be low by the blue (it had very little).

Thanks.

You would run into the same problem with scraping an angle plate. One trick I tried is to start hollow along the unbalanced axis, then creep into a flat. using a counterweight might help but you cannot always place it on the part. Theoretically, if the part is truly flat it should spot even if it is unbalanced. the problem is getting it flat without being balanced. I also used small surface plates or SE to flatten unbalanced objects. the trick is to take the part out work-holding so you do not distort it while spotting. Balancing an L shaped long object on the point of the L is a pain, need a loose fitting fixture. Once you have it stable just use an SE and spot it like you would spot a lathe bed or machine way.

The only way I know how to avoid scraping a bow into a part requires that you can hinge it. and apply pressure only at the airy points. Again starting concave, is better than starting convex, unfortunately, every machine with mating surfaces will have one surface worn convex and one concave. If you machine the part before scraping it make sure you err on the concave side. In reality, it is a hard thing to do, because worn machines like to make things fat in the middle. Richard told us when scraping dovetails to use a dovetail straight edge that fits the best, it is long enough and wide enough to cover the dovetail, but not too large to hinge it in the dovetail. Develop a feel for the "hinge" in the dovetail. You cannot really see where it is hinging, you need to feel if it is rocking on a bump, or scraping on the ends. I hope this bit helps.

Since I have been playing with this, I can tell you that a class is invaluable to correct all the things that you learned to do wrong :). You may not even know that you are doing something wrong but a guy who dreamt. ate, drunk, shat this stuff for 40+ years like Richard did, can take one glance and see it a mile away.

dee
;-D
 
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