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Watch how you clamp stuff when scraping

Peter.

Titanium
Joined
Mar 28, 2007
Location
England UK
Today I was scraping in the sole of a straight edge. So I could get around it I had it loosely held in a vice on the end of my milling machine table - same as I usually do with parts that will fit there. I'm always careful not to over-clamp stuff when scraping because - you know - everythng bends. Just for kicks I clamped this straight edge up tight in the vice and took a print. Take a look at what happened!

Here it is just nipped in the vice. The straight edge rests on the two blocks and just held enough so it won't move. I rattle it in the jaws when closing them and stop just when it stops rattling.

clamp loose.jpg

Now look at the vice handle where I've pulled it up tight.Haven't turned the handle very far but look at the effect it's had on the straight edge:

clamp tight.jpg

Stuff is hard to hold for scraping sometimes, this is just a warning to think before you screw a part down hard or clamp it solid for scraping - you might be making life very difficult for yourself.
 
It is probably OK to hold it securely for scraping, as long as you do not clamp super-tight on lighter structures. But I at least, always remove it from the vise and spot it when it is free and clear of any clamps etc.

I usually whap it a time or three with a plastic hammer to dislodge particles as well, which may or may not also have an effect of stress relief, I have heard several opinions, and actually doubt the effect myself.
 
Here are a few that my students made in some classes. Tight but not to tight to
bend them. Wood devises C clamed to the side of a table. The one is threaded rod drilled hole through the side of the table and nuts on each side of the table side. Then 2 nuts and 2 flat washers on each side of the straight edge. I used to do a similar set up on the side of my 3 x 5 cast iron plate.
 

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See if I can find a few more: I love using those 2 screw woodworking clamps.
 

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You can do a lot with a woodworker's vise... the wood faces don't mark up the item, and they open far enough to hold a lot of things. Plus you can put wood blocks in to hold at an angle, etc.

Big ones are expensive, but can be found used for much less.
 
Peter, are you marking the straightedge while it's still clamped in the vise, or are you saying it's become permanently deformed from having been clamped up once? I'm not quite clear on the sequence.
 
Peter, are you marking the straightedge while it's still clamped in the vise, or are you saying it's become permanently deformed from having been clamped up once? I'm not quite clear on the sequence.

Not at all, I have the part completely free when spotting. This was just a demonstration because I don't recall the subject ever coming up here before so there might be people unaware that it could be a problem.
 
Not at all, I have the part completely free when spotting. This was just a demonstration because I don't recall the subject ever coming up here before so there might be people unaware that it could be a problem.


Thanks. That's a related subject to the "support on three points" discussions, that parts move a surprising amount in response to different situations. In other words, stay keenly aware of all the ways the mundane stuff you're doing might affect your results.
 
How tight are you clamping the straight edge in the second picture?

I am a bit surprised. I often hold parts in my bench vise and have not noticed this problem. I am conscious of clamping force, but sometimes it needs to be quite tight to keep the part from slipping. I do have aluminum jaw covers on my vise, which might help a little.

Now spotting while the part is clamped in the vise is another story. That never works.
 
I’ve had similar experiences when I first started scraping long narrow things like straight edges. Learned to clamp in a vise only for roughing then make a crude cradle to hold the part without clamping forces for finishing. For 40 ppi on 48” or longer, I also take care to support at Airey (Bessey? Can’t remember which) points for final scraping passes. Doesn’t seem to matter for 3’ or less.

L7
 
Pete is no rookie to scraping as he has taken 2 of my classes, one in Norway and he helped host a class in England. He knows what he's doing and I have personally seen him scrape and he is an excellent scraper and rebuilder. Pete is the fellow - how do I say this and be politically correct....lack of hair... 2014 class he attended in Norway

Richard King scraping class Norway August 2014 - YouTube
The UK class was held at a farm and the host had a nice machine shop and his pet pigs...lol

20171216_131228.jpg20171215_122152.jpg20171213_103224.jpg20171213_150112.jpg20171214_145811.jpg
 
In the same vein about excess clamping pressure warping parts, it drives me nuts to see some youtubers clamping unmachined rough casting, such as thin easily warped straightedge castings onto a mill table without bothering to shim below clamp points (or just file a datum reasonably flat). Then have to spend time surface grinding when a little care first would prevent an extra step.

A teacher years ago tried to instruct me to keep parts unstressed while machining. A lesson I sometimes need to re-learn....

L7
 
Like TGTool, my first thought was that the part was being marked while held in the vice. Peter clarified that it is “free” when spotting.

As demonstrated by bending gibs to straighten them, cast iron will move a surprising amount without permanently bending. Clamping in the vice on the arch- which looks fairly parallel- should not bend the cast iron beyond its elastic limits.

So--- what is going on???

My guess is that if Peter looks where the jaws of the vice contacted the straight edge he will find vice marks on the cast iron from flattening the normal irregularities of a cast surface. The jaws of the vice being harder than the casting impressed the casting protrusions into the casting. The vice “peened” the sides of the arch, inducing localized surface stresses, which warped the casting. If this is correct, then filing or scraping off those areas would remove those surface stresses and the straightedge would print as it did before.

If someone has a partially scraped straightedge and wants to test this idea, here is how to proceed.

1 - Start with the straightedge roughed in enough to be printing along almost all of the edges. Take a print or photo of the markings.

2 - Put the straightedge in a vice with hard jaws and devise a way so it can be put back in to the same placed.

3 - Tighten vice with a torque wrench while periodically removing and taking a print to see if it has warped.

4 - When it has warped, look for vice marks showing flattening, then file or scrape those away while taking prints to see if
they return to the “pre-warp” pattern.

5 - Repeat the above- but with aluminum jaws in the vice. My guess is that you will need to go well beyond the torque needed with the hard jaws.
 
Or to carry that experiment one step further. Stick a couple small bearing balls on one side of the straightedge where it's clamped in the vise and indent or peen the cast iron. The diameter of the ball impression versus the change in print pattern might show something useful. (Thank you Brinell)
 
I just pulled on the handle a bit :D

My original 'un-stressed' print was unaffected. I didn't think it would cause a permanent set and it didn't. You're right, the arch is very uniform, but it's still just 'as cast'.

I have had straight edges that had moved slightly over time, one I bought used must be 50 yrs old had a definite half-thou or more of twist, plus it was hard as hell to scrape. This one sprang right back when the vice was released.
 
I am now unsure of what you did.

My impression was that you were lightly holding the straightedge in the vice to scrape it after blueing it on a surface plate. (the top picture) You then tried pulling the vice tight, and the next time you blued it on the surface plate the print changed, you then put the straightedge back in the vice, and took the bottom picture.

Your comment "This one sprang back when the vice was released" Leads me to think that you were gently holding the straightedge in the vice to scrape- and bring a blued "master" to the straightedge, bluing it while held in the vice (top picture) and found tightening the vice changed the way the straightedge marked up (bottom picture).
 
Another important this one should do while scraping an old, new or dropped straight edge is to "ring" it by hanging it from aa rope and hitting it with a soft blow hammer, leather hammer or a chink of wood. I have been doing this for years as I was taught to do it from my Dad. It will change some and won't others. I have seen it change on my King-way, Brown and Sharpe, Busch, Foster, and Martin. I do it on lathe saddles that crashed. My friend Professor Alex Slocum atMIT calls it vibration stress relieving. Many of my students have seen them move too. You print the SE, see where it pivots or hinges, mark that spot with a magic marker, take a picture of the bluing, lift the SE up and hang it vertically and hit it. The metal vibrates and changes from a millionth to a few thousands. Blue it again and 99% of the time the hinge pivot point changes, then you compare the photo of the bluing before and after. You do this a 1/2 dozen times until the SE stops moving.

I misplaced my glasses today, so I can only find the one picture for now.

Checkout this You Tube minute 18
Scraping class 2017 in Denmark - YouTube
 

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