PDA

View Full Version : Scraping conundrum, now contemplating suicide..



.RC.
02-23-2011, 04:02 AM
At my wits end trying to scrape in a Craig Donges 18" straight edge...

I have got it down flat to the point that it will float across a surface plate..

I spot it with a thin coat of blueing on the surface plate.. It shows it is high on the ends which confirms with the hinge test..

I do very light scrapes, so light next to no swarf is produced...

First two cycles very little happens... Next thing I know with the third cycle it is high in the middle...

OK I leave it a while and respot, still high in the middle.... I scrape that and next spotting cycle it is high on the ends...

I am at my wits end trying to scrape this in... I have a good 30 spots per inch marking where it does touch but there seems to be no pattern as to what will touch after a scraping cycle... The amount I take off per cycle barely takes off the blue...

What am I doing wrong? It is so frustrating, I do some cycles get no-where, try again, get no-where, then walk away and do something else for awhile before coming back to it...

PeteF
02-23-2011, 04:21 AM
I'm just a beginner at scraping as you know, but have you ruled out heat input from somewhere, that's certainly what it sounds like to me anyway.

Pete

Forrest Addy
02-23-2011, 04:22 AM
Heat from your hands warms the camelback and bows the face concave. First time or two you handle it nothing happens. As the back gains heat and expands, you have trouble. Or you if you hand rub the face clean, it bows the other way. Handle the work uing oven mitts or a couple of chop towels to pad your hands. Keep the swarf off them.

Also, cleanliness. Inevitably a crumb of scraping swarf gets in the blue on the plate. If you have inexplicable trouble, clean the plate and re-blue it. Scraping cleanliness is easier if you use a shop vac to pick up the swarf.

How are you handling the work (your straight edge)? Is it in a vice and you bring the plate to it or do you have the plate nearby and bring the work to the plate? Second way is usually better because anything held in a vise is easy to deflect slightly with the jaws no matter how careful you are.

Been there, done that, got the T shirt, read the book, saw the movie, plagerized the story, paid taxes and insurance on it, took credit for it, denied its existance, exposed it as treason, sold it to organic farmers, etc.

Peter.
02-23-2011, 04:41 AM
I'm perplexed. If handling it so little can cause it to bow either way, how is the tool any use when pressed into service? Do you have to handle it with oven mitts? I gather that this is a heavy item, and would naturally be lifted close to one's body? Wouldn't body heat affect it also in use?

.RC.
02-23-2011, 04:47 AM
Heat from your hands warm the camelback and bows the face concave. First time or two you handle it nothing happens, then the back gains heat and expands and you have trouble. Or you hand rub the face clean and it bows the other way.



I have wrapped it up with rag and insulating tape to minimise heat input... Yes I do wipe it with my hands once per cycle to detect any material on the face....

Surprised that this minimal touching would make it warp so much...

dian
02-23-2011, 05:27 AM
just curious. what grade of surface plate do you use?

.RC.
02-23-2011, 06:04 AM
It is a workshop grade B, but I have a larger 00 awaiting delivery...

Greg Q
02-23-2011, 06:22 AM
RC, are you trying to just split the spots with a very small radius tool? Are you using the Biax at this stage or are you hand scraping?. You need a ski store up there in the tropics so you can get serious insulated gloves:p

.RC.
02-23-2011, 06:34 AM
Not trying to split the spots yet... Just want to get an even covering of spots in one spotting... As connelly says, there is no point getting a 40 ppi bearing in one area while you have a 2ppi in another... If the markings are not turning out correct, get aggressive and get the material off..

I was using the Biax, but went to hand scraping, but have gone back to the Biax with only a 2mm stroke as at this stage the difference between convex to flat to concave is not a lot...

If I put slightly too heavy a layer of Canode all I get is blue from one end to the other and I nearly need a 2 tonne crane to pull the straight edge off the surface plate.

BTW I just heat treated my 36"..... It has now got what looks like a 2mm convex bow in it... It is in the freezer now...

As for the gloves.... Yes I think I am going to need a set http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=110649528579&ssPageName=STRK:MEWAX:IT .. Plus re-evaluate where I am currently working

Greg Q
02-23-2011, 07:21 AM
Hmmmm. i wonder about your ink thickness. I have chased my tail this way until I went to a barely discernible thin blue mark-up. Are your spots still too coarse to switch to a metho haze marking?

The other possibility is your stoning off between spottings. I had to learn, then re-learn the softly-softly touch with the stone when de-burring. I bought a very hard Arkansas for fine stoning that seemed to help*

*seemed to help=I hate being in the final stages of a scrape. You work so hard to get there, and no sign lights up to announce when you've met your goal.

dian
02-23-2011, 08:47 AM
so the plate will be flat to about 0.0002" over a 18" distance. i never really scraped, but i reckon very light scraping as you describe is taking off less than that, maybe even under 0.0001". are you trying to scrape to more precision than you plate offers?

J Tiers
02-23-2011, 09:39 AM
I can demonstrate heat input with a straightedge I scraped some time ago..... made from an old Stanley CI level. There is not a lot of mass in the level body, so it responds quicker, both ways (heating and cooling).

If I pick it up (insulated) and spot it against a surface I scraped to my granite flat, it spots evenly.

if I pick it up by the center of the top, hold it 10 sec, and spot it against the same surface, I get contact at the ends only. The top expands and humps up the middle of it.

That REALLY threw me off a while back, had me wondering if the flat had a big hump in it, etc. But I tried two pieces that I had scraped to the same granite flat, and found that 'cold" they spotted each otehr, so I knew then that there was no big issue with the plate.

As for tiny errors of 0.0002, scraping and spotting is precise enough to show that. But normally, unless you have moved on to the "haze" spotting level, you are probably looking at a thicker medium than that, and you will probably see that level of variation only in the "density" of each actual spot..... you may see blue, around a more brownish area that indicates the actual contact spot.

Unstable markings in your case might be heat from hands, or from teh scraping process. They might be due to not realizing how much you are taking off, or to differences in the way you perform the spotting, differences in how you place the S.E. for spotting, etc.

It can also be that you really ARE getting "there", with some confusion due to heating, possibly not a consistent heating, which may have tossed your "system" into instability. Then you are just 'rocking back and forth" between high "here" and high "there".

Maybe you should try moving onwards to the spot-splitting for a couple cycles. Instead of actually scraping "off" the spots, split them. Make sure you split the CONTACT areas, the brown spots in the middle.

That is effectively removing even less than light scraping. Try it.

Peter.
02-23-2011, 09:46 AM
I wonder why if reducing heat input is so critical they don't fit cast iron staight edges with non-heat-conducting handles? Or do they?

Greg Q
02-23-2011, 09:52 AM
I wonder why if reducing heat input is so critical they don't fit cast iron staight edges with non-heat-conducting handles? Or do they?

Yes, they do. The 36" castings that we own have a handle boss included. The traditional handle is made from local hardwood. The 18" that RC is working on does not have that feature though, so body heat is more of a factor.

My own spotting master handles are made from space shuttle tiles, but I understand they are no longer available ;)

Peter.
02-23-2011, 09:58 AM
Surely there must be some 'slightly used' ones floating around :D

Greg Q
02-23-2011, 10:10 AM
Surely there must be some 'slightly used' ones floating around :D

You'd think. In fact I think I have the 1/2 of the only spotting masters to appear on OzBay in the last few years. It is a 24" B & S. The other one I saw was a ten foot Starrett, no bidders.

GQ

Rosco-P
02-23-2011, 10:18 AM
Seems that the next logical next step would be milling two flats on the back and installing two wooden or plastic handles.

duckman
02-23-2011, 10:34 AM
You also need to get rid of the stone when removing burrs, make a dead file take a well use file and stone the teeth, you don't want to file the burrs just rub them off. Stoning will spread the spots and make flat spots and also give you false bearing. My B&S 36" camel back has wooden blocks screwed to 2 points on the back for handling.

Black_Moons
02-23-2011, 12:57 PM
Heat input can be from other sources too. Iv heard of precision levels drifting from heat just because a bright halogen light was shining on them.

Id also wonder how much heat is produced from scraping itself.

Maybe next time you spot it, Leave it for an hour or two with any nearby bright work lights turned off, with the block supported by foam insulation first?

gwilson
02-23-2011, 01:31 PM
My B&S straight edges from 18" to 36" all have factory wooden handles.

Mcgyver
02-23-2011, 02:10 PM
just curious. what grade of surface plate do you use?

that's where my thoughts went is well....its not difficult to scrape beyond the surface plate's accuracy, especially if tooroom grade and its label is more accurate that the granite - worn out or lying manufactures. I noticed this as I used to use a cheapo composite plate then one day started using a Starrett; there was quite a different blueing pattern left on the work between the two!

That and as Forrest mentions the heat factor. if there are not wood handles, handle with a cloth, minimize handling, don't leave in direct sunlight etc. You don't have to baby it too much, but remove the worst/obvious offenders

gzig5
02-23-2011, 05:48 PM
Heat input can be from other sources too. Iv heard of precision levels drifting from heat just because a bright halogen light was shining on them.

Id also wonder how much heat is produced from scraping itself.

Maybe next time you spot it, Leave it for an hour or two with any nearby bright work lights turned off, with the block supported by foam insulation first?

Geez, what kind of hoops do we have to jump through to use the darn things once they are scraped in? I've got an 18" waiting for finish that came from the run on PM a couple years ago, but it sounds like I need to finish it and use in in a temp controlled clean room.

RC... What was your heat treat process and what angle did you put on it?

2ManyHobbies
02-23-2011, 06:42 PM
It isn't about overall temperature, it is about temperature differences over the the entire tool.

Cast iron coefficient of expansion:
.000006 inch per inch per degree F
.0000108 m/m per degree C or K

So, over an 18" edge, one degree F will cause .000108" expansion. If the temperature isn't uniform, you'll notice it.

My larger cast iron cookware starts flat, bows noticeably while preheating, and returns to flat once the heat has normalized throughout the entire pan. Same as what is being discussed here, but I'm not measuring my cookware to .0001" and nobody is scraping an edge while it sits on a burner! ;)

jeffatdortek
03-10-2011, 06:25 PM
At my wits end trying to scrape in a Craig Donges 18" straight edge...

I have got it down flat to the point that it will float across a surface plate..

I spot it with a thin coat of blueing on the surface plate.. It shows it is high on the ends which confirms with the hinge test..

I do very light scrapes, so light next to no swarf is produced...

First two cycles very little happens... Next thing I know with the third cycle it is high in the middle...

OK I leave it a while and respot, still high in the middle.... I scrape that and next spotting cycle it is high on the ends...

I am at my wits end trying to scrape this in... I have a good 30 spots per inch marking where it does touch but there seems to be no pattern as to what will touch after a scraping cycle... The amount I take off per cycle barely takes off the blue...

What am I doing wrong? It is so frustrating, I do some cycles get no-where, try again, get no-where, then walk away and do something else for awhile before coming back to it...

Was your straight edge stress relieved? Internal stresses from the foundry can be released as you scrape.

.RC.
03-10-2011, 07:36 PM
Yes it has been sort of stress relieved to the best of my abilities.

fredf
03-10-2011, 09:47 PM
...Scraping conundrum, now contemplating suicide..
...

Have faith .RC.! That is a rather permanent solution to a temporary problem

I think this is why I have no desire to try scraping :eek:

Mcgyver
03-10-2011, 10:45 PM
I think this is why I have no desire to try scraping :eek:

heed the inner voice, be it wise

.RC.
03-12-2011, 02:18 AM
Think I have finished this side.... I have the dovetail to do yet... it is 70% done... Just hope it does not spring the side I have already done

The picture is not the best... Not an easy thing to take a photo of..

http://img.photobucket.com/albums/v606/OzRinger/straightedgefinished007.jpg

J Tiers
03-12-2011, 11:53 AM
Looks like good distribution and spot count.... nice work.

Tony Ennis
03-12-2011, 12:01 PM
I think this is why I have no desire to try scraping

Same with me. Perhaps it isn't so bad once you have the skill to measure to that level of precision.