Camelback Straight Edge - It Moved On Me

rgerlach · Feb 13, 2019

Holy Crap Batman. I spent the day getting my 18" camelback straightedge close to where I wanted it and went in the house and had dinner. An hour or two later I went back out to the shop and did a few more touch-ups, cleaned it off and spotted it again. My nice flat straight edge was now bowed and only touching on the ends. Was this due to my handling the straightedge with my bare hands and the heat it induced into the casting?

I would never have guessed it would be that sensitive.

RonG

bmikkalson · Feb 13, 2019

Reclean the table and straight edge. Retest..
Also hold and ring the straight edge see if it has a crack .

No it should not move, u may have a tiny shiner too.

sfriedberg · Feb 13, 2019

Thermal expansion is a real thing, yes.

Cast iron's coefficient of thermal expansion is roughly 0.000006" per inch-degreeF. So if you made a 10" section 10 degreesF warmer, that section could be expected to expand 0.0006", which is certainly enough to notice.

Making the back longer would have some not-obvious effect on the face due to the webs, lightening holes, reinforcing ribs, etc. However, I would have wagered a quarter that it would move in the other direction. Can you show a side-view or an "isometric" photo so we can see the location and approximate thickness of the back and the webs that connect the back with the working surface? At this point, I am speculating there's a more or less complete web between the back and the face, based on how it moved.

Demon69 · Feb 13, 2019

rgerlach said:
Holy Crap Batman. I spent the day getting my 18" camelback straightedge close to where I wanted it and went in the house and had dinner. An hour or two later I went back out to the shop and did a few more touch-ups, cleaned it off and spotted it again. My nice flat straight edge was now bowed and only touching on the ends. Was this due to my handling the straightedge with my bare hands and the heat it induced into the casting? View attachment 249509 View attachment 249510 I would never have guessed it would be that sensitive.

RonG

Things can move due to bad handling, think temperature gradients. Can also move if trapped stress is released when bumped or dropped.

I did a couple videos showing the effects of bad handling/different grips etc, gives an idea. You have to go full on numptey but is very possible.
over 24" test 1
over 24" test 2

Anything that causes a major temp differential can get things moving, that includes your surface plate. When you take your straight edge to bed with you and stroll into the shop which is at 10ºc, then dump it on the plate, youll get a print like you have in a couple minutes. Leave it a couple hours so the temps in the SE can settle out youll get your print back.
Dont sleep with your straight edge!
Left your SE out in the garage all night (catch me twice shame on me). Throw it up on the plate and a previously perfectly hinging SE is now spinning like a top, cos the morning sun coming in through the window has temporarily domed your plate.

In short, its not nearly as bad as some folks like to make out, but it can lead you into the woods if youre not aware.

Cheers
D

rgerlach · Feb 13, 2019

"Can you show a side-view or an "isometric" photo so we can see the location and approximate thickness of the back and the webs that connect the back with the working surface? "

Here is a side view:

M.B. Naegle · Feb 13, 2019

I had the same issue with my 24" straight edge.

How old is the casting? New castings take a while to settle. Stress relieving them helps a lot, but it might take a couple rounds of scraping and checking before it stops moving.

rgerlach · Feb 13, 2019

They are 10+ years old and have been stored in a non insulated garage type area so they have seen temperature cycles of 45 to 100 degrees F over this period. They were supposed to be fully stress relieved before I bought them. So I don't think it is an issue with internal stresses.

Larry_D · Feb 13, 2019

If you were warming it up while working on it first, and then you let it cool, I'd expect the opposite behavior. So, if the ribbing was warm while you were working on it and expanded, it would bow the edges down. Then you scrape those down. It cools and the ribbing contracts, and pulls the edges back up. Now it's high in the middle...

Makes me think it actually got warmer sitting. Is that a possibility?

I think what Demon mentioned may be likely too. If you left it setting on a cool surface plate while you were away, it may have pulled heat out of the face, contracting it. Which is the same net differential as heating the ribs, causing it to bow the edges down.

Demon69 · Feb 13, 2019

By all accounts machining can put stress into as well remove stress from a workpiece :scratchchin:

. Hanging it and ringing it cant hurt.
I think if it were mine id leave it on the plate overnight, touch it back in, check it again the following day. Taking another look at your SE, id rough the whole thing in before I started going for the finish also.

Try not worry about things too much, you can see in the video it took the right conditions and some real handling abuse to get things to move a couple tenths in 12". Keep in mind theres no such thing as flat, just variations from. So easy to get :willy_nilly:

about numbers that in the real world dont matter that much.

Happy scraping.

Mark Rand · Feb 13, 2019

It's not just the straight edge that can distort. I had a case where a gib that I was scraping flat apparently distorted over-night in a similar manner. It turned out that the surface table itself was distorting because I wasn't running the A/C all the time. Once I realised this and arranged for a powerful fan to equalise the temperature on the top and bottom of the reference, the readings became predictable again.

swatkins · Feb 13, 2019

Yes keeping the temps under control is very important. My 4x3 and 3x6 tables live in a heated and a/c control part of the shop. I always let something I've scraped come to equal temp before testing it or working on it...

I found that all stress relieving is not created equal and you really have no control over how someone else does it.. People seem to cut corners

I get the best results by treating the edges before I plane them, then treating again after the major metal is removed. After the treatment it's back to the planer for the last couple of thousands then on to scraping.

It does help to have your own treatment oven

Paolo_MD · Feb 14, 2019

When I scraped Dennis Foster's Featherweight I did notice a similar behavior with drastic changes of temperature (I'm talking of everything soaked at the same temp, not while moving from one temp to another): the straightedge would touch at the ends in a warmer environment, and would touch in the middle when it was cold. But I'm talking passing from high 80s to low 40s and the deflection was at least a couple of tens/ft.
I haven't tested yet the behavior of my other camelback straightedge.
These precision instruments are designed to be used in a constant temperature environment, not at extremes.
In your case, before worrying too much, I would quantify first the variation. It doesn't take much to change drastically the print when the ink is thin.

Another observation I made is that, the lower is the camelback, the more the straightedge is sensitive to heat from handling.

Paolo

Richard King · Feb 14, 2019

Cast Iron Straight edges move especially newly machined and scraped ones. Not knowing how it was stress relieved 10 years ago and only taking the word from someone isn't good enough. How were they setting for 10 years? As Steve Watkins points out he has seen SE's move, He is lucky and bought a great oven that a straight-edge fits in perfectly. I sold one of my 48" camel backs to a company several years ago after I had used a well know heat treat company and the customer had stability issues with it, 2 different times. They sent it back 2 times. I had it stress relieved again buy the first company again sent it back to the company in NY 2 times. They sent it back and I sent it to another Treater and that one worked. So they move. Also one needs to ring (Vibration Stress relieving) them during scraping. I have been selling and scraping SE's sense I was 12, so for 56 years I have been working with them. I have seen Brown and Sharp straight edges change from humidity and heat / cool. It moved but how much? .00005" . Most jobs and .00005" in 12" is more then good.

M.B. Naegle · Feb 14, 2019

One other thought: In your 2 pictures, you're supporting the SE from the ends. Shorter SE's may not sag as much as bigger ones, but as Paolo noted, if it has a shorter hump that is more sensitive, and if you're fine scraping it flat (looks good by the way), it may be more prone to show a sag. I wonder if when you flipped it onto the plate, it held the shape it made sitting on the blocks?

I've had better luck holding small SE's in a vise with padded jaws as it lets the working face rest more naturally. You could also make a fixture that lets it sit on the 1/3 points through the holes in the sides.

Lurk · Feb 14, 2019

For what its worth - if you are able - leave it part scraped for a couple of weeks. Then go back to it especially if its first time scraping that SE.
One useful bit of advice passed on to me last year(from this forum) was - after pulling some old SE out of storage - hang em, ring em and leave em hung for as long as possible. I never had one move to that extent mid scrape - I took around 3 weeks to scrape each one over 3ft.
I covered the workshop windows with a sheet to cut any (rare and seldom seen in the winter) sun, and had a small heat source sat in one corner of the shop running on the same heat 24/7 it held the shop at a consistent temp around 8 deg C - which is only a couple of degrees of the summer temps in there. Not ideal, but it reduces the degree of changes.
mat

rgerlach · Feb 14, 2019

Update

OK, I went out the next day and re-spotted the SE as someone suggested. To my surprise it looked much better:

I then picked up the SE by the grip portion on the camel back with my bare hand as I had been doing. I held it for about 1.5 minutes. A long time but this was just an experiment. I got this:

Re-spot after holding grip with bare hand.jpg

I then let it sit for about 20 min and then again re-spotted it using leather gloves to hold it and I got this:

Re-spot 20 minutes later wearing gloves.jpg

Hardly scientific but it certainly demonstrates the effect that I was facing. I will proceed with the leather gloves when I finish it though I may let it sit for a week or two as someone suggested.

What is the ringing that mentioned?

Ron G

swatkins · Feb 14, 2019

rgerlach said:
OK, I went out the next day and re-spotted the SE as someone suggested. To my surprise it looked much better:

View attachment 249692

I then picked up the SE by the grip portion on the camel back with my bare hand as I had been doing. I held it for about 1.5 minutes. A long time but this was just an experiment. I got this:

View attachment 249691

I then let it sit for about 20 min and then again re-spotted it using leather gloves to hold it and I got this:

View attachment 249690

Hardly scientific but it certainly demonstrates the effect that I was facing. I will proceed with the leather gloves when I finish it though I may let it sit for a week or two as someone suggested.

What is the ringing that mentioned?

Ron G

Take a rope and tie it loosely to one end of the edge. Hang it up from a rafter, tree limb, engine hoist, whatever... Start hitting the straight edge with a rubber hammer, sharply, about 10 times. Let the edge vibrate "ring" between each hit...

What brand of edge is this? There is so much mass in the base that I would think holding it it bare hands for 2 minutes should affect it that much!

Mark Rand · Feb 14, 2019

I suspect that you are seeing repeatable distortion due to temperature change on the top compared with the base of the straight edge. Ringing, hanging, ageing etc. won't change this. You could think about using gloves all the time when handling or making a couple of wood/fibre-board/foam handles to eliminate any heat transfer from handling the straight edge,

BrianB · Feb 15, 2019

I would be worried about that right hand edge. It is on hard in the one print but totally hollow the next. I am wondering if you aren't getting some rock in your prints. You really need to work those edges as you have nearly lines of contact in several places.

Richard King · Feb 20, 2019

Mark Rand said:
I suspect that you are seeing repeatable distortion due to temperature change on the top compared with the base of the straight edge. Ringing, hanging, ageing etc. won't change this. You could think about using gloves all the time when handling or making a couple of wood/fibre-board/foam handles to eliminate any heat transfer from handling the straight edge,

Mark I agree with you most of the time, But I know for a fact ringing helps. We had this discussion a year ago and I asked my friend Professor Alex Slocum of MIT about it and he said it is a know fact that Vibration Stress relieving changes things. I learned the ring method from my Dad 50 years ago when we cast, high temp stress relieved, rough machined, high temp stress relieved again, finished machined and then hand scraped. We would ring them 4 or 5 times during the scraping process and they changed more in the beginning and smaller amounts as the better it got. If I rescrape them I always ring them and they change. It may only be 1/2 tenth and not measurable, but the hinge changes. If you look at Stefan G Norway you tube he shows us scraping a 48" camelback and we hung it and rang it and Stefan and Jan saw it move. You have your opinion and we have ours.

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