Camelback Straight Edge Scraping

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wcunning

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I have a 6' camelback that I bought on eBay a while ago with no confidence in its scraped surface -- probably not too far out given that it has been scraped before, but likely not Grade A anymore. My biggest surface plate is 18"x24", so I picked up a 5' granite parallel when it dropped in my lap for reconditioning my straight edges. It's also not particularly recently certified, but it was used by a CMM installer, so I have some reasonable confidence in it.

My question is this: how do you blue/scrape to get a flat surface on a very long camelback when you don't have an 8'x8' plate? I've seen sideways mentions of carefully overlapping passes and checking one half and then the other, but never a real description of how to do it with some certainty. I can scrape for flatness pretty damn well, and I have reasonable measuring equipment, but nothing that I'd 100% trust to do 6' x 2" of a camelback surface. Does anyone have any guidance on this? Rich?

Thanks,
Will
 
wcunning, I'm DEFINITELY a newbie for your questions. I'm not sure if any of this helps, but section 9.9 through section 9.13 (pages 54-57) of Connelley's Machine Tool Reconditioning might be a good place to start. As a beginner, I'll be watching this thread to learn more on the topic.
 
Any precision flat tool that is long and skinny and made from metal is always suspect WRT flatness. Metal has residual stresses, and it moves over time, even when stress relieved multiple times and properly stored. It probably should be checked using a oversize high accuracy (AA) surface plate before use each day. However, the things also move with heating and cooling, and when the temperature is not the same throughout. If a guy is selling a granite parallel that was used for CMM calibration, I would ask, why did he sell it? Machine tool repair specialists use smaller plates to test and calibrate longer tools, but also have the knowledge and experience to interpret the prints as they move the longer tool along. They also do the reverse, use a shorter camelback to test and scrape in a long lathe bed or other surface. I KNOW that I do not have the skills and experience to do that, and so I would not try to do so on anything important. In a few weeks I will be attending a Richard King scraping/machine tool conditioning class and hope to start to learn and practice some of those skills. All I know for sure is that guessing is just guessing, and proof is derived from valid steps of proving the work.
 
The CMM calibrator was retiring, sold it to a neighbor of mine who used the parallel as the base for an exceedingly accurate autocollimator setup. It's likely not AA, but I'd be pretty shocked if it was below the requirements for grade A. He got rid of it because he, like all of us, needed the room in the shop. Again, not *guaranteed* and *warrantied* but more accurate than my worn out machine tools for sure.

Honestly, if I could find someone who had a serious granite plate, guaranteed to be A or AA who would let me, I'd go rent time and scrape those straight edges in in a better environment, but I don't even know where to start to ask that question.

As to temperature, my basement shop is 60*F +/-1*F over basically the whole thing. It gets a bit warmer in the summer, has a bit more of a temperature gradient in the winter, but the duct work for the house runs all through the basement, the furnace is in the middle and the mass of everything averages it all out quite well. I should probably do more testing with the IR thermometer to know for sure that it won't be a major source of inaccuracy, but the machines are definitely more worn than the coefficient of thermal expansion issues I might run into. When I get down to finish scraping in a couple of years on the mill and lathe, I'll worry about making sure that it's done in optimal conditions, but right now it's not too much of an issue.

I'm only 5% better than absolute amateur. I have the proper equipment -- surface plates, camelbacks, Biax scraper and flaker, AccuFinish sharpener, etc -- and I can scrape for number of points. I cannot interpret the bluing with any real confidence, and the granite is close enough to the width of the camelback that I can't do much hinging. That's why I'm trying to get the discussion going and hopefully learn a few things.

Thanks,
Will
 
Quote::Honestly, if I could find someone who had a serious granite plate, guaranteed to be A or AA who would let me, I'd go rent time and scrape those straight edges in in a better environment, but I don't even know where to start to ask that question.

I would think in Detroit there should be someone with a good plate, might post on CL to rent time on one.
 
I have a ton of chores today and will write more later. I would build a platform to set the Granite SE on so it sets on the Airy points, so you can rub the SE on it with confidence it won't fall on the floor. I would also drill and tap a hole in one of the out side ends so you can hang it and ring it as storing straightedges this way is the way I have found they stay straight better then any other way. Please post a picture of the SE and a pic of the end before drilling it so we can talk about it. You will have to lap scrape it.

Also if you haven't scraped before look at the one post I linked to here about a Studer grinder I taught the owner how to scrape over the internet. Got to go....Rich

PS: On the You Tube Show Stefan G who attended the Denmark class last year....he shows you how we rang a straight-edge ,,
 
It's a 30+ year old Challenge camelback, so I'm not sure it needs much stress relieving or ringing I'd hang it from one of the hand holds, personally. I haven't quite figured out where I'll have space to build a rack to hang my collection of straight edges, since it's a pretty complete collection and thus will take up plenty of space. I'll post a couple pictures of the collection tonight.

I do, sorta, know how to scrape, at least scrape flat and for bearing surface. I got lessons from one of your students, Rich -- OtherBrother from PM.

Thanks,
Will
 
ahhh well Daryl can help you out...one of my A++ students....I have seen 60 year old SE's take a set or twist if they are sitting cocked eyes for a while...
When you become a bit more experienced you will understand a ringing helps maintain all straight-edges as the get work hardened just from rubbing them on parts. I have a few more years experience working with SE's. You can do what you want, but for others reading this I would hang it and ring it to vibrate the stresses out while scraping old SE's. Also I have learned over the years hanging them from the holes and not an eyebolt they bend . But as I said, your choice.
 
I didn't realize they built up that much new stress from use/sitting, and all the discussion I'd seen on ringing was about taking stress out of fresh castings. I'm always happy to listen to experience. I'm a little unsure how to setup and drill the eyebolt hole in the narrow ends of the simple flat straight edges, as opposed to my dovetail ones. I'll take some pictures this evening, and see where you'd recommend putting an eyebolt/how to drill for it.

Unfortunately, Daryl hasn't found a place to use a big surface plate on his straight edges either. I should check in with him and see if he'd be interested in looking again. Honestly, I need to get a hold of him and see if he'd be interested in using the big granite parallel to check his straight edges, too, though his longest is 5 ft, if I remember right.

Thanks,
Will
 
Sure makes sense that hanging them straight up and down will avoid the gravitational forces and keep it straighter.

Rich: When you say ringing, are you saying like a tuning fork, or like gauge blocks. I'm taking it that because your talking stresses, you are saying tap it with a brass hammer or something to ring the stresses out.
 
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