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    Default How straight are straight edges?

    Any ideas?
    variance by price?
    Variance by suppliers?

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    Depends on the spec, like anything else, right? 0.002 per 6" would be an easy spec. 0.0001 per 6" quite a bit more expensive! Should be a named or numbered grade from NIST (nee NBS) or DIN, like there is for gage blocks.

    Tru-Stone had this info about granite straightedges:
    Laboratory grade AA accuracy is .000025” per 12” and inspection grade A accuracy is .000050” per 12”.
    Mitutoyo series 528 knife-edge straightedges (totally different beast, obviously) are catalog rated at ±.00005”, with the largest model being 10" long.
    Last edited by sfriedberg; 05-07-2014 at 05:47 PM. Reason: fix typo in FLA

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    Less than .002 error in 48 and 60 inches was a common straight hard and ground parallel type. precision .001. camel backs were almost zero. Starrett was a fair to good brand. Brown and Sharpe considered better by most.

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    My 48" granite straight edge is certified to be straight within 50 microinches in any foot. I've scraped straight edges closer that that - 90 microinches overall - by using the Whitworth averaging of errors technique and hysterical control of heat. These straight edges were 8 ft long and the process took days because of time to allow them to reach equilibrium before each printing cycle.

    A plain vanilla steel straight edge (you didn't say cast iron or hard steel type) can be straight as 0.0002 per foot.

    The biggest factor contributing to error in a straighness reference is heat from handling and the environment. The sun in the window, radiant heat from nearby sources (including your body) the heat of your bare hands, evaporation of solvents used for cleaning, etc all contribute to unequal heat in the material from which the tool is made. This uncontrolled heat input causes unequal expansion of its parts and consequent temporary distortion in the reference face or edge.

    Skeptics will scoff but those who have used a straightedge near its limits of calibration learn to be especially careful of exposing it to uncontrolled heat.

    Want a vivid demonstration? End support on 1-2-3- blocks a 2 ft long piece of 2 x 2 square tubing on your granite flat. Set a 0.0001 reading dial test indicator to register "zero" at the underside of the center. Make sure the tube is solidly seated (no burrs or teeter) on its support. Set a paper cup of hot coffee on top of the middle of the tube so the heat from the cup bottom radiates across the 1/4" gap formed by the brim. Watch in fascination as the DTI needle starts to creep over a period of one minute. Remove the coffee. You will not be able to feel the localized difference in temperature but the steel tube will. Do not disturb the setting because the lesson continues. Note it takes several minutes to return to zero - but never quite gets there. Actual equilibrium may take an hour. Mathematically it can be called asymtotic

    So sweat the calibration of your straightedge and preserve it from abuse, shocks, and negligent use. Also be aware if the lingering effect of uncontrolled heat when you use it near the limits of its accuracy.

    It aint all or nothing. Proportion your precaustions. Casual use for a quick check requires only routine care. Checks to a tolerance of 4 times or less the tool's certification requires the special care. And appropiately care between. YMMV

    I see on review I didn't answer the OP question: How much? How good?

    Hard straifght edges, the good kind are easy to price from the maker's website. Expect the price to double with each grade of improved precision.

    How good? The usual use is with a single single feeler or sighting the gap in grazing light. While good results can be attained on a surface of uniform reflectivity the results are non-quantified in gap sighting and errors of less tha the smallest feeler are undetectable or unmeasureble. One can approach the limits of a hard straight edge's calibration using the three feeler method (cut three pieces of feeler from the same roll, support the ends on two peices, feel along the middle with the third).

    Cast iron scraped straight edges can be damn expensive if bought new from the few stiil making them. $1500 (check this; I may be talking through my hat) for a 36" camelback was my last information. Less so if bought used on eBay, Craig's list, pawn shops, cold calling long establish shops, hard shopping the used market. I remember one used as a wall decoration in a barber shop. I thought is was a foundry pattern for years until I ran into a fellow I used to see there. He said it was real and heavy as hell.

    You also can use them with feelers and sighting the gap in grazing light but they are usually used either spaced above and parallel to the work and scanned with a dial indicating height comparator or a surface gage and a DTI. The other and more frequent use is as a transfer tool where a thin fim of transfer medium (Prussian blue for instanced) is use to mark the work under consideration.

    How good depends on care andmaintence. Straight edges have to be calibrated, Their accuracy may degrade in time dure to wear and metallurgical changes. If not veridied against a reference of known accuracy you may be stabbing in the dark come time to exercise the tool to it's limits of accuracy .

    Hard straight edges have to be ground straight. You don't just throw the work up on a surface grinder and touch up the edges. Thats just a recipe for parallel but crooked. There's a 5 or 6 step process to regrinding the edges of a hard straight edge to ensure straightness and to certify calibration.

    Cast iron straight edges have to be scraped flat and straight against the best part of a receently calibrated surface plate (look at the diagram on the plate's calibration sheet some part of the plate is will be flatter than another). This is also skilled work not only to produce the desired precision but the bearing quality of the calibrated surface.
    Last edited by Forrest Addy; 05-08-2014 at 03:57 PM.

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    i wonder how heat affects this type of straight edge:

    Tuschierlineal 500mm / Werkzeug in Niederhasli kaufen bei ricardo.ch

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    gravity effects all objects and they can sag depending on how supported thats why they have airy points

    Airy points - Wikipedia, the free encyclopedia
    .
    what is straight depends on its orientation and how supported. i see this all the time setting up machines for cnc machining to 0.0005" flatness tolerances over total length

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    Those will sag to gravity as DMF explains. When I use this type I use a camel-back with it too. lets say your part is concave and this non ribbed angled straight edge (SE) was scraped on a precision plate and it hinges or the airy points are 25 to 30 % from ends. You set it on the concave and just the pressure of you hands will bend the SE into the concave and give you a false read. I have seen where some will bore holes into the flat side along it's length approx 1 1/2" apart to lighten it plus make it a bit more rigid.

    Another tip when using this type of SE when you set it down on a worn surface on each end holding with your hand with light pressure hinge the straight edge to see where the airy point is on the opposite end, note where it is, then switch hands and hinge the other side. Then that is where you can hold it to rub it back and forth. This will cut down on your hand pressure bending the SE and giving you a false reading. This techniques is also a good way to hold a thin gib or hold down when bluing it up.

    I use the spec's of .00005" per 12" when scraping a straight edge or a precision machine like a Studer Grinder. and .0002" / 12" when scraping a conventional machine like a SB lathe or Bridgeport. As others have said. How are you measuring this? What is the temp of your shop? Temp and humidity controlled is so important when working on precision machines and tools.

    Even the heat of your hand will affect straight-edges when working on precision parts. I will wear cotton gloves or wrap a rag around the ribs of a camel back. better safe then sorry. I recall what an Engineer at Professional Instruments told me once when he said a new machinist told him he could grind a shaft to .00005" round. He asked to see the guys measuring instrument and he whipped out a regular micrometer. ....It isn't going to happen eyeballing a mic when measuring that fine.
    Rich

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    we use to call it the salesman bullshit factor.
    .
    they would sell a optical instrument and say it is accurate to 1/40 of a arc second
    .
    the instrument cross hair even if only 4 arc seconds is thin and hard to see so somebody saying it was accurate to 1/160 of a crosshair is like saying they are measuring 0.001" with a $2 scale

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    well, i wonder why this type of straight edge is used over here almost exclusively, and the "camel" seems to be the standart for the "anglosaxons" (sorry about the expression, dont want to insult anybody, thats what we say in german).

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    Quote Originally Posted by Richard King View Post
    I have seen where some will bore holes into the flat side along it's length approx 1 1/2" apart to lighten it plus make it a bit more rigid.
    Does boring the holes actually make it more rigid, or does it just bend less because it's lighter?

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    This is one of those cases where the term "rigid" needs to be defined. If it means gravity-induced sag if the edge is supported at each extreme end, then the solid form will be less rigid as the increased resistance to a given force of the solid form is offset by the increased weight of the solid form so it will sag more. I think that is the definition that has more relevance to ordinary camel back use. If the term means increased resistance to deflection due an externally applied force after self-induced sag has already occured, then the solid wins. The small gain in rigidity of a solid form is offset by its own increased mass and material expense--thus the popularity of I-beams and the rarity (ever see them?) of solid iron members in buildings, bridges, aircraft, etc.

    So, it bends less in the first definition sense because it is lighter.

    Denis

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    Quote Originally Posted by Mud View Post
    Does boring the holes actually make it more rigid, or does it just bend less because it's lighter?
    I can't imagine material removal making a structure stiffer....

    [edit] denis beat me to it, I think.... although he took the time to actually explain.

    [edit #2] I just modeled up a quick and dirty camelback, 3 feet long. Solidworks tells me it's about 80 lbs if it were gray iron. The version "with holes" sags more when I do a really basic cantilevered FEA... I'll try it approximating the "concave surface", too...

    [edit #3] I just tried the simply supported version as in Richard's example. Got the same result - the "lighter" camelback deflects more.

    Of course, I'm sure there is some inflection point where the opposite will occur (I'm imagining a completely hollow camelback), but my cruddy model shows that some haphazardly placed holes just makes it worse. Having never actually held a real camel back, or even seen ones with "rigidity holes", take all this with a few blocks of salt.

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    Dian it will work if you use a a level and do both at the same time. It takes less time then having a rigid camel-back that is longer then the part your scraping then lap scraping section to section using a level and non ribbed straight edge. Do an experiment like Forrest mentioned. Set the straight-edge on 2 one two three blocks that you have double checked the are the same. Try to slide in the 3rd block in the middle and see if it slides in. Take it out and then put a surface gage with a .00005" indicator on the granite surface plate and lay your hand on the top of the straight-edge and watch the indicator move. if you want to get a bit more technical set some weights on it and let us know the results.

    I have been scraping for as far back as i can remember and my Dad and I both made Camel-back and Non ribbed Straight-edges straight-edges from scratch to finish 40 PPI /.00005 per 12" flatness and I guarantee that non ribbed straight-edges sag do you gravity. Same principal as a 2 x 4 laying flat or vertically.

    Mud I am not exactly sure on that one. That would be a great question for Archie the Professor. I may be wrong about the holes and rigidity.
    Rich
    Last edited by Richard King; 05-09-2014 at 10:17 PM.

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    My Rahn Planekator has measured calibration of +13 -12 micro inches over 2'. Note the insulated hand grips. http://www.normanmachinetool.com/data/257RAHN_PDF.pdf

    Don Clement


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    Default Long since retired . . .

    Richard is teasing me -- I retired from the title "Professor" fifteen years ago. Most of my manufacturing engineering students called me "Archie". At that time I could not have contributed much to this discussion, but for the last six years my retirement hobby shop has taken me deeper and deeper into scraping ways and other things related to rebuilding the machine tools that I love. I am currently restoring an American Pacemaker lathe and you can read about it in this thread.

    First, I agree with most everything that has been stated above and I am not looking to start an argument. I will share my view on some things about straight edges that do not seem to be much discussed in these sorts of threads:

    o My answer to Mud's question about lightening holes: Folks who have scraped all day long know why a straight edge cannot be allowed to be too heavy -- picking it up a hundred times a day gets old even if it is fairly light. If you cannot lift a straight edge, it really does not matter that it is more accurate. I have one six-foot cast iron-faced aluminum Busch and a bunch of B&S cast iron "bridge" (camel-back if you wish) straight edges ranging from 18" to 60". The 5-foot B&S is heavier than I can pick up more than once a day -- it is the standard that I use to scrape in my 6-foot Busch (& the others too). The 5-foot B&S appears to have only have been lightly used -- I can understand why. I suggest that the way to think about lightening holes is that they allow a straight edge to be much deeper (distance from the working surface to the opposite side) so that it will be much more rigid because the material is farthest from the neutral axis, while keeping the weight to a manageable amount.

    o Airy Points. Wikipedia says just about all I know, but most folks do not seem to notice that the mathematics behind the calculation of the Airy Point locations is for a uniform rectangular cross-section bar and hence the actual numbers would not translate directly to most of our straight edges. Also the Airy Point calculation is to determine where to support an end-measuring standard so that the ends are parallel for precision measuring. I do agree that these support points would be near those that give the least departure from straightness along the length of the straight edge. If you do not know how to support a straight edge, the 25% to 30% numbers commonly used are reasonable.

    o When using the straight edge for spotting, I suggest that something else is going on that bears keeping in mind. Any straight edge that is scraped by printing on a master (e.g.: surface plate) is not "simply supported" (i.e.: by two points), but is supported all along its length and the more carefully it is scraped in, the more evenly it is supported, allowing it to assume a completely relaxed condition with regard to internal elastic stresses. I think we can all agree that this is a good situation. As the part being scraped approaches perfection, once again the straight edge is in the same uniformly supported condition it was when it was scraped in. To me this means that one can do better work with a straight edge that the numbers one might measure under simply supported conditions. (Note that there are many other uses for a straight edge that require it to be simply supported during use.)

    o I store my B&S straight edges in their factory shoes which support them simply. I think that B&S knew what they were doing. I checked a friend's 4-foot B&S against my 4-foot B&S AND against the Lucas HBM bed I was scraping at the time -- sort of a modified "three-plate method". With all temperatures equalized both straight-edges would print the same uniform 20-points per square inch pattern on the Lucas bed and on each other. Both straight edges would "pivot" around the 25% points. Life was good . . . I then left the friend's straight edge supported on a ground plate that had been installed at the center of its bridge -- it seemed like a good idea at the time. When I came back from lunch, this straight edge would spin on its mid-point when placed on the Lucas bed. An hour later, after being supported on its 25% points, it again pivoted properly as it had before. (By the way, I store my Busch straight edge by hanging it vertically from the lightening hole at one end. This makes sense to me for a non-bridge type straight edge as it hangs symmetrically on its neutral axis.)

    Archie

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    Thank You Archie, it is great that we can be honored to have you contribute to the forum. There are so many who guess at what they write, but not you! Your not only a Professor of Engineering (retired), have worked along side other professors training the top minds in our country you come here to share your knowledge and when we read it we know it's facts. . Like our old mutual late friend Fulton Holtby longtime professor at the U of MN and Stout State in WI. who I visited with usually on Saturday when he and other great minds would gather for breakfast at Midwestern Machinery and share their love of the Machine Tool Industry.

    I am positive, He and You are / were loved by their students and became a friend. I know many considered him an Uncle as I did. I know I have enjoyed our talks when we met. You have a wonderful personality and learning from you is a dream. I would suggest any PM members if you need some advice Archie is the man. I call my students my "kids" or they are my heritage and expect them to pass on what I teach them too. Thanks again and Happy Scraping. Rich

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    On that "heat" issue.... it's quite real, and you can see it.

    I scraped a short (18") SE blank that I got from a member here. It fooled me for a while.

    I had it scraped what I thought was flat, and it was pretty flat, but then I noticed it would just spin on its center. Picked it up, brushed off the surface, brushed off the granite, put it down, all was perfect. "Just dust" I thought.

    So I came back later and checked again... it spun on the middle, just slightly, as it had before. it had been sitting on the clean surface, and no way was there dust, so I smelled a rat.

    Sure enough, I picked it up with a rag, and looked at it, set it down, and it was perfect.... the hand heat thru the rag expanded the camelback side just enough to make it look good.

    I made and put on the wood handles, and never had any more trouble after I corrected the crown. I had scraped-in a slight crown due to the heating and expansion.

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    Cool! At last someone with major engineering creds is in the loop to cut through the fog

    The burning question I've harbored for years: Camelback straight edges resemble bridge trusses but few camelbacks I've seen are trinagulated. Why is that? Most are bayed with vertical stiffeners framing lightened shear penels. Easier to mold I expect and fewer hard-to-cast intersections.

    I was tauight that a triangulated truss is stiffer per unit of weight. If I had my 'druthers, I'd design a triangulated cast semi-elliptical truss having L/6 chord, cruciform posts, flat (read pattern draft) diagonals, and an L/30 height web stiffening the sole. My thinking is a triangulated truss would be stiffer for the weight than a post and panel truss. I can't help but think the triangulated truss would be stiffer in torsion as well, provided the triangulation and the cross-axis posts webs were wide as the arch of the camelback.

    Don't you love word descriptions? I need a USB port in the side of my head - but what would I hook it to?

    So, Doctor Cheda: Thoughts? Analysis?

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    Default Head affects the work as well as the reference . . .

    JST,

    I had a similar experience with my first major scraping project on a 1912 Lucas HBM. I was well into the finish scraping phase with the 2,000# HBM bed "parked" in front of my translucent main shop door. It was winter and the southern sun was on my back to supplement the heat in my shop and provide good lighting -- this was my standard winter mode of operation. I would have a great print, with the straight edge pivoting (or hinging, as Rich calls it). I had the HBM bed far enough from the door that the sun was hitting the lower half of the bed and found that I had the same experience of having the printing/pivoting change significantly. Once I realized the problem, all I had to do is to move the bed a few feet so that it was out of the direct sun and everything stabilized. Connelly even warns about drafts in a large building causing selective cooling or heating of one part of a large machine and effecting the process -- after my experience with the Lucas I can believe it.

    My B&S straight edges seem to have their wooden blocks in line with the support pads in their shoes, so I use them for handling and also use them to support the straight edge when inking the working surface prior to printing. It has always puzzled me that most of the blocks are not "proud" of the high point of the bridge hump so that I need to place some other blocking under them so that the hump will clear a bench surface. I usually use two saw horses to support the straight edge during inking for this reason. I have not come up with a satisfactory answer to this, other than the pads in the shoes of some of the straight edges seem to work to support the straight edge. In any case, I find the straight edges to be very precarious when inking. Does anyone have any light to shine on this minor part of scraping? ( This minor annoyance grows larger as the scraping day gets long . . .) I have thought of making a stand that would support the straight edge by its handling blocks and prevent it from falling over.

    Archie

    P.S.: Rich, Thanks for the kind words -- I will not post what my wife said when I read them to her . . .

    P.P.S: Forrest, we were composing our posts at the same time. I'll think about your ideas, but for now let me "lower expectations" -- I am not a "Doctor", although I do have two M.S. degrees, one in materials. I am an "old school" teacher who taught at a state school that focused on hands-on training of engineering students. My background is very "blue collar" and I do try to bridge the gap that often forms between the practical and the theoretical. I find that the theory folks are correct in their analysis, but there is often a misunderstanding of the system they are studying, while the practical folks know what they see, but do not understand the details of what is going on -- my favorite example is "crystallization" of metals. (Do not get me started on that . . .)

    Final shot: Plug the USB cable into a spelling checker -- I cannot figure out a few of the words in your post . . .

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    Quote Originally Posted by Forrest Addy View Post
    <>I need a USB port in the side of my head - but what would I hook it to?

    <>
    You know, forrest, your head MIGHT look better that way. Sorry for the very cheap shot. Could not help it....

    Seriously, I think you have posed a very interesting question with respect to the optimal bracing of the c-back sole vs. the bow. I also suspect that require considerable analysis to answer properly. I wonder if it has ever been done formally.

    Denis


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