[machinist] Making Accurate Straight-Edges from Scratch by John A. Swensen

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Thread: Making Accurate Straight-Edges from Scratch by John A. Swensen
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   1.   12-14-2014, 10:06 PM  #1
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      [image: Default] Making Accurate Straight-Edges from Scratch by John
   A. Swensen

   Making Accurate Straight-Edges from Scratch by John A. Swensen
   https://home.comcast.net/~jaswensen/...ight_edge.html
   <https://home.comcast.net/%7Ejaswensen/machines/straight_edge/straight_edge.html>

   "I adapted the techniques for making straight-edges from those described
   for making accurate surfaces, as described in Wayne R. Moore's "Foundations
   of Mechanical Accuracy", Moore Special Tool Co., 1970. This fascinating
   book describes how reference tools and machines with accuracies on the
   order of millionths of an inch are built."

   "The fundamental principle used to create precision straight-edges is
   that the only way that three curves can match each other is if the curves
   are all straight lines. By repeatedly comparing the three developing
   straight-edges with each other and averaging their shapes where they don't
   match, the three developing straight-edges will become arbitrarily
   straight."

   "Why Three Straight-Edges Are Necessary "

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    2.   12-14-2014, 10:18 PM  #2
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     [image: Quote] Originally Posted by *LFLondon* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439222>
   Making Accurate Straight-Edges from Scratch by John A. Swensen
   https://home.comcast.net/~jaswensen/...ight_edge.html
   <https://home.comcast.net/%7Ejaswensen/machines/straight_edge/straight_edge.html>

   "I adapted the techniques for making straight-edges from those described
   for making accurate surfaces, as described in Wayne R. Moore's "Foundations
   of Mechanical Accuracy", Moore Special Tool Co., 1970. This fascinating
   book describes how reference tools and machines with accuracies on the
   order of millionths of an inch are built."

   "The fundamental principle used to create precision straight-edges is
   that the only way that three curves can match each other is if the curves
   are all straight lines. By repeatedly comparing the three developing
   straight-edges with each other and averaging their shapes where they don't
   match, the three developing straight-edges will become arbitrarily
   straight."

   "Why Three Straight-Edges Are Necessary "
    Is there more that you intended to post on this?

   Denis

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    3.   12-14-2014, 10:45 PM  #3
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     [image: Quote] Originally Posted by *dgfoster* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439230>
   Is there more that you intended to post on this?

   Denis
    No. The full article is at Swensen's web page and I am sure there is
   plenty more across the Web from other sources. I thought it might be useful
   to some.

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    4.   12-14-2014, 11:27 PM  #4
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     [image: Quote] Originally Posted by *LFLondon* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439253>
   No. The full article is at Swensen's web page and I am sure there is
   plenty more across the Web from other sources. I thought it might be useful
   to some.
    Oh. (puzzled look)
   Denis

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    5.   12-15-2014, 12:06 AM  #5
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   I thought it was a good post. I certainly learned something about
   straight edges.

   thanks

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    6.   12-15-2014, 12:57 AM  #6
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   Not exactly a new idea, is it?

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    7.   12-15-2014, 01:25 AM  #7
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     *Forrest Addy*
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   The OP needs to look back in the archives for the acheivements of Steven
   Thomas, D G Fortser and some others whose names I will recall in the middle
   of the night.

   They've made straight edges by starting with their basic concept,
   particular requirements, and research. They made a pattern, got castings
   made then machined and scraped them.

   There's another fellow who make patterns to make wax segments that when
   joined, dipped, sanded, and melted out produced a box straight edge casting
   molds that when cast and stress relieved producted castings similar to that
   showcased in Moore's "Fundamentals of Mechanical Accuracy" I think the same
   guy cooked up some fine cast iron set-up squares. He took my scraping class
   in Oregin in 2007 as I recall. Beautiful work. There's also a slug of us
   regular posters who've collectively scraped in acres (seeming) of reference
   tooling.

   It's good when a guy takes the bull by the horns and made his own
   reference tooling in complince with if not certified to commercial
   standards. He should do this knowing he's not breaking new ground however
   laudible and successfull his efforts. Meet the challege even if you have to
   re-invent the wheel. Do so, and your entitled to strut and preen a little.
   Those of us who have been there will applaud your arrival.

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    8.   12-15-2014, 04:29 AM  #8
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     [image: Quote] Originally Posted by *Forrest Addy* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439330>
   The OP needs to look back in the archives for the acheivements of Steven
   Thomas, D G Fortser and some others whose names I will recall in the middle
   of the night.

   They've made straight edges by starting with their basic concept,
   particular requirements, and research. They made a pattern, got castings
   made then machined and scraped them.

   There's another fellow who make patterns to make wax segments that when
   joined, dipped, sanded, and melted out produced a box straight edge casting
   molds that when cast and stress relieved producted castings similar to that
   showcased in Moore's "Fundamentals of Mechanical Accuracy" I think the same
   guy cooked up some fine cast iron set-up squares. He took my scraping class
   in Oregin in 2007 as I recall. Beautiful work. There's also a slug of us
   regular posters who've collectively scraped in acres (seeming) of reference
   tooling.

   It's good when a guy takes the bull by the horns and made his own
   reference tooling in complince with if not certified to commercial
   standards. He should do this knowing he's not breaking new ground however
   laudible and successfull his efforts. Meet the challege even if you have to
   re-invent the wheel. Do so, and your entitled to strut and preen a little.
   Those of us who have been there will applaud your arrival.
    Thanks a million for your reply, absolutely amazing and inspirational!
   That is where the rubber meets the road to say the least, that anyone can
   tread the sacred ground of making their own accurate reference tooling. I
   was not aware of those particular accomplishments of Foster and Thomas and
   will search PM for their posts on this subject. Much credit to them and to
   you. I need to spend some time exploring the world of scraping and pick up
   a few tools to experiment with in my shop.
   LFLondon

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    9.   12-15-2014, 05:50 AM  #9
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   It's interesting this has come up now. Down here long straight edges are
   hard to come by. I have a 3' and that will probably do me, but I did wonder
   just the other day if it may be possible to make up a longer straight edge
   with a weldment, and then attach a shoe(s) on the bottom of cast iron to
   form the straight edge itself that could be scraped in. It seems to me it
   could save an expensive one off casting that in itself could be of dubious
   quality.

   Has anyone ever seen this type of thing done before?

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    10.   12-15-2014, 08:07 AM  #10
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   He doesn't seem to show how to avoid twist...

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    11.   12-15-2014, 09:43 AM  #11
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     *Richard King*
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     [image: Quote] Originally Posted by *Pete F* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439373>
   It's interesting this has come up now. Down here long straight edges are
   hard to come by. I have a 3' and that will probably do me, but I did wonder
   just the other day if it may be possible to make up a longer straight edge
   with a weldment, and then attach a shoe(s) on the bottom of cast iron to
   form the straight edge itself that could be scraped in. It seems to me it
   could save an expensive one off casting that in itself could be of dubious
   quality.

   Has anyone ever seen this type of thing done before?
    I have thought about extending cast iron plates but not SE's over the
   years that way Pete or setting 2 or 3 granite plates side by side to make
   longer ones when all you have are short ones. I never have gotten around to
   trying. Not enough spare time yet. I have read about people trying to use
   steel scraping straight-edge and again have not tried it myself. I suppose
   if your not holding tight tolerances it would work if you can control the
   temp so they don't grow. I have seen steel weldment machine tools come and
   go. Never heard of one being very stable. Pete I would be interested in
   what you discover if you try it.

   A old friend patented a portable mill expandable round way by using what
   looked like tapered sections that fit together and held with a device that
   resembled a D mount lathe chuck. Maybe you can make a expandable SE like
   that?

   I have thought about experimenting with making a cast straight-edge and
   cylindrical grinder beds with a threaded steel rod (or cast Iron), like a
   press brake back gage that has a straightness adjusting bar. When I scrape
   the bed of a grinder concave when the base is shorter then the table I have
   thought it would be nice to be able to adjust it concave, as scraping it to
   tenths is a pain. The bending contraption would help in non temperature
   controlled areas. As the temp outside changes the SE bends and you would
   adjust the bolt to compensate.....something for you young inventors to
   think about.

   I have seen this issue working in Missouri where it is cool at night and
   early morning and by noon it's 100 F degree's and your straight edge
   changes. Not a lot but a few tenths. On one job years ago at a Federal
   Mogel shop in SE MO, we had 2 straight-edges. One for AM and one for PM.
   They were identical Brown & Sharpe 48" er's. It seemed silly then and now
   but that worked better then scraping the one after lunch everyday. lol..
   The centerless grinders we were rebuilding had mass and didn't change as
   fast as the SE's They eventually air conditioned the rebuilding shop.

   The Moore book has been around for years and showed how back before
   modern methods of testing were invented they used 3 plate method. The only
   way there was to achieve flatness way back then. I wonder who came up with
   the idea? I recall when I was a teenager lapping 3 lapping plates together
   for Control Data for some project they had (my Dad's Company and we
   contracted to many companies rebuilding equipment). That was when we didn't
   own a Granite plate and all we had was hand scraped surface plates. That
   was a real pain scraping plates back then with HSS (1960's) I can only
   imagine how hard it would have been in the early years...

   When teaching machine building /Scraping in Taiwan one of my students at
   YCM did an experiment and scraped 3 HKA-24 straight-edges (King-Way or my
   Family Brand Straight-edge) with a BIAX power scraper and got the 3 he
   scraped together to 60 PPI . When he was finished they tested them in their
   Metrology lab and they were all identical and within .00004" flatness. They
   are a work of art, and are now on display in the company show room.

   I met Greg Dermer from Portland who told me he made Moore duplication
   patterns out of wax Straight-edges, when he took one of my scraping classes
   a few years ago. He told me he wanted to learn to scrape 40 points per inch
   using a hand scraper and a BIAX. He also made some nice non ribbed angle
   straight-edges. Bushe Precision makes an Aluminum with cast iron scraped
   sub plate epoxied on Straight-edge A6100 Series - Aluminum Parallel
   Straight Edges On Busch Precision, Inc.
   <http://machining.buschprecision.com/viewitems/straight-edges/a6100-series-aluminum-parallel-straight-edges>.


   Thanks LF for starting the thread. It's nice to remember the old days
   and not re-write history. Rich

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    12.   12-15-2014, 10:52 AM  #12
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     *DMF_TomB* <http://www.practicalmachinist.com/vb/members/dmf_tomb/>
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   I often mill parts to .0005" per 40" flatness and straightness tolerance.
   You would only need to scrape if you wanted straighter
   .
   On a cnc mill I often uses a marker to make sure I mill last .001", often
   the mark is removed only partially on some of it which usually means
   the milling is removing only .0002", getting finer is difficult because
   of metal deflection and cnc servo errors. The cnc mill often oscillates
   .0001" or more. Still the need to hand scrape for finer tolerances

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    13.   12-15-2014, 11:23 AM  #13
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439553>
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     [image: Quote] Originally Posted by *Pete F* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439373>
   It's interesting this has come up now. Down here long straight edges are
   hard to come by. I have a 3' and that will probably do me, but I did wonder
   just the other day if it may be possible to make up a longer straight edge
   with a weldment, and then attach a shoe(s) on the bottom of cast iron to
   form the straight edge itself that could be scraped in. It seems to me it
   could save an expensive one off casting that in itself could be of dubious
   quality.

   Has anyone ever seen this type of thing done before?
    Not much experience here but it occurred to me that with a welded piece
   the welded zone and adjoining area affected by heat might have a different
   crystalline structure (even after post weld heat treatment and stress
   relief) and that might cause unwanted minute deformation of the tool with
   temperature changes rendering it useless except for low tolerance work.

     Richard King
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    14.   12-15-2014, 11:30 AM  #14
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439559>
     *Forrest Addy*
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   A temperature compensated fabricated steel/cast iron straight edge. Hm.
   Interested concept, Richard. There's enough difference betweeen the thermal
   expansion rates of cast iron (varies with composition ) and steel (pretty
   constant) to make composite metal scrape reference tooling an unfulfilled
   dream. Many times I've looked at a steel tube camelback faced with cast
   iron and got bogged down with the expansion problem. Weight and stiffness
   advantages await but the thermal stability problem trumps all.

   Richard's musings and a flash recollection of a cased clock's thermal
   compensated pendulum (where brass and steel elements are subtractively
   combined to produce a pendulum whose length and period does not change with
   temperature) led me to musings of my own.

   Imagine a straight edge whose upper limb is comprised of brass and steel
   in a way that its expansion rate exactly equals that of the cast iron shoe.

   The proportions of brass to steel would depend on the particular cast
   iron's expansion rate. As I said, that rate varies slightly enough to screw
   up the straighness tolerance over a scraped reference tool's likely thermal
   range in service (40 to 100 degees F).

   For example, look at generic thermal expansion coefficients
   (in/in/degree F 10E-6)

   Brass 11.3
   Steel 7.8
   Cast iron 5.8

   These figures will vary depending with the actual materials selected.

   So brass and steel will have to be combined subtractively so their
   combined rates equal that of the cast iorn selected.

   One could work the math to obtain the proportions but the length of
   steel subtractively compensated by a length of brass to equal that of a
   unit of cast iron will have to be determined experimentally from samples of
   the actual materials. Book values are all over the map.

   Before the nay-sayers start shouting down the concept of a - um -
   "tri-metal" straight edge they should consider accurate thermometers based
   on bi-metal strips have been in use for generations. I'm proposing the
   bi-metal principle be applied in the opposite sense. I vaguely perceive a
   clever design and metalurgical stability are just over the horiaon.

   The design of the posited structure will have to incorporate elements
   leading to stability and rigidity equalling that of an equivalent cast iron
   straight edge but with reduced weight, in range of a home workshop
   rescources, etc.

   A tall order but do-able in my fevered imagination. Hmm indeed.

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    15.   12-15-2014, 11:46 AM  #15
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439572>
     *waynes* <http://www.practicalmachinist.com/vb/members/waynes/>
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   I suspect the straight edge varies due to changing temperatures - if
   given time to stabilize at any particular temperature, I would expect it
   would end up straight.

   If correct, having the base with a very similar cross section to the
   arch would probably prevent most of the problem - though it may make the
   straight edge rather heavy and unwieldy.

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    16.   12-15-2014, 11:46 AM  #16
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439574>
     *DMF_TomB* <http://www.practicalmachinist.com/vb/members/dmf_tomb/>
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   i remember a straight linear rail bolted to a concrete floor under a
   skylight window at roof.
   .
   the next day the linear rail was bowed or curved when checked with
   optics. we had wanted to
   use a non contact capacitance sensor connected to a lap top to see if a
   20 ton cast iron wheel
   was round.
   .
   we waited til the next day when it was cloudy and the linear rail was
   straight again. we measured the
   20 ton wheel and found it was not circular or octagon but partially
   both. the theory being when the
   wheel was ground the grinder as on the side. when the wheel turns the
   part between the spokes is
   on the top of wheel during rotation that part sags from the weight.
   .
   basically the wheel was changing shape every time it rotated. the wheel
   was in a air pressurized
   chamber at only top and the had a air pressure gage showing the pressure
   fluctuation every time wheel rotated
   the gap at wheel sides sealing the leaking air was changing so the air
   leakage was changing as it
   rotated
   .
   parts sag from their own weight. a straight edge flat when horizontal
   may not be flat when held
   vertical. sort of like a 20 foot long 1" round steel bar sags more when
   held horizontally than when
   held vertical, when someone says they have a straight edge flat and
   straight within .0002" per 40"
   i just think maybe if held horizontally it is straight. when same
   straightedge is turned on its side
   and horizontal again it may bend differently and no longer be straight.
   .
   we used to use a precision invar tape measure. it depended a lot on
   whether it was continuously
   supported, sorted on ends only, supported on 3 points, 4 points, etc.
   there was a hundreds of
   pages long length compensation table showing true values as the support
   method was change

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    17.   12-15-2014, 11:48 AM  #17
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439576>
     *Richard King*
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   Per Dennis's post #4 on Forrest's last post #14.

   Oh. (puzzled look)

   Rich

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    18.   12-15-2014, 12:08 PM  #18
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439598>
     *Richard King*
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     [image: Quote] Originally Posted by *DMF_TomB* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439574>
   i remember a straight linear rail bolted to a concrete floor under a
   skylight window at roof.
   .
   the next day the linear rail was bowed or curved when checked with
   optics. we had wanted to
   use a non contact capacitance sensor connected to a lap top to see if a
   20 ton cast iron wheel
   was round.
   .
   we waited til the next day when it was cloudy and the linear rail was
   straight again. we measured the
   20 ton wheel and found it was not circular or octagon but partially
   both. the theory being when the
   wheel was ground the grinder as on the side. when the wheel turns the
   part between the spokes is
   on the top of wheel during rotation that part sags from the weight.
   .
   basically the wheel was changing shape every time it rotated. the wheel
   was in a air pressurized
   chamber at only top and the had a air pressure gage showing the pressure
   fluctuation every time wheel rotated
   the gap at wheel sides sealing the leaking air was changing so the air
   leakage was changing as it
   rotated
   .
   parts sag from their own weight. a straight edge flat when horizontal
   may not be flat when held
   vertical. sort of like a 20 foot long 1" round steel bar sags more when
   held horizontally than when
   held vertical, when someone says they have a straight edge flat and
   straight within .0002" per 40"
   i just think maybe if held horizontally it is straight. when same
   straightedge is turned on its side
   and horizontal again it may bend differently and no longer be straight.
   .
   we used to use a precision invar tape measure. it depended a lot on
   whether it was continuously
   supported, sorted on ends only, supported on 3 points, 4 points, etc.
   there was a hundreds of
   pages long length compensation table showing true values as the support
   method was change
    Tom it must have be a learning experience working at one of the most
   precision machine builders, Gleason. I enjoy reading your posts (now that I
   know you..lol) What your telling us reminded me of Lyon Precision and their
   work with Air Bearings Inc. They have developed products and employee's
   have written books detailing spindle rotation and testing it.. Here you can
   and watch this and understand it.
   http://www.lionprecision.com/sea/revolutionVideo.html &
   Precision Spindle Metrology: A book by Eric Marsh
   <http://www.lionprecision.com/tech-library/psm.html>

   Reminds me of when I was touring a plant in Taichung and one of the
   engineers described a situation they had. They were building CNC lathes and
   would move them in an assembly line. One station was for scraping and
   alignment, then it would move to electrical install and then guarding and
   final testing run and at that location the machine was out of tolerance. I
   figured it out at once..The machine was under a huge window and the sun was
   shinning on it. We took a temp gun and the front of the machine was hotter
   from the sunshine then the back that was in the shade. Rich

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    19.   12-15-2014, 12:27 PM  #19
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439625>
     *DMF_TomB* <http://www.practicalmachinist.com/vb/members/dmf_tomb/>
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     [image: Quote] Originally Posted by *Richard King* [image: View Post]
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439598>
   Tom it must have be a learning experience working at one of the most
   precision machine builders, Gleason. I enjoy reading your posts (now that I
   know you..lol) What your telling us reminded me of Lyon Precision and their
   work with Air Bearings Inc. They have developed products and employee's
   have written books detailing spindle rotation and testing it.. Here you can
   and watch this and understand it. Revolution in Spindle Measurement
   Video Documentary <http://www.lionprecision.com/sea/revolutionVideo.html>
   &
   Precision Spindle Metrology: A book by Eric Marsh
   <http://www.lionprecision.com/tech-library/psm.html>

   Reminds me of when I was touring a plant in Taichung and one of the
   engineers described a situation they had. They were building CNC lathes and
   would move them in an assembly line. One station was for scraping and
   alignment, then it would move to electrical install and then guarding and
   final testing run and at that location the machine was out of tolerance. I
   figured it out at once..The machine was under a huge window and the sun was
   shinning on it. We took a temp gun and the front of the machine was hotter
   from the sunshine then the back that was in the shade. Rich
    .
   i used to work for Kodak. they had many big multi story tall machines
   that moved from temperature
   changes or different weight on a floor
   .
   anybody looking through a optical level with a fork truck driving by
   cannot miss the floor moving.
   .
   when taking some measurements i had to ask people to stand away from a
   optical scale as their
   weight standing on the floor was making the floor go down distorting the
   optical scale reading

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    20.   12-15-2014, 12:37 PM  #20
   <http://www.practicalmachinist.com/vb/general/making-accurate-straight-edges-scratch-john-swensen-295849/#post2439639>
     *Richard King*
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   I do that with the classes Tom, We put a level on the granite plate and
   have the class crowd around behind the plate. Have one student watch the
   bubble..usually a Starrett 198 .0005" / 12" and then we all walk to the
   other side of the plate and the bubble moves. It is so interesting when you
   share the stories of where you worked. Thanks. It would be a great service
   to all or to the history books if all of the machinists were to share all
   these stories so younger generations can read about them. Have to Thank
   Milacron for giving us this forum! Rich

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