expected work time for machinist exersize of filing a cube flat? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 03:06:17 am
content: I am working on an aluminum block which I cut out with a hand aluminum type nicholson file.I have been working on it for around 3 hours with draw filing and it looks pretty good surface finish wise, but if I balance something on top of the hump, (old sine bar) I can still see it rock a bit back and displace oil. I can slide paper under the bar on top, and the square lets a bit of light shine through.It's good enough for what I need (decent appearance) , but if I wanted to do the old machinist exercise of hand filing it as flat as it can be, how many hours would it take? just curiousI heard its something they used to make tool and die makers do way back when (and maybe the military mechanics branch still teaches this)Also, at what point do I need a non flat curved file and indicators vs just draw filing (the limits of dragging a standard flat aluminum file on it).It's dimensions are 3.75x3.75x2 inchesWhat I did is draw the file at a perpendicular angle across the axis I want to draw file it, so its scuffed up, then draw file and watch the scuff lines slowly dissapear.However, I noticed that while the middle looks flat, if you do a light inspection its still kind of a hump. I think the file I am using is too thin and is bending (so I would need a convex file). It also appears to take exponentially longer as the flat area is increased in size to get the corner marks to file down.I don't mean to make it square, just so the sides are flat (making it some fucked up quadrilateral shape). I also don't want to put it on my surface plate to try to measure its flatness because it is unworthy.

author: Gyro
author_rank: Super Contributor
timestamp: August 03, 2019, 03:07:20 pm
content: A lot of your recent questions would get a much better (ie. some) response on a mechanical engineering / machinists forum.

author: mc172
author_rank: Frequent Contributor
timestamp: August 03, 2019, 03:40:21 pm
content: I was made to make a C-H block when I was an apprentice with nothing but a file. The blocks were given to me roughed out on a bandsaw, all sides sawn.A C-H block is actually two blocks, one C-shaped and one H-shaped, and the C piece must fit in both sides of the H, in every orientation possible. The C is a 2x3x1" block with a 1" cube filed out of it. The H is a 3x3x1" block with two 1" cubes filed out of it. In imperial of course, because you've got to suffer the imperial system before you graduate to the much more sensible metric system.The point of it is not about skill, since you haven't even got any of that yet - it's about seeing whether you have the mental ruggedness to just get on with it despite it taking forever even when the bloke standing next to you is allowed to use a milling machine to remove material quickly. The drawing specifies something absolutely ridiculous like +/- 0.02 mm (or whatever in imperial) with flatness, perpendicularity and parallelism requirements all over the place.After a good few weeks of whittling away and thinking that I'm coming close, they stopped me working on it and put me on a job that made them money. I don't think they've ever had a finished block.I would say chuck it in the scrap bin and do something infinitely more useful like learning how to play a recorder with your nose.

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 04:05:56 pm
content: hmm,I understand those are typically made with steel, and I am working with aluminum,3 weeks is awfully long though, but I guess it will still take a very long time. I was wondering if my technique was off lolIt's just a base for something, to be fair draw filing actually gave me better results then trying to sand it down but takes much longer. That sounds like its way past 100 hours of work.difficultly level:

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2019, 04:34:56 pm
content: Neither hand sanding nor filing can ever get a totally flat surface, as either the work or the file or abrasive will always rock slightly, so it always ends up convex. The only ways to get a good flat surface manually after youv'e done the best you can with files/abrasives are *either* by using engineer's blue (e.g. Dykem) and a reference flat surface to mark the high spots, then hand scraping them. 'Rinse and repeat' until the high spots are as small as possible and uniformly and randomly distributed over the whole surface, *or* by lapping three flat surfaces of the same material, equal area and similar shape against each other in turn. If you only use a pair of surfaces you end up with a pair of closely fitted convex and concave surfaces, but the only way three surfaces can mutually fit is if they are all flat.For a good enough reference flat for blueing, set a sheet of float glass in plaster of paris (to support it so it doesn't bend) on a concrete slab. Don't use any sort of hardened, toughened or tempered glass as they are very rarely flat.

author: mc172
author_rank: Frequent Contributor
timestamp: August 03, 2019, 04:36:15 pm
content: Most of the time is spent going off to take measurements and deciding what to do next, rather than the filing itself. And wondering why you chose engineering or even woke up that morning.With some practice you could probably flatten your block within a few hours. I thought you might have had plans to go and do the other five sides, hence why I didn't just answer the question.The first part of the problem is knowing which way to go. Which parts are high, and removing those parts. That's all there is to it.I have found that the best way of filing something flat is to take cuts in one direction only and then measure it, whether that is with an engineers' square or blueing up a surface plate. Then, take your next cuts in a different direction. You can then see where you have removed material and where you haven't even once the blue has come off.You need to have a range of different files in terms of tooth coarseness or fineness, so that when you get to the point when you need to take a small amount off, you can do so with a very fine file.I would only choose to do this with a rectangular section file, with completely parallel sides. It's called a "hand file" but that doesn't describe it very well, since they're all hand files, right?If you are suffering from what appears to be a bending file, just apply less pressure and take more cuts. It could also be that you're rocking the file a bit as you take your cuts.Use a wide file, rather than a needle file. You can control which side of the file cuts when you get good at it. About as wide as your thumb would be reasonable for your job.To go the whole distance and make it as flat as possible, you'd need to scrape it, which I find really satisfying to be honest. It can look really good if you get the technique right and has a real, practical purpose, too.

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 05:29:17 pm
content: to clarify its aerospace scrap from a bandsaw or something that I cut into shape with a battery metal saw and filed. The worst measurement is 0.01 inches off right now but once polished it will look good enough, so your spec of 0.02 is 12 times tighterI was only thinking about going further for the experience which sounds like its unpleasant (and my mind starts going in strange directions too while I am doing the filing and checking with square etcmaybe I will try to take it further based on this information but it feels a bit depressing, I would rather hunt t-rex in a mechwarrior then with a hatchetmaybe I will try to 0.1mmneed blue though

author: KL27x
author_rank: Super Contributor
timestamp: August 03, 2019, 07:05:03 pm
content: QuoteMay be true for that company, but another purpose is what you and IanM have explained about getting the surface truly flat.Even in Coppercone2's own words:QuoteSo, Copper, you have already learned part of what is meant to be learned by this exercise. If you extrapolate, you can imagine that you can't get any surface truly flat with a flat file. But given certain tolerances, you can meet that with a smaller area cuz... given a certain radius you can get it to be "flat to within 0.02mm, provided it is small/short enough," same way float glass is flat... but if the piece of glass were 25,000 miles long it would have a hell of a curve to it. This is only partly true, though. You can actually get a small piece of metal flatter than the larger piece, using the same file, even adjusted for the length.*No, you don'tneedto scrape to get it flat. Any method you can reasonably think of will work, so long as you are identifying the high spots and working towards flat. As mc172 has suggested, you can preferentially focus pressure on the end of a file. Or you could use the end of a machinist abrasive stone (Like the silicon carbide "sticks" that some folks try to use like a file; most of the time when working with flat surfacs, these are more useful to use the end like a pencil, which the end quickly wears into an angled bevel) to selectively remove material. Essentially this is using a relatively smaller abrasive/cutting surface to selectively work down the high spots.The reason lapping will get pretty near flat vs a flat file is actually significant, if you give it enough thought. Yes, there's the caveat that the lap will also wear, thus potentially resulting in spherical convex/concave surface. But provided the lap actually stays flat, the lap can automatically make a surface much flatter than a flat file. Why is this? If you can't figure it out, I can elaborate; there's a very simple and obvious reason for this, once you put the pieces together.**Yes, convex files are a thing, and that is another way to get these surfaces flat. You still have to identify the high spots and whatnot. You can't just file away like you are doing now, and magically end up with a flat surface by using a convex file.QuoteTrue. But you can lap two surfaces together and get a near flat surface. You can do this even if the two materials/laps wear at different rates. It's a matter of technique.*, ** : These two things are related

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 10:05:19 pm
content: is it possible to weigh down the underside of a lap and spin it upside down on something with a flex shaft?say you make a bowl over the top so you can hold some hand pressure on it to control it a little bit. and maybe let it sit ontop of a nother lap?this is boring as FUCKthere is a few patches of nice polish coming along, and now i want some kind of giant gauge block thing

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 10:24:21 pm
content: or put it between two lapping plates, with the top one weighed down evenly, and glue some kind of strands, strings, rods, etc to the side of the piece being machined so it can be moved around in a figure 8 between two spinning plates?idk how people can do this shit, its so fucking boring holy mother fucker

author: KL27x
author_rank: Super Contributor
timestamp: August 03, 2019, 10:27:26 pm
content: By all means continue to not learn anything.I'm sure the purpose of this exercise was to come up with this:https://youtu.be/6F0JF5bwa10?t=597Quoteok... instead of polishing these patches over and over until they spread and cover the entire surface, why don't you do what has already been explained and work those shiny (high) spots down, first? Using a scraper or localized abrasive you can take those down faster than trying to polish the entire surface to flat. Unless you want to stop and build a motorized lap.Forgive me for posting this, again. I feel like it is too appropriate to pass up.

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2019, 10:31:38 pm
content: its alot of work to smear ink and scrape this shit all day

author: KL27x
author_rank: Super Contributor
timestamp: August 03, 2019, 10:46:31 pm
content: QuoteAnd how do you think large metal mill beds are made to be straight? Do they just mill/surface grind them in a larger mill? How did anyone make that one?Scraping may take a long time, but if you do it right, every minute you spend scraping can save you many more minutes of lapping. You are not trying to scrape the high spots perfectly flat, you are just hatch marking them with scratches/scallops, so that when you lap it, the "exponentially slower" thing is reversed. You can go back and forth with it. In larger interfacing pieces, the surface finish might not be important, and you can just purely scrape the surface in. But we know you want this to be nice and shiny all over without any low spots.If you go too far and end up with a slightly concave surface, by scraping or localized sanding/abrading, ok then. Great, actually. That will lap out easier than convex. The convex surface will be harder to lap without inadvertently rocking, and you can end up taking 2 steps forward, 2 steps back, chasing your own tail. Doesn't matter if you do figure 8's or figure 9's.Make it flat, first, then make it shiny. Once you start seeing that shine you seem to get fixated. Going from duller to shinier is not necessarily the quickest path to getting to your destination. From your posts is seems certain that all you are shining/lapping, endlessly, are the spots that need to be removed. Attack that high spot. No, it's not automatic. Hand eye, judgment, and intermittent measuring/referencing are involved. Deal with it. At least it will give you something to do rather than bore yourself to death whilst chasing your own tail. If it's taking too long, you can afford to get more aggressive!? Hell, if you can't scrape and lap a 3x3" aluminum surface flat as an afternoon project. You're doing it wrong.Forget scraper. For this big a piece of aluminum, all you eed is a piece of coarse sandpaper and your hand to get a major leg up. Fold the paper over and use 2 or 3 fingertips to back it. Put some oil on the block and start sanding. Spend the most time in the center, spend the least time near the edges. And stay completely off the very edges; in fact, you might want to mask them off with tape. Do that a little while, then take to your lap, and just lap it a little bit to see where your high spots are. Repeat until your lowest spots are shallow enough. Again, you're making a judgement call. You might want to gauge your lowest spots to see what you're actually up against. When you think you finally have a bite small enough to chew, you can start lapping for real. Starting with the most aggressive grit that doesn't leave new low spots (here, the major concern will be rounding of the edges of the block). If you have very coarse marks/scratches completely saturating the surface where many/most end up the same depth (grossly flat but very rough), this will lap faster/easier than 90% flat with only a few area with small low spots. So you can periodically stop lapping to use the most aggressive method you can to rough up any large flats that have developed but in a way you won't accidentally create new super lowest spots.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 03, 2019, 11:26:59 pm
content: Quote from: coppercone2 on August 03, 2019, 10:31:38 pmbuy cnc machine...Quote from: KL27x on August 03, 2019, 10:46:31 pmmaybe a tool like this may help (if no larger reference flat surface available)otherwise for small piece...i can think of other method and techniques if flat surface is not available, one classic element to get straightness is using thread + with other stuffs but, i dont think i'll elaborate since its not for the faint hearted.

author: KL27x
author_rank: Super Contributor
timestamp: August 03, 2019, 11:38:08 pm
content: ^In the bottom video, lapping with sandpaper over a surface plate. Notice that you have to have a slurry of particles for this to work. Silicon carbide sandpaper fractures and makes a slurry with oil/water, so it will work better for this than trying to use alum oxide paper. If you use alum oxide that is firmly bonded to the paper, it won't work as well. A more extreme example would be a diamond plate. If you tried to "lap" your cube on a flat diamond plate, it will produce a convex surface (once the surface area of contact exceeds the point where the file can cut that specific material for the given force you can apply), just like you get with a flat file. This is technically no longer "lapping," but filing with a large file.Take a perfectly flat and polished (and rigid) aluminum surface of maybe the same size you have there, and rub it over a broken in, perfectly flat (and rigid), fine grit (fine enough... for the surface area we chose in the first place) diamond plate. You will only produce scratches around the edges of the surface. You'll scratch all the way around that border, leaving a rounded bare spot in the center. You can't put a scratch in the center of that flat spot. Unless a rogue diamond breaks off and rolls around. Or unless you gain 1000 lbs. Or until you've abraded away a wee bit of a radius onto the rest of that surface.

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2019, 02:40:25 am
content: its clean and polished because it will go on a surface plate (the bottom is flat enough now) and I want it to wipe down easily when I clean the plate and not accumulate bullshit (yes I checked with a fowler digimax)

author: Rerouter
author_rank: Super Contributor
timestamp: August 05, 2019, 03:27:09 am
content: It comes down to how small a tolerance you need. When i needed to file my 3d printer base plate flat i was just using the corner of a metal ruler over points around the circumference to mark the highest points via the light through the gap (if it pivots anywhere but the edge it is a high point. Otherwise file down the edge)While i did not have anything to measure it. I can say its within 0.1mm due to the uniformity of the width of a printed line.Took me about 4 hours to get a quite concave 30mm diameter aluminium plate that flat using that method

author: KL27x
author_rank: Super Contributor
timestamp: August 05, 2019, 11:06:34 am
content: I lapped a 4x6" translucent arkansas stone a few years ago, starting with a raw chunk of rock. This isn't something I have ever done before, and there might be a better way. But I got 'er done.I sawed around the stone the 3/4" deep or so I could reach with a diamond blade tile saw, then cracked the top off, which didn't go so well. I nibbled away the gross deformities with the saw, basically blind, holding the stone over the saw. This wasn't even close to flat, at this stage, but it was going the right direction.What I did to make it grossly flat was set a board over the saw so only a few thousandths of the blade poked out at it's high spot (not concentric at all). I passed the stone over this, rocking it to get it to cut, but always avoiding the corners. Eventually the corners were the high spots, and it would sit still. Then I could slide the stone over the board/cutter, still avoiding the corners, until it stopped cutting anything.I dunno what it would have looked like if the blade ran true without wobble/orbit. But the result from this left the surface completely covered in little consistent craters/fractures, best I can measure maybe 7-8 thou deep. This lapped out in relatively no time. A few hours spread over an afternoon. The flat spots came in all together.After making it perfectly flat, I later changed my mind and convexed it. :0 It was sometime after doing this that I discovered how useless it is to have a flat "file" this large and fine. I'm pretty sure the folks that have any use for a flat-lapped hard hone like this use it with lapping paste of some sort or intermittently scuff up the surface with a diamond plate. With a bit of a convex surface, it actually sharpens things just the way it is.

author: duak
author_rank: Super Contributor
timestamp: August 06, 2019, 05:17:09 pm
content: There are things not mentioned about get good results with just a file:1.) get the right file - rather than draw filing, get a file with wide, non-interupted teeth, ie. not double cut, I have a "Flat" type with curved teeth that leaves a remarkably smooth, flat finish. The chips are more like tiny tendrils than powder. Files wear out and get dull. For best results on aluminum, don't ever use it on steel.2.) make up some hard steel guides to support the file. You can use two hardened steel shafts clamped on V blocks on either side of the workpiece and then use a height gauge to set their planarity. Files are high carbon steel and are harder than just about any other type of steel. The file should slide across the shaft and not mark it. Just a note, most bench vises have hardened steel jaws that can be used as cutting or filing guides. They will work as filing guides but their planarity isn't guaranteed.3.) aluminum sticks to a file, and causes it to ride up. Some alloys are worse than others. Use chalk on a new or cleaned file to prevent this.4.) get a file card or wire brush to clean the file when debris starts to build up.5.) don't drag the file back over the workpiece as it breaks off the fine edges of the file.If you want flat aluminum with parallel surfaces, look for Tooling Plate. One brand name is MIC-6. It's cast and ground flat and parallel. It's not particularly strong, and doesn't cut or machine easily because it smears, but it is stable.On a related note, if you are into making precision prototypes have a look at Dan Gelbart's videos. Dan is a friend and colleague and I learned a number of things about materials and techniques over the years.https://www.youtube.com/user/dgelbart/videosDan's grinder video is quite interesting because he used a number of semiconductor fab pieces to make a CNC grinding lathe. He used it recently to help me modify a part for an out of production auto transmission.

author: coppercone2
author_rank: Super Contributor
timestamp: August 06, 2019, 10:13:48 pm
content: I am actually a b ig fan of dan's lathe that uses air spindles, I watched all those videospart of why I wanted the kiln so bad too, but you don't need to pour anything to make the braze joints

author: duak
author_rank: Super Contributor
timestamp: August 06, 2019, 11:13:58 pm
content: Dan is an amazing person and has developed a number of interesting things over the years. I wish he would make a few more videos about them but then he wouldn't have time for the new stuff he's working on.

author: eKretz
author_rank: Frequent Contributor
timestamp: August 08, 2019, 07:22:52 pm
content: Holy cow. I didn't read all of this, but having done this exercise, I will relate a trick that may be very helpful to you. Working to create a flat surface and ending up with a convex one is a given if working by hand like when using a file. Without a guide for the file, this will happen pretty much every time without fail, that's just the byproduct of the mechanics of the human body and the way it moves. When I did this exercise we weren't allowed to use file guides. You got a file, a micrometer and a vise - and no you could not use the file jaws as a file guide.Once you are close to flat, use a narrow file to remove the center portion of your surface so that there's a dip or relief in the middle. Now your file will rest on the two high points, making it far easier to create a final flat(ter) surface. Create that first flat surface, then work from there. Hold the opposite side parallel, then use a square for the next side and hold its opposite side parallel. Rinse and repeat.

author: KL27x
author_rank: Super Contributor
timestamp: August 09, 2019, 01:54:01 am
content: QuoteAll excellent practical info, but forgive me for one nitpick. I assure you that it is more than body mechanics at work. When you exceed a certain surface area of contact for the hardness of the material and bite/grit, a flat (and rigid) file can no longer cut in the center of that contact patch.Even if you have a large flat file, 1 foot wide and 1 foot long, and you carefully rub a large, stable block over it that begins perfectly flat (and you never let the workpiece go beyond the edges of the file)* it will immediately start becoming convex unless you stop and cut your losses. To cut, there has to be a minimum pressure for the bite/grit of the file. And we can only push so hard. In this example we are intentionally exceeding the limit where we can cut the entire surface. Theoretically, if the workpiece were perfectly rigid and riding on rails that were perfectly rigid, and you were limited in downward force to some maximum, you might have the situation where the block just slides/burnishes over the file and it doesn't cut anywhere. But in practice, when you're getting around and beyond the point where the file will cut that amount of area with the given force, it will abrade all around the flat face, but not in the middle. If the cut face is circular, you will leave a perfect circle in the center that won't scratch. If the part is square, you will get a square with rounded corners in the center. Hence, if you continue filing, the face will become convex, automatically. And vice versa, as you approach flat from the other direction, you will peak once you reach that level of arc/convexity. You can't get any flatter without changing your technique.A filing robot couldn't do it, either, not without resorting to some tricks.*There are ways around that, by using the edges of the file. Esp if you are using a cornering file that is flat but is rounded/tapered on the side. By concentrating force towards that curved edge, you have that "reach" and local discretion. Or by pressing/bending the file to use just the tip. But the overall practical point is that unless you're lapping, achieving true flatness often takes some measuring and judgement calls and fudging rather than just using guide rails and jigs.Hogging out a little in the center it a great idea. It not only gives you two points to rest the file across, but it also removes some of that surface area of contact, allowing the file to cut where you need it to.Lapping is different. Lapping is slower. But the particles roll around, and a certain percentage of the time they happen to create few enough random high points to actually cut in any given place on that face.

author: eKretz
author_rank: Frequent Contributor
timestamp: August 09, 2019, 12:25:41 pm
content: Yes, that is so, but there are ways around your problem also. One is intentionally bowing the file by applying a torque to convex the filing surface. (Nothing in this world is perfectly rigid, it is actually very easy to flex even a 1/4" thick file). Another is to apply pressure with one's fingers not at the ends of the file but in the center where it is making contact with the work.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: August 09, 2019, 02:11:26 pm
content: Quote from: KL27x on August 03, 2019, 10:46:31 pmThey might put moglice (some polymer compound that hardens) on one side, but the other side needs to be flat anyway.The question for how fast your methods work depends on if you adapt the methods to the precision you want to achieve when the time is right. At some point filing brings you nowhere nearer or takes you longer than the next method, so you get to sand, grind, lap or scrape the workpiece.

author: Rerouter
author_rank: Super Contributor
timestamp: August 09, 2019, 02:56:09 pm
content: a larger grinder can be built using a smaller one, but it takes a lot of careful measurements to make sure your keeping at minimum the same level of flatness for parts over the normal travel. it generally involves more rigidity,and as things get longer you need to pay more attention to the center of mass of parts,

author: KL27x
author_rank: Super Contributor
timestamp: August 09, 2019, 10:20:10 pm
content: So you can take this machine apart and put it in Copper's Proxxon milling machine to true the ways? In some kind of custom vice? Then you just move it over and somehow re-level exactly right for every X inches of the ways that you flatten? You will end up with a nice straight surface that doesn't need scraping?https://youtu.be/3N20fttRgKM?t=565(At this angle, you can see the surface very clearly; it looks scraped in, to me. I imagine someone spent a lot of hours "smearing around ink and shit.")I mean, yeah, you want to get it in shape as fast as possible. But I think you will end up with hand fitting/finishing if it actually needs to be straight, because that will be the most efficient way to do it... At least until the world needs a crap ton of these things, then someone will build a machine to build these machines.Less than 100 years ago almost all very low tolerance precision steel parts requires hand-fitting. Even the small ones.QuoteA lot of the guys that just spent 20 years in a classroom taking computers and CNC machines for granted have a funny way of "machine thinking." Thinking that anything precise has been made by a machine or process, and humans can only screw that up. This guy is gonna try to let the flat file make things flat and will assume any non-flatness (in particular the very slight convexity that is actually what you automatically get) is because of "human biometrics" or lack of repeatability/consistency. They may think they need guide rails or something equally stupid. They may be adverse to utilizing these "tricks" because of this way of thinking. They are still trying to remove sources of human input/error, thinking that is the problem. Knowing that the convexity is natural and not human error is part of breaking through this way of thinking. Human input can improve this result. Just gotta get over it. And the fact that the surface finish is gonna get screwed up in the interim. It's gonna get worse in that regard before it gets better.It's like trying to cut a thick piece of stock with a saw with too many TPI. It won't do it; at least not if we need the cut to remain in a single straight line. When the span of metal is too big for the file, we might use a "file" with one tooth called a scraper.

author: coppercone2
author_rank: Super Contributor
timestamp: August 10, 2019, 12:24:41 am
content: I kind of wonder if you can embed a benchtop grinder into a giant block of epoxy sand with a table underneath to make a surface grinder. Perhaps if you managed to stick a bearing on the existing shaft and build a structure around it out of glue to stiffen up the shaft without alignement.Or like a polishing machine since they have extra long shafts. Is it possible to reinforce something with multiple bearings?Granted it would not have serviceable parts. The shaft on the grinding wheel probably needs to be way thicker right?

author: eKretz
author_rank: Frequent Contributor
timestamp: August 10, 2019, 01:11:00 am
content: Yes, if you need real precision, scraping is the only way to get there. It is possible to achieve better accuracy with scraping than any machine tool can achieve. However, the OP was only inquiring about a filing exercise, not how to make the most precise square block possible.

author: KL27x
author_rank: Super Contributor
timestamp: August 10, 2019, 01:13:40 am
content: QuoteThe part where he suspected it was very slightly convex, and explaining how he determined that with a straight edge, though. It's like he doesn't even trust his eyes and straight edge and doesn't understandhow that would even occur. Me is telling him. (Well, I'm telling anyone else reading the thread, since copper never acknowledges learning anything new in any of his thread.)If you trust the machine/process to make a flat surface, you will instinctively equate surface finish with flatness. The most regular and consistent finish you can make across the piece of aluminum that large with that given file will be slightly convexed. Therein lies Copper's Conundrum. We knows he didn't have a problem filing the basic shape out. He was reading the newspaper upside down in the reflection and wondering why it wasn't perfectly flat.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: August 10, 2019, 02:21:55 am
content: I think you got the title wrong: it is rather a masochists' exercise.

author: coppercone2
author_rank: Super Contributor
timestamp: August 10, 2019, 02:29:20 am
content: yea its unpleasant