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What's the best way to make a hemispherical 4 foot radius concave Mirror?

I want to make a two piece reflective (on the inside) sphere big enough for a person to comfortably stand in. What would be the best way to go about this?

Question by jcox12    |  last reply


Need Concave Wheels 3/8" Wide.?

Basically it needs to be 3/8" Wide at the concave part, and preferably from amazon. Can't find anything that wide though. Help please? I need it soon, to meet a project deadline for school.

Question by Batryn    |  last reply


Where to find biconcave lens?

I want to make the google cardboard VR, but I cannot find biconcave lens. Is there any household thing that contains biconcave lenses with about 45 mm focal lengh and can the spectacle lenses be used for this project? Also buying online is not an option for me.

Question by Ahtasham Ahmad Mohtashim    |  last reply


WANTED!! large frensal lenses, parabolic lenses(concave), and projection TV's

I need some parabolic lenses, frensal lenses, and projection TV's to make a death ray from thanks

Topic by ostomesto    |  last reply


Sharpening a concave blades like Kukri or similar

Every now and then you have someone approaching with the odd job.This time it was in the form of an old Kukri / Gurkha knife.Wasn't expecting this when I was asked a few days ago if I could sharpen some old knife so it can be used for camping.The knife had a few marks from hitting hard stuff or maybe the occasional nail.But the worst was that for as long as guy had that knife it was only "sharpened" using a belt sander.You know, these tiny machines advertised to give your (kitchen-) knife the perfect edge.We could now argue about the pros and cons of having a knife edge that is literally rounded.But once it was done so many times that the edge really looks rounded it becomes obvious why this method only works for thinner blades.Adding to the problem was the fact that the belt used was just over 1.5cm wide.Appereantly so it is easier to do the concave part of the blade.Lets just say lengthwise it looked like someone created a wave pattern LOLThere was nothing "straight" on this nice blade anymore.Now, if you look up how to properly sharpen thise Kukri knifes then you can find all sorts of really useful tips.One I really likes was to use some eraser and toglue sandpaper on it.Small and flexible enough to cause minimal damages to the curve towards the handle.Another nice one is to use half round diamond file, preferable of a finer grit in the 600 region.Should work fine - if you plan to invest an awful lot of money on such a file.I however like things quite often done the old fashioned way.The oldest trick in the book....Whether you are using chisels and work on wood, just love to keep your knifes sharp or go on long camping or hiking trips - sharp knifes and tools just become your thing.The main thing everyone tells you is a super flat surface for whatever is supposed to sharpen your blade.For the normal stuff that is fine and good and you only need to flatten out your stones every now and then.But what about these odd jobs?Imagine you would need to sharpen a long paper cutting blade on some machine.Might be over a meter long and it has to stay with a perfectly straight edge.Back in the day this task was not done with some very expensive stone of large size....Instead sandpaper of various grit was used on a flat steel surface.I actually prefer a small pane of glass and tape my sandpaper on it.Hard to find anything finer than 1000 or 2000 grit but you might be surprised how well this stuff polishes onces clogged up a bit.Its all about the right level of wetness...Anyways, for our Kukri in question I decided it is time to do the same but in a way that does not harm the blade, constantly cut into the eraser and still is solid and "flat".If you still work with a sickle then you already know where I am going here ;)I used a small diameter spray can as my surface to hold the sandpaper.Of course a piece of PVC pipe, round wood or similar would warok the same way....Sticky tape does not work well with sandpaper unless you use double sided stuff.But it is enough to wrap one round on the top and one on the bottom of the sandpaper on the can to hold in place.So much for the basics....If you know how to sharpen a knife then you also know that there is a prefered way of doing it.Depending on the blade and stone in question you literally try to cut a thin slice out of the stone with every stroke.Either stright or with a cutting motion.This works fine with sandpaper on a flat surface, not so much however on a round surface.Try it and you see how you cut off the sand from the paper and constantly ruin your edge.The only way to do it is to move with the edge.You start from the heel and stroke to the tip.The can is used likea sharpening rod and shall always stay at a 90° to the curve of the blade.Takes a bit of practice to find the right grip to hold the blade while moving and twisting the can but well worth it.The rounded surface only allows for a very thin area of the sandpaper to work on the edge.I started with 120 grit!!!It left a trail of destruction on the edge, at least in the rounded up section....Once I only had a very thin bit left on the edge from the old sharpening I switched to 240 grit until a flat edge formed.As the Kukri was a disaster this process still tok over 4 hours to complete.That blade was properly hardened too...The start of the finnishing was done by jump right to 600 grit paper.The first can was just slightly smaller in diameter than the concave bend in the blade - perfect to smooth out those nasty bumps.But with a burr forming now on the edge and minor mishap with angle of the can towards the curve of the blade would mean cutting into the can while sharpening the concave bit.Meant I used my emergency insect repellant can as I did not like the idea of hoping my pepper shaker would start leaking while sharpening ;)If you blade is not too damaged you can of course start right away with a smaller diameter.The process is the same as before.Move along the blade and keep the can at the 90° angle towards the curve.Once you feel a burr forming on the side turn over until you have a bur on the previous side again.Repeat until all the marks from the coarser grit are gone and the edge has a uniform shine.Switch to a finer grit and go as high as you can here.I had to stop at 1000 grit as my supply of 2000 and 4000 grit is out.Hints and tricks along the way....It really helps to do this sandpaper sharpening under running water.The paper won't clog up, you won't risk a losse grain making really deep marks...But on a bad blade this can take several hours and would do it with a small aquarium pump or so and some gloves.A fine but stiff brush and soapy water however do wonders to clean up used sandpaper!I prefer to use these re-used pieces before switching to a finer grit.In most cases they are already finer than the next grit and create a nice polish that makes the visual confirmation of your right angle and angle of attack easy.A kukri is a working blade!It is mot meant to make fish filet or shave you legs.It is somewhere between axe, big bowie knife and hatchet.That mean if you would dare to give a 8° angle either side of the edge you would have a pretty damn wide edge...Stick to the original in width but keep it nice and flat.It is good compromise between cutting sharpness and durability when for example chopping wood for your camp fire.DO NOT USE A BELT SANDER!!I said it before but have to repeat it again as there is people using a big belt sander with enough free space to add a set of wheel that creat the curve I got from my spray can.The guys in India that make these knife do this blind folded....It takes years of practise to get the steady hand required not to cut through the belt.The beginners start in reverse, meaning the belt runs towards the edge.These guys only to the basic forming of the edge with really coarse grit.Basically to remove the marks from the forging.After that the pro takes over the blades and he has the belt running towards the edge!If you are silly enough to try it at home be prepared to have the belt flying in your face very violently!!The reverse sanding can't be used to finnish a blade as you never get a proper sharpness and flatness right on the edge.So just stick to manual and take an hour or so longer but then be able to enjoy a cold drink when done.You need surprisingly little sandpaper in terms of clogging up and getting useless until you get to the finer grits.If you use a wooden dowel or similar then make it a bit longer and add strips about 6cm wide of sandpaper.This way you have all the grits you need in one place and can take them with you to keep your blade sharp ;)If you glue it onto the stick it is also quite easy to give it quick brush clean when done.The really tricky part starts from about 800 grit onwards.Every mishap on the concave part can mean damage to your paper or to your edge.When using stone most beginner think that using a lot of pressure is a good way to remove the material quickly.In reality however it is just a sure way to wobble the blade over the stone, especially if the blade is not fully straight.Sandpaper can be more aggressive than your stones as in our case you only work with a little area and every time you turn the can only a little bit you have a fresh piece of paper working instead of a slurry building up.This mean you really do't need much pressure at all.It is the repetition, not the pressure that gives you the edge if you don't mind the pun here. ;)For a real working knife stopping at 1000 grit once you do single strokes either side of the blade is sufficient.The tiny burr left will disappear quickly during use and the Chakmak can be used for a quick refurbishing after every longer use.Should mean you only need to get the sandpaper out once you edge actually started to get blunt again.The final stroke....There are those people that don't have a kukri to go camping...Some people like to collect them.Restoring an old kukri can be done like with any other knife.That is until you want a razo sharp edge that is also highly polished.This is quite possible with the original edge width on the kukri.But of course you can only go so far with sandpaper....Modern technolgy provides us with the solution in several options.Firstly we have the ceramic sharpening rods.Unless you can do with kitchen variety thickness you need to pay a lot of money.A short 8cm diameter rod can set you back over 100 bucks with ease.Especially if you want something that provides a mirror like finnish.And alternative that is often available relatively cheap is a ruby rod.They can often be found with slight damages that make them useless for laser applications.Even burnt out rods are still fine as long as they are not cracked.It is quite hard (literally) to give them a satin finnish but I found that good quality sandpaper is sometimes capable of doing it.I like one side smooth and the other half of the rod with a satin finnish to prepare the edge.On the budget there is quality wet and dry sandpaper as commonly used in paintshops.If used dry the finer grits tend to clog up on such a wide edge.Once you have a piece of 1000 or finer grit that is fully clogged up you can use to give the edge a final polish.With this you won't even need a leather strop anymore but as said it takes a lot of practise so you won't cut the paper in the concave area.The steel rod....If you happen to have a hardened steel rod, like from a motion rail, small drive shaft or a big drill then give it a try.When using a drill:Of course use the end of the drill, not the working part ;)Also make sure it really is motth as any burr from the chuck or such will cause deep scratches on your blade.If it starts to feel sticky after a few good stroke you know the drill method is working.If it continues to feel very smooth and you don't see any polishing effect at all if tried on a small area only then you blade is of really good quality.But then again you would have confirmed that already by the ongoing swearing during the endless hours trying to remove some material from the edge...A word of advise for the first time user of a kukri:Although a good kukri is hard to damage without hitting a stone or metal, you can make blunt very quickly.It is top heavy blade and requires a steady hand when working on other things than meat.Chopping into some wood and letting the blade slip can deform your edge.A little mishap can be fixed with chakmak but not if hit hardwood badly a couple of times.And tempting as might be to use it as a small hatchet or axe to split your kindling:Never hold a piece of wood and then hack into it from the top with your kukri!Not only can you miss the wood and hit your hand, the wood can also split far easier or in unexpected directions!If the kurki is sharp you then have a good chance to loose a finger or two!

Topic by Downunder35m  


Do the flatface g11 and g12 decks usually have concave?

I just got my g12 in the mail and it has pretty good concave. It has about as much as my berlinwood elias assmuth. The reason im asking is because my fiend has a g12 and it has little or no concave at all. So i want to know if the g12's usually come with concave?.

Question by GianniMora  


Building Longboards?

Has anyone made their own longboard? post pictures of press and board!! here is my homemade cruiser! it has a tiny bit of concave....lol

Topic by TeacherOfTheWays    |  last reply


Laser focusing and colimiter

I want my high powered laser (that i am going to make ) to be super powerful and if i understand this theory right than i should be able to keep it in a focused beam correct?description:first the high power laser is activated shooting out the beam- then a concave lens is placed in front of the beam- at the most focused point you put a flat lens not at all concave then the laser will be shot at its most focused capacityVisual: \ l ==========\ ====--- -- -- --- l / l beam continues at focused rate / Correct or in correct if incorrect is there any way to do this?

Topic by HOMEPIE64    |  last reply


Green Tourmaline Crystal as a natural battery- Answered

-  I know that a Green Tourmaline Crystal, when heated, will produce a negative and positive charge  at respective ends of the crystal-so, if the heat source was a  super glass lens ( convex or concave, i am not sure which would work most intensely, with the sun going through  the lens and hitting the crystal-  ( for the ecologically minded)-or a laser- or microwave blast,  would we then have a natural battery?  It may have to be a  large crystal!

Question by kathleencleans    |  last reply


Help/guidance needed: Seven colour laser spirograph with lens'

So I'm trying to make a seven colored laser Spirograph, with lens' (that turn the beam into a crosshair for example) and I can't for the life of me work out how to make it work properly. I can design it so that I either have the lens or the Spirograph, but I cannot make it so that the Spirograph fits through the barrel of the lens thread. I think my best option might be to have two holes in the front of the device or use concave mirrors, what do you guys think?

Topic by 5225jackiechanman    |  last reply


PINHOLE CAMERA Q? - I am trying to build a pinhole camera with multiple pinholes that still takes one seamless image.

The final version of this camera will be used to take photos of people's bodies. I want to create a "topographic map" of the body, as though that person had been split down the spine and unfolded to view in full. If you have ever seen a large snake skin framed, or the saved skin of a person with a traditional Japanese tattoo, you might have an idea what I mean. I have built many cameras, but never one that is concave or with multiple pinholes. Silhouetting is fairly easy to eliminate, but creating on image with multiple pinholes is what is stumping me. Frustration mounting...

Question by draweli    |  last reply


57-Storey Death ray

Concave, polished, and 57 stories tall - the Vdara Hotel in Las Vagas could be the world's largest death ray... Reports are cropping up around the web that the smooth curved surface of the hotel focuses the Sun's rays into a patch around 10 feet across in the pool area, soon after noon. Fortunately, the glass disperses around 70% of the solar energy falling on it, otherwise lawyer Bill Pintas might not be here to tell the tale; After a brief dip in the hotel pool, Pintas was sunning on a recliner. He was on his stomach, relaxed, eyes closed. But suddenly, the lawyer became so uncomfortably hot that he leaped up to move. He tried to put on his flip-flop sandals but, inexplicably, they were too hot to touch. So he ran barefoot to the shade. "I was effectively being cooked," Pintas said. "I started running as fast as I could without looking like a lunatic." Then he smelled an odor, and realized it was coming from his head, where a bit of hair had been scorched. It was about 12:20 p.m., as best Pintas can recall. The hot-spot, which sweeps the entire pool area during the day, spikes at least 20 degrees above ambient temperatures, and has been seen to melt plastic bags and cups (made of plastics which melt at around 70°C (160F). Other guests, including newspaper reviewers, have also observed the burning beam. The hotel management doesn't call it a "death ray", they prefer the more friendly distinction "solar convergence phenomenon". Gordon Absher, a spokesman for MGM Mirage says the hotel is addressing the problem, and comments, "Because of the curved, concave shape of that hotel, they sometimes get isolated pockets of high temperatures." ---------------------------------- Never mind dispersing the light - get those windows polished up, and let's see how many lawyers a fifty seven storey burning glass can really deal with!

Topic by Kiteman    |  last reply


VERTICAL AXIS WIND TURBINE FEASIBILITY HELP

Guys need help to realise its feasiblity and pros & cons. This project will comprise of a renewable energy source which is controlled by TI msp430 controller by closed loop system. This renewable energy source will use a magnetically levitated vertical axis wind turbine with single phase permanent magnet alternator made by our team. There will be 3 stationary wind guide vanes/flaps of concave shape on the outer periphery of turbine. These vanes will be controlled by dc gear motor for constant wind flow and protection against high speed winds. Using perturb & observe MPPT algorithm on DC-DC boost converter, constant DC voltage for GRID integration is made. A lower energy Bluetooth module will send data on user computer about whole system parameter, hence making the whole system to be smart and hence data is saved in database. https://www.youtube.com/watch?v=fC5hYLjhO9U

Topic by nitcool    |  last reply


VAWT a new way to wind generation

Guys need help to realise its feasiblity and pros & cons. This project will comprise of a renewable energy source which is controlled by TI msp430 controller by closed loop system. This renewable energy source will use a magnetically levitated vertical axis wind turbine with single phase permanent magnet alternator made by our team. There will be 3 stationary wind guide vanes/flaps of concave shape on the outer periphery of turbine. These vanes will be controlled by dc gear motor for constant wind flow and protection against high speed winds. Using perturb & observe MPPT algorithm on DC-DC boost converter, constant DC voltage for GRID integration is made. A lower energy Bluetooth module will send data on user computer about whole system parameter, hence making the whole system to be smart and hence data is saved in database. https://www.youtube.com/watch?v=fC5hYLjhO9U

Topic by nitcool    |  last reply


how to make a rubber part that doesnt exist ?

Hi good peoples, i work at a place that makes and sells beef jerky and smokies. quite recenty one of the machines we use to cut them from links quit working properly and needs a new rubber pad for the clutch that stops the blade. the problem is the company we bought the machines and all parts for them it died with the owner so we cant replace them. my question is how hard would it be to mold rubber into a new brake pad for the clutch. i will put up pictures of the clutch and pad as soon as i can but i might as well start with the first problem how to cast and mold a rubber sturdy enough to do the job. all info on this would be greatly appreciated side note: now that i have the parts this is what they look like. two very soft rubber wheels with a concave treaded surface. the rubber tread is really worn down hence the need for replacement 

Question by snowfox222    |  last reply


Does anyone know of a compound slide for a metal lathe which is also capable of precise ball turning?

I have recently become interested in machining metal, and I am now looking at lathes to buy. However, the lathe I want does not come with a compound slide, which I consider to be a necessity for the types of things I want to make. Another necessity is a ball turner. Is there a compound slide with ball turning capability? If possible, I would like it to be able to cut convex and concave and also to use a small winding handle to control it's angle (like it uses to control cutting depth and position). I have considered making my own, as I know I am unlikely to find something that exact, but milling isn't an option - I don't have access to a mill or the knowledge of how to use one. I am willing to accept a quote for a custom made one, although my budget is limited (I'm 15).

Question by scarabeetle101    |  last reply


VAWT a new way to wind generation

Guys need help to realise its feasiblity and pros & cons. This project will comprise of a renewable energy source which is controlled by TI msp430 controller by closed loop system. This renewable energy source will use a magnetically levitated vertical axis wind turbine with single phase permanent magnet alternator made by our team. There will be 3 stationary wind guide vanes/flaps of concave shape on the outer periphery of turbine. These vanes will be controlled by dc gear motor for constant wind flow and protection against high speed winds. Using perturb & observe MPPT algorithm on DC-DC boost converter, constant DC voltage for GRID integration is made. A lower energy Bluetooth module will send data on user computer about whole system parameter, hence making the whole system to be smart and hence data is saved in database. https://www.youtube.com/watch?v=fC5hYLjhO9U OR YOU CAN SEE https://www.instructables.com/id/Building-a-new-type-Vertical-Axis-Wind-Turbine-VAW/

Topic by nitcool    |  last reply


K'nex Ball Machine Contest - Elements

Greetings! I've decided to run a small contest, in the hopes of us folk here at instructables coming up with some new and exciting ball machine elements and lifts. All you need to do is post a photo and description (video would be awesome too!) of your newfangled ball machine element or lift in the comments below. The criteria you will be judged by are originality, execution and aesthetics. The deadline is October 31st, giving you a whole month to work on your elements/lifts. Judging will be done my myself, and the winner will be announced shortly after. The winner will receive a 3 month Pro Membership, and if there are 10 or more entries, the winner will receive a 1 year Pro membership, and the runner up a 3 month Pro Membership. Good luck to all! -IaC Update: Winner announced! Judging has taken place! There were many good entries - I thank you all for your participation. The winner I have chosen is Martijnd95 with his concave bowl. He will receive a 3 month pro membership for his creation. Thanks everybody!

Topic by I_am_Canadian    |  last reply


Update on the layered metal weapons/props

Hello to everyone who took the time to read this update/list. So I have not forgotten about my promise to upload the Solidworks files and cutting plans. However that will be taking a backseat to finals until summer break. I will however go ahead and list off the items finished, in progress, on hold, or currently only in Solidworks form. Finished -6in Axe: 3lb head on an oak clad 1/4in steel flat bar about 20in long total -3in Tomahawk/Axe: 1.25lb head on a para cord wrapped 1/4in steel flat bar about 20in long total -27in BoneSplitter: 3.2lb aluminium reinforced steel construction. -18in BoneCracker: 2lb aluminium reinforced steel construction. -18in All steel BoneCracker: 3.5lb steel reinforced steel construction. More square handle and better center of balance than previous version In progress -10in Bearded Axe: 3.25lb head. To be mounted on oak clad 1/4in steel flat bar handles 18in, and 30in long. -10in blade Pugio: Model made of mild steel for practice and training. Needs to have bevels, and fullers ground plus handle and guard mounted. -8in blade Pugio: Model made of mild steel for practice and training. Needs to have concave edges, bevels, and fullers ground plus and guard mounted. On hold -14in Blade tester: Simple test bed modeled after BoneSplitter to test viability of various band saw blades(Metal and Wood cutting) for use as cutting edge for mild steel blade carriers. On hold due to the band saw blades being too hard to drill through. Only existing as Solidworks files --27in Revised BoneSplitter: 4.5lb Aluminium reinforced steel construction. Better center of mass and more square handle than MY           previous version. --37in blade Bastard sword: Aluminium reinforced steel construction. Should be very close in terms of center of balance and stiffness when compared to its hardened steel companions.  --20in blade Stacked steel Gladius: Twin riveted steel support plates with detoothed metal cutting bands saw blade used for striking edges.  --blade Hispaniensis Gladius: Single piece of metal for the blade. --blade Cinquedea: Single piece of metal for blade. Multi fuller blade. --8in blade Pugio curved: Same as other 8in Pugio except for the convex curved dagger tip --14in blade Spear from Fullmetal Alchemist: May upload the variant of this in which the "wings" are separate pieces of metal. --3.25in square Warhammer: 8.5lb head Assembled from 13 1/4in steel plates pinned together. Front is covered in 3/8in spikes poking through a steel striking face to prevent separation of the plates. Back end is a 4in spike. --5.25 square Warhammer: 16.25lb head Assembled from 21 1/4in steel plates pinned together. Front is covered in 3/8in spikes poking through a steel striking face to prevent separation of the plates. Back end is a 4in spike. --Straight edged War Axe: 7lb head Inspired by Ironclad Industries' Riftclaw. Made from 7 1/4in steel plates pinned together. I am aware that the riveted and pinned construction of these items makes them inherently less durable and less corrosion resistant than single piece construction. However I am a college student who is working with mostly scraps and whatever has to be purchased. Since mild steel thick enough to carve the Warhammers or War Axe is not common scrap material around the college's machine shop the only way to economically create them is to use stacked plate construction. As for the BoneSplitter, BoneCracker, and Bastard sword the stacked plate construction is used to both reduce the weight of the item and stiffen it at the same time.  

Topic by tytiger33