Precision Mechanics

[Music] welcome back doc stools I'm calm so today we're going to talk about precision mechanics this Ron repite meter collaboration series with saunders machine works NYC cnc has brought up an excellent discussion about precision mechanics so we're going to take a look at differential screws quarter twenty thread resolute ors quarter twenty eight thread resolution a membrane type flexure and some other stuff so let's check it out and learn something about precision mechanics okay these are all examples of what I call membrane flexures and what I mean by membrane is it's an added piece okay so it's an it's really it's an added on piece you know what just stay okay so it's an added on piece so we have the body of the of the instrument here and then we've added a membrane that's attached that is the actual flexure so this is the hinge that flexes and becomes the flexure okay so there's that's one example this is a one that I made this is a another very similar to that one now there's a real interesting one in here and we'll take this up and I'll show you the inside of this and this is a what's called a parallel membrane flexure here I'll bring in a little closer but the way this works here is it's got two flexors here that are clamped and then a screw adjustment so what it does is it's moving this indicator up and down and it actually moves it in a parallel line it's actually an excellent an excellent type of movement there's no very little mechanical hysteresis it's very smooth and there's just no stick-slip stored energy in the system so it's kind of neat that way so now we have an adjustment screw here that's pushing against this and that's another subject that we're going to talk about today is you know how fine does a screw have to be right real fine screws are not the easiest thing to make and you need special taps form if they're little holes and so there's all kinds of things so you'd be surprised how good of a resolution that you can get with a properly designed screw mechanism the kind of resolution that you can get with just common screw threads so we're going to take a look at that but let's look at these membranes a little closer and you guys can get an idea how this thing works all right these are the these are the flexure elements here so these are you know thin shim stock spring steel shim stock and they're clamped on either side with a with a bar that has a bunch of screws in it you know much the same way these this guy here is is clamped okay that's a thicker flexure so it doesn't need it doesn't need the the over clamp bar I should say you know the it's actually a long washer is what it is anyway to just prevent stress concentrations around the holes in the in the membrane there but you can see how this one works here okay it's got a little return spring to keep tension against the against the adjustment screw and this adjustment screw let's see if I can see what that looks like it's not super fine I want to say it well you know what let's just dip it up and take a look at it in there you can see in there I'm getting it turn a little more there that's where we're looking right there and I'm a nod it might be 32 threads per inch 20 I don't know so it looks about like a quarter 20 honestly so and the amount that it's moving I don't know might be 32 threads per inch something like that actually is probably metric I think this is a midotaur here stand and we've got our cej indicator here now keep in mind that on that Ron repite meter we have a pretty short range of motion and this only has six tenths plus or minus six tenths travel okay so that little flexure that's in the Ron repeater meter really is not moving very far so let's go take a look at something else on that all right let's take a quick look at the how that Ron repeater meter is set up there okay so it's got a it's got a bar and then we have a we have a cut through the bar okay like so and then it's got some length this way okay so this is the area that everybody seems to have a problem with now keep in mind that Ron repeater meters been around since 1953 and they work fine and now I would also agree that this is a very this is a crude solution and from a design standpoint it's kind of lame okay so we can do better than that and let's just take a quick look at this though so this thing's moving up and down at this end right but honestly it's not moving very much so if we just assigned some quick numbers here I think somebody said that this is roughly a 5 inch radius here somewhere in that region okay so if we just draw a little triangle here like this okay and we say that this leg is five inches here getting all that right yeah okay and we looked at the cej indicator and it has plus or minus six tenths okay so what that means is this thing can move up and down about 1.2 thousandths of an inch but let's just look at one side let's just analyze one side so let's just say it's a 6/10 right so 0 6 over 5 let's get the handy box out here 6 and I've defined I can so that angle there is point 0 0 6 8 8 degrees alright for our degrees minutes and seconds folks that is it's 24 arc seconds okay that's not a lot of movement so we're well you know without doing the stress analysis we're well within the elastic range of that so this joint is probably not subject subjected to excessive stress okay but we all agree that it's kind of a lame design so let's do something a little bit better and I think we can re-engineer this a little bit and get a membrane flexure out of that okay and do a nice job of it and have a much more elegant mechanical solution [Applause] all right a little bit below flush all right let's go cut this thing in half try it out alright let's put this little monkey together Oh holes don't line up it works pretty good yeah it's more than enough movement right there now you just need a clever over travel mechanism alright here's what I'm thinking so we want it to be wanted flush with the surface so we don't want it sticking up ideally so let's let's draw one side of this so we're looking at we're looking at one surface there actually let's do this let's draw that in there just so so folks get the idea all right I'm going to care about that so well the foot the stop has to prevent travel in two directions so down and up so what I'm thinking is what if we just kind of slot through like this okay yeah let's do a little erase there okay and then we cut a something that looks like that that's a little bit hard to draw so the idea is we take a fastener something like that and what it can do is they can drop into this it can drop into this slot it's threaded into the other member okay but when the flexor flexes it pulls up against that shoulder that or that's shoulder there and then if it extends the other direction it butts up against the back of that slot and then it's all nice and flush and below surface so let's try that that's a simple one another way you can do it is you can you can insert a piece in here that spans across that has a little bit of clearance this is nice because it's adjustable it's a little bit adjustable so you can you can change the relationship a little bit now the only negative on this to give me again hiccups is there's nothing that keeps it from turning necessarily so potentially it could get out of adjustment and well potentially you can get out of adjustment so we might add a some threadlocker to it or something like that to prevent it from moving or make it the movement stiff I should say is what might be a better way to do but let's say let's get a little slot in our sample piece and see how it works [Laughter] [Applause] [Applause] okay here's our our little over travel stop that we did it's got a little socket head cap screw in there and it's actually just a right thickness here that it gives me a little bit of free play in either direction when it's in the kind of the null position of the of the flexure so it does its little job and keeps us from from injuring the flexor or yielding the flexure by by hyper extending it in either direction so now we can make minor adjustments by just reaching in there and turning it that way that seems to work there's nothing that keeps it from rotating although there's no particular forces on this and make want to make this rotate in one direction or another so I think that's a reasonably elegant solution for kind of on the fly in the shop but simple and easy to machine let's take a look at the the adjusting screw mechanism next because there's a lot we can do in that area to make it much cooler alright so here's the other little thing that we wanted to talk about so this is this is a quarter twenty eight socket head cap screw and I suggested that this would probably be a reasonable adjustment screw for the Ron rápida meters or the what do we call them the ultra I don't know John and I have to come up with a name for the versions that the heats building anyway I suggested that this might be a good adjustment screw now my old buddy Forrest addy from the practical machinist he chimed in and correctly pointed out that that this is this is probably a two course to make a good adjustment screws since our indicator resolution is you know in the 20 millions per division range okay so making an adjustment it's meant to to null the indicator we probably need a finer screw and he's probably right but as an exploration and as a as a kind of a demonstration we're going to check out just how good you can do with a with a quarter twenty eight okay and the other thing that we're going to show here in a sec is something that's called a differential thread okay so we have a another quarter quarter twenty eight here but this one's been very sneaky ly modified and it has a tapped hole in the center of it that this 1032 screw can go into I can get it in there okay so so this is a right-hand thread that's a right-hand thread and we can actually show how we can create a very simple differential thread with these two screws so this is quarter twenty eight and ten thirty two and what I've done is I've made a little I'll zoom out a little bit and I'll show you in a sec I made a little demonstration piece that it's very similar in geometry to the the Ron repite meter upper upper limbs so let's say let's go over on the mill and let's take a look at this and let's just see how well we can do with this and talk about some of the design elements around how we apply screws for precision adjustments this is a I don't know I would call it an analogue of the Ron repite meter upper limbs so it has two bars that have a actually a saw cut through them that the adjustment screw spreads apart as a flexure and adjust the position of the indicator or allows you to know that the indicator reading or actually take pressure off of the off of the indicator so it's just made out of steel and I just kind of whip this together this is kind of a demonstration piece so I didn't show the construction of this but we're going to try it out here and see how it behaves so what we're going to do is for starters we'll put our our quarter twenty eight in there okay actually I need another another screw there I'm just going to put this tap 10:32 on the bottom and quarter twenty eight on the top so I'm just going to put this in there too to make a little spot that this other screw can bear against there so run that in and I've left a big space in there so it's just easier to see doesn't really affect the performance of that okay so when we tighten this up it's spreading these these two limbs apart and this is a giant flexor is what this is so let's set that in there I'm going to clamp it in the vise and then what we're going to do is setup an indicator here and to see what how it behaves so let me get that going and we'll be let's this this is a this indicator is 50 millions per tick there oops okay I think I'm against it yeah there we go alright so this is our ambiguous quarter twenty eight screw here let's just uh let's just try it here okay so you know it works okay this diameter is pretty small so the you know the radial movement is actually kind of tiny there so and it's also not very smooth right now and I'll show you a way around that all right but we can easily adjust it within 50 Millions with a quarter 28 screwed it straight up okay now what can we do to improve that well the first thing we can do to improve that oops is take this out let's get rid of that and we have another quarter 28 screw here I'm going to show you okay and what we've done is we put a large knob on the top of it okay what this does is it gives us this take a better tactile feel okay and it gives us better radial resolution with our adjustments so let's let's do that again all right so it's much it's easier to turn and it's also easier to hit our number okay because there's a home there's more space between the divisions of the larger the diameter you get here okay so so it looks like our quarter 28 may work out for adjusting our 20 million our indicator okay but we can also do better than that so the next step the next upgrade here is let's let's take this out the next simple upgrade that we can do in this particular system with the quarter 28 here is it's important these surfaces here the surface that bears against the other surface that it's pushing against if they're polished and smooth and hard then the whole mechanism behaves much better alright so what we can do here and I polished see the end of this quarter 28 screw here what we're going to do is we're going to set up around and I'm going to put try not to drop it is I have a small hardened steel ball alright and what I'm going to do with that let's get this kind of in position first is I'm going to use my handy-dandy tweezers here is I'm going to put this as the contact point for the screw okay in there and then I'm going to this is going to run down and bear against that against that screw so let's try that and see how that behaves and I can already feel the difference it's just much smoother okay so smoothness is important and in precision mechanics we call it a stick slip okay what that means is you know this herky-jerky motion that you get out of certain kinds of mechanisms and it's called stick slip and what that is is you're building up energy in the system and then it's it's releasing it in kind of an uncontrolled way right which is really bad for precision precision mechanics I'm going to mess that up I can tell you so let's get down close again alright we're pretty close there and let's mess around with our adjustment here yeah and it's just a lot smoother and you know I can't really show you this in the you can't feel this in the video obviously but you'll just have to trust me guys okay and it makes sense so what you want is you want a hard to hard contact surfaces differential hardness is good one slightly softer than the other but hard on hard is generally pretty good so okay so we got our large resolution knob in we're quarter twenty eight and we got a hard ball under there it's pretty tasty right now I think we can do better than that though let's take a look at the differential screw next okay this is the the differential screw here and what we we have quarter 28 on the top 10 32 on the bottom and the inside of the quarter 28 is tapped for the 10 32 so what you have is you have two different thread pitches and as one advance advances the other cancels part of that travel out okay and that's how differential screw works now in this case the 1032 is fixed and the quarter 28 is the moving member okay they're both right-handed threads in this case there's a way to do it with right-hand and left-hand threads and the way you can figure out there's a excuse me an equation for figuring out the effective pitch between when you have two different thread pitches and well I'll show you that right now let me put this in here can I get it in position all right and so the way you determine the effective pitch okay let's let's just call this effective pitch is equal to 1 over P 1 minus 1 over P 2 okay so this is our quarter 28 okay and this is our our 10 32 okay and I won't I won't bore you with the the rest of the math but it comes out to an effective thread pitch of 224 threads per inch okay I did the Matt off-camera so but you guys can do it yourself so effective pitch is equal to and this is you take 1 divided by 28 1 divided by 32 okay and you get to 224 threads per inch now I don't know if you've ever seen a 224 threads per inch screw but it basically looks like a human hair wound around a cylinder okay you can barely see it alright so let's let's try it out here I'm all set up here and unfortunately I didn't put a knob on this one but I got a wrench here but this will illustrate the radial movement that I'm doing to get indicator movement over here so let me get this going and we'll take a look at just how cool these are all right this is the differential screw here I'm just going to actually let me see if I get turned by hand I can turn it by hand it's a look it's a little snug turning it by hand I would you know if I had a nice knob like that it would be better but let's put this on there and you guys can get it now watch the indicator and the arm of the wrench to get an idea of the resolution that we have here it's actually almost the same rate of change as the indicator needle which is pretty cool right I'm just going to stop on the floor there okay alright so that's that's the differential screw that's effectively 224 threads per inch alright with simple common hardware okay and this has more than enough resolution to to do what we want to do on the run with Peter meter setting it up now one thing to keep in mind with differential screws is this is about a increase of seven fold on the the effective thread pitch of this particular screw but it also reduces your travel by that same amount so you don't get a lot of travel out of them all right so that you got to be careful so they can be a little bit tricky to set up because you have limited range of movement and especially if you have a kind of a tight a tight arrangement there but anyway they're pretty neat and you know you can use them for where you need a really fine screw now you know where you going to buy a 224 thread per inch tap right it's a custom it's going to be expensive so this is a sneaky way to to do that you [Music]

Channel: oxtoolco

Views: 78,861

Keywords: Rahn, Repeat meter, Surface Plate calibration, Precision mechanics, Metrology, Ultraprecision mechanism, Flexure, Differential screw, NYC CNC, Saunders Machine Works

Id: 7RC8WL2ngfA

Length: 31min 41sec (1901 seconds)

Published: Mon Jun 05 2017