How To Cut Threads On A Lathe

one squeaky clean lead screw coming up let's cut some threads hello internet my name is quinn and this is blondiax there's lots of great videos out there that will show you all 58 steps to cut threads on your lathe this video is a little different i'm going to show you how to learn to cut threads this is an operation with a steep learning curve all its own so follow me up that curve and by five minutes into this video i will have you cutting your own threads so let's go this is a lead screw unless your lathe is very old indeed you have one of these when you cut threads what you're actually doing is copying the lead screw in fact the very earliest screw cutting lathes just swapped out the entire lead screw with one that matched the pitch that they wanted to cut but then later on we figured out that it's actually easier to just have a constant thread pitch that's a factor for a lot of other thread pitches and then use gears to change the ratio of the lead screw to the spindle you'll need a few things to get started first is a thread cutting tool this is a commercial one by grizzly there's a link to this below it's high speed steel it's ground with a 60 degree point on it and that point is biased slightly to the left as you can see here and i'll explain why that is later and then i also have one that i ground myself here and this one is biased to the right and i'll explain why later but just to show that you can grind your own tool if you like or if you prefer you can use a carbide insert that has the correct shape on it already and then you'll need one of these this is commonly called a fishtail gauge it's got these cutouts in it that tell you the exact angle to which to grind your tools you can use it as a template for grinding your own threading tool and it's also used to align the threading tool later so you'll need one of these there's one of these included in the back of the engineer's black book if you'd like and there's a link to that below as well and lastly we will need a thread pitch gauge now for amateur threads like we're making it doesn't have to be a fancy one just a basic set here that has the type of threads that you're going to want to cut pick an easy thread to cut for your first time we're going to do 3 8 16. i recommend something in the like 16 to 24 pitch range or in metric something like 1.5 to 0.6 somewhere in there finer threads are actually easier to cut for beginners because things happen slower on the lathe as you'll see first we have to prep the material i strongly recommend brass because it's very easy to cut for your first few threads it's really an advantage give yourself like an inch and a half or so of stick out here not so much that you'll lose rigidity but you don't want to be working too close to the chuck for your first couple of threads because it's a little intimidating so we're going to face the end of course and then we're going to turn down the od to the od of the thread which is 3 8 in this case or i recommend actually going a couple thou under that it'll make your life a little bit easier don't go too small though as you'll see here later what happens if you do that and then i recommend a generous chamfer on the end as well that'll make the thread work better in the end and makes it a little easier to test fit it next step is to set up the lathe for cutting threads if you have some kind of fancy machine then all you got to do is look at the little chart throw some levers bob's your uncle if you're a hobbyist schmuck like me then you probably have to deal with change gears if you do all this change gear stuff before you prep the stock by accident which i have done don't worry about it you can still feed by hand you just won't be able to power feed correctly with the change gears in there set for threading safety first go ahead and whack that e-stop and or unplug the machine and in this case the change gear cover also has a safety interlock in it getting in under the cover here we can see the change gear banjo that goes from the spindle up top and then the power is transmitted down through the gears and into the transmission and ultimately to the lead screw and that's what sets the ratio of the two your lathe will have an intimidating looking chart on it like this that tells you how to set all of the gears for metric and imperial threads so in our case we want to cut 16 threads per inch and you can see that there's actually two entries here for 16 so i just pick the one that has the most gears in common with what's already on there saves us a little bit of effort and then this little schematic diagram here shows you how to arrange the gears on that banjo and don't be ashamed if this makes no sense to you because it made no sense to me the first few times so there may also be a diagram in the manual explaining how to do this that's a lot easier to follow but once you've done it a few times the little schematic on the machine will make sense and then you can save yourself digging out the manual so in my case there's two gears that are different from what's on there now we need the 60 and the 65 so there's these little clips and the gears just slide off like so we're going to need that and we're going to need that one so set those aside so as i pull the gears off i'm just setting them in a keep pile and a rejection pile and you may also have spacers like this and those allow you to set the gears on different positions inboard and outboard on the banjo so the little diagram will tell you how many spacers you need and where to put them all then start sliding the gears back on in the new order and you'll note that all these gears are sliding together very easily and that's because i've spent a couple of hours cleaning up and deburring all of these components if you buy a new import lathe the fit and finish on these gears will not be very good at first so it's worth taking the time to do this of course you'll need to adjust the spacing of the axles there as well once all your gears are in place then you need to set the backlash between them and for the threading gears just a quick and dirty setting is all you need so just push the gears together until they're jammed up and then back them off slightly and tighten the axle this will be a bit of a noisy setting there's a lot of backlash there but it really doesn't matter for the threading gears because they're only going to be on there for five minutes and running at low rpm so it's not worth the time to get a perfect backlash on there but when you go to put your feed gears back on then take the time to get that backlash just right there is a backlash setting on spur gears that is both smooth and quiet and then for any new gears that are dry you're going to want to lubricate them these are two things that work well crc open gear lube and wheel bearing grease they're both equally terrible in different ways the crc seems to run a little quieter so i've been using it lately but it comes in this spray can that seems determined to cover your entire shop in black sludge so you know use with caution do your best to keep it off the belt so it doesn't make the belt slip clean things up put the cover back on and just give it a little test spin and see how those gears sound again if they're rattling because they're loose that's fine as long as they're not grinding because they're too tight then you can proceed with cutting your threads again don't sweat the backlash until you go to put the feed gears back on and then it's worth putting the time in to get it perfect careful though there may be one more step if like this machine you have a hybrid of change gears and transmission then you got to make sure you're also on the right transmission setting so i got to set my transmission to b to complete the thread gear setup next verify that your thread cutting tool is on center so just throw it on the back of the tool post there so it aligns with the tail stock and you can see that mine is a little bit high here so i'll lower it down make sure you check the height with the tool post tight because that will affect it get down low so you're looking right at it and you want the top surface which is what's doing the cutting aligned with the point on the tail stock and then make sure your tool post is square and a one two three block on the face of the chuck is an easy way to do that now let's talk about the threading dial it should be disengaged when you're not using it otherwise it's just wearing out the lead screw for no reason so go ahead and engage that now let's talk about how this guy works normally it just sits there and turns but then when you engage the half nut it stops turning on whatever value you had it set on when you engage the half nut so how easy is it to hit the number just like you're supposed to it's actually pretty easy and here's how you can get the feel for that so put it on a number and you can feel the half nut engage and then just wind the carriage back just a hair and the half knot will no longer engage because now it's between two threads and then just wind the carriage back with a little bit of pressure on it until it engages again and that gives you your engagement zone if you engage that half nut anywhere between there and the line when the lead screw is turning you will hit your number so you can see that it's actually pretty generous and the faster the lead screw is turning the easier this is to do your lathe manual will have a diagram explaining which numbers you can hit for each thread pitch but for your first thread just pick a single number and use that every time just always go on one that will work for any thread it's not the fastest way to do it but it will always work and if you're a gamer while you've been training for this your whole life so you'll be fine next lightly touch off on the work then mount an indicator on your carriage and put it on the tool post with a good full turn of preload there that's going to be really helpful here and then zero it out right there it's also very helpful to mark up the surface with a sharpie it really helps see what you're doing when you're cutting threads then with the lathe running at a modest rpm for your first time 150 is fine wait for that one to come around engage that half nut and let it ride and then when it gets to the base of the area that you prepared just unlock that half nut and you can let it sit there at the end for a minute it'll just cut a little circle that's fine and pull it off the work and move back to the start and that's what we call your scratch pass you've just been touching the work with the end of the tool there and now you can come in with your thread pitch gauge and see if you're cutting the right pitch so you can visually see and you can also feel the little teeth engage with the scratch marks that you made because the tool was just touched off on the work if this doesn't line up then there's something wrong with your transmission settings or your change gears okay we're ready to start making cuts but how deep should we go well if you look at 3816 on a thread geometry chart you'll see that the od is of course 3 8 and it'll also tell you what the minor diameter of this thread is which is 0.2983 in this case so a little bit of math we subtract that from the major diameter we get a total depth of 76 thou that's on all sides so divide that by two and we get 38 thou which is the final depth of cut we want to see on the indicator now this is a ballpark and it's not accounting for all the proper tolerances that are supposed to be in a thread but for your first time that's a good ballpark when you get within a few thou that number you can start test fitting and you'll learn from there where to start and stop based on those numbers so we had pulled the cross slide away from the work so now push it back into zero now we're back on the surface of the work and now dial in five thou for your first depth of cut you want to take light cuts when thread cutting and then wait for that number to come around engage that half nut and do just what you did before don't worry too much about stopping on exactly the same place each time again for your first thread doesn't matter you'll make a bit of a mess at the base there really doesn't matter and then again pull it off the work come back and then dial in another five thou for your second cut and that's about as deep as you want to go from now on so you want to have kind of logarithmically decreasing depths of cuts you can get away with a couple of five thou cuts then you're going to want to stick to like three thou for a couple of passes and then two thousand don't be afraid to do lots and lots of one thou passes basically the lighter cuts you do the nicer your thread is gonna be so you can see there's some patience required here because if you're doing one thou cuts 38 depth of cut let's see carry the 5 62 and divided by that that's 38 passes but don't let your impatience make you greedy if you try to take two aggressive cuts here you will regret it you can also use that blue on the peaks of the threads to monitor your progress when you're getting down to just wisps of visible blue then you're probably close to your final depth which will show as within a couple of thou on that indicator of where you calculated and it's time to start test fitting with your nut if you have a go no-go gauge for the thread even better but for your first thread a cheap commercial nut is just fine when you get to this point where the nut threads on but it's weirdly tight then just do a spring pass that's another pass with the same indicator reading that you used last time and there will be less deflection in everything and it'll be a lighter cut but it will still be a cut because you're taking some of the deflection out that was there on the previous pass and that's usually enough to where it fits nicely and sometimes you can do multiple spring passes so this is kind of what you're aiming for you've got nice clean 60 degree vs with little flat spots on the peaks there which you can see with the remaining blue there if you don't see any blue left that just means your initial od was probably a little too small and so you had to cut deep enough to eliminate those flats at the tops of the peaks if you have chatter marks little ridges in the sides of the v's there that just means you had a little bit too much depth of cut you got a little bit too greedy but it's okay if everything's not perfect it'll still work as a thread [Music] with the basics down let's mix it up a bit so i've been using this standard cutting tool now let's switch to the other one that i showed at the beginning this one is biased to the other side because it's intended to be used upside down and we're going to be cutting the thread left to right away from the chuck the tool height here is a little unintuitive because the tool is upside down so you want to set it to align with the bottom surface there now previously we just squared up the tool post but really you should use your fishtail gauge to get the v properly square to the work because the tool grind might not be perfect the sides of the tool might not be correctly aligned with the angle of the grind so the fishtail gauge eliminates any error with that and then the real key here is to run the lathe in reverse now health and safety warning do not do this if your lathe has a threaded spindle and your chuck is threaded on because of course you will unthread your chuck by doing this but if you have the type of chuck mounting that allows you to run the lathe in reverse this method of thread cutting is really really nice because it allows you to run at a higher rpm you're not charging towards the chuck with the half nut engaged which is a scary thing so you'll get nicer threads much lower anxiety i first learned of this method from joe pie so thanks to him for this i think once you cut threads this way you'll never go back to the quote unquote normal way people do it again assuming your lathe supports it this time let's step up our game on prepping the stock i'm going to use this round nosed tool here to create a groove you want a round nose to tool here and not like a grooving or parting tool which would leave square edges in here because those would believe stress risers which will weaken the thread so make a nice curved groove at the base there that's a little bit deeper than the minor diameter of your thread you'll see how this is going to help us here in a minute now we're also going to step up our game by learning about the role of the compound so go ahead and set this thing to is it 30 degrees or 29 and a half whatever like everyone always says maybe but maybe not i am now going to tell you the number one most important thing that nobody ever tells you about chinese lathes they typically have the compound markings 90 degrees off from american lathes so don't set it to 30 or your threads will be misshapen you want to set it to 60 if you're using a chinese import lathe if your thread comes out looking like sawteeth then it means this angle wasn't correct so try the other setting the 30 versus the 60. then it's a good idea to get in there with a protractor and just set it to precisely somewhere between 29 and 30 degrees because the markings on the compound aren't that precise the reason that we do that is to lower tool pressure by moving the tool in at the same angle as the v groove that we're cutting only the leading edge of the tool is doing any cutting instead of cutting on both sides like we were previously just feeding with the cross slide when you're cutting really fine threads or just cutting in brass doesn't really matter that much but if you're cutting deep threads or something like steel this compound does start to matter quite a bit now as before we're going to touch off on the work and then we're going to zero the cross slide that's important and we're also going to zero the indicator on the tool post just like before so far so good and now you can see what that groove does for us it gives us a nice place to start without touching the material for cutting our threads on each pass or if you're cutting towards the chuck it gives you a nice safe zone to disconnect the half nut where you're not touching the work and now you can also see why that left bias on the threading tool is so valuable because it lets you get closer to the chuck and still get into that thread groove for each pass so you do your scratch pass as before verify everything now when you're going to start making cuts you're going to feed your depth of cut with the compound now otherwise everything is the same you're going to feed in 5 for the first pass but using the compound and the indicator takes out the cosine error of the depth of cut that's introduced by the angle of the compound and now when you're going to feed for your next pass you're going to wind up with the cross slide you're going to go back to the start with the carriage just like before and then you're going to feed that cross slide back into zero that's why we zeroed that so the cross slide is book marking the top surface of the thread if you like and then once that's zeroed out the indicator should now show the same depth of cut you had before then you can feed in for your next pass with the compound and so again i'll do another five thou because it's just the second pass but then you're going to want to do smaller passes after this otherwise it's the same process that you learned previously but you can see we've just added this additional element that we're feeding back to zero on the cross slide and then using the compound for our depth of cut each time so it's just a little bit more to keep track of but once you've cut a couple of threads the first way then adding this element of the compound is pretty straightforward and you just kind of get into a rhythm here and this thread is a good example of what happens if your od when you start is a little too small you can see that i've lost the blue on the peaks of the threads and the peaks are pointed but that's okay again for a cheap commercial nut like this still threads just fine and it'll be adequate for hobby work now remember at the top i said how you're copying the lead screw when you cut a thread what that means is it's difficult to create metric threads with an imperial lead screw or vice versa because the two are not even multiples of each other there is however a way to do it and it involves a special gear so i have an imperial lead screw so let's look at the metric chart and you'll notice something interesting here all of the patterns have this weird 39 gear in them except one there's one place here where they happen to line up but all the other patterns have a 39 tooth gear in them and that 39 tooth gear is nowhere on the imperial chart so that 39 tooth is what we call the conversion gear on larger lathes it's often 127 teeth but it'll be a weird number of teeth and what that does is it introduces the fractional error between metric and imperial thread pitches so it allows you to cut the other type of thread with a little bit of error so we start off in reverse as before because i'm cutting away from the chuck and you just power yourself away cutting your threads get to the end and you disengage that oh no we've just ruined the thread unlike cutting matching threads where you can re-engage with a multiple using the threading dial and you'll land in a common denominator of what you're cutting once you've got that fractional adjustment in the system you have to keep it the same and thus never disengage the half nut so the way that we do this is we just power back and forth instead when you get to the end just power down the lathe and let it spin off the end of the thread so cutting left to right is much easier when doing mismatched threads because you don't have to worry about coasting into the chuck and then reverse the lathe power your way back in when you get close you can shut the machine down and then wind it the rest of the way by hand to get back into your starting groove and then proceed for the rest of the cut now there are methods for cutting mismatched threads that do involve disengaging the half nut but those are out of scope for this video here so you can go look those up if you are interested so i hope this method will help you learn to cut your very first threads there's so much more to learn about thread cutting it's a very complex topic you know we haven't touched on how to properly measure threads or how to ensure the correct geometry at the base and the root of the thread and so on tons more to know but this will get you started down the road i hope you will give this method a try learn to cut your own threads don't be intimidated by it it's really enjoyable if you like these videos throw me a little love on patreon and i will see you next time you

Channel: Blondihacks

Views: 322,601

Rating: 4.9462337 out of 5

Keywords: blondihacks, machining, machinist, abom79, this old tony, vintage machinery, steam, electronics, making, maker, hacking, hacker, lathe, mill, woodworking, workshop, shop, model engineering, engineer, engineering, live steam, machine shop, metal lathe, vertical mill, metalworking, metal shop, jewlery making, diy, home improvement, how to, do it yourself, do it yourself (hobby), mini mill, mini lathe, tutorial, threading, single point, how to cut threads, manual lathe, cutting threads, threads

Id: q7scadYptTI

Length: 20min 36sec (1236 seconds)

Published: Sat Feb 06 2021