Metal Lathe Tutorial 12 : Surface Finish

hello internet my name is Quinton and this is blondie hacks this is lathe skills a series of quick videos on getting started in machining this is episode 12 surface finishes twelve twelve I started doing this finger thing at the beginning of the series and I didn't think ahead what was gonna happen above ten so let's start by taking a quick look at the extremes of surface finish this is a very poor surface finish and you can see it's actually corroded and this is one of the very first things I ever machined so as you can see it's a skill that you need to develop at the other extreme the very best surface finishes come from surface grinding and this is a piece of old one tool steel that's been precision ground at the factory so this is about as good as it gets so why does surface finish matter well obviously this part is a lot prettier than this one and the true deep-seated desire every machinist is to make things pretty but it matters for a number of practical reasons as well first and foremost is precision if I'm taking a dimension off of this part when you get down into really precise dimensions you get into the tents and the hundreds of thousands then what happens is the depth of the tool marks or the fine grooves on the surface start to overwhelm the precision that you need out of the reading and so what you're actually reading then is the tops of your tool marks and you're kind of averaging them out so the better your surface finish is the the better you can hit dimensions and the more precise and consistent those dimensions will be an example of where that starts to matter is it was something like a bearing fit up where this might be pressed onto a shaft and the specifications for this bearing might call for an interference fit if you know plus and minus a few tenths in which case your surface finish needs to be good so that you can hit that dimension and have the exact right press on interference without damaging the bearing and in fact that's part of why high quality bearings such as this one have very high quality ground surfaces on inside and out another example is heat treating this is oh one tool steel that has been hardened and then tempered back with a torch and you're tempering by eye you need to be able to see the color of the surface so this is a kind of a yellow brown and this is kind of a blue verging on indigo and the color tells you the hardness of that material as you're tempering it so you know when to stop and if the surface finishes pour as on this guy it's quite difficult to tell you know what color it is and where you've got gotten to and you know when to stop tempering so in order to get an accurate temper you really need a smooth clean surface and lastly I'll call out further surface treatments that you want might want to do so this part here this is a shop made tailstock dye holder and this was cold blood with Brownells Oxbow blue and cold bluing is a process that really responds well to a good surface finish basically the better your surface finish is the better the cold bluing process is going to come out another important reason is corrosion resistance so you can see how this part has a lot of flash rust on the surface you know surfaces rust when moisture collects in imperfections in that surface so these two parts here are actually the same age and this guy has flash rusted quite badly and this guy looks factory-fresh so the better your surface finish is the longer unprotected steel can go without rusting so when you're making tooling or things like that that you want to be unpainted a good surface finish is very important there as well now I showed the extremes of surface finish earlier but the average home machinist is going to be working kind of in this middle range of mediocrity and within which there's a fair amount of variation as well so the first way you can tell how good your finish is of course is just by feeling it if it feels smooth it's probably good but sometimes things will feel smooth but then if you run your fingernail on it I'll just hold it up with a microphone here you can hear that it's actually not smooth and you can feel that with your thumbnail as well it varies a bit by material as well so while smooth and reflective are always rules of thumb steel is always going to show those tool marks a little bit more than some other materials brass tends not to show tool marks so it looks good on camera brass makes everybody look like a good machinist and aluminum on the other hand is interesting because it really shows the toolmarks you know you can very clearly see on camera that this was done with a half inch end mill but in fact its glassy smooth to the touch so even though you can see the tool marks and actually they show up more on camera than they do in person but even though you can see the tool marks the part is actually very smooth so learning to recognize surface finish on different materials by feel and by sight is is helpful now claiming to be an authority on surface finishes on the Internet is a bit like showing your welds and expecting nobody to criticise them but I do want to show you at least what a decent surface finish on steel looks like so as I said it's smooth and shiny but the key here is that when you look closely the grain of the material is kind of the dominant feature you see the grain of the material more than you see tool marks so that's kind of a good rule of thumb there no but look at this as good as that surface finish is magnification is what separates the girls from the women so here on the macro lens you can actually see the tool marks as well as the grain of the steel in this surface so even though something might look really really good and feel a really really good a close-up look at it gives you a pretty good view of what's actually happening to the material choice of alloy also matters this the quality of surface between these two parts is different not just because there's years of experience on my part between them but also because this is mild steel and this is 12 L 14 free machining steel this has a very small amount of lead in it which makes it machine very very nicely makes a good chip action and makes it much easier to achieve a good surface finish so pro tip if you want to look like a good machinist on YouTube do everything in 12 o 14 steel okay and I show until now what actually creates good and bad surface finishes well I think the easiest way to demonstrate that is to create a bad one something that I am uniquely qualified to do [Music] okay so I've pretty much done all the things that lead to a bad surface finish here just shy of actual chatter as you can see I got lousy cheap action I ended up with a bird's nest my chips weren't breaking so that means my speeds and feeds weren't right and I get this mess out of here right oh alright remember now this is actually twelve or fourteen steel too so this is the stuff that's easy to get a good surface finish with yet I've still managed to bungle it up definitely fails the fingernail test so this is this is a lousy surface finish in fact you can even just feel it okay I've brought you in real close so we can see what's actually happening now remember that lathes are single point cutting tools and that means only the very point of this tool is doing the cutting so if you think about what that means this material is spinning and this point here is moving across the surface at a regular speed what the lathe is actually doing is cutting a very fine thread it's making a helix and the lathe is always doing this even when you're not cutting threads intentionally so a bad surface finish is effectively a really fine thread which you know you might actually brag about look at this incredibly fine thread I made but nope that's not what we want now the other effect that you can see if you look really close here is that there's actually like little balls or little lumps on the surface and that's where the chip action was bad so the material wasn't cutting cleanly as the tool passed over it the material was actually kind of getting torn off and that creates like little birds and little balls and little lumps of steel that are a lot of what you can feel as well so the fact that a poor surface finish is actually a very fine thread is the key to understanding how to improve your surface finishes now if you think about thread cutting you do that with a very fine pointed tool if you tried to cut a thread with a with a round tool it would just sort of smear your threads all over the place and in fact that's exactly what we want to do when we're trying to create a good surface finishes smear the thread all over the place so if we use so here's an exaggerated round nosed tool so you can kind of see what we're talking about imagine this tool moving over the surface and you can kind of see how as it moved the blunt nose on this guy would kind of overlap previous passes and kind of blur them all together and so this is why a round nose tool is always going to give you a much nicer finish is because it's going to kind of average out the the tiny thread marks that you're making now in steel this might be a more typical nose radius that you would that you would use it's hard to see from the top but if I show you the end here you can see how the end of that tool has kind of been rounded over so that little bit of nose radius there will give us some much nicer surface finish and in contrast here's the tool that we used for that poor surface finish demonstration and you can see on the end that it's actually completely sharp there's no no radius there at all and in fact to make matters even worse the point of that guy is actually rolled over a little bit so this tool needs sharpening okay a round nose tools are so great then why wouldn't we use a comically large radius like this all the time well the answer is tool pressure the larger your nose radius is the higher your tool pressure is and we talked about that in the chatter video and that's why a larger radius tool would have to be reserved for your finishing passes where you're taking light cuts and not removing as much material because you need to avoid chatter okay then why would you ever use a very sharp nose tool like this guy if you know it's going to lead to a lousy finish well there are lots of times when you want to minimize tool pressure at all costs so for example on a small machine like this when I turn tool steel dual Steel's really tough stuff and I have to use the sharpest tool I've got to prevent shatter and also if you're doing roughing passes and we want to remove a lot of material as much as you can at a time you might use a sharp nose tool because you don't care about surface finish since you're going to be doing finishing passes after the fact and then of course sharpen those tools are also good for making clean inside shoulders and various other things but most of the time a compromise tool like this with a modest radius on it will will do just fine for all of your roughing and will achieve a satisfactory result on your finishing passes now it's not all about tool nose radius surface finish is largely about getting your speeds and feeds right and in fact the sharper your nose radius is the more important this is larger to radii are more forgiving for having not quite perfect speeds and feeds and that comes back to what we said about those those little balls those little tear marks that are on the surface here that you may not be able to see but you can feel those are due to poor chip action to get good chip action we need this material to shear just right when it encounters the tool if our speed is too slow the material is just kind of gonna get pushed or plowed and if it's too high the material is gonna start tearing so getting it just right is key so that includes a lot of factors of course the surface speed has to be correct and you can look it up in the books what your surface speed for each material should be though honestly a lot of the books are written for maximum production on large machines often with carbide if not otherwise specified so I think it's better to just kind of experiment in your own shop and get the feel of it but a number of rules of thumb apply first and foremost your tool needs to be sharp and your tool angles need to be correct so if you're grinding your own tool bits make sure that you know all your angles are right it's part of why when learning I strongly recommend buying pre-ground sets to eliminate that learning curve when you're trying to improve your surface finishes you need to eliminate as many variables as you can and you don't want to be trying to learn the skill of grinding tool bits and perfecting surface finishes at the same time and the other interesting thing about speeds and feeds is that we can manipulate them within the desired range for the material to get the effect we want so remember again that this is what the sharp nose tool is effectively cutting a very fine thread well the faster this material is spinning and the slower this tool is moving across the material the finer that thread will be and if that thread gets fine enough it actually starts to just become one surface so you can picture that when deciding on your speeds and feeds now the slower the surface is moving to a point not you know not to slow but if this is moving slower and this tool is feeding slower then you can take deeper cuts now if you're in a hurry to remove material you're going to want to take a deeper cut which means your surface speed is going to need to be a little bit lower and you're also going to want this feed to be higher because you're in a hurry and so that will allow you to remove as material as possible in a short amount of time but of course your surface finish is going to be poor so you put all that together and it gives you your basic rule of thumb for speeds and feeds which is higher surface speed slower feed gives a better finish and lower surface speed higher feed will remove material more efficiently okay so putting all that together when you're going for best possible surface finish on your final pass you want to do a shallow cut maybe ten thousandths and you want a higher spindle speed and you want a slower feed so high speed slow feed round-nose shallow cut it's going to maximize your surface finish quality but wait there's one more tool in our surface finish arsenal and that's cutting fluid cutting fluid improves your chip action and also helps clear chips away from the cutting tool so that chips from previous passes don't scratch up the material if they get caught under the tool bit that sort of thing so when you're really worried about your surface finished a little bit cutting oil will always help so something like tap magic I like for steel and for software materials aluminum copper and brass I like wd-40 cutting fluid has other magic powers as well it's useful in roughing passes because it will burn off and carry the heat with it and keep the temperature down on your cut which is always a good idea and of course helps clear chips for roughing as well so cutting fluids never a bad idea okay so let's see if we can put our learnings to use I've got a fresh hone on my tool here and this is the moderate round nosed tool I have increased my spindle speed a little bit I've slowed down my feed and we're going to do a sixth out and we'll get some cutting oil on there and let's see if we can put our surface finish where our mouth is [Applause] okay the debris off of there okay so that feels very very smooth you can probably still see the toolmarks on camera because they always show up on camera but certainly passes the fingernail test I can't feel any any marks on that surface at all it's very very smooth you know there's always room for improvement but hey showing something less than perfect on YouTube is a surefire way to get negative come I mean engagement so let's say you ended up with a service finish that isn't as good as you need but you don't want to redo the whole part is there anything we can do to rescue it well remember at the top of the show we said that the very best service finishes come from a grinder the reason for that is because the the size of the tool is effectively dust and so your tool marks are microscopic and we can get there ourselves with every paper now anytime you're doing any kind of grinding or sanding on the lathe using any kind of stone based tools you always want to protect your ways because all stone based tools shower grit and that grit is harder than everything on the lathe so that grit is going to collect on your on your precision surfaces and damage them over time as things move around so always protect your ways and now we can get busy so we're going to start with 400 grit emery paper and when you're doing this you want to hold the paper with your fingertips just in case something grabs you don't want your fingers to get pulled in anywhere so you want to hold it such that the lathe is just going to pull it out of your fingers if anything happens I'm just kind of work it back and forth you can already see it getting shinier there and in fact a longer piece of Emery paper is a good idea but this is what I've got so that's what I'm using now if you're going to do this you know make sure you've got room left in your dimension because you can easily take off a half-hour or more by doing this and in fact if you need a really precise dimension say in the tenths range finishing up with Emery paper is a really easy way to do that so now we're up to 1000 here and the goal of each pass is just to effectively remove the sanding marks from the previous pass I mean we're going to go up to 2,000 now and so now a little bit of oil never hurts an oil just keeps the sanding area clean it keeps the grit from collecting in the paper okay so now the surface is just like butter now the toolmarks actually look worse but that's an optical illusion in a sense because what you're seeing is the tool marks that were a little deeper say you know a couple hundred thousands deeper than the other areas here which you know we've made really shiny if you really want to get rid of all the tool marks you need to go further with the 400 grit until all tool marks are gone but remember if you do that you're probably gonna take you know three four tenths off of your dimension if not more if you get carried away with the 400 you can easily take off a thousandth or two so be careful with that but if you need to rescue a mediocre mediocre surface finish emery paper can definitely do it so that is surface finishes in a nutshell I hope you found something in here useful and please do consider supporting me on patreon there's a link down there in the description and we will see you next time thanks for watching you

Channel: Blondihacks

Views: 128,935

Rating: 4.9317298 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, resin casting

Id: 1Fgj7m4MTbw

Length: 19min 6sec (1146 seconds)

Published: Sat Mar 23 2019