Essential Machining Skills: Working with a Lathe, Part Two

tapping a hole in a part is done on a lathe and very much the same way that it's done on a milling machine or a drill press we started here by drilling a pilot hole in preparation for putting threads in the hole I'll start by putting the part securely in the truck because during the threading operation replying torque to the part I start by locking the spindle and on this machine I do that with this knob pushing in until it clicks into place and now see I've blocked the spindle I start by putting this little tool in the Chuck to use as a pilot for supporting and guiding the back end of the tap handle like this it's also possible to use a regular taper this is a live Center can also be inserted in the tailstock and used to support the back of the tap handle we start by inserting the tap into the hole and bringing the small taper into contact the back of the tap handle now I'm moving towards the back of the machine this gives me better position which to turn the handle the tap handle a while I'm turning this I'm also applying just enough torque to this wheel to maintain contact between the taper and the back of the tap handle if I don't keep up with this handle what happens is that the tap handle unseats itself from the taper at this point it's very easy to break the tap so make sure you maintain contact here after several turns the tap handle it's a good idea in many materials to turn the tap backwards about a third of a turn this breaks the chips inside the tap and frees them up so that they can be forced out of the flutes that run up the side of the tap after going in about half the length of the threaded portion of the tap it's usually necessary to remove the tap from the hole to completely clear the chips very much the same way that you do when you're drilling a deep hole now you can see when I was removing just the last few threads of the tap from the hole we're shaking the tap handle around quite a bit if you're working very soft materials it's easy to damage the last few threads that way so then it's a good idea when withdrawing the tap to remove the tap handle and withdraw the tap by hand we clean the flirts to the tab and carefully start the tap it's easy at this point to start the tap in the wrong direction and start producing another set of threads if you feel the tap starting to bind at any time you've probably started to tap incorrectly should back out and try again many materials can be tapped dry to get a better finish or to tap particularly tough materials such as stainless steel it's a good idea to use a little bit of cutting fluid you continue this process until you have completely threaded the hole a drill will produce a hole in a part like this that's within a few thousandths of an inch of the nominal diameter of the drill if you want a more accurate hole than that a common way to achieve that is with a boring tool like this this long thin tool allows you to get down inside a hole like this and machine that surface to set up the lathe for boring start by putting the part in the Chuck put the lathe tool in a holder like this clamps it securely and this holder placed on the tool post and lock down start by bringing the end of the tool just inside the hole and then with the cross slide bringing the tool out until it's nearly in contact with the side of the hole spin the Chuck to make sure that there's no interference between the part and the tool or between the Chuck and the rest of the lathe now I'll bring the tool out till it just contacts the inside of the hole you can hear it making a very light cut now I'll back the tool out of the hole dial in a desired cut in this case all I want to do is clean up the surface of this hole so I'll remove about ten thousandths of an inch during this pass this is an operation that's very difficult to control by hand the variations in feed rate that you can avoid when you're feeding buy and produce variations in the cutting force on the tip of the tool these tools are typically very long and thin so those changes of cutting forces produce relatively large deflections in the tool which then change the diameter of the hole so for any holes other than very very shallow holes it's best to get the tool set right make sure the depth of cut is what you expected and then engage the power feed for the saddle because you cannot see the end of the tool during the cut it's very important to know how deep the tool is in the part if the hole that you're making is not a through-hole you can do this very simply and roughly by running the tool down by hand until it touches the bottom of the hole a very quick way of setting some indicator of the depth is simply with a marker or a piece of tape that you can put on the boring bar at this point in that way during the following passes you make as you enlarge the hole up to the desired size you can then disengage the power feed just before you get to that mark and then continue feeding the rest of the way by hand another more accurate way to do this is with a dial indicator you can adjust the micrometer so that the dial indicator reads zero back the tool out make another pass and when the dial indicator starts to move that's a warning that we're getting close to the bottom of the hole and it's time to stop the power feed and continue from that point by hand another common way of setting a stop for the depth of a hole is with a saddle stop which is simply a clamp that attaches to the bed of the lathe and generally incorporates a micrometer adjustment like this you can see after the first pass I've made with a boring bar that the finish the boring bar is produced is much better than that that was produced with a drill the hole is now very round but not yet to the final size so we now measure the hole we've made and then using the dial on the saddle we can enlarge the hole until we reach the desired size this tool is not very stiff so you can't take a large cut so typically you have to enlarge the hole in several steps until you approach the final size when you're very close to the final size say within five thousandths of an inch it's a good idea to take a very light pass re measure the hole and then take a final pass of a few thousandths of an inch knurling is often used on tools such as the center punch or this drift to improve the grip you have on the tool and here if you look closely you can see there's this little diamond pattern over the surface of the tool this pattern is produced with this tool this tool is inserted in the lathe and then brought up in contact with the part and then fed down the length of the part there's some fairly careful setup that's required initially to get the angle of the tool set right this way and also the height of the tool I'll bring the tool into contact slowly and then have a look at the pattern it produced if you look closely here you can see that the tool is offset slightly from being perpendicular to the part so the tool is cutting in deeper this edge then at this edge that's about the right angle we want so we're going to plunge into the part this way and then feed down the length of the part if you look very closely you'll see though that rather than a diamond pattern with cross hatching there only one of these rollers has come into contact that's because of the height of the tool isn't set quite right you want to keep adjusting the height of the tool until you get a uniform crosshatch pattern okay now both wheels are coming into contact with apart we are getting a uniform crosshatch now it plunge the tool into the part and wait several revolutions for the teeth to settle down into the part before engaging the feed now you can see the tool feeding down the part leaving knurling behind it not so good what's happened here is that the wheel producing these grooves settle down into a uniform pattern if you look at the cross hatching and see that the wheel in the other direction is actually producing about twice as many grooves per unit distance this wheel is producing now you will get this behavior you think about it in almost every case because unless the wheel diameter is a multiple of the diameter of the shaft in one revolution this tooth on the wheel won't come back to exactly the same place and the way to avoid that problem is to plunge quickly enough so that you produce deep grooves during the first revolution deep enough so you form these that allow this wheel even though it's not going to register exactly to catch the V produce in the first revolution and then fall down in place now after several more revolutions some averaging takes place so that the wheel produces uniformly spaced grooves around the part so we'll try again come in a little faster this time okay much better you can see here that this pattern is symmetrical as it is on this Center punch and this is the desired effect the compound on the lathe is often used for cutting tapers do this we start by loosening the bolts that secure the compound to the cross slide see here the base of the compound other degrees marked out to rotate the compound to get the desired angle lock the compound in place and use a tool that is typically the same tool that's used for turning before you begin the cut advance the compound to the limit of its travel so you make sure that when you finish the cut you haven't used up all the available travel in the compound back the compound off until you clear the end of the part start the machine and with a compound and cross-slide you just the tool until you're taking the desired cut now with both hands on the crank on the compound it's possible with practice to maintain a fairly constant speed you can see after making this first pass that we still have plenty of travel till we run to the limit of the travel where the compound which we will need as a speech it becomes wider take another cut and you continue in this way until the feature has reached the desired dimension turning long thin parts such as this one can be a problem due to the deflection of the part due to the cutting forces produced by the tool I'll show you what that looks like and sounds like I try to take a pass this part you can hear the noise the chatter and if you look closely you can see a very irregular surface finish what's happening is through the deflection of the part the part rises up a little bit rides up onto the tool and then back off the tool and vibrates in order to deal with this problem start by machining a center to the end of the piece now have the center machined into the end of the piece and lock the piece back into the Chuck remove the drill chuck from the tailstock and replace it with a live Center the center has ball bearings in it or roller bearings that allow the end of the center to rotate freely so we can lock the tail stock in place with the hand wheel bring the center firmly into contact with the work piece and then lock the tail stock in place now we have this part supported both by the Chuck and by the center but the past I'm making this time is the same in depth and speed as the past that I just made this time you don't hear the chatter and if I stop the machine you can see they have a much better surface finish the most common way of threading apart is with a die in a die stock like this I prepared this piece of stock by turning it down to the desired diameter which in this case is 3/8 of an inch I was using this die we'll just start by pushing the die up against the part and turning the die stock proceeding down the part to produce the thread it's a very quick and easy way of producing a thread but the thread is not particularly accurate in terms of diameter of the thread or also in terms of the straightness of the thread this die is fairly short so it tends to wander a little bit as it's cutting the thread there are times when you want a much more accurate thread than that and those times you would typically use a tool like this if you look closely you can see the 60 degree angle the thread is machined into the end of this tool and this tool is used for single point thread turning and put the tool in a tool post make sure it's perpendicular to the axis of the machine so that the thread ends up being symmetrical apart the next step is to couple the feed of the headstock with the feed of the table so that we produce a thread of the desired pitch to do that there's a transmission that couples the headstock to the table of the lathe the first step in setting up the machine here is to select the correct transmission ratio between the headstock of the machine and the table feed in order to produce the desired number of threads per inch if you look closely here you can see two sets of numbers the bottom set of numbers is the number of thousandths of an inch traveled per revolution for cutting the numbers on top are the equivalent number of threads per inch for that transmission ratio in our case we're looking for 24 threads per inch by disengage transmission and move this indicator over until it's in the desired column reengage the transmission now coming down this column 2:24 come across come to B and C this is another part of the transmission it needs to be set the a B knob is here set that in the B position and down here is a knob marked C D and E not knob wants to be set in the C position not quite done yet a couple of other levers need to be set the lever here Marc belt feed and gear thread we want to set that and thread and there's another lever here it also needs to be set in the threading position not done yet it's another lever needs to be set we're going to be making threads that are standard or right-handed and set that knob in place we're finally done this is the most complicated machine that I'm familiar with to set up for threading you'll find that there are no industry standards for how this is done some machines are this complicated other machines are far more simple but you'll find no consistency some machines have knobs other machines have levers they'll put them in different places take the time anytime you're working on a machine unfamiliar to you to determine the function of all of the controls be certain that they're right before turning on the machine and using it in the machining operation now if we come over to the table we have an indicator here see as I move the table indicator turning indicating the relationship between the thread feed screw and the table down here you have a lever marked threads only use this lever by setting the forward reverse knob on this particular machine in neutral now you find push this lever down in certain positions it will engage the table with the thread feeds now let's turn the machine on if you look at what the thread feed indicator is doing now it's coupled to the screw so it's indicating that the screw feed is turning if you look closely there's a mark here as a number approaches the mark you'll find you're able to engage the thread feed at that point and you'll feed the thread until you reach the end of the thread and then disengage the feed at that point you back the tool out of the thread back the tool away from the part and now wait for that number to come around again and you proceed in that fashion until the threats complete again there's no consistency from one manufacturer to the next the safest way to do this is to always use the same number on many machines there are some threads that you can cut on any number there are some other threads that you can cut only on even or odd numbers and then there are some other threads that can only be cut on a single number become thoroughly familiar with the machine before you start experimenting something like that now show you in more detail how to set up the cutting tool to begin the threading procedure before we actually start cutting the thread got a little bit more preparation to do do this with a parting tool I'm coming in right up to the shoulder with a parting tool then cutting down into the shaft slightly deeper than the depth of the thread this is called a thread relief and its purpose is to give me a place where I can stop this tool at the end of a threading pass take the threading feet out of gear and then back the tool out before we can actually start threading the part there's one more setup operation that needs to be performed when you need to set the compound to the thread angle which in this case is 30 degrees loosen up the compound and rotate it you look closely here you can see the degrees marked off in the compound so rotate this around the 30 degrees and lock it in place we want to make sure that the threading tool is perpendicular to the axis the rotation of the part and lock it in place the purpose of setting the compound at this angle is so that so we're taking deeper and deeper cuts producing these threads we take those cuts by incrementing the compound in so that we're only cutting on one side of the tool if we were to increment the tool in during each pass this way with a cross slide the tool would be attempting to cut on both sides of the thread the two chips basically come off and fight each other tend to stack up on the tool and damage the threads so by setting the compound this way and only cutting on one side of the tool we get a nice clean cut we can start the machine now I'm doing here you see the tool moving around is I'm running the cross slide ahead until it comes up against a threading stop many machines don't have such a stop this machine does is very convenient so I run the clock cross slide up against the threading stop and then bring the tool in with the compound until it just touches the part now back the tool off look at the setting on the compound no take a relatively light cut this time we'll take about five thousandths of an inch now we'll wait until a number comes around to the mark engage it at one this is where threading gets pretty exciting see the thread being cut going to watch closely and be ready as soon as it runs off the end of the thread to disengage the feed wait a little too long you stuff the tool into the larger diameter of the part shut it off too soon and you damage the back of the thread now that it's disengaged we back the tool out of the part using the cross slide back up bring the tool in again with a cross slide up against the threading stop dial in another five thousandths or so wait for a number to come around engage the feed it's a good idea to keep a hold of this feed engagement lever so you don't have to go looking for it because you don't have much time so now we've completed another pass acting's out back it away from the part bring it again up against the feed stop dial in another five thousandth and again wait for the number to come around engage the feed stop very easy feed pop the feed after feed cutting away to lose track of what to do next resist the temptation to try to go too fast but will almost always happen is that you'll goof up the order what you're doing things what typically happens if you're in a hurry is you finish a pass you forget to withdraw this quick grab a hold of the carriage feed and you can look here and see what I've done to this thread it's unlikely that the person using this part will be satisfied with the thread that looks like that take your time this is a relatively tedious operation but you can expect that it's also a relatively precise one you you

Channel: Open Source Machine Tools

Views: 221,016

Rating: 4.8983297 out of 5

Keywords: Lathe, Machining (Taxonomy Subject), metal, steel, Metalworking (Visual Art Form), Manufacturing (Industry), open source

Id: jXET1-g6CJA

Length: 47min 3sec (2823 seconds)

Published: Fri Aug 30 2013