SHOP TIPS #192 Intro. to Gears & Gear Cutting on Bridgeport Mill tubalcain

howdy is tubal-cain again and in this edition of machine shop tips entitled introductions to gears and gear cutting I'm going to talk a little bit about the different kinds of gears and the gear nomenclature and gear formulas and all the related information that is really necessary for you to know to learn about and to study before we get into the next few chapters which we'll deal strictly with the cutting of the gears in an upcoming chapter I will be cutting a gear rack and a gear rack is nothing more than a straight or a linear gear and they can be cut on the bridge sport but only very very short ones like I will show you in the video longer ones of course have to be cut on a horizontal type of milling machine bevel gears are pretty common gears sometimes called miter gears and you will be able to cut these on a bridge port with the special equipment as special cutters but it's probably not something the average home machinist is going to do here we have a large angular gear with the smaller gear remember the smaller gear is often called opinion such as the rack and pinion these are called internal gears or ring gears and there's a complete planetary gear system these are helical gears they run so much quieter than common straight to their spur gears but it's not something you can do at home this is an interesting type of gear called a herringbone gear and this one is very large six feet in diameter again something that you can't do in your home shop these are worm gears actually this is the worm these are very large compared to a man and this is the worm gear or the worm wheel they reduce the speed greatly but again you can't do this in your basement shop these are common spur gears now spur gears have the teeth straight across and this is the type of gear that you can cut on your Bridgeport mill of course you need a dividing head and special cutters which I'm going to talk about here in a minute but you notice that these gears have different sized teeth and that is the pitch of the year or the diametral pitch I'm going to cover that too the interesting thing is that the the way that I'm going to show you to cut gears here on the bridge board is not the way they're cut in industry industry and mass production at all it is really a rather slow process it would be used in a machine shop to make replacement gears or one of or by you guys in your home shops that are made possibly making some lathe gears and I will show you how to do that and some upcoming chapters gears are mass-produced and cut in industry with a cutting tool called a hob and it's done on a hobbing machine not a milling machine you might find some interesting YouTube videos on that if you want to study on that but that is the way that they are mass-produced certainly not by this method that we can do in our own shops in order to cut gears on your standard Bridgeport milling machine here's what you need you need a dividing head of some sort and I have shown you several different kinds possibly a tail stock depending on your setup and you most certainly need the correct cutters which I'm going to talk about in a moment and then you need an arbor to hold the cutters in the vertical position and these are available with a straight shank and this is a 3/4 straight shank or an r8 shank they'll fit directly into the spindle and the diameter of these is either one inch or 7h that is the arbor is to hold the cutters our 7/8 or one-inch and these are 7/8 cutters that's probably the most popular size more in these gear cutters this of course could be homemade but it's preferable to buy one because there are only about $40 this is an 1850 gear cutter probably made by Brown & Sharpe they say but most interesting and look at that dividing plate this is a dividing head can you see the little holes the whole circles and up here is the Chuck and then you can see the gear cutter there so that's where the actual cutting took place right up here and the work held in that circular Chuck I can't get any closer to show you that and mounted on a rather elaborate ornamental type of base let's talk about resources and where you can find information on this besides the internet some of these old catalogues here and I like these Boston gear catalogs you still find these around there's quite a bit of good information in these and several of pictures that I took or showed you a little while ago came out of this older book not will any longer available it's quite old but Machinery's handbook regardless of what Edition you have has a very large section on gears and matter of fact starting here on this page there are approximately 200 pages on gears and quite a few pages just on spur gearing so that can be one of your major resources in addition to that this is a very good book called you're cutting practice and you can get that on eBay or maybe even downloaded I paid $4 for this book but there's a lot of good information on gears and gear cutting here I have shown you this before and Cincinnati a milling machine company and that of course is the dividing head that I'm going to be using and this information in here tells you how to cut gears and how to use the dividing head so that's a good resource and there are just any number of standard machine shop books that you can find either new or used that covered gears gear cutting dividing and so on so be sure and look at some of these resources here because they are very valuable in worthwhile in these videos were really only interested in cutting involute spur gears with the 14 and 1/2 degree pressure angle and in the reality often you will have a drawing that will tell you exactly what diameter to cut the gear how many teeth what the demet religious and possibly even what cutter to use but in case you do not get that on your draw or your specifications going to show you how to determine all that information and along with that goes some of this terminology that we need to cover and I'm going to only cover part of it there's much more than what I will show you and you can certainly look that up and read about it in the resources that I have shown you but in reality much of that information is engineering information and may not be needed at the milling machine but when you look at a gear like this you can count the number of teeth and often the number is stamped on that for instance this is a 44 but we need to know the outside diameter because we have to turn the gear blank to whatever diameter is necessary and it must be very precise when you make your gear blank in some cases it's easier to show you this on a toy gear but there's two circles drawn here that I drew on it and the outer circle is called the pitch circle and it is simply the curve of intersection of a pitch surface of revolution and a plane of rotation according to theory it is an imaginary circle that rolls without slipping with the pitch circle of a mating gear now is that clear and the inner circle is called the root circle and it is tangent to the bottoms of the toothed spaces and a cross-section I don't really expect you to understand that but the pitch circle is an important circle now here's a good picture out of one of these real old books you need to know with the addendum and the D dindim is the addendum is the distance from the pit circle to the outside diameter and the D dindim is the distance between the pit circle and the roof diameter and that's why I just gave you those diameters on that toy gear a moment ago the circular pitch is the distance from the center of one tooth to this of the next tooth and circular pitch is the same as linear pitch on a gear rack when we're cutting we will cut to the whole depth in that I can be found either by formula or it might be indicated and marked on the gear cutter that's the hole depth but in fact the gears do not mesh all the way to the bottom and that's called a working depth there must be a little space left so that there is not too much wear or noise or clashing and that is the clearance a little space that is left there is the thickness of a tooth now there are many other terms and a great deal of terminology that I am NOT going to cover and I don't think it's necessary for you to know when you're cutting a common spur gear probably the most important term of all and I haven't mentioned it yet is the diametral pitch and that is the number of teeth on the gear per inch of diameter of the pitch circle and it will be sometimes indicated by a DP for de mettre pitch but most often just a p on this gear cutter notice that it's the number 4 cutter 12 P well that's 12 diametral pitch not all of these gears have the same diametral pitch notice that this gear will not mesh with this one or with this one whereas these two mesh and they are the same diametral pitch so when you see the teeth that are different size that's what it means is that there are different than natural pitch so you much must match up your gears if you're making gears or replacement years so that they will have the right pitch and will mesh with one another if we need to make a replacement gear and there there is no drawing how do we determine that a Metro pitch well most all of the catalogs have a age with silhouettes this is the Boston gear catalog and you merely match the gear up with the silhouette until you get one that fits there are also gauges for this that are available this company Chicago gear Works has the same thing and the small gears are way here on these little inner circles and the numbers you see are that a metro pitch and in this other catalog they go on for pages as the gears get larger to determine diametral pitch and other dimensions but there's another method mathematical and it's quite accurate take this gear for instance it is 40 teeth it's marked on it but not all gears will be marked if not count them up and be very accurate do it twice so we take that number and then we also measure it with a micrometer or a caliper and there is my dimension slip just a bit I'm just a couple thousands off there but it won't matter then using this formula for 40 teeth and the diameter below it P or DP equals the metro pitch is n plus 2 over D and as the number of teeth D is the diameter and if you work that out that's 40 plus 2 over the diameter simplify it 42 over 2 point 3 3 2 divide that out and we get 18 point 0 1 and it was carried out even further than that but forget about the change that is an 18 diametral pitch gear next i want to talk about gear cutters there are eight gear cutters in a set of cutters for each time natural pitch well why are there so many and why do we have to go to that expense and the hour expensive because the shape of the teeth very starting with a gear a very small gear has which we sometimes call opinion has teeth that are quite curved and as we approach a larger gears and then all the way up to a rack and this is a rack here which is a straight gear their teeth are almost trapezoidal or straight lined and all of the other cutters gradually change from this straight line to curved a little bit more curved and so on and that's why there are so many cutters in a cept in this page from a very old brown and sharp catalog from the 40s here are the cutters that they sell and any given that Metro pitch from a number one two or three four five six seven eight you can see that a number one allows you to cut large gears from a 135 teeth up to a rack a number four for instance allows you to cut gears with teeth anywhere between 26 and 34 teeth and as we advance here and get all the way to number eight notice that that cutter is only capable of cutting 12 and 13 - it's ears so depending on the job you may not need all of these you can purchase or obtain just the cutter you need for that particular job because these are pricey sometimes of $50.00 and more each and you can buy them either individually or in sets in fact what I just told you a moment ago was a bit of a fib because there are actually 15 cutters in a complete set but I don't believe they do that anymore but in the case of what I told you a moments ago on that other page some of those cutters were only an approximation of what you really need so you can see here if you want to study this that these other cutters are kind of in between and allow you more exact gear geometry that's free of charge and you probably do not need to know that actually because of the cost of these cutters old tubal-cain himself doesn't even have full sets but I have some common ones here they can be used and I will be using these in demonstrations but let's take a look at what it says on an individual cutter this is the number 4 cutter in a set 12 diametral pitch and it will cut anywhere between 26 through 34 teeth and the 180 there is the depth of the teeth so you would write that down of course before you start because once this it's mounted on an arbor you can't read it anymore it's hidden and on the backside there's usually some other information such as keep it sharp it's made of high speed steel and it was made by brown and sharp and examine the cutter this is a form cutter to make sure it's not dull or chipped now these can be resharpened at a tool and cutter grinder and all they do is grind the end here so they don't change the form of the to shape most of these cutters are approximately 2 inches in diameter and you need to know that remember that when you set your spindle speed that you're actually dealing with a rather large cutter so you have to slow it down especially if you're cutting steel or harder materials and you need to know the bore side size mentioned previously this is 7/8 and one other thing that you might need to know and write it down before you mount it is the thickness of the cutter so check it with a micrometer right here the thickness and they vary none of the limb will be the same the reason you need to know that is that when you go to set your cutter on the centerline of your Arbor when you're cutting it you will you might need to know that dimension or half of that dimension so that you can find your Center there are rules and formula for dimensions of spur gears and this information is available in many different books or Machinery's handbook and another excellent book is a booklet put out by cincinnati milling machine company and I use it when I was teaching and as a student simply called cutting spur gears and it is available on the internet as a download I believe from vintage machinery but here are the formulae that you probably need to know many more are available because sometimes what you your Givens are not the same so diametral pitch is simply number of teeth over the diameter there are other formulas for that often you need to know the outside diameter and that equals the number of teeth plus 2 over the de mettre pitch the hole depth is usually given to you on a cutter but if it is not use this formula 2.15 7 over the Metro pitch and you can find the hole depth and that's strictly for 14 and 1/2 degree pressure angle now you want to know more about pressure angle you're gonna have to read up on it because it is incredibly complex and finally to determine the number of teeth its diameter times diametral pitch that will get you started but be sure and read through all of these books that i have recommended and that concludes this video on spur gears and much of what I have just told you will be used in later videos where I cut spur gears by several different methods on the Bridgeport milling machine so be sure and watch those other videos and this is tubal-cain saying so long for now

Channel: mrpete222

Views: 136,548

Keywords: south bend lathe, atlas lathe, cincinatti milling machine, index head, dividing head, sherline lathe, myford lathe

Id: JMhYuXx35Ik

Length: 21min 45sec (1305 seconds)

Published: Sun Feb 08 2015