Some problems with lathe run outs and reaming a hole
Most of the experienced people know exactly what it is but the majority of people don’t, mainly because they have had no formal machining education and had to learn everything the hard way, by trial and error, and in most cases a lot of errors are made. Runout on diameters is derived from TIR,total indicator reading/deflection) which is the swing of the indicator needle to its extremity on both the plus and minus side of the diameter you are checking. Runout is exactly half of the TIR. If the TIR is .005 then the runout for this particular piece is .0025, or eccentric by that same amount. Not to be confused with elliptical/out of round conditions etc .When 2 or more diameters on the same shaft,part) have one common center, they are said to be concentric to each other but they also have to be in alignment for the length of each diameter. These two criteria must be met for concentricity. Conversely if they are not, they are said to be eccentric to each other and just how much is determined by the TIR and runout is then established. A diameter can still be considered concentric even if it is tapered as long as the two centers are coincidental. This is mostly for round parts . TIR plays a very important role on flat surfaces too. The total swing of the needle tells you how much it deviates out of flatness, alignment and straightness on the surface to be regulated by its tolerance alone. A part print tolerance of “flat within .001†means exactly that. The TIR has to be .001 or less. If it’s out of tolerance one can say it runs out, outside its tolerance.
Here’s where a lot of the problems start, right at the lathe that’s doing the boring/reaming /turning etc. Some of the older lathes that are of higher quality ,Hardinge/Sheldon/Monark/P&W/Clauson etc) have no problem with this, but many of the imported ones and I class some of the Chinese clunkers in with this, but not all. Not every lathe is centered correctly but it is never noticed by the inexperienced because he doesn’t do really close work. This is where it really shows up even though gunsmithing is not close work. “Close†is relevant. Some think .005 is close, others, .0002 and under is, all depending upon the person/experience and knowledge.. To check lathe runout it’s done with set up tapers which are a set of two hardened diameters built on a usually #2 Morse Taper or what have you, and made in sets. They have a 4.00/6.00 long OD that is ground to the same diameter. One is inserted in the head and the other one in the tailstock and an indicator run along them to see if there‘s any TIR deflection. In many cases the two do NOT match up on both the YY and ZZ. This means that one may be higher than the other. The tailstock setover controls the YY,but not alignment) but now we do not have any adjustments for the ZZ. They HAVE to be at the same height and the best way is to shim the headstock. Moreover, if the tailstock is set so it will not cut a taper on a turned bar, it can still be out of alignment and will show up immediately on reaming as the reamer will cut oversize as the hand wheel is advanced. This condition will enable any reamer, whether it be a plain /spiral or chambering to cut large. Some lathes have been as much as .0015 out of alignment with the tailstock and it’s a nightmare to get it right. You can readily see if the tailstock be lower/higher than the headstock and you want to chamber ream, the reamer will cut oversize if held in the tailstock chuck. Even free floating it OC, will produce a strain on the reamer and it will cut larger near the end even though it be piloted. Yes, the diameter if piloted will be concentric to the bore but it’s not the chamber/or hole that you want. You have to your advantage a 4 jaw and end control that you can align the bore OC but the tailstock center has to be bugs nuts in relation to this to achieve a perfect ream. The law also says that if a trued and rounded part whether it be OD/ID is indicated in on three of its quadrants then the fourth HAS to follow. If it does not, you have a problem somewhere else and it may be related to the machine itself. If the headstock is not perfectly parallel to the ways/alignment of the tailstock the resulting chamber will not be in alignment with the bore, thus increasing throat erosion quicker on the one side. If the head is out of alignment even a small amount it can create a real PITA to indicate in a barrel prior to chambering. Giving this condition, the threads and any other diameter that you turn will be in perfect concentricity at the same set up, but once an unaligned tailstock is employed, the reamer will again cut large, but still be concentric. Both head and tailstock must be perfectly aligned to achieve a perfect reamed hole. It’s basically the same thing as boring/reaming a hole on a vertical,that’s not in tram) and bringing up the table. The reamer will actually bend and you will produce a larger tapered/out of square hole, all the time. It’s not much, but when you’re after accuracy, it will NOT be there. A simple way to check this is to put in a plug gage in the tailstock ,or a set up taper), lock it down, then run the indicator down the length of it for any lateral runout . Any deflection means you have tailstock runout on either YY or ZZ. There’s a lot to say about the reaming of any kind, but to produce a perfect chamber/ID that is right on the money, the whole set up has got to be right on the money too. “Olde†Timers and experience know all about this and will be bored. I hope I didn’t wander too much but if it helps someone, it’s all worth it. I’m sure many may disagree.
Thanx Tec
Most of the experienced people know exactly what it is but the majority of people don’t, mainly because they have had no formal machining education and had to learn everything the hard way, by trial and error, and in most cases a lot of errors are made. Runout on diameters is derived from TIR,total indicator reading/deflection) which is the swing of the indicator needle to its extremity on both the plus and minus side of the diameter you are checking. Runout is exactly half of the TIR. If the TIR is .005 then the runout for this particular piece is .0025, or eccentric by that same amount. Not to be confused with elliptical/out of round conditions etc .When 2 or more diameters on the same shaft,part) have one common center, they are said to be concentric to each other but they also have to be in alignment for the length of each diameter. These two criteria must be met for concentricity. Conversely if they are not, they are said to be eccentric to each other and just how much is determined by the TIR and runout is then established. A diameter can still be considered concentric even if it is tapered as long as the two centers are coincidental. This is mostly for round parts . TIR plays a very important role on flat surfaces too. The total swing of the needle tells you how much it deviates out of flatness, alignment and straightness on the surface to be regulated by its tolerance alone. A part print tolerance of “flat within .001†means exactly that. The TIR has to be .001 or less. If it’s out of tolerance one can say it runs out, outside its tolerance.
Here’s where a lot of the problems start, right at the lathe that’s doing the boring/reaming /turning etc. Some of the older lathes that are of higher quality ,Hardinge/Sheldon/Monark/P&W/Clauson etc) have no problem with this, but many of the imported ones and I class some of the Chinese clunkers in with this, but not all. Not every lathe is centered correctly but it is never noticed by the inexperienced because he doesn’t do really close work. This is where it really shows up even though gunsmithing is not close work. “Close†is relevant. Some think .005 is close, others, .0002 and under is, all depending upon the person/experience and knowledge.. To check lathe runout it’s done with set up tapers which are a set of two hardened diameters built on a usually #2 Morse Taper or what have you, and made in sets. They have a 4.00/6.00 long OD that is ground to the same diameter. One is inserted in the head and the other one in the tailstock and an indicator run along them to see if there‘s any TIR deflection. In many cases the two do NOT match up on both the YY and ZZ. This means that one may be higher than the other. The tailstock setover controls the YY,but not alignment) but now we do not have any adjustments for the ZZ. They HAVE to be at the same height and the best way is to shim the headstock. Moreover, if the tailstock is set so it will not cut a taper on a turned bar, it can still be out of alignment and will show up immediately on reaming as the reamer will cut oversize as the hand wheel is advanced. This condition will enable any reamer, whether it be a plain /spiral or chambering to cut large. Some lathes have been as much as .0015 out of alignment with the tailstock and it’s a nightmare to get it right. You can readily see if the tailstock be lower/higher than the headstock and you want to chamber ream, the reamer will cut oversize if held in the tailstock chuck. Even free floating it OC, will produce a strain on the reamer and it will cut larger near the end even though it be piloted. Yes, the diameter if piloted will be concentric to the bore but it’s not the chamber/or hole that you want. You have to your advantage a 4 jaw and end control that you can align the bore OC but the tailstock center has to be bugs nuts in relation to this to achieve a perfect ream. The law also says that if a trued and rounded part whether it be OD/ID is indicated in on three of its quadrants then the fourth HAS to follow. If it does not, you have a problem somewhere else and it may be related to the machine itself. If the headstock is not perfectly parallel to the ways/alignment of the tailstock the resulting chamber will not be in alignment with the bore, thus increasing throat erosion quicker on the one side. If the head is out of alignment even a small amount it can create a real PITA to indicate in a barrel prior to chambering. Giving this condition, the threads and any other diameter that you turn will be in perfect concentricity at the same set up, but once an unaligned tailstock is employed, the reamer will again cut large, but still be concentric. Both head and tailstock must be perfectly aligned to achieve a perfect reamed hole. It’s basically the same thing as boring/reaming a hole on a vertical,that’s not in tram) and bringing up the table. The reamer will actually bend and you will produce a larger tapered/out of square hole, all the time. It’s not much, but when you’re after accuracy, it will NOT be there. A simple way to check this is to put in a plug gage in the tailstock ,or a set up taper), lock it down, then run the indicator down the length of it for any lateral runout . Any deflection means you have tailstock runout on either YY or ZZ. There’s a lot to say about the reaming of any kind, but to produce a perfect chamber/ID that is right on the money, the whole set up has got to be right on the money too. “Olde†Timers and experience know all about this and will be bored. I hope I didn’t wander too much but if it helps someone, it’s all worth it. I’m sure many may disagree.
Thanx Tec
