I was taught 29 deg so that the bulk of the material is taken with the left side of the tool cutter. The principle being to keep the forces clearly on one side of the tool so that the backlash in the system always on one side. The light cut with the right hand side is essentially a finishing pass so that you don't get small ridges in your part.
The lathes at your work probably have enough horsepower to handle a plunge cut. But even if my lathe had enough HP I still find the threads to be ragged if I plunge straight in rather than taking a cut off of one side by turning the tool to 29.5 degrees.
Yep, that is the way I was lead to understand it. The primary concern is to keep the cutting forces clearly on one side of the backlash and the secondary was the light pass on the RHS of the tool.
So are you saying that the light cut on the right hand side (the part shown in red on the schetchup drawing at 4:20 onward) is something like a finishing pass that happens on every pass? If so, I guess that makes sense. I hadn't thought of it that way. I was thinking that the purpose of setting the tool at 29 or 29.5 was to prevent the right hand side of the tool from cutting at all.
Hi Keld, The only thing that matters is the destination, not how you get there. There is no single correct way to thread, the "best" way will depend on many things how you were taught being one of them, others being the material and the machinery. I remember demonstrating threading at one of the shows here in UK and a chap said that when an apprentice he was taught angled top slide but when he got a job in industry they laughed him for complicating the process saying just go straight in!!
Based on reasoning 29 degrees is a safety margin. Machining is a process of exact values where as you cannot error on the plus. Meaning that you cannot replace material once removed. 29 degrees is a form of "sneaking up". This difficult operation that takes time to master. 29 degrees allows one to achieve success from a novice perspective. As the novice Machinist masters the threading operation he/she will no longer need the safety margin. They will soon set their compound to prescribed 30 degrees as they will know their machine and its quirks. Any value over 30 degrees will over shoot the 60 degree thread profile, causing a jagged edge by a factor of one degree on each pass. The illustration does not focus on the resulting thread profile. 29 + 29 = 58 , 29.5 + 29.5 = 59, 30 +30 = 60 workable profiles. 31 + 31 = 62 stair steps to the right eating into the previous pass.....
I was taught that the reason for the 29 or 29 1/2 degree angle of the compound is to cut mostly on the left side of the tool while the right side of the tool would act as a support to prevent/minimize chatter. Doing most of the cutting on one side also minimizes the cutting force required again helping to reduce chatter to form a thread with a better surface finish. (600 hour Machinist Course, Aberdeen Proving Ground, Maryland 1965)
If it's not too late to join this discussion, some time ago I was in contact with a really old old-timer who was taught to set the top slide at 30 but make the threading tool 59 degrees. The forward cutting edge being set to cut at the correct angle. leaving clearance on the backside. Well, in truth it was 27.5 and 54 (for Whitworth threads) but some I didn't want to confuse the issue.
You just have to choose an angle LESS THAN half the angle of the thread. This way you can be sure that the shape of the cutting groove is right in every pass - you just have to make the groove deep enough. You have to be sure too that the cutting tool is perpendicular to the turning axis. There is no need for "cleaning pass" or "last pass with cross slide" - that's pure rubbish !
By using the 29 degree angle you are going to get the 60 degree thread. If you used 31 degree you would end up with a 61 degree or greater thread. By rubbing the side on one side and cutting on the other you will get the 60 degree of your threading tool. what you want is for both sides to the tool to always be touching the material that you are threading. (note that on the rub side you maybe getting some cutting but not enough to cause a problem with the chip of the other side, but by cutting a little on the rub side you are guaranteeing a 60 degree thread ).
Your looking at it first of all wrong in that your illustration is of a already cut thread. So now visualize it before it is cut and as you move the cutting tool into the work its actually cutting the left hand side. Think about it, as you crank the tool in at that angle of 29deg the tool is moving inward and leftward in the direction of the 29d line, which means its cutting on the left side of the tool not the right side, just the opposite of what you would have thought.
@crcarlsonUT So are you saying that the light cut on the right hand side (the part shown in red on the schetchup drawing at 4:20 onward) is something like a finishing pass that happens on every pass? If so, I guess that makes sense. I hadn't thought of it that way. I was thinking that the purpose of setting the tool at 29 or 29.5 was to prevent the right hand side of the tool from cutting at all.
Not disagreeing with you but there are few "absolutes" in engineering except perhaps for zero degrees Kelvin. Personally. if I hear of a different way of doing something I give it a try and if it works I file it away in the old brainbox for future use. My current favourite for general purpose single pointing is an 1883 design clapper box tool holder and going straight in, but I do understand why others have their own favourite ways.
@Films4You Yep, I've seen many of his videos. They are very good. I'm not so much questioning the 29.5 degree concept, I'm more wondering if it shouldn't be 31 degrees instead of 29 or 29.5 - especially after drawing the whole concept up in sketchup (4:20).
I am just an inexperience hobbyist, but the two friends who taught me to thread were both master machinists with vast maching experience in both job shop and r&d/production management..they both taught that the tool should be advanced by the compound set at 1/2 the thread angle, e.g. 30' for a 60' thread angle until the light finishing cuts when the cross slide should be used to feed the tool straight in. using their advice I've had no trouble, brain fart moves excepted, even at my novice stage of development, producing threads both internal and external to specs as measured by thread mike and peedee wires. looking at your drawings I don't believe that they accurately reflect the two threading processes. you may wish to view a you tube vid on the subject by marc lecuyer aka thatlazymachinist.
I have been a machinist for 14 years now and i was always taught 29 degrees in school. And yet when i go into the workplace, at EVERY place ive worked, they leave it at 90 degrees haha. Dont ask me why.
ups ... I clicked send too early ! ... and lots of people DON'T know why - they just learned it blindly - and that's not good ... but lots of people don't care, and that's bad too .... ! No Wonder the world is running like it does ...
Does a 29 degree angle constitute a 60 degree angle cut ?It is close Probably good for most practical purpose, But to take advantage of the threaded surface to give even wear, You have to make sure the tool bit is at the correct angle and your feed in is exact. Then when you use the thread it will be a uniform fit and it will tighten better then if it is not, as it will slowly cut into the other thread, Probably not noticeable in most cases, Just cut it close and you will have sufficient use from it
nope, a 30 or less in-feed angle will still yield exactly the angle that the FORM of the cutter profile has, so exactly 60 degrees, because it is a FORM tool and the last cut will cut on the entire perimeter of it's form (at least the part that is below the o.d of the stock.
