I posted this message to uk.rec.models.engineering, but it be of interest to this group as well.
Has anyone had success in screwcutting left-hand threads on a Myford S7B where both flanks of the thread come out looking good? I've gone through several feet of leaded steel, trying to cut a 7/16" x 20 tpi,
60 degree vee thread. I'm cutting under power, from left to right, back-geared low speed; tool upright,facing the rear of the lathe and ground for left hand cutting.
I assume this is correct for left hand threads. However, while the leading flank always come out looking great, the trailing flank is torn and pitted with occasional chatter marks. Any comments?
Yes, it will, for it has the potential to yield a drunken thread (I love saying that!). The pressure of the cut, when not loaded against the propelling face of the lead screw, can, and often does, force the tool away from the cut. That in and of itself may not be the reason you're getting a poor finish, however.
I suggest you chase your thread until you are near size, then take successive passes feeding with the cross slide only, not the compound, and taking roughly a thou each pass. That generally yields a superior finish on your thread. The chip, when your tool is very sharp (and honed) and the lathe is rigid enough to track properly, will be a perfect reflection of the cut, a very thin V shaped chip.
It is very important that you have ground the proper side RELIEF on your threading tool, and also important that you have not altered the top of the tool. It should have 0 rake, otherwise you'll effect included angle. That you're cutting a left hand thread may be creating the illusion that you have plenty of relief, but you may not have any and the tool may be dragging. Take a close look at the side of the tool to see if you see any signs of rubbing.
You didn't mention any lubrication. I suggest using sulfur based cutting oil.
Are you threading between centers? I avoid that like the plague. Running between centers is generally a poor way to make anything, albeit more accurate in most cases. Rigidity suffers when running between centers.
For chasing an external left hand thread, be absolutely certain you have your compound turned with the handle towards the headstock, and set at no greater than 29½°. I favor setting at 29°. Your threading tool should be DEAD on center, not above, not below.
Good luck with the thread. Any more problems, tell us more about your setup. You should be getting a very nice thread with that material.
Good questions, all! The topslide is set at 29 degrees (pointing torwards 1:00 o'clock); and the cut is being taken the orthodox way: .010 for the first couple of passes, gradually reduced to .001 as the "vee" gets closer to "depth".
Running between centers is never as rigid as holding parts in a chuck. The degree of precision is superior, of that there is no doubt, but, again, rigidity suffers. Consider that the item being held is much longer when held between centers, and is supported on, generally, very small cones. Having worked as a machinist, I know that there are ways to get around turning between centers, and commercial shops do so in the interest of making time. One way is to hold parts in soft jaws. Concentricity doesn't suffer, and material can be removed at a much faster rate because of the more rigid setup. Chatter becomes a non-issue. When running a machine of quality, one can use both, a center and a chuck, which is the best of all worlds where rigidity is concerned. It all depends on the nature of the project, and the equipment at hand. I didn't mean to imply that no one should be turning between centers. Hope that helps to clarify my statement.
This toolbit works great for easy to machine materials, but I've had problems with hard steels like 4140, O1, W1, S7 etc. with chatter and deflection.
To make a one side thread cutter, start almost like the standard bit you mention. Then make the point angle slightly less than 60 degrees. The next grind is real hard for me to explain: On the top of the bit grind a 15 degree top rake angle relative to one side of the cutting edge. Then stone the point so it has at least a 15 thousandths or more radius. Now the bit cuts on only one side, doesn't touch on the other, and the radius makes the successive cuts smooth on the side of the thread you're not cutting.
Use LOTS of threading oil for this method. Also, you must index the tool bit in at a 30 degree angle, using the compound, for this to work. You can't pussy foot around with this bit, it won't do light cuts well.
Just another trick to try. Works fer me, others have cussed it.
-- An apple a day keeps the doctor away. Two apples a day gets the doctor's OK. Five apples a day makes you a fruit grower, like me.
The only difference would be the amount of side relief, and on which side it is ground, to account for the helix. One needs a lot more on the advancing side because the bottom of the tool will drag when there's not enough relief.
I do NOT recommend altering the top of a threading tool. You can get away with it when making home projects, but it's a terrible habit to get caught up with. If you ever have to cut threads to specifications, you'll get threads rejected endlessly. Tolerance on thread form and pitch diameter is relatively tight and doesn't allow for all the "Kentucky windage" that so many seem to enjoy. Learn to cut threads properly and none of it is necessary. When a tool doesn't cut properly, there's a good reason. If you understand the geometry, you'll figure out the problem and correct it so you get good results without effecting proper form.
Threading tools should ALWAYS cut on both faces, with the majority of the cut generated on the advancing edge, that's why you feed with the compound, and at less than the included angle of the thread, regardless of the angle, be it 55° or 60°. It's an old and proven concept and nothing has changed in that regard. Nose point is not optional if you expect to cut proper threads. There's a formula for the degree of flat permissible. Moving away from the formula can yield threads that bind because there is insufficient clearance on the minor diameter, thus forcing the operator to cut undersized threads to generate enough clearance for the thread to fit. Again, learn to cut threads properly. When in doubt, check your Machinist's Bible. There's a wealth of threading information, all of which is very valuable if you expect proper results.
"Harold & Susan Vordos" firstname.lastname@example.org
I wasn't trying to do a treatise on single-point threading. I'm purely amateur, and only cut an occasional sloppy thread. (My best work is to die for; it needs a die following my attempts.) Just trying to determine from the original poster if the problem might be in the way he ground the bit. FM
No problem at all, FM, but the advice is good. Regardless of your purpose, learning to chase threads properly is its own reward. It's not really difficult, it just requires a given amount of dedication. Give it a go when you have some spare time, trying it by the book. What you'll discover is that once you master the techniques, it's actually faster than all the dodges one uses to get to a useable end. I've cut my weight in threads on a lathe, both single point and with die heads, and I'll take a chased thread hands down over any other thread except for a rolled or ground one. You can get them that good if you try!
I appreciate all the good advice that's been posted here. But I don't believe the problem is with the cutting bit or the set up. I have done a lot of thread cutting on a South Bend Lathe as well as an Atlas lathe. This Myford I'm dealing with now has replaced them both, but I've only now gotten around to cutting threads on it. In my original post I had asked if anyone had cut left hand threads on a Myford Super
7B. The Myford's lead screw is 3/4" x 8 tpi, left hand thread. I believe there is something very subtle going on with the lathe that is interferring with the screw cutting process. I'm just not seeing it. To eliminate all possibilities, I'm going to go back to square one and run a series of tests with right hand threads in aluminum, brass and then steel. Maybe a clue will turn up.
Very perplexing, indeed! Makes me wonder if the spindle has slop, or if you're turning between centers, if the tailstock quill is not rigid enough, perhaps due to wear. Just thinking out loud. I'd be interested in hearing what you discover.
That bothers me. A workpiece in a chuck is, assuming the chuck and headstock are perfectly rigid, a cantilever beam. For a given load, deflection is much greater than with a load supported at both ends. Even better would be a chucked piece *plus* a center at the tailstock end.
The problem with this (and this was one of Robert Bastow's favorite no-no's) is that unless the tailstock axis and the spindle axis are spot on, the center in the end of the workpiece will deflect and pick up the tailstock center.
Once the piece is turned and removed, it may no longer be cylindrical because of this.
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================
Agreed, Ted. My point is to have the item held in a chuck, yielding a part that is shorter overall than when held between centers, and with the tailstock in theoretical alignment with the spindle. Running the tailstock center is very much a part of what I propose. That's the best of all worlds, and that's why I made mention of the nature of the job and the equipment on hand. It's entirely possible that the home shop machinist may not be able to do the same things that a production shop does. Condition of equipment is important to avoid other problems.
Aside from precision grinding, it's VERY unusual to find production shops running parts between centers. It's just too limiting, particularly time wise. I did work in one shop where that was not the rule. Eimco crawler tractors turned the final drive shaft on an American tracer lathe (pre-CNC days), all between centers. Very large lathe, with appropriate centers (#5 or #6, can't recall now) in the parts. The shaft was 6" diameter. We had to second handle the shafts to cut threads, so in this instance it was somewhat better than chucking the parts. That's not always the case, however, especially in small diameter things that love to chatter. That was my point.