I'm doing facing cuts and the tool is grabbing because the rake is too positive (I think. There's a graunchy noise and the feed goes slack for a bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm cut, brazed carbide tool on cast iron).
It's not really practical to change or regrind the tool. I'm dead on center height, and wondered whether changing the height might make it better by changing the effective rake somehow?
If so, which way? And why? I tried to work it out but got confused, and came up with two different answers.
Is this a BIG facing job? (more than a couple of inches in diameter?) If so, you may have to increase the spindle speed to keep the cutting speed in the desirable range as you move from or toward center.
90 mm x 55 mm rectangle shape, and that may be a reason for it getting worse at the middle - I can't get the speed up very high, as it's an odd shape with the CoG off-axis in a 4-jaw independant and it vibrates too much.
I can get an okay finish with a very fine finishing cut or two, but I have to do a few of these and they don't have to be measured, just flat.
Sounds good when it's hissing, or actually more like air whooshing through a carb in a sweet engine :) and it's one more nice point about having a variable speed - you don't have to stop the lathe to change gear or even stop the feed, you just turn the speed knob up until the swarf starts burning.
Is that last bit right by the way? I've been doing it that way all day and the carbide tool seems undamaged, I figured if the swarf wasn't burning the tool wouldn't get too hot. No lube used (or needed), I'm cutting good quality fine-grained grey cast iron, and the lathe doesn't have a coolant system.
What is the material? I have a hunk of steel round of indeterminate ancestry (probably hot rolled...) that cuts like a piece of concrete and, no matter what I do, I can't get a decent finish on it. OTOH, other materials cut beautifully, using the same setup...
Hot rolled? That's not a material, it's a method of producing material. It could be 1020 or 4340 just as easily.
From the description, what you have is chrome-moly. It likes to tear and will usually yield a good finish only when run fast enough with carbide. If you're using carbide and getting a finish that varies between shiny spots and torn spots, you're likely using the wrong grade of carbide and experiencing premature tip failure.
I understand "hot rolled" is a method, not a specific material. My basic point was, however, that different materials cut VERY differently and, if you try to use materials of "indeterminate ancestry," you never know quite what you are getting into. And, for the benefit of the original poster of the question, unless you know what the specific material is, it is difficult to suggest an approach to machining it (or even if it can be satisfactorily machined for the application...).
But hot rolled steels are, in general, much less uniform in hardness (hard spots and soft spots) and tend to machine poorly. Most of the more "machinable" steels are cold rolled. Your friendly metal dealer will tell you he mostly sells cold rolled steels to machine shops and (cheaper) hot rolled steels to welding shops...
Agreed. While you may encounter 1020, it's likely it is something else, especially if it has been machined. That's why industry has color coding of materials, and certification that is available when the specific alloy must be known and used.
And, for the benefit of the original poster of
satisfactorily
Hard to argue with that! However, unless a part is subjected to heat treat or welding, as long as it can be machined, the material choice usually makes little difference. I can't help but think that's why so much material comes from scrap sources for the home shop types. Economy must be a chief concern.
I've machined one hell of a lot of 4130/4140 and 4340 in my years in the shop and have to say I've never encountered that problem. In fact, I'd suggest to you that the material tends to be very uniform. Didn't say it machined well, just machined consistently the same. Because such material usually is accompanied by certs, it tends to be of high quality. If you'te talking about turning an axle from an auto, you're more likely talking about tool failure from heat treated steel than material variations.
Depends on the nature of the work, wouldn't you say? If you're engaged in making parts from alloy steels, the vast majority of it *is* hot rolled material. Machine shops may very well consume a larger volume of hot rolled material than cold processed materials (rolled or drawn). Screw machine shops are notorious for choosing free machining steels,
1117,1213,1215,12L14, 11L17, all of which lend themselves to such operations. You might say that work of that nature migrates towards those that have the equipment. You can't generalize much beyond that. Further, if you're doing work for people that know what they need, they typically provide material specs that must be met. You generally have little choice of how the material is produced.
One material is particularly good machining *because* it is cold processed. Stressproof. LaSalle Steel developed the process by which it is produced. Surface finish is generally very good, and it cuts without tearing.
A lot of shafting isn't cold processed (drawn) at all, but ground and polished hot rolled. You can't generalize.
What I really wanted was a quick rundown on how changing tool height might change the effective rake, and why. I'm sure I've read something about it, but I can't remember where.
Changing tool height has a bit of an affect on rake, but very little. You generally can't change the height of the tool enough to make much of a difference. It has a profound affect on front clearance, however, especially on small diameter cuts. If you're experiencing the tendency to hog, reduce the front clearance a little, which will lower the hogging to some degree. You can achieve a delicate balance where the feed rate is just slightly less than the amount of clearance, so the tool isn't dragging, but if it hogs, it bumps into the cut below the tip and stops the advance. That's not a great way to go, but on a light machine it can be a useful dodge. Good luck.
One more thing. Cast iron responds fairly well to negative rake, so if you're having trouble with hogging and chatter, consider reducing your rake to 0 degrees. I wouldn't go negative on a light duty machine, but no rake would likely be quite useful.
Deviating from dead centre height only makes sense when turning at approximately constant diameter. It makes an equal change change in both front clearance and front rake angles
This angle change is almost directly proportionate to the cutting radius (arcsin h/r) and so would be extremely variable with a facing cut. It has no effect on the side clearance and side rake angles.
Its only real use in a facing cut is to set the tool a few thou high so that, at dead centre, it pushes off the tiny residual stub rather than passing underneath it and leaving a pip.
With cast iron, sufficient packing under the tool to achieve about 10 deg negative rake is a possible improvement but the quick and dirty method is to stiffen the setup :-
Lock the carriage to the bedways.
Fully tighten the topslide gib strip to lock down the topslide (or remove it altogether!).
Overtighten the cross slide gibstrip to the point where substantial force is needed to produce crossfeed.
It sometimes helps to start the facing cut from the centre rather than the periphery.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.