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View Full Version : Nose Radius, theoretical v. practical



RussZHC
08-08-2011, 10:27 PM
Still working at better finishes, so I have seen a bit of the practical side of changing the nose radius on a lathe bit. One remedy suggested has been to increase the nose radius.

However the theoretical side of me has a question, if you are cutting with a point, as opposed to an edge, how is it that changing the radius does work? I mean if it is a point, would not, theoretically anyway, the actual radius not matter as just the very apex is cutting anyway?

What am I missing?

Black_Moons
08-08-2011, 10:31 PM
Just because its a point, does not make it infinately small. The amount of cutting surface in use depends on the radius, feed rate and depth of cut.

Your tool also does not cover the work an infinate number of times, but cuts a thread* like pattren, Even if ever so fine, It still does. The height of these threads is reduced with incressing nose radius and lower feed rate. Less depth of cut also helps reduce vibration/deflection/etc at the cutting edge and hence improves surface finish.

*threads: In this case refers to just a continious helix pattren along the work, Not the shape of V threads.

Toolguy
08-08-2011, 10:40 PM
Also, it usually cuts easier if the nose radius is larger than the depth of cut.

BadDog
08-08-2011, 10:45 PM
Imagine the details of the cut. The depth of cut sets the difference between pre-cut diameter and post-cut diameter. And that difference determines the width of cutting edge. So it's not a point contact. Depending on machine, tool, target size, and material; that width of cut may be anywhere from 0.002 up to 1" or more.

If you had a perfectly sharp pointed bit, and no lead angle with sufficient clearance on the back, then a cut with that bit would produce a result looking much like a buttress thread, or perhaps a primitive zero-rake saw blade if viewed in profile (like on a comparator). (Edit: ignoring what happens on the clearance side)

If you took the same cut with the same bit after putting a small radius on the tip such that the diameter of that point radius was roughly equal to the feed per rev, you would get a distinct scalloped cut resulting from roughly the same rate of material removal, but cut forces would climb a bit over the first cut.

As your nose radius gets larger still the scallops theoretically get more and more shallow until they are barely perceptible when the diameter is several (or many) times larger feed per rev. This continues up to a practical limit where you no longer have enough rigidity (machine, tool, and/or material) or power to handle the extra force produced by the longer cut line combined with additional lateral forces and energy consumed in shaping the more complex chip (caused by the non-linear cutting edge). Lateral forces (along the cross slide) can be particularly bad when using larger radius and shallow cuts because the end result is almost the same as excessive lead angle. Then you get chatter and are forced to reduce nose radius, depth of cut, slow/gear down, or possible a combination.

HWooldridge
08-08-2011, 11:00 PM
We regularly use radius inserts at work and finish is greatly affected by changing the radius. For example, we run a 1.25" diameter part in 12L14 and must generate a 32 Ra finish or better in production. We started with a .015 radius tool which typically would start in the high 20's but run over the 32 limit in short order. We jumped up to a .064 radius insert, which left finishes well below 20 Ra - but the insert failed within 100 parts and finish went to hell again. We then went to a .032r insert and hit a home run, as the finish stayed in the 20's for almost 1000 parts.

In my home shop, I usually grind my HSS tools sharp then hone a small radius on the tool by hand. Leaving a dead sharp point usually results in the tip crumbling.

RussZHC
08-09-2011, 01:34 AM
Thanks for the replies so far, I hope there are more to come. Each reply has been slightly different and all help with my attempts to understand.

The question, other than of course wanting to improve finish, was sort of prompted by my being out of "feed rate", as in to go any slower would require larger gears than "standard" which, if made or found, would also mean redoing the covers since the fit is already really tight (not saying it won't happen, just not super high on the to do list). Spent a fair bit of free time today trying to come up with a solution and so far, no go.

I am enjoying all this so much and one of the few materials I have more than enough of are lathe bits so I think the place I will start is to go to a larger tool (3/8" to 1/2") and play with size of radius more as well as DOC.

One of the things I am still wrapping the brain around is how all of these variables interrelate...does not matter how often I read it the personal observations are still so few...

Your Old Dog
08-09-2011, 08:34 AM
.....................does not matter how often I read it the personal observations are still so few...

Sounds like we had the same reading teacher. I read aloud like Sophocles but my comprehension is near zip. That's how they taught in the 50's

To solve most physical problems I magnify them in my minds eye. I take them to an extreme that lets me get a better look. As for your radius question, I make sense out of it by thinking that as increase the radius I at some point approach the function of a plainer bit.

How I keep the inner relationships making sense is to realize that absolutely everything in machining, as in life, is a compromise. I also keep in mind that perfection is simply a goal, something to shoot for to keep you moving in the right direction. If the real goal is 1" you will never achieve it. It exist but we can always measure an error. So, in compromise, how much error are you able or willing to accept?

In electronics we have Ohm's law; E = I x R to guide us through our work. I wish they had a simple rule like that for lathe work !

Carld
08-09-2011, 09:44 AM
Using a radius on the tip really helps on finish cuts. Sometimes a small radius on a roughing cutter helps keep the tip from breaking down. It's hard to have to big a radius for finish cuts but it's easy to have to big a radius for roughing cuts.

For finish cuts the radius should exceed the feed rate and DOC so that the tip overlaps each pass on the helix of the cut. That way the surface will be smoother.

If you want the best finish of all use a shear cutting tool, it produces a near ground finish and is better than a filed finish as it will be consistent the length of the work. A shear cutting tool is simply a left hand turning cutter turned upside down. The side relief angle is about 30-40 deg though. Use a DOC of about .002", rpm about 150, feed .002-.004".

http://i82.photobucket.com/albums/j276/yeathatshim/P2210016.jpg

Boucher
08-09-2011, 10:33 AM
We regularly use radius inserts at work and finish is greatly affected by changing the radius. For example, we run a 1.25" diameter part in 12L14 and must generate a 32 Ra finish or better in production. We started with a .015 radius tool which typically would start in the high 20's but run over the 32 limit in short order. We jumped up to a .064 radius insert, which left finishes well below 20 Ra - but the insert failed within 100 parts and finish went to hell again. We then went to a .032r insert and hit a home run, as the finish stayed in the 20's for almost 1000 parts.

In my home shop, I usually grind my HSS tools sharp then hone a small radius on the tool by hand. Leaving a dead sharp point usually results in the tip crumbling.

This type of information is what makes this site so great.

Thanks for posting it.

Boucher
08-09-2011, 10:38 AM
As mentioned previously there are a lot of interrelated variables that influence surface finish. The variation in surface appearance as the cutting speed goes through the sweet spot has always intrigued me. This is easily viewed by taking a facing cut from the OD to the center. The cut starts fast and slows down as it progresses. At the correct cutting speed the surface finish dramatically improves. Try this with a few different nose radii and it will help to answer the original question.

RussZHC
08-09-2011, 09:22 PM
Boucher: agree, 100% as to why this site works; as example I think I found all of Carld's threads re: shear tool but I am not certain the others have actual number and numbers sometimes really help me put things together (see below, re: YOD)

Carld: its on the short "do" list [I thought of this while puzzling and the discussion of radius details...I "extrapolated" and began thinking if one were to go to the largest radius possible, you get a straight line, that shearing edge?...I think its maybe one of those math thingies I like but perhaps don't understand the full implications of...]

YOD: I do the same thing LOTS and in this case, being way short on experience, I do it for even the simplest operations...as a made up example, if you make a heavy wall cylinder out of a solid round bar, what do you do first? Remove until near finished dimension the "interior" of what will be the cylinder...OR do all until near finished dimension on the exterior and then work on the "interior"? I mean to some more knowledgeable it may be "6 of one half dozen of the other" but I don't know that I see it that way. The discussions about turning taper help me A LOT since I get to work on visualization skills.

vpt
08-09-2011, 10:14 PM
Using a radius on the tip really helps on finish cuts. Sometimes a small radius on a roughing cutter helps keep the tip from breaking down. It's hard to have to big a radius for finish cuts but it's easy to have to big a radius for roughing cuts.

For finish cuts the radius should exceed the feed rate and DOC so that the tip overlaps each pass on the helix of the cut. That way the surface will be smoother.

If you want the best finish of all use a shear cutting tool, it produces a near ground finish and is better than a filed finish as it will be consistent the length of the work. A shear cutting tool is simply a left hand turning cutter turned upside down. The side relief angle is about 30-40 deg though. Use a DOC of about .002", rpm about 150, feed .002-.004".

http://i82.photobucket.com/albums/j276/yeathatshim/P2210016.jpg



Does work nice.

http://img197.imageshack.us/img197/3491/lathewood024.jpg

HWooldridge
08-09-2011, 10:52 PM
Boucher: agree, 100% as to why this site works; as example I think I found all of Carld's threads re: shear tool but I am not certain the others have actual number and numbers sometimes really help me put things together (see below, re: YOD)

Carld: its on the short "do" list [I thought of this while puzzling and the discussion of radius details...I "extrapolated" and began thinking if one were to go to the largest radius possible, you get a straight line, that shearing edge?...I think its maybe one of those math thingies I like but perhaps don't understand the full implications of...]

YOD: I do the same thing LOTS and in this case, being way short on experience, I do it for even the simplest operations...as a made up example, if you make a heavy wall cylinder out of a solid round bar, what do you do first? Remove until near finished dimension the "interior" of what will be the cylinder...OR do all until near finished dimension on the exterior and then work on the "interior"? I mean to some more knowledgeable it may be "6 of one half dozen of the other" but I don't know that I see it that way. The discussions about turning taper help me A LOT since I get to work on visualization skills.

Re your "example", I'm not sure if there is a right way but as a rule of thumb, we typically spot, drill, bore, turn OD and part off (unless there are other intermediate steps such as threading or milling). We do this because drilling usually requires the most stock removal. In fact, we often buy special size ground bar stock so exterior turning can sometimes be eliminated.

Carld
08-10-2011, 01:02 AM
If I were making a cylinder out of oversize stock I would bore it to .030-.050" undersize and then turn the OD. The reason is you have less chatter during boring if the wall is thick. When you have the OD to size you can then indicate it back in and finish out the bore.

Because of all the metal removal you may have distortion of the bore so you need to leave enough to clean everything up to be concentric.

It seems to be easier to stuff a bore with rags or paper to dampen chatter to turn the OD than it is to wrap the OD with rubber strips to dampen chatter and turn the bore.

RussZHC
08-10-2011, 01:21 AM
My fault for making this thread wander but who cares ;)

As you mention distortion, still playing around with the test bar...blue chips, yeah OK hot, carrying heat away, get that but still surprised at the amount of heat and for how long it stayed in the remaining bar itself...can sure see how distortion plays a role in fit, why one needs to give time for work pieces to "rest" (to use a food term) and can also see how the rubbing of edges (cutter or work) can have an effect such as producing chatter and IIRC from product info, that is one of the roles of fancier blends of HSS bits (to absorb heat, not distort or as much and to continue to cut) OR, for that matter, how heat can be used to make a very tight fit part to part...same coin, different sides?

Carld
08-10-2011, 01:32 AM
Regardless of how blue the chips come off there is still a lot of heat put into the work by the turning process. Even light tan chips will put a lot of heat in the work.

If your trying to turn a test bar, which I have done a few times, you need to turn it between centers and try not to get it real hot. Use hot roll to make the bar because it has less stress in it than cold roll.

Having learned how to use the shear cutting tool I think I could make a much better bar now but the two I still have are pretty accurate and I seldom need them anymore.

PixMan
08-10-2011, 08:35 AM
In my experience there's a trade-off between the improved finish of using a larger tool nose radius and the likelihood of getting increased chatter as the contact area of a larger radius grows.

If I'm trying to hit a target Ra value, I occasionally use this calculator:

http://www.americanmachinist.com/Calculators/Turning.aspx

I admit I'm more-often-than-not using carbide insert tooling, but the principles don't change based upon cutting tool material or cutting speeds (usually.) The only thing that can change the final look is that with certain coated inserts, the coatings tend to work better with generating some heat in the chip and you get a shiny finish on otherwise dull materials (cold rolled steels in particular.)