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Tips & Techniques Pages
Lathe Cutting Tools & Surface Finish
Herein are my tips and techniques notes on selecting the right cutting tool and achieving a decent surface finish with it. They are to date a rather disorganized collection of anecdotes found elsewhere. Over time I will organize these and test each one, discarding those that don't work so well and emphasizing those that do. For now, I am using a font convention to differentiate the tips:
If I have written a tip in this font, I have personally verified it.
If it is written like this, I found it on the Internet and am awaiting verification. These are the ones to take with a grain of salt.
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Should I Use Carbide or HSS? |
Let the great debate begin! Should I use carbide or HSS (which incidentally stands for "High Speed Steel")? The answer is, "It depends", as is true for many things in life. All professionals start out cutting with HSS--that's what they learn on. That's not to say a Home Machinist has to start that way, but you should probably learn to cut with HSS at some point. HSS will cut smoother for finishing. Some authorities recommend HSS exclusively for aluminum or finishing steel and use carbide only for roughing steel. This is less true with more modern positive rake inserts such as the "CCGT" inserts.
Still others claim they do everything with carbide, and that this is the way the "pros" operate too. That's particularly true of CNC machining, where throughput is expected to be very fast in a production operation. Carbide will cut a lot faster than HSS. In fact, a common mistake made by beginners is to be too timid with carbide. They run the spindle too slowly, feed too slowly, or take too mild a depth of cut. Be a little bit gentle with carbide, especially when making interrupted cuts--it's brittle and can chip. An interrupted cut is one were the tool is not in contact with the workpiece for the entire spindle revolution, for example if you were turning some square stock to make it round. I've managed to chip a carbide insert myself, owing to excessive chatter before I managed to find the right combination of feeds and speeds while boring my QCTP.
A real advantage for HSS is you can grind these tools to special shapes on an ordinary grinder. That's hard to do with carbide insert, probably impossible for a home machinist. During my QCTP project, I made the following parting tool:
A parting tool I ground from HSS-Cobalt steel...
This particular tool is made of HSS-Cobalt, which is an even stronger alloy than basic HSS. You will also find these tools are a lot less expensive than carbide, about 80 cents is what I paid for the rectangular blank I ground to make this tool. But getting back to carbide's advantages, not having to grind tools is a comfort to many beginners who are unsure what to do (go ahead, try it, I had not troubles at all). It is much faster to get going making some chips when you don't have to worry about grinding your tools too. Another advantage for HSS is that it will cut with less pressure than carbide, it therefore requires less rigidity.
Some say it is hard to achieve precision with carbide. The reason is that it doesn't like to take really fine cuts, cuts that remove very little material. The edge on a carbide tool is actually not quite as keen as on a hand ground HSS tool. Especially an HSS tool that you also hone on a stone. This is also the reason it's harder to get as nice a surface finish. Really, it's just an issue of thinking ahead and setting up your lathe to make exactly the right cut, instead of going at it with a lot of little cuts. If you are to be proficient, you should be able to do this anyway. As to the finish, my attempts with carbide CCGT inserts yield fantastic results. CCMT is good for rougher cuts that take more material off. By the way, try to minimize the number of different inserts you need to keep in inventory as you are buying tooling.
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Speeds and Feeds |
Why the heck is there so much focus on speeds and feeds? Because business needs repeatable machining processes that produce near optimal results, and this is how the art of machining has been boiled down to a science. Below is a cut at spindle speeds to shoot for when machining based on the material being cut and the diameter of that material. These numbers should be good for turning and facing. Keep in mind when facing, that the diameter changes as the tool moves closer to center. Fancy CNC machines will speed up the spindle as you move closer. If you have a variable speed motor, you can consider doing the same.
Let's try an example. Suppose you want to turn a piece of 1" steel. We're talking mild steel, not drill rod, stainless, or anything too hard. The speed range for this operation using an HSS tool would be 152 to 611 rpm, with an average speed of 382. I think I'd start out at about 200-250 rpm and see how things behave as I tried to work the speed up as close to the higher end as possible. Note that these numbers reflect an extremely aggressive depth of cut of 0.125". Check the footnotes. If we're only going to cut 0.010", we can multiply by 1.4x, which would have us starting in the 300-350 rpm range. What about carbide tools? Whether you are using a brazed carbide or insert tool, you can double or triple these speeds, putting us between 600 and 1000 rpm! You can begin to see why industry likes carbide. It's capable of cutting a lot faster than HSS.
It's interesting to note what a dramatic range of speeds may be called for. The highest speeds in this table are over 9000 rpm when cutting small diameter (1/4") aluminum. Imagine a shallow cut and carbide and you can soon understand why CNC tools can run at tens of thousands of rpms. You can trade spindle speed against feed speed, so if you can't turn fast enough, feed the work faster or take a deeper cut. At the low end, we find that a 6 1/2" piece of one of the superalloys like Inconel would have to be turned at 4 rpm. I can just hear the fingernails on chalkboard sound of that horrendous process now. These extreme ends are probably not something we're going to be doing on our little hobby lathes, but it is nice to put things into perspective. The main takeaways here are that the material and the diameter we're working with can really affect the optimal spindle speed. You can also get a sense of what the optimal working range is. Depending on what you're doing there can be a wide sweet spot of a couple hundred rpm or an extremely narrow spot. HSS and carbide do not overlap much at all because their properties are so much different. Translation, if you just swap an HSS for a carbide tool or vice versa and change nothing else you are probably going to get poor results.
Here is another way we hobbyists can go very far wrong. I've boiled things down to a simple set of basic materials in these two charts. The real authorities have pages of different materials carefully categorized. A hobbyist is going to use a lot of scrap material rather than buying expensive alloys that are just right for the job. This scrap is of unknown composition, and may consist of a material that machines very poorly. If we don't know what we've got, how can we get good results from it? We can try a few simple tests by looking at the color and whether the stuff is magnetic. If we are really lucky we have some ability to test the basic hardness which can help a lot (have you picked up your hardness tester on eBay yet?). Mostly we're going to have to guess and experiment to find the sweet spot where we can get a good surface finish.
If you want to create your own tables like this for all the lathe operations, focusing on the specific allows you are machining, and coming up with a much more accurate result, it isn't hard to do. Get your copy of Machinery's Handbook, find the appropriate tables and formulae, and build an Excel spreadsheet. One thing this convinced me of was the extreme usefulness of a variable speed DC motor setup together with a tachometer. Otherwise it seems we hobbyists would be doomed to spend most of our time turning work far outside the best performance envelope, making our lives even harder. There are those who feel these "textbook" speeds are too aggressive for the Asian hobby lathes. This is a hypothesis I intend to test very shortly. If a correction factor needs to be applied, I want to know what it is. So I'm going to take a variety of cuts at differing speeds in several materials and see what the result is.
The Trade-off Between Feeds, Depth of Cut, and Speeds
This is one of those mystical relationships that you really need to get comfortable with. There is a pecking order here. If you want to cut faster, first increase feed speed, then increase depth of cut, and increase spindle speed last. Why? Because this is the optimal combination to keep tool wear at a minimum. If you stress your cutting tools less, you will stress your machine and workpiece less, things will cut more accurately, and the cutter won't wear out as fast.
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Heavy Cuts and Finishing Cuts |
By now you are beginning to get the idea for how feed, depth of cut, and speeds all relate to one another. Let's be sure to also understand that we may want a different strategy for a heavy cut versus a finishing cut. It is an old machine shop adage that you make your money on the heavy cuts--getting a piece roughed to nearly the end result quickly is most of the work. Adding a finishing pass or two is easy.
What is a heavy cut? This really depends on your machine, and you'll need to experiment to find out. On my little Lathemaster 9x30, 0.050" is a heavy cut. A finish cut would be 0.010" or less. So, when roughing, I will be taking 0.030-0.050" roughing cuts until I get within less than 0.020 - 0.030" of my desired measurement, and then I'll finish from there on out using much lighter cuts of less than 0.010". I like to try to arrange for the final finish pass to be about 0.005". Less is not helpful, especially with carbide inserts, as it is hard to take a cut that is too little.
How do you know if you are cutting heavy enough? Chatter is a good indicator, although I always try increasing feed speed first when I get chatter. It is a harmonic effect and you mostly just need to change the cutting parameters to knock it "out of tune". Watch your chips. They're going to be blue with carbide, or at least straw colored with HSS on a heavy cut because they're getting really hot. BTW, they're also carrying a lot of heat away from the workpiece and cutting tool, and that's a good thing too. Another telltale is the chip. If your chips aren't breaking off into little C's or 9's, you need to kick it up a notch. Crank up the feed until the chips start breaking.
There are four factor that limit aggressive stock reduction. They are motor HP, tool type and rigidity, part configuration, and material limitation. The limitations are more or less self explanitory. Motor HP is obvious. Long boring bars, delicate form tools, skinny or overhung parts and you won't be able to take a max HP cut on it. Some materials are so hard or tough the best you can do is nibble on them. On low carbon steel you can expect to remove a bit less that one cubic inch of stock per motor HP, harder steels or aluminum and brass more or less in rough proportion to their strength.
A big lathe will make a Hardinge or Monarch user cringe at the degree of heavy cut that can be taken, let alone my poor little Chinese lathe. Roughing on those machines starts at 0.125" and can go up from there. Folks talk about the sound of chips hitting the window on a CNC turning center as sounding like hail when it's working right.
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Tips for Traditional High Speed Steel (HSS) |
Buy a grinder, read some books, check out a video if you prefer, and make some tools. As you grind, the tool blank will heat up. Keep a cup of water next to the grinder and dip the tool in every so often to cool it off. According to Machine Shop Trade Secrets, the is very little danger of softening the steel by getting it too hot. Before it reaches those temperatures, it would be too hot to hold. When you've got the tool cut to the shape you want, take a fine oilstone to it and hone it to a fine edge. If you catch it before it dulls too much, you can restore the edge with that stone pretty effectively.
Dull = Sharp, Shiny = Dull
Look for any light reflection from the cut edge. If you see light, it's dull. Thats because a sharp edge has no reflection, but once it is rounded, it is able to reflect the light and conveys the looks of a shiney edge. You see a variation when turning shafts in a lathe. Say you wonder if the toolbit is getting duller when you are in the middle of a cut. Just look at the light reflection off the shaft. If the light reflection is wider , the tool is duller and your finish is rougher. As you get close to a "mirror finish, notice that the "light line" reflection gets narrower, until it becomes a very fine line left to right, which is significant of a beautiful smooth finish.
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Tips for Cutting With Carbide (in no particular order) |
Some of my Carbide Tooling for the Lathe...
Rigidity is always important, but it is absolutely critical for carbide
Carbide cuts with more pressure than HSS, so rigidity is paramount! This is where hobbyists often miss out with carbide. Their machines are inherently less rigid so attention to how they're used is essential.
Interrupted Cuts Are Okay With Carbide
"Avoid interrupted cuts--carbide is too brittle to handle it."
You read this adage on the Internet a lot but I've found it to be poppycock with modern inserts. I've turned both hex and square stock with my carbide tooling and seen very little issue with interrupted cuts. Just go easy and it'll be fine.
A Fine Finish Needs A Small Cutting Radius
To achieve surface finish on light cuts and teary materials you need
a small
cutting radius. Hand ground steel tools tend to end up with sharp points
and
even trying to radius them usually leaves a series of tiny flats rather
than a
smooth radius. This works fine on small machines. Carbide inserts designed
for
steel usually have either a 0.4 or 0.8mm radius which is HUGE by comparison.
Just try grinding a rad that big on a steel tool. It takes a while. Also
they
tend to have radiused edges to give strength for deep cuts. With a rigid
machine you can still get surface finish because there is almost zero
chatter
taking place.
For the small lathe you need to chuck the CNC advice out of the window.
You
will certainly get a better finish with a 0.4mm insert than a 0.8 but
that is
not the answer.
What you need is a razor sharp cutting edge with lots of rake to offset
the
effects of the large tip radius. Seek out the inserts designed for NON
ferrous
turning and use them for finishing steel. I can't give a make or part
number
because I have never bought an insert in my life - I just ponce them off
my
friend when I visit his CNC workshop.
The non ferrous ones I use are bright silver - almost like polished
stainless
steel in appearance. Forget the black or gold ones - seek the bright silver
and
ye shall find.
Don't use them for roughing and never on interrupted cuts - so turn
square
stock to round with an old black radius nose insert first and then put
the
secret weapon in the toolholder for finishing.
If anyone who reads this goes out and buys a decent sturdy toolholder
and finds
these proper non ferrous tips I defy them to not achieve perfect surface
finish
on absolutely anything - stainless steel (yucky stuff !!), brass, bronze,
soft
teary mild steel, plastic - anything you like. Then you can throw out
all your
old brazed tip crap and HSS bits coz you won't ever need them again. (well
maybe once in a blue moon for grinding up special shapes)
Uncoated Inserts Give A Finer Finish
Uncoated inserts leave a better surface finish, although they won't last quite as long.
What About Brazed Carbide?
What about brazed carbide to indexable inserts? I have some of each and find them to be fairly interchangeable. The brazed carbide seems to have a wider range of shapes, which may be helpful, and I believe it works better for really fine work. Despite the latter, I will not go in search of my brazed tooling unless I need one for its shape. The difference in fine work isn't worth changing tools.
Softer Carbides Resist Chipping
Prefer softer (c2-c5) carbides because they resist chipping.
No negative rake!
Negative rake is done largely to allow all the cutting edges to be used (there are some other reasons, but this is the biggie), so you see it a lot on industrial tooling. Avoid it like the plague for hobby tools because they just don't have the rigidity. Lots of positive rake will produce great results.
Carbide Surface Finish Checklist
Improving surface finish with carbide:
- Reduce feed rate
- Use a larger nose radius insert. The larger the radius, the smoother the finish, until the onset of chatter, at which point you need less radius.
- Increase cutting speed (should be about double to triple HSS). I am always surprised when I increase speed and things cut better!
- Flood cutting zone with coolant
- Try a deeper cut. Carbide doesn't like really shallow cuts that just produce dust rather than swarf. Plan accordingly if cutting to a particular size so you finish cut isn't too shallow.
Don't "Shock" Carbide WIth Cold Coolant
Some articles caution against brush on coolant or other coolant approaches that will "shock" the carbide by changing temperature too quickly. It causes tiny cracks to form. I am forever brushing on coolant. I think the key is to start early and be consistent with it. After all, any time you see a CNC machine with carbide tooling it is always blasting coolant on as though from a fire house. It's shock cooling that you need to watch out for. The other thing to keep in mind is that most home shop machinists don't get things cranked up to full speed with carbide anyway, so it usually doesn't get hot enough to suffer from shock cooling.
Look for Blue Chips with Carbide
Many writers say you know the speed is right on steel for carbide when the chips are "blue". He suggests a minimum cutting speed of 200 ft/min. I admit that in my early stages I am still not very good at judging chip color as a way of deciding when I'm cutting fast enough. I've been hit by many very hot chips and they feel like ants biting! Eventually, I got to where I naturally ran things fast enough that all the chips started coming off blue.
Don't Pause, Take Too Shallow a Cut, or Feed Too Slowy With Carbide
Pausing, taking too shallow a cut, or too slow a feed speed will cause carbide to stop cutting and start rubbing which leads to a poor surface finish and a dull cutter. A good cutting depth will produces endless swarf with clean looking edges. Score marks on the workpiece are a sure sign of this problem.
Get the Right CCMT Inserts for Small Lathes
While many experts prefer HSS for a finer finish on aluminum, here is a good tip. There is a CCMT-compatible insert of fairly new design that is optimized for aluminum and leaves an extremely fine finish. Look for CCGT 060202, made of "AK20" material. Arno is one (German) company making these. Apparently some are also set up for left and righthand cutting, so you don't get so many edges. You have to get the inserts that look like little crowns as these have the high positive rake. Normal diamonds are nearly identical to CCMT and you won't see a difference. There are probably thousands of different kinds of inserts and it pays to sniff around the catalogs and try a few new things.
I find these CCGT "Little Crown" inserts always work very well on steel. Lately, I see them sold as CCMT too. The nomenclature will tell you there is little difference between CCMT and CCGT except for manufacturing tolerances, so you have to look at the shape as much as anything to find them.
Standard CCMT insert is okay, but not great on small lathes...
CCGT (that's a misnomer, they're also seen as CCMT, you have to look at the shape!) is intended for non-ferrous, has huge positive rake, and cuts everything on my lathe better than vanilla CCMT...
The little buggers are hard to find, though, so I spent a little time doing some research. For some reason, they tend to be identified as "CCGT" rather than "CCMT". According to ISO, all the "G" is supposed to mean is that the insert was made to tighter dimensional tolerances. It is a plugin for CCMT. Likewise TCGT fits TCMT for triangular inserts used on something like a boring bar.
I went deep into the manufacturer's sites to track down this insert shape, because that's the key. Regular CCMT's often have some positive rake, but nothing like this. What I discovered is that the major insert makers have a special line of this style insert:
Valenite CCGT-1L
Seco CCGT21.51F-ALKX
Each one has a slightly different sales pitch about why you'd use the insert. Iscar is pushing them as offering such a fine finish for aluminum that no grinding is needed, for example. The recommended materials even vary across the lines. What started out as an aluminum super finishing insert can be had in formulations that extend to high temperature allows, stainless, and other possibilities.
Now the bad news. Since they aren't nearly as common as regular CCMT's, and they seem relatively new, they cost more money. Carbide Depot has a page offering many of these inserts. You can find them much more cheaply on eBay, but they are often poorly identified. My rule is if I can't clearly see the high positive rake design in the picture, I won't take the chance on eBay. By shopping carefully, I've managed to buy 20 or so of these inserts, which will last me for quite a while.
Relationship of depth of cut to tool nose radius
If your DOC is less than your tool nose radius, the part of the insert that is "in the cut" is not perpendicular to the cutting force. The radius on the insert will tend to spring the work piece away from the tool. You will get more deflection if your DOC is less than the tool nose radius. This might make it difficult to maintain a consistent size on your diameter on a long unsupported work piece. So, the issue is not surface finish, but rather maintaining an accurate diameter. For best finish results, keep nose radius no less than 2/3 of depth of cut, or, depth of cut should be 1.5x nose radius.
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Tips for Coolant (See Also Mill Tips & Techniques for More General Info, This Section is for Lathe Specific Info) |
Ketchup and Mustard, Please?
When manual machining, I use a set of cheap condiment squeeze bottles to hold various coolants in, together with a chip brush to apply the coolant. Keep a variety of coolants on hand, and experiment. I use heavy sulfurized cutting oil, WD-40, my way oil, and kerosene. All have their place. Aluminum likes thinner coolants such as kerosene or WD-40. Steel tends to like thicker coolants.
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General Tips |
Remove any burrs created during roughing with a die grinder before moving on to finishing.
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Use a faster feed and slower spindle to rough quickly, switching to a slower feed and faster spindle for good finish.
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Nifty Carbide Tools |
Aloris No. 71 Parting and Cutoff Tool Mini Review: Expensive, but I swear by the thing (not at it!) when it comes time to part off.
Circle Machine Boring Bars: Really sweet boring bars. No comparison to imports. I broke my Glanze bar and chipped an expensive drill bit trying to bore the spring steel of a helical beam coupler for my CNC Lathe Conversion. This bad boy cut perfectly smoothly without a hint of chatter or other trouble. They're over $100 new, but can be found for half that on eBay.
Tiny little Circle Machine boring bar performs really well...
Glanze Indexable Turning Tools: Great tools available from Grizzly. The boring bar isn't worth much, but the others are great. I love the CCMT inserts. Micro 100 is also selling these under their name. They're the same tools, and Penn Tool has them on sale from time to time. I bought the 1/2" set and find it is more rigid. Be careful though. There is barely room to get the cutter on center on my lathe with this size.
Indexable Threading Tools: The experts recommend the "laydown" type here, rather than the "on edge" style. This just means the insert sits on top rather than on the side of the tool. Dorian is highly recommended. There are a couple of types that Enco sells such as Carmex or an SER-type insert tool. You'll want tools for outside as well as inside threads. The latter are kind of like boring bars.