Are lathe tool carbide inserts supposed to be sharp?

[m12lrpv]

I've got a tu2004v lathe inbound (they're on backorder) and it's my first lathe. I'm a hobbyist and not a trained machinist so I've never been educated in any of this stuff. I suppose you could say I'm somewhat of a stereotypical newbie having sat through a ton of you tube videos trying to learn all I can. I do however have a lot of grey in my beard and a shed full of tools that do get used so to my credit I'm smart enough to have not killed myself doing something stupid using them. I would like to continue that pattern

Following that stereotypical newbie process I've bought one of the typical carbide insert tooling sets that you get from the usual chinese import sites. (And for those that always choose to chime in about HSS I have some of that also and will grind it up when I get to it).

Back to the carbide inserts though, I'm genuinely surprised at how blunt most of the inserts are. I can easily push my finger onto the cutting edge without cutting myself. Except for the threading inserts and the grooving tool which do feel a bit sharp.

My past experience tells me that blunt tools can dangerous tools so I want to make sure that this is normal before I try and turn some steel with them and potentially do some damage.

I do understand that different materials require different insert types/grades/sharpness. I have a small cnc and all the milling cutters i have for it (even the carbide ones) are extremely sharp.
From the CNC I know that Aluminum, plastic and wood need sharp cutters and from drilling I know that steel likes sharp drill bits but maybe that's a HSS drilling thing only. I've never drilled steel with a carbide.

So are these cutter inserts meant to be sharp (drill bit sharp) or is the rounded looking top of the cutting edge actually normal? Am I worrying over nothing?

I know straight up that I wouldn't go near aluminum with these. Plastic I expect will be safe to try it on because failure there won't damage anything other than the plastic. I expect that blunt inserts on steel are probably just going to chatter and/or screech loudly, like trying to drill with a blunt drill bit or a drill on reverse.

If there's anything I think that the typical youtubers who have machinist backgrounds and training leave it's it's the really simple stuff like the answers to questions like these

 

[Zeb]

Carbide inserts do tend to feel that way. You should have zero problem with aluminum and you can push them much harder than HSS, as the heat won't touch them. Just make sure the inserts are secure but not overtorqued.

 

[m12lrpv]

Thanks. I'll give it a shot as is and see how they go.

 

[Mechanicboy]

If you want a fine and detailed work at material who are not possible with insert carbide tool, use HSS tool who are easy to form with bench grinder and keep sharp with hone stone. To keep "long life" of tools --> select cutting speed depending om which material and diameter.

 

[Jasonb]

Most generic sets tend to have a general purpose insert with a code **MT Where the ** refers to the shape and the M the tolerance. For fine work on steel, and all work on brass, bronze, stainless, plastics and Aluminium you would do well to get some **GT inserts which have a higher tolerance that is achieved by grinding or polishing and typically have a bright "chrome" look rather than the golden colour of the **MT. This makes them much sharper so they cut better without loading up the small hobby machines and less likely to weld aluminium to the edge. A 0.4mm tip radius will do for most things.

 

[Richard Hed]

There are many different shapes of carbide bits. I generally use the long-nosed diamond shape for a lot of cutting but there are other shapes I also use. Triangle, square, rectangular and even round ones for certain -purposes. You should try different types for different purposes/materials. Also, many peeps recommend giving the carbides a touch up with a hand stone. I find this helps with difficult materials, other wise , I don't bother. Also, it really IS a necessity that the cutting speed is much higher or you can easily crack the carbides. Also NEVER turn your chuck backwards with the carbide touching the work--you are almost guaranteed to break it. I know, I've done it too many times and am sure I will do it again.

Oddly, I just was reading a perfect beginner book, thimpfking it might have somethign useful for me in it but it didn't. It's a great book, but told me nothing new for myself. Horribly, I went to look for it on my computer and can't find it. However, there are lots of places to get good books on the subject.

this is Z-library: [Magazine] Scientific American Mind. Vol. 19. No 6 | | download

Here is Archive . org: Free Books : Download & Streaming : eBooks and Texts : Internet Archive

and also PDF drive: The Gospel of Barnabas by Lonsdale Ragg - PDF Drive

These all just happen to be on subjects I was interested in, you can do your own searches, of course. There are others too, but I thimpfk these three are the best.

HSS bits might be a better general usage for you if you have a small lathe. I don't know waht a tu2004v is. Do you have a photo? be sure to get a cou0ple sets of protective eye-wear. It's the num,ber one essential safety tool. Also, you can read about various of us who have NOT LOST THEIR FINGERS yet! but have had various accidents that it is a good thing to learn from. Don't wear baggy long sleeves particularly on yuour left arm. Never leave the chuck key in the chuck unless you want it as a decoration stuck in your head.

What kind of hobby work do you intend?

 

[barryc43]

Might I suggest that you buy a bit of nylon bar for your first turning adventures? It will allow you to see how the lathe and various tools function without danger of doing any damage to yourself, the lathe and of course the tooling?

 

[Bushranger]

Just my 2 bobs worth, but I'm using a 1970 Hercus 9AR most of the time now. Shop made toolholders with no built-in back rake. Sick of grinding the rake into HSS bits, bought some brazed carbide tools to run on the lathe. They had to be sharpened before use, but work a treat. Might be worth considering something similar. Again, just my opinion.

 

[simonbirt]

I would echo what has been said, inset tooling is really not intended for hobby lathes. Watching YouTube videos can be helpful, but there is an awful lot of bad practice being proglamated. Inset tooling is intended to be used on rigid machines using high rates of feed and speed together with flood coolant; all difficult to achieve for hobbyist.

You say that you have some HSS to experiment with, I would make that my starting point, once you understand the geometry and can grind it into a working tool you will have a much better understanding of machining. It is easy to grind, in fact a 1” belt sander is great for shaping HSS, better in many ways to a bench grinder.

Brazed carbide tooling is a good option, but it is hard to grind and easy to chip.

Have a look at some of the books by the likes of George Thomas, what you will see is simple tooling producing outstanding fits and finish. It is also very satisfying to see a tool you have made moving metal.

 

[Jules]

Carbide tends to be used on large rigid machines. They are designed to remove metal at high rates and just bulldoze there way into the material and break nice chips.
Most hobby lathes aren’t really rigid or heavy enough but you can still turn metal with them.
I tend to use inserts designed for turning aluminium because they are much sharper and load the machine less.
You can also try a smaller radius tip as that will increase the effective cutting pressure.
stay safe and have fun.

 

[clockworkcheval]

Cutting metal is in close-up not really cutting but a discontinuous rip-off proces. The sharper the tool the finer the rip-offs, the lower the forces and the smoother the resulting surface. The cutting speed of carbide inserts is roughly up to three times the cutting speed of high speed steel HSS. And this higher speed gives a much smoother rip-off proces. Compare with plowing snow at low speed and high speed. So the sharpness of the insert will not affect the process much, and a slightly less sharp cutting edge is stronger and more wear resistant and thus allows for bigger chips, that produce higher cutting forces. For small precision parts it is however beneficial to give the insert a really sharp cutting edge using a fine diamond grinding/honing wheel.
The use of carbide inserts depends fully on those higher cutting speeds. Your lathe tu2004v has I believe a maximum spindle speed of 2500 revs. This means that you have a good carbide cutting speed for steel down to diameter 20 mm, and for brass and aluminum down to diameter 40 mm. For diameters smaller than this you would be well advised to switch to HSS tooling. The HSS cutting speed for steel is OK down to diameter 6 mm and for brass and aluminium down to diameter 12 mm. Several members of my horological society use a small second lathe capable of high revs like up to 15.000 - 20.000 revs for the small parts with diameters to be cut say from 1 mm up to 10 mm.
In my own workshop I have pampered myself with a Schaublin 102 VM lathe which goes up to 3000 rpm with a frequency controller, a Myford Super Seven of similar size which also with a frequency controller goes up to 5000 rpm and a TAIG with a DC motor and DC control that goes up to 18.000 rpm spindle speed.
Overall it is my strong believe that the hobby machinist and the hobby equipment reflect the workshop practices of 60 years ago. So it is very helpful if you have acces to a workshop engineering manual of about the 1960's.

 

[Ken I]

Carbide inserts almost never have "razor sharp" edges as they chip too easily then - they are even rumbled after sintering to remove sharp edges.
The cutting edge radius is obviously going to "rub" but as the depth of cut becomes bigger, this become less of an issue.

So insert tools require greater depths of cut, higher speeds and much greater machine power and rigidity to get good results. Hobby machines often don't have the power or rigidity to drive insert tools very well.

https://www.researchgate.net/figure...ed-cutting-edge-images-of-edge_fig1_257336237

Light "finishing" cuts with carbide insert tools generally doesn't work - you need to work your way down to size taking cuts that develop a good finish and your last two passes must be at the same depth of cut (let's say 0.5mm) - such that when you are aiming for Ø25mm diameter you need to go to Ø27 then Ø26 and finally to Ø25 - this is so you get to size with the same reaction forces and cutting dynamics. Change depth of cut on your last pass and you are almost certainly going to get it wrong.

Imagine you aimed for Ø 27.00 but got Ø 27.05 - dial in Ø 0.53 depth of cut and you get (say) Ø 26.02 - so for the final cut dial in 0.54 depth and you should come out at Ø 25.00.

Hobby machining often requires light finishing cuts and for that you need "razor sharp" edges - use HSS or brazed carbide properly sharpened and polished.

Regards, Ken

 

[m12lrpv]

Thanks guys.
I'm fully expecting that I'm going to end up with a draw full of different cutters. After all that's what hobbies are all about. Filling your shed with stuff.
I had actually been somewhat dismissive about the brazed carbide cutters but i've seen a few references to them being able to be sharpened so I'll probably get some of them too.
At the moment though I was just wanting to get a handfull of tools to enable me to get started with a few small cuts while I worked out what it was i really needed. Buying the insert holder kits is really just buying the holders with a couple of inserts thrown in so it's a safe option because even if the inserts that come with it are garbage they're only temporary.

I've been looking at lathes for a long time but have never been able to justify it. I've been able to make enough $$$ from other hobbies to justify the spend so I'm finally making the leap. I could never justify buying the lathe i really want (a gunsmithing lathe) but this will do to get me started.

 

[Jasonb]

I find I can take of a 0.001" depth of cut with a DCGT in steel. I could not easily do that with a DCMT or CCMT particularly on long slender work like crankshafts or valve stems where the "blunter" carbide tends to deflect the work or cause chatter when the required deeper cut is taken.

Probably best on full screen

 

[clockworkcheval]

Asking about the economic justification of hobby machinetools is asking the wrong question, as I often have to explain to my resilient wife Eleonora.

 

[Steamchick]

I like the expression "resilient wife"... we all need one of those!
I sometimes wonder at the motivation of these girls to remain wives of hobbyists and engineers... But understanding sub-nuclear particles and black-holes is easier than delving into the minds and motivation of Wives and Girlfriends.
K2

 

[Steamchick]

Bucket-loads of information here... a lot I was never taught.
But I was taught in the 1960s, on machines that had home-ground "tool-steel" tools and speeds in the hundreds, not thousands. For machine stiffness versus size of cut, it was always the workpiece that was least stiff, so cuts were adjusted to stay below distortion limits of the workpiece.
That is not appropriate training for HSS or carbide tool use.
Also the huge lathes that I used as an apprentice (biggest was a 16 ft long bed, 18" swing) do not compare to hobby lathes that you can twist round your little finger with a heavy cut... so cuts are often limited by the combined machine and work-piece distortion that causes inaccuracies as explained by Ken - post #12 above.
Thanks all. Education comes every day...
K2

 

[L98fiero]

Carbide inserts almost never have "razor sharp" edges as they chip too easily then - they are even rumbled after sintering to remove sharp edges.
The cutting edge radius is obviously going to "rub" but as the depth of cut becomes bigger, this become less of an issue.
View attachment 130441

So insert tools require greater depths of cut, higher speeds and much greater machine power and rigidity to get good results. Hobby machines often don't have the power or rigidity to drive insert tools very well.

https://www.researchgate.net/figure...ed-cutting-edge-images-of-edge_fig1_257336237

Light "finishing" cuts with carbide insert tools generally doesn't work - you need to work your way down to size taking cuts that develop a good finish and your last two passes must be at the same depth of cut (let's say 0.5mm) - such that when you are aiming for Ø25mm diameter you need to go to Ø27 then Ø26 and finally to Ø25 - this is so you get to size with the same reaction forces and cutting dynamics. Change depth of cut on your last pass and you are almost certainly going to get it wrong.

Imagine you aimed for Ø 27.00 but got Ø 27.05 - dial in Ø 0.53 depth of cut and you get (say) Ø 26.02 - so for the final cut dial in 0.54 depth and you should come out at Ø 25.00.

Hobby machining often requires light finishing cuts and for that you need "razor sharp" edges - use HSS or brazed carbide properly sharpened and polished.

Regards, Ken

Typically with carbide you shouldn't take a cut less than the insert tip radius though if you are finishing with the xxGT inserts intended for non-ferrous metals, because of the sharpness and high top rake you can take smaller cuts.

 

[Steamchick]

I have also used a diamond disc or hand dresser to put an edge on carbide tips for finer machining and finishing cuts. (But didn't know the "Why?"!).
K2

 

[ChazzC]

Might I suggest that you buy a bit of nylon bar for your first turning adventures? It will allow you to see how the lathe and various tools function without danger of doing any damage to yourself, the lathe and of course the tooling?

I suggest Delrin™ (Acetyl) rod for first attempts, Nylon can be gummy.