Heavy cuts

[Water-mark]

What is the limiting factor in how much material you can remove in a single cut?

I have a 3" lathe that can only take light cuts, even in brass
I'd like to know if an upgrade is possible or will i need a bigger lathe.

It's currently running a 400w motor and i'm using carbide tip tooling sharpened on a green wheel.

The surface finish is generally good hut as soon as you get greedy it just stalls.

Thanks

[pete]

Horsepower and machine rigidity. Increasing the HP on a light duty lathe will just start breaking and over stressing parts. Ultra sharp tools and the correct tool grind for the material will help a lot. Industry rule of thumb before highly specialized cutting tool coatings and grinds for CNC came in was approximately one cubic inch of mild steel removed per minute per HP. But that's for industrial sized machines.

[leeko]

Hi, a couple points that should help without any upgrades:

- a green wheel sharpens dull carbide, but only to a certain degree. Under a microscope, the edge likely has lots of microfractures and will cut suboptimally. To properly sharpen carbide, after the green wheel it should be touched up on a diamond wheel or a diamond hone by hand. It should then cut much better, and maybe give you a little more depth of cut before stalling

- a step beyond this would be to set the carbide aside, and use HSS tool bits instead. They're easier to sharpen to a razor edge, and will almost certainly cut much more freely than carbide, even properly sharpened carbide, on your small lathe. A 3" hobby grade lathe really isn't designed to withstand the higher cutting forces involved when using carbide tooling.

Try the HSS, and make sure to hone the edge. Look into correct rake angles for the material you're cutting (generally zero top rake for brass, but a high positive rake will give better results for steel), and you should see some reasonable improvements without spending anything.

Hope this helps

Lee

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[John Evans]

Go to correctly sharpened HSS tooling,will work much better !! You need a diamond wheel to finish sharpening carbide,green wheel by itself won't cut it. [LOL]

[ctwo]

What are light cuts and what is a reasonable expectation for a machine like this?

On my 10" Logan with 1 HP motor, I'm able to take a 1/4" in steel, but that depends on the specific material, its diameter, and of course the speed and feed. I usually stay well under 0.100" and most of my sneak-up passes are around 10-20 thou...

[BadDog]

I didn't see it mentioned, but for those who may not know, he's in the UK so what he calls a 3" lathe is what most of us are more familiar with as a 6" lathe.

And I believe HSS is very much the answer you need. If you have hard or very tough materials, carbide would be the answer, but for anything over 1/2" diameter your lathe may be limiting. For abrasive stuff like cast iron, carbide sharpened on a diamond wheel might be an answer. But using a green wheel on carbide for anything less than a 20" lathe doing roughing operations in castings, I would toss it in the trash. Another options might be the nice "up sharp" polished positive rake inserts (not the cheap import sets). Or Tatung (sp?) or Stellite bits, which are basically equivalent to upgraded HSS. And of course there are different grades of HSS for different purposes.

If going to HSS, you would be very well advised to read Harold's extensive dissertation on both grinder configuration and grind profiles (including chip breakers). You'll find those in the forum stickies. Again, I think this is the key to solving (or at least dramatically improving) your problems.

[Harold_V]

i'm using carbide tip tooling sharpened on a green wheel.

I can't think of much that would be worse, although cutting tools made from mild steel come to mind.

I can't imagine using a green wheel for carbide (been there). I was trapped (briefly) in a commercial shop that did just that, and soon came to realize how poor the results were. The resulting surface on the cutting tool, in my opinion, is not acceptable.

One of the hardest lessons to learn is that carbide is not a "magic cure-all"! It has its place, even on small machines, but it must be accompanied by adequate horsepower and rigidity, otherwise the only real benefit is abrasion resistance or for cutting extraordinarily hard materials. I can think of no good reason to use it when HSS would be better. For it, you will be required to reduce spindle speed, which, if accomplished via belt change, will result in an improvement in torque, resulting in more reasonable cuts.

Harold

[Gary Armitstead]

In my 45 years as a die sinker (making forging die cavities), I used the green wheels to sharpen carbide lathe tools. But the finish "grind" was always done with a 6 or 8 inch diameter by 1 inch wide diamond impregnated steel wheel, which are not usually affordable for the hobbyist machinist.

[BadDog]

Actually the diamond wheels that fit the carbide grinders (A6 I believe?) and 7" surface grinders have become quite affordable. Well under $100, and often closer to $50, which is well within the range of a lot of other stuff we buy for this hobby, and a LOT cheaper than many less used tools (if you use brazed carbide). I use them for brazed carbide, though I've gone to inserts in many appropriate places, or HSS as needed. About the only brazed carbide I use any more are on the "Bokum" style boring bars for my boring head, but it got used 2 weeks ago for touching up a bar for just that. I have a medium and fine grit (forget the actual numbers) on my carbide grinder for the last 7 years or more, they were used when I got them, and I use them for shaping too. After all this time I still see no sign of wear, so I don't see much reason to add a green wheel to the mix.

However, I wouldn't advise anyone to fool with grinding HSS without the right wheels (see Harold's sticky posts). I tried and it was miserable, WELL worth the small cost to make grinding hub adapters to mount standard 7" surface grinder wheels on my 7" bench grinder.

[Glenn Brooks]

Hi Water-Mark,

I have a similar sized belt driven lathe - a Dalton 7x36" (manufactured in 1919) to which I mounted a 1/2 hp motor. These lathes are old time, stout, small, cast iron machines that did yeoman service in WW1 in and near the trenches. I generally use 1/4" HSS bits and can comfortably take cuts up to .030" depth of cut without chattering or stalling the lathe. But it cuts nicely and leaves a good finish-if I do my job right. These lathes have been known to take 3/4 hp motors, although 1/2hp and less is most common.

Maybe if you can include a make and model lathe, maybe a photo or two, you are working with, someone can advise if you could replace your motor with higher wattage. Unless yours is a lightly built machine, or specifically designed for high cutting speeds with carbide bits -e.g. a carbide mini lathe, Iam guessing you could upgrade to a larger motor and improve the metal removal rate of your machine. But again, as others have said, depends on how stiff is the bed, engine mounts, headstock bearings, and designed operating speeds.

Regards
Glenn

[choprboy]

Using my old Craftsman 6"x18" lathe with a 1/2hp, I was once able to take about a 0.080" deep cut with about 0.0035" feed rate in 1018 steel off a 2" round. But that was in backgear with a very large tool approach angle and sharp HSS, made a long chip about 0.1" wide with a descent surface finish (for 1018). Everything just worked out perfect that time. More typical was 0.020-0.030" max depth with HSS for "hogging" material. Carbide at half or less than that do to the increased speed required and resulting loss of torque.

[Water-mark]

Thanks for the replies.

The lathe is a 1950's randa type b long bed.
I work mainly in brass but also in copper and aluminium, almost no steel or stainless.
Looks like i'll be revisiting the gibs and grinding up some hss tooling.