I understand how much everything on a lathe, including the work can flex, but this just doesn't make sense to me,
If I turn down say a 1/2 aluminum rod that's chucked and extends only a half inch and dial in 0.01" of feed and measure the before and after diameter with a micrometer the diamter decreases by 0.02" Perfect.
The part I don't get is if I take a smaller cut of say 0.001" it will remove more than 0.002" of diameter. For example if I need to remove 0.003" of diameter, feeding the crosslide 0.001" will come pretty close.
It only seems to happen with very thin cuts. I'm only turning the crossslide dial in one direction- there no backlash weirdness going on.
This is spring. The stack of carriage, cross slide, compound, toolpost, toolholder and tool becomes quite flexible. It loads up as you take initial heavy cuts, then when you take a very fine last cut, the spring unloads and drives the cutter deeper than you wanted. All machinists need to learn to compensate for this.
Generally, the more rigid and massive the lathe, the smaller this effect gets, but at some level it shows up on any lathe.
At .001 infeed you are basically making a spring pass.
Suggest use an indicator on the cross slide and once you get to be about .050 away, taking the nearly the same exact amount off every time, I generally use about .005 per pass on small machines...run the slide back very quickly to the start position so that it only leaves a shallow spiral mark on the part, and then adjust the indicator by a thou or less after each pass, such that your final pass will end up being EXACTLY .005 deep...thereby making deflection a fairly predictable amount...after the final pass, quickly lift the tool away from the part so that you don't gouge it up.
Hmm forgot about this. I've been fiddling with toolpost height, setting the cutter so that a facing pass doesn't leave a nub at the end of something. Too low definetely makes the parting blade very unhappy as the work starts to climb and chatter like crazy.
I'm able to compensate for this behavior, it just seems opposite of what I'd expect.
My first guess is that a deeper cut would tend to be less than you dialed in as the machine everthing is flexing apart. I'd then assume a wimpy fine cut would be about what the handwheel would indicate there's less stress on the machine, work and everything in between. Going over what I dialed in would be possible due to backlash if the cutter is being pulled INTO the part.
I was moving cutter over to the right past the work, so there wasn't a buildup of tension or springiness that way. The post about backrack makes sense though. There's nothing to keep the cutter from being pulled into the work on a sherline. Mine has about 2-3 mil of backlash after some adjustments, but there's no real positive lock of any sort, just tension on a wedge shaped plastic gib. You can only get it so tight before it just acts as a brake and nothing moves at all anymore.
It's interesting process to try to tame thing thing.
material does some off again when you go backwards. I was measuring after this second pass and the tool cleared the work so I could fit the micrometer in place.
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There's an awful lot that can go wrong on a light "hobby" lathe. Banquer said it right, about having to "baby" it to prevent flex and spring. The other responses about spring in the system were also spot-on.
Also, if you're using a lantern toolpost, it's hard to control the rake without re-grinding the cutter. Raising and lowering the tool only serve to alter it's "cut per increment".
It's not "kosher" on aluminum to use a zero back-rake, but it will tell a tale if you try it. The finish won't be all that great unless you use high speed and have a VERY sharp cutter, but if the problem goes away, it indicates that hogging (due to spring in the system) is the problem.
A zero back-rake springs away from the work -- if it's precisely on the centerline.
You might also check your mounts. If the whole bed can spring and twist, it will give similar disgusting results.
Note too that with some material/tool combinations there's a minimum depth of cut that you can achieve. This mostly happens with materials that have a lot of flex for their strength, and tools that are not as sharp as they need to be for that material.
I mostly have this issue when machining plastic, or when I have not been diligent about sharpening my HSS tools.
are there any sort of rules of thumb for cutting vs rubbing depths?
I have noticed that the generic looking 1/4" HSS tools with all sorts of bevels behave quite differently than one indexable HSS insert (little diamond shaped things)holder I have where the top of the insert is completly flat. The only bevel is on the edge of the cutter itself.
It seems to leave a nicer finish on aluminum under pretty much any condition. Is this possibly due to the less backrake?
Good point. I use my cutoff bit for finishing to tenths (0.0001") and was thinking of mentioning that here. So I went down and cleaned up the rough-sawn end of some Delrin rod with it, 1 - 2 thousandths per pass, the bit angled 9 degrees toward the face (Multifix toolpost), and had no trouble with the bit sliding before cutting.
I hollow-grind the end of the cutoff bit so it can be touched up easily with a stone, which contacts and cuts at only the very top and bottom. jsw
There's nothing to keep the cutter from being pulled into
SHERLINE??!!
I now have a 3500 Lb. Sheldon, and I STILL have to worry about things like this, but somewhat less. You CERTAINLY have to be VERY aware of cutting forces on a light machine like the Sherline or Taig (or ghastly Chinese mini-lathes.)
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That is a zero back rake tool (assuming the tool holder isn't tilted)... usually used for brass, bronze, and cast iron, but suitable for testing the principle. As you can see, back rake makes a big difference in how a tool hogs -- especially on a fairly flimsy machine.
Reminds me of turning a 1" rod of 6060 in the lathe - dry as a bone - and as it heated up Al expands and you cut more. Then you come off it and it soaks into the chuck and air - shrinks back to where it was and A slot I cut not a smoothing cut!
Al needs coolant and plenty of it to stay cool so it won't expand.
Mart> I understand how much everything on a lathe, including the work can flex, but
The main advantage of having an exaggerated positive rake angle is reduced horsepower usage per unit material removal....if your tool is "hogging in" then I would suggest tightening up the slide lock..
Disadvantage of exxagerated rake being it results in a fragile tool edge, which will be more prone to premature wear via micro fracturing.
Additional issue is material property; a soft flexible bar that bends fairly easily is more likely to climb up onto the tool under negative or neutral rake than under positive.
R15-6. A VERY advanced lathe. It looks like a geared head, but the spindle is driven by a belt drive (3 matched belts) that runs on a separate input shaft suspended on its own bearings surrounding, but NOT running on, the spindle, so as not to transmit vibration to the spindle. It drives the spindle by two clutch dogs that are hand-scraped to put exactly matched torque to the spindle. The gears are on the motor, down below.
The threading leadscrew has double universal joints at the left end so it doesn't transmit any radial torque to the carriage and make waves in the parts. Carriage and crossfeed drive is done by metal plate clutches so they can slip if you have a crash, no broken gears.
The QC box has 80 feed settings! They have two big drums full of gears that can be rotated by cranks to select the desired feed or thread.
There's more, LOTS more, sophistication in the design of this machine, I was just in awe of it when I did the rebuild. Every time I took something apart I was amazed at the great ideas that went into it.
As for use, it works amazingly well after my rebuild of the main bedways and applying Moglice to the bottom of the carriage. I did some
1"- 20 TPI threads in 1018 steel in 3 passes, and could have done it in TWO! I also made some 1" ball joint sockets with a quarter-round form tool. I drilled to 7/8" (my largest Morse taper drill) and then just plunged the form tool in, expecting at least a little chatter. This would have made my 12" Atlas dance and the neighbors would have heard the noise, but all I heard was a faint crinkling sound as the chips crumpled inward on the cutter.
I'm embarrassed to have such a fine machine and use it so little!
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