Lathe - Poor surface on cuts

OK I'm still new to lathes & machining, but I have spent the last 12-18mths
really frustrated with my Myford 7
I bought it 2nd hand, and it is only a small lathe, but I have seen othere
peoples work & that looks OK.
So I have persisted tring to solve my cutting problems, with no lucks so
far. I purchased several books & have search the net
but can't seem to find an answer. I don't have time for tech courses ( I
barely have time to get into the workshop) which are a
fair way away, and charge a lot anyhow.
In a nut shell, I can't seem to get a smooth cut, on Aliminium or on Mild
Steel. I'm not being picky here, the best I've done has ridges in it.
But a 'normal' cut is just so rough. It looks like the metal is being
'pulled' from the surface rather than cut or sliced.
I've tried HSS & cobalt & carbide tools. I've tried fast & slow, manual and
auto feeds. I've spent ages grinding tool bits
being careful not to over heat, and NOT water cooling so I create cracks.
Then only to see them 'burred' when used.
sort of like they are made of cheese. None of my efforts has given me a
smoothly cut surface (forget mirror finished here,
just smooth enogh not to cut your hand will do!!) I don't get any bad noises
when I cut, and while I'm cutting it looks OK,
but when I turn machine off, I see a really rough (not as bad as a golf
ball, but one the way visually) surface
So could someone write me a checklist or something, in order that could lead
me to the cause?
eg
Should I buy indexable tips?
Am I setting the tool to low? or high? (I'm trying real hard for centre)
Should I use positve rack or negative? (I'm using positive)
Faster or slower? (tried both, slower seems worse however)
Different material? (tried Aliminium & mild steel)
Heavier / lighter cuts? I probably tend to light to very light cuts. 1/4
turn on crossslide would be a big one for me
or maybe just junk the myford & buy a 'real' lathe? will that fix the
problem?
Any help appreciated. I would just like to be able top cut a nice
smooth(ish) surface on a piece of metal (any colour or type will do!)
Thanks in advance
Reply to
Richard
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Well, it is hard to tell from here what the problem is. First, maybe you can have someone ekse try the same material on their lathe. There are certain alloys that do NOT cut well, and nobody will get a decent surface on it.
Two, you can cause work hardening on many materials. It usually is not a problem with aluminum, though. Work hardening happens when the cutter advances too slowly per revolution of the work. How much do you try to reduce the diameter of the workpiece each pass?
On lighter lathes, you don't want a wide part of the cutting tool to be contacting the work all at once. So, having a cutter advante toward the work such that the trailing edge is sticking out the most will set up vibrations that can foul up the surface. You generaly want the leading edge to be the farthest out.
The burr you refer to above is called "built-up edge", and it is workpiece material that has deformed in the plastic state and welded to the cutting edge. This indicates the cutter is getting hot. You may be taking too deep a cut, or advancing the carriage too fast. Finally, some cutting oil is a big help. I brush "thread cutting oil" on with a toothbrush as needed. A thin film really helps the built-up edge problem.
I don't get any bad noises
You definitely want positive rake, with a small tip radius, either .016 or .032"
Yes, that sounds right. Aluminum should be turned at about 600 SFPM, which works out to about 4000 RPM on a 1/2" piece. You can certainly go slower, but ought to stay above 1000 RPM or so on that size. Mild steel would be about 100 SFPM, or 750 RPM on a 1/2" part. With carbide, very roughly, you can go about 5-6 times faster.
Hmm, 1/2 turn would be .025 on radius? That's pretty reasonable.
You say you don't get any "bad" noises. But, from the description, it has to be making some kind of noise. When I turn softer materials, I rarely can hear any sound but the lathe's motor. Except when threading and parting-off, if I hear nearly any sound from the cutting operation, it means something is going wrong.
I'm wondering how you are holding the cutting tools on your lathe, and whether your chuck is holding the part well. Poor rigidity of the cutting tool support can cause this kind of trouble. You might bring the cutting tool a few thousandths from a workpiece while the lathe is off, and apply force with your hand to the tool holder. If ten Lbs of force on the holder changes the gap visibly, you have a REAL problem with some looseness in the lathe, somewhere. (The one place this isn't necessarily a problem is backlash in the leadscrews, as the cutting force generally is always in the same direction.) But, any rocking of the toolpost on the compound, the compound rest itself (a COMMON problem), the compound swivel, cross slide or carriage can cause a lot of trouble. Sometimes you can see the oil film right around the slides "pumping" as you try to rock the toolpost. This clearly indicates what part of the machine has the looseness. If it is in the sliding ways, you may be able to tighten gibs on that slide, but if there is wear in the center of the slide, it will bind at the ends of travel. (This is a common problem on well-worn machines, and can't be easily fixed.)
Jon
Reply to
Jon Elson
A little radius on a tool can be a good thing, too much almost never is. Best advice I can give would be to find one of the old lathe manuals, Atlas, SB, Sheldon all have good ones. (Lindsay is one source) Second, what feed rate do you have? Too high and it's going to look like a file when you're done.
THird, any play in the machine? IF there is, you're fighting that play, find out how to adjust it out.
Tool on center? The smaller the work and the smaller the machine, the more critical it becomes.
Clearances and rake? Both will make big differences in finish and tool life.
Is the chuck solid on the spindle, and the spindle bearings with almost no play? Don't guess here, check with an indicator. If the chuck moves on the spindle, check the back face of the chuck adaptor plate to see that it locates solidly.
My first guess would be with tool angles, they're not completely critical, there is a pretty wide range they will work, but outside of that range, nothing works. You might also try putting a tiny radius on the tip if you don't have one already. Try using a little cutting oil, if it makes a major improvement, then it's tool angles.
Rich.
Reply to
Richard
Try 12L14. If you can't get a smooth finish on it then something is wrong with your lathe or tool bits.
With carefully honed HSS I can get a smooth turned finish on O-1 drill rod and 303 stainless but not on mild steel or especially cold-rolled hardware-store rod.
jw
Reply to
jim.wilkins
Can you make a decent cut on someone else's machine?
Is your tool dead center on height or just slightly below? It should be.
Are you cutting too fast? By this I mean spindle RPM. It should be figured with the surface feet per minute formula for the material you are cutting and the tool you are cutting with (HSS or carbide).
What are you cutting? Mild steel is hard to get a decent surface finish on. 4140 will give beautiful results. Aluminum needs a coolant like kerosene or WD-40. The later will yield a mirror finish, if done right.
Ron Thompson On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA
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Severe stupidity is self correcting, but mild stupidity is rampant in the land. -Ron Thompson
Reply to
Ron Thompson

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