Beginning Turning Issues

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Reddmax

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Thanks everyone for the efforts on the board. This place is pretty cool.


After fixing lots of niggling problems with my old Logan Lathe (2557 12 X 35), I'm practicing turning a non-tapered work piece (I just untwisted the bed and leveled it), and I've run into a problem. On a 10 inch piece of stock, I'll begin turning with good finish, cut about 2 inches in, and then my finish gets rough for about an inch, and then it becomes smooth for the rest of the cut. This occurs with a .010 depth of cut or at .002 depth of cut, and and seems independent of SFM. I'm running carbide tools at about 170 sfm on .820 piece of 12L14. It is an unsupported cut.
 
You have a an .820 bar 10" out of the chuck unsupported? If so that will not work very well at all, that's about 8" to much.
 
As said above, you should either move the piece further into the chuck or, if the piece has to be that long, centre drill and use a centre in the tail stock to support it.
A good rule of thumb is no more than 3-4 diameters of stock unsupported. Longer than that you need the tail stock or steady rest to provide rigidity.
 
I think if the finish is acceptable for the first couple of inches of the cut its probably not a support issue? Sounds like your leadscrew might have some sort of damage, bent possibly?

Shawn
 
If you are using a centre to support the bar I've had issues when the work piece heats up a bit from cutting and pushes harder against the dead centre and produces an un smooth finnish.

Stuart
 
Thank you all for your replies. I'm working on doing a barbell cut to make sure everything is in alignment. I would use the tailstock if I was working on a piece of stock in that length. To complicate the story, when I do use the tailstock and turn at 200 to 230sfm, the chatter/surface roughness is significantly decreased, but I do have an error in that spot of roughly 2 X the errors in the rest. I took measurements every inch, and in most places the diameter of the shaft was .7872 to .7878. In the spot referenced above, it was .7863. This I find very strange.
 
12L14 has a machinability rating of 193% and will run 325SFM with high speed tooling. Your speed is too low for carbide. However, I wouldn't twist it any faster with that much extension unless a tailstock center was used.

I think your finish problem may be a harmonic that is setting in at that spot in the bar when under pressure of the cut due to the unsupported length.
 
I was thinking that. As noted, when supported most of the irregularity goes away. Is there a machine speed chart for tooling anyone can supply me with? Or should I just google it?
 
My old Logan had a step in the bed about 7 or 8 inches from the chuck. Apparently the previous owner(s) used it a lot in that spot. Check your bed with a good straight edge.
Ed P
 
Turning speed is based on material being cut, and cutter type, and part diameter. If you google for "feeds and speeds" you will come up with many usefull charts, but these will list cutting speed in surface feet per minute (sfm). You then have to calculate the rpm based on the part diameter and sfm. The equation is pretty simple

Rpm = 4*SFM/D

Where D is the part diameter (or the diameter of the cuter if you are milling).

If i am turning 1inch diam steel with a hss cutter, we'll say my sfm is 50. That would give me an rpm of 200. Dial that into the machine, or use the next slowest speed on the machine.

Keep in mind this is just a starting point, and there are a number of things that will effect the actual speed. In particular if you are experiencing chatter (the surface of the work has a regular pattern thatlooks kind of like scalloped potatoes) then the general rule of thumb is to "increase feed or decrease speed". Basically take a heavier cut, or lower the rpm. Improving rigidity can also reduce chatter, but that is not always an option.

One other thing to mention is that, depending on the type of carbide you are using (mostly the common types of inserts) then 0.010 is a pretty light cut. You can run into problems where the cutter can't get a bite, and starts rubbing instead of cutting. This makes the material in that area harder, and makes things more difficult on successive passes.
 
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