Turning 3/16" Mild Steel

I noticed a bit of taper also. Real evident if you are trying to get a nice slip fit inside a tube. I was doing about half an I ch at a time with no problem. I even did a few at one inch but I could tell I was asking for trouble. I am using the cheesy cold rolled stuff you get at the box stores so it is probably not as hard or ridgid as anything else.

Jim
 
brino said:
Tom, the references I usually go by say 60-80 sfpm for cutting steel with HSS tool bit, so for 3/16" stock around 1200-1600 rpm.

-brino
Click to expand...

brino, good catch. I keep forgetting that some tables apply more to production machines and carbide tools. Maybe that's why half of recommended speed works so well for me and my HSS tools. ;)

Tom
 
Why do you want to take a full depth of cut as opposed to several lighter cuts? If it is about saving time, it seems to me that pulling the part out ~3/8 at a time wouldn't really solve the problem, even if it does cut well. And, you would end up with "steps" (may or may not be an issue)

I would think that increasing the RPM and taking 2-3 lighter cuts would more efficient.
 
I think my actual full DOC was only .015, I thought one pass seemed plausible, like it would have been on anything in a larger OD. I was able to do it about 1/2" at a time with no problem. I didn't notice any step at the continue point and the finish just a bit rough is a good thing since I will want to put a bit of grease on the shaft for lube between the shaft and the brass bushing.

Please keep in mind I am really absorbing a lot of this information and it is all valuable and sure to be used on future projects.

Jim
 
It appears you've got it figured out and it's working for you, that's the main thing. I'm no expert by any means, but I use the 3-times-diameter as a basic starting point for maximum stick out from the chuck unsupported. Sure you can stretch it, but it seems to be a good ratio for anticipating whether there might be deflection issues to watch for or not. Kind of prepares me for it, I guess is what I'm saying.

It's also interesting that you're getting better results using your parting tool for the whole operation rather than a normally ground tool with a nose radius. But again, it's working so that's the main deal!

-frank
 
To much unsupported length and not the best choice of tool, also too slow on the spindle speed.

I would leave only 1/16" clearance at the chuck (set a carriage stop), use a live center and a 35º diamond insert tool with the smallest nose radius you have to clear the center.
At a minimum 2000 RPMs, .008-.012 IPR feed rate to rough leaving .004 for finishing at .004-.006 IPR to finish, 2 passes and part off.

2000 RPMs is 33.3 Rev. per second, 1" at .oo8 per rev. is 125 revolutions, divide by 33.3 SPR and you have a 3.7 second cycle time for the rough, twice that for the finish pass, That is 12 seconds of cutting time, figure 10 seconds to change to the parting tool, 5-10 seconds to part off.

Good luck
 
Hi, Jim. Sounds like you've got it figured out, and I'm glad the tip to only extend a bit at a time helped, but I think the slow RPM and parting tool aren't making your life any easier.

I'm definitely still a noob as well, but I recently started taking speeds and feeds a lot more seriously with much better results because of it. Basically slower speeds are for heavier roughing cuts, and faster speeds for shallower finishing cuts, but you still want to be in the ballpark for your material. At 500 RPM you're only getting around 24.5 SFPM (about 1/3 to 1/4 as fast as you should be turning). Since you only need to take 0.025" off the diameter, you're basically just making a finishing cut. The references I use (laugh) say around 60-80 SFPM for roughing cuts and around 100 SFPM for finishing cuts on plain carbon mild steel (as they say, that's the great thing about standard references — there are so many to choose from!).

A center would, of course, help prevent deflection, but on such a small part it seems unnecessary. On really long thin rods you need a follower rest anyway — a steady rest still lets the part bend in the middle) — or continue to use the "step and extend" trick you're using now. The "three times diameter" rule of thumb says no more than 9/16" should extend from the chuck, but I'm pretty sure you could get away with even a full inch extended as long as you take off the full amount in one pass with a sharp turning tool (per my earlier comment: more axial load that way and less radial load).

I bet with a freshly ground turning tool, 3 or 4 times faster RPMs and taking the full 0.025" off the diameter you'll get much, much better results. I'd feed by hand, but if power feeding on my Logan I'd use the slowest or next slowest feed (0.004" or 0.008").

By far the best article I've ever read on sharpening HSS tool bits is Conrad Hoffman's treatise on the matter. Really great article, well worth reading and re-reading several times — some great info there.

Lastly, can you adjust your 3-jaw chuck for runout (sta-tru back or whatever the trade name is)? Otherwise, unless you're really lucky, you've almost certainly got enough runout to make it hard to turn small diameters and take the same amount off all around. Small, precise stuff is usually best handled with collets, the 4-jaw (independent) chuck, or old-school turning between centers. I only use my 3-jaw for rough and ready larger turning jobs. Buying some 3C collets and a closer for my old Logan was one of the best things I ever did — utterly changed my opinion on just how "loose and worn out" my lathe was.

Enjoy!
--
Rex
 
do you have a tail stock center and steady or follow rest? I would be inclined to get the work piece well supported and turn down the diameter first using a very sharp RH HSS toolbit (don't use a cut off tool for turning). Get it close to diameter and then sand/ polish to the final dimension given that these will be axles. Then put your cut off tool back on, stick the rod the minimum distance out of the chuck to part off and crank them out. Go much faster than you think you should for turning and be very very careful to get your cutting edge on center to avoid the rod riding up onto the tool.
 
I think he fixed it 3 months ago. ;)

Tom
 
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