rings in work piece

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senorgilamonster
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rings in work piece

Post by senorgilamonster » Tue Mar 13, 2018 3:00 pm

I am a novice and very poor machinist. I am not particularly mechanically gifted either.

While turning a piece of steel bar, I got a series of evenly spaced "rings" along the work piece. They are sloped by as much as .008. I suspect that the frequency of the rings is related to the rotation of the lead screw. The work also seemed to be hardened.

I could not find any swarf in/on the lead screw, and repeated the effect after cleaning. Just prior to this I had nice 6' piece of curly shaving, so I thought that I had the settings pretty good.

Harbor Freight (yes, I know) 7x14 lathe with a (Little machine shop)16" bed and lead screw upgrade.
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ringing.jpg

Harold_V
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Re: rings in work piece

Post by Harold_V » Tue Mar 13, 2018 3:08 pm

It would be hard to diagnose the problem without seeing the machine, but I couldn't help but note that I'm seeing a negative rake insert in the picture. If you'll pardon me for saying so, that's insane. Negative rake requires a reasonable depth of cut to perform well, and is poorly suited for finish cuts. The use of negative rake requires extreme rigidity as well as horse power.

Have you tried using HSS for your machining? If not, why not? Yeah, there's a learning curve involved (in learning to properly shape the tool), but you'll be rewarded with superior performance, and the understanding you'll gain of tool geometry will serve you extremely well for the rest of your life.

Mean time, is the tool you're using on center? It's entirely possible that its dragging slightly under the (theoretical) cutting tip and is floating the tool. Not much else makes sense, although with a very light duty machine nothing would surprise me. Take a look just below the cutting edge to see if there's any sign of either galling, or it is's shiny, as if it's been rubbing. If so, lower the tool until it's on center.

Might not hurt to mention the material you're machining. Is it mystery metal, or do you know the alloy?

H

Edit: While it's a long shot, I have to ask. Does the tool holder present the proper geometry? I've seen imported insert holders that had no included angle, so (negative rake) inserts had no clearance and wouldn't perform properly.
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GlennW
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Re: rings in work piece

Post by GlennW » Tue Mar 13, 2018 3:48 pm

Harold_V wrote:
Tue Mar 13, 2018 3:08 pm
but I couldn't help but note that I'm seeing a negative rake insert in the picture. If you'll pardon me for saying so, that's insane. Negative rake requires a reasonable depth of cut to perform well, and is poorly suited for finish cuts. The use of negative rake requires extreme rigidity as well as horse power.
Looks like one of those tool bit sets that use TT style inserts which are actually positive rake.
Glenn

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Magicniner
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Re: rings in work piece

Post by Magicniner » Tue Mar 13, 2018 5:03 pm

GlennW wrote:
Tue Mar 13, 2018 3:48 pm
Looks like one of those tool bit sets that use TT style inserts which are actually positive rake.
Dunno, there doesn't seem to be much clearance on the insert, it looks like the front edge is vertical.

John Hasler
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Re: rings in work piece

Post by John Hasler » Tue Mar 13, 2018 5:18 pm

Speed? Feed? Depth of cut? Material?

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senorgilamonster
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Re: rings in work piece

Post by senorgilamonster » Tue Mar 13, 2018 6:11 pm

I purchased new 1.125" A36 Hot Rolled Mild Steel Round Bar from a reputable supplier. I do have a couple of sticks mystery metal laying around. so it is possible that one got chucked in by mistake. I'm guessing around 1800 rpm with .008 cuts.

I don't think there is a negative rake angle, it may be an optical illusion in the photo. It is a TT insert

What I have found though is that the gibs on the cross slide had loosened up. Tightened it back up and the problem resolved. I hypothesize that as the tool moved forward, the resistance caused the cross slide to rise up until it would not go farther. It then forced itself back into place and the cycle repeated. I would have expected it to just stay "up" but...

Thanks all.

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Re: rings in work piece

Post by John Hasler » Tue Mar 13, 2018 9:04 pm

A36 can be frustrating. It doesn't work harden but it "tears". It's kind of like machining bubblegum. I get the best finish on it with a HSS tool with a lot of back rake and lots of relief.

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BadDog
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Re: rings in work piece

Post by BadDog » Wed Mar 14, 2018 12:46 am

Whatever other problems might exist, depth of cut is way too low, particularly if that's 0.008 on the diameter, and even if not. The only inserts I've seen that can manage that with good outcome are the cermets with crazy high speeds, and other somewhat exotic options. "Up sharp" (as I've heard it called) inserts bordering on what you use on aluminum can also manage it, but loose an edge quickly on most steels. But even with sharp ground inserts with tiny lands, 0.020 on the diameter for common steels is about the limit for me (with my ability, lathe, tools, etc). I more often use negative rake inserts to take much heavier cuts (anywhere from -0.200 to -0.500 dia) with fairly fast feeds and get very nice finishes even in common steel. But that depends on tool, lathe, material, diameter, and so on...

So for small depth of cut, properly ground HSS would be my choice if something like Cermets were not available. And for even smaller DOC, look up something called a "shear" ground bit. Very different approach to grinding HSS, but can yield successful cuts smaller than traditional grinds. It's been a while, but I used them generally around -0.002 dia, or even less, per pass with relatively slow feeds to get some very fine finishes on low carbon steel (like A36). But shear bits suffer quite a bit of heat and abrasion (particularly when the edge isn't perfect), which is part of the reason for so small a DOC, and your usage has to take that into account .

You don't mention feed rate (that I saw), but obviously that must be considered as well. But in general, one thing I learned the hard way, there is a practical limit to how light a cut will be successful, and that exact limit varies determined by pretty much everything...
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EOsteam
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Re: rings in work piece

Post by EOsteam » Wed Mar 14, 2018 12:51 am

Please keep in mind that I’m an amateur and ask many questions on this forum. It seems that 1800 RPM is way too fast for that diameter. You might look at roughly 340 RPM for HSS and maybe double that for carbide.

I have a 7.5” x 19” lathe that is roughly 250 lbs in weight and fairly sturdy and it really likes HSS. The only carbide used on that lathe is positive rake.

Harper

earlgo
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Re: rings in work piece

Post by earlgo » Wed Mar 14, 2018 9:08 am

After having been thru the brazed carbide phase, these kind fellows here convinced me to return to using HSS.
On the economics side of things, one can buy 2 or 3 HSS blanks that can be reground dozens of times for the price of 1 insert, not counting the insert holder. Of course you would need a bench grinder (with proper wheels) to do the shaping, but what shop can exist without a bench grinder?
These gentlemen know how, so....
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senorgilamonster
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Re: rings in work piece

Post by senorgilamonster » Wed Mar 14, 2018 3:01 pm

ironically, I don't want that shallow of a depth of cut...I am roughing out stepped axles, 1/8" between each step. A simple axle is taking me 6 hours because of 1) depth of cut 2) frequent stops to let the material cool down. As pointed out - obviously I am doing it wrong.

I think that I need to read up a bit. and experiment some more. Unfortunately I don't recall how I came up with this set up. I started several years ago and put everything in storage for the last 2 years. I did go with the TT inserts originally so that I was eliminating one variable while learning (grinding my own), but I do understand the economy of HSS.

I've considered going to take some community college machining classes. The 2 big trade schools nearest to me have wonderful CNC machinery, but I don't think teach "old school" since they are focused on getting their students into today's workforce. Ironically, I can design in CAD and generate the G-code to run such machines...but I don't own any.

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Re: rings in work piece

Post by Harold_V » Wed Mar 14, 2018 3:31 pm

Learning to grind proper cutting tools can be intimidating, of that there is no doubt. However, once you understand the logic involved, it's no big deal, especially if you have good hand-eye coordination.

Economy aside, the real benefit is in wringing work out of a machine that is not well suited to serious machining. Once grinding is mastered, material can be removed to the machine's capacity, with better results.

It's simple. Fractional horse power machines don't really benefit from the use of carbide. The exceptions being when attempting to machine either hardened or abrasive materials.

H
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