Tool bit help

[TRS]

I read some posts a while ago about tool grinding. I tried my hand at it and I'm having some problems. I'm using hss blanks and a rocker tool post. The cutting bit works pretty well for facing, and when I go to turn a shoulder for the tenon it does really well for the first few cuts. However, when the tenon starts to get smaller and nearer proper tenon size, the tool bit seems to 'push' against it, leaving a wobbly, uneven surface or just outright cutting gouges into it. I've tried adjusting my angle, resharpening the bit, changing speeds.....I'm pretty sure the problem is with the way I ground my bit, but I can't figure it out. I rounded the nose off, but this seems to be the biggest culprit. It's either the bit, or my procedure is way off. On the included sh*t diagram, the left hand is how my cuts start and the right hand is how they tend to end. Also, by the time they start going awry, the material ceases to come off in ribbons and starts looking ugly and pocked/melted. Any help would be GREATLY appreciated.
Tim

[SchmidtN]

This might sound stupid, but are you sure your lathe is spinning in the correct direction? I was working in a metal shop once and a guy couldn't understand why he kept burning up bits and his work pieces. He had the headstock spinning the wrong way and was just making a bunch of friction and heat. His "round" pieces look like your drawing on the right.

Other than that, maybe check google for "lathe tool bit geometry" or "lathe cutting tool geometry" might help.

[TRS]

Yes, it's spinning the right way. I'm convinced I'm committing an error in my tool grinding, but I can't figure out what. I've since reground the tool a few times and tested it. The tool LOOKS good, but the radius always seems to 'rub' the piece. It can burn big grooves or, after I really gave it a smooth grinding, it leaves thin circles, almost like it's threading the piece.

[LAH]

Have you tried turning a tenon with an already made cutting tool, like a carbide tool, just to narrow the problem down to the tool that you had made. Did you grind a front relief angle on the tool? Is the tool level with the center of the spindle?


Abe

[Alden]

I was just searching old posts about grinding lathe tools.
Heres a couple I bookmarked
viewtopic.php?f=43&t=4628&hilit=grinding+bits
viewtopic.php?f=43&t=4240&hilit=grinding+bits
Hopefully there will be some new answers here too.

[TRS]

I was just searching old posts about grinding lathe tools.
Heres a couple I bookmarked
viewtopic.php?f=43&t=4628&hilit=grinding+bits
viewtopic.php?f=43&t=4240&hilit=grinding+bits
Hopefully there will be some new answers here too.

That first link is what I used (along with the tubalcain youtube series) to learn how to grind it. I'm going to go dig up some carbide cutters I know I have lurking about and give Abe's experiment a test. Thanks all.

[caskwith]

What material are you cutting and is the tool exactly on centre?

[e Markle]

I'm convinced I'm committing an error in my tool grinding, but I can't figure out what.

You are.
I know what it is.
I'm not telling you.

Hope this helps!

[baweaverpipes]

The tool has to be dead center. Well, actually, I keep mine about .001 below center.
I have about a 5% rake on my tools.
I turn @1260rpm

[JonBood]

Hehe.. I actually drilled a moose horn in the wrong direction on my lathe once, took me some tome to figure out why the drill bit seemed so dull......

[taharris]

I have very little experience with metal lathes, but the limited experience I do have supports your belief that your tool is not sharpened correctly.

I believe you are getting a good cut initially because you have more material so support the cut. As you get to smaller and smaller diameters the material you are trying to cut is deflecting.

Is there a machine shop in your area? I'll bet that you could find someone there who could help you with tool sharpening.

Todd

[TRS]

Chris and Bruce were spot on; my tool wasn't quite centered properly (no more eyeballing it for my lazy ass) and I fiddled with my countershaft setup until I found a proper speed. It amazes me the dramatic difference that some very small adjustments made. Thanks to everyone who offered help, with the typically glaring exception of Ernie.

[sethile]

Looks like you've discovered the culprit, but I thought I'd add a link to a YouTube series that I found very helpful recently while reviewing how to grind tool bits:

1st: http://www.youtube.com/watch?v=hrDr4rYLiAk
2nd: http://www.youtube.com/watch?NR=1&v=dRyqIm5JR5s
3rd: http://www.youtube.com/watch?v=HTQ46NMMc88
4th: http://www.youtube.com/watch?v=rZbr7Sh8jQw

I just made a new shank facing tool that cut so square and cleanly that it completely amazed me! No more sanding shank and stem faces for me! Nothing like understanding the basics to get better results...

This guy goes by Tubalcain, and his videos are great! He's a retired High School metal shop teacher. Here is an index of his videos that can be used to easily find them on YouTube:
http://neme-s.org/Tubalcain/machine_shop_tips.htm

I've found several of them extremely helpful while setting up my new lathe and retooling for it.

[caskwith]

Glad you got it sorted. Off centre bits are fine if you are turning large diameters, in fact in certain cases you actually want to set a little high or a little low, but on small diameters you have to be exact or if in doubt as Bruce says just a tiniest touch under centre. Parting tools however should be a little above to stop digging in and snapping.

[smokindawg]

Ok, stupid question here from a newby metal lathe guy.

How do you make sure your on center without eyeballing the cutter?

[nolo]

You can put a center in the head stock or tail stock and line up your tool bit with the center's point.

I've seen people use a rule between the work piece and tool bit. If the tool bit is on center the rule will be vertical.

There are specialty centering tools out there too. But I don't see the need.

I just line the tool bit to a live center in the tail stock. With proper lighting it is effective. Putting a piece of white paper behind the points as a back drop helps me too.

[TRS]

Yeah, I put a live center in the headstock taper and line the cutting bit up with the point of that.

[Mark Beattie]

I had been running into a similar issue with my Taig. I had sharpened my bits over and over again and still couldn't achieve a clean cut. It turned out that the spindle in the headstock was slightly loose. All it took was a little tightening and those tools cut through my acrylic like butter. This issue did force me to re-learn how to sharpen bits. It might be a valuable experience to investigate the URL's included in previous posts if you're having this issue. Cheers.

Mark

[kkendall]

Ok, stupid question here from a newby metal lathe guy.

How do you make sure your on center without eyeballing the cutter?

That's how you do it - by eyeball .... either something in the tailstock or headstock with a point on it - and eyeball the top edge cutter to be at the same height as the point.

[staffwalker]

You can chuck up a piece of rod and take a small cut with the cutter. If it is at the correct level the cutter will cut cleanly across the end of the rod and leave no material. If it is at the wrong height it will leave a tiny circle of uncut material in the center of the rod. If you have something left you need to shim the holder until it doesn't leave a center. bob gilbert