Turning finish

[markba633csi]

Agree with everything above- I rarely hone my tools anymore, often the honing will dull the tool if you aren't careful.
Make sure you have plenty of positive rake on the tool. Are you sure of the material? Just sayin'
-M

 

[Ken226]

When I cut 12L14 I use aluminum tool geometry.

Same here. I use tcgx inserts, and while the chip breaking sucks the surface finish is fantastic.

 

[kmanuele]

Thanks to all. Sorry for late reply, again.

Comment re: back rake. That was the main problem. Didn't have any, only side rake. Increased it significantly and much better results. It seems that steel needs some shear action? Still get a bit of build-up, but doesn't seem to affect the results.

Tool as modified with large back rake (about 20 degrees) below. Top view also. Tool is presented 90 degrees to work.

Also was getting tool angles a bit less than expected because my grinder tool rest is below the wheel center. So a 10 degree rest angle results in only a 7 degree tool angle, for example.

Comment re: RPM. I'll try a lower speed, though LMS table for 12L14 shows 200 - 260 FPM for HSS. Are these not correct?

On my lathe, I can't adjust feed rate independent of spindle speed as they are geared together. Is this not the norm?

 

[mikey]

Good job including back rake.

Your nose radius is way too large. As it is now, that tool will deflect and you will have a lot of trouble bringing a work piece in on size. I highly recommend you re-grind the tool and put a nose radius of about 1/64 - 1/32" on it. You will find it will cut far more accurately and will still finish quite well. Look at a 1/32" diameter drill bit. That radius is what you want on your turning tool.

 

[Mitch Alsup]

On my lathe, I can't adjust feed rate independent of spindle speed as they are geared together. Is this not the norm?

Definitely NOT the norm:: how would you cut both ¼-20 and ¼-28 threads if the pitch was not independent of the RPM ?

 

[Jim F]

On my lathe, I can't adjust feed rate independent of spindle speed as they are geared together. Is this not the norm?

You can, you need to change the gears.
It is called a change gear set.

 

[WobblyHand]

@kmanuele is correct, on a mini-lathe there is no way to change feed rate independent of spindle speed at least as delivered. Little Machine Shop sells a 3D printed plastic gear set that does lower the feed rate compared to stock. I bought one for my mini-lathe and use it often. It helps get better surface finish. I recommend it. https://littlemachineshop.com/products/product_view.php?ProductID=1137&category=

 

[Firstram]

Build a new banjo that adds some gear reduction. Difficult to get pictures of, but it works well!

https://gadgetbuilder.com/FineFeed.pdf

 

[kmanuele]

Definitely NOT the norm:: how would you cut both ¼-20 and ¼-28 threads if the pitch was not independent of the RPM ?

Maybe I was confusing 'feed rate' and tool speed. Newbie terminology confusion.

Feed rate, defined as inches per revolution (and not inches per second), is always constant for whatever thread pitch or power feed is selected -- regardless of RPM. Right, so far?

So the pitch, or feed rate, is independent of RPM, but not the tool speed.

Increase RPM, and the tool speed increases to maintain the pitch.

What I was trying (poorly) to say is that I can't change tool speed independent of spindle RPM.

K

 

[jwmelvin]

What I was trying (poorly) to say is that I can't change tool speed independent of spindle RPM.

You mean you don’t have additional gears with which to change the threading pitch or power-feed ratio?