Finishing passes on the lathe

[The Smoking Yeti]

Hey, so I have a big question for you metal lathe junkies. How should I do my finishing pass on Acrylic/ ebonite? I know it's possible to get a great polished finish without even touching it with sandpaper. Can I get some advice on tooling bit profiles/ angles of attack for making the final passes? I'd rather not have to come back and sand, especially on an integral tenon.

Pictures might be the biggest help here for sure.

[RadDavis]

I always sand my integral tenons. It get all the tool marks out.

Rad

[KurtHuhn]

I meant to take these photos the other day, but got wrapped up in other stuff.

This the bit I use for stem tenons. I don't use it for anything else.

From the top:


The side:


And the bottom:


Using this bit shape, and setting it slightly above the center line of the tenon, and I get a smooth finish with no tool marks. All I do after that is hit it with a bit of EEE (penturners polish) while it's still spinning and that's all it needs. YMMV, not available in all states, use at your own risk, probably only works for me because nobody told me it wouldn't.

[caskwith]

No top rake on that tool Kurt? Or do you use it in an armstrong type holder that has integral top rake?

[KurtHuhn]

You are correct sir! My lathe has a "lantern" style toolpost and I use a reversible yoke Armstrong holder that works wonders. I can set the rake anywhere from nothing to many several degrees depending on how I position everything.

[RadDavis]

You are correct sir! My lathe has a "lantern" style toolpost and I use a reversible yoke Armstrong holder that works wonders. I can set the rake anywhere from nothing to many several degrees depending on how I position everything.

What in THE HELL are you talking about!!?? Reversible yoke Armstrong holder!!?? Rake!!?? WTF?

Rad

[Mark Beattie]

I recently purchased a metal lathe and have been slowly navigating the learning curve on it. I've ran into the same questions you have 'Smoking Yeti'. I've gotten some really good information from Kurt and Tyler ( and some others ) that have helped. The main issue being keeping your tools sharp. I use a HSS 1/4" bit when turning tenons with acrylic. You can tell the difference between a dull bit and sharp bit with only a couple passes. It should be possible to create a polished face with minimal tool marking when you bits are sharp. This link helped me a lot while learning how to sharpen my own bits. I use a basic $50 bench grinder with a fine wheel and can sharpen a new bit or freshen the edges of an existing bit in a couple minutes.

http://www.sherline.com/grinding.htm

Make a coffee and take 10 minutes to read this over. It will save you time and improve your finish.

Let me know if this is helpful. Good luck.

Mark

[caskwith]

You are correct sir! My lathe has a "lantern" style toolpost and I use a reversible yoke Armstrong holder that works wonders. I can set the rake anywhere from nothing to many several degrees depending on how I position everything.

What in THE HELL are you talking about!!?? Reversible yoke Armstrong holder!!?? Rake!!?? WTF?

Rad

It's ok Rad, I know what he is talking about.

[Sasquatch]

Rad, Chris knows what Kurt is talking about.


Hope this helps.


Todd

[caskwith]

Rad, Todd knows that I know what Kurt is talking about. Whether Kurt knows what he is talking about is another matter.

[Sasquatch]

PS I can change the angle of my cutting tool any time I want to obtain the ideal approach. Don't need no reversible Armstrong choke holder thingy neither.

[RadDavis]

Todd,

I know what you're talking about.

Rad

[Sasquatch]

I'm talking about skill. Guts. Determination. A melding of man and machine.

I'm talking about the New Order. I'm talking about the WAY THINGS ARE.


I'm talking about having a beer and it's only 10 in the morning.

[wmolaw]

I'm talking about skill. Guts. Determination. A melding of man and machine.

I'm talking about the New Order. I'm talking about the WAY THINGS ARE.


I'm talking about having a beer and it's only 10 in the morning.

LOL, or four!

[KurtHuhn]

What in THE HELL are you talking about!!?? Reversible yoke Armstrong holder!!?? Rake!!?? WTF?

My metal lathe is super old, so I gotta make words up to describe the various doodads, thingamabobs, and whatsits that sprout from it.

[SimeonTurner]

What in THE HELL are you talking about!!?? Reversible yoke Armstrong holder!!?? Rake!!?? WTF?

My metal lathe is super old, so I gotta make words up to describe the various doodads, thingamabobs, and whatsits that sprout from it.

Ha! I heard Rad telling Bruce The same thing once, only I think he was talking about...errr...something else...

[ToddJohnson]

You goat scrumpers are so childish. Anyway, I'd like to bring this back on topic as this is a serious forum, and this sort of nonsense gives pipe makers a bad name. So, as I was saying, you slobbering, crayon-eating, morons . . . wait, what were we talking about? Oh yeah, cutting tools.

I grind zero-relief cutting tools for virtually everything, and the rake angle is either zero degrees, or ninety degrees, depending on where you live, and if you put the day in front of the month, or the month in front of the day. Either way, the cutting surface is perfectly parallel to the ways. This allows for very quick and easy honing of the cutting edge without altering the cutting height of the tool. If you alter the profile, and that alters the cutting height, then you're constantly having to adjust the height of the tool holder in your quick-change. This is a very cumbersome endeavor.

Also, you want to face your material with a very rounded, bull-nosed cutting tool that has roughly a 10-15 degree rake below the cutting surface. This way, you have as little surface contact--and therefore less friction--between the cutting tool and the face of the material as is possible. If you're cutting integral tenons--like a crotchety old bastard--this becomes more difficult, but the same principle applies. In this scenario, you want to use a somewhat more narrow V-shaped cutting tool (same 10-15 degree rake) with a rounded point. The radius should be about 1/16" rather than coming to a perfectly sharp point. Cut at an acute angle (that is to say pointing toward the headstock) and only concern yourself with one axis at a time. In other words, cut the tenon to its proper diameter and length first (using long passes on the X-axis, and only incremental adjustments after each pass to decrease the tenon's OD), and then cut the face of the stem perfectly smooth. If you first make several passes down the length of the tenon (X-axis) and then back the tool out to rough-cut the face (Y-axis), you can then make one final skim-pass from the tip of the tenon all the way down its length, and then outward until the bit clears the OD of the rod stock. If your bit is sharp--which means that it should basically have a mirror-polish--then you will have no need for sanding at all. I never shave, but if I did, I could do it with my cutting bits. Remember that the best possible finish you can expect on your shank face is a mirror image of the finish on your cutting tool.

TJ

P.S. Rad, after reading this thread, I'm afraid you may have early-onset dementia--well, I guess it's not really that early.

[e Markle]

P.S. Rad, after reading this thread, I'm afraid you may have early-onset dementia--well, I guess it's not really that early.

Just wanted to add a hearty "hope this helps" to TJ's comments.

Hope this Helps,
EM

[Alden]

" I don't always shave.....But when I do, I use TJ Toolbits"

[RadDavis]

P.S. Rad, after reading this thread, I'm afraid you may have early-onset dementia--well, I guess it's not really that early.

Just wanted to add a hearty "hope this helps" to TJ's comments.

Hope this Helps,
EM

That wasn't very hearty. Give it another go.

Rad