good lathe tool tip forms for machining an extruded bar.

[Owen N]

I am making a small engine cylinder head on a "Micro" lathe.
I am working of a domed recess in the main chamber, which is the squish area of a two-stroke.
I found that if I come straight in from the front side, then cut towards the centre,
I need zero top rake, a fair bit of front rake, and a big radius cutaway to clear the the curvature of the chamber.
The material is 6063, I think, and fairly soft.
It is very slow going with the tiny lathe.

It is very chattery with just weak coolant (1:50) but removes material at a reasonable rate.
With spray cutting oil, it makes a smoother surface, but barely scrapes the material off.
It does remove some, eventually.
Questions?
1) How do I deal with increasing curvature as I get closer to the middle?
2) are there more favoured tool nose shapes? I have one narrow and one wider tip.
3) I have found if I feed in at about 15 degrees inwards by using both feeds, but a square tool, it seems to cut better.
4) should I revert to a more spade-shaped tip? This one is optimised for the mating seat for the barrel,
52mm in diameter, outer, 3.5mm recessed, 3mm cylinder wall thickness.
5) what is a better tooth form to mount at an angle in the holder? Do tools need to approach the work at an angle, or just be square on?
6) do I have to cut inwards from the outside?
7) should I bore a shallow central hole, then take 15-20 degree slices into the hole?
8) I see some tools have several other rakes. Are these worth bothering with?
a) a cross-tool "curling" rake.
b) a back-rake across the front of the tool,- away from the direction of cut.
I don't think there is any chance of getting a continuous coil of swarf with this material.
9) should I add more side-rake in the direction of cut? - towards the centre of the head.

I am quite an amateur lathe-user, and the more "brute force" approach doesn't really work with such a tiny lathe.
I tried using top rake, but the tool seemed to dive into the workpiece, then knock back the longways travel bed locks.
This leave deep tool marks.

I suppose on a bigger machine it would jam or break the tool.
The whole machine is noticeably flexible, being made of fairly thin extrusions.
The other tool tip I have is just full width, which was good for smoothing the cylinder mating area, but not so good at removing a lot of material.

Is there any online documentation going into this area in detail?
What titles should I look for?
It is sort of over-specialised for a YOUtube video.

 

[Compositepro]

I doubt that rake angles are your your problem, but rather relief angles. As you approach the center of your work, the radius of the concave surface you are cutting gets smaller and smaller. You need a pointy cutting tool to reach into the concave without some other part of the tool interfering, and rubbing before the cutting point touches. Look at how boring bars are shaped to avoid this.

 

[BrianE22]

You also have the problem of the velocity going towards zero as you approach the center of rotation. The tool actually breaks off the material at the center point (zero velocity). Sharp tools help to keep the thickness of the remaining material small and easier to break. Sometimes raising the tool tip above center is helpful.

 

[Owen N]

1)I did find a video illustrating the brute force approach. this looked like a triangular tungsten tip about 3/4" wide,
and about an 8 inch chuck.
My chuck is 2 inch...

2) I did use the boring relief on the outward edge.
I don't need it going the other way.

3) I am using about a 5mm square tool, not tungsten.

There are some ways of finessing these tiny tips for really low torque lathes.

Probably most lathe operators have never tried to use a micro lathe, so haven't developed a series of favourite ground tool steel tips for
this kind of aluminium.

This softer 6063 aluminium is a bit tricky . It must be moderately hard. This alloy hardens with age after extrusion.
6061 is also used in extrusions, and tends to be even softer.

 

[Owen N]

Got the inner dome part of the cylinder head done.
I used a square tip tool ,with front and side rake, then angled the tool holder back against the direction of cut.
I think about 5 thou per cut.

Finished with a rotary round-nosed mill/burr/file in a drill while spinning the head in the lathe, then 80 grit Carborundum paper.

Now I have a significant gouge in the 3mm seating area.
A getaway flat-faced 10mm drill, facing tool.
It fell out of the chuck, at an angle, still spinning. There was probably just enough room to stop it falling out completely.
-I should check chuck tightness better.

I hope J-B Weld likes combustion chambers.
It seems to be good for 550 deg f, so should be OK in the enclosed seal area.
Possibly exposure in the direct combustion flame area may tend to erode it??

I have seen some different epoxy used in drag car exhaust ports, and it does gradually erode away, but it still had that white-yellow epoxy look.
that was a big 6 with chopped-up V8 heads- I think it even held water! You may have seen it on youtube.

 

[Sparweb]

IME, aluminum never puts up a fight on a lathe. I've never used a lathe smaller than 12" swing, however. Something is loose or dull if your toolpost kicks back.

Forgive the back-to-basics question, but are you sure your tool is sharp?

You must have rake to clear the chip. You must also have an undercut angle, and relief on the cutting edge, too. If you're grinding tool steel, dunk it in water to harden the tip. If you're using carbide, the tip will be sharp until something ships off, then you throw it away. Or set it aside to have it professionally re-dressed for many years, then forget, and then throw a box of them away.

https://duckduckgo.com/?q=lathe+cutting+tool+angles&ia=web

https://www.machinemfg.com/tool-angle-selection-in-lathe-cutting/

 

[Owen N]

Part of the problem is extreme flexibility of the lathe.

Even with a bigger lathe, I have had built-up edge, which makes the lathe "slam" and cuts a gouge in the metal.
If I had a lathe that used those 3/4" tungsten triangular tips and a huge amount of torque, then things would be a bit quieter.
Tungsten does not seem to need lubricant on many materials.

It seems to depend a lot on the quantity and quality of the coolant/cutting lubricant.

If the 6063 aluminium was a bit closer to T6 condition, it would probably behave better.

Relatively small drills on a drillpress don't misbehave like this, but it needs a fair amount of torque and pressure to cut well.
You get nice curly swarf. Larger drills tend to chatter on the drillpress. possibly the low speed is not low enough?