turning small dia brass

[speedyb]

Is quite difficult!!

the flex is quite the problem is it not ?
makes me wonder how you guys do it, and even made me think of a custom made follower rest.
I am using the 3-jaw and live center. even then...hmmm... lol.

Diameter is approx 0.200", length about 2 inches of it turned.

 

[mklotz]

Lots of small diameter turning problems can be solved with a redesign. Use standard size material and fit a small diameter piece to a larger to form the intended shape.

If that's not possible, nibble the problem to death. Fix parent stock so that only two diameters project from the chuck. Turn end down to finish size. Pull another two diameters out of the chuck and turn to finish size. Repeat as needed.

 

[Don1966]

Just like Marv said and I would also use a small diameter cutting tool and keep it sharp,then nibble at it to size. Make sure you last pass is repeated without more in feed. The other method I would use is graving if it is not a straight turning.

Don

 

[rake60]

I made a connecting rod once that gave me fits.

It was .250 on the ends to be milled flat and .125 in the middle for a length of 2 inches.
I had some 1/4" thick flat brass stock to use up so I sawed a pieces off of it 1/4" wide to
make the rod. It broke close to finish on 3 separate attempts.

I finally gave up and grabbed a piece of brass round stock.
That's when I noticed a slight difference in the color of the two materials.
That old flat stock was bronze not brass.

I still had some problems turning the brass that small.
I ended up using as very sharp parting tool set dead on center. I'd gouge in .010 at a time
and hand feed it very slowly to turn the undercut to the .125 diameter.

Rick

 

[Sshire]

Check this thread look for Tel's post with pix
http://www.homemodelenginemachinist.com/index.php?topic=17787.0

Best
Stan

 

[Dr Jo]

If it is an end you are turning have you thought of knocking up a rose bit with a hole in the center the desired diameter and working it up the parent stock until you get the required length?

Have you tried using a traveling steady?

Jo

 

[speedyb]

the "Gizmo" in the other thread is real neat

aand heck yes on using more standard material sizes...this is part of a sterling kit.. and I really dont have the knowledge to know what I can change, design wise.

I ended up turning it, about 3/8" at a time. not real pretty ,but it was going inside of an Al block, so its not seen.
this little sterling is really taxing what little skill I have...but its suppose to, since its my first multi-piece machining project.

 

[lensman57]

Is quite difficult!!

the flex is quite the problem is it not ?
makes me wonder how you guys do it, and even made me think of a custom made follower rest.
I am using the 3-jaw and live center. even then...hmmm... lol.

Diameter is approx 0.200", length about 2 inches of it turned.

Hi,

I have had the same problem when making parts for my hobby engines. The biggest mistake that I used to make was starting with a too small a diameter raw stock sticking too far out of the chuck. I strongly advise you to use a collet and just leave very short lenght of the stock out and take deep cuts using a very sharp hss tool not carbide as these require a lot of lateral pressure to cut material, repeat the process till you get the required lenght.

Regards,

A.G

 

[Florian]

the flex is quite the problem is it not ?

The flex as itself is no the problem; rather cutting edge geometry and speed. Possibly also the material;

Its best to use free-cutting brass as it is quite brittle.

Then, your cutting edge should have NO radius as every radius increases cutting force. This is because the pressure of the cutting edge is always at right angles to the tangent of the edge. (And this means you will have forces across the rotation axis of your rod if you have any radius or chamfer on the cutting edge)

Based to this picture:

(source: http://www.mfg.mtu.edu/cyberman/machining/trad/turning/geom.jpg )

-side and end relief angle should be about 5°
-side cutting edge angle 0° (or use side cutting angle but then you will need to rotate your tool so the cutting edge is just parallel to a turned face)
-end cutting edge angle about 1 to 2° more than side cutting angle (very small so you still reach a good surface although working with no nose radius)
-side rake angle should be about 5 to 10° (unlike 0° when usually machining brass !!! )

The cutting edge should be razor-sharp (as already mentioned)

Then, you will need enough rpm, I would say at least 1000, to do this Job.

I Think with this configuration and live center; you will be able to turn your rod to the required diameter.

Oh and by the way; for roughing, it is better to take one large cut with small feed rate than taking few small cuts with higher feed rate. The reason is, that the big diameter of stock material will be more rigid than the constantly decreasing diameter when taking small cuts.
Finally you can take a very small cut (like 0.05mm or 0.002 inch) to finish.

Regards Florian

 

[speedyb]

Thanks very much, and that was a great post, Florian.

Indeed, I was using carbide!

IIRC, I was using a spindle speed of about 1500rpm, and a feed rate that was fairly slow, but could be slower.

I came across a post elsewhere showing a homemade follow rest, and that could come in handy also.

Really apreciate all the help. It was getting very frustrating yesterday. I almost gave up, when on a second part, it was destroyed.
But the help here makes me realize to keep trying with these suggestions

 

[Florian]

Hey Speedyb

Carbide is almost never the best choose for machinig free cutting brass.
Except if you use an indexable tool with CCGT, DCGT or VCGT inserts and also with a very small tip radius. (0.1mm or 0.2mm)

But still, I rather like to use HSS for machining free cutting brass, also because if the tool starts to get dull, you chan just resharpen it in a few seconds and you will have a sharp cutting edge again.

Regards Florian

 

[mzetati]

What worked for me was chucking a much larger diameter workpiece and turning small lenghts of it to size and finish.
A D.I. fitted to the cross slide is in order, and an adjustable stop for the carriage on the ways would help.

Marcello

 

[tel]

Brass cuts best with flat top (no back rake) tools.