Facing piston tops..best feed/speed?

G

GregSY

Diamond

Joined

Jan 1, 2005

Location

Houston

• Oct 25, 2007

• #1

A basic question:

I have to reduce the height of some forged aluminum pistons and I think I'll use my lathe.

Since the diameter will vary greatly (4.375" down to 0") as I make the cut, and I have to choose a fixed speed, what's a good recommendation for speed (and feed) to get a good finish?

Thanks.

 

macona

Diamond

Joined

Jun 20, 2006

Location

Beaverton, OR

• Oct 25, 2007

• #2

Carbide or HSS tooling?

 

Forrest Addy

Diamond

Joined

Dec 20, 2000

Location

Bremerton WA USA

• Oct 25, 2007

• #3

It's only aluminum. It's tough but still aluminum. Make a pot to hold them if your 4 jaw won't swallow enough skirt to grab solid metal.

The varying surface speed isn't a surmountable problem. Use HSS and start at about 300 Ft per minute (about 270 RPM). There will be a change in the texture of the machined surface as you cut to the center but the dimension change will be nil for your purposes. Use about 0.010"/rev radial feed. Depth of cut will be however much the set-up will stand.

Button your shirt collar. The chips will be hot.

 

G

GregSY

Diamond

Joined

Jan 1, 2005

Location

Houston

• Oct 25, 2007

• #4

Thanks Forrest. I have a speed around 270.

I also have a solid way of mounting the piston that won't rely on chucking up to the skirt.

I'm just planning to use the insert that's in my Kennametal holder...I think it's a CNMG as I recall which I believe is carbide. Will that kill me?

Maybe I should mist it with CRC oil as well?

 

jim rozen

Diamond

Joined

Feb 26, 2004

Location

peekskill, NY

• Oct 25, 2007

• #5

As you suggested don't chuck up the skirts.
They're pretty fragile. Use a fixture that
pulls down via the wrist pin holes.

At least that's what they show in HTRAL.

Jim

 

G

GregSY

Diamond

Joined

Jan 1, 2005

Location

Houston

• Oct 25, 2007

• #6

The device I will use is a ho-made rip off of this:

http://www.kb-silvolite.com/feature.php?action=read&F_id=14

 

macona

Diamond

Joined

Jun 20, 2006

Location

Beaverton, OR

• Oct 25, 2007

• #7

You can run a bit faster with the carbide. Might want to use a bit of WD40 as a cutting fluid to keep build up down on the top of the insert.

 

eKretz

Diamond; Mod Squad

Joined

Mar 27, 2005

Location

Northwest Indiana, USA

• Oct 26, 2007

• #8

That's a sweet looking fixture.

 

C

carbideinsertman

Aluminum

Joined

Sep 27, 2007

Location

Michigan

• Oct 26, 2007

• #9

Greg -

If the insert is just any type of CNMG with a coating on it, I would definitely follow the recommendations from Forrest Andy.

You may want to get a hold of your local tooling person and ask about getting a sample of an uncoated, polished insert specifically for aluminum. Such as this:

A tool like this would allow you to run 600 SFM + at feeds .004 on up. the polished face keeps the edge from gumming up, and the rake helps with the chip evac.

Most carbide companies have them, so I'm sure your local supplier will know what you mean.

Regards

 

G

GregSY

Diamond

Joined

Jan 1, 2005

Location

Houston

• Oct 27, 2007

• #10

Just finished my first piston...I didn't want to wait to try a special insert (though it is a good suggestion) so I just gave my toolholder a good cleaning and put in a new Kennametal CNMG insert.

I experimented a little since I was making multiple passes. The cut was an interrupted cut thanks to the valve reliefs and I wound up with a 315 RPM and a feed of .012". A .010" cut gave a better finish than .015" so I went with that. I removed a total of .058".

I cheated a little and spent about 10 minutes with some 320,600,800, and 1500 grit paper with oil to polish the face. When I got done the finish was definitely 'mirror' - I started to buff it with a clean old t-shirt and I realized that was dulling the finish.

Now, 7 more to go!

 

T

Troup

Titanium

Joined

Jun 18, 2007

Location

New Zealand

• Oct 27, 2007

• #11

Has anyone else noticed how a frequently interrupted cut - in a turning situation - often yields a better finish than a constant cut, with gummy materials?

I imagine that any built up edge barely has time to form before the next "penetration event" smacks it to kingdom come.

 

D

donie

Diamond

Joined

May 17, 2003

Location

Walla Walla Wine and Wild Turkey

• Oct 27, 2007

• #12

A "very sharp" brazed carbide tool bit will produce a smoother finish, one you wont have to polish.

 

eKretz

Diamond; Mod Squad

Joined

Mar 27, 2005

Location

Northwest Indiana, USA

• Oct 27, 2007

• #13

Yeah troup...sometimes I notice that on steel parts...the continuous surface might have a dull finish, but as soon as the interruptions start it gets smooth and shiny. (Roughing cuts).