Help machining C260 brass plate.

[68rustang]

I have a part that historically has been made from 6.5"x6.5"x.75" Muntz metal (C365 or C370 don't know which) plates. Due to availability problems we have had to find a new alloy to work with. Long story short I have a whole skid of C260 plates I need to machine. The machining involves milling a .3125" deep circular slot/groove into the face as well as poking 7 holes all the way through. 4 of the holes are .34375", 1 is 1.15625", and 2 are .4375". EVerything is done with one tool running inside offsets.

The machine I am using is a Tormach PCNC-1100 and the cutters I have tried with not very good results were from Niagara Cutter, .3125" DIA, 4-flute, 1.375"-flute length, HSS, I am using flood coolant. Using my speed and feed calculator for the given alloy and cutter:

RPM: 2750
IPM: 42
Req HP: .85 (well within the Tormach's abilities)

Max radial depth of cut: .3125"
Max axial depth of cut: .07813"

Using those settings I broke the cutter halfway through the second .07" cut and the finish was terrible. I loaded another endmill and tried again but this time cranked my feed way down, like 10 IPM. The cutter was chattering quite a bit and the finish was less than pefect but at least I didn't break anything.

I know the cutter is too long but that is all the supply shop had that will cut all the way through my plate. I am waiting on some 1"-flute length cutters (both 2 and 4 flute) to come is and try those.

I am fairly new to CNC machining and most of my limited experience has been with polypropylene, and acrylic. Does any body have any ideas of what I should be doing or have been doing wrong?

In the past these were done with a 4 step process. The groove was cut manually on a lathe and the holes were drilled at three manual drill stations.

**EDIT

The new cutters came in this afternoon and I tried the 2-flute 1"-flute length and they chatter pretty badly like I thought they would. Everybody says brass is easy to cut. What am I doing wrong?

[big_mak]

Brass like to grab long cutters. Try a three flute endmill if you have some. The core is a bit heavier than a two flute, and should still give you alot of chip clearance. You may also want to look at solid carbide. Choke up on the cutter as much as you can this will help some of your vibration.

Another thing to consider is your fixturing. How are you holding the part. That has to be rigid as well.

[68rustang]

I have a three flute end mill but I am reluctant to try it. It is the same extended length and it the the last one I have for my plastic parts. I also have a four flute solid carbide 1"-flute length that I will try. Though being a $30 cutter I am hesitant to try it before I get a little better luck with the HSS.

I am trying to be cautious in my approach to cutting this brass plate. This is my first experience cutting metal with my PCNC-1100. Using my speeds and feeds calculator and a four flute cutter I get an RPM of 2750 and a IPM of 42. Taking my cautious approach I have been leaving the RPM at 2750 and cranking the feed rate down. Doing so I haven't snapped any more mills in half but I did knock all the corners off my last four flute. Should I be turning the RPM down along with the feed?

[Geof]

Originally Posted by 68rustang ..... Taking my cautious approach I have been leaving the RPM at 2750 and cranking the feed rate down. Doing so I haven't snapped any more mills in half but I did knock all the corners off my last four flute. Should I be turning the RPM down along with the feed?

Sometimes chatter can be reduced by increasing feed but your work must be well secured.

Do you have overrides on your machine so you can increase or decrease speed and feed on the fly. What you do is knock the spindle speed down 10% and see if the note of the chatter changes.

From your description it sounds a bit like the chattering is building up and the tool end is really vibrating around wildly and banging the sort of sideways.

Another thing to try is get an old cutter that has done a lot of work on steel. This will have taken the extreme sharpness off the cutting edge and on the brass it will not have the same tendency to grab.

[68rustang]

Today I made some progress. I do not have any mills that have been used for steel. Most of the millwork we do here is in plastics. I set up a test piece and using my overrides bumped my feeds until things looked and sounded good. In the end I ended up pretty close to what the calculators were giving me for feeds. My RPM of 2750 is spot on and my feed of 22IPM is just shy of the 26 Machinist's Toolbox was giving me for slotting. The best part is at the end of the day I had a few good parts and I didn't break any more cutters. Monday I plan on try the cobalt mill now that I have some confidence in my program.