bearing bores

[eschless]

I'm a bit new to cnc. I'm trying to cut a bearing bore in steel tube (heavy walled) and am having a hard time with concentricity. Does anyone out there have a pretty good idea of depth of cuts for best accuracy? I've got sharp tools, rigid holding and so far have best luck with 2 finishing cuts at .002 depth, after my .050 roughing cuts. But I'm barely holding .001 tolerance.
I know on my lathe, I get best repeatability with ~.010 cuts, but this is a different beast!
The bore is 1.350 and is .440 deep.
Any suggestions?

[HuFlungDung]

Concentricity has to do with the accuracy of chucking the part. But I'll assume that you're saying that its just generally not a good bore.

What style of insert are you using? Are you having chip wrap nest problems that scuff the surface up? What feedrate and sfm?

[eschless]

I'm using a 1/2" 4 flute coated HSS end mill,climbing, finishing at 2500 rpm and slow 5 fpm feed
At this slow feed rate, the chips are nearly non-existant!.

I am holding it in a very rigid lathe chuck, mounted to the table. No flexi.

I guess I'm really looking for some general advise about interpolating circles accurately with cnc. Its a Haas vfo

[Ken_Shea]

eschless it sounds like you know what you are doing and I cannot say I do, but .001 ain't shabby for a interpolated cut with an end mill. I would think it would be extremely difficult to get much better without using precision boring tools.

Ken

[HuFlungDung]

Here I had the notion that you were turning the part in a lathe!

Okay, for really accurate holes in a mill, you would be best to use an adjustable boring head. If you have many holes to do, this would be the best way to make lots of good holes.

HSS tools cannot be run at any old speed... you're going much too fast for it. The keen edge will be ruined. The dullness of the tool causes loads of cutter deflection. I'd recommend that you try to finish with a carbide endmill, if you must interpolate the finish cut.

[eschless]

That's what I'm trying to find out. Thanks.It sounds like I should use something like a Criterion boring head. I needed to hear it from someone who does this more than my limited experience. Have you ever heard of a Cat 40 Criterion head adapter, or a good way to adapt R-8 to Cat 40?

[eschless]

Sorry, I missed that last post before I replied.
It sounds like maybe I'm asking a lot from HSS tools. Problem with cnc is ya can't feel when a tool is cutting poorly. A friend suggests using indexed carbide insert tools only, and especially Iscar. Maybe I shoulda listened!

[Ken_Shea]

Checking some software, the feeds and speeds are coming in at about 500rpm using a HSS .5 4 flute end mill cutting .100 deep at 6 IPM.

[eschless]

Wow. I probably toasted the mill. It still feels sharp, tho. Why so slow with all that coolant?
Any advise on tool speed software?

[HuFlungDung]

I don't think an insert cutter will do a good job. Buy a high helix carbide endmill and save it to do only the finish cut. It should last a good while. That's not saying that it will be as good as a boring head.

I don't know what kind of mount a Criterion adapter uses, might be threaded? I imagine most anything is available in Cat40, though.

[HuFlungDung]

HSS runs at about 100 sfm max. Maybe slightly more on a very light cut, but there is not much leeway. Carbide is more forgiving on light cuts

[eschless]

I just ripped my criterion apart. Its got a 7/8 thread , so there's a start.

Found an adapter for $26 at Travers. Thanks for all the help!
I guess I felt that you can interpolate anything perfectly...a bit idealistic there..
uhhm. whats sfm? (proof I'm not a real machinist)