Lathe tool brass forming tool grind

[redvan22]

I'm trying to make a run of small brass knobs from 1/2" brass rod and I am having a lot of trouble eliminating chatter and sudden dig-in.
The tool bit I have designed is an elongated tear drop with the point shortened for drilling and tapping for the screw and the round end flattened.
I make the profile with two bits, one for the back end and back side of the knob and another for the front round and flat. Both give me trouble.
Zero side rake
Zero back rake
Front relief should be 8 degrees, as suggested but that caused issues so I have reduced it gradually to 3-4 degree.
Still get chatter and dig-in which makes a mess and in most cases ruins the piece.

Any suggestions?

Michael

 

[DavidR8]

I've never made a tool for brass but I understand that there should also be zero top rake.
Maybe post a pic of your tools?

 

[erikmannie]

I remember in my Intro to Machining classes, both instructors referred to the challenging property of brass wherein the workpiece wants to suck in the (in this case) hand-ground HSS tool bit.

Maybe tightening the gibs would provide the added resistance you need to prevent your “sudden dig-in”.

In my limited experience, I have wondered if an exact same setup on (1) brass or (2) mild steel will allow more chatter on the brass. Are you sure your setup is as rigid as it can be?

 

[mattthemuppet2]

snug everything up, reduce stick out of the work and the tool, and make light cuts. Chatter is inherent in forms tools and brass likes to dig in, unfortunately.

 

[cross thread]

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[Janderso]

snug everything up, reduce stick out of the work and the tool, and make light cuts. Chatter is inherent in forms tools and brass likes to dig in, unfortunately.

Good advice. I like working with brass. Yes it likes to grab the tool.

 

[Illinoyance]

You didn't describe your lathe. Is it large and rigid enough for the wide form tools?

 

[redvan22]

You didn't describe your lathe. Is it large and rigid enough for the wide form tools?

I have a mini lathe 7x16 that I've done extensive work on to eliminate slop;
Have installed brass adjustable carriage gibs.
Have scraped the compound slide and honed the gib.
have done work on the cross slide but it does need more.

I'm not trying to do the forming with one tool. I am using two, one for the back end and another for the front.
As for stick out, barely sticking out enough to get the job done.

 

[Illinoyance]

I am not sufficiently familiar with mini-lathes to offer advice. Sorry.

 

[Huub Buis]

Using form tools on a mini lathe is always a challenge due to the lack of rigidity.
Position the top slide and tighten the gibs to improve rigidity and reduce the dig in.
As already mentioned, grind the tool at zero top rake and reduce the front to 1° to reduce the dig in.
Place the tool upside down (at center height) and flip the rotation of the chuck to eliminate the dig in. That is probably the only way to do it besides CNC turning.