From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Gnarly finish!
Date: 09 Aug 1999 01:01:30 GMT

>Subject: Re: Gnarly finish!
>From: "Judy Swinney" <judybob@home.com>
>Date: Sun, 08 August 1999 04:03 PM EDT
>Message-id: <Aolr3.28946$WX1.195916@news1.rdc2.tx.home.com>
>
>Nigel,
>The mysteries of carbide machining continue to befuddle me!  The stuff is
>great for removing lots of metal in a short time but usually at the expense
>of a good finish when we get down to the final cut.  I've given up on trying
>to understand it, metalurgically speaking!  Probably carbide is too sharp -
>by that I mean it is so hard with respect to the workpiece, that it effects
>a tearing action no matter how light the contact.  This is exacerbated by
>any lack of rigidity, no matter how slight, in the machine and tool holder.
>My theory, and I'm sticking with it!
>
>There's a lot to be said for the old HSS, or even CS tool bits with a
>correct radius ground on the nose.  This fits right in with my theory,
>above, about the hardness (virtual sharpness) of the bit.  All else being
>equal, a radiused nose on the tool, (carbide tools too) presents a kinder,
>gentler cutting surface -- Hell, it might even accomplish this by slightly
>wearing itself down rather than tearing the work.  May explain why old style
>bits wear faster than carbide.  Am I on a roll, or what?  The opinions are
>my own and should not be construed to reflect on those of my employer,
>friends, or the scientic community at large.
>Good luck,
>Bob Swinney
>
>

"Hmmmmm" is about all I'm going to say to that. This carbide v HSS debate seems
to surface at regular intervals and I feel compelled to throw my 2 pence in.

No full time professional turner that I know, would dream of using anything
other than replaceable carbide bit tooling and have no problem achieving
surface finish on any normal material or bar size. I use one general purpose
turning/facing tool donated by a CNC turner friend many years ago and do 99% of
my lathe work on my knackered old Colchester Student with it except for parting
off and boring of course. It is a 20mm square shank item so it's solid as a
rock but even so I use it as close to the toolholder as possible. It takes the
55 degree rhomboid inserts DNMG.

What has to be borne in mind and rarely seems to be is that carbide bits are
designed for rigid, high power machines with the requirement to generate very
rapid stock removal. Even if you ask an insert supplier for recommendations you
won't necessarily get advice that is the best for hobby machinists on small
manual lathes. Here's the deal.

To achieve surface finish on light cuts and teary materials you need a small
cutting radius. Hand ground steel tools tend to end up with sharp points and
even trying to radius them usually leaves a series of tiny flats rather than a
smooth radius. This works fine on small machines. Carbide inserts designed for
steel usually have either a 0.4 or 0.8mm radius which is HUGE by comparison.
Just try grinding a rad that big on a steel tool. It takes a while. Also they
tend to have radiused edges to give strength for deep cuts. With a rigid
machine you can still get surface finish because there is almost zero chatter
taking place.

For the small lathe you need to chuck the CNC advice out of the window. You
will certainly get a better finish with a 0.4mm insert than a 0.8 but that is
not the answer.

What you need is a razor sharp cutting edge with lots of rake to offset the
effects of the large tip radius. Seek out the inserts designed for NON ferrous
turning and use them for finishing steel. I can't give a make or part number
because I have never bought an insert in my life - I just ponce them off my
friend when I visit his CNC workshop.

The non ferrous ones I use are bright silver - almost like polished stainless
steel in appearance. Forget the black or gold ones - seek the bright silver and
ye shall find.

Don't use them for roughing and never on interrupted cuts - so turn square
stock to round with an old black radius nose insert first and then put the
secret weapon in the toolholder for finishing.

If anyone who reads this goes out and buys a decent sturdy toolholder and finds
these proper non ferrous tips I defy them to not achieve perfect surface finish
on absolutely anything - stainless steel (yucky stuff !!), brass, bronze, soft
teary mild steel, plastic - anything you like. Then you can throw out all your
old brazed tip crap and HSS bits coz you won't ever need them again. (well
maybe once in a blue moon for grinding up special shapes)

When I next see my mate I'll get the make and part number and post it here. No
doubt there are many suppliers of similar stuff though.


Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm