Re: [CAD_CAM_EDM_DRO] Re: Very Confused!! Need help.

Johann,

I also suspect that you problem is in the rigidity of the spindle. Small
cutters are very unforgiving of deflections that can increase the chip
load by 200%-500% instantaneously.

If you have a dial indicator, set it up with the stylus against the
cutter and slowly rotate the chuck by hand. Check the runout with no
load on the spindle, for a 3mm cutter it should be less than 0.01mm.
Imagine if you trying to take a cut with a .05mm chip load as another
post suggested. If the spindle is running out by .1mm (the indicated
runout is twice the off center amount) the cutter would be taking 0.0mm
cut on one side and .15mm on the other, three times what you had planed.

Now with the indicator against the cutter push and pull on the chuck in
the direction of the indicator stylus and record the max/min travel.
Reset the indicator 90 degrees to the first test and repeat the
push/pull and record the numbers. Now take the difference of the max/min
min in each direction, square these two differences by multiplying each
by itself. Add the two squares together and take the square root of the
sum. This is the mechanical "slop" in the total axes/spindle system.
This slack maybe causing the "shuddering" when the cut starts. Some
material, especially with sharp cutters can pull the cutter into the
work when a cutting starts. the mechanical slop may allow the machine to
jump into the material increasing the tool load and snapping the cutter.
There are some work arounds for this. The cutter can be fed in either a
"conventional cutting" or "climb cutting direction". Conventional
cutting tends to be more tolerant of a loose machine. I have cut parts
on manual machine which have had slop that could be measured with the
medium marks on a tape measure by always approaching from the same
direction and making sure that the backlash or slop was taken out before
cutting.

Conventional cutting can be thought of as feeding the cutter into the
part in a direction opposite the rotation of the cutter. Think of the
cutter pushing away from the part rather than climbing on top or rolling
across the surface of the part. Conventional cutting should help with
mechanical looseness of the axes and spindle combined but you may still
find that you break cutters when the cutter finishes the cut if it
suddenly springs forward when the cutter goes off the part. To counter
this, consider reducing the feedrate as the cutter comes tangent to the
end of the cut.

If you have spindle runout and excessive deflection on the spindle,
these must be addressed mechanically. There is no "electronic fix" for
iron problems, only patches and compromises. One thing that may however
ease the pain if you are cutting wood, plastic or foam. Consider using
RotoZip Cutters. These come in two flavors, one with a pilot on the end
for cutting drywall and another with flutes to the end like and
conventional end mill. Choke then up as much as possible to reduce the
cutter overhang and try using these. They may still break but they are a
lot cheaper than end mills.

gary

aka nitewatchman
dba SynerTech LLC



Mike wrote:

> Johann,
>
> My guess is that your main culprit is the fact your using a tool that
> wasn't designed to
> handle side loads. There should be no shuddering when the chips are
> flying. As long as a
> gantry is built rigid enough for the loads it's expected to handle its
> design is no worse
> than any other.
>
> Calculating the cutting speed takes a piece of info you need to get
> from the bit
> manufacturer which is chip load. Aside from that the calculation is
> simple. Feed rate =
> RPM * Chip load * number of flutes. The manufacturer will also tell
> you what you
> maximum cut depth should be, but in my experience it's usually 1/2 to
> 1 x the diameter,
> but smaller bits can be less.
>
> I've estimated your feed rate should be 190 to 380 mm/min given hand
> drill speeds are in
> the 1250 to 2500 rpm range and a conservative chip load of .05 mm.
> Since you're not
> coming anywhere close to this I think you need to work out the
> shuddering problem
> before you'll be able to get satisfactory results.
>
> Regards,
> Mike Nemesh (Aerowright)
> Roswell, Georgia USA
> http://webpages.charter.net/mnemesh <http://webpages.charter.net/mnemesh>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>, "Johan Van Wyk" <wykj@...>
> wrote:
> >
> > I'm using a bosch 800 watt hand drill fixed to the gantry, it has a
> > variable speed and torque controller, so what I usually do is to adjust
> > the speed till I get the best results on the plastic, so far I've been
> > too chicken to actually try and put it through any metal type material.
> > My favorite bit to use is a 3mm straight 3 flute HSS bit. At the price
> > of those it's a unnerving experience when one of them break, and I've
> > gone through 2 already.
> >
> >
> >
> > One thing that I saw is that there is a bit of a shudder on the drill
> > when the chips actually start flying, I suppose that is one of the
> > drawbacks of using a gantry style setup, the Z-axis is "hanging" off the
> > rest of the machine, and I couldn't get it as rigid as I was hoping..
> >
> >
> >
> > One of you asked whether I calculated the cutting speed, and I have to
> > confess, I'm really no machinist so no I haven't is there some specific
> > formula for that? So far I've just been slowly in increments trying to
> > ramp up the speed, I try to keep my cuts less that 5mm deep at a time,
> > am I being too aggressive in how much I cut?
> >
> >
> >
> >
> >
> > Thanks so far anyways
> >
> >
> >
> > ________________________________
> >
> > From: CAD_CAM_EDM_DRO@yahoogroups.com
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> > [mailto:CAD_CAM_EDM_DRO@yahoogroups.com
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>] On Behalf Of lcdpublishing
> > Sent: Thursday, January 04, 2007 8:01 PM
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> > Subject: [CAD_CAM_EDM_DRO] Re: Very Confused!! Need help.
> >
> >
> >
> > Johan,
> >
> > Along with all the other questions, let us know what spindle you are
> > using? Is it a router, a dremel, etc.
> >
> > Seeing as you are breaking tools I am guessing you are using a
> > dremel motor with tiny dremel bits?
> >
> > Chris
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> > <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com> , "Johan Van Wyk" <wykj@>
> > wrote:
> > >
> > > I recently finished my first homebuilt router pouter project, and
> > was
> > > doing some trial runs for a while now, just figuring out the best
> > > software solution etc. etc. So basically I've been cutting air for
> > 2
> > > months now just to suss things out..
> > >
> > >
> > >
> > > One thing I now realized that I made a mistake on is my ever
> > increasing
> > > need for speed, so at the moment I got a good mix between speed and
> > > torque.
> > >
> > >
> > >
> > > Now I started putting some wood and plastic under that cutter to
> > see the
> > > chips fly, and lo and behold all that speed was completely
> > unnecessary,
> > > I'm not able to cut anything at speeds more than approx. 5mm per
> > minute,
> > > what am I doing wrong??? I keep breaking bits if I go faster, or
> > the
> > > cuts become very dirty and especially plastics start chipping etc.
> > So
> > > why do I keep reading on these forums about xxxx ipm if all that
> > speed
> > > is wasted on everything except G00 rapids??
> > >
> > >
> > >
> > > Maybe one of the boffins on here have some idea of what I'm talking
> > > about, and maybe able to help me understand this...
> > >
> > >
> > >
> > > Any help will much appreciated.
> > >
> > >
> > >
> > > BTW. To all the lurkers out like me out there, it really isn't that
> > > complicated to make a machine so start already.....!!!! You'll be
> > amazed
> > > at the feeling you get inside the first time that machine starts
> > moving
> > > on it's own..
> > >
> > >
> > >
> > > Thanks
> > >
> > >
> > >
> > > Johann van Wyk
> > >
> > >
> > >
> > > [Non-text portions of this message have been removed]
> > >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>

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