Re: [CAD_CAM_EDM_DRO] Re: Re:Re: CNC'ing a Mini Mill

Comments below... I don't mean to offend anyone. I maybe have a
different prospective than most. I'll share anyway.

On Wed, 2003-11-19 at 14:05, jharnedy@... wrote:
> Hi Jon, Ese and Steven
>
> Thanks for your feedback.
>
> >rpm = 20*1000 / pii * D
> >mm/min = rpm * pii * tooth feed (to be safe use .05mm to start with)
>
> >A gross value for HSS is 20 m/min, so with your 25mm 2 flute? (tooth)
> HSS the rpm would be ~250 rpm and the table feed ~25 mm/min.
>
> So for 10mm 2 flute End mill the values would be 100 mm/min feed and
> 630 rpm.
> With a depth of cut of 0.5mm, this would result in metal removal rate
> of 500mm^3 / min (0.03 in^3/min)
>
> What would be the thrust for these values? or it it over the top for a
> 250kg bench mill?

The cutting rate is not over the top for a mini mill. We have a Grizzly
G0519 and it easily cuts a 6IPM at 25 thou with a 3/4" endmill.
Something to remember is that when you CNC anything you can take
lighter, faster cuts rather than taking big, deep, slow cuts.

> What are the experiences that others have got from applying steppers
> to bench mills of this size - 250kg ( 550lbs) ?

First of all, use servos, not steppers. Stepper systems are totally
open loop. If the motor misses a step for any reason, you won't know
about it. With a servo system, the driver counts steps from the
controlling software and compares that with the motor response from the
encoder. If the motor misses a step, the control loop will catch it
up. If the motor gets too far behind the drive pulses, the drive will
error, telling your software there is a problem. This is much, much
more secure/robust than a stepper setup, especially when the speeds go
up.

Secondly, I think it is a waste of time to CNC a mill of this size and I
qualify that by saying we've done one that weighs 750 pounds.

My reasoning is that once you get the machine CNC'd, you'll do far more
with it than you ever thought. In spite of finding our minimill too
small, it runs parts several hours a day, while our bigger mill sits
idle. We are in the process of CNCing the second mill.

Once you CNC a mill, you'll increase your feedrates, depths, size of the
parts you want to work on, etc. You'll want more rigidity. You'll want
more travel, etc. Do yourself a favor and buy a bigger mill.

Minimills are quite light and prone to chatter. They aren't strong
enough. The column ties into the base via a few bolts in a small
circle. The column is quite small compared to a BP clone, even a mini
BP clone.

Another problem with mini mills is that they don't have a ram on the
turret. As soon as you get a workpiece that is very wide, the back of
it will hit the column before you can work on the front of it. With a
turret mill you can move the head in and out via the ram to reach the
front and the rear of a large piece, even if you have to reset your x,y
coordinates to do so.

Enco and Grizzly sell a mini BP clone 8x36 that weighs 1650 pounds and
costs about $2300 when on sale. BTW Enco sometimes has free freight in
January... In hindsight, this mill is the smallest one that I would
consider CNCing.

Such a mill has a lot more meat to it than does a mini mill and has a
number of important advantages:

a) a traveling knee. With a round column mill, you can't raise the head
or you'll lose the x,y position. This is a terrible detractor from
their usefulness. It is one thing to reset the X,Y coordinates once in
a while on a turret mill if you move the ram. It is a whole different
matter to do it at every tool change.

a2) a travelling knee. It is very nice to be able to CNC an axis and
keep the quill available for hand use. With a BP clone, you can CNC the
knee and still have full use of the quill for delicate operations.

b) a built in lubrication system. Your CNC mill will provide a lot of
value for someone for a long, long time if it is done right. The first
thing that will wear out are the slides. A lubrication system is a must
on any CNC machine that is going to be used a lot.

c) a coolant reservoir. You'll find you want coolant on your mill and
the reservoir/drain system on a BP clone is a definite advantage. It is
also easier to fashion splash guards on a turret mill than a mini mill.
BTW: The table isn't drilled and tapped for a coolant drain on most mini
mills.

d) Lots/more table travel.

e) Room for a big vice.

f) A tilt-able head.

g) A spindle brake for changing tools. Most gear head mills require you
to shift into low gear to hold the spindle to change tools.

h) A smoother/wider way system for the table.

i) Way more room to install a zero backlash ballscrew setup ! I speak
from experience here !


> TIA
> John
>
>
>
>
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