Re: [CAD_CAM_EDM_DRO] Re: Cast resin for a cnc mill design - good idea?

Hi Tom:
It sounds like you are intending to have a long table with a saddle
underneath.(like most milling machines)
A reasonable rule of thumb is that the saddle be at least 1.5 times as long
as the guideways are wide, and that the saddle be 1/2 the length of the
table.
That means you could reasonably proportion your machine to have a 36" long
table with 9" of overhang at each end and an 18" long saddle for a total
stroke of 18".
If you cut it down to the bare minimum and go with a saddle 1/3 the length
of the table, you would still have a 27" long table and a 9" long saddle to
get your 18" of stroke.
I woudn't dream of going this skinny unless I was running on linear
guideways in order to get zero play in the ways.
If you extend the stroke of the machine so that the end of the table goes
past the edge of the saddle you will create all kinds of problems that you
will have to solve.
A Bridgeport mill does this by making the saddle deep enough to house the
guts that make the table move, and accepting that the ways will be
unprotected at the extreme ends of the stroke.
The machine is also noticeably less rigid and very inaccurate compared to
its performance when the table is reasonably centered.
You can't even do this if you have chosen linear guideways, or the balls
will fall out at the ends of the stroke.

To start your machine design:
I encourage you to get yourself some pieces of cardboard and mock up what
you are intending at full size.
Then go to the lumberyard and get some cheap timbers that you can use to
make a rough 3D mockup.
Next, go to a steel supplier, and try to pick up one end of a billet roughly
the size of your table.
This exercise will help you get the whole thing into some kind of mental
proportion.
Cheers

Marcus
-----Original Message-----
From: Tom Murray <thomasm923@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com <CAD_CAM_EDM_DRO@yahoogroups.com>
Date: Monday, April 23, 2001 9:07 PM
Subject: [CAD_CAM_EDM_DRO] Re: Cast resin for a cnc mill design - good idea?


At the risk of flogging a dead horse, I'd like to make one more post
before I go off and ponder all that has been said in this thread.
First, I'd like to thank everyone that contributed, there has been a
lot of good advice given.

Marcus wrote:

>Hi Tom:
> I think you have a good basic approach for this project, but I am
>concerned that you have underestimated the size of a milling machine

with

>the work envelope you have described.
>A capacity of 18x12x8 implies a pretty large machine; all up weight

is

>probably going to run several thousand pounds if you want to get

decent

>rigidity.

Wow! Talk about biting off more than I can chew!

I've had the new Taig mill in mind when I came up with those
dimensions. One of their mills has an envelope of 18"x 9?" x 7?". The
mill probably weighs in at under 100 lbs. Of course, I would like to
end up with a mill that is tighter than the Taig, but I
have no use for a Bridgeport sized mill…

I am in the jewelry industry and would use a mill to make models,
molds, rapid prototypes and various one-off items. I would be
machining non-ferrous metals, for the most part. So obviously I don't
need something that I can mill transmission housings on. Why have the
extra length, then? Well, this probably won't be the only machine
that I will ever build. Having some extra travel would help out in
the future. Of course, if I am being unrealistic, I can always scale
everything down. It's time to sketch this out in Rhino.

Marcus continued:

>For an 18" stroke machine you need to have a minimum of 36" of

guiding

>surface if you want to keep the table fully supported

Is it alright to have some overhang on the table? Let's say about 5
to 6 inches on both sides (remember - no transmission housings)?

Thanks and a tip of the fez to all,
Tom Murray