Re: [CAD_CAM_EDM_DRO] Re: Hot rolled router frame: added note

Yes, I think you are on the right track. You could attach pad support points
to the tubing and then mill just the points flat- averting stress induced
warpage
since you are not machining the tubing itself.

I did find the previous message about grouting pads
on tubing in my archives so I will include it here with
some added comments:
-------------------------------------------------------------------

To achieve a precision mounting surface for linear rail ways on
rectangular structural steel tubing I use epoxy grouted pads
on one face.

As an example I will describe the method used on the y axis of
my latest machine. It is welded up 4"x6"x1/4" hot rolled steel.
the rails mount on the 4" face.

First the bolt holes for the rail are marked, drilled, and tapped. For
thinner wall tubing a nut might be used rather than tapping.

Then I cut out many 4"x6"x 3/8" pads out of stress relieved 1018 hot rolled
steel. These were drilled and surface ground on both sides. Pre-ground
low carbon stock would work as well.

On the tubing I epoxied 3 small flat head copper rivets upside down where
each pad would go. They were about 1/8" high and formed an equilateral
kinematic support. In other words the pad never rocks when it is placed
on these little copper nibs.

Now I get out my $80 master precision level. Each bubble division is
0.0001" per foot slope. These things are great. Very high quality.

I start filing the copper nibs down with a fine file then place the pad
on and check level both longitudinally and transversely. A few light file
strokes
takes off a fraction of a thousandth. It takes a little while to get the
trig
right but by selectively measuring and filing the individual nibs the pad
ends up dead level lengthwise and crosswise. The nibs can just have a
rounded
contour at the contact points- it's not critical because it is kinematic.

Now the next pad is laid on its supports. This has to be level AND the same
height. To monitor the height I used an indicator with a mag base on the
first pad. This and the level are used to get this pad dead level and
exactly
the same height as the first. Errors can accumulate so I tried to get it
right on
to the tenth.

The exact sequence of filing the nibs to get level and height might seem
mysterious, but it ends up being pretty easy. The first ones I often mess
up and have to glue on another copper nib. After doing a few it is almost
automatic- I could get a pad to come in in less than 5 minutes. I suppose
I could write a math program to give a filing sequence, but I just didn't
need it.

After doing all the pads we have an accurate flat surface. We can take off
a pad and lay it back down without affecting accuracy due to the 3 point
support.

The last thing to do is to apply epoxy putty to the pad area. The bolt hole
must be taped up or plugged so it will stay clean. Then the pad is applied
and lightly pressed down, perhaps squeezing out a little of the epoxy around
the sides. The pad will stop when it hits the nibs. Line up the holes and
lightly
clamp. Too much clamp force will actually warp the pad.

After curing remove any plugs and clean up squeezeout and it's done!

>>>The pads and grout are firmly held in compression by the rail bolts so

the adhesive bond doesn't have to be that strong.

>>>The structural tubing will not have warped from machining stresses

since almost no machining was done to it.

>>>The pads serve to transfer the load to the corners of the tubing where

it is very stiff.

>>>With the right epoxy grout compound you get very effective constrained

layer damping of the structure!
----------------------------------------------------------------------------
----------------------------------------

Ok, so that is a brief description. Adding some pictures with this on my
site
would be better, and I will try to do this when time permits.

There are many variations of this...

The nibs can be just machined on a big mill.

In the eighties I used three little jack screws (made from setscrews)
instead
of nibs... just turn the screws to level the pad. Lots of holes to tap.

A similar method can be used to create parallel sets of ways.

One final note... I always pin the master rail with 3 dowels.

Les

Leslie M. Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
Engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus CNC for sale:
http://www.alltel.net/~leswatts/forsale.html

----- Original Message -----
From: <vrsculptor@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, February 28, 2003 11:00 PM
Subject: [CAD_CAM_EDM_DRO] Re: Hot rolled router frame: added note


> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Leslie M. Watts"
> <leswatts@a...> wrote:
> For the grouted
> > pads on structural tubing my write-up is in the message archives
> of this
> > group..
> >
> > This method is increasingly popular with machine
> > builders as no machining (or machining stresses)
> > are applied to the structural member. One can
> > routinely achieve split thousandth straightness
> > over many feet .
>
> Les,
> By grouting I assume you mean attaching an add-on strip to the frame
> with a gap filling substance such as filled epoxy or maybe
> polyester. This never occuured to me and does offer some significant
> benefits. The add-on piece doesn't even have to be steel. If you
> bolted on aluminum strips on the top and bottom with countersunk
> screws (with plastic grout) set between the final rail attachment
> points you could probably flatten and square the rail using a mill
> cutting one side then the other. When you have cut the entire X
> travel slide the cut section on parallels spaced closer than the
> ends of the table to cut more of the end. Keep flipping, sliding and
> cutting until the whole surface on both sides is cut.
>
> As you are not really doing much to the original tube except
> drilling holes you don't cause stress and everything should stay in
> line. Neat, I like it. Am I on the right track?
>
> Roger
>
>
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