what should be the fixed frame? if we select our base link of our robot then we get a werid output

edit

we wrote the URDF and lauch file to visualize the robot in rviz but if we select our base link(or any link) we get weird output as below:

before selecting fixed frame our model is fine as shown below:

our URDF is:

<?xml version="1.0"?>
<robot name="cham_bot">

 <link name="z_axis">
    <visual>
        <geometry>
            <box size="0.2 0.2 6.5"/>

        </geometry>
        <origin rpy="0 0 0" xyz="0.25 0 7"/>
        <material name="green">
            <color rgba="1 0 0 1"/>

        </material>
    </visual>
    <collision>
        <geometry>
                <box size="0.2 0.2 6.5"/>
        </geometry>
            <origin rpy="0 0 0" xyz="0.25 0 7"/> 
    </collision> 
 </link>

 <link name="cross_slide">
    <visual>
        <geometry>
            <box size="0.05 1.7 1.5"/>
        </geometry>
        <origin rpy="0 0 0" xyz="0.125 0 8"/>
        <material name="green">
            <color rgba="1 0 0 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
                <box size="0.05 1.7 1.5"/>
        </geometry>
            <origin rpy="0 0 0" xyz="0.125 0 8"/>
    </collision> 
 </link>

 <joint name="z_axis_to_cross_slide" type="prismatic">
    <parent link="z_axis"/>
    <child link="cross_slide"/>
    <limit effort="1000.0" lower="-0.38" upper="0" velocity="0.5"/> <!--specify exact values -->
    <origin xyz="-0.127 8 3.9"/>
    <axis xyz="0 0 1"/>
 </joint>

 <link name="beam">
    <visual>
        <geometry>
            <box size="0.2 8.5 0.6"/>
        </geometry>
        <origin rpy="0 0 0" xyz="0 0 8"/>
        <material name="yellow">
            <color rgba="1 1 0 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
                <box size="0.2 8.5 0.6"/>
        </geometry>
            <origin rpy="0 0 0" xyz="0 0 8"/>
    </collision> 
 </link>

 <joint name="cross_slide_to_beam" type="prismatic">
    <parent link="cross_slide"/>
    <child link="beam"/>
    <limit effort="1000.0" lower="-0.38" upper="0" velocity="0.5"/> <!--specify exact values -->
    <origin xyz="-0.125 8 3.9"/>
    <axis xyz="0 0 1"/>
 </joint>

 <link name="beam_bracket_1">
    <visual>
        <geometry>
            <box size="0.25 0.5 0.7"/>
        </geometry>
        <origin rpy="0 0 0" xyz="0 3.9 8"/>
        <material name="yellow">
            <color rgba="1 1 0 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
                <box size="0.25 0.5 0.7"/>
        </geometry>
            <origin rpy="0 0 0" xyz="0 3.9 8"/>
    </collision> 
 </link>

 <link name="beam_bracket_2">
    <visual>
        <geometry>
            <box size="0.25 0.5 0.7"/>
        </geometry>
        <origin rpy="0 0 0" xyz="0 -3.9 8"/>
        <material name="yellow">
            <color rgba="1 1 0 1"/>
        </material>
    </visual>
    <collision>
        <geometry>
                <box size="0.25 0.5 0.7"/>
        </geometry>
            <origin rpy="0 0 0" xyz="0 -3.9 8"/>
    </collision> 
 </link>

 <joint name="beam_to_beam_bracket_1" type="fixed">
    <parent link="beam"/>
    <child link="beam_bracket_1"/>
    <origin xyz="0.10 8 -3.9"/>
    <axis xyz="0 0 0"/>
 </joint>

 <joint name="beam_to_beam_bracket_2" type="fixed">
    <parent link="beam"/>
    <child link="beam_bracket_2"/>
    <origin xyz="0.10 8 3.9"/>
    <axis xyz="0 0 0"/>
 </joint>

 <link name="beam_bracket_plate_1">
    <visual>
        <geometry>
            <box size="3.75 0.7 0.05"/>
        </geometry>
        <origin rpy="0 0 0" xyz="0 3.9 7.625"/>
        <material name="yellow">
            <color rgba="1 1 0 1"/>
        </material>
    </visual ...