Re: A "simple" Hexapod project - Joints

Do keep us informed!

Have you decided what working envelope you ar going to aim for and
what sort of machining you want to do?

Graham



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "stephank97" <stephank@c...>
wrote:

> I have done the inverse kinematics for a triaglide system (general
> closed form) as it is fairly simple and just a 4th order polynomial
> to solve.
> A couple of comments:
> These machines are fairly large i.t.o space occupied for a similiar
> xyz machine. So you are trading floor/table space for building cost
>
> 2 things play a dominant role, working volume and joint range.
> As the working volume is irrelevant in the x direction (you simply
> add more rail length), the problem is deciding y and z ranges and
> these are interrelated. If you want a wide y with a shallow z, to
> minimise the length of rail, you need a tall machine. For a deep
> working volume you need to increase the x direction.
> More important, if you choose to use ball joints, the limit of the
> sideways range will decide your minimum leg length. Plus you would
> like some clearance above the spindle holder.
> Another problem creeps into the system. If you want to minimise

rail

> movement, you have to operate with the legs as near to vertical as
> possible, but this creates the problem that your sideways stiffness
> is at a minimum. Conversely if you operate the machine with maximum
> sideways stiffness, your vertical stiffness is at a minimum.If you
> operate the machine with both vertical and horizontal stiffness,
> your machine size is at a maximum.
>
> another thing to look out for. Since this is a non linear movement
> machine, there are points where a small movement in the rail can
> cause a large movement in the tool position, So the resolution is
> not equal over the entire working volume. Steppers would not be the
> best of choices.
>
> Despite these difficulties I am working on mine and will keep you
> informed
>
> Stephan
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "alex" <telecomt@t...>

wrote:

> > Graham,
> > I would go for a ball joint.
> > If you place compression springs between the plate and the base,
> you can
> > adjust
> > stiffness of it.
> > Alex
> > ----- Original Message -----
> > From: Graham Stabler <eexgs@n...>
> > To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> > Sent: Tuesday, June 17, 2003 3:06 PM
> > Subject: [CAD_CAM_EDM_DRO] Re: A "simple" Hexapod project - Joints
> >
> >
> > > Although I am undecided as to whether or not to build a 3 axis
> or 5
> > > axis machine I have decided on a vertical slide type set up.
> This
> > > means no actuators as such to design, just bog standard rails

and

> > > leadscrews. See files section "hexapod ideas" if you don't

know

> the
> > > configuration.
> > >
> > > This means the mechanics consist of a stiff frame, 6 (for a full
> > > hexapod) or 3 (for a triaglide) linear slides, 3 or 6
> leadscrews, 6
> > > fixed struts a table/router mount. And 12 joints.
> > >
> > > Carl has used gimbles and these have the advantage that they are
> > > simple to make and decent bearings can mean they have little
> slop. I
> > > am just wondering what other possibilities there might be,
> looking at
> > > large machines doesn't seem to help much as the joints are often
> > > covered or are mysterious ball joints that don't seem to be
> holding
> > > enough ball.
> > >
> > > I have just seem some ball joints on the web where the ball is
> held
> > > by a magnet, the largest was abotu 25mm in diameter and would
> hold
> > > 25kg.
> > >
> > > What about universal joints if also allowed to rotate?
> > >
> > > Another thing I have noticed is that sometimes joints are
> combined.
> > > Struts that join at the moving table form a triangle with two
> pounts
> > > fixed so they can be jointed together and then both put in a

ball

> > > joint of some sort.
> > >
> > > Ideas appreciated, especially good ones.
> > >
> > > Cheers,
> > >
> > > Graham
> > >
> > >
> > >
> > >
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