Re: Z-Axis Knee Woes

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jbordens <jake@a...>"
<jake@a...> wrote:

> Hi folks. I'm retrofitting a G3102 Grizzly Knee Mill, a smaller

type

> mill somewhere bigger than a drill mill, but smaller than a full
> bridgeport.
>
> I just finished my Z-axis, after success with X and Y. I decided to
> motorize the knee for various reasons. I'm using Camtronics 600-in-

oz

> motors and a 2:1 ratio on the pulleys.
>
> The knee is driven by a shaft that is at 90-degrees to a jackscrew.
> The shaft is connected to the jackscrew via a set of helical spur

gears.

>
> Here are my problems:
>
> (1) the motors aren't powerful enough, apparently, to raise the

knee.

> They stall and eventually cause my geckos to fault. I'm pretty

sure

> I have the "limit" set to Max on the geckos.
>
> (2) when lowering the knee, the motor turns and every now again
> there's a 'thump' from the mill, as if the knee is not lowering
> smoothly, but in little increments.
>
>
> I guess I have two options:
>
> (a) go to a higher ratio, but 2:1 was the most practical. 3:1 is
> possible but expensive, and anything more is really not possible due
> to pulley size constraints.

Seams to me a ratio of 4:1 is more likely to succeed here.
Well you could use a double pully arangement with 2 2:1 pulleys
if 4:1 is not possible, the can still be mounted on the same axel
but with the intermediate one allowed to spin feely.

>
> (b) counterweight the knee. I think this is complicated and

probably

> not a good idea.

Well you got two problems here one is the weight hanging off the
ways which induces the binding when lowering the table.
The second is the weight itself wich is exessive and will vary
depending on raising or lowering the table.

A solution to the first problem could be mounting some bars to
the sides of the knee protruding back towards the back of the
mill past the ways.
Then mount some weights on the ends of the bars untill the
load is Balanced evenly front to back.

This fixes problem one, unfortunetly it makes problem two worse
so something will definatly need to be done about this now.

You mensioned it counterweights sorry if you dont like them but
I think they would work well.
Like a steel cable pulley system that picks up the load at the
center of gravity, probably somewhere on the bars you added and
run straight up over a pulley and down to a convienient place
behind the mill.
Now the table should not bind and require nearly the same torque
going up or down.
You may find another one of the pitfalls of powering the knee
Backlash may become a problem now.
You may be able to get by when adjusting the counterweights so
that there is aways a positive gravity on the knee.

>
> In either case I'm not sure it would solve problem #2. I'm

currently

> at a loss on how to proceed. Any suggestions are greatly

apprecaited.

>
> Thanks,
> Jake

Hopefully somebody can recommend a simpler aproch that will lead to
a acceptable solution.
Most People do not consider automation of the knee to be trouble free.
There are some compromises to be made, consider all your options
and determine what you can live with.
Let us know what you come up with and how it worked out so others
can learn from it.
Good Luck