Re: [CAD_CAM_EDM_DRO] part holding

At 10:50 PM 9/2/2004, you wrote:

>The hardest part about cnc is not the electronics, the servos, the
>ballscrews. The hardest part is holding down the friggen part! I
>just ruined yet another brand new 1/2" made-in-usa endmill when my
>program had a rapid that went .010 too close to a holddown clamp at
>120IPM.

Unfortunately, I know very well what you are talking about.

Not sure how other controllers work, but here's what I do on a part with
lots of different cuts.

I do it in sections. Here's an example. Making a throttle body. This
requires cutting the center section out completely at some point. I drill
a couple of half inch holes roughly along the center line. Put a piece of
half inch aluminum with corresponding holes under the blank. Bolt the
entire thing down to the table using half inch allen head bolts with no
washers. Those have the smallest head diameter. The ends of the part have
lowered sections that protrude outward along the center axis.

Two or three passes on each end lowers the ends to finished size. I then
lock the ends to the table with regular hold down clamps. This locks both
inside and outside to the table so that removal of the center section will
not allow the chunk of metal in the middle to bang around when it is
finally cut clear of the outside section. The plate under the part keeps
everything level and steady and allows the cutter to go all the way through
the part and still not hit the table.

Now here's what I mean by sections. I program just those two end cuts in a
single program called "xxxx1.cnc" where the "xxxx" is the particular part
name I have assigned to the entire process. Put a M99 code in just before
the end process. When I know that process works, I open the "xxxx.cnc"
program which is the parent program. Do an M98 call to the xxx1.cnc
program and make the cut. Also include multiple lines of comments in each
program. Comments in the 1 program are specific. ".75 cutter with no
cutter comp", "Small end cut", "Large end cut", etc. Comments in the
parent program will be less specific. "End cuts" etc.

The next process might be the outside oval of the top where the air intake
fits. I'll program that into "xxxx2.cnc" until it is right. Make more
comments. When it works properly, I'll call that sub routine from
"xxxx.cnc". In the mean time I will have stopped "xxxx1.cnc" from being
called by putting a ";" in front of the M98 call for xxx1.

By doing it in sections I can make sure each cutting routine works. That
includes not hitting clamps etc. I generally do a dry run through each sub
process with no cutter mounted. Run the feed up to 20 or 30 ipm,
disconnect the spindle and do a run through. I can stop the dry run or
slow it to a crawl to check clearances and positions. A dry run through
single sections is very fast. Also the graphics on the computer screen
only show the sub routine so it's easy to see it without having to try
to pick it out of all the other cuts.

As each sub routine is finished it is added to the parent program as a sub
call. A ";" in front of the line that calls the sub will prevent the sub
from being called. This allows me to combine subs in whatever combination
I want and look at the results graphically.

Any sub may contain any number of subs of its own, and those subs may
contain subs of their own. If the part is really complicated, I simply
start with the least sub, get it running right and add it to its parent sub
or the parent of the entire part. Sub subs are named the same way. Parent
= xxx.cnc. Subs that report directly to the parent are "xxx1.cnc",
xxx2.cnc" etc. Subs that report to parent subs are named, "xxx11.cnc",
"xxx23.cnc", "xxx231.cnc" etc.

I realize this does not address your question specifically, but it does in
a round about way. If you do your parts in small sections it's a lot
easier to see that each set of cuts performs properly before it's added to
the parent program. It also makes alteration of the part much easier in
the future. Instead of plowing through hundreds of lines of code in the
parent program, simply open the sub having to do with the process you need
to alter. Alter it and don't even mess with the parent. The parent
program is very short too as all the cuts are simply sub routine calls. If
the parent has any lines of its own, it is generally only rapids between
sub locations. Tools can be in the subs or in the parent. If the subs
require a different tool, then it's better to put the tool change in the
sub. If many of your sequential subs use the same tool, then put the tool
in the parent and save the lines in the subs.

Hope this is helpful.

Doug C.
Houston, Tex.