Re: [CAD_CAM_EDM_DRO] part holding

Hi Doug:
Is this your routine for production jobs only, or do you do one off jobs
the same way?
My personal habit has always been to program my one off parts in smaller
chunks too, but I just run the programs one after another, rather than
stringing them together with sub calls.
I'll typically run the programs straight out of Mastercam with only the
verification utility in that program to verify that my program is OK.
I run an optional stop routine after every cutter, so I can check the tool
height offsets by dropping the rapid and feedrates in the control and I stop
the feed just above the job to see if I'm in the ballpark.
If my cutter is roughly in the right X-Y location and at roughly the right
height, I assume everything is good, and have at 'er.
I'll be programming the next bits while the roughing is going on, so I can
constantly keep the machine cutting.
Typically I'll invest less than 15 minutes in the first program, just to get
the ball rolling, and I never dry run it.
Of course, my production jobs are never run this way; those get the full
meal deal, so I can take full advantage of the fixturing and the
toolchanger.
I do have some tricks to keep the unexpected at bay:
First, I always set the top of the job to be Z=0.
That way, all feed moves are Z- moves and I can set a low clearance height
with confidence.
Whenever possible, on one off work, I leave myself 0.100" of clamping stock
and squeeze the block in the vise, if I have both inside and outside
contours to cut.
The waste of 0.100" of stock is trivial compared to the cost of multiple
reclampings and the cost of crashes into clamps: I've found this to be true
even for exotics like titanium.
My individual programs are always dirt simple, typically one or two cutters,
doing one or two operations.
Holes are always done as a separate program, and my first operation is
always a facing pass at Z=0.
So far, touch wood, I've never had a crash, except when I ran a long drill
in the carousel into the side of the job because I commanded the block to
move under the carousel, and neglected to verify the height of the tool in
the carousel. (one of the biggest shortcomings of the Haas Minimill is the
low Z axis clearance between carousel and table).
I mostly build injection molds and production tooling, so most of my work is
one or two off.
Cheers

Marcus

----- Original Message -----
From: "Doug Chartier" <doug@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, September 03, 2004 12:23 AM
Subject: 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.
>
>
>
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