Dealing with metal plate that is larger than table size

[pmurdock]

I have a job at hand which is a large metal plate (3/8" thick) that needs a variety of tapped holes all at various locations throughout the plate. However the plate is larger than my CNC table (a Haas VF-2 with probing option).

I'm trying to work out in my mind how to accurately cut all these holes in the plate. I know this is going to require several setups. I guess my question is how best to approach this. Should I cut in some features deliberately for the purpose of alignment on the table and what type of features are commonly used for this? Maybe a small slot so that I can use a dial test indicator to square the plate up again when there are no edges of the plate on my table?

I would appreciate any tips and wisdom from experience!

Cheers,
Paul

[Geof]

If the piece is larger than your table how do you plan on clamping it down? Or is it just longer than the table with space for clamps on the sides.

To give yourself some reference points for re-alignment after doing one set of holes and moving the plate you could drill some pieces of flat bar to attach to the holes you have tapped. With the flat bar bolted on before you do the first move, machine a straight edge along the flat bar and interpolate a hole in it. The straight edge gives you alignment, and the hole gives you a position reference.

[little bubba]

You didn't say how much bigger the plate was, but its not all that hard to do, just pick up on features that are already there.

If you have two tapped holes in line in the same axis, ream the minor and use gage pins to square it, even if they are not in line you can do the same thing, it just takes a bit more time. You have a probe, use it, pick up on a couple of holes, rotate in your CAM and go, no squaring needed. Use a third hole at a know location to check it.

Here is one oversize one I did quite a while ago. It only overran the travel in one direction, but it was fun anyways.

.

[pmurdock]

You guys have such creative genius..

You're right that the plate is not so big that it covers the entire table - so there will still be some t-slots exposed.

thanks for all the tips.. incidentally all the holes will be tapped and fairly small (6-32) or so. so I'm thinking the probe route won't work directly on the holes unless I use the flat bar as the guide because the probe has a fairly fat ruby tip.

So my next question is how can I line up the spindle directly on top of an existing hole (assuming I've squared the work up with a dial test indicator). It must be a simple technique but I just can't see how it can be done.

cheers!
Paul

[Geof]

Originally Posted by pmurdock .....So my next question is how can I line up the spindle directly on top of an existing hole (assuming I've squared the work up with a dial test indicator). It must be a simple technique but I just can't see how it can be done. cheers! Paul

You don't try to line up over the hole; do as I suggested, bolt something on and make a hole big enough to use the probe, a known distance away from the hole you want to line up to.

[pmurdock]

ahh... duh.. of course! I see clearly now.. that's fantastic really - simple but elegant.. it seems as if work holding is just as much a science as machining. Like a little puzzle just waiting to be unlocked.


Cheers!
Paul

[Geof]

Originally Posted by pmurdock ..... it seems as if work holding is just as much a science as machining. Like a little puzzle just waiting to be unlocked. Cheers! Paul

I think more of a science...or maybe art, certainly sometimes more of a challenge.

On many of the parts we make the time spent fixturing the parts was as much, or sometimes more, than the machining time.

You have to train yourself to look for the non-obvious, and often very simple solutions; simple after you have found them that is. Human brains always seem to look for the complicated solution first.

[little bubba]

Pmurdock, I'm pretty much with Geof on this one, workholding is the science of machining. Hand somebody a nasty little part you have made, that took 9 setups, even with a 4th axis. Ask them how to make it, and of course they say cut this, cut this, cut this. Then stare at them blankly for a minute like you are retarded, cock your head like a puppy does, then calmly ask them

"how do you hold it?"

You will get back a giant blank stare.

The fixturing and the holding is where the money is, the tooling and techniques is where the money is on simple stuff, anybody can hold stock in a vise.

For your info, the picture I posted earlier of the old Wells, you can 't see the control but it is there, behind the head.

How we did that one, notice the slats bolted to the table, they were bolted down them milled, so there a was nice stop. .250 holes where drilled and reamed some distance off of the stop(.25"). The stock was prepped by drilling and reaming .251 holes at the correct distance, I think it was 26". There were also a ton of 3/8-16 holes in those slats to clamp the stock.

the part was run in 10 seperate operations, using the doweled holes that were prepped into the stock. First was the -Y side, reclamp and cut leaving one dowel hole so that indexing was easy(lined up on 2 dowels). Definitely not for production, but onesy twosy, it works, 3 on that part, but it sure did pay good.

Notice the cutout in the half wall, just for that job.

[drwc]

Amen to the workholding issue, as a hobbyist, this has been the most challenging aspect of the jobs I have attempted.

Wayne