Importance of squaring workpiece and/or vice

[outsider787]

I just discovered the G68 (and G68) g-code command, (which rotates the coordinate system of the machine) and am beginning to wonder about the importance of squaring off the vice (or generally speaking) the work piece/stock to one axis of travel of the machine.

If I can adjust (rotate) the coordinate system by any amount, why not just slap the work piece on the table roughly aligned, find the rotation of the work piece (by doing an edge finding operation in two spots along one side of the stock), calculate the angle, enter the g68 command, and begin machining!
Heck this could easily be added as a screenset to Mach3 and automate a lot of the steps.

Seems pretty simple compared to having to do a lot of fiddling, and running an DI along the edge several times and adjusting/tapping the part with a mallet in between, until the DI reads a very small amunt...

What am I missing?

[691175002]

You can do whatever you want if it is accurate enough for the work you are doing.

Does your method really have any advantages though? Squaring a vice only takes a minute or two. Cheap edge-finders have garbage repeatability.

With a DTI you are measuring a line as you move across the vice jaw. If you are edge finding, you are measuring only two points on the vice jaw which has a higher potential for error. Any DTI will match or beat the accuracy of an edge finder.


There is no real reason why you shouldn't use G68 instead of squaring your workpiece, but it seems like it would save very little time and potentially cost a lot of accuracy.

[christinandavid]

You are correct that rotation can overcome this issue on simple CNC setups - but most professional machinists have worked on manual mills/lathes (no rotational offsetting available here) and even CNC operations using a 4th or 5th axis, where co-ordinate rotation in one plane can open up a right can of worms in another. You must also consider the accuracy of setting the workpiece origin in X and Y if you are coming off the edges/corners.

The only viable option in a lot of cases is to clock things up parallel to mc axes (and in the case of multiple vices, in line with each other). You would also ideally want be accurate beyond a very small amount, ie zero

The best option ultimately is a cracking good fixture that holds the part in exactly the correct attitude - but then again, making a good fixture usually requires the skill to clock things up anyway...

DP

[SCzEngrgGroup]

You'll spend the same time determining the angle as you would just putting the piece straight, so what do you gain? It takes only a minute or two to get a piece straight to within a fraction of a thou.

Regards,
Ray L.

[TXFred]

It will work so long as you remember to plug the correct rotation value into every program you run. The moment you forget, oops!

Setting the vise up correctly is more error-proof, and it allows you to use hand jogging to make last minute modifications to a completed part. Imagine that you just spent 6 hours machining a part, and then the engineer comes to you and says,"I need one more hole, drilled at this XY coordinate."

These things and worse can happen. I've had the engineer come back with a change to the design after we had finished part 95 of a 100 part production run.

If your part is skewed in the machine, you're going to be pretty miserable.

Also, if you've skewed the part, a simple X move to square off a face is now an X-Y move, which means that it is actually a series of tiny steps. And the Y axis ballscrew now becomes a factor, which increases overall backlash. The end result is that overall quality suffers.

Frederic

[txcncman]

Or...put a couple of precision keys in the T-slots of the table and bump material against them. Or dowel pins or whatever works best for your situation.

[Fixittt]

here is a simple rule of thumb that I fought for a long time.
The rule is "If you dont start square, you wont end square"
As many have said above, your adding in way to many variables to the work. What you want to do is develop a consistent workflow. With as little deviation as possible.

now say for instance, you setup the machine for a job, do all of your calculating ect to compensate for not being square. Then you go away on vacation. Now you have to setup again because you dont remember where you are at square wise.

now lets say that you square your vice in. do your job, then go on vacation. You come back in 6 months. You know exactly where you are and that the vice is square because you always square it.

Now, lets say you have someone over, and he/she needs to bust out a part. They have cnc skill and need little help from you. You point to the machine and say "Get after it" Or do you tell them to find X Y and Z points do some math add the gcode to get almost square............

[bogiestl]

Buy the best, and you only cry once.

A guy I know has two matched Kurts on his mill. Dead nuts same for height, thicknesses, etc., etc... Comes in handy.

[109jb]

Key the vise and it will always be square. I keyed mine and it is within .001" along the face of the fixed jaw every time. That is close enough for 99.99% of my machining tasks.

[khass0410]

Keys takes maybe a minute to put on a vice and our very cheap, then all you need to do is put it in the machine, if you don't have keys then indicating a vice in takes maybe 5 minutes indication two vices in takes 10 minutes not that hard to do....why not do something the correct way then having to do something over and over??? Why not save time and money?

I just discovered the G68 (and G68) g-code command, (which rotates the coordinate system of the machine) and am beginning to wonder about the importance of squaring off the vice (or generally speaking) the work piece/stock to one axis of travel of the machine.

What am I missing?

[Geof]

.....A guy I know has two matched Kurts on his mill. Dead nuts same for height, thicknesses, etc., etc... Comes in handy.

I know its a few months since this post but I had not seen it earlier.

I have two matched Kurts on my machine and the best I can do is get them lined up to within about 0.0003" of each other. Does that qualify as "Dead nuts same"?