fixture offsets vs machine home

[ragoodale]

I would like to get opinions from people in the CNC industry regarding program creation using fixture offsets versus program creation from machine home.

Currently have a couple dozen VMC's with thousands of programs for existing product. All programmed with fixture offsets.

It has been suggested that all programs be written from Machine Home. This would be supported by subplating all tables and modifying vises to pin to subplates. I believe the thought is to eliminate/reduce need for registering part location.

It only takes a few moments to touch off part/fixture. I cannot see any real advantage in creating program with origin at Machine Home.

Welcome any input regarding this configuration.

[Geof]

I cannot see any advantage either, anyway it would be redundant to rewrite the programs in machine coordinates.

When you have fixtures and vises located at repeatable known positions on the table in machine coordinates you can define fixture or vise offsets or part location offsets by using a G10 command to enter work offsets or G52 commands to create local work offsets for the parts.

[broby]

I do not see any real advantage for referencing to machine home.
If the fixture is relocated, then all references are invalid!
The majority of our programs are referenced using a probe to locate the job, first pass is done by the operator to setup the "rough" location of the job, then the probe steps in and locates the zero point to the exact location of the part.
Obviously this is referenced against machine home position, but since we use Okuma's, this is different to the Fanuc world of requiring "Referencing" all the time. (the Okuma machines are smart enough to know where they are all the time! )

[ragoodale]

The intent behind this is to eliminate the need to touch-off (register) locating surfaces. Really was hoping for more than two responses. Anyone work in this type of environment? Advantages/disadvantages?

[beege]

The idea of using machine home to program has been replaced with using fixture offsets. If you use a sub-plate with reference pins and an origin hole, then you can locate that hole, and know where everything else will be, and plug in your work coordinates without touching off, as long as you're SURE that the same vise, same indexer or fixture is going to be used, and that it locates as well as you need it. Everything would have to be documented, so that no oopses would happen.

Even then, depending on the fussiness of the part, you must indicate fixtures, to be sure it locates as closely as you need it to.

A program nowadays reflects the part being made, so that when you see a dimension on a print, you can see that number in the program. If you go from machine zero, those numbers no longer are recognizable, which in my book is just plain nuts.

[Geof]

The intent behind this is to eliminate the need to touch-off (register) locating surfaces.....

You have to register all the locations on the first set up after starting to use the dowelled fixtures. These are the coordinates that are then included in the programs in the G10 commands which reads the values into the machine offset registers. After the first set up when all the positions are defined, and repeatable you don't need to register anything again, except, possibly, for tool lengths when a tool is replaced.

Having the fixture locations defined and in the program does work and saves a lot of time. My production shop has been using it for years. For many parts our changeover simply involves pulling the fixture of a shelf, loading the parts and calling up the program. We leave the toolchanger full of tools and the programs are written to call the ones they need from whatever location they are at. With a carousel changer this means the carousel shuttles back and forth and adds a second or two to toolchanges, The tool change time on the side mount machins is unchanged because the machine moves the next tool into place ahead of time.

[judleroy]

Two thirds of my mill table is covered with a 3" thick steel plate that has been face milled and then drilled and tapped with a grid pattern that is referenced off of machine home. The other third of the table is where I mount my vice. I use machine offsets to reference parts in the vice. That just allows me to basicly use either coordinate system as I see fit for the job. That also means I can't vote in your pole since I use both systems on a daily basis when machining. My thought is always be flexable in your programming methods. What works for one part doesn't always work for another. Judleroy

[HuFlungDung]

I would think it would be a practical impossibility to literally use the machine home as a reference because of the difficulty of fine tuning the master fixture (ie., dragging it around and squaring it up and clamping it down all the time). Plus, if you need two sets of fixtures for the same project, then you'd have to make perfect duplicates.....which may or may not be difficult to accomplish.

When dealing with the basic machine coordinate system, if you don't like where it is, you can move it with a G92 offset. That is the basic hand-me-down principle of having G92 still available on a modern cnc. Then you can keep all your fixture offsets as one unchanging set of values. That is the theory anyways. While you can do your level best to make perfectly repeatable fixtures based on the machine zero, I would still expect a small tweak would be required for whatever reason, and G92 would do it.

Of course, one wants to avoid the situation where the G92 command gets executed when the machine is out of position. However, that would normally only occur at the beginning of the main program. It is simple on a modern machine to execute a return to machine home on the line immediately before the G92 line, so then your G92 should never get accidentally executed at some random position.

[snkdriver]

If your fixturing/part location is capable of using machine home locations, then the setups can just as easily use fixture offsets. A fixture offset is just a distance from machine home and if necessary is easily adjusted. If for some reason machine home location changes, you will have to reestablish the exact same machine home using grid shift or mechanically adjusting something. If there is concerns about fixture offsets being changed by someone, incorporate these numbers into your program files (G!0). I can't see any reason to use machine home. If someone can enlighten me, I welcome the comment.

[tommyo]

Hello,

Is it possible to have 20 different machines with the exact
same machine zero? It creates a nightmare if a leadscrew
motor, encoder or microswitch is replaced. Ever had a coupling slip?

The only case I have ever seen this work is when all the tooling
reference points were drilled oversized on each machine from
home reference. Bushings on 1/8 dowel pins were lowered
into the holes using every individual spindle head. Heated
lead was poored around the bushings and allowed to cool.

The result was multiple heads in every machine had the same
reference location relative to the existing tooling method. If
the table top, motors or even machine was replaced the reference
could be recreated by simply re-heating the lead.

This exact tooling method was used to eliminate registration
problems making multilayer circuit boards. The panels needed to
be taken on and off numerous times in the process. Repeatability
was difficult unless the same head continued each step.

I hope this old field service story helps.

Thank,

Tom O'Connor

[ragoodale]

In response to comments inquiring about the ability to have all machines with the same zero reference point (relative to the sub-plate), the implementation of programs from machine home will be on machines which have had a precision sub-plate mounted and timed (squared and located) to machine zero.