Correct use of fixtures?

Hi All,
I never used fixtures before but have a need to do so now. From a practical point of view, what is the correct way to use them. Do you have the fixture setup to locate at the same place very time and then reference from home or do you clock the fixture every time that you use it.
Secondly, do you generate your gcode to be at an offset to the fixture location and never loose the gcode or do you clock the part location every time?
Any instruction here will be welcome

Thanks John. From you response I gather that you clock up every time that you use the fixture. You dont have holes drilled into you bed to hold the fixture or anything like that hey?

I want to make a permanent plate to fit over my bed and onto that I want to make fixture placements with dowel pins etc. So I only have top clock the base plate and then swap fixtures as I go.
Does that sound feasible to you?

By "clock up" I assume you mean to use an edge finder or something else to find XY zero?

Yes that is correct. I will clock once only after homing the machine and then just record the offsets for every fixture. I have almost finished my kit form plasma machine and the fixtures are for manufacturing some of the parts for that.

Thanks again for your kind help as always.

You can also drill and ream holes in the table of the milling machine, that way your fixtures will have always the same position from the machine home position. I think it's only worth it for parts that you have to make a lot, but with some time between them. At work we used it for large sand casting models, the shape was always different, but they had only 2 different frame sizes. We had to make a pair every couple of months, so it was worth it.

However, if you only have to make 1 series of parts, you can just as well touch off your fixtures, only if you have to make other parts between them is has a advantage.

I have a fair number of parts that will be made on a continuous basis. Therefore I thought of making a fixture plate that will fit over my mill table and it will be clocked and never removed. I will then drill and tap a range of holes for general work holding as well as the pins for regular fixtures that will fit on top of the plate. This will include my 4th axis as well. Hopefully I will only have to clock the plate once and after that I could use the offsets.

an old topic but just chiming in with how I use my fixtures. My little miill has 2 vices on it, so I work down from G59 rear left first, G58 etc. Also I have found it usefull to make G54 standard machine coordinates (IE is the same as G53, but now it is modal and doesn't need to be called on each line) and G55 is referened to the rear left corner of the stock being worked on, which can be set from G59 or G58 or with an edge finder or whatever is convenient. Below I layed out how i do it, as the above paragraph really got away from me in editing...


G54 Machine coordinates
G55 Raw Material rear left (usually the same as G59 or whichever fixture I am using)
G56 Raw material rear left in second vice
...
...
G59 is rear left of first fixture,
G58 is second fixture
etc.

I still touch off fairly often, just in case and to verify no body has tried to move on me, either the fixture themselves or the home switches. I generally just check one fixture, if it is still right, the rest probably are too, but if its important it only takes a second to check em all. The tedious part is getting em square when you put them back on. some day I'll pony up some cash for a sacraficial bed that I can fixture up properly.

mariusl wrote:

Hi All,
I never used fixtures before but have a need to do so now. From a practical point of view, what is the correct way to use them. Do you have the fixture setup to locate at the same place very time and then reference from home or do you clock the fixture every time that you use it.
Secondly, do you generate your gcode to be at an offset to the fixture location and never loose the gcode or do you clock the part location every time?
Any instruction here will be welcome

I have encoders that have index pulses, and so when I home my machine, the
position is VERY repeatable, to within one encoder count. LinuxCNC records
the offsets between machine coordinates and the work coordinate system
(such as G54, G55, etc.) So, when you touch off the first time, it can recall the
same position even when LinuxCNC has been shut down and started
again the next day. But, this requires a repeatable way of homing
the machine.

Jon

Missed a huge question in your post, let me chime in on how to setup your gcode the way I think is best.

what you are going to want to do is write your g-code relative to something on the part you are machinging that makes sense. For 90% of what I do its the top left corner, which is also the edge of the fixture I zero to, so my gcode is using the same cooridnates as my G5# that I am using at the time. the other 10% of the time I have parts that havea locating feauture, say a drilled hole in them that is how I mount them on whatever it is they are going to be used for. so my gcode is centered around that feature. now if the peiece of stock I am working with changes in some way, I still only need to zero my working coordinates around the new location of the pin hole, and mycode can be used with 0 modifications. The biggest tip I can give you is to deffinatly keep your gcode as generic as possible. learn to write subroutines (easier than you maythink!) and paly with everything linuxcnc has to offer, NGCGUI is awesome and there is a lto of work other people have put into it htat you can rip off for your own uses.