Emco Compact 5 PC X touch-off

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17 Jun 2014 05:03 #48016 by krlynn
I’m a beginner at CNC and machining in general. I have an Emco Compact 5 CNC PC that I acquired with a purpose to turn small handles. It’s running linuxcnc, configured with files I downloaded from this site.

After reading everything I could find on the subject, I spent several hours yesterday struggling to figure out how to properly set X0 in the G54 coordinate system. Getting the Z axis dialed in wasn’t a problem.

I’m testing with a program written for me by an expert (and member here) who used it to demo the lathe before I bought it. So I know the program can work to turn a piece of 1.5” diameter oak dowel into the intended shape. I came close one time, although the curves turned out to be much less pronounced than they’re supposed to be. I suspect that’s because the offset still wasn’t quite right and I don’t have tool-path compensation sorted out yet (at all). But I’ll get to that in due course.

The fact I came somewhat close to having X0 properly set on one pass and then couldn’t repeat that result (although I know I followed the same steps at least once again) makes me wonder if I failed to clear some settings at points when I need to. I closed and re-opened linuxcnc a few times and repeatedly homed to G53 X0 Z0, but that didn’t appear to make any difference.

My approach to testing was as follows. There is a block of code early in the referenced program that instructs the cutting tool to travel to X.75. After homing to G53 X0 Z0, I mounted a work-piece in the chuck identical to the one used to demo the lathe when I bought it. Then I touched off for both X and Z. After that I re-homed, removed the work-piece, and then executed the program line by line until I got to the X .75 instruction. Upon completion of that movement I could easily measure where I was in relation to the X axis.

My understanding is that linuxcnc runs in Radius Mode as default, so it was a little confusing to me when the dialog box that opens during touch-off requests a value for diameter. In any case, I tried entering both radius and diameter, and neither yielded the intended result.

I wish I had taken detailed notes from the start, but I didn’t. At one point, however, I noted that after I’d set radius to .75 (and so diameter to 1.5) in a touch-off, the X .75 instruction moved the tool to a point where DRO showed radius as .0370 and diameter as .75.

So I doubled the value I entered at touch-off. That didn’t work either. I tried entering .75, 1.50, -.75, -1.50, 3, and 0. None got me close to true .75 or served to replicate the single promising run through the program, although I know I had to have repeated the setting I used in it at least once. Again, that makes me wonder whether I needed to clear out values somewhere along the way.

I’m probably missing something obvious, but I’m out of ideas to try. Any help would be much appreciated.

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17 Jun 2014 09:34 #48020 by krlynn
I think I got it sorted, at least well enough to turn my first well-shaped handle.

I had read somewhere in the LinuxCNC Wiki that Radius Mode is the default. Somewhere else I'm sure I read that in Touch-off a diameter value should be entered unless in Radius Mode.

I finally figured out I must be in Diameter Mode tonight when I cleared everything and re-homed, set G54 to the same as G53, and then instructed a -.75 movement that measured an absolute .375 and read-out as .375 radius and .75 diameter.

So I entered G8. Reset everything. An it worked.

I also figured out what I'd done wrong in the tool settings, which fixed the issue I had yesterday with curves being smoothed out.

Now I need to get to the bottom of why my default appears to be Diameter Mode and how to change that.

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17 Jun 2014 13:53 #48026 by ArcEye
Hi

You can work in either mode, but I always use diameter mode since you turn work to a particular diameter, not radius.

There have been reams written about touch off on a lathe, I always set X roughly, then take a light cut with the tool I will be using, measure the diameter carefully and set that figure.

There will be some who argue once X0 is set you don't need to reset it, but the above method caters for tool wear, machine inaccuracies etc. I am not into production turning, just one offs I want to be the correct dimensions

regards
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17 Jun 2014 18:46 #48030 by BigJohnT
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17 Jun 2014 19:02 #48032 by piasdom
what you have as your initiation string ? if you have G53 or G54 in there , it will be in that mode till
you change it. if you keep changing G5? you need to make sure of what it was LAST set to.
hope this makes sense ? Good Luck

I think I got it sorted, at least well enough to turn my first well-shaped handle.

I had read somewhere in the LinuxCNC Wiki that Radius Mode is the default. Somewhere else I'm sure I read that in Touch-off a diameter value should be entered unless in Radius Mode.

I finally figured out I must be in Diameter Mode tonight when I cleared everything and re-homed, set G54 to the same as G53, and then instructed a -.75 movement that measured an absolute .375 and read-out as .375 radius and .75 diameter.

So I entered G8. Reset everything. An it worked.

I also figured out what I'd done wrong in the tool settings, which fixed the issue I had yesterday with curves being smoothed out.

Now I need to get to the bottom of why my default appears to be Diameter Mode and how to change that.

The following user(s) said Thank You: krlynn

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21 Jun 2014 20:31 - 21 Jun 2014 20:32 #48165 by tenaja

... I always set X roughly, then take a light cut with the tool I will be using, measure the diameter carefully and set that figure....

This is a good method. To add to it, I think the phrase "light cut" should be defined for newbies.

The light cut should remove the same material as your finish pass. Also, light cuts on a lathe should always (i.e. if at all possible) be no thinner than the radius of the cutting tool. For example, if you are using a cutting tool with a radius of .016", then you should try to remove a minimum of .016" of material, which is .032" in diameter on your part. Sure, you "can" make a .002" cut with a tool having .016" tool radius, but the finish will be nowhere near as good as a deeper cut.

The reason is easier to understand if you picture the tool cutting into the material like a snow shovel digging into snow. If your shovel edge has a 1" radius, then it will want to lift up and skim on top of a thin 1/4" layer of snow. But if the snow is 2" deep, the shovel has no reason to try to skim. It is all about the pressures; when the tool radius is large compared to the cut depth, the forces pushing the tool away from the centerline are greater than those pushing away from the spindle chuck. If there is any play in your x axis (and there always is) then you will get a poorer finish with an excessively light cut.

As a rule of thumb, you can "push the limits" to half the radius, but the best finishes will be material removal depth at the radius size, or deeper.

This is why (especially for small machines) it often works better to use a tool with a smaller radius and interpolate any radius you might need.
Last edit: 21 Jun 2014 20:32 by tenaja. Reason: fixed typo
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24 Jun 2014 02:14 #48206 by BigJohnT

what you have as your initiation string ? if you have G53 or G54 in there , it will be in that mode till
you change it. if you keep changing G5? you need to make sure of what it was LAST set to.


Just to clarify G53 is only effective on the line it is on.

www.linuxcnc.org/docs/html/gcode/gcode.html#sec:G53-Move-in

JT

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