TLO Questions

[idahoan]

I’m having trouble wrapping my head around all the TLO scenarios; I have a converted knee mill with only slightly less that 5” of Z travel. I have been making parts since getting my quill drive up and running a while back, and have been reasonably successful with setting my tools; but every once and a while I hit a road block.
I only have the basic version of MSM which I love; I set my tools at the top of my part using a gage block (.100”). I have read and re-read the manual and still can’t seem to come to grips with all the equations; sometime I end up with TLO values that are larger than my machine has travel?

Today I ran a job, set 4 tools; all the TLO numbers were positive (not sure why) I used one of the tools to set my G54 Z0. The first four tools ran fine ,they were all drills and a reamer. Then I need to do another operation with an end mill. So I loaded up the end mill and set it to the top of the same stock as all the rest of the tools and came up with a TLO of -6.1369? All the other tools were in the +1.2 to 1.8 range and the end mill in its holder was about the same length. I tried a couple time and always got the same TLO each time. I re-started Mach 3 and went back to the same spot and now my TLO for that tool was +.3114? This happened to me twice today and has happened before; each time after re-starting Mach the number were more in line with the other tools; any ideas?
Do I have to set all my tools every time for each job? If I have tools in holders that I have set before will setting the Z fixture offset with a tool that has a valid TOL adjust all the other tools in the table?

I would like to get a handle on this so I can have some more confidence in my machine; I’m not new to CNC or MACH 3 but I am relatively new to having the Z axis running on this mill and MSM. It would be nice to set a tool and by looking at the TLO be able to say yes that looks reasonable; but when the numbers are larger that the machine has travel somethings wrong.

Thanks for any help.
Dave

[DaveCVI]

Hi,
I'm at a bit of a loss as to how to help you.
There are multiple techniques in use in the industry for handling Tool Length Offsets (TLOs).
When I wrote the MSM manual, I covered multiple techniques (as MSM supports several common approaches). This makes it a tad confusing as one needs to know which technique they want to use in order to pay attention to only those parts of the multiple techniques covered in the manual.

What stumps is is that when I wrote the manual, I did my best effort to explain the details. If the manual does not help you, I don't know what else **I** can add that will help (as my personal best effort is what's in the manual). I understand that this sometimes happens... different people gain information different ways.
For some, my style of explanation is fine, for others it is next to useless. When this happens, I usually suggest trying a different book (as sometimes a different writing style will help).
One text that many people have found useful is Peter Smid's "CNC programming Handbook".

Dave

[idahoan]

Hi Dave
Thanks for taking time to read my post and responding. I was pretty frustrated when I wrote it and I’m not always very good at putting my thoughts down on paper. You manual is very well written and any problem gleaning the information from it I’m sure lies with me.

I did attempt to tell you what method to set tools I have using and also asked some specific questions relative to what I have been seeing; this method is to set the tools at the top of the part or stock and then using one of the tools with a valid TLO to set the G54 Z0. I have been using this method mostly with success. I have concentrated my reading to this section of the manual but as you indicated everyone learns differently and in my case I probably do better being shown how to something opposed to reading it.

I seem to recall that you indicated in the manual that using the above method would return TLO values that are all negative numbers; is this correct? I ask this because that is not always the case with my machine. My mill homes at the top of the Z travel and it only moves down in –Z. I would like to understand what I’m doing wrong if anything to get positive values.


Thanks,
Dave

[idahoan]

Hi Dave

Can you please help me understand the text in red; this may be the root of my troubles. I have not been setting WC Z0 before touching off my part Z0; I was under the understanding that they were one in the same; and that touching off the part Z0 also was setting the WC Z0? If not do you have any guidelines on how and where I should be setting my WC Z0?

<The touch off technique provides a TLO relative to the Part surface for each tool. However it does
<not yield Part_WCz0 (as we don’t know the PTL). This technique uses Part_MCz0 as a common
<relative reference plane without knowing the plane’s Z coordinate value.

<The touch off technique requires that the WC Z0 has been set before the tools are touched off to
<Part Z0.

The way I like to run my machine at work is to set my tools at the bottom of the part. ie. fixture plate, mill vise, parallels, etc. then set my G54 fixture offset to my part and/or stock thickness. I'm not sure MSM will allow me to work this way. This way you are never setting tools on rough or raw stock.

Thanks again for your help,
Dave

[DaveCVI]

hi,
I'm traveling this week so my responses may be at irregular times...

I seem to recall that you indicated in the manual that using the above method would return TLO values that are all negative numbers; is this correct? I ask this because that is not always the case with my machine. My mill homes at the top of the Z travel and it only moves down in –Z. I would like to understand what I’m doing wrong if anything to get positive values.

OK you have MC Z0 at the top of Z travel. That is the most common case.
When touching tools off to a common surface to determine their TLO values. you should only get negative TLO values.
The TLO value is actually the amount that the tool tip had to travel from MC Z0 to the reference surface, and the amount of travel is measured on in machine (MC) coordinates.
Since MC Z0 is at the top, and we can only travel down (-Z direction) from there, the TLO value has to be negative.
Therefore, when using this technique, a positive TLO value is a big red sign of something is wrong.

The question then becomes what is wrong....

I find that looking at the WC offset page is often the key to figuring that out.
In the upper panel, MSM shows all the numbers that are added together by Mach to get from MC to WC. Just read across the row for the axis - in this case the Z row.
Look at each number and see if they all pass the sanity test.
Note that a longer tool will have a smaller (smaller in the sense of the Absolute value) TLO value - as a long tool has to move less distance to reach the reference surface than a shorter tool.

Dave