Hold me, I screwed up.

[cobrakai]

I've got that terrible sinking feeling right about now.

I was making a new fixture plate for my mill. Basically it's a 1" thick piece of aluminum that is 1/4" narrower than the machine limits, so I cut at the X-Y limits using a 1/2" end mill. This provides a nice sacrificial work surface and also gives me a visual of my usable workspace.

Well I carefully measured my tool lengths, set the Z-zero on the table, checked the tram and ensured it was perfectly flat everywhere. Checked my tool length, tightness, collet tightness, everything. Brought it down to .0005" from the table and it just skimmed some sharpie from the surface. Perfect. I made my program bottom level .003" so that it would not get close, and leave a small amount of flash that I could remove with a razor blade. I took light depths of cut in a helical 2D-contour program to minimize tool deflection.

And yet... someway, somehow, the tool managed to cut into the table on the finishing level. Only about .001", but STILL! HOW DID THIS HAPPEN!? Yes, the groove is outside of the usable working area and shallow, but it's still a blemish on my table I managed to keep perfect for 6 years. After kicking myself for the last 3 hours, I am still confused as to how this happened. Could spindle/tool temperature cause a .004" growth in tool Z height? I now have a constant reminder for the future, but unless I know what happened it can happen again.


Now... even though I am 99.9% not going to do anything about it, is there a way to fix this? Could I remove the table and have that top surface ground flat in the future? Like I said, it's only about .001" deep.





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[LeeWay]

Did you measure your stock thickness first?

[cobrakai]

I set the bottom level at .003" above the table surface, and referenced the zero off the table, so it shouldn't matter what my stock thickness was as long as it didn't do some rapid plunge or something.

[ger21]

Did you measure your stock thickness first?

+1.
Your 1" aluminum is probably less than 1".

[smokediver576]

+1.
Your 1" aluminum is probably less than 1".

But he referenced off the table surface........

[toyshop]

Backlash in the Z axis? Check with an indicator if it feeds down consistently. If the mill wasn't warmed up first may be an issue with the ways or the spindle growing. Expensive machines use a chiller to keep the spindle and ball screws from growing under use.

As far as fixing it. Scraping it in would be an option. I've scraped in small pieces but nothing this big.

Dave

[cobrakai]

Backlash is a simple one I didn't really think about. I could probably account for .001" there, since I referenced while traveling in the downward direction, but the tool tends to pull itself downwards. Not long ago I went through the whole gib adjustment so I think the maximum I saw was .001 or .0013 backlash (lost motion I think they call it) in the Z, and far less in the X and Y.

So perhaps .003" offset from the table, .0015 backlash, .0025" growth from heating up. I guess that could explain it. Ugh. In hindsight, I would have crept up on the bottom level. There is no reason to do it in one pass. Lesson learned I guess! Just very frustrating.

toyshop, what does scraping involve?

[84scrambler]

Double check your tool height and see if it matched what is in your library and check every single flute. Not sure what software you used but in HSMexpress you can offset you vertical height when you select heights tab and also again when you select your passes tab so make sure you dont have an extra offset in one of those section and check you signs on everything. My spindle height usually screws with me on days when it gets over 90 degrees and I can can see it grow by about .002" but it is extremely hot to the touch on those days. Today its in the low 80's and it just gets on the high side of warm and it is in the noise of my error. How did you set the zero on the table?
You can also repeat your steps for the top of your aluminum plate and do an x offset and cut a profit pass and see if it clips it. At least you will see if it is repeatable.

[cobrakai]

Good idea, worth checking. I had to turn the machine off for the rest of the day today.

I set the Z zero with a dial indicator tool in the spindle that has a certain height. On the tool I set it with the Tormach Z touch probe with a carbide surface. I verified the tool length matched the dial indicator by bringing it to .0005" of the table and ensuring it didn't touch, but only grazed a layer of sharpie. So I am 100% confident it was set up correctly. Something definitely moved between start and finish of the job (tool growth, slip, etc).

[toyshop]

Hand scraping in a surface is a labor intensive and highly skilled talent. A machine table is not the place to start to learn.
Google hand scraping. there a a lot of interesting videos. That is only the start. Practicing a lot is what it takes.

If your spindle was perfectly lined up you could use a large diameter fly cutter that would reach both ends of the table at max travel. Reality is it would probably make the table less accurate, due to sag at both ends of travel.

Dave

[cobrakai]

Yeah, I wouldn't even attempt that.

I wonder if the dovetails are parallel to the top surface. If so it could easily be skimmed using a magnetic table on a surface grinder.

[74BurkeMVN]

Now that you've got that out of the way, move on.
I've got a doosie in my table where a corncob sucked itself into the table. TTS pulled out.

[Jim Dawson]

My condolences. The for next time and for others who may have this happen, I put a spoil board under the workpiece when I have to work close to the table. Normally I use a piece of 3/4 mdf. When bolted down to the table and surfaced, it is a very flat surface, stable to work from. But it doesn't like water based coolants. If you are wanting to use water based coolants, Delrin or even polycarbonate would make a good spoil board. Laminate countertop material (Formica) would also work well if you need to work close to the table, at least it will give you 0.030 or so before you hit the table.

I built a tooling plate the other day just about like what you show in the your picture. I needed to screw the parts down to the plate so having that extra 3/4 inch of clearance helped a lot. After surfacing the MDF, I removed the flathead cap screws, and bolted the tooling plate down using the same hole pattern. Then surfaced the tooling plate. Then drilled and tapped all of the hold down holes in the tooling plate, full depth. Worked great. MDF is wonderful stuff, good dimensional stability, and holds up well to petroleum based coolants (kerosene in my case). Can also be drilled & tapped for holding light work.

As far as trying to fix the table, don't do anything. Just leave it, unless you are going to pull the table off and have it ground by a shop that has the proper equipment to do it. Looks like it would take about a 30 inch surface grinder to do that job.

[LeeWay]

You mentioned that you were referencing the table. You didn't say that you were referencing the tool plate. That matters a lot.

Now you know there is an issue with the Z axis or at least some issue exists there. Sooo, you did not actually screw up as you mentioned in title.
You said it was a sacrificial tooling plate, so face it off.
Kind of a happy accident if it let you know there was a machine issue that needed correcting.

[smokediver576]

After I drilled into my table and ran a Shear Hog into it as well, I said, screw it and ordered a fixture plate. I personally think a new mill should ship with one of these!

https://saundersmachineworks.com/col..._fixture_plate

If I do screw up - just clean it up with a facing op and you're back in business. Anyone new to the CNC game, this is a must have item to add to your list.

Awall

[Steve Seebold]

I would have put your plate on some parallels to get it off the table then cut the ends. I assume cutting the tends of your plate is purely cosmetic.

[LeeWay]

Steve, thanks for that post. It caused me to take a second look. I took that line as a shadow.
I see what the fuss is about now.
My apologies and condolences.

[IMT]

You mentioned that you cut it in 1 pass. Did the TTS holder pull out? At 1" DOC it doesn't take much WOC to exceed the limits of the TTS, especially if you don't regularly lube the outside of the collet and clean the ID and tool holder shanks. I had a tool pullout with .75 DOC, .05 WOC in 6061. After that I adjusted the compressor kick on point to 100 psi, tightened the PDB so it just barely releases at 100 psi, and keep the collet OD lubricated, collet ID and tool holders cleaned with acetone. Made a big difference.

[74BurkeMVN]

You mentioned that you cut it in 1 pass. Did the TTS holder pull out? At 1" DOC it doesn't take much WOC to exceed the limits of the TTS, especially if you don't regularly lube the outside of the collet and clean the ID and tool holder shanks. I had a tool pullout with .75 DOC, .05 WOC in 6061. After that I adjusted the compressor kick on point to 100 psi, tightened the PDB so it just barely releases at 100 psi, and keep the collet OD lubricated, collet ID and tool holders cleaned with acetone. Made a big difference.

Good reminder to maintain that stuff. The release point of TTS holders is a little more tricky with ATC and it's alignment/repeatability. That's where I got bit. In my case, I did have a sacrificial plate under the work, and had run the particular job a few times on top of many others prior. Confident that all was running well, I was 15ft away deburring parts while the job ran when I heard the spindle rapidly bog and stall. By the time I got to the estop it was already over.
Kudos to the Lakeshore Carbide 1/4" tool, as it didn't break. The scar in my table augured in to full cutter depth. I've since bought a SMW fixture plate and of course addressed issues that caused the pull out in the first place.
In related news, I broke my first Haimer tip last week.

[cobrakai]

Thanks for the input. In hindsight I have come up with about 20 ways I could have done it differently. It's just that frustration that comes along with machining, as I'm sure you all have felt before.

The toolpath was very gentle. It was a 5000RPM 25IPM, full width, .050" DOC helical contour. The machine didn't even struggle at all. I am positive the TTS didn't pull out and checked on it as the program ran. I haven't had one pull out ever since I started keeping the holders clean and the bellevilles torqued properly. Even during some very heavy cuts that started to bog down the machine, the tools stayed put.

The reason for running along the perimeter is to give a visual of where my machine limits are. I plan to face off the top of the work plate and machine a 1" grid pattern into the top so I can easily set workpiece origins. I picked up this idea from an experienced machinist I know.