Why do tiny endmills cut deep?

[Matthew Hajicek - Singularity]

I've noticed that small endmills around 1/32" tend to cut deep compared to larger ones, even new tools freshly touched off.  I've seen this on different machines at different companies.  You can have a fresh 1/4" or 3/8" bull or flat cut a floor right to nominal, then try to blend into it with a fresh 1/32" ball and the ball will overcut by a thou or three.  Does anyone know why this happens, or a rule of thumb on predicting it?

[GoetzInd]

Possibly higher RPM of smaller tools causing thermal expansion of spindle in Z axis? What is your process in touching your tools off? Using a presetter for all tools I haven't really noticed this.

 

Mike

[jlw™]

I don't run a machine much any more but I always matched tools using a 1in gage block. It doesn't take much more time than using a tool probe. I never got burnt a single time matching a tool but I got burnt quite often trusting a presetter or tool probe.

[Colin Gilchrist]

Those tools also have a really small shank. I'd be surprised if you weren't getting some pull out when the tool first engages the material. What kind of holders are you using? I'd be wary of any type of collet holder, especially if you aren't using the "high precision" collets. If you are using collets to hold these tools, its worth the extra bucks for the good quality ones, with the actual "nominal diameter", not the ones that are designed to fit a "range" of sizes. For example, on a .125 diameter shank, I've had the best luck with the ".125" I.D. High Precision collets, vs. the 3-4mm expandable ones.

 

Also, the other issue I've had with very small tools is the aperture of the laser being "too big" to get an accurate "blocking" of the beam with a very small tool. You're relying on the tip of a very small tool to block the beam. Try doing a test where you run your "tool measuring routine", and check the Tool Length Offset value. I'd be very surprised if it didn't vary by .001 or more, as you measure it several times. If you are getting the variance with the laser, then I'd suggest bolting a small block of material to the bed of your machine, someplace off to the side. Then you can do a test cut with your larger finishing tool, and then manually adjust the "blending" of the small tools. I think Rickster mentioned doing this for the Mold Work he does, and I liked the idea so much I incorporated the process into my work that requires blending small tools...

[civiceg]

On super critical blend parts I would back up small diameter end mills .0005 to .001 high. Run a few features and check the blend and rerun. This is when I needed less than .0002 mismatch. 

[slyym]

Nope! Its usually the way operators touch off tools. Happened all the freaking time at my last job because they used a piece of paper to touch tools off. On tools like that that just do corners I would touch the tool off to the bigger area already machined with a gage block. If that area was -1.000 then after touching smaller tool there ad +1.000 to the offset and part would be perfect every time until I left and they went back to using paper that is.

[FROZEN]

GoetzInd, on 11 Jul 2016 - 7:51 PM, said:

Possibly higher RPM of smaller tools causing thermal expansion of spindle in Z axis? What is your process in touching your tools off? Using a presetter for all tools I haven't really noticed this.

Mike

 

 

I 2nd that.

 

[Rstewart]

If your using a touch probe the 1/32" endmills are flexing on touch off

[FROZEN]

Matthew Hajicek™ - Conventus what tapper and RPMS are you using??? I think the centrifugal force changes the tappers. The research I have seen HSK gets rid of this.

[BenK]

How are you touching off the tools? If you use a laser they are never accurate for small tools. Back when I was running a machine I had a test program I used to adjust the length and control the blend. I would cut small round pads in a piece of material that I could take to a height gage to measure the difference. 

[specv]

I second a lot of the things said, but there is also significantly less tool pressure on smaller tools as well. Tool pressure on larger tools is actually pushing away and the small tools are cutting more free.

[Matthew Hajicek - Singularity]

Just got back in after taking a few days off for a baby.

 

Touch-off is by contact touch-off probe.  Machine is a Haas VF-3SS, CAT-40, 15K RPM.  I'm using ER-11 and ER-16 collets for the smaller tools, and the parts are on a TR-160Y trunnion.  I'm going to have my lackey run a touch-off repeatability and thermal growth test Monday morning.

[Matthew Hajicek - Singularity]

So we ran a thermal growth experiment.  We found tool touchoff repeatability to be <.0002” for a given temperature, but .0014” change from cold (74°F) to hot (111°F).

[FROZEN]

ouch

[civiceg]

A decent warm up program will cut that down considerably. Moving all axes and running the spindle at operating speeds. After a decent warm up you should see less than .0005. 

 

This is why I would set all my tools at the same warmed up condition when ever possible. Also having tools that run for awhile, run them close to the same rpms when possible. 

[Matthew Hajicek - Singularity]

I asked Productivity about it and they said I saw such a big change because I ran my test dry, since the spindle is chilled with coolant flow.  The machine could be setup with a separate chiller, but I'm sure that would be a bit pricey.  So far my solution is to run a part with the tools at nominal, measure the mismatch, and adjust stock-to-leave and such to blend it out.  Fortunately I don't have to get things perfect on the first part like some of you moldmakers.

[THEE THAINZ™]

" to be <.0002” for a given temperature, but .0014” change from cold (74°F) to hot (111°F)."

I see similar results on all axis where I work on our VF 3 now on our VF 8 it's a little less but it has scales.

[Rstewart]

What I do on our Haas is always run the coolant.  If I'm machining dry, I'll just aim the nozzles away from the tool/part.

[Brad St.]

Matt, one other thing we used to run into and still do is if it's a carbide tool post in the tool setter look real close at it and see if you see small pitting from the tool to post (carbide to carbide). We'd see this quite often and have to pull the post and re-grind them flat. One other option is to move the tool to different placements randomly to see if your getting different measurements.