CNC router overload

What I wanted to do

I had a request to cut and engrave high pressure laminates. These are a lasagna of compressed paper and melamine resin, coated in a thin layer of (colored) melamine. HPL is really so durable that it’s even taken for facade planking. It is dishwasher safe and some variants can be heated to 200°C without taking any damage. You can find HPL as boards on playgrounds, garden tables, as panels in the tram etc. So I was like “cool stuff, let’s get started!”

Cutting data in High Pressure Laminates (HPL, “Resopal”)

I used the cutting data recommended by a couple of websites with values for “hard wood”. 2-flute upcut wood carbide endmill, speed of cut 450m/min, 6mm, Z+ 6,5mm, S24000rpm, F3500mm/min, full slot, climb milling.

Cutting results

And I was alarmed by the way the cut looked when I was done. Here are the symptoms:

• While cutting, spindle RPM would drop repeatedly but catch up to desired speed again quickly.
• high chipload on the bit, big chips, looking all fine
• endmill didn’t follow a straight line: it somehow “wobbled”.
• when cutting taps to keep the material in place, the cutter would move sideways a bit although it shouldn’t

Searching the culprit

First, I checked the spindle. No overheating, still tightly clamped to the portal milling machine, no bearing clearance, endmill securely fastened. Then I thought “Maybe something is wrong with the portal? Can I bend it?” No, I couldn’t. Then I looked at the endmill.

It didn’t look good at all. The cutting edges were practically gone where they had touched the material. Strange. It was brand-new when I started the job and it didn’t even run for 20 minutes.

Analysis results

• I used much to aggressive cutting data
• Because of that, the router motor was operated weigh outside its capabilities
• This caused sudden drops of the motor’s rotational speed
• This again led to increased chipload of my endmill
• As bearings and attachment of the router to the portal was “soft”, the cutter tried to evade from its cutting path
• This probably saved me from having a shattered endmill though

Improvements

Researching HPL parameters (again)

I did some more research on the internet. HPL is incredibly hard and tough. To compare some values (Brinell-hardness):

1. Beech: ~3.7N/mm² (Source)
2. HPL: ~185N/mm² (Source)
3. Mild steel ST70: 215N/mm² (Source)

I have found some recommendations for cutting HPL like “Carbide endmills should have a cutting speed of up to 150m/min” and “Use diamond-hardened cutters or 1-flute ones that are optimized for Aluminium”.

Interim containment

To correct things, I just did all cutouts again at finishing settings in CAM. With a fresh endmill and less aggressive feed rates of F2500mm/min and an updated depth per pass of Z+1,5mm. The result looks much better now.

Optimized cutting data

Still, let’s recalculate cutting data to take material differences to hard wood into account. I’ll choose a 4mm single flute (z=1) carbide coated aluminium cutter and assume a cutting speed vc = 150m/min for HPL at a feedrate per tooth fTooth = 0,025 similar to cutting mild steel.

Target RPM: \(n=\frac{v_c}{\pi d}=\frac{150000\frac{mm}{min}}{\pi \cdot 4mm}=12000\frac{1}{min}\)

Target feedrate: \(F=n \cdot f_{Tooth} \cdot z = 12000\frac{1}{min} \cdot 0,025mm \cdot min \cdot 1= 300\frac{mm}{min}\)

I chose a 4mm endmill because my router motor torque diminishes over-proportionally when I reduce RPM. With larger endmills, this effect would probably lead to insufficient cutting power. If you have a spindle with an asynchronous machine, your torque curve is essentially flat so that shouldn’t be a problem of yours then.

The feedrate seems so slow - I hope that this wouldn’t heat up the material so much that it melts. I’ll complete this post once I bought and tried the respective coated carbide endmill.

For your reference: Here is the related thread in the forum that I am visiting regularly (all in German).

• 2022 12
• 2021 18