First Cuts

It only took 8 years, but this machine has finally made a few test cuts. This page starts with some failures and ends with satisfaction....

My first test involved surfacing a block of aluminum. I took an endmill that I purchased years ago. I wanted to use some conservative feeds and speeds, but still get chips and not dust, and so I set the spindle to about 7500 RPM with a 50IPM feed. It was advertised as 1/4 inch, but it clearly is a reground and recoated item. The shank is 1/4, but the rest is not. This does not bother me, especially for a roughing end mill, as long as it cuts well - it didn't. There was a a lot of loud squeaking while cutting and clearly a lot of vibration visible in the surface.

The video at the end shows includes an example cut. My first thought was that the cutter was dull, so lets check that out:

Not the sharpest cutter, but not clearly dull either. I am not sure what is going on there. Maybe the geometry is different enough on rouging end mills to cause this? Anyway, I did order a set of new carbide end mills. Lets see if they look any sharper under the microscope:

And that certainly is a much sharper cutter! Unfortunately, the coating was much worse at repelling aluminum. While I certainly got a much nicer cut (video at end), the cutter loaded up with aluminum in a hurry:

Next, I added just a little lubrication by painting the top surface of the block with oil. That worked well, and I did some test cuts at 15, 30 and 70 mil cutting depth. I think it can do more, but this is really an unsatisfactory situation with respect to cutter lubrication - at least for aluminum cuts. I'll have to work something out. Still, I got chips, not dust which is a good start.

Lastly, I want to get an idea of how much more milling volume I can get out of the Bosch spindle. With a width of cut of about 0.2", a depth of cut of about 0.07" and a feed rate of 50IPM, the metal removal rate is about 0.7 cubic inches per minute. There is a handy table here to calculate the power requirements:

0.7 cubic inches of aluminum per minute requires approximately 0.2 HP, according to this.

The router is advertised at 2 HP at 25000 no-load RPM, so even at the lower RPM I use for testing, I still have quite a bit of room. I think the bearings will probably be more of a limit than the HP. Interestingly, with a coefficient for steel of 1, this same cut would have taken 0.7 HP! Probably at or past the limit of the spindle....

As I write this, I am reminded of the ridiculous labels on shop-vac type vacuum cleaners. They vastly overstate the motor power. A quick check on the Bosch website for the 1617 router reveals the following information:

  • "The Bosch 1617 2-HP Fixed-Base Router has the power to get results, with a 11 Amp motor"

But at 11A motor at 120V is only about 1.8 HP.... certainly sounds fishy. For steel the RPM of the spindle has to be lower than for aluminum. At 3800 RPM I have only got 0.3 HP left of the fictional 2HP rating. This assumes a flat torque curve, but I did rewire this to be a shunt motor, so that is a reasonable assumption.

In any case, I mean to answer the obvious question people who build CNC mills get asked all the time: "But can it cut steel??"

And I myself am curious too. So I glued a 1-2-3 block down and set one of those 1/4" end mills to work on it. I messed something up on my first cut. I don't know what and it chipped the carbide and just left an ugly gouge on the block. On my second attempt I did a couple of passes across the block with a 5 IPM feedrate cutting a 5 mil deep slot one way and deepening it to 15 mils on the way back. I did get the sparks one commonly sees with hard milling, which scared me at first, but the slot was cut very nicely and the end mill was ok. I really do need to work on a lubrication system.

Clearly visible on the right is the first cut which broke the end mill and then gouged along. On the left is the slot cut on my second try. The surface roughness is comparable with the grinding marks, but I guess that the feed rate is probably lower than it should be.

Lastly, I tested on a stainless steel block and also tried out a larger diameter cutter for increased rigidity.