Maximum side load

Mostly Printed CNC - MPCNC Advice
1

I was looking into recommended feeds and speeds for MDF. I found a datasheet at onsrud.com for MDF, where they state that a 1/8" end mill should have a chip load of 0.006" when using a 1/8" depth of cut. (I looked at the numbers for the “good” end mill, which is $30, so it should be a good one!).

At various places I found that an RPM of 18k is recommended for MDF. A 1/8" single flute end mill at 18k RPM and 0.006" chip load and a depth of cut of 1/8" should have a feed rate of 18,000 * 1 * 0.006 = 108 inches per minute, which is 45 mm/sec.

When I put those numbers into FSWizard, I get a cutting force of 4.08 kg for a slotting operation, and 0.4 kg for a finishing pass that takes off 0.3 mm (10% of the diameter of the end mill). I’m not 100% sure that all the numbers I put in are correct though, but it makes sense to me.

My steppers are rated for 4 kg.cm. Two of those are pulling a single axis, so that should be 8 kg.cm. You need to subtract some torque that is required just to move the gantry around (no idea how much). Moving at 45 mm/sec should be easily doable without (much) loss of torque as far as I understand.

What I’m not sure of is what the “arm” is for the stepper. Is it the radius of the 16T GT2 pulley, which is roughly 0.5 cm? If so, the two steppers should be able to plough through even 16 kg side force! (Minus the friction of the gantry).

I know the deflection of the Z axis will be huge so accuracy is out the window. But for roughing that doesn’t matter.

When I made the shoe tree I posted earlier, I used a feed rate of only 8 mm/s and the RPMs were probably around 24k, with a 3mm end mill and 2.75mm DOC. At that RPM, the recommended feed rate is 60 mm/sec, so I was going very very slow. There was a slight burning smell, which indicates that the feed rate was indeed too low. Everything worked out fine though, nothing got black and the cuts were fine.

Of course I can just experiment to see whether these numbers are in the ball park, but I’d like to understand the numbers behind the recommendations and what the theoretical performance of my steppers is.

2

That is missing a lot of info actually.

Calcs are ridiculously tricky. See the one I have listed on the milling basics page. Use my numbers and pay attention to the force it gives. Depending on probably a thousand things, most MPCNC builds can do a 1.9kg load in that calc.
I promise you will be much better off not using one. You need to learn what sounds good or not. it is easy to do, watch a few cuts change one variable at a time. Pick a project that is quick and useful, coasters or something and make a ton of them and figure it out. Calcs are for exotic projects and maximizing a standardized machine, they are not for us.

How you can use them is max out your machine by ear/eye. Then put every detail of that cut into it and see what load it comes out to be then you can use that number to prep new cuts to see if they are in the ballpark of your builds capabilities. After a handful of cuts you will never waste you time with a calc again unless you buy a giant machine.

1 Like
3

That’s the one I used, the FSWizard calculator. It gave me a 4 kg cutting force.

That’s a very good ballpark number to know, thanks. I’ll definitely experiment with my machine to understand its limits!

In your experience, does it make sense that the steppers are never the weak link? Can they easily handle that 1.9kg side load without ever skipping steps? Is chatter the limiting factor?

4

The steppers should never even come close to being a factor. Rigidity induced chatter should always be first.

1 Like
5

But sometimes, there will be skipped steps. Once the bit isn’t going where it should, the loads can get big quickly. The bit can bite too deep, or get stuck in the material. Then the steppers will skip steps.

6

I tested my machine with MDF. I was pleasantly surprised to see how fast it could go!

I made a small 3cm x 3cm rectangle toolpath, with a 2.75mm depth of cut. The maximum depth was 13mm (so 5 passes, where the last pass was slightly less than 2mm deep instead of 2.75mm). I used a 2 flute 4mm straight HSS end mill. (It happened to be in the router already and I saw no real reason to use another bit for this test.). The router was to 22k RPM (not sure how accurate that is, I have a cheap Katsu router). I have stainless steel tubing (25mm OD, 2mm wall thickness).

The fastest feed rate that still worked was 64 mm/sec (!). At 72 mm/sec I think the bit got stuck or something.

I also tried a finishing pass of 80 mm/sec, also with a depth of cut of 2.75mm, but a feed rate of 80 mm/sec. It really zips around at that speed, and the final dimensions were still very good. (Outer dimensions of the 30x30mm square were 30.10x30.10mm. With a 8 mm/sec finishing pass the dimensions were 30.00x30.00mm.).

When I put these numbers in FSWizard, I get a side load of around 4 kg. I used “Wood” as the material in FSWizard, and MDF is pretty soft, so the actual side load may be less than that. I might try with aluminium another day, to see whether I can match the 4 kg side load with that.