Cutting thin gauge steel

Does anyone have any experiance cutting 20G steel? If so, I’d love to chat, I have several questions about the process.

as an amateur machinist I can tell you that even at it’s lowest speed you cnc router is like five times the speed a milling machine would spin at, and that would not be good for any bit cutting metal. it will either overheat and or break. it could even throw sparks


Hey Nick,

if you want to mill steel, the usual spindles shown here that have two magnetic poles and a speed range from 6,000 rpm – 24,000 rpm are not the right thing. While they are excellent for high chip loads on wood, because the have constant torque up to very high speed, mostly steel is milled with rather slow speeds from 1,500 – 3,000 rpm, but with high torque. For this, I would choose a spindle with 4 or more magnetic poles (2 or more pole pairs) which are rated 3,000 – 12,000 rpm or even 1,500 – 12,000 rpm. Because of the double number of magnetic poles, at the same VFD frequency, they run at half the speed than those with 2 poles (=1 pole pair), as explained here, but with double the torque.


For comparison, a spindle with 2 magnetic poles with characteristic as the most spindles shown here (good for wood):

Compare the speed/torque diagram on the bottom of all the datasheets!

More spindles

Pole pairs on three-phase asynchronuous induction motors:

Biezl, Public domain, via Wikimedia Commons

Biezl, Public domain, via Wikimedia Commons

2 magnetic poles (1 pole pair) 4 magnetic poles (2 pole pairs)

While the Onefinity frame itself is up to the task of light steel milling, the typical spindle choices are not (whether we’re talking Makita router or VFD type spindle).

You can cut steel with it well enough to struggle thru a one-time small project, but I definitely would not recommend planning any sort of repetitive work with steel. As mentioned, the slowest spindle speed is far too fast. Consider that even IF we had access to a proper RPM range spindle, the Z axis may still show weakness under the demands of machining steel.

That said, I did cut steel with mine: I cut the oval pocket in the front .090" deep as well as the 12mm keyways in the center of the table. On the oval, I used a 1/4" carbide 2-flute end mill @ .005" [0.127mm] depth of cut, and flooded with Cool Tool cutting/tapping fluid. I used the same DOC when cutting the keyways thru 7 ga. steel, but I used a 1/8" end mill. I was able to take a few good finish passes to achieve good dimensional tolerance once the slugs were cut out. The tip of the end mill was toast, but I still had good existing side flutes for the finish size passes. The whole process was very slow and very messy.

Now, allow me to circle back to my comment regarding the Onefinity frame. While it is up to the task, it’s a less than ideal frame for milling steel due to exposed ball screws & somewhat sealed linear bearings. While you do see ball screws & linear bearings on high-end commercial machines, the vast majority of smaller mills (Bridgeport size & smaller) typically use dovetail style guides & wipers, which are much more steel-chip friendly & cleanable than ball screws & linear bearings.


Machinist, thank you for your detailed reply, i didnt even think of the chips getting into the bearings, i guess i had planned on using a dust boot if it would even suck up and chips at all after having lubrication for the bit… would a spindle with a vfd be any better or do they still not provide a low enough rpm and torque to get the job done?

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I have no direct experience with a VFD spindle, but from what I’ve read, there is basically no usable torque down at the lower RPM range. In other words, it may be able to spin @ 5000 RPM, but you couldn’t possibly cut anything with it.

Prior to moving from MI to AZ last year, I had one of these (but sold it), and it was pretty good for machining steel, although it wasn’t CNC.

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Ok, so thay little guy looks to be used for machining parts correct? I guess ill need to look into a plasma table or laser for doing some steel sheet work… for now i guess, send cut send my sheet steel requests… Thank you!

Yes, correct. The mill is capable of real (albeit small) machine work. A plasma table would be a most excellent choice for doing thin sheet metal work. Laser better yet, but probably cost-prohibitive for anything hobby related that’s actually capable of cutting steel.

Hey Bill, hey Nick, hey all,

The spindles I refered to above do have maximum torque at 1,500 rsp. 3,000 rpm, both the HPFD and HFPM series are designed to mill steel (see the torque/speed diagram in the datasheets).

However a spindle, which is an induction motor, can never compare to the router, which is a so-called universal motor (a commutated series-wound motor). Only a spindle delivers constant high torque over its entire rated speed range (CT mode), while with the router, where the speed is dependent on the load, you only have an extremely narrow combination between usable torque and speed. Also a spindle is never slowed down by any load, unlike the router. If you want to know about the fundamental differences between the two, see here

You know I’ve often questioned how an experienced machinist like you, of all people, still works with this hand trim router instead of a spindle. However, knowing the kind of work you do, that could be why you’re still happy with the router: first, perhaps that narrow torque/speed range happens to be one you need, and second, you’re probably very far from demanding much power from the motor :slight_smile:, and of course thirdly because the spindles usually shown here are not rated for speeds below 6,000 rpm, as they target people who want to mill wood. However the hand trim router does not offer you low speeds either.

But I totally agree that if you put one of the spindles that are able to deliver high torque at very low speeds, and which are designed to mill steel, on the Onefinity, the Onefinity is the limiting factor then, at least if you expect steel to be milled at this speed and at this MRR, as also impressively shown here. It is not a machine designed for it.

Do you remember when we discussed another interesting machine here and here, we described the problem on the Onefinity with the router slid far downwards into the mount, the problem with the Z assembly in general, but also that the other machine refered to, which is sold as being a versatile machine perfectly able to mill steel, has plain bearings instead of linear ball bearings which would be more resistant against the high forces and vibrations when milling steel (by the way that machine had some other advantages)

What a nice machine. Why did you sell it? Your Onefinity Machinist isn’t really able to replace it (in no case in matters of milling steel)?

In this mill, as they describe here, you have a type of motor that is far superior to the commutated (brushed) universal motor like your hand trim router. A high torque from 100 – 2500 rpm, and totally silent, that’s ideal.

However the power they show here is sufficient for a mill. For a CNC, where you have much more speed of operation, it would have to be much stronger.

Note: If one should not only be interested in what spindle manufacturers offer to consumers and hobbyists, like Mechatron does, it is worth a look at this manufacturers list.

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Unless you’re looking at much higher $ (20K type dollars), you can skip the laser research and head straight to plasma :slight_smile:

I use a Langmuir Crossfire Pro for my sheet steel work with a Hypertherm plasma cutter. One nice thing is I can do prototyping using the laser and chipboard before moving to cutting the much more expensive steel. I just need to take the SVG file I use on the laser to a GCode generator for the Crossfire for toolpathing. But that’s only because I had the laser before the plasma.



Yet another exquisite post that is as informative as it is eloquently written. Hats off my brother, your posts are always appreciated.

Yes, I’ve given thought a time or 2 to something different than the router. However, as much noise as I make about wanting to have a proper lower-speed spindle, my work doesn’t merit me putting much serious thought into getting one. The router has served well for cutting plastics/composites and such, and I know it isn’t placing stresses on the Z axis that would be of concern regarding accuracy. Even if I had a spindle with a more appropriate RPM range for metal/steel, I have little faith that the Z axis could deliver an appropriate level of accuracy due to deflection. All that said, I think the Z axis is well designed and adequate for the target users that primarily cut wood at high RPM’s.

I’ve basically eliminated aluminum from my “menu” on Xometry, and only take the occasional parts that fit my profile. For example, I just took on a polycarbonate part that fits nicely on my machine (it’s on there now as we speak).

Yes, I do recall that. I’ve always been a fan of that type of spindle that is secured at the bottom and has good bearing support as well as hits the target of a lower RPM range. I’ve also been a fan of plain bearings which are better suited for heavier machining. Typical linear bearings don’t provide the surface-area contact that is critical to reduce vibration. I had started playing with a design to accommodate that type of spindle, starting with getting the Z axis stepper out of the way and driving it with a generous size timing pulley (as I had on my X-Carve). This would allow clearance for the spindle motor, but I would still require a new spindle carrier, which likely would not resemble the current one.

I loved that machine! However, concerns of how much room I’d end up with after our move as well as justification for the space it would require didn’t add up, so I sold it before the move with the assumption I would replace it at some point if after the move if I could justify it. In my current shop (shed), I really don’t have room to absorb it, nor do I really have much interest in machining steel. For the rare occasions when I need to machine steel, I can make do with the Onefinity. Case in point is when I modified my controller housing (steel box). I was able to cut the front controller cutout larger to accommodate the 8" monitor which was larger than my previous DDCS controller.


Hey Bill,

By coincidence, while searching for something else in the depths of the forum I found that you have shown the machine here! It was in the thread where you showed how you built your base with steel top for your Onefinity Machinist in December 2020. Really a nice machine!

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Yes, it was a pretty nice machine, and had a bluetooth DRO. It helped me cut the original steel table plate down to the final size & configuration I wanted. This was another (way smaller) mill I built before I got that one (also sold this one).

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I just went through that old thread and stumbled on this:

Do you have these extrusions between bearing block & Z axis assembly still in place?

No, I ended up taking those out as I deemed them not necessary, and it added an additional 20mm of distance (as well as the associated leverage) to the Z axis. But it was nice to have the adjustment, and kept the Z axis in near-perfect side-to-side alignment due to 6mm dowels in the Z axis that engaged the slots in the extrusions.

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