I need to cut various sizes of structural steel sections (angles, box, etc). I would welcome advice on the best tool to use.
I have angle grinders, but these seem a bit too casual and inaccurate?
I have a Bosch Professional circular saw (bought for wood), 190mm, 30mm bore. Are there metal-cutting blades for this anyone could recommend?
Does one need to get a purpose-made saw (eg the Rotabroach Element 9)
https://www.rotabroach.co.uk/element-saw-range/element-9-circular-saw/
Any advice appreciated.
The "best tool to use" depends on what can be afforded.
The colder the cutting process, the less the material is affected (hardened, blued, annealed) where it is going to be welded.
True cold saws (ultra low RPM, liquid cooled, strongly fixtured, often hydraulically actuated) and big metal cutting bandsaws generally provide colder, non hardened, dross free cuts, as opposed to abrasive cutting, flame cutting, or plasma cutting. But even better than cold saws and band saws are water jets and lasers. And there are even more esoteric processes that are better.
But I get that you mean best cutting by manual means, within reason and realm of an individual fabricator on the farm or home.
The accuracy of a cut off wheel affixed to an angle grinder is up to the operator's skill, patience, and creativity. Sometimes, a lot of pre planning a cut sequence is necessary, especially on 3 dimensional cross sections such as H beams, where the flange of one dimension interferes with tooling access to the perpendicular dimension to complete the cut.
Colder, cleaner cuts can often be obtained with a circular saw like your Bosch, when outfitted with a metal cutting blade. 190mm blade diameter is 7 1/2 inches, and most circular saw blades in that size range available in the USA are made as 7 1/4" blades, with 5/8" (16mm) diameter hole and an alternative diamond shaped antispin pop out for arbors so equipped. I can confess that I've never seen a circular saw blade as small as 7 1/2" with an arbor diameter as large as 1 3/16 of an inch (unless you are measuring the long axis of the diamond?). Anyway, if Bosch of Europe sells a steel cutting blade RATED FOR YOUR SAW'S RPM, and RATED FOR THE SOLID CROSS SECTION THICKNESS for the materials you wish to cut, I wouldn't hesitate to use it.
It's the blade that does the cutting. I buy good blades. I don't give a muck in May about the saw itself. In fact, I put metal cutting blades on an old circular saw that a neighbor threw away. Cord was compromised, and it didn't work. The neighbor was also throwing away an old vacuum cleaner, that had a good cord, so I put the cord of the vacuum cleaner on the saw, and voila, a "$400.00" metal cutting circular saw, for free.
I write the saw's RPM and draw an arrow for arbor rotation on the blade shroud, because I'm old, can't see, and can't remember too good. The nice thing about this older Skilsaw is the blade shroud and guard are made of metal, so sharp edges of the gullet flung fresh hot chips don't dig into and collect inside of the blade shroud like barnacles, as what could happen with plastic guards.
On another thread, a WeldingSite member suggested that only "worm drive" circular saws could be repurposed for cutting metal. I decidedly disagree. While worm drives do make it easier for right handed operators (I am right handed) to see the cut lines due to the blade being on the left hand side of the gear box... worm drives have a starting flaw, because of that gear box, and because of the orientation of the worm drive saw motor.
The rotational axis of a worm drive is parallel to the cut, and inline with the rotation of the wrist. This means that when the trigger is pulled, the saw jerks initially, due to the instantaneous torque rise inherent with electric motors (especially those lacking soft start, like a traditional worm drive Skilsaw). The motor is rotating in the direction that the forearm can least resist, and that initial jerk means that the saw must be turned on first to get the jerk over with, and then positioned on the cut line whilst the saw is running, for best accuracy. One could spend all day aligning the saw perfectly on the cut line with the saw off, but once that worm drive saw kicks on, that initial jerk is highly likely to require repositioning.
On the other hand, a direct drive circular saw has a motor rotation perpendicular to that of the forearm, and rotates axially along with the same rotation as the blade. Once the saw foot is placed on the material and the cut lined up, turning on the trigger to the saw does not jerk the saw tangentially away from the cut. The saw blade simply starts rotating. And I don't need to see the blade following the cut line, because I always clamp bars to the work for guidance, and simply push the saw foot along the guide, because I placed the guide the exact distance away from my desired cut line as the blade is distanced from the parallel edge of the foot plate on the saw itself.
So yes, any saw that spins no faster than the RPM the steel cutting blade is rated for will do. Even a saw rescued from the dumpster.
However, you mentioned and posted a link to the Rotabroach "Element 9", which is a (direct drive, blade right, NON worm drive) 230mm blade diameter metal cutting circular saw.
To be sure, Rotabroach is a well respected name in the business of cutting holes into metal. But this Taiwanese built imported saw is no different than, and in fact is identical too, the same Taiwanese manufactured saw imported and rebranded as the:
Rotobroach Element 9
Fein Slugger 9
MK Morse Metal Devil CSM9NXTB
Steelmax S9 XP 9"
Taiwan 9" Metal Cutting Circular saw generic as shown in different color schemes on taiwantrade.com, which is the Taiwanese equivalent of China's Alibaba Express
Arcon Tool PMM-230
Evolution EVOSAW 230 (Evolution has a slightly different blade shroud... have not determined whether it is the older or newer design)
These are all the same saw. Pick your color, brand name of choice, price, availability, warranty offered, and/or proximity of service after the sale. Same Taiwanese saw though.
Which isn't a bad thing. I have found that machinery built in Taiwan has been vastly superior than machinery built in China. Most all of Bosch power tools are now made in China. At least the last 6 Bosch tool purchases I have made over the last 5 years were made in China, including a 12" circular saw blade that turned out to be bent brand new out of the box. Not visible to my naked eye, but my blade sharpener saw it right away. He straightened and retensioned the blade for me. I have found better quality with Taiwanese built tools.
Just recognize that despite well proven brand names like Rotabroach and Fein and MK Morse... the saw itself is the same. One could probably import it directly nowadays. What does the cutting however, is the blade, and that is where the brand names like Freud (under the Diablo brand for metal cutting) and MK Morse are worth their reputations. I also use metal cutting blades from Tenyru, which are made in Japan. (Not all Tenyru wood cutting blades are made in Japan, but the metal cutting blades I buy are).
Yet unmentioned are reciprocating saws, jig saws, and portable band saws... all viable options for accurately cutting steel without inputting a lot heat. Each tool type depends on the shape of what is being cut and the access available to make the cut. Bosch still makes jigsaw blades in Switzerland that are of high quality. I use a variable speed Bosch jig saw to cut large holes in thin sheet steel that is difficult to cut, like stainless. Speed control helps with directional control and vibration.
No matter how you make the cut, you can always cut generous, and "sneak up" on your final cut line with sanding. Flap disc, wheel disc, and belt sanding can all remove a remarkable amount of material quickly, leaving a smooth finish. Not that critical I suppose for a 3 point tractor implement hitch, but since the topic is about various ways to cut metal, the idea of finishing the cut with a sander should not be discounted.
