BS250 >> metal cutting

[GrahamRounce]

Hi all - I have a Record Power BS250 which has served me well for wood and aluminium.
I'm now more & more wishing that it would cut mild steel, say up to 6mm thick.

There are plenty of VFD speed controllers for synchronous (= induction?) motors, but they all output 3 phases, I don't know why. Can I just use one of the phases? Would the wattage then be only a third?

Apart from doing rebuilding to allow various belt arrangements, has anyone else managed it successfully somehow else?

Thanks v much,

 

[Fergie 307]

The problem is that a wood saw is just far too fast, you need to aim for a blade speed somewhere in the region of 200 ft/min for steel, so typically around 1/10 of the speed it normally runs at.
I was in a similar position using a Kity with a metal cutting blade. I was able to change pulleys to get it to run at about 1/3 speed, but still much too fast really.
I ended up converting a little Clark CBS 190 by replacing the motor with a 12v 350w one intended for an electric bike. It has a built in 9:1 reduction gearbox, together with a variable speed control. I had to make a new gearbox output shaft to suit, but got the motor from a local guy who repairs electric bikes. The screw on the end of the shaft that holds the sprocket was knackered so scrap as far as he was concerned, and so free!
As I was going to replace the shaft anyway it didn't matter to me. I paid about 15 quid for the saw on e bay. I already had some big mains to 12v DC power supplies so the only other thing I had to buy was the speed controller, which was less than £10.
I put the power supply and electronics in a project box on the back and the speed controller has a pot and LCD, which fits neatly in the space previously occupied by the on off switch.
Very small, and not a great quality machine, but works a treat for cutting the various steel and brass plates I use for repairing clocks. It will happily cut up to about 5-6mm steel and 12mm aluminium using tuffsaw bimetal blades.
So as an "I wonder if" type project I am quite pleased with it, and so is my Kity !
The other issue is that if your machine uses rubber tyres then metal will get embedded in them, and very difficult to get rid of, and may then get into any wood you cut with it subsequently.
My Kity is an old 612 and doesn't have tyres.

 

[ChaiLatte]

Doing it today, one of these might be a good candidate to replace the existing motor:

https://vi.aliexpress.com/item/1005007683855333.html

That is the first one I found; they are available at lower prices and on Amazon.

Maybe have a think about the format in which the steel you wish to cut arrives in your workshop. That will tell you what percentage of required cuts can be made on your existing bandsaw

If the steel is similar to a sheet of plywood, a variable speed jigsaw (run along a straight edge guide) and good blade could be an option.

Another option is to mount the jigsaw upside down, in a table, with a guide for the top of the blade. That allows you to use a fence.

On a vertical bandsaw, cutting a 24" length off a 3m long stick of material will not be easy. For that, an angle grinder and 1mm slitting disk is good (and can be done outside - take the tool to the work).

 

[Sideways]

A vfd will not do this.
If you did buy a single to single phase vfd (there is one, it will cost £ a few hundred) when you dial your machine down to the 1/10 speed that you need from @Fergie 307 and others' experience, your bandsaw motor which only outputs 200W at full power will now only be outputting 20W.
It will barely scatch the metal and stall if you look at it harshly.

A VFD isn't a gearbox.

Portable bandsaw (these are geared for metal cutting), variable speed jigsaw with a metal cutting blade or a special "cold saw" - a circular saw with low speed high torque motor/gearbox.

And remember the feel of a hacksaw blade if you twist it in the cut, these are straight line cutting solutions, not for curves, except maybe the very shallowest.

 

[guineafowl21]

A 3ph VFD on 1ph motor would just trip with a phase loss fault.

You can get VFDs for 1ph, but most 1ph induction motors have a start circuit that is switched out above 70% or so of rated speed. Running them below this will likely burn out the start winding and switch, or blow the cap.

You’re better off with mechanical speed reduction here. Taperlock pulleys are very handy for this.

 

[Fergie 307]

Doing it today, one of these might be a good candidate to replace the existing motor:

https://vi.aliexpress.com/item/1005007683855333.html

That is the first one I found; they are available at lower prices and on Amazon.

Maybe have a think about the format in which the steel you wish to cut arrives in your workshop. That will tell you what percentage of required cuts can be made on your existing bandsaw

If the steel is similar to a sheet of plywood, a variable speed jigsaw (run along a straight edge guide) and good blade could be an option.

Another option is to mount the jigsaw upside down, in a table, with a guide for the top of the blade. That allows you to use a fence.

On a vertical bandsaw, cutting a 24" length off a 3m long stick of material will not be easy. For that, an angle grinder and 1mm slitting disk is good (and can be done outside - take the tool to the work).

The discs sold for stainless steel are very good and last longer than other types.

 

[GrahamRounce]

Thanks!
I'll look at that Clarke thing. "Very small" isn't necessarily a disadvantage!
And that servo motor, too.

Surely with a VFD the power is the same regardless of the speed. It's still the mains voltage, after all, just a different frequency?

I have and do use a jigsaw, but it's noisy and of course vibrates a lot. Actually, that's what I'm trying to get away from, sorry!

I should have said that at the moment what I'm trying to do is, starting with a 50mm x 6mm x 1m mild steel bar, making a dozen or so 3-pronged things like tuning forks, about 40mm long, and each square prong and each space 30x6x6mm. Jigsaw works, but is slow, and needs a painful amount of filing!
(I'm also looking at casting, but that's a lot of bother! Forget I said it!)

Thanks all, again,

 

[ChaiLatte]

Do you have a sketch of the plan view of the item?

The fastest way to remove metal might be a twist drill - especially if the top of the prong can be semi-circular.

Having read what you describe, a cold-cutting metal saw (Evolution Rage 4 is a popular one) might work well. As you are making multiples of the same thing, making a jig (flag stop) to position the work optimally will be worth the effort.

Instead of working with 40mm long items cut one at a time from the big bar, it might be possible to make a double-ended part out of 80+mm long and then cut in half at the end.

With the chopsaw above, you can gang pieces of the parent stock together (it should cut 3 at a time), chopping off 40mm increments. That alone doubles, triples or quadruples your throughput.

 

[Fergie 307]

Portable bandsaw or donkey saw would also be good for the OP's needs, could stack multiple lengths to speed things up.

 

[--Tom--]

Look at water jet cutting - if you can make a vector graphic like an SVG there’s a few places that’ll cut and ship to your door. If you’re doing enough to warrant investing in a saw may work out better to just outsource.

 

[Sideways]

Surely with a VFD the power is the same regardless of the speed. It's still the mains voltage, after all, just a different frequency?

No. Absolutely not.
Skipping a load of details, a VFD basically maintains a constant ratio between frequency and voltage at it's output.
If it didn't reduce voltage as it reduced frequency to slow the motor down, any motor would overheat and burn out.
The result is that power is essentially proportional to frequency / speed.
As you slow the speed down, the power is reduced.
So at 10% speed you have about 10% power. Near useless.

As I said, a VFD is not a gearbox.

 

[Fergie 307]

My Kity is the type with the motor next to the saw. The belt arrangement on these doesn't lend itself well to modifying it to run much slower.
However they did do a version with an integral stand, where the motor sits underneath. This type is actually far more common than the one I have. These are ideally suited as there is plenty of room for the large pulley you would need on the wheel.
Not a great picture, but you get the idea. You can pick these up for around £80-100. 612 and 613 are very similar.
Modifying it would be pretty easy.
If you think a bandsaw would do the job then this is potentially a cheap solution.

 

[GrahamRounce]

No. Absolutely not.
Skipping a load of details, a VFD basically maintains a constant ratio between frequency and voltage at it's output.
If it didn't reduce voltage as it reduced frequency to slow the motor down, any motor would overheat and burn out.
The result is that power is essentially proportional to frequency / speed.
As you slow the speed down, the power is reduced.
So at 10% speed you have about 10% power. Near useless.

As I said, a VFD is not a gearbox.

"Skipping a load of details...
...If it didn't reduce voltage as it reduced frequency to slow the motor down, any motor would overheat and burn out."

If I recall school science correctly, the ac "resistance" (reluctance?) of an inductor goes up with the frequency, and vice versa. So at a reduced frequency the coils would let more current through?

So a VFD reduces the voltage at the output to stop that happening, and keeps the current sub-burnout, ie at the normal level?

But the magnetism that makes the rotation is dependent on the current. If the current is the same at a reduced speed, surely so will be the magnetism and hence the strength?

I'm sure I'm wrong, but why? Thanks,

 

[GrahamRounce]

The problem is that a wood saw is just far too fast, you need to aim for a blade speed somewhere in the region of 200 ft/min for steel, so typically around 1/10 of the speed it normally runs at.
I was in a similar position using a Kity with a metal cutting blade. I was able to change pulleys to get it to run at about 1/3 speed, but still much too fast really.
I ended up converting a little Clark CBS 190 by replacing the motor with a 12v 350w one intended for an electric bike. It has a built in 9:1 reduction gearbox, together with a variable speed control. I had to make a new gearbox output shaft to suit, but got the motor from a local guy who repairs electric bikes. The screw on the end of the shaft that holds the sprocket was knackered so scrap as far as he was concerned, and so free!
As I was going to replace the shaft anyway it didn't matter to me. I paid about 15 quid for the saw on e bay. I already had some big mains to 12v DC power supplies so the only other thing I had to buy was the speed controller, which was less than £10.
I put the power supply and electronics in a project box on the back and the speed controller has a pot and LCD, which fits neatly in the space previously occupied by the on off switch.
Very small, and not a great quality machine, but works a treat for cutting the various steel and brass plates I use for repairing clocks. It will happily cut up to about 5-6mm steel and 12mm aluminium using tuffsaw bimetal blades.
So as an "I wonder if" type project I am quite pleased with it, and so is my Kity !
The other issue is that if your machine uses rubber tyres then metal will get embedded in them, and very difficult to get rid of, and may then get into any wood you cut with it subsequently.
My Kity is an old 612 and doesn't have tyres.

This seems like the best way, apart from the rubbery tyres, which my CB250 (identical to the Clarke, it looks like!) has, but I'd hope to live with that.

Does the speed now vary up to the original woodcutting speed & power, so it can be used for both?

How's the noise difference?
Thanks again,

 

[Sideways]

I'm sure I'm wrong, but why? Thanks,

You did ask ... and it's still skipping a few details because vector control VFDs are more subtle, principally at slow speeds, but this doesn't change the basics.

https://electronics.stackexchange.c...duction-motors-controlled-with-a-constant-v-f has a good enough explanation, but just google for "why constant v/f" for many more explanations with more or less of the maths.

"
Magnetic cores saturate when the flux density reaches a certain limit, determined by the core material, size, and geometry. When the core saturates, the windings stop looking like an inductor and start looking like a wire. This generally makes the current through the windings shoot up dramatically, resulting in higher losses and possible hardware damage. And since there's no more magnetic flux being stored in the core, no actual useful work is being done with that extra current. Saturating the core, in general, is a bad thing.

A given coil around a core will have a maximum volt-second product; this tells you the point at which the core will saturate. If the volt-second product for a winding was 10 Vs, You could apply 1V for 10 seconds, or 10V for 1 second, or 100V for .1 seconds, and the flux density would end up the same. (V=L di/dt, so if L is constant, and the product of V and dt is constant, di is also constant.) Since a motor is a winding around a core, the motor also has a volt-second product limit. Once you reach that limit, you have to reverse the voltage and flux up the core in the opposite direction to keep doing any useful work.

Since we're applying a sine wave to the motor winding, the half-period of that sine wave is the seconds in our volt-second product. If the frequency goes down, the period goes up, so the volts must go down to keep the volt-second product the same. If the frequency goes up, the voltage can also go up, because the period went down. Since frequency and voltage vary in the same direction, we can say the volts-per-hertz remains constant.
"

 

[guineafowl21]

"Skipping a load of details...
...If it didn't reduce voltage as it reduced frequency to slow the motor down, any motor would overheat and burn out."

If I recall school science correctly, the ac "resistance" (reluctance?) of an inductor goes up with the frequency, and vice versa. So at a reduced frequency the coils would let more current through?

So a VFD reduces the voltage at the output to stop that happening, and keeps the current sub-burnout, ie at the normal level?

But the magnetism that makes the rotation is dependent on the current. If the current is the same at a reduced speed, surely so will be the magnetism and hence the strength?

I'm sure I'm wrong, but why? Thanks,

*reactance, not reluctance. Otherwise, you have it right.

Since power is proportional to torque and speed, reduced speed means reduced power, given constant torque.