How to slow down RPM of miter saw?

The saw is designed to run at a certain RPM and blade size. Changing either is not recommended. I know I didn't answer your question. My response was in the name of safety.

John Doe prodded the keyboard

This would depend upon the type of motor ! If we assume a universal motor then a thyristor or triac phase controler that can handle the maximum current plus 100% could be used. An induction motor would need a variac (variable transformer) or one of the electronic devices used specifically for this purpose. But I would doubt that what you have uses an induction motor.

--
Best Regards: 
                       Baron.

Have you ever even used a miter saw?

More often the design of a common inexpensive tool takes into consideration the cheapness of its parts. A more expensive tool might run at a faster or slower RPM and be designed to do the same thing, only better. Safety doesn't necessarily have anything to do with lowering the RPM. If you want to go on a crusade for miter saw safety, you should talk about wearing eye protection and keeping your hands away from the cut. In fact, I am using it for aluminum. Obviously that's not what it's designed for but that happens to be what many of us, including professional metalworkers, use miter saws for.

Miter saws are not for people who are overly concerned about safety. They are inherently hazardous. It also isn't for people who have no basis for their understanding of safety. I've come across people who don't really know what they're talking about that say you should use gloves "in the name of safety", but that actually increases the risk.

I'm trying to solve a problem that has to do with a potential hazard. And it requires a lower RPM.

I think there is a safety consideration with lowering the RPM, but the reply author probably doesn't even know what that might be. If the idea were to increase the RPM, then there would be an obvious possible concern for safety that any Joe might recognize.

I just bought a cheap abrasive cutoff disc made by a well-known manufacturer that's supposed to cut metal. The thing doesn't cut aluminum and therefore trying to do so is a safety hazard.

Using a miter saw is not for the meek.

Tom Biasi wrote in news:Lxoew.792810$ snipped-for-privacy@fx05.iad:

If the rotor is AC/DC (most inexpensive power tools like drills, circular saws, and miter saws) a diode is an excellent choice to just drop some voltage and slow it down.

Since the thing can probably pull a few amps you want high current diode; 20+ amps would be my guess and whatever line voltage you run at.

AC/DC motors are easily distinguished because they make lots more noise than induction motors. AND you are saying that you have 5500 RPM? That is too fast to be an induction motor on 50 or 60 cycles.

Yes, I'm sure it's an AC/DC motor. It's a cheap miter saw. So if it's an AC/DC motor, using a diode drops the voltage which in turn reduces the RPM? Using a diode won't just reduce the power?

It's obviously the easiest possible attempt, that and providing a simple (properly rated) switch to change between the two power levels.

Unplug it! Do you see what lack of detail does to the quality of the answers you can expect.

That will probably work, but this path (reducing supply voltage) is a good way to burn out the motor. if you follow this path your warranty is void.

If this is an important tool, replace it with one designed to run at the correct RPM.

--
umop apisdn

Yes it will reduce the "power" since power is RPM versus torque. You will reduce the RPM to something less than half with less power (all things being equal) but the motor will still behave like an AC/DC motor and draw more current and produce more torque as the load tries to slow it down - something that won't happen with all simple lamp-dimmer triac/thyrister schemes to slow down a motor.

Yeah, single pole double throw center off toggle switch would be a good choice providing you can't accidentally bump it on.

OR A diode and single pole single throw switch; a safer bet with the switch shorting the diode - shorted is full RPM, open half.

I put a large size lamp dimmer in place of a powerstat (variable transformer) in a laboratory centrifuge with no problems - but a centrifuge has a constant load, unlike a saw. My substitution saved several hundred dollars and a week of downtime - and was still working

5 years later. With lamp dimmers and AC/DC motors you can slow the motor down well enough with the dimmer, but unless the dimmer is designed to handle inductive loads, the torque falls off as the load increases and so the motor slows down faster than the load increases. Undesirable in a saw...

starting (stall) current could be 100A or more.

Pretty-much all types of circular saws have some sort of gearchain or belt drive between the motor and the blade, the blade rarely runs at the same speed as the motor. One reason for this is to offset the bulk of the motor from the plane of the blade axle, allowing deeper cuts with a given size of blade. So, blade speed is not indicative of motor speed.

--
umop apisdn

I don't know about New Zealand, but in United States English the expression "slow down" does not mean "stop". And in fact I am not asking for a specific RPM, therefore that specification is not needed, as the helpful replies already indicate.

As for "voiding the warranty". I can void a warranty before even touching a product...

-- Jasen Betts wrote in news:m5dd6r$c6d$2 gonzo.reversiblemaps.ath.cx:

Thanks. Was wondering about that, too.

If the motor stalls, it wont cut very well, either. And then there is the fact that fuses/breakers tend to blow...

Gearing between the motor and the blade makes no difference since the specifications are for the motor. If the troll wants people to be specific, it should at least pay attention to the given information...

-- Jasen Betts wrote in news:m5ddup$egh$1 gonzo.reversiblemaps.ath.cx:

I won't even begin to address such a nasty rant against someone who only pointed out some basic safety concerns. Yes I have used a miter saw. They are not inherently dangerous if the safety instructions are followed.

Only if it's a universal motor, and you want to do it at the motor lead connections, in case you have something else in the saw that expects AC.

14 amps isn't a small amount so the selected diode will be a stud type of some heat mountable type.

Jamie

You could always use a variac.

Jamie

I will check that.

--

Thanks to the replies.

** Does it really pull 14 amps ?

A standard triac light dimmer will do the trick - but it must use a 16amp triac at least.

Or a nice, big Variac.

** Crude and will only drops the speed a small amount.

... Phil

One hopes that it wouldn't be expected to run stalled for very long, and the inrush/peak current of diodes is much higher than steady state

- since we see the same thing on linear power supplies with large electrolytic capacitors.

1N4XXX series of diodes are good for 1 amp and can carry a surge current of 30 amps.

You might be correct with table saws where they use an induction motor and keep a small sheave at the blade for that reason, but AC/DC motors used in bench top table saws, hand held circular saws, miter chop saws, etc. are direct drive (blade goes on the motor shaft - and frequently the field and windings are offset a bit so the motor shaft is not concentric with the housing, but of course, still concentric with the field iron)

On the subject of safety...

Here you have a major retailer Lowe's hardware showing a guy wearing gloves while using a miter saw...

Hover your pointer over "Ideas and How-tos".

In fact, as the vast majority of professionals will tell you, wearing gloves while using such a power tool is a bad thing.

Stuff unnecessarily amputated...

At approximately 11:30 a.m. on July 8, 2010, Employee #1 was operating a CTD model M25R chop saw (powered miter box type saw) and cutting aluminum I beams. She was wearing gloves when the saw blade caught the gloves and pulled her fingers into the blade. The blade amputated her left thumb, and index, middle and ring finger at the distal interphalangeal joint. Additionally, the company had rules about not wearing gloves while operating the chop saw. After the accident, she was transported to a medical center where she received treatment and was hospitalized.

Every power miter saw I have seen in the last 25 years had a universal motor. They also have a circuit that dynamically brakes the motor when the trigger is released. I wonder if that circuit would have any negative effect on certain speed controllers. Probably not. In any case universal motors have poor speed regulation. And lowering the voltage or current significantly my make the speed regulation really bad. As I recall from reading about it this is partly because the brushes are located in the best place for the least amount of sparking and changing the voltage or current will change this location. Eric