Whats the best ratio for 4th axis

[Georgie]

I'm looking to build a 4th axis using T5 timing gears,
I have chosen 36 tooth and 20 tooth giving a ratio of 1.8:1

I could have chosen 60 tooth and ten tooth giving a ratio of 6:1

Which is best ? Which is most common ?

Or is there a better and more common ratio to what I'm proposing to use ?

Thanks in anticipation.

George

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[mcphill]

Which is better, Ford or Chevy?

Your question is kind of unanswerable. It will depend on your machine, the motor for the 4th you use, the design of the 4th, what materials you intend to machine, and so on. I don't see any way to give a "one size fits all" answer to a question like this...

[Georgie]

Which is better, Ford or Chevy?

Your question is kind of unanswerable. It will depend on your machine, the motor for the 4th you use, the design of the 4th, what materials you intend to machine, and so on. I don't see any way to give a "one size fits all" answer to a question like this...

Thanks for the response,
My machine is a 3 axis router, bed size 1100x600. running Mach3. 1.5kw spindle with max speed 24000rpm. Max collet size is 7mm shank.
I'm mainly cutting wood but would also want to cut aluminium.

George

[ger21]

You need to answer about 50 more questions.
What's the biggest diameter part you machine on it?
What resolution do you want with the biggest part?
How much will it weigh?
How fast do you want to accelerate?
And to what speed?
How fast do you want it to spin with the smallest part you'll machine?

Motor?
Drive?
Power supply?

....?

[Torsten]

I'm looking to build a 4th axis using T5 timing gears,
I have chosen 36 tooth and 20 tooth giving a ratio of 1.8:1

I could have chosen 60 tooth and ten tooth giving a ratio of 6:1

Which is best ? Which is most common ?

Or is there a better and more common ratio to what I'm proposing to use ?

Thanks in anticipation.

George

You want to configure so all your slides will be able to hold/push at about the same force. Your weakest Axes will set the limit of how fast or hard you can cut.
This will also be true for a 4th Axes.
If this is to be a rotary axes the most important criteria to consider will be the maximum radius or diameter that cutting would apply to.
Calculate the circumference of that radius or diameter and match that to be equal to the motor power and transmission ratio of your other slides.

Once the ratio is known you can calculate the speed of the rotary table based on your motors usable rpms.

Evaluate your setup, may make adjustments to the expected cutting radius or motor power then recalculate until completly happy with results!!
Good Luck

[Georgie]

You want to configure so all your slides will be able to hold/push at about the same force. Your weakest Axes will set the limit of how fast or hard you can cut.
This will also be true for a 4th Axes.
If this is to be a rotary axes the most important criteria to consider will be the maximum radius or diameter that cutting would apply to.
Calculate the circumference of that radius or diameter and match that to be equal to the motor power and transmission ratio of your other slides.

Once the ratio is known you can calculate the speed of the rotary table based on your motors usable rpms.

Evaluate your setup, may make adjustments to the expected cutting radius or motor power then recalculate until completly happy with results!!
Good Luck

Thanks,

Looks like I have opened up a rats nest here. All I know is I want to try and build the 4th axis (Rotary). I've seen many on utube and some on ebay so I'm looking for something similar.

I will use a Nema 23 with a holding torque up to 3NM and a 1.8 degree step angle. (M60STH88-3008DF) I have chosen a timing gear ratio of 1.8 to 1 which will give me 1 degree per step ( I think)

I was trying to find out what the differnce between 1.8:1 and 6:1 would be
I think that 1.8:1 would need 200 pulses per 360 degrees and 6:1 would only need 60 pulses per 360 degrees. A ratio of 6:1 would also result in a drop in speed.

I realise with harmonic drives and ratios of 50:1 we can get much higher accuracy, but that's a a price.

As for what sort of diameters I will be machining, well under 6" I would think.

[ger21]

People are constantly asking what motors to use for their machines, and the reality is that it's an impossible question to answer, because 99% of those asking can not provide the information required to properly choose motors. Anyone who tells you is purely guessing, unless they they've used a similar setup themselves.

Be aware that with a 6" diameter part, your 1.8:1 ratio will give you .05"/step, which is not very good resolution. 6:1 will give you a little more than 3 times better resolution and holding force.

[Georgie]

People are constantly asking what motors to use for their machines, and the reality is that it's an impossible question to answer, because 99% of those asking can not provide the information required to properly choose motors. Anyone who tells you is purely guessing, unless they they've used a similar setup themselves.

Be aware that with a 6" diameter part, your 1.8:1 ratio will give you .05"/step, which is not very good resolution. 6:1 will give you a little more than 3 times better resolution and holding force.

Thanks Gerry, thats the quality info I'm looking for. Looks like 6:1 may be better for me after all.
Maybe I've got it wrong, I was thinking that the more pulses from the stepper to turn the job through 360 degrees was better.
Using a 1.8 degree/pulse stepper and using a gear ratio of 1.8:1 then I reckon that it would take 200 pulses to turn the job through 360 degrees. I.e 1 pulse = 1 degree of rotation.

However using a 1.8 degree/pulse stepper and using a gear ratio of 6:1 then ...... 1200 pulse per 360 degree of rotation ?

Hmmm, I'm getting a bit knotted here, help

Hold it, I think I have it worked out now. Using a 6:1 ratio would be 648 steps per 360 degrees of rotation. Is that correct ?

[ger21]

No, 6x200 = 1200 full steps per revolution. Most likely, your drives will have microstepping, which will increase the number even more.

[Georgie]

No, 6x200 = 1200 full steps per revolution. Most likely, your drives will have microstepping, which will increase the number even more.

Thanks Gerry. I got it now.

George

[jimmyjames1981]

1.8:1? If i were building a 4th axis i would gear it down considerably more, like 6:1 or even more, theres no reason to spin a 4th axis that fast, and the more you gear it down the higher the resolution you will have in your part and also the higher accuracy you will have. I dont think i have ever seen a 4th axis spin faster than maybe 100rpm.... say your optimal speed for the stepper is 600rpm, and any of your cutting speed will be far less than probably 20rpm, so i would gear it down between 15-30:1 with a gear box. again theres no need to spin a 4th axis at 3-500rpm...

[CaptainVee]

Let me approach it from the other side.

I built one with a 1 Nm motor that has a reduction gear attached. I thought a reduction gear would add strength to the setup.

I was wrong. I needed speed because I'll be running it for lathe duties in plastic and found that my surface speed was too low, melting plastic and burning wood (for practice).

I did not realise that when you run steppers at high revs torque goes out the window. When the thing runs at speed I can just grab it, even though the 8.something reduction is further reduced 50:20....

So I say, start with what you have in mind now.

[jimmyjames1981]

Well 99% of the time a 4th axis on a mill is not used as a lathe, if you gear up the stepper to say 5:1 the resolution is going to be horrid....

[Uguessedit]

Old post but my two cents for others. Rarely is a 4th axis used as a lathe and if so a mechanical brake is typically put in place. I’ve seen low ratios work fine and there is even a company running a 1:1 now with a harmonic drive and servo which has amazing resolution, low torque, and high speeds. Steppers are not my first choice but if you have no room to gear otherwise buy yourself a planetary adapter made in many ratios to further compensate. I have a 4th axis indexer converted to hob Gears. It is a 2:1 belt drive but a 50:1 planetary adapter giving me 100:1 besides I can also change if I feel the gears and belt. This indexer also doubles as a rotary table, it can have a lathe chuck, Morse taper, or 8” face plate depending what I am doing. The high resolution is important unless you have expensive electronics to compensate. If you want turning capabilities have a separate unit just for that and don’t use it for anything else. That’s my two cents on an old post for new readers searching.

Thanks,

Looks like I have opened up a rats nest here. All I know is I want to try and build the 4th axis (Rotary). I've seen many on utube and some on ebay so I'm looking for something similar.

I will use a Nema 23 with a holding torque up to 3NM and a 1.8 degree step angle. (M60STH88-3008DF) I have chosen a timing gear ratio of 1.8 to 1 which will give me 1 degree per step ( I think)

I was trying to find out what the differnce between 1.8:1 and 6:1 would be
I think that 1.8:1 would need 200 pulses per 360 degrees and 6:1 would only need 60 pulses per 360 degrees. A ratio of 6:1 would also result in a drop in speed.

I realise with harmonic drives and ratios of 50:1 we can get much higher accuracy, but that's a a price.

As for what sort of diameters I will be machining, well under 6" I would think.

[generaldisarray]

FWIW
My Troyke Nc8 model rotary has 180:1 reduction with a 2000ppr encoder on the servo motor.

[asuratman]

What do you think the best ratio on gearbox reducer for 4th axis with NEMA 30 stepper motor ? I am going to use 1:30 ratio. Do you think higher ratio have better resolution ? I want to use 5" rotary.

[Goemon]

What do you think the best ratio on gearbox reducer for 4th axis with NEMA 30 stepper motor ? I am going to use 1:30 ratio. Do you think higher ratio have better resolution ? I want to use 5" rotary.

Nobody is going to be able to answer that question. There is no "best" ratio even if you have all the info on the machine, motors and the application.

I tried digging into this question while shopping for my gear head but I got the sense that few people here (if any) have a deep understanding of the topic (it's complicated). People seem to find out if the ratio was enough for their application and motor choice through trial and error.

In this type of scenario, I like to base my choice on what manufacturers of other machines are using for similar purposes. Most rotary tables (for milling) on eBay have ratios of around 90:1. low end manufacturers like Sherline use about the same for their CNC rotary axis.

I ended up buying a gear head made for a Fanuc Robot with a 119:1 ratio. I don't know if that is the "best ratio" but I think it will work.

The ratio is just one factor though. They need to handle the torque, the part weight and have sufficient precision / low enough backlash for the application.

[asuratman]

Yes, its difficult to answer. I finally will use 1:30 ratio with nema 23 425 oz in stepper motor. I will see what will happen.

[Goemon]

Yes, its difficult to answer. I finally will use 1:30 ratio with nema 23 425 oz in stepper motor. I will see what will happen.

I guess a lot will depend on how you plan to use it and what your expectations are.

I believe people use lower ratios like that when using the 4th axis at higher speeds for lathe-type work.

[mactec54]

Nobody is going to be able to answer that question. There is no "best" ratio even if you have all the info on the machine, motors and the application. .

This is an easy question to answer, the industry uses 40:1 has been the standard since rotary tables where invented, the machinery handbook you will find how the calculations are done for Gear cutting when using a 40:1 and many other operations

This does not say you have to use 40:1 the Megatorque Motors / Spindles are 1:1, for anything hobby for a rotary table I would not expect anything below 20:1 would not be of much use, if you where only doing Engrave where there would be very little loading then anything would work