A and C axis for a 5 axis milling machine

[Goemon]

Is it not better to get a gear box (harmonic or otherwise) with a shaft output and then connecting it to a decent rotary table (bypassing the rotary table's built in gears)?

It seems like connecting the chuck directly to a flange output HD would really limit the size of chuck you could use. Most of the used HDs on eBay are fairly small. Would you be able to connect an 8" or even a 6" chuck to a 4" HD flange? Also, it seems like it would require a custom face plate (which I am not capable of making with any precision).

It's annoying that there doesn't seem to be a single good deal on a complete used HD on eBay. I haven't seen any that look like the ones other people have used to make a 4th axis that come complete with housing and a way of mounting etc. Maybe I just missed the boat on this one.

I need to investigate what other types of gearbox are available with both low bachlash and are available in larger sizes. I should probably start by trying to understand what the backlash ratings mean. E.g., the gearboxes sold on the Automation Technologies site have backlash rating numbers but I don't know what a BL rating of 7 actually means.

Everyone seems to describe their gearboxes as "precision" - the most over-used and least usual description in the CNC world...

[RCaffin]

I very deliberately chose a hollow shaft version of the range so I could run a length of rod through it - just like on a lathe.

No, I would probably not suspend a an 8" chuck off a 4" flange if I could avoid it. 5" was OK in my case, but I think 4" would have been better.

Custom face plate: yes, that is needed. No choice.

As for getting exactly the HD you want - you may have to camp on eBay for a bit. They do turn up, but it is a bit of a buyer's market.

If there were other sorts of small high-reduction gear boxes with negligible backlash on the market, we would know about them. I don't know of any.

'Precision' - for a given meaning of the word.

Cheers
Roger

[dixdance]

My 8" chuck weighs 55 lbs, seems like a lot to hang horizontally on a 4" flange HD. Plus the forces involved in cutting metal are pretty significant. The flex spines look so delicate, or are those things sturdier than they appear?

I'm not too worried for my application, I'll only be taking very light cuts in wood, and my table will be horizontal, and supported by a large cross roller bearing. Seems like one of the smaller (cheaper) drives will be fine.

One question: some of the ebay HD's come with servo motors and drivers - is it asking for trouble to try to use a servo with Mach 3 when the rest of my motors are steppers? Instinctively ot seems like something for a beginner to stay away from, but is it really? Any big advantage to a servo, other than closed loop performance?

[skrubol]

One question: some of the ebay HD's come with servo motors and drivers - is it asking for trouble to try to use a servo with Mach 3 when the rest of my motors are steppers? Instinctively ot seems like something for a beginner to stay away from, but is it really? Any big advantage to a servo, other than closed loop performance?

Depends on the driver. Most modern servo drives can be driven by step/dir signals just like a stepper. If it's just a servo amplifier though, you need to close the loop externally which Mach can't do on its own (boards like kflop can do it, but they aren't cheap and adds quite a bit of complexity.)
Advantages of the servo depend a bit on the servo and the drive. You won't be getting completely closed loop with Mach, but even if it's closed in the drive, servos tend to be more accurate, more powerful for their weight/size, can be smoother, and won't lose steps (will fault if they fall too far out of sync, and Mach can use the fault signal to trigger e-stop, etc.) They're quieter as well, but the rest of your machine is steppers that won't really matter.

[RCaffin]

My CNC has (DC brush) servo motors on X, Y & Z axes. Works very well. The motors must have encoders on them as modern drivers are Step/Dir. I would expect the ones on eBay to be thus.

In reality, steppers are only used on the low end of the market. They do not have the power or speed of servo motors.

Cheers
Roger

[Goemon]

My 8" chuck weighs 55 lbs, seems like a lot to hang horizontally on a 4" flange HD. Plus the forces involved in cutting metal are pretty significant. The flex spines look so delicate, or are those things sturdier than they appear?

I'm not too worried for my application, I'll only be taking very light cuts in wood, and my table will be horizontal, and supported by a large cross roller bearing. Seems like one of the smaller (cheaper) drives will be fine.

One question: some of the ebay HD's come with servo motors and drivers - is it asking for trouble to try to use a servo with Mach 3 when the rest of my motors are steppers? Instinctively ot seems like something for a beginner to stay away from, but is it really? Any big advantage to a servo, other than closed loop performance?

I wonder how significant the cutting forces for softer metals like aluminum really are when using a high speed spindle with relatively small tooling. Nobody seems to know how much force is involved or how that data can be used to specify the required bearing size, motor and gear reduction ratio. Most of us appear to be in the trial and error / assumptions realm.

I looked into the benefits of AC servos over stepper motors recently because I got some with some actuators I bought. There are a number of advantages. Specifically, they are available in larger sizes / more power, faster speeds, higher resolution and the closed loop feedback thing. People don't agree on how noticble these benefits are over a properly sized stepper.

The major disadvantage of AC servos is the cost and complexity. Having just tried to piece together what I need to run my AC servos, I would say that they are absolutely something that beginners on a budget should stay away from. If money is no object then that's a different matter as you can buy them new and ask the vendor to include all the right cables and drives.

[Goemon]

I very deliberately chose a hollow shaft version of the range so I could run a length of rod through it - just like on a lathe.


If there were other sorts of small high-reduction gear boxes with negligible backlash on the market, we would know about them. I don't know of any.

'Precision' - for a given meaning of the word.

Cheers
Roger

I'm not sure that necessarily follows. There are a lot of very expensive machines that use 4th and 5th axis rotary tables that don't use harmonic drives. Plus, there doesn't seem to be many (or any) gear reduction experts here.... I am going to assume that there are other options that will produce exceptable performance for my needs.

I'll keep an eye out for a worthy harmonic drive deal but I am leaning more towards buying a complete used 4th axis from an old VMC. I think that will produce the best results just from the fact that it doesn't rely on me machining a custom face plate... plus, I like that they are designed for heavy metal work so my aluminum jobs shouldn't challenge one.

[RCaffin]

There are a lot of very expensive machines that use 4th and 5th axis rotary tables that don't use harmonic drives.
I did mention (somewhere) that when the worm&wheel reaches a certain size, one can start to get a bit fancy about backlash elimination.

Alternatives to HD? Usually worm&wheel. Other options do not normally have the torque rating for a VMC.

Gear reduction experts? If you don't like the options offered, happy hunting.

Cheers
Roger

[Goemon]

There are a lot of very expensive machines that use 4th and 5th axis rotary tables that don't use harmonic drives.
I did mention (somewhere) that when the worm&wheel reaches a certain size, one can start to get a bit fancy about backlash elimination.

Alternatives to HD? Usually worm&wheel. Other options do not normally have the torque rating for a VMC.

Gear reduction experts? If you don't like the options offered, happy hunting.

Cheers
Roger

I'm not criticizing. I'm just saying that gear reduction backlash is a fairly specialist topic and that harmonic drives can't possibly be the only viable option as there are so many machines that do fine with other styles of reduction. It's just a fact and an obvious one.

[mactec54]

I'm not criticizing. I'm just saying that gear reduction backlash is a fairly specialist topic and that harmonic drives can't possibly be the only viable option as there are so many machines that do fine with other styles of reduction. It's just a fact and an obvious one.

Yes there are better options, the best one is the Megatorque Servo Motors direct drive, no gearing needed at all. most Servo Motor / Drive Manufacturer's offer them

Almost all manufactures that are building multi axes machines are using Megatorque Servo Motors which are direct drive, these are used also for lathe spindle for Mill Turn Machines

[Goemon]

Yes there are better options, the best one is the Megatorque Servo Motors direct drive, no gearing needed at all. most Servo Motor / Drive Manufacturer's offer them

Almost all manufactures that are building multi axes machines are using Megatorque Servo Motors which are direct drive, these are used also for lathe spindle for Mill Turn Machines

I was reading about some of the direct drive rotary tables with high resolution encoders. They seem to be the best option in terms of accuracy and simplicity (if you can find a setup you can afford). It raises the question for the diy builder; just how much torque and resolution do you really need to be able to produce acceptable results with 4 axis aluminum milling with a 1/4" end mill... I noticed that some guy on eBay is selling direct drive rotary tables (for CNC use) for a Nema 34 stepper motor frame. I assumed they were terrible and just skipped over them but I'm kinda curious of how well they work.

I have some 400watt AC servos with drives. I have been wondering if I could use one to (directly) drive an R8 spindle I have with an R8 shank lathe chuck.

In addition to direct drive and harmonic drive options, I found various gearbox manufacturers claiming to offer 100% backlash free gearboxes that are capable of withstanding high torque. There is also no shortage of articles explaining the various techniques for reducing or eliminating backlash on more traditional work gear drives.

Net net, there are various options which is good because good deals on suitable (and complete) harmonic drives are few and far between...

[RCaffin]

I found various gearbox manufacturers claiming to offer 100% backlash free gearboxes that are capable of withstanding high torque.
The URLs would be of great interest!

Cheers
Roger

[dharmic]

Just how much torque and resolution do you really need to be able to produce acceptable results with 4 axis aluminum milling with a 1/4" end mill...

More than anything else it depends on the radius of your workpiece and how you plan your cuts. If you can get away with indexing and you're working pretty much directly into and along the axis you can get away with very little - my machine has a throwaway 2:1 pulley and NEMA 23 motor 4th axis and it copes fine. But once I get off-axis or otherwise throw any significant tangential load at it, it skips. Can't even hold an indexed position, let alone drive continuous reliably. For my machine even 10mm off axis or a good heavy sideways cut on the top and I have problems.

Work out the cutting force of your heaviest expected work and applying it at the largest expected workpiece's diameter gives you your torque. Double it for safety. As for speed, well, if you're doing continuous then you want the smallest likely circumference's 'linear' feed rate to match your linear axes, ie divide your x/y/z max rate by (? x small diameter) and that's the revs per unit time you want in speed. Too fast and you are wasting money, unless you want a 4th that can act like a lathe spindle as well. Too slow and your fast linear axes are going to be waiting on it during coordinated moves.

[RCaffin]

I went to the other end of the spectrum: 0.001 degrees resolution and huge angular stiffness (50:1 HD), but a little slow. Well, I don't regret it.

Cheers
Roger

[Goemon]

I found various gearbox manufacturers claiming to offer 100% backlash free gearboxes that are capable of withstanding high torque.
The URLs would be of great interest!

Cheers
Roger

Just Google "backlash free gearbox". There is no shortage of results.

I can't verify any of their claims obviously but if they are to be taken at face value then there are plenty of options.

[Goemon]

More than anything else it depends on the radius of your workpiece and how you plan your cuts. If you can get away with indexing and you're working pretty much directly into and along the axis you can get away with very little - my machine has a throwaway 2:1 pulley and NEMA 23 motor 4th axis and it copes fine. But once I get off-axis or otherwise throw any significant tangential load at it, it skips. Can't even hold an indexed position, let alone drive continuous reliably. For my machine even 10mm off axis or a good heavy sideways cut on the top and I have problems.

Work out the cutting force of your heaviest expected work and applying it at the largest expected workpiece's diameter gives you your torque. Double it for safety. As for speed, well, if you're doing continuous then you want the smallest likely circumference's 'linear' feed rate to match your linear axes, ie divide your x/y/z max rate by (? x small diameter) and that's the revs per unit time you want in speed. Too fast and you are wasting money, unless you want a 4th that can act like a lathe spindle as well. Too slow and your fast linear axes are going to be waiting on it during coordinated moves.

How do you work out your heaviest expected cutting force? Doesn't it vary massively based on the end mill size, length, spindle power and speed, the depth of cut and the hardness of the material?

If I took all that information for a typical cut, I'm still not sure how I would convert it into a useable "cutting force" number so I could select the right size motor. A typical part for me would be 2" x 6" x 20" or 2" x 6" x 30" and all in either 6061 or hard woods. My spindle is 7.5hp / 24000 rpm 2.5nm of torque and I'll mostly use 1/4" or 1/8" end mills. How do I use that info to size a 4th axis motor?

Perhaps, if the rotary axis is to be used only for indexing (as it would in my case), the best approach would be to use an encoder (to ensure correct positioning) and then to use macros to operate a break while cuts are being made instead of relying on the motor's holding torque.

[RCaffin]

No shortage of results, but too often 'backlash-free' translates into 'well, just a few arc minutes'. On a CNC, arc-minutes of backlash are gross.

Some admitted that you need to do regular adjustment of the spring loading to maintain the zero backlash - those ones usually has very low torque ratings due to the spring loading.

A few did claim 0 arc-minutes, but they turned out to be rather big (min dia 500 mm in one case) and rather expensive. Many of them would not even indicate a ballpark pricing before being able to do an extensive sell campaign on you. A bit like 'if you have to ask the price, you can't afford it'.

I dream on.

Cheers
Roger

[dixdance]

Perhaps, if the rotary axis is to be used only for indexing (as it would in my case), the best approach would be to use an encoder (to ensure correct positioning) and then to use macros to operate a break while cuts are being made instead of relying on the motor's holding torque.

If you are just using the rotary as an indexer, that sure seems like the least expensive way to go. You wouldn't even necessarily need a low backlash rotary, just some way to load it and then keep moving in the same direction against the pre-load. The brake would handle the cutting forces, and a light weight rotary would be fine to just handle indexing.

[dharmic]

How do you work out your heaviest expected cutting force? Doesn't it vary massively based on the end mill size, length, spindle power and speed, the depth of cut and the hardness of the material?

Milling calculators (HSMAdvisor, FSWizard, G-Wizard etc) often tell you the expected cutting force based on work material, tool material, spindle speed, feed rate and stepover. They need this number to calculate tool deflection anyway.

FSWizard is telling me (for example) that a 1/4" two flute HSS cutter running flat out (8881 RPM) in 6061-T6 with a stickout of 16mm, DOC 11mm, WOC 2.78mm, feed rate of 1200mm/min will use 500W, 0.54Nm and cutting force of 17.5 kg (172N).

From there, it's just maths and basic principles - torque is force times distance from axis, reduction in speed almost equals increase in torque, etc.

Following the example, worst case, that 172N applied across transverse to the axis at full radius of 3" (say 75mm) from a diameter of 6" is going to generate 172*0.075 = 12.9Nm of torque. If it's along the axis it'll be zero. But for that example I'd want the 4th to be able to deal with at least 25Nm (or in stepper terms, 3500 oz-in). This is achievable with a 5:1 reduction pulley and 700 oz-in NEMA 34 stepper, for example.

Then, as Roger mentions, resolution becomes an issue. Again working on the max radius, if you had 6" (150mm) diameter pieces that's pi*150 = 470mm circumference. If you want a resolution of a thou at that size workpiece ie 0.025mm, you need at least 470/0.025 = 18,800 steps per revolution. Which in this example is going to be a bigger ask than the torque requirement.

Whether you're going encoders or steppers, you need the resolution. And nigh on 20k divisions or steps per rev is going to need some reduction. Can you get away with a belt? Do you need something like a harmonic drive?

You can say "Ok, I don't want to go nuts, give me a 1.8º step and run microstepping at 4" ie 800 steps per rev, means you need a reduction of 18800/800 = 23.5:1 or call it 25:1 which is in the realm of a 30:1 HD or a two stage 5:1 pulley and belt rig. At that kind of ratio, your 3500oz-in torque requirement becomes 140 oz-in and any old NEMA 23 stepper can run it.

Or, if you want to stay at a single 5:1 belt rather than two stages, you'd need to run 18800/5 = 3760 steps per rev which is a squeak over 16 microsteps on a 1.8º stepper or 8 on a 0.9º stepper - doable if it and its driver can manage 700 oz-in per step at 8 microsteps.

Don't forget that stepper torque seriously drops off as
- the speed goes up: what might be fine for holding indexed work might be rubbish for continuous fast work, and this gets worse the higher your gear/belt reduction ratios are; and
- the microstep count goes up.

So you'll need to do some iterations working out feedrates - in the example FSWizard gives us a linear feed of 1201.4mm/min which, at 150mm diameter, is 2.5rpm - or 62.5rpm on the stepper if you're running 25:1 reduction.

Get the idea? Calcs are a PITA but you only need to do them twice. Once, now, when you're planning your build and purchases. Then once again, when you put it all together and it skips like a kangaroo and lacks sufficient torque to pull the skin off custard and you need to work out what you got wrong the first time so you can do it again

Damn, you went and made me do maths over my first coffee on a Saturday morning. So the numbers are probably wrong.

[RCaffin]

it skips like a kangaroo and lacks sufficient torque to pull the skin off custard
I had to laugh. Good one.

Cheers
Roger