Opening Questions

[yydoctt]

Hi I'm very new to the world of CNC. I make a lot of prototype models (Product Designer), materials range from blue/yellow foam, to ureal, to wood.

I've looked at Joe's gantry router and although its impressive it wont do what I'd actually want without the 4th and perhaps 5th axis.

I can, in most cases get away with a 4th axis which leads me to the question is it as simple as buying a rotary table and adding a stepper motor? backlash issues etc?
Therefore how hard would it be to make a trunnion table? two stepper motor controlled rotary tables attached?<-- really hoping it could be this simple..

I know very little about the whole thing and perhaps should find somewhere to read up on the basics, but if someone could tell me how much a setup like this could cost (based on me producing most of it) and answer the previous questions I'd be very grateful

Regards

[yydoctt]

So a bit of google searching told me that simply adding a stepper motor to a rotary table wont work due to the lack of brakes! Is there anyone who's fashioned a set of 'brakes' or made a 4th axis from scratch?
Purchasing a 4th axis doesn't appear to be an option for the immediate future.

[MrWild]

Better to use a servo as it gives force against motion

[ger21]

Therefore how hard would it be to make a trunnion table?

That would depend on your abilities and what you have to work with. MIGHT be really easy for some, and impossible for others..

With enough gear reduction, a stepper will work connected to a rotary table, especially when your cutting foam. Really depends on the cutting forces involved. But, backlash may be an issue.

A big factor with a 5 axis machine, is software, which typically can run upwards of $10,000.

[yydoctt]

Thanks for your responses. I've been googling more and seen using worm gears can zero the backlash or minimise it. As you pointed out Gerry it is way beyond what I know, but am capable of learning.

Taking a rotary table, changing it to worm gears and adding a stepper or servo could work? I wont worry about the trunnion, well not untill I've actually made a cnc machine and a fourth axis!

For my CNC it's going to be mounted as an enclosed box (foam is messy), the dimensions undecided (will make a solidworks model at some stage). I plan to have the gantry running along the ceiling, hanging down. The fourth axis will be standing vertically. Hopefully when I've done all of that I will know how to make a zero backlash trunnion.
The software I will use will be the cnctoolkit or perhaps mach3 - based on what I've read so far.

Regards

[ger21]

The software I will use will be the cnctoolkit or perhaps mach3

You'd need both. CNC Toolkit creates g-code, and Mach3 reads the code and controls the machine.

[yydoctt]

ahhh.. my lack of knowledge must be pretty apparent! I guess I'll read up all I can before I do any more 'planning'. I think I just needed to know whether I could get a low cost 4th axis or not, as the whole thing is pretty pointless without.

So could someone please clarify that using worm gears, a rotary table and a stepper motor will produce a rotary axis with minimal backlash, suitable for foam, ureal and wood?

Regards

[CarveOne]

Originally Posted by yydoctt ahhh.. my lack of knowledge must be pretty apparent! I guess I'll read up all I can before I do any more 'planning'. I think I just needed to know whether I could get a low cost 4th axis or not, as the whole thing is pretty pointless without. So could someone please clarify that using worm gears, a rotary table and a stepper motor will produce a rotary axis with minimal backlash, suitable for foam, ureal and wood? Regards

Rotary tables will always have some amount of backlash. Machinists usually compensate for it by always rotating in one direction when drilling a hole pattern. Typically, they have a 90:1 ratio, but some have different ratios. There is usually no adjustment for minimizing the backlash. High end units made to close tolerances are very expensive.

For CNC uses the gcode will need to rotate the rotary table in one direction as well. Reversing the direction during cutting operations will make the backlash very apparent in the resulting cuts. The material being cut will have little impact on the backlash because it's a function of the worm to gear tolerances.

You would be better off using a direct drive stepper (or servo) to rotate a part, or use a timing belt drive arrangement.

CarveOne

[jsheerin]

Random thoughts:

You can use a harmonic drive gear head which has (in theory) zero backlash. In reality, the backlash should be very small. Of course the downside is that these are expensive. For less money you can get a planetary gear head which will have a bit more backlash - how much depending on the model.

Some drive systems are self locking - they can't be back driven. Worm gear systems are usually like this (I'm not an expert on this area, so research it on your own to confirm). Rotary tables typically use a worm gear setup inside them already, but the problem as others have noted is that they have some serious backlash and are made to only be turned in one direction for precision work. I have a dividing head like this. Pics here:


The input is to the worm gear on the right, and the output is from the pinion gear.

I would disagree strongly about using a stepper to directly drive a part being the best way unless the part is very small or the stepper is very large, and the cutting forces are minimal.

[hemsworthlad]

Go have a look at Hoss's trunion table here.
http://www.hossmachine.info/forum/ya...Table.aspx#139

and his 4th Axis here.

http://www.hossmachine.info/projects_5.html#4th%20axis

[yydoctt]

CarveOne -
thank you for your suggestions, I have been googling recently and seen builds as you have described. Direct drive seems to be a good-value way to do this. I do however need to find a build where I'm told exactly what parts are required and why. Machining and designing it wont be a problem as long as I know what it is I'm designing and why. So back to google for a highly detailed build search! Sorry my knowledge is that limited!

JSheerin -
When doing a search for 4th axis builds I came across a thread (i think on this forum) where people had used harmonic, planetary or dual worm gear drives to reduce backlash. As you said harmonic drives came out top. I found this amazing build when googling last night:
http://www.franksworkshop.com.au/CNC...is/4thAxis.htm

The issue is the expense. I looked on eBay and the Uk site aren't selling any for starters, which isn't necessarily a bad thing as importing often works out cheaper, depending on weight. Do you know how powerful the drive would need to be, as the price range was vast?
I see in your pictures you are using worm gears but it doesn't look powered?

Also, if I were to make a trunnion table, the 4th axis could be as large as needs be, but how would I make a low profile 5th axis to sit on the table?
I'm very keen on the prospect of harmonic drives so thank you for the information!

hemsworthlad -
I have bookmarked that trunnion build, great find, thank you!
That is certainly the first step towards a possible 5th axis. Like I said above I would need a low profile axis running on the trunnion table though.
Anyone with any ideas or links?

Regards

[ger21]

Direct drive seems to be a good-value way to do this.

I also would not recommend direct drive. It might work OK for foam, but will be underpowered for wood. You probably want at least a 10:1 reduction. This can be done with a double belt drive.