Author Topic: To the dark side CNC  (Read 19276 times)

Offline jadge

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Re: To the dark side CNC
« Reply #75 on: August 13, 2015, 10:15:18 PM »
It is interesting that my (commercial) 4th axis drive has a stepper motor directly connected to a worm and worm wheel, as per most manual rotary tables. While that limits the maximum rotary table speed to about 5 rpm it has the advantage that the cutting forces are not directly opposed by the motor holding torque but by the worm and worm wheel. It never occurred to me to reduce cutting parameters when using the rotary table. When I roughed out the worm shown previously I used speeds, feeds and DOC that I would have used in 3-axis mode for the selected cutter.

It also means that the resolution (if not the accuracy) per step of the motor is about 1 minute of arc.

Andrew

Online Vixen

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Re: To the dark side CNC
« Reply #76 on: August 13, 2015, 10:44:12 PM »
Andrew

My first 4th axis unit was also manual rotary table. It was a poor quality thing and I could never get rid of the backlash in the worm and wheel. That is why I am about to make a 4th axis spindle, based on a small lathe headstock. My mill is only a small desk top EMCO F1 so I have learned  from day one. the art of reducing the cutting parameters to live within my machines capabilities, I can barely believe the feeds and speeds of your Tormach working gauge plate.

Are you far from Fareham in Hampshire?

Regards

Mike
It is the journey that matters, not the destination

Offline jadge

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Re: To the dark side CNC
« Reply #77 on: August 14, 2015, 11:46:16 AM »
Mike: Here's a general shot of my 4th axis rotary table and tailstock. It's not as rusty as it looks, just a consequence of the camera settings:



My CNC mill is nominally the same horsepower as my manual Bridgeport. So my expectations were that it would be comparable in terms of removal rates, with some caveats of course. The CNC spindle speed is much higher than the Bridgeport, so I tend to run smaller cutters at faster speeds than on the Bridgeport. This also helps as the CNC mill is not as heavy, and hence rigid, as the Bridgeport. The other main caveat is the CNC spindle is driven by a 3-phase motor from a VFD, whereas the Bridgeport is a 3-phase motor via a varispeed belt drive. So the Bridgeport is essentially constant power, whereas the CNC spindle is constant torque (not constant power) below the base speed of the motor. I've made a note of what rpm corresponds to base speed and take this into account when selecting cutting parameters.

I live near Cambridge, the closest I get to Fareham is Horndean as I have an uncle who lives there. However, a couple of times a year I go to a traction engine meeting south of Reading, which is probably a bit over half way.

Andrew
« Last Edit: August 21, 2017, 10:50:34 PM by jadge »

Offline MMan

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Re: To the dark side CNC
« Reply #78 on: August 14, 2015, 07:35:03 PM »
Thanks All,

I have changed my milling operations to be a roughing operation (parallel), leaving about 0.5mm. Followed by a scallop op to finish the shallow surfaces and a contour to finish the sides, both with small step overs. I have also changed cutter to a bull nose, although it will probably not make as much difference now the step overs are reduced. Cutting time is still not too bad at 2 hours.

I have added one more op which is a 2d operation to separate the work from the stock. This needed tabs and the only op that can create tabs is a 2D Contour which is much like a Profile in CamBam. In order to use this I needed a 2D outline for it to follow. You can create a 2D sketch from a 3D body with Project in the Sketch menu. The sketch is then visible in CAM and can be selected to create an outline. This might be a useful way of getting at 2D operations for other reasons, don't ask me what for but I am new to 3D and not giving up on 2D yet.

I will post a pic once I get some more workshop time to have another go. This could take a while, our baby daughter takes most of my time at the moment,

All the best Mman.

Offline Dave Otto

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Re: To the dark side CNC
« Reply #79 on: August 15, 2015, 01:10:02 AM »
Nothing wrong with extracting 2D geometry from your solid to be used in tool paths; I do it routinely both at home and the day job. Sometimes with a CAM program you have to go after it with a big hammer to get it to do what you want.  :lolb:

Dave

Offline MMan

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Re: To the dark side CNC
« Reply #80 on: August 25, 2015, 12:38:00 AM »
Thanks Dave.

I got some time today and so recut with a bullnose cutter and the modified CAM settings, including the 2d operation. It is a whole lot better. The only thing I would change if I was to do it again is there are two finishing passes, one for the near horizontal and one for the near vertical and they overlap leaving me with patterning. There is probably a control for it but I did not see it (or understand its use).

For this application the finish is fine (the previous version would have been too, except I needed the 2d op to cut down and through). A quick rub with a hand pad and it will be perfect.

Mman

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Re: To the dark side CNC
« Reply #81 on: August 25, 2015, 10:14:33 AM »
Mman, That looks like a fine piece of machining. 
When the surface is bright and shiny and newly cut it, always seems to emphasise the scollops. Sometimes you can see them but not feel them. Give it a rub over with a Scotchbrite pad and it will look completely different, much smoother.
It is always a trade off when it comes to the finishing pass, you can half the final step over distance to try and improve the surface finish at the cost of doubling the machining time. Even die makers need to hand finish to achieve the final mirror smooth surface finish.
It is the journey that matters, not the destination

Offline MMan

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Re: To the dark side CNC
« Reply #82 on: August 25, 2015, 01:15:27 PM »
Thanks Mike,

It certainly shows me that Fusion 360 works and that I can start moving from 2D to 3D.

Mman.

Offline kvom

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Re: To the dark side CNC
« Reply #83 on: August 25, 2015, 01:54:53 PM »
Thanks Mike,

It certainly shows me that Fusion 360 works and that I can start moving from 2D to 3D.

Mman.

Each has its place, and it's usually wrong to do 3D machining where 2D will work.  3D modeling works for both.

Offline MMan

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Re: To the dark side CNC
« Reply #84 on: August 25, 2015, 05:45:30 PM »
Hi Kvom,

I completely agree, I was very relieved when I found the 2D operations in the CAM section.

All a question of picking the right tool for the job - much like the CNC vs. manual discussions. Both have their pros and cons and in both cases we have to work out how to use them to make what we want.

All the best,

Mman.