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### Author Topic: A-axis rotary lathe question for Mach 3  (Read 8397 times)

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#### halfmill

• 101
##### Re: A-axis rotary lathe question for Mach 3
« Reply #10 on: November 04, 2017, 03:28:50 AM »
whoops apparently you can only attach one jpeg... so here one and I will post the other...

#### halfmill

• 101
##### Re: A-axis rotary lathe question for Mach 3
« Reply #11 on: November 04, 2017, 03:29:28 AM »
and the last jpeg

#### joeaverage

• 5,604
##### Re: A-axis rotary lathe question for Mach 3
« Reply #12 on: November 04, 2017, 03:51:47 AM »
Hi Bob,
extremely nice work, but where do you plugin the big fat amp?...LOL

My guess when I asked you about what you wanted a rotary axis for was that the subtle curve in the bridge and nut were what you had in mind.
Any decent CAM program can follow a gentle curve like that to well within the resolution/accuracy of your machine. We are used to manual milling
and cutting a curve by co-ordinating two axes by hand is beyond a human operator. Not so for a CNC machine. A rotating axis could potentially offer
accuracy in line with the Truth In Rotation of the axis, a few um with care. I would expect a decent hobby router to achieve some thing like 0.02mm
or 20um if the curve were generated by co-ordination of two axes.

I imagine 20um would be considered pretty fair when compared to hand worked.

Craig
My wife left with my best friend...
and I miss him!

#### joeaverage

• 5,604
##### Re: A-axis rotary lathe question for Mach 3
« Reply #13 on: November 04, 2017, 04:10:02 AM »
Hi,
I believe a vertical router with a small diameter endmill probably with a modest radius if not a ball mill would make the bridge pictured easily. The resulting
curve would be close enough to circular that you couldn't measure the deviation from ideal. Of course you may decide to go with a parabolic or elliptical
shape, given how radical you are! LOL

Where fourth axis machining really pay dividends is things like truing cylindrical surfaces of engraving on a cylindrical surface.

You may have noticed that dude1 chimed in that Fusion 360 can't do continuous 4 axis but can index up to two for 3+1 and 3+2.

You can by the way attach up to four images in one post. When you attach one image click on <more attachments> to browse for another pic.

Craig

My wife left with my best friend...
and I miss him!

#### RICH

• 7,367
##### Re: A-axis rotary lathe question for Mach 3
« Reply #14 on: November 04, 2017, 09:02:03 AM »
What is a "Rotary lathe for A-axis"? ie; is it just a Rotary table and how driven?

RICH

#### garyhlucas

• 663
##### Re: A-axis rotary lathe question for Mach 3
« Reply #15 on: November 04, 2017, 12:59:19 PM »
Half,
Are you programming by hand or using a CAM program?  Do you use a 3D modeling program?  The big advance in CNC is really in 3D modeling and CAM programming. I have lots of hand programming experience but life is too short for that!  I use CamBam, both on my hobby machine at home and the big machine at work. I wouldn’t even think of making your bridge with a rotary axis, it’s just too much work that way. It is mostly a 2D job with a little surfacing of the curved part using a ball mill.

#### halfmill

• 101
##### Re: A-axis rotary lathe question for Mach 3
« Reply #16 on: November 04, 2017, 01:17:57 PM »
Thank you for your response.  Attached are 3 jpegs...

The "basic setup" is just to show how a mock up of what I am trying to do looks like... so you can see my general issue.  Now in a complete set up  I would have the wooden log with a 1 inch wide slot cut just deep enough along its length so that  the bridge blank would match up with a 3" diameter circle.  Then as the A-axis rotary turns it would cut the curve onto the wings of the bridge.  The "log" which is made of Spanish cedar is very light... slightly heavier than balsa, is 3 1/8" in diameter.  Now my first step is to turn the log itself and true out the log to be an exact 3" diameter.  So the log must turn at a speed that is slow enough that the end mill can keep up with it.  The mill end spindle runs up to 24,000 rpm and I simply turn it on manually for the time being while I am learning.  So I have to have the rotary turning at one speed and then the y-axis progressing down the length of the log at another speed.  All of this is just to get the log trued out.  Then I start facing the real issues of the bridge factors.  Now I am brand new at all of this and the learning curve is a bit daunting...but I will hang in there.

Now as you see, there are two slots of different widths and depths in the middle of the bridge.  The thinner of the two holds a piece of bone, (used to be Ivory, but we need our elephants now so its bone), and that slot leans slightly toward the back of the bridge to hold the bone at a slightly stronger position relative to string tension... so the rotary could then be moved to whatever portion of a degree etc., that would give the slot its proper back-tilt.

The transition from the wing of one side to the wall in the middle part of the bridge has to be ninety degrees so I do not have a good image of how that could be done with a ball end mill.

I am open to everyone's considerations and ideas on all this.  Right now I need to figure out how to get the lathe aspect to turn at a slow speed, and then have the mill end move down the length of the log so that it cuts the log and trues it out so there is no wobble that then translates to a wavy surface on the curve of the wings.

Sincerely guys,   bob

#### halfmill

• 101
##### Reply to Gary Lucus
« Reply #17 on: November 04, 2017, 01:26:20 PM »
I am just starting.  I bought a cnc, a 4th axis lathe and mach 3.   I am slowing learning Fusion 360. And I am taking a cnc machining class at community college... so I can say I am swamped with learning all this...  so for now I am just trying to slowly move most of my hand skill over to the cnc processes without losing any tonal qualities in the instrument.  Machining is precise, but it can't "hear"  or experience the feel of a professional concert quality instrument. So I will have the cnc do all the rough in work and I will handle the art side. Thank you for your responses.  bob
« Last Edit: November 04, 2017, 01:28:36 PM by halfmill »

#### halfmill

• 101
##### Re: A-axis rotary lathe question for Mach 3
« Reply #18 on: November 04, 2017, 01:30:10 PM »
Hi Bob,
extremely nice work, but where do you plugin the big fat amp?...LOL

My guess when I asked you about what you wanted a rotary axis for was that the subtle curve in the bridge and nut were what you had in mind.
Any decent CAM program can follow a gentle curve like that to well within the resolution/accuracy of your machine. We are used to manual milling
and cutting a curve by co-ordinating two axes by hand is beyond a human operator. Not so for a CNC machine. A rotating axis could potentially offer
accuracy in line with the Truth In Rotation of the axis, a few um with care. I would expect a decent hobby router to achieve some thing like 0.02mm
or 20um if the curve were generated by co-ordination of two axes.

I imagine 20um would be considered pretty fair when compared to hand worked.

Craig
yes learning all of this is the bugger in the butter...bob

#### garyhlucas

• 663
##### Re: A-axis rotary lathe question for Mach 3
« Reply #19 on: November 04, 2017, 03:40:45 PM »
Half,
If you have fusion 360 you can make a 3D model of that part and fusion should easily handle machining it with just 3 axis.  The way I would approach this is to 2D mill the rectangular raised protrusion and slot, and the top of curved area for length, width, and a little extra on top.  Then use a 3D milling operation on the curved portion with a ball mill.  In CamBam from a 3D model I would create a single mregion by putting a slightly more than tool diameter rectangle around the whole part, and a rectangle equal to the raised portion.  Then CamBam would recognize to machine only the area within the outer rectangle and outside of the rectangle around the raised part. You might use a routine typlcally called waterline rough and waterline finish.  Rough will make multiple passes at a specified depth to remove most of the material leaving a defined roughing clearance.  A waterline finish would then cut the surface to finished size and you would specify a small stepover so the ripples are minimized.  There is usually other 3D methods that may do the job better.