CNC Crankshaft Machining

[Stuart]

I think the excessive pass tally is what it was done for an exhibition , I remember John saying they slowed it down so the great unwashed could follow the movement and therefore the material would last longer less cost   


at least it’s given a starting point for a more rounded gcode to do the job for what it’s intended

Good work guys

[Zephyrin]

Most machines are too dumb to know the A360 and A0 are the same place, so will unwind back to A0. Need a way to reset A360 to A0 without actually moving

No way to use a Modulo function for the angular variable in this G code programming ?  sort of MOD((alpha+increment);360))

[gadabout]

I ran it again this afternoon on Mach3 while I was doing something else and it ran for 38 minutes before I estopped it, so maybe it's not quite right?? 
Mark

[Jasonb]

I'm with Stuart's comments on the speed, Talking with Ketan from ARC he has said before that Adam always wanted to push the mill to run as fast as possible but it was not good for showing punters what was going on and he did not have an endless supply of materials or cutters to break when he pushed things a bit too far!

looking at the Code Mike originally posted it looks like it should go round 18times as #4 the passes is set to 18. That sounds about right for 1mm off per pass as the inner edge of a pin will be 17.5mm from the furthest edge of the web

[Vixen]

See attached file

I believe I have shaken the bugs out of the LinuxCNC version.  When I run it with debug statements the Z values at A=0,90,and 180 look correct.

The program assumes Z0 is the centerline of the bar and that Y0 is the top of the bar at A0.  I don't know why there is a move in X, so I'd just remove it, and position the X axis along the bar where a pin will be located.  No other X moves are generated.

The starting value of Z should be the radius of the bar.  The first Z move will thus be a plunge to the value needed for A0, and the rest of the Z moves are coordinated with A and Y axes.

Program logic is fairly easy to follow if you you visualize a circle representing the Pin turning around the A axis with a constant radius equal to the PCD.  Then the position of the top of the PIN is the position of its center with its Z value incremented by its radius.

Using the example values, the top of the pin at A0 is the sum of the pin and PCD radii or 12.5.  At A180 it's the PCD radius minus the pin radius or 2.5.  At A90 and A270 its the pin radius or 5.

The program allows specification of passes; each pass increases the DOC.  In the example the excess material at A0 is 2.5mm, so 18 passes means removal of .139mm, probably an excessive pass count.

Hello Kvom

I loaded the attached file from reply #28.

It loaded successfully without error messages

It will execute

It produces a toolpath plot in the Y,Z axes. see below



I set the program to run 4 passes. You can see the y,z toolaths are now circular and correct. The four tool paths for the four passes are all 15 mm diameter and do not diminish in diameter as I would expect. You can see the Z axis incrementing the depth of cut but the corresponding tool paths do not decrease in diameter. There does not appear to be a sin, cos function (#51,#52) in the O003 subroutine to recalculate the new toolpath diameter

Here is my 4 axis machine setup. The piece of metal is from a previous iteration of this program.



Regarding depth of cut and number of passes. We need to remember that at the A180 position, 20mm of material needs to be removed. Therefore 20 passes, each removing 1 mm  of material is not unreasonable. I would expect the toolpath to show an outer circle of 30 mm less the depth of cut, followed by 19 more concentric circles of deminishing diameter until the final 10 mm diameter is achieved.

Setting the Z axis to zero on the material centre line is an interesting challenge. It's easy to set the Y axis, simple touch off both sides and divide by two. Without a centre line reference block, the Z axis is not so easy, as you can only touch off the top surface. So I touch off the top surface, zero Z, move down by half the material diameter and zero Z again. Not as accurate as a reference block but it will do for these initial experiments.

Hope this feed back will help you refine your LinixCNC version of this interesting program

Regards and Respects

Mike

[gadabout]

With M99 at the end will it keep running in a loop? As that's what it seems to be doing for me
Mark

[Jasonb]

Regarding depth of cut and number of passes. We need to remember that at the A180 position, 20mm of material needs to be removed. Therefore 20 passes, each removing 1 mm  of material is not unreasonable. I would expect the toolpath to show an outer circle of 30 mm less the depth of cut, followed by 19 more concentric circles of deminishing diameter until the final 10 mm diameter is achieved.

Mike with the PCD of 15mm eg 15mm stroke the pin will not be so far over, 17.5mm at the most as per my previous post. John's photo shows the pins are not flush with the edge.

[kvom]

With the default params there is 2.5mm difference between the edge of the bar and the pin.

[Jasonb]

Mike, I think your simulation is showing the correct movement.

The idea of the program is that the cutter stays vertical above the ctr of the crank pin during the full 360mm rotation. As the throw of the crank is constant at 15mm PCD the Y axis should not need to move more than 15mm. Likewise the rise and fall over 360deg will also be the same 15mm PCD.

Think of it like an eccentric, you can make it whatever diameter you like but the follower will always only move by twice the offset or the total throw.

All your simulator is showing is the tool following the ctr of the pin and gradually working it's way down, the first pass will just kiss the opposite side of the stock to the pin then you will gradually get a larger arc of engagement until the last 3 passes where you are cutting on the full 360deg.

Attached sketch shows that Z movement is only 15mm (95-80) for both the finished pin and another pass (red circle) of 20mm dia, turn it on it's side and Y will also stay at 15mm movement

EDIT

I've asked Ketan if he has a copy of the program, a video of it in action or can remember how long it took to cut each pin.

[Vixen]

Hello Jason,

We need to be careful about what we are comparing and commenting on.

I have no doubt the John Stephens program runs perfectly on an Arc Mach3 machine. It's been well demonstrated as working. And that program was the subject of my initial post. Since then, we have discovered it  will not work on LinuxCNC because that operating system handles subroutines in a entirely different way. Kvom is working hard to convert the original program for LinuxCNC ( and Pathpilot) use

Therefore please remember,  I am only commenting on the much modified LinuxCNC version that Kvom is working on. It has become so modified, it bares little resemblance to the original.

 The LinuxCNC defaults were: bar diameter 30, PCD 15 and pin 10 which may be different to Sir Johns defaults, and would account for the different depth to be cut numbers.

I have completed a machine run of Kvoms Reply #28 version, from the beginning through to the end on my LinuxCNC 4 axis machine and this is the result of 18 passes.



You can see that only a small segment (about 120*) was machined away to a depth of about 2.7mm and located either side of A180. The curved segment appears to be centred on the required pin position but is oversize Clearly, the modified program is not behaving as we would wish and still needs some more development work.

My feed back to Kvom is intended to allow more trouble shooting of his LinuxCNC changes and should not be taken out of context as comments on the original John Stephens work.

Mike

[Jasonb]

My comments were based on the same perameters of a 15mm PCD used by both John and Kvom which would not leave 20mm to come off, as confirmed in post #37 by Kvom. If you are looking for an incorrect finished item then it is hard to comment whether the program is giving the right results. And that the path would not be a diminishing circle unless the revised program does not follow the same principal as JS's original.

On your test cut did the first or second of the 18 passes start removing metal or was it just the last few?

[Vixen]

Hello Jason

Ok it's 17.5mm not 20 mm, but still enough to need multiple passes. I have not seen John's program in operation only a modified one, which gives strange results, so I am guessing how it should work.

All 18 passes appeared to take a small amount off the A180 sector.

Mike

[Jasonb]

That makes it more complicated as it may have been a simple Z height issue as your 2.7mm could be 2.5 and add that to 15mm half (dia of bar) would give the 17.5.

It would seem that the second half of the code is not feeding down far enough, the first part gives the movement about the pin which as you say seems to be correct, will have a look to see if I can spot anything obvious but I probably know the least about G-code but whatever machine it is running on the basic maths and geometry would be the same.

What do the 18 passes look like on your screen, I would expect a stack of circles 18 high like you posted for the 4 passes with 1mm drop between each, last may be 0.5 or first cutting 0.5mm above the bar.

[Vixen]

Here is a screen-shot showing (in red) the first pass of the 18 passes while running Kvoms Reply #28 version of the LinuxCNC modified program.



On pressing GO the tool does a rapid (G00) plunge to Z 2mm (just clear of the workpiece). It then moves (G01) to Z 15 mm. then the A axis rotates 360* while the Y and Z axes move +/- 7.5mm. When A 360 is achieved, the tool plunges (G00) back down to Z 2mm (just clear of workpiece) and the A axis unwinds to A0, then everything repeats. The Z increments are each approximately 0.14 mm. All measurements are taken from the screen while everything is in motion, so there may be some small errors in the numbers.

Mike

[gadabout]

I have just done a full cutting run on Mach3 using the original code, each pass took 3 minutes but I ended up with a quarter moon segment not a crank pin, so it appears something is amiss
Mark