Spiral design advice needed

[myxpykalix]

Here is a left/right hand barley twist spiral.

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However if you look at where the design starts and ends

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It looks very unfinished and awkward and I wondered how others dealt with that or what you did to make it look like it transitioned into the other designs better? In other words how would be the best way on the indexer to try to cut away that unfinished part?

[porscheman]

Jack,

All the barley twists that I cut on my Legacy ornamental mill start and stop with a full rotation of the barley-cutting router bit around the circumference of the piece at the same depth that you cut the barley twist.

This is pretty easy to do on the Legacy, and should be extraordinarily easy on the indexer. At whatever point you are starting your barley spiral, plunge to the cutting depth, rotate the indexer one full revolution (360 degrees) and then proceed with your spiral cut as shown in the photos. At the end of the cut, add another full 360 degree rotation of the piece without moving the bit along the axis. I really like the look of this barley twist style and believe that it's in line (stylistically) with traditionally carved barley twists.

Finally, you could achieve some really nice symmetrical left/right twist designs as you've shown in your sample turning by inserting the "full revolution" cut described above in the center of the piece, and then reversing the direction of your spiral as you move to the far end of the turning. This is a bit harder to do on the Legacy, but would appear to be an easy extra few programming steps for the indexer.

Another cool thing to try with big, simple spirals like the barley twist is to make a wood blank with two different wood species (say maple and cherry), with one of the species at the center of the blank, surrounded by the second species. First, use the indexer to turn a 2" diameter "dowel" from the first species (let's says the cherry). Set that piece aside. Then, take two pieces of maple and use a large 2" ball nose bit (or you could use the carving capabilities of the bot) to hog out a semicircular 2" groove along the length of each piece. When placed together, these two pieces of maple will create a blank with a 2" hole down the center. Glue up the maple blanks with the cherry dowel in the center. When you turn this with a barley or rope bit, you'll pierce the outer maple layer and expose the cherry material at the bottom of your spiral cuts - the two-toned effect is quite dramatic and takes people a long time to figure out how it was done.

Hope these ideas are useful.

John

[myxpykalix]

John,
Thanks for the design ideas. I too have a Legacy 900. And yes it is fairly simple on the legacy to make the designs at the top and bottom as you describe rotationg the piece 360 degrees, however it isn't quite as simple on the indexer since you don't have independent control of the Z and B axis.
Sometimes using the legacy i like to make my designs on the fly deciding how deep a particular cut might go but i can't do that on the bot.
For example, this barley twist file was made with an initial depth of only .25 because the true bit profile depth was 1 inch. Well i can't go 1 inch with a cut. So what i have to do is take it down .25, stop bit back at beginning, rezero to surface, rerun file till i get to desired depth.
I guess i will have to adopt that strategy to this design challenge.
I will take some of your design ideas and run with them

[porscheman]

Jack,

As with one of your indexer forum postings from a week or two ago, I'm probably always running the risk of making a fool of myself for two simple reasons - first, I'm a new shopbot owner and I'm just learning how to use the machine and second, I don't even own an indexer (yet). So I clearly can make some dumb assumptions, as I did when I assumed in your earlier posting that you could just turn on the indexer axis and keep it spinning while you then make X and Z moves. When Christian mentioned the M5 command and how the axis moves were all linked, I had one of those "Aha!!" moments.

Having said that, it would seem that cutting the round and spiral passes that have been mentioned above are easy to do with the M5 command. Again, that may be completely naive, but I'm pressing on with my example....

Let's assume that you are using a 2" diameter barley twist bit for your design. The barley twist is a single start spiral, and from the Legacy world, I know that I need the X axis to travel 2" for every 360 degree rotation of the indexer. For simplicity of the math in this example, let's assume that I want to make the length of my spiral 12" (or anything evenly divisible by 2). Finally, let's assume that the X starting point of the top of the spiral is 3" down from the top of the turning.

Using the same process that I would if using my Legacy mill, here is the sequence of M5 commands that I would use to cut the spiral:

M5,3,0,Z Safe Height,,0

This should move the bit above the starting location of the spiral cut at whatever "Z Safe Height" keeps the bit from contacting the workpiece. I'm assuming you've determined this value by previously zeroing to the surface as you mentioned above.

M5,3,0,Z Cut Depth,,720

This should plunge the bit down to your cutting depth WHILE rotating the piece 2 full revolutions. If the axis moves are coordinated, this should give you a gentle plunge to cutting depth spread out over 2 full turns, and will create the round cut at the top of the spiral. I guess to be on the safe side, you could follow that command with this one:

M5,3,0,Z Cut Depth,,360

which would give you another full rotation of the piece now that you are at full cutting depth. If burning of the wood is a factor, then you could use "Z Cut Depth - .005"

Now to cut the spiral section that is 12" long, I need 6 full rotations (2160 degrees) of the piece with the 2" barley bit, so I use:

M5,15,0,Z Cut Depth,,2160

This should cut the spiral and leave the bit at cutting depth at the end of the spiral, so I can finish up with the round cut at the bottom of the sprial:

M5,15,0,Z Cut Depth,,360

And finally, we should be able to raise the bit up out of the cut using:

M5,15,0,Z Safe Height,,0

You mentioned in your last comments that "you don't have independent control of the Z and B axis", but I think when you use the M5 command and leave some of the parameters unchanged from the last command (your X values, Z values, or B values), you are effectively controlling individual axes. If for some reason, that logic is fatally flawed, then I apologize for jumping into one of your threads.

Regards.

John

[bill.young]

Hi John M.,

Good explanation...that's exactly how it works. To make it a little simpler the Indexer Virtual Tool gives you input boxes for those values so that you don't have to type the commands manually, but it creates almost the exact same set of commands.

Thanks for jumping in,
Bill

[porscheman]

Thanks Bill, I'm glad I didn't make any incorrect assumptions.

One other basic question for you or Jack - in order to get the indexer to change direction of rotation (for left hand spirals or right hand spirals), do you just enter negative values into the last field of the M5 command?

Thanks.

John

[bill.young]

Hi John,

Let me correct one thing in your posting above (and somewhat answer your question).

Remember that ShopBot moves are by default done as Absolute moves. So your first example moves..

M5,3,0,Z Safe Height,,0
M5,3,0,Z Cut Depth,,720

...are fine because the B axis starts at 0 and ends at 720 degrees. The next command as it's currently written, though...

M5,3,0,Z Cut Depth,,360

...would actually turn backwards because it would be going from 720 degrees to 360 degrees. This wouldn't be a problem in this case since it's just spinning with no other axis moving, but your next move....

M5,15,0,Z Cut Depth,,2160

would only actually turn 5 revolutions because it's starting a 360 degrees. And the next command would spin backward 5 revolutions...

M5,15,0,Z Cut Depth,,360

You can keep adding and subtracting the angle if you want like this...

M5,3,0,Z Safe Height,,0
M5,3,0,Z Cut Depth,,720
M5,3,0,Z Cut Depth,,1080
M5,15,0,Z Cut Depth,,3240
M5,15,0,Z Cut Depth,,3600

...but that's kind of hard to keep track of what's really happening. The easiest way to deal with it if your manually writing commands is to Zero the B axis between the commands so that it's easier to read, using the ZB command like this...

ZB
M5,3,0,Z Safe Height,,0
M5,3,0,Z Cut Depth,,720
ZB
M5,3,0,Z Cut Depth,,360
ZB
M5,15,0,Z Cut Depth,,2160
ZB
M5,15,0,Z Cut Depth,,360


Sorry to add another level of confusion to things but I missed it the first time and wanted to clarify.

Bill

[porscheman]

Thanks Bill - that's another piece of the puzzle falling into place. Since the indexer axis "remembers" it's zero position (when in Absolute mode), this would make it easy to handle multi-start spirals by referencing each start relative to the zero point.

Going back to my original M5 commands, I wonder if it's possible to place the system into relative coordinate mode, but only in the B-axis dimension. In other words, all X and Z moves would be absolute, but the rotational B moves would be relative. This would eliminate all the ZB commands (though that's clearly not a big deal).

John

[bill.young]

Hi John,

Currently it's not possible to "split" the axis like that...have the B-axis Relative and the rest Absolute...but we have been talking about it off and on. It would solve a couple of problems...the question with all changes like this is if it would cause more problems than it solves


Bill

[myxpykalix]

John,
The only assumption you made wrong on my part is thinking I am writing these moves by hand and I am not. I have a wizard that i input the basic info into and it spits out my sbp file.

I feel less confident with my manual writing abilities since i think, if i leave a comma out or place a wrong value somewhere i can't visualize what it will look like.

However I am going to see if in my "start" and "end" boxes if i put the same thing (0) whether it will turn it or not. If it does that may solve my problems.
Although Scott Cox, in another theread, wrote the code to spin and plunge in place and i believe i cut and pasted and saved that somewhere.

[myxpykalix]

My theory on inputting 0 in start/end boxes to just rotate didn't work. Now what i thought of doing is this. If you look at this picture you will see where arrows are it is .080 higher than the barley cut.


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What i want to do is take an endmill and just cut down .080 to make it even with the cut surface. The problem is that when i measure a cut I can Zero from the surface for spirals.

If i want to use the indexer virtual tool to "round the blank" I have to measure from the center. Then calculate the radius, deduct half the depth of cut for my inputs.
The weak link in this is if my gauge block is off (and i do the math right), it still won't give me a parallel surface cut.

So before i go and ruin this I wondered if anyone had a suggestion on how to round the blank using the surface as a measuring point?