Turning a tapered column

[myxpykalix]

Rose,
Take plenty of pictures and post and write down as much as you can to share so we can learn from you!!

[knedert]

Thanks Jack,
My first thought was:
"Oh great... now I'm a guinea-pig!" lol
We'll have an update our latest adventure:
"How we found our Z-zero on the lathe" posted soon, I'm sure!
till then...
~Rose

[johnm]

Folks -

Okay, well - we've got it, sort of.... We've been comparing our cut file sizes with the actual diameter of the object being turned and have been fine tuning.

Here are pictures of the second attempt at turning - the first was in foam. We do wonder what is causing the fretting and lack of the clean cut right at the shoulder - the slight scalloping you see. We had it also on the round tenon end, but I had sanded that off before I thought to take the picture.

Rose and I think it is something to do with the interaction between the cutter geometry and rotation speed of the lathe. Any ideas on how to prevent this artifact? We are using a straight 3/8" plunge bit, and the scalloping repeats at ~5/16" intervals - I think it has something to do with the plunge part of the bit, since this is the radius of the cutter. Any ideas?

Thanks

Rose and John


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[myxpykalix]

That problem has to do with the rotation of the bit in relation to the direction of cut. It's hard to tell the direction you are cutting based on the picture. However try it in the opposite direction from what you have there.
If you look at your piece, the opposite side of the cut is clean while the leading edge is rough.

It seems to me that in the indexer instructions it tells you to center the bit over the workpiece. Now i'm not sure how to do what i'm going to explain on the shopbot but here is what we do when we want to turn something round on the legacy (same principle and setup)

We center the bit (say a 1 1/4" bottom cleaning bit) over the workpiece. Lower the bit till it touches the surface. Raise our "stop" the height of the depth we want to cut to get the piece round.
The machine turns the piece while we plunge down and it cuts, however what we do is back the cutter off from the center by 1/2". If you come into the piece from the side (so to speak) as opposed to straight down you get a better cut. If you come into the cut from the backside your cut will be clean on the leading edge of the cut. If you come in from the front side your cut will be clean on the trailing edge (or vice versa)
Now how you program that into the indexer, I don't know and the more experienced users might explain why we wouldn't want to do that if thats not ok to do. I know it works for us on our Legacy Ornamental mill and its the same basic setup only more manual.

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[bill.young]

Hey Rose,

Looks like you're getting awfully close!

If you were just cutting a straight cylinder then a straight bit should work fine, but in general with a profile cut the geometry gets funny with a straight bit because it's cutting with the edge of the bit instead of the center as it moves along the blank. It's also hard to accurately deal with any "hollows" in the profile using a straight, flat bottomed bit.

My vote would be to create a file in Part Wizard using the same method as the Extruder...creating an offset with an endmill tool with the same radius as the ballnose bit that you'll cut with...and then using the Flipper to stand it the file on edge. Then cut it with the same sized ballnose bit. Any sharp inside corners will be radiused, but since they're already on a lathe they'll be easy to clean up by hand with something like a parting tool or sandpaper. Remember that since the center of the toolpath is at the center of the ball of the bit, you'll have to adjust your Z-zero by the radius of the ballnose to get the right cutting height.


Bill

[johnm]

Bill/Jack -

Okay, I understand that moving off of "TDC" would give a smoother cut - the wrinkle then, is that by moving off of TDC, you are altering the geometry and your tool cutting position will now be modified and give you a slightly larger diameter cut, since you are now working on the hypotenuse of a triangle, rather than in the straight line of TDC. Now, you could figure out what the position of the cutter on the angle is vs TDC, but some of this is already giving us a headache....

What do you think?

John

[bill.young]

I'd still suggest trying it with a ballnose bit along the centerline of the blank, though I must admit that my experience is with blanks turning at indexer speed and not at lathe speed. At indexer speed it works very well.

There are a handful of ShopBotters that have a lathe setup like this...maybe one of them will chime in with info that's less theoretical than ours!

Bill

[myxpykalix]

John...Pass the BC powder! I don't how about a hypotenWhateverthingamajiggy but you might get some info to give you a better idea of how to accomplish what you want to do by going here:
http://legacywoodworking.com/techniques.cfm

Now this is based on doing it manually on their machine but you might pick up some nugget that might help you in your situation, if not....
YOU'RE ON YOUR OWN! lol

[rnels]

Rose and John,

Might be a little late but here is a file to try.


36 inch 3 to 1 inch taper .5 ballnose zip
36 inch 3 to 1 inch taper 3 inch round blank .5 ballnose.zip (0.8 k)

Please check it out before running. I was stepping down by .1 inch. You could change the file as you would like.

I am assuming you have a 3 inch already rounded blank. Change the MS, JS to whatever you are comfortable with. Zero x at start of taper. zero y at center of blank. ZZero at top of material.

Randy

[rnels]

Rose and John,

I assumed you have your lathe running with the x axis if this is not correct this file will not work.

Also noticed I went too deep I should only be going 1 inch down not two because we take an inch off of each side as it turns. Here is a new file.


36 inch 3 to 1 inch taper .5 ballnose zip
36 inch 3 to 1 inch taper 3 inch round blank .5 ballnose.zip (0.7 k)

Randy