Wavy blank!

[Woodchipper]

I am getting back into penturning after a long time doing other things. I just turned a camo blank for a bolt action pen. The ends came out great with care and calipers. However, I have a couple of spots that are high/low in the blank. I use a 3/4 inch roughing gouge sharpened on a Rikon grinder and touched up with a DMT diamond file. The tool rest is just a bit below center. I tile the tool and hold the tool to slide along the tool rest. I start the cut just shy of one end and work to the opposite end. Just puzzled about not getting a straight line all along the blank. Your advice is appreciated.

 

[leehljp]

My first thought is that you are using a mandrel. Are you? If so, there are several items in a mandrel's characteristics that can contribute to that.

Using your roughing gouge, and applying just a minuscule too much pressure can cause flex to the mandrel. You didn't say it was out of round, but the description implies it. A second problem with mandrels is when the wrong live center is used. With Very pointed live centers, the very tip will ball up inside the cup or the mandrel end and cause a wobble. This often does not happen until a number of pens have been made. A bent mandrel can also cause it.

 

[Woodchipper]

Didn't check for out of round but from end to end, it is a miniature washboard in a couple of places. This is one of the long shaft mandrels; had it for years. A penturner in our old club had a shorter mandrel, lessening the bend of the shaft. I can turn two blanks for Slimline pens. He turned one at a time. Thanks.

 

[JohnU]

It could be flexing in the mandrel. I alway turn one at a time. There's less of a chance to flex it. Regardless of what I turn the blank with, I usually do a final cut with a 1" screw, positioned flat on the tool rest like a negative rake cutter. Just a thin cut to smooth out the blank from end to end. It's just what works for me.

 

[leehljp]

Didn't check for out of round but from end to end, it is a miniature washboard in a couple of places. This is one of the long shaft mandrels; had it for years. A penturner in our old club had a shorter mandrel, lessening the bend of the shaft. I can turn two blanks for Slimline pens. He turned one at a time. Thanks.

Out of Round does not alway go OOR from one end to the other, but most often on one end only, where the flex in the mandrel is happening. You didn't mention it but are you using a 60° live center? If not, that most definitely is contributing to the problem. And most people do not have this problem initially, it just happens later on, making people think that it is Not the live center Nor the mandrel, or too much pressure applied.

Lots of threads/ post in the past on this same problem, and lots initial assumptions that it can't be the mandrel or non 60° live center because it worked fine last week or last month.

 

[Woodchipper]

Will go back and check. I'm thinking that I might be starting the cut in the middle of the blank instead of
going end to end, thus leaving high and low spots. Lee, I have a 60 degree LC.