Parting tool disaster!

[Woodchipper]

I watched a video by Sam Mayotte (watched it three times) on making a parting tool from a knife. I had an old chef's knife and followed the steps in the video. However, when I applied the tool to a piece of rounded scrap, it burned rather than cut. Tried it at different angles, each side up and still nothing. Any ideas, suggestions, etc. here? Thanks.
Buying a tool is out of the question as I just blew my allowance for the next year with buying an expensive camera lens.

 

[monophoto]

First, I think you mean Rick Mayotte and that this is the video you watched for inspiration -

I assume that you understand that the intersection between the two cuts that Mayotte made at the end of his tool is the cutting edge. It needs to be sharp, but all it takes is a few swipes across the shorter of the two cuts with a diamond card to clean up and sharpen that edge.

Second, I suspect the problem is not in making the tool, but in how you are using it. The most likely cause of burning when using a parting tool is that as the tool cuts deeper into the wood, it binds against the wood, causing friction that burns the wood.

The secret to using any parting tool is to not try to part off entirely in a single pass. Instead, start the cut and go in a few mm, then start a second cut about half the width of the tool away from the second cut. Make that cut about twice the depth of the first cut. Then move the tool back to the first cut, and go in even deeper. Alternate between the two cuts until you complete the parting cut.

You can use a narrow parting tool to make a narrow groove in the surface of a turning, but if you need a deep groove, or if you need to part off, then the widtth of the cut will have to be greater than with width of the parting tool to avoid having the tool rub against the cut surface and cause burning.

 

[SteveG]

Typical parting tools are made with the widest part of the tool to coincide with the cutting tip. "Diamond" profile is one example. If the knife blank is wider at the top, tapering to thinner at the original cutting edge, then when you cut and form the parting tool profile, the blade thickness may be thinner at the cutting tip, causing the thicker portion above (spine) to rub and cause friction and burning. So it would be important to select a knife blank that does not taper across the profile from spine to original cutting edge.

 

[monophoto]

Steve's point is subtle but important, and is illustrated here (drawing not to scale):

Diamond parting tools are designed to have a cross-section that is fat at the cutting edge. This is to minimize the degree to which the sides of the tool can bind in the cut made by the tool. Even so, there is a risk that the fat portion of the blade will rub on the sides of the cut leading to friction and burning.

The thickness of a kitchen knife is generally greater at the back than it is at the cutting edge. so when you make a thin parting tool from a knife, it is possible that reshaping the blade will result in a cross-section that is slightly trapezoidal. With the tool used as shown, the upper portion of the blade is slightly wider than the lower portion. As a result, as you cut deeper, the upper-half of the blade can become thicker than the width of the cut, which can then lead to binding, friction and burning.

Most commercially-made thin parting tools have a uniform thickness which almost assures that the blade will bind and rub.

So regardless of the style of parting tool you are using, it is necessary to do a relief cut next to where you are parting to allow space for the tool to penetrate the workpiece without rubbing.

 

[Woodchipper]

OK, it's Rick. Senior moment. Yes, the burn was on the sides (been out of the shop too long) but also in the cut. I'll play around with the one-plus-half width to see how that does. My original parting tool is 1/4 inch wide and not real good for some applications. Thanks.
I'll check the thickness of the blade to see if it's tapered.

 

[howsitwork]

OK, it's Rick. Senior moment. Yes, the burn was on the sides (been out of the shop too long) but also in the cut. I'll play around with the one-plus-half width to see how that does. My original parting tool is 1/4 inch wide and not real good for some applications. Thanks.
I'll check the thickness of the blade to see if it's tapered.

REALLY worthwhile calling into any local engineering works or steel stock holders and asking if they have any old machine hacksaw blades. You grind the teeth off and these make EXCELLENT parting tools about 1/16 wide by 1 1/2 " deep. Make a handle from two halves and bind it on and voila .


PS snap the blade in half by grinding a small line across it , wrapping in a cloth then hitting it when secured in a vice. DO NOT do this without a cloth and the line or it might shatter dangerously.

 

[monophoto]

REALLY worthwhile calling into any local engineering works or steel stock holders and asking if they have any old machine hacksaw blades. You grind the teeth off and these make EXCELLENT parting tools about 1/16 wide by 1 1/2 " deep. Make a handle from two halves and bind it on and voila .

Reciprocating saw blades also can be recycled as parting tools

 

[crokett]

Reciprocating saw blades also can be recycled as parting tools

I need to try this then. I've been wanting a thin parting tool.

 

[Woodchipper]

There is a machine shop close by. Also, recip saw blades are cheap. I have a couple and will see if they are worth keeping.

 

[monophoto]

Here's a very old video of Capt Eddie on recycling a recip blade to make a parting tool

But let me emphasize that Rick Mayotte's idea of recycling a kitchen knife works just as well. I have a very thin parting tool that I made from a bit of high-speed steel that I bought from Harbor Freight (part of a set intended for users of metal lathes). I also made one from a conventional hack saw - it worked, but those blades are super thin and probably a bit too flexible to be practical as parting tools.

 

[Woodchipper]

Cute and informative video using the fire and rescue blade. Now to find the blade. I notice that his blade doesn't have an "offset" but is an angle similar to a Bedan tool. Might try that. Measured my PT and the blade is a constant width.
BTW, that isn't an old video. It's ancient! Look at the TV in the background. Thanks for the link!

 

[Woodchipper]

Well, finally got it solved. I set up the grinder and put a better "edge" on the two faces of the end. Went to the lathe and put the parting tool to a sample of wood that has been there since my dilemma. Anyway, it worked. I found if I lower the back, I can get a good cut. Raising the back result in burning as I had before. Now to put a handle on it. Hmmm. Wonder where I can get some wood?