Shars retrofit parting blade

Ken226

Ken226

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I've been using one of these for a long time:
shell gas station open near me


It was a big upgrade from HSS blades and made parting quite a bit easier, but I kinda always wondered how the same gtn-2 inserts would work if it wasn't extended so far from the toolpost. It always seemed to kinda suck for rigidity.

I came across one of these retrofit blades which allowed me to use gtn-2 inserts with the HSS blades holder. It moves the blade about 1-1/4" closer to the toolpost.

I just finished taking my first cut with it, and am very impressed. The extra rigidity made a significant difference. It was definitely worth the price.

Anyone else using one? Opinions and experience?

 
I have the -3 version. I haven't snapped a HSS blade since. I still suck at parting, but I haven't broken anything with the GTN-3.
 
RandyWilson said:
I have the -3 version. I haven't snapped a HSS blade since. I still suck at parting, but I haven't broken anything with the GTN-3.
Click to expand...


I'm loving it so-far. I've been using the same inserts for a long time, but having the blade over an inch closer to the toolpost center made a remarkable difference.
 
i should reevaluate carbide parting tools
i purchased a used blade, new inserts , and found it to be less than stellar on my 1236 lathe
maybe a new blade with new inserts may be the answer
thanks for the report!
 
I think the difference I'm seeing is due to the new blade being much closer to the toolpost. Less cantilevered length along the z axis.



The flexural rigidity of a cantilevered beam is a function of (elastic modulus times second moment of area) divided by length cubed.

So, the denominator being a cubed number (length cubed), a small change in the amount of offset would have a huge effect on rigidity.

At least, that's my theory anyway.

The old blade had 2" of offset, and the new blade has 1". According to that formula, reducing the offset by half should make it 8x more rigid. Along the z axis anyway.
It would probably be alot more accurate to find the distance from each blade to the toolposts center of moments, and calculate the difference based on that, but I don't know where that is. Whatever the actual amount, I'm sure it's a good bit more rigid.

My blade stickem-out on the X axis is the same on both, so that shouldn't have anything to do with it

It seems to cut about 8x better anyway. :)



The new blade sits in the holder with a few degrees of rake as well, rather than horizontally. That could be playing a role as well.

Kinda makes me wonder. How many loved or hated tools/inserts may have performed just based on how they were mounted.

Edited to add a correction:
Earlier I guestimated about 1-1/4" difference in offset. Looks like 15/16" is more accurate.
 
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As I understand it, the tool blade is designed specifically to mount in the angled/raked BXA holder. The cutting insert is held with less rake than a normal tool so it's relative position to the work ends up as intended.
 
RandyWilson said:
As I understand it, the tool blade is designed specifically to mount in the angled/raked BXA holder. The cutting insert is held with less rake than a normal tool so it's relative position to the work ends up as intended.
Click to expand...

Since you mention it, I went and eyeballed them side by side.

Your absolutely right, I can clearly see that the insert fits into the new blade at at an angle to the blade, keeping it's rake relative to the part the same.
 
@Ken226 - Please give us a link to this parting holder. I've tried searching the Shars website for "P3N" ... no joy. Thanks!
 
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