Various HSS tool size needs?

T

tmenyc

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A friend brought me a number of HSS bits from an estate sale, which made me finally ask: my QCTP is AXA, I've been cutting 3/8" bits, but are there reasons why I might want to go up to 1/2 or down to 1/4 or even 3/16? I have a bunch of 1/4 already that came with the lathe.

I'm guessing that the 1/2 bits need greater rpms and torque than my Logan might provide and I might need even if it could, and does AXA really go to a full 1/2"? But the smaller bits? When would 3/16 be useful?
20190520_093851.jpg

Also, the 3/8 parting tool is interesting...when would using that be preferable to a blade?

I think the tiny boring bar is very close to the size I bought from Micro100.
Thanks!

Tim
 
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Size of bit will not matter as long as it fits the holder and can be centered on the part. As long as you do not try to take a cut deeper than what you have been currently taking with your 3/8 bits the larger bits will be fine. Parting tool will probably be sturdier than standard cutoff blade but will be limited on depth of cut.
 
Use them all. Anything that will fit the holders you have. A larger size bit does not use more HP because it is big, it only does so if it has a larger corner radius and you use it all, or if you just make deeper cuts with it. A 1/2" tool with a 1/64" or smaller radius and a shallow feed uses very little horsepower. The extra mass and rigidity helps a lot. The only real down side is that the larger size can get in the way when cutting smaller features in the work. Another issue can be if you can get the smaller and larger tools cutting on the center line of the work with the tooling you have. There are ways to address that situation, though you may need to make or buy some extra tooling to do it, but quite doable.
 
Smaller bits, less grinding to form the shape you need. Bigger bits more rigid, can take a bigger cut.

Bottom line, use the size of bit that fits your holder and allows you to get the cutting edge at the proper working height with your holders.

I use a KDK holder on a South Bend 10. The vast majority of my bits are 3/8, because that is the size that fits the holders.

I inherited enough 1/2 inch bits that I made a 1/2 inch holder but I really had to thin out the bottom of the holder to keep the cutting edge at the proper height.

The size of the bit has nothing to do with RPM or torque. That is driven by the diameter of your part and the depth of cut you take.
 
Many thanks, all...makes sense!
best,
Tim
 
At some time or another you'll probably try single point threading. The smaller tooling will make turning finer threads easier. It would be cumbersome to say the least to try to cut a 40 tpi thread with a 1/2" tool. On the other hand it will be a lot easier to cut a 3 or 4 tpi with a 1/2" or 3/8" tool.
 
Have a look at whether the 1/2" tools are the correct height relative to the spindle centerline. On my Logan, they're too
high by a bit, even with the tool holder as low as it can be adjusted. Some Logan compounds can be a bit "tall" for a QCTP.
 
This is like a textbook, thanks guys!
Tim
 
Tom et al -- Know that it's much, much appreciated. It's not lost with me, and although I got off to a very late start, I hope to be in the position to pass it on in time. The interplay of all the variables is not yet second nature for me, but I'm working on it!
Tim
 
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