the 1 st tool bit

[Ray C]

Ok, here are two basic HSS bits that I use frequently. These are all right cutters but, I have them in left side too. The first has a slightly rounded point. These are not good for cutting up against a tight shoulder but, they give a nice smooth finish in most cases. These are not fancy grinds and they are easy to make.



In all these test cuts, we're working on A36 Hot roll. The outer hard skin and scale was cut off with a carbide bit. Always use a sacrificial bit or junky piece of carbide to cut that outer scale off as it will eat-up a fresh piece of HSS. Since we're using A36, you will always get a dull gray finish. It's not really possible to get that mirror glossy look with A36 -so don't even try.

The first thing to do is measure the diameter so you can calculate RPMs.

RPMs = 4 x SFM / Diameter. SFM for A36 is about 125 and the Diameter is 2". 4 x 125 / 2 = 250.

I'm leaving the RPMs at 250 and am taking cuts of 0.008".

[EDIT: BTW, I'm adjusting the overhead lighting so that every line and imperfection shows up. This is to help demonstrate what the finish looks like].

With IPR (Inches Per Revolution) at 0.004, it's a very smooth matte finish. If you wipe it with a towel, it will not get snagged or leave lint behind. This is a finished piece of metal that needs no touching up. You could polish it with emery cloth if you wanted too.



With IPR at 0.006 it's still a smooth finish but, will snag a towel lightly but not leave a lot of lint behind.



With IPR at 0.012", still smooth to the touch but, will snag a towel and leave lint stuck in the finish.




Now comes the spear-point. I use these to get right up against a shoulder and make a clean edge.



Same RPMs (250) with IPR at 0.008". Keep in mind, all DoCs are 0.008" and that not enough to significantly change the diameter so, I'm leaving the RPMs at 250. Anyhow, at IPR 0.008, to the touch, this is a very noticeable ridged finish that will snag a cloth.



IPR at 0.004". This is much smoother and refined; still snags a cloth but not as badly.



There's more to discuss about what to use, when, where and why. All kinds of factors also come into play if you're working with low, medium or high carbon metal but there's a similar theme across the board when using these two common HSS cutting bits. For now, I just wanted to get some photos up so you can see that once RPM is set properly, feed-rate and bit configuration are what control surface finish.

Ray

 

[chuckorlando]

Thanks guys. So it seems slower feed is a better finish all things considered? To a point I am sure. If so, thats the exact opposite of what I was trying to do. ahahaha. I never thought about using old tool to take the scale. That would likely explain why the face mill inserts did not seem to last as long as I thought they should on my last tool.

The teacher just got us our speeds and feeds paper work last week. Unfortunately the school dont have as much interest in our needs as him. So almost all the paper work we get are copies of from his apprenticeship or copies from his personal books and notes.

Our instructor told us to start at about 50% of what is in the machinist hand book and work up till we think it's proper. Logic being all machines are not equal and your better safe then screwed.

 

[Ray C]

Thanks guys. So it seems slower feed is a better finish all things considered? To a point I am sure. If so, thats the exact opposite of what I was trying to do. ahahaha. I never thought about using old tool to take the scale. That would likely explain why the face mill inserts did not seem to last as long as I thought they should on my last tool.

The teacher just got us our speeds and feeds paper work last week. Unfortunately the school dont have as much interest in our needs as him. So almost all the paper work we get are copies of from his apprenticeship or copies from his personal books and notes.

Our instructor told us to start at about 50% of what is in the machinist hand book and work up till we think it's proper. Logic being all machines are not equal and your better safe then screwed.

For HSS, yes... Slower feed usually gives smoother finish.

It's very important to have the proper SFM charts available so you can calculate the proper RPMs. Here is a good chart: http://www.southbaymachine.com/setups/cuttingspeeds.htm

Equally important is knowing how to set the lathe for different Feed Rates. Some lathes have a couple levers and some you need to change gears. Some are hybrid (like mine) in that there are only a couple feed-rate gear combinations and each one supports quite a few different feed rates which are selectable with a dial setting.

Also for HSS cutting bits, your feed rates will almost always be between 0.003 IPR to 0.015 IPR. Anything slower than 0.003 and you'll fall asleep before the cut finishes and anything faster than 0.015 will be too coarse and wear-out the bit too fast. (The same speeds are true for carbide but, for different reasons).

And finally, high RPM rates will break, wear out or burn-up HSS real fast. The typical chart that I showed you above is a very good starting point that, depending on the batch of material, will only need to be tweaked about 5-15% up or down to find the sweet spot. Most of the time, unless you have a variable speed lathe, you can't get it perfect anyhow.

Go ahead. Experiment but, I urge you to practice on particular finish style and focus on nailing a given diameter within say, +/- 0.001". Once you can do that, switch to a different finish style and practice again nailing a particular diameter. Guess what? Sometimes the techniques of nailing a diameter are different given the type of finish you're after.


Ray

 

[Ray C]

Something to keep in mind... If you have a machine with a lot of vibration, slower feed rates (regardless of RPM) will kill your surface finish. If the machine is shaking like a leaf while taking it's graceful time to make the cut, that gives plenty of time for the vibration to show-up in the surface finish.

I've written much about how to stabilize a lathe but, if it needs to be reviewed, please ask the question by starting another thread.


Ray

 

[chuckorlando]

Thanks for the link. I see I have been using to much spindle speed on the face mill. Lesson learned. On the bright side it gave me a chance to practice my lapping.

Thanks

 

[jam]

Good morning Jam,

That finish looks great from here... :thumbzup: You must have had a smile of satisfaction on your face after your first tool bit grind worked out so well.

Cheers....

Brian

thanks man yes I do

 

[jam]

Very good... Keep practicing. For extra credit, make-up a target diameter and see if you can turn down precisely to that value -regardless of what the finish looks like. And for extra-extra credit, practice turning it down to a known diameter with different finish types.


Good work... Be Safe, Have Fun.


Ray

I can do a target diameter I need all the extra credit I can get what finish type do I need to do I have 1 EXTRA CREDIT I need 2
no bs I Need All the Advice I Can Get...
:LOL::help:

 

[Ray C]

OK, see if you can make a spear-point bit like the one I showed. -Not the rounded edge (soft point) one, the spear point. Do that then, make a couple cuts at different feeds and show us the results. Take nice close-up pictures like you did last time. Keep your DoC (Depth of Cut) at 0.008". Tell us what diameter stock and what RPM you'll use...

Start there and we'll play it out...

Ray

I can do a target diameter I need all the extra credit I can get what finish type do I need to do I have 1 EXTRA CREDIT I need 2
no bs I Need All the Advice I Can Get...
:LOL::help: