HSS cutoff tool with aluminum bogging motor

[BentTooner]

12-5-11
Hi,

I'm new to machining.

I'm trying to part off a 0.900" aluminum rod on the lathe of my HF5980 "3 in 1 tool."

I've got a new HSS parting tool (straight out of the box) set to cut at the midway point of the rod, sticking out about 1/2", I'm cutting near the chuck and the lathe is set to the slowest speed.

I'm using cutting oil but the cutting tool bogs down and stops the lathe motor when I advance it into the rod.

I put a straight, pointed lathe bit in the tool holder and I can cut 90 degrees directly into the rod with a nice ribbon of aluminum coming off the bit. No problem. I could cut off the part with this bit and then face the end to get a square end, I guess... Seems wasteful.

Was I supposed to sharpen the cutoff tool or is there something else I'm missing?

Thanks so much for any pointers!!

[Torch]

I'm trying to part off a 0.900" aluminum rod on the lathe of my HF5980 "3 in 1 tool."

I've got a new HSS parting tool (straight out of the box) set to cut at the midway point of the rod, sticking out about 1/2", I'm cutting near the chuck and the lathe is set to the slowest speed.

I'm using cutting oil but the cutting tool bogs down and stops the lathe motor when I advance it into the rod.

I put a straight, pointed lathe bit in the tool holder and I can cut 90 degrees directly into the rod with a nice ribbon of aluminum coming off the bit. No problem. I could cut off the part with this bit and then face the end to get a square end, I guess... Seems wasteful.

Was I supposed to sharpen the cutoff tool or is there something else I'm missing?

Parting is probably the most finicky of all lathe operations. From what I've read, the proper term is "The Dreaded Parting Tool".

Seriously though, your machine should be able to part aluminium. At a guess, something is flexing / shifting and the tool is digging in. Make sure your gibs are carefully adjusted so there is no play in the table. Make sure there are no chips under the tool post when you mount it (can allow the post to rock slightly). Lock the carriage, and you can even try slightly snugging the cross-slide lock -- just enough to apply a little drag, not enough to actually lock it down of course.

It's also possible that you are turning too slow. I find parting is best done at the low end of the range for a given diameter, but overly slow isn't necessarily better. Parting 1" aluminium, I think I would probably start off around 500 - 800 rpm and only reduce it further if I experienced chatter. Lots of oil is good -- you can't apply too much oil when parting!

I would definitely examine the tool and make sure it is properly sharpened. What kind of tool and holder are you using? Are you using a thin parting blade, designed for a quick-change tool post holder? Those holders position the blade at an angle, so the tool effectively has back-rake "out of the box", so if the blade is mounted in a straight holder, there is insufficient rake. Without rake, the tool is rubbing more than cutting.

Speaking of the rake, make sure the blade centre line is vertical, not the side! Those thin parting blades have a slight clearance angle ground into both sides. If you just slap it in the holder, the blade will have no clearance on one side, and excessive clearance on the other.

One last thing about the blade, make sure the blade (not the holder) is square to the chuck. Absolutely dead-nuts square. If it's kicked at all, the blade will quickly start rubbing on the side of the cut and if you continue to force it, the cutting tip will probably shatter.

One technique I have heard of that is apparently useful on a less-than-rigid machine is to mount the tool upside down and come in from the back side of the work piece. That way, if things flex, the tool is pulled away from the work instead of digging in.

When you do get everything just right, keep a steady pressure on the infeed and apply lots of oil and the blade will just sink steadily in with a steady stream of chips flowing off the cutting tip. If parting off a longer piece, keep the tail supported with a centre, but don't apply pressure with the tailstock -- just use it to support the end and keep it from tipping and pinching the blade.

[BentTooner]

Hi,

Thanks so much for the help!

Of everything you said, I think I had checked all of it except the back rake. I am using a thin parting tool. I made the blade holder myself. The blade is very rigid in the tool holder and I carefully squared it up to the part. But, the parting tool is simply horizontal.

Parting Tool in tool holder on lathe

PartTool1.gif (27.84 KiB) Viewed 9621 times

Should I make an angle so the 'rear' (the end of the tool that is nearest to me) of the tool holder is lower than the 'front'?

Thanks again!

[Harold_V]

Should I make an angle so the 'rear' (the end of the tool that is nearest to me) of the tool holder is lower than the 'front'?

That isn't a good idea unless you have means of adjusting the tool for center height. Otherwise you'll have to use the tool with a given projection from the holder, which may or may not be right for a particular application.

I've read your post with considerable interest. Having done my share of parting through the years, I've never experienced a parting tool that would stop the machine. Chatter and breakages are common issues, however. Something isn't adding up, at least for me.

You might consider posting your pictures larger. 1024 x 768 is now very acceptable, with the updated board software. It will size them as a thumbnail, which can then be clicked by readers to enlarge them such that detail can be observed. A picture of the cutting edge, looking at the tool from the end, would be very helpful. It may be that you have an issue with tool geometry that can be easily resolved. Hard to say without being able to see the cutting edge and sides of the tool.

Harold

[Torch]

First of all, I'm pretty new to all this too so anything that Harold advises is worth 10x whatever I have to say!

Here's a pic of my parting tool holder:

Notice the knurled nut used to adjust the height as required? I think that's what Harold is referring to. If you don't have something like that, the only way to adjust the height is by sliding the blade in and out along the "ramp" -- hard to be precise and may not place the blade far enough out to part a larger diameter. However, that angle provides rake without grinding the top of the tool -- less blade wasted when re-sharpening. Without it, I think you need to grind a bit of relief in the top of the tool, otherwise you are scraping rather than cutting. (I'm sure Harold will jump in here if I'm wrong!)

This picture of from the business end:

Note how the blade is tapered, but the holder is square-sided. When the blade's centre line is vertical, there is a small gap at the bottom between the blade and the holder. (I understand that some holders have that angle ground in, but my Chinese one doesn't).

BTW: Looks like you made a nice job of your holder.

[BentTooner]

Hi, Harold and Torch

Thanks for the replies and the great photos.

I can see Torch's holder is way more complicated than I was thinking. It seems very nice.

Also, I'm not sure but the blade end looks different... Torch's looks tapered to me. My end is just square. I did NOTHING to it. I just took it out of the Harbor Freight box and used it.

I don't have a camera that will get close enough to show anything useful in focus. My son has a nice camera and I'll ask to come over and take some pictures.

In the meantime, is the thin parting tool supposed to be ground to a tapered point? Because, mine is definitely not tapered at all.

Thanks again for the help!

[Torch]

The taper provides side clearance. I wonder if the lack of clearance is what is causing your stalling? Wait to see what Harold advises, but my bet is a) grind a slight back-rake on the top and b) grind some side clearances.

[Harold_V]

The taper provides side clearance. I wonder if the lack of clearance is what is causing your stalling? Wait to see what Harold advises, but my bet is a) grind a slight back-rake on the top and b) grind some side clearances.

I'd say the same thing you've been saying. The only thing I see with your (Torch) parting tool that might send up a red flag is the front relief. Too much relief can yield hogging. While the relief is intended to provide operational clearance, too much lowers cutting resistance and allows the tool to hog when it might not otherwise. I've pretty much always tried to connect rake and front/side relief in such a way that they work in tandem. Each has an optimum point where cutting pressure is minimized, yet the tool isn't inclined to hog.

Material choice makes a huge difference when parting. Some metals simply machine well, while others are difficult. Mild steel is actually one of the more trying of materials, due to its sticky, tearing properties. When you can machine mild steel with success, you can usually handle most other materials in stride. Unfortunately, for me, I had almost no experience with that material until I left my first place of employment. I'm not convinced that I am all that good, even today, machining that lousy stuff.

A comment on a cutting tool right out of the package----in particular, one made in China. (Note that I am not down on Chinese equipment---I am not critical of those that make such things their choice. A Chinese tool of any description is better than no tool).

The point being, with items such as lathe tools, they should be considered as blanks. They are not (normally) intended to be used out of the box---it is expected that they will receive at least a minor tune-up, so they provide the required geometry to cut well. Without seeing the cutting edge up close, I would expect it is neither sharp, nor straight. It may be deflecting to one side or another, or it may be rounded at the point of contact. Such a tool won't cut, and is likely to stall the spindle on a modest machine, in particular if the belt is kept slack, as some prefer. A good friend, an excellent self made home machinist, prefers his belt slip when he gets in trouble. That wouldn't fly in industry, for you can't get the proper work out of a machine, but it makes sense for the guy that is not well seasoned and, maybe, inclined to do "stupid stuff", things we've all done at one point in time or another.

Sharpening a parting tool can get complicated. That's why I grind my own, from blanks. I prefer they have relief in two directions from the cutting edge, both top to bottom, and front to back. That's easy to do when you hand grind the original form, but preformed parting blades don't offer that option. If you modify the blade in the way of tapering towards the rear, you render the ground portion useless once the cutting edge is slightly dulled because the tool now creates a kerf narrower than the blade. It can be used ONLY for cuts as deep as the grind is long.

It's also nice to have back rake. That makes for a much easier and nicer cut. In a case where the tool can be adjusted vertically, as it can be with a quick change, I heartily endorse a tool that provides a few degrees, much like the one shown by Torch.

Bent, it's really easy to know if a parting tool will cut, or not. Look for these things. The widest part of the tool MUST be the cutting edge. If that is not the case, the kerf created won't accommodate the body of the tool in use.

The cutting edge must be sharp, and on center. When the tool makes contact with the part, the only thing that should touch is the cutting edge. If contact is made anywhere else, the tool can't cut--it will use the contact point as a bearing, albeit a damned poor one, and float the cut. That means if you have the least bit of rounding at the cutting edge (a radius or deformation that runs from the top of the tool towards the base of the tool, on the front face), the tool will drag at the contact point and often refuse to cut, or cut only with unreasonable pressure, often when the material climbs the tool slightly. That's a very dangerous condition, one in which you risk breaking the parting tool, scrapping the job, and, on very slender spindled machines, bending the spindle (been there, done that, on a Craftsman 109).

Another consideration is if there's any rounding of the sides of the tool. Any rounding on either corner that isn't consistent, top to bottom, once again, forms a cut that is narrower than the tool, so binding is the result.

Do not attempt to part dry. Very few materials will permit such a cut, and you may not encounter then aside from leaded steel. Lubrication assures that the chip will flow easily, and that there is no chip welding at the tip of the tool. That's a common problem.

Hope some of this helps.

Harold

[Torch]

The only thing I see with your (Torch) parting tool that might send up a red flag is the front relief. Too much relief can yield hogging. While the relief is intended to provide operational clearance, too much lowers cutting resistance and allows the tool to hog when it might not otherwise.

Yes, I should have swapped ends again before taking the picture. The day before I was working Delrin, and I've found it likes a very sharp angle; otherwise it tends to fold and jam rather than cut. So I keep one end of the blade sharpened with a little extra. The other end of the blade is more suitable for most metals.

[BentTooner]

Hi,

Thanks again for the help!!

Wow, that's a lot of info...

I'm trying to digest it all. For now, here's the part that I can grab onto at the moment:

"The widest part of the tool MUST be the cutting edge. If that is not the case, the kerf created won't accommodate the body of the tool in use. "

I can see that the cutting tip of the tool (which touches the part) must be wider than the rest of the tool. Otherwise, the kerf will not be wide enough to let the rest of the tool move into the groove being created in the part. That seems to be happening to me.

If I have that right, I think I can see why the wider, tapered, square-tipped tool I have works so well. That wider tool is straight out of the box, too, but it is 0.140" wide at the cutting tip, tapering to 0.125" at the back. Is this the concept of " relief " ?

The parting blade I have, on the other hand, is just straight all the way and is only 0.058" wide with NO TAPER AT ALL. You can measure it anywhere along its length and it's the same 0.058".

So, how do you get a skinny parting tool like this to work? Do you leave the front 1/2" as is and grind back the rest of the tool's length to 10/1000's less than the tip? That seems odd... and tricky.

If I'm getting this point, it would make sense that all parting tools should have a wedge shape built in to them.

Maybe I'm still missing something but it's hard to see how this parting blade I have would ever work straight out of the box. And, dressing it nicely along the entire length (4") would be tricky. Maybe I could mill it down...

[BentTooner]

Hi, again

I just wanted to correct a thought I had earlier...

I don't think it would be necessary to mill down the entire length of the parting blade. You would only need to mill down enough to get it to cut as deep as you plan to go into the part.

Since I'm currently working on a 0.900" aluminum rod, I only need about 1/2" of taper to part off the rod.

Even with this, it still seems tricky to mill the blade down on a taper. I'm probably still missing something 'cause this seems too complicated...

[Torch]

Ok, first off, you don't want to mill your cutting tools -- you want to grind them. If you look in the Resource Library, you will find Harold's discussion on Grinding Wheels and HSS tools, where you can learn all about the best wheels, techniques, etc. But the long and the short of it is you need a bench grinder and you use that to shape the cutting tool to suit the application.

Now, think of "relief" as referring to space between the tool and the work. "Rake" can be thought of as the angles ground to form the cutting tip. So "back rake" is from the cutting tip away from the work on the back (top) of the tool and is the measured angle below horizontal. "Side rake" is from the cutting tip away from the work on the side of the tool and is the measured angle behind perpendicular from the work.

Since a parting tool is not used to cut sideways, it does not require side rake. It does require back rake. The ideal angle varies as to the material: The steeper the angle, the more the tool will try to "hog" or pull in to the work. Some materials (like brass) are prone to hogging, so a 0° back rake might be appropriate. Some materials need a steeper rake to help the tool cut instead of rub. 5° or 7° is usually a good "general purpose" angle for a wide variety of metals and should work reasonably well with your aluminium.

Now, we also need three clearances: one on each side of the blade (that's the taper from top to bottom shown in my second picture above -- and yes, my tool came that way. Some come in a "T" shape instead). With a parting tool, you have very little choice as to the angle because the blade is so thin. The third clearance is on the front of the blade, between the cutting tip and the work, measured from vertical.

Harold pointed out that the front clearance in my picture is excessive. You don't normally want that much clearance because it makes a thin, weak cutting tip that is easily chipped (there's no support under it!) If you look at the opposite end of the blade in my picture (ie the end held in the tool) that angle is more suitable for metals.

Of course, the back rake and the front clearance are measured from imaginary lines. Measured between each other, the included angle is called the "angle of keenness" and is a measure of how sharp the tip is. (Which is why I had excessive front clearance -- I was cutting plastic and needed the extra sharpness. More rake would have been even better, but... )

Clear as mud?