HHS tool holder question

[Abner]

Newbie question
I have a SB 10 with a 4" sq. turret tool holder.
What I want to do is hold 3/8" sq. HSS tool bits.
Watching Tubal Cain on youtube he shows how to grind bits and mentions that the tool holder gives you a 16 deg. backrake.

My turret tool holder is to short to accept the taller tool bit holders I have. Just clamping them in gives no backrake. I don't want to grind a 16 deg. backrake into the top of my bit.
Are there holders for this situation?
Have you guys made holder adaptors? or holders?
what can I do?

[whitis]

Angling the tools, like in an armstrong/williams holder and lantern toolpost, can increase the probability and severity of accidents. When a straight tool grabs, it deflects away from the work; in an angled holder, it deflects into the work and grabs harder. Next thing you know, the work is ripped out of the chuck and becomes a projectile or you are stripping teeth off your gears or ripping your toolpost off the compound.

Ocassionally, you may need the armstrong style tool holder to get into tight spaces. I would be tempted to use the carbide style holders, which hold the bit straight, in such cases and grind the rake on the tool.

Effectively, you are cutting above center even when the tool bit is adjusted on center. The horizontal line from the axis of rotation of the work isn't what is important; what is important is the line between the axis of rotation and where the tool bends. The radius of the workpiece vs the geometry of the tool and holder also factors in as small diameter work can give you some clearance when the tool bends.

This could also be a problem with diamond style HSS toolholders, though in some cases the bit will slide in the event it digs in rather than dig in further.

[Arthur.Marks]

I don't want to grind a 16 deg. backrake into the top of my bit.

Why not

[Paul Alciatore]

Here we go again with grinding tools.

Ovservation one: 16 degrees of back rake is a lot. Depending on what you are cutting, probably way too much. Back rake values in the range of +/- 10 degrees are probably the normal range. The greater the backrake, the more likely the tool will break or wear rapidly. The less the backrake, the stronger the tool edge is. Negative values are considered less and produce the strongest edges.

Observation two: until you are more experienced, I would suggest you start with rake angles in the range of 4 to 10 degrees for HHS tools. You can get some experience with your lathe and your materials and then start to modify the angle as needed.

Observation three: the main thing you need when cutting metal is rigidity. This is an advantage of a turret style holder. It is solid, sturdy. You really don't want to throw this advantage away by adding any extra devices or adapters between the turret holder and the tool. Simple is best.

Now, how to get the proper geometry with your tool holder. Many, but not necessairly all, turret tool holders will hold the tools in a horizontal position. This is not necessairly good or bad, but it is a neutral starting position for the angles so whatever angle you want, that is the angle you must grind. I suspect from your comment "Just clamping them in gives no backrake." that your holder is of this horizontal type. So, you MUST GRIND whatever angle you need to cut with into the tool: both rake and clearance. This means that you should grind the backrake angle on the top of the tool. And grind the clearance angle on the front and side edges as needed.

Another consideration is how you will bring the tip of the tool to the center line of the lathe. This is important because if it is above or below the center line, then the rake and clearance angles will change with the diamter of the work as you cut it. In order to actually use the angles you ground the tool to, you must be on the center line. If you are above it, the rake angle will be greater and the clearance angle will be less. This can become extreme and the clearance angle could become negative. A negative clearance angle would mean that the tool is rubbing on the OD a bit below the cutting edge and the cutting edge is not even in contact with the work. It can not cut under these circumstances. You must have clearance in order to cut. This is true for any cutting edge. On the other hand, if you are below the center line, then the tool may have a lower rake angle and cutting can be harder in a smaller machine. If taken to extremes, the tool can even be drawn under the rotating work with distasterous results. Oh, and if you are facing a part, you must be on the centerline to cut to the center of the part. Too high and you get a bit of a mess. Too low and you leave a button.

Your turret holder has been designed with a particular sized tool in mind. Most such holders do not incorporate any means for adjusting the height of the tool. So you must establish this height to put the cutting edge on the center line by several parameters. First, the tool size. If the tool is too small, it can not reach the centerline without shimming. If too large, it will have to be ground down (on the top side) to the proper height. So it is best to use the size that the holder was intended for. Before standarizing on the 3/8" size, be sure this is the intended size for your holder. Second, the tool must be ground properly. A larger tool can be ground down to the correct height. Third, you can use shims under the tool to bring it up if it is below the center line. A bunch of sheet metal shims in several thicknesses can be very useful. If you use shims, be sure they extend to the front edge of the holder or the tool will be unsupported for whatever amount it is short of it: another loss of rigidity.

Final observation: a turret holder is great for a production situation where several tools can be ground and mounted properly and then hundreds or thousands of identical parts can be made by just rotationg the turret. However, it is not very good in a small or home shop environment where parts tend to be made in singles or very small quantities. A far better tool holding system there is the quick change style holders. These allow a wide range of tools to be mounted and adjusted for the proper centerline height and then removed and remounted in the exact same position in seconds. You are not limited to the four or six tools that a turret will hold and tools can be removed for sharpening and returned to the same position (minus what was ground off) to continue the job with minimal adjustments. And if you must remove a tool from a turret holder, you instantly loose it's position. In a quick change holder, you can remount it and continue working on the same or an identical part with the same dial readings.

[Abner]

whitis - Is it ok then to use a 3/8 HSS without backrake- I'm confused here. I understand your comment about digging in with an armstrong holder- I hadn't thought about that.

Tubal cain showed an @ 83 deg R hand cutter grind. 10 deg relief on the vertical faces and a 10 deg slope on the top. He mentioned the 16 deg tool holder angle (16 deg) multiple times. I assume this is for chip breaking?

Still confused.

AM - My concerns are the constant need to adjust the height after sharpening with shims and how far back would that 16 deg need to be? Is it for chip breaking or ? And that would make a compound angle on the top - 16 deg from the tip back and 10 deg from left to right at the same time? Am I being a wus?

Paul - Ah! Thanks I thought I understood what the issues were. The turret came with the lathe and carbide insert holders as well. My experience using carbide is with a table saw and wood. I was very unhappy with the finish results of carbide on cold rolled shaft steel, and I chipped a lot of inserts.
From your description I could use HSS whit the 10 deg relief(s) and a 10 deg slope on the top, once shimmed to center it could be resharpened and should be aligned to center when reinstalled. Increasing the backrake would help it 'peal' the metal?

[Carld]

The only time you may use a 16 deg back rake is on aluminum and even then it is under certain conditions.

You don't need holders for HSS cutters in a turret tool post unless your using real small, say 1/4" sq., cutters.

As others said, 10 max is all you need and you can use "0" back rake for almost everything you do and be ok. Look in the Machinery Handbook for the tool shapes and start grinding.

I suggest you put the carbide insert holders away until you learn how to grind and use HSS cutters. I also suggest you make a BUNCH of different thickness shims that are the same size as the slot in the side of your turret tool holder, you WILL need them.

Get a bench good bench grinder and have a coarse and fine wheel on it and don't use it for anything but the lathe cutters.

[Arthur.Marks]

No, no offense intended! Grinding rake, if any is to be used, is just how it's done. I'm not too great at the off-hand compound angles either. FWIW, when I grind HSS from blanks, I use this little handy device: http://alisam.com/page/1gu8x/Metalworking.html It comes with instructions---which I believe are a copy from "How to Run a Lathe" book put out from South Bend long ago. Any case, it gives a nice chart with example grinding angles (including back rake suggestions) for different applications which may help you out.

[Carld]

I want to tell you a secret. The back rake toward the tool holder and the back rake away from the leading cutting edge is NOT CRITICAL. I know this will be hard to believe but it's true.

The average front and side clearance is about 5 deg, did you notice I said ABOUT.

The angles on the top of the cutter can be from about 5 deg negative to 5 deg positive. Notice the word ABOUT.

The angles can be more than 5 deg to suit the job it will do. Experiment with it.

Here is the rest of the secret, the angles will vary according to the material being cut and the rpm, feed and depth of cut.

There is nothing magic about lathe cutters. You grind one and if it don't work you regrind it until it works.

Slap the damn cutter on the grinder and grind it and try it. Eventually you will learn what works.

[whitis]

whitis - Is it ok then to use a 3/8 HSS without backrake- I'm confused here. I understand your comment about digging in with an armstrong holder- I hadn't thought about that.

Generally, you will want some positive rake on HSS tools. Different angles are better for different materials. You will find some tables of angles to use for different materials, though the numbers vary a bit. Generally, the more positive the rake angle, the better it cuts but the more fragile the cutting edge. For harder materials, interrupted cuts, or more brittle cutting tools, decreasing the rake angle gives better tool life.
http://www.metalartspress.com/PDFs/S...athe_Tools.pdf
http://www.ic.polyu.edu.hk/student_n...%20Turning.pdf
http://www.rulezman.com/rulezman_wor...nderManual.pdf

Threading may be done with 0 degrees rake; for small diameter work, the rake angle may actually affect the thread profile.

Tubal cain showed an @ 83 deg R hand cutter grind. 10 deg relief on the vertical faces and a 10 deg slope on the top. He mentioned the 16 deg tool holder angle (16 deg) multiple times. I assume this is for chip breaking?

No, the 16 degree tool holder angle gives the top (back) rake on a tool that has not had a top rake ground into it.

AM - My concerns are the constant need to adjust the height after sharpening with shims and how far back would that 16 deg need to be? Is it for chip breaking or ? And that would make a compound angle on the top - 16 deg from the tip back and 10 deg from left to right at the same time? Am I being a wus?

This is one of the reasons for the popularity of quick change tool holders that don't need shims.
Yes, the top face of a cutting tool will generally be a compound angle.

Paul - Ah! Thanks I thought I understood what the issues were. The turret came with the lathe and carbide insert holders as well. My experience using carbide is with a table saw and wood. I was very unhappy with the finish results of carbide on cold rolled shaft steel, and I chipped a lot of inserts.
From your description I could use HSS whit the 10 deg relief(s) and a 10 deg slope on the top, once shimmed to center it could be resharpened and should be aligned to center when reinstalled. Increasing the backrake would help it 'peal' the metal?

Lathe cutting tools don't peal the metal, they shear it. It is a little complicated. But the thickness of the chip is not the same as the thickness of the metal removed.

[jugs]

Here we go again with grinding tools.

So much good info has been posted on here again & again on this & other subjects, that then gets lost in the info-mess, so constantly needs repeating.

How about creating a sticky thread on the Basics of tool grinding that we can direct pepole to.

john