Grinding Lathe Tools

Grinding Lathe Tools

Tool grinding is part science and part art, but can be an enjoyable side activity to working with the lathe. My goal here is to teach beginners enough to get them started with a few basic tools. Here are some additional resources:

You can, of course, buy ready-made carbide-tipped cutting tools.  If good quality, these work well and last a long time. The real advantage to grinding your own tools is not cost savings, but having the ability to make a tool for whatever purpose you may run across in your work.  For example, I have made some very small boring tools from 3/16" tool blanks which are quite handy for boring out a small hole, say .373 in diameter, to press-fit a 3/8" shaft.

The essence of lathe tool grinding, as I think of it, is to undercut the tip of the tool to provide 'relief' so that the metal just below the cutting tip does not contact the work. This concentrates enough cutting force on the tip to cut into the metal of the workpiece.

Most of the regular cutting tools I make are undercut on the front and left edge of the tool. Since most tools are designed to cut while moving from right to left (towards the headstock) it is not necessary to provide relief on the right side of the tool. Additionally, I usually grind a similar relief, or rake, on the top surface of the tool.

When you order your lathe, be sure to order about 10   5/16" x 2 1/2" High Speed Steel (HSS) tool blanks. (Note: the older, non-Sieg, Homier/Speedway lathes use 3/8" x 2 1/2" tools) I usually get mine from Enco. They are normally about $1 each but often go on sale for about $0.80.

The four sides of the blank are ground to a smooth, shiny finish. The ends are a coarse finish with a preformed angle of about 15 degrees.

  

We will use a simple four-step procedure to make our cutting tool

1. Grind the end relief
2. Grind the left side relief
3. Grind the top rake
4. Round the tip

Grinding the End Relief

First we will grind the end of the tool blank. Use the coarse wheel of your bench grinder and hold the tool blank angled downwards from the tip to the rear and with the tip pointing to the left about 10-15 degrees.. The tip of the tool blank should be a little below the center line of the wheel.

    

Grinding causes the tool blank to get quite hot so you will need to dip the end of the tool into a water bath every 15 seconds or so during the grinding operation. When you see the tip of the tool start to discolor from the heat its a good time to make a cooling dip. Fortunately, HSS does not conduct the heat to your fingers very fast, but you can get burned if you go too long between cooling dips.

 

Here's a picture of the tool after grinding the end:

 

Grinding the Left Side Relief

Now we'll grind the left side of the tool. The procedure is essentially the same except that we hold the tool with the side at about a 10 degree angle to the grinding wheel.

 

Grinding the Top Rake

Now we grind the top surface to form the rake. Be careful during this operation not to grind down the cutting edge or you will end up with a tool whose tip is below the center line of the lathe. If this happens, the tool will leave a little nub at the center of the workpiece when you make a facing cut. The usual remedy is to use a thin piece of shim stock under the tool to bring it back up to the center line. A much nicer solution is an adjustable-height tool holder.

After this operation we have a working tool with a very sharp tip. This tool is useful as-is for operations that need a sharp tip to turn down to an interior edge such as a shoulder.

Rounding the Tip

We will round the tip to form a tool that is useful for facing and turning. Hold the tool so the tip touches the wheel and with the tool tilted downward. Rotate the tool against the wheel to round the tip to about a 1/32" radius. 
 
 

Here's the finished tool in action making a finishing cut on a facing operation:

As a final step, you may wish to smooth the cutting tip on a fine diamond hone or oilstone. I've found that the tip tends to get smoothed pretty quickly after a few cuts, so I usually skip this step, but it does make a difference if you need a fine finish.

So now you know how to grind the most basic of cutting tools. There are many other types of tools that you can grind including shaping tools, cutoff tools and boring tools. Here's some additional info posted by Brian Pitt:

You usualy want to set the tool on center to about .003-.005 above center but almost never below (the workpiece will try to roll over the top and chatter) cutoff ,threading and most carbides should be on center while HSS for general turning can be a hair above to make up for the deflection of the workpiece and machine which will bring it back on center, it can also add a little burnishing action as the work rubs the front face of the bit and will give smoother finishes the side and top rake reduce the amount of cutting force and heat generated and help to control the chip by giving it a difrent angle to curl on depending on the material being cut , softer and stringier materials like aluminium and 1018 steel need more rake to give a knife like edge while materials that break up easily like brass and cast iron use less rake to keep the tool from digging in and grabbing

One of the most important facts often overlooked by beginners is that the cutting face of the grindstone wheel quickly becomes dulled and clogged with metal particles.  To maintain an agressive cutting face it is essential to refurbish the wheel face frequently with a dressing tool.  I use a dressing tool with a single industrial diamond point for this purpose. I got it from Enco for around $15.  I use this tool to refresh the grinder wheel after about every 10 minutes of grinding time. It makes a big difference.

Here's some great information on various types of tool rake, posted by Dub Thornton:

No, rake angles and front relief are NOT the same. Front relief is the angle ground into the front of  your tool which allows ONLY the front cutting edge to contact the workpiece. If the tool contacts the workpiece below the cutting edge, you will get a "rubbing" action, and the tool cannot bite into the workpiece. Side relief (or clearance) is the angle ground into the side of the tool which allows only the side cutting edge of the tool to contact the workpiece.

There are two rake angles, both on the top of the tool. Back Rake is the angle from the tip of the cutting tool toward the back of the tool. It may be either positive, neutral, or negative. If it slopes down from the tip of the tool toward the back of tool, it is positive rake, an upward slope would be negative rake, neutral is self explanatory.

Rake angles, particularly back rake, may be built into the tool holders. The old lantern type holders I grew up on usually had a positive back rake angle built in. This was a help when grinding your tools, as you did not have to grind any back rake into the tool itself. When grinding threads tho, a neutral back rake angle is desirable, thus one had to grind a negative rake angle (point of tool pointed downward) to compensate for the positive back rake angle built into the tool holders.

Side rake is the angle from the side cutting edge of the tool toward the opposite side of tool (across the top of the tool). It can also be ground for negative, neutral, or positive side rake. A negative rake angle is usually used for brittle materials, such as brass, which are notorious for "hogging in" as you cut. A positive rake angle would increase this "hogging in" action, while a negative rake angle will push the tool away from the work, eliminating the tendency to "hog in". "Hogging in" is an old term for the material grabbing the tool, pulling it into the material for a deeper cut than you are set up for. Backlash in your machine increases the possibility for such"hogging in". Many projects are spoiled, as well as tools broken, etc, by this action.

A very general rule of thumb. For heavy, roughing cuts, use less clearance and rake angles. This leaves more material in the cutting tool to withstand the pressures of heavy cutting, plus more "beef" in the tool, means more ability to carry heat away from the cutting edges. For finish cuts, and turning such "soft" materials as aluminum, use more clearance and more rake angles for a better finish. I have seen may references on this reflector to Top Rake, which is confusing to me, as it does not define whether side rake, or back rake is being referred to.

Compiled by Rahul KP 

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Awhile back Mcgyver posted an excellent writeup of his build of a Universal Grinding Fixture on the HSM forum:

I wanted a universal fixture for the T&C grinder, capable of holding drill bits and lathe tool bits so that I could easily to facetted drill sharpening, acme or V thread bits etc. I had fun taking pics through the build and thought you guys might enjoy them

The assembly was fabricated using my stick welder





Here's an overview of the unit after welding



I sent everything out to be normalized and had the drill V-block case hardened at the same time. Well worth the $30 to know its going to be reasonably stable



For accurate work, you need a reference surface. I decided to scrap in one side of the elbows so there would be no distortion from clamping. Started with the power scraper




and finished with a hand scraper. Almost complete blue-out - good enough for the girls I know!



I bolted each elbow to a tool makers block (made by yours truly, hardened and ground square to 1/10 thou all over and done as a matched pair).



The elbows are now all squared up.



I then made an arbor for the lathe to clean up the circumference.



Here's an elbow, square and ready for divisions



I graduated each axis in degrees - TEDIUM! Gotta get the 4 axis cnc built!



With one disk graduated, the other needs a witness mark. Here I'm using an indicator to get the disk's face parallel to the machine's axis before make a witness mark on the base



Graduating all done!



Here's progress to date, I mainly included this shot to show the tool bit holder attachment.



Now things get messy. I used auto body filler, spot putty and a couple of coats of cellulose primer with sanding in between



This is chronologically a bit out of order - machining on the V-block was done prior to case hardening. Here am drilling the two longitudinal holes, one for the tooth rest gizmo and the other for rest at the end of the drill. The two bars the make up these mechanisms are held via cotters - you can see the brass cotter the intersects the hole and the 1/4" ready rod holding it in place



Here the V is being cut.



Here's the drill stop - an adjustable gizmo that the end of the drill rests against. There are a couple of different attachments for this - ie a long thin adjusting screw for very small dia drills and different lengths of the long rod. It will hold drills from 1/16 to 1" or more



I made this "third hand" rig and posted pics of it here awhile ago, and a few wanted to see it in use - here you go. It's a real time saver, otherwise to do this job properly you'd be pinning or such the pieces together. The work piece is the {rest missing}



I get the best silver solder results by placing small pieces of solder in the flux and then just warming everything up



Here's the tooth rest body after pickling and slitting



The tooth rest is almost done. There will be a micrometer feed on the end, I still have to make the screw and end piece. The idea is that between its built in adjustments and the convenience of the cotter holding it in the V block, its quickly adjustable to any position and then the micrometer adjustment can bring the tooth rest do the final position



The V block is being set up for grinding. It was ground all over after case hardening and an oil quench.



Here's the completed unit. I used the Tremclad hammer finish paint - I don't think I'd recommend it and probably won't use it again but don't have energy right now to refinish.



More Comments:

Graduations were done with the work held in a dividing head and a sharp V with some positive rake ground on a round hss blank held in a collet in the mill. The mill has a friction brake, I locked this on (with duct tape, handy mans secret weapon) and cut each line by moving mill table, cut was about 5 thou deep, 5 & tens marks received longer lines. I've not yet stamped the numbers, will probably use the pillar tool with some sort of rig, but haven't thought that out yet.

It's all mild steel except for the obvious brass pieces. A lot of the photos do look like AL, that's because after normalizing the heat treater kindly ran them through his sandblaster.

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In an update, Mcgvyer posted the following:

For those of you not sick of my grinding pics, here's a follow up and some shots of it in use

First off, I redesigned the micrometer tooth rest - it needed more of an offset and longer range. Here it is ready to go - 5 separate silver solder joints on this assembly





The V block is case hardened and ground all over, also finished the cotters so both the drill stop and tooth rest is very easy to adjust.



The angles are set on the graduated dials on the universal swivel, I was doing a 118 point with two clearance facets, one at 15 and one at 30 degrees. The drill is placed in the v block and the stop and tooth rest adjusted then tightened down. I generally index the drill and do both cuts on one facet then change the angle of the base to do the second facet or bevel. Over the past couple of week ends I made a mobile stand for bench top T&C - I found it really hard to set up (ie tooth rest) if I couldn't get access all around the machine to view the set up.



Grinding grinding grinding



By making different lengths of rod for the drill stop, the rig can hand almost any size drill



The final result. Yeah, I can sharpen a drill by hand but it will take me a lifetime of Sundays to get so that they are as nice as this. Since they are virtually perfectly symmetrical, the cut very accurately. The one I tried and measured was within a thou, give the limits of small hole gauges.

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