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By Mikey, on August 30th, 2011
Using tip geometry
Here is a table that presents the table angles we need to set when we grind standard lathe tools. Note that the headings at the top of each column eliminate the word “angle” after each heading but otherwise matches the diagram above. The front relief (angle) in the table is the same as the end relief angle in the diagram above. The angles here are very important starting points for modifying tools. If you simply wish to grind standard tools then grind with the angles listed here.
 The values in the table are the number of degrees to which you must angle the grinder table for the particular feature you are grinding. For example, when grinding a tool for aluminum you will angle the table of your grinder to 12 degrees and then grind the side; doing so will create a side relief angle of 12 degrees (your table setting) and the side cutting edge angle (dictated by the angle you hold the tool at as you grind the side) at the same time. You then reset the table to 8 degrees and as you grind the end the table angle creates the 8 degree end relief angle as you shape the end cutting edge angle at the same time. You then reset the table to 16 degrees and as you hold the tool at a 35 degree angle (to the belt) you feed the tool straight into the grinder to cut the top face; doing so cuts the side rake at 16 degrees while creating the desired 35 degree back rake at the same time.
In order to alter the tip geometry of our tools to lower cutting forces: Use the largest angle listed in the table as a baseline, and then add about 25-40%. This works out to about 2-5 degrees higher than the largest recommended values and your tool will work better on your little lathe without endangering the tip excessively. While these changes may seem very small and insignificant the reduction in cutting forces they produce are anything but. The tool used for our cutting force demonstration earlier was ground with these conservative changes and it works well.
Keep in mind that none of the tip angles works in isolation.
- Increasing the relief angles without changing the side rake gives you a sharper interface at the edge so cutting forces decrease and finishes improve but you also limit chip clearance and lose some edge strength because the edge is not supported as well.
- Increasing side rake without altering the relief angles reduces cutting forces even more while improving chip clearance but finishing potential drops off a bit. Because the edge is better supported if relief angles are not altered increasing side rake does not sacrifice edge strength as much.
- The effect of increasing back rake is not nearly as noticeable as when altering side rake. However, increasing back rake will tend to focus the cutting forces at the tip of the tool while helping side rake to flow the chip out of the cut.
- If you increase both the relief and side rake angles then cutting forces plummet but the tip and edge strength is reduced a lot more so be aware of it when the tool will experience high cutting loads. This is less of a concern for finishing tools.
- The shape of the tool has a significant impact on strength. The more mass you have in the tip the more latitude you have with angle changes.
Table Notes:
- Side rake for brass and bronze is not zero in the table but a flat, un-ground top works well for these materials. Profiling tools also have zero top rake. A flat rake keeps the tool from digging into soft, grabby materials or when there is a lot of edge contact between the tool and the work. You don’t have to grind the top but you should hone it.
- Side rake is the angle that varies the most with different materials. In fact, side rake is the key variable at that tip. If you look at the relief angles in the table you will see that they don’t vary much between materials because clearance on a round part is still clearance and once you have it then, well, you have it. When you vary the side rake the included angle of the side and end suddenly becomes more, or less, acute. It is this alteration in included angle, also known as the angle of keenness, which changes to accommodate various materials. Seemingly small changes to side rake can have a major effect on cutting forces and how the tool cuts. Pay attention to side rake and use it to your advantage – it’s a major player.
- Side rake for Stainless and Back Rake for Aluminum are very aggressive. That’s a lot of steel to grind off. You may want to look into grinding chipbreakers when grinding tools for these specific materials. Chipbreakers break the long, stringy and dangerous chips into smaller pieces and are much easier to grind than the whole top of the tool. I won’t go into chipbreakers here but you should know about them.
For optimum performance you can grind a set of tools (rougher, facing and finishing) for each material you commonly work with. They will work far better than general purpose tools and are worth the time it takes to make them. If you grind them as you need them you will have a complete collection in no time. If you do make sets I suggest grinding the same face of each tool before changing your table angle to save setup time.
On the other hand, you can actually get away with less than optimal geometry. For example, you can grind a general purpose tool that will work on mild steel, aluminum, stainless steel, plastics, tool steels, semi-hardened steels and brass. The tool we will grind as an example below is one of these general purpose tools and the tool angles used are typical for a tool that has to cut multiple materials under varying conditions. These tool angles are a compromise, just as the shape of a general purpose tool is a compromise but they work fairly well.
General purpose tools allow you to minimize the number of tools you need to grind. For example, if you had to minimize the number of tools in the drawer you could have a general purpose right hand and left hand tool, a RH and LH general purpose knife tool for facing (see the end of this discussion), a zero-rake 60 degree threading tool and a zero-rake round nose tool with a 1/64”-1/32” nose radius (for between-shoulders work and general cutting of brass). Add a HSS blade-type cut off tool with a 7 degree face used in a rear-mounted tool holder and you can handle most general jobs in a hobby shop.
An important angle you may see that isn’t in any table is called the Lead Angle (LA).
 The LA is not ground into the tool; it is set by positioning the side cutting edge relative to the work piece. In general, a tool is used with the shank positioned perpendicular to the work; in this case, the LA is equal to the side cutting edge angle. However, increasing or decreasing the LA of the tool to suit your purpose works better.
Increasing LA can significantly improve finishes; for this reason MHB recommends using the maximum LA possible as long as there is no chatter. Remember that increasing LA also increases cutting forces because more of the side cutting edge comes into contact with the work. The effect of this is most readily seen on thin, flexible work pieces in the form of chatter. On larger pieces that don’t flex much a large LA can really improve finishes.
If you develop chatter at any time, especially on thin pieces or on hard materials try reducing the LA (move the side cutting edge more perpendicular to the work) and things will improve. In some cases going to a negative LA really helps, especially on thin work or an ambitious roughing cut. The effect of reducing LA is equivalent to reducing depth of cut and increasing feed, which has a positive effect on reducing chatter. Try playing with LA to further modulate cutting forces on the lathe – it can be useful.
Another term you may see is the Angle of Keenness. This is the included angle formed by the side and top of the tool as you look at it from the tip end of the tool. For harder materials this angle is less acute and for softer materials it is more acute. Back in the day, before angle tables became widespread, machinists ground their tools with this angle: less acute for hard stuff, more acute for soft stuff. Since the hardness of the material being cut is accounted for by the angles found in the angle table I ignore this angle but mention it here for completeness. If you were paying attention, this is the angle that changes as you alter your relief and side rake angles. Those old guys knew what they were doing.
Lathe tools come in many shapes, each suited to a particular task.
Continue reading → Grinding Lathe Tools on a Belt Sander – Part 2
By Rob, on August 24th, 2011
 I tried making some machinable wax last night. It’s supposed to be harder than plain wax and mine wasn’t, or at least not noticeably. But I believe I can fix it by re-melting it and dissolving more plastic into it.
Machinable wax is made from ordinary paraffin (candle) wax that has HDPE or LDPE plastic added to it to make it harder, more dense and to raise its melting temperature. I used HDPE that came from milk bottles. The ratio of wax to plastic is about 4 to 1 and I heard you could achieve that by just adding plastic to the wax until it won’t absorb any more. I did that and then used my electric fryer’s basket to strain out the extra. I realize now that the basket had too much undissolved plastic in it and that I probably should have waited more patiently and stirred the mixture more.
Here are some tips if you want to try making some
- I used 2 pounds of wax. I think it was just about the right amount for the 9×9-inch non-stick baking pan I used for a mold. Although it could have held more.
- Both my grocery store (in the canning section) and local Hobby Lobby store sold 1-pound packages of paraffin wax for $4. But Hobby Lobby also had 10-pound slabs for $15 ($1.50/pound). They also occasionally have their candle making supplies on sale and they sometimes have 40%-off almost anything coupons.
- Machinable wax has many uses. I was going to give this batch to a maker/hacker space for their members to use to practice machining. Machinable wax won’t damage cutting tools, it doesn’t create sharp swarf and it can be re-melted and reused. It’s also often used to make prototypes, molds, lost-wax castings and to test CNC programs.
- I wanted to dye my machinable wax a fairly dark blue like you see in the picture. But when it cooled it was a much lighter blue like the spilled wax on the cardboard. I bought my dye from Hobby Lobby, which only offered one choice, a pricey ($5) package containing small amounts of red, yellow and blue. I used up all the blue on just this one batch. I’m going to find a candle making forum and ask the experts where I should go to buy more. I also want to ask them if copper pipe would make a good mold for round wax rods for the lathe.
- DO NOT try to use food coloring to dye your wax. It contains water and when you add it to the melted wax it will boil and spit creating an unsafe condition.
- DO NOT melt your wax over an open flame because it’s very flammable, especially before you begin adding plastic to it. And DO NOT use any kind of heat source that you can’t easily and precisely control the temperature of. Many people make wax candles and it’s not a particularly dangerous hobby. But if you aren’t careful you can burn your house down and/or severely burn yourself.
- You should have an accurate way to measure temperature because pure paraffin wax can burst into flames at 390° (F), although its flash point will increase as you add plastic or other additives to it. Candy thermometers usually cost less than $10 and a non-contact infrared thermometer, which is what I used, can be purchased for $20-$30. I was very cautious and never let the mixture get hotter than 290° because I was making it indoors. I may have turned the temperature up more if I’d been making it outside in my driveway. By the way, paraffin wax melts somewhere around 125° to 165° and HDPE melts at about 266°. The flash point of HDPE is about 650°.
- I wore protective clothing and safety glasses to protect myself in case I spilled any wax. I also wore welding gloves when I poured the wax into my mold.
- I saved plastic milk bottles for a long time to get enough HDPE. I used scissors to cut them up and I didn’t even try to get their paper labels off. I just cut them out and threw them away. Many other containers and bags are made from HDPE and you can identify them by looking for the triangular recycling symbol they almost always have. It will usually tell you exactly what kind of plastic it is made out of. If you want you can buy HDPE from Enco and other machine shop supply companies. They will probably call it “High Density Polyethylene.”
By Rob, on August 22nd, 2011
 I needed a large and very flat work surface to build my CNC router table on. I didn’t have a big enough workbench and I didn’t want to take the time or spend the money to build one. And concrete floors aren’t as flat as you might think, especially mine. So I used a 3/4-inch thick piece of MDF, some 3/8-inch tee nuts and bolts to build a temporary flat work surface on my basement floor. Here’s how I did it:
- I marked the locations for the tee nuts by drawing lines 3-inches from the edges of the MDF to leave room for the pieces of my router table. I also found the center of the board by drawing diagonal lines from the corners.
- I installed 3/8-inch tee nuts at the corners of my drawn rectangle, at the center of each side, and at the center of the board. Then I put bolts in just the four corners and screwed them in about half-way.
- The next step was to level it, which was easy. I placed a long construction level on the diagonal between two corners and adjusted the bolts in those corners until it was level (at least one of the other two bolts should not be touching the floor). Then I leveled the other diagonal by adjusting its corner bolts.
- I double checked to make certain all the edges were level and made some small tweaks. When I was happy with the results I screwed in the other bolts until they just barely touched the floor. Their only purpose was to make certain the MDF didn’t sag where it wasn’t supported by the corner bolts. They may not even be necessary, especially if you’re only going to use it for a short time.
A machinists level is more precise
I have an 8-inch Starrett machinist’s level that I’d never really used before this. I put it on top of my construction level and was amazed at how much more precise it was. And when I put it directly on the MDF it showed small raised areas around the tee-nuts that I’d probably created by pounding them in with a hammer. They weren’t enough to matter but I could have avoided them by hammering the nuts in just enough for their teeth to make contact with the MDF. Or I could have ground or filed their teeth off leaving only the flanges because there was probably enough friction in the holes to keep them from turning.
By the way, don’t buy a traditional machinists level unless you can get one for a very good price like I did. Digital levels cost much less, they’re at least as accurate and they can do more tricks. I’ll say more about that when I talk about getting my router’s rails parallel both horizontally and vertically.
MDF or Particle Board?
I was going to use particle board instead of MDF because it’s stiffer, stronger, cheaper and the sheets I’d bought in the past always seemed to be very flat. I’d put a sheet of particle board in my shopping cart but when I went to have Home Depot cut it I noticed it had a very noticeable curve. An employee was waiting so I quickly decided to use MDF instead. MDF seemed to work well for this project but I’m not sure it was the best material to use. I’d like to know what you think.
Giving credit where it’s due
I didn’t come up with this idea myself. I learned it from this video which shows it being used to create a level surface for candle molds.
Continue reading → How to build a flat work surface
By Mikey, on August 15th, 2011
 I’m a believer. As a new guy I totally sucked at grinding lathe tools. It’s almost painful to admit how many stubby, misshapen, multi-faceted, overheated and just plain ugly lathe tools I made back then. The amazing thing is that some of those tools actually worked as well as the pre-ground tools that came with my lathe. I started to believe those guys that tell you, “… just get it close and it will work.” Of course, I was too embarrassed to call myself a hobby machinist with those Franken-tools so I bought an expensive set of inserted tip carbide tools that I thought would make a major difference but was disappointed. They couldn’t rough as deep or finish as well as my ugly high speed steel (HSS) tools, at least not on my lathe, so then I believed those guys that tell you, “… stick with HSS on a hobby lathe.” Hey, desperation can make you mighty receptive, you know.
Fast forward 15 years and now I believe, after having ground many experimental tools, that the best lathe tool for a hobby-class lathe is a HSS tool with its tip geometry modified to reduce the cutting forces it produces, ground on a belt sander.
Since standard tools are intended for use on industrial lathes their geometry can produce cutting forces that are excessive at times, especially when roughing but this can also affect sizing and finishing cuts. To be clear, cutting force is that force produced by the tip geometry of the tool that must be overcome to make a cut. We can look at it as a continuous resistance generated by the shape of the tool as it is pushed through the material during the cut. A standard HSS tool has a broad, wedge shape and creates a lot of resistance (carbide is even worse), so we must dial down our control inputs (depths of cut, feeds and speeds) to use them. If we alter the tool so it has a narrower included angle it cuts with less resistance, thus lowering the rigidity and power needed to make a given cut. We still have to manage our control inputs but since the cutting forces are lower we would expect to be able to cut deeper, achieve better accuracy in sizing and finishing cuts, and finish better with greater ease before running into the rigidity and power limits of the lathe. And this is exactly what happens.
To demonstrate this I took a 0.050” deep cut in 12L14 mild steel on my admittedly older hobby-class manual mini-lathe using a very good quality inserted tip carbide roughing tool with a new insert, a sharp freshly ground high speed steel (HSS) roughing tool with standard tip geometry ground for steel, and the general purpose tool with modified tip geometry (not optimized for steel) that was ground for this discussion.
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The carbide tool made the cut but chatter was excessive. Speed was as low as I could reasonably go and the feeding force was very high. The finish is really ugly due to all the chatter and the chips are tiny and powdery. Reducing the depth of cut to 0.010, the proper depth of cut for this insert, allowed it to cut as it should. |
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The HSS roughing tool did fair but there was a lot of chatter. Speeds and feeds had to be adjusted almost constantly to make it this far. Finish is rough but not excessive for a roughing cut. Chips are tightly curled due to the standard side rake. Reducing the cut to 0.030” allowed the tool to work much better. |
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The modified tool cut easily, speed was about 100 RPM higher than with the other tools and there was no chatter at all. Finish is much better compared to the other tools. The chips look more like loose shavings due to the sharper included angle at the tip. |
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Chips from the carbide tool, HSS rouging tool and the modified tool are distinctly different. You absolutely cannot fake a chip. They tell you exactly what is going on at the tip of the tool. |
Continue reading → Grinding Lathe Tools on a Belt Sander – For the New Guy
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