Cutter Geometry

I am sitting in O'Hare airport on my return flight from the 2010 OTI conference in San Jose. OTI is dedicated to the art of ornamental turning, including engine turning. It was great to catch up with some fellow engine turners that I haven't seen in a few years, as well as meet some talented turners who are new to me. One of the conversations that came up regularly was cutter geometry and profiles. The geometry that I am using was proposed by George Daniels in his excellent book Watchmaking, and is appropriate for many non-ferrous metals. I have found this to be the best geometry for brass and silver, and should work well for copper and gold.

The starting point of any cutter is the material of the cutter itself. Traditionally engine turners would have used a high carbon steel, and eventually high speed steel (HSS) for their cutters. These metals will work fine, however, with the introduction of high quality carbide, they have become a little out dated. Last year David Lindow, a clock and rose engine maker in Pennsylvania, turned me on to Micro100 carbide tools. These tools use a micro grain carbide that is exceptionably strong, and can maintain a remarkable polish. I purchase their brazed tools meant for metal turning on a traditional lathe (Travers part# 21-100-907). The square profiled cutters are great as they allow the most flexibility when reshaping for engine turning. Micro100 tools are three or four times the cost of many other brazed carbide tools. The cutter geometry discussed below will work just fine with HSS cutters, or lower quality carbide. Considering the amount of labour involved with engine turning, the additional expense of Micro100 cutters is justified. Each of these cutters will last the lifetime of most engine turners, and the difference in the quality of cut is noticeable. I keep a few cutters for different applications, however, a single cutter will get you started.

The next consideration is the material you intend to cut. Harder metals will require a smaller included angle, while softer materials will require a larger angle. Some experimentation will be required with new materials. I work primarily in silver, and test patterns in brass. Since copper has cutting properties similar to silver, the same cutter should work ok. Moving further away from silver to aluminum (Al) or steel would mean a change in cutter geometry. If you intend to cut Al, maintain a cutter specifically for that purpose. Do not cut silver with the same cutter used for Al. Small amounts of Al remain on the cutter and will affect the quality of cut.

Cutter-Geometry

The first part of shaping the cutter is removing a lot of the unnecessary material. Diagram 1 shows in red the initial material to remove. There is no need to have a cutter that is 0.250" wide. None of your cuts will be that wide, and often times it would get in the way. I use a bench grinder to do the bulk of this shaping. It doesn't need to be precise, and you should have approximately 0.060" left for the cutter. Next remove some of the material from underneath the front of the cutter. You will be cutting the front face with a down angle of 10 degrees. There is no need to polish the tool holder. Diagram 2 shows the top and side profile of the remaining cutter. The left side of the cutting face should be made first. I use a side angle of 18 degrees on both sides. If you are working on a softer metal, like Al, try an angle closer to 14. I use a Glendo Accu-Finish for shaping and polishing cutters. It is a great system for maintaining repeatable angles while polishing. Starting with a 240 diamond wheel, work up to 1200. I form the left face right up to right hand edge. Now I switch over to the ceramic wheels and move through 6, 3, 1/2 and 1/4 micron. At each stage check the surface finish under a loop. The 6 micron wheel will remove all visible scratches from the carbide. By the time you get to 1/4 micron, it should look like a mirror. Now go back to the 6 micron wheel and form the right face of the cutter. The distance from the right hand edge to the point should be around 0.015". Work your way up to the 1/4 micron. It is time to form the relief angle. Tilt the table up to 10 degrees. The relief face should be formed only with the 1/4 micron wheel, and does not need to be very large. At most it should be 0.004" wide (about the thickness of a piece of paper). Be sure that the left and right face are the same size. If they aren't, you will get some striations in your cut.

The cutter is now ready to make some chips. If you take care not to drop or chip the cutter, you should be able to just touch it up with the 1/4 micron wheel. It is important that any touch ups be done before the start of a project. Your cuts will look different if you polish the cutter half way through the project.

Vision

Gear is good, vision is better I wish that I could claim that saying for my own, but the credit goes to David Du Chemin. David is a Canadian photographer out of Vancouver who is known for his humanitarian photography. As well as running an excellent blog about photography, he runs craftandvision.com. Through Craft and Vision he publishes inexpensive ebooks, written by himself and others, discussing the art and business of photography.

I often end up hearing discussions about how such and such new tool is all a person needs to be able to make some awesome piece of art. Or how they could do the same level of work as that artist if they had access to the same quality of tools. Or my personal favourite, people telling me my camera takes great pictures, as if I had nothing to do with it. I regularly get involved in debates about the merit of CNC tools in making art. All of these discussions boil down to the simple truth; gear is good, but vision is better. Artistic vision is the only thing that you need to make great art. You don't need the most expensive camera, or an antique rose engine to produce excellent work.

My shop has an odd variety of tools in it. I have both manual and CNC metal lathes, manual wood lathes, a 4-axis CNC mill, antique straight line engines, a vacuum casting machine on top of hundreds of other hand and powered tools. Some of them are home made and only make sense to me, some require advanced computer knowledge to control, others would not be out of place in the work shops of Faberge or Cellini. But none of these tools are what make me an artist. Some of them allow me to perform repetitive tasks easily, others are the best tool that I can afford to have at the moment. But for all of the hundreds of tools I own, they are not what defines me as an artist. They allow me to turn my artistic visions into reality, but they do not create that art out of thin air.

Regardless of the art you make, you will need some tools to work with. They may be as simple a pencil and paper, or as complicated as a shop full of tools. Some tools will make your life easier. Some tools will allow you to create effects that you can not create any other way. Some tools are the bare minimum that you can afford right now to be able to practise your art. At the end of the day, they are all just the means of creating your artistic vision.

The next time you are questioning the tools used to create a piece of art, consider instead the vision of the person who made it.

Engine Turning Pt 2

As I discussed in my last post about engine turning, I am using a straight line engine in my work.  This post will cover how an straight line engine works in more detail.

There were many variations of engines built, however, they all follow the same basic principles in their operation.  There needs to be some sort of headstock to hold the work piece, and there needs to be a tool slide to secure the graver and the guide, which controls the depth of cut.  As the head stock and piece are moved down, the graver is pressed into the material and cuts a line.  A basic engine such as this is typically called a bordering engine, and was used for creating borders on picture frames.  The variation in the patterns that a bordering engine can produce is very limited.

Most straight line engines have a few other critical features that allow them to create an infinite number of patterns.  The first is a pattern bar holder.  Pattern bars can be bolted in place, and using a few adjustments, can be changed to alter the final pattern that is engraved.  It is important that the pattern bar holder is very sturdy, that it allows multiple bars to be held at the same time, and that the bar can be moved up and down independently from the headstock.  My next post on engine turning will discuss the variations created through this important feature.

The tool slide on the Plant straight line engine has several important upgrades.  The first is an index system for moving the graver from left to right.  Using a ratchet and pawl system along with an adjustable stop, the cutter can be advanced by a known amount between each cut.  Because the human eye is excellent at picking out flaws in repetitive patterns, it is critical to keep the spacing consistent from cut to cut.  A radial adjustment is also a useful addition to the tool holder.  If the work has a curved surface on the edge, such as the side of a case, it is important to keep the graver perpendicular to the surface that it is cutting.  If the graver isn't perpendicular to the surface, one side of the cut will be deeper than the other.

The most important upgrade to the headstock is the radial adjustment.  The most common system to allow radial adjustment of the piece is through a worm gear.  The worm provides very fine control over how many degrees the piece is turned from one cut to the next.  A further improvement is to add a ratchet and pawl system to allow indexed fine control over the radial movement.  In the case of the 14" Plant, each notch on the ratchet wheel moves the piece 1/4 of a degree.  The sunburst patterns created using this upgrade were very popular on cigarette cases and picture frames.

The next post on engine turning will discuss what pattern bars look like, and how variations in their use can lead to an infinite variety of patterns.

Multi-lead threading

In an old blog that I stopped maintaining several years ago, I had a few partial posts discussing multi-lead threads.  To my surprise those posts still attract a large number of hits from Google.  Since those posts really don't do a great job of explaining how to use multi-lead threads in pen making, I thought an updated post here would be beneficial to my readers.

Multi-lead or multi-start threads are commonly used in consumer products to decrease the number of turns required to screw on lids and caps.  It would be very frustrating to require eight or nine turns of a jar lid to tighten it, or three or four turns of a pen cap to remove it.

My main use of multi-lead threads is on pens, so the example I will show is for a 4 lead thread that is based on an outside diameter of 0.492" on the pen barrel.  I will also be talking about how to code this thread in Mach3 for turning on a CNC lathe.  I have done it once or twice on a manual lathe and I'll talk briefly about how that works.  Please understand I am not a master machinist.  There are people out there who really do know what they are doing, and I expect that some of my practises would frustrate them.  This is what works for me and many other pen makers that I know, and was developed primarily from experimentation.

A few things up front that I recommend you get if you are serious about experimenting with these types of techniques.  First off you need a metal lathe.  This may sound obvious but let me explain.  Many pen makers are using wood lathes.  Unfortunately these do not have the ability to thread using a fixed cutter.  If you want to thread with them you are reduced to using taps and dies.  That is ok for many applications, but really forces you to use standard sized threads.  Custom taps and dies are available for multi-lead threads, but you are usually looking at hundreds of dollars to have them made.  Being able to cut threads yourself on the lathe allows you to use odd diameters and thread pitches to accommodate what you want to design, instead of designing to a standard.  A small metal lathe is fine as long as your work fits on it.  Just remember that it is easy to make small parts on a large lathe, and usually very dangerous to make large parts on a small lathe.  I usually recommend a 9x20 as a good size, and variable speed control will make your life much easier.  There are plenty of resources out there with detailed reviews of the different 9x20 lathes on the market.  Please learn how to use the threading feature of your lathe on standard threads before you try multi-lead threads.

Buy a copy of the Machinist's Handbook.  Unless you need all of the latest and greatest CNC info in it, an older copy will do just fine.  Yes a new copy will set you back $100+, but it is worth the money.  I keep one in the shop and one in the office for quick reference when designing parts and coding.  Unless you have great eyesight, avoid the small toolbox edition.  Get a good set of digital calipers.  The $30 set you bought is not giving you accurate measurements.  A good pair of Mitutoyo calipers will be expensive, but as long as you don't drop them, they will provide you with accurate measurements for years.  I've saved myself the cost of those calipers several times over in the past 3 years just by avoiding remaking parts that weren't measured properly.

Normal threads are expressed using a diameter-pitch notation.  For instance 1/2-20 or 1-8.  Lets take a look at the 1/2-20 thread first since it is the closest in diameter to the one we're discussing.  The 1/2" diameter refers to the outside diameter of the male thread.  It is also called the major diameter.  The bottom of the thread is referred to as the minor diameter.  For a 1/2-20 thread the minor diameter is 0.438".  I'm not going to go into all of the details about theoretical and practical threads, but the minor diameter is the theoretical bottom if you used a sharp "V" cutter.  The reality is that the bottom of a 1/2-20 has a diameter of 0.446" (you can read all about the math behind threads in the Machinist's Handbook.)  That means that the thread is 0.027" deep at any point.  With a pitch of 20 threads per inch, it will require 20 turns of a nut to move it 1".  I usually thread 1/4" of the barrel.  If we use this thread on the cap of a pen, it will take 5 turns of the cap to close the pen.  That is just not acceptable.

To close the cap faster, we need a coarser thread.  An 8 tpi thread is going to allow the cap to close in 2 turns.  Unfortunately the smallest standard thread that uses a pitch of 8 tpi has a major diameter of 1".  Not exactly ideal for the barrel of a pen.  On top of that, an 8 tpi thread is more than twice as deep as a 20 tpi thread.  Such a coarse thread doesn't look very attractive on a pen.

This is where a multi-lead thread comes in handy.  We can combine 4 threads that each have a pitch of 8 tpi.  When you look at the finished thread and use a thread gauge to determine the pitch, it will look like a 32 tpi thread.  Because the combined pitch is 32 tpi, we'll use that to determine the minor diameter.  I use a major diameter of 0.492", and minor diameter of 0.460".  This is a little deeper than a 32 tpi thread should use, but I've found through experience that it works well for the plastics that are common in pen making.

The first step in actually cutting this thread is to turn down the outside diameter to 0.492".  In the example photographs below, I've turned down the first 1" of a brass rod.  Next step is to turn a single thread that has a pitch of 8 tpi, to a depth of 0.460".  The next bit is to figure out how to cut the second thread so that it is offset from the first by an appropriate amount.  The easiest way is to start the next thread 1/4 of thread pitch farther way from the piece.  In this case 0.031" (there is 0.125" between each peak of the thread, divided by the number of leads we have, gets us to 0.031".)  Same thing for the next two leads.  Keep moving the start point of the cutting cycle 0.031" farther back until you have cut all four threads.

Lets take a look at the g-code used to thread my pen barrels.  All of these examples work with Mach3 Turn.  If you are using a different control software, things may change slightly (although the dimension numbers should all be the same.)  Here is the code that I used to turn the piece in the photos below:

01: G18 G40 G49 G90 G95 G80
02: M3
03: (TOOL# 6 THREAD 4 LEAD 8TPI THREAD FOR 0.950)
04: T0606
05: (SPINDLE 500RPM)
06: S500
07: #1 = 0.492 (START X)
08: #2 = 0.460 (FINISH X)
09: #3 = -0.950 (LENGTH TO THREAD)
10: G00 Z0.1
11: G00 X#1
12: M08
13: G76 X#2 Z#3 Q5 P0.125 J0.001 L45 H0.002 I29 C0.025
14: G00 z0.131 X#1
15: G76 X#2 Z#3 Q5 P0.125 J0.001 L45 H0.002 I29 C0.025
16: G00 Z0.162 X#1
17: G76 X#2 Z#3 Q5 P0.125 J0.001 L45 H0.002 I29 C0.025
18: G00 Z0.194 X#1
19: G76 X#2 Z#3 Q5 P0.125 J0.001 L45 H0.002 I29 C0.025
20: M09
21: G0 X2 Z2
22: M5
23: M30

If you drop this code into Mach3, please remove the line number: from each line or it won't work.  Any line with ( ) is a comment and will be ignored by Mach3.  It is very useful to comment your code so you can remember what you need to do 6 months down the road when you want to re-use it.   Line 1-2 are the initialization strings that I use for my lathe.  Those will probably change for yours.  Line 4 is telling Mach3 to change to tool 6.  That is my threading tool.  Line 6 is setting the spindle speed.  I'm using a speed of 500rpm because that is what I'm comfortable using for on my lathe and this material.  You will need to experiment to find a good speed for your lathe.  Start slow (no more than 100rpm,) and slowly work up to something more comfortable for you.  Remember to take into account that your stepper motors must be able to accelerate fast enough to keep up with the spindle.

I am using variables in this code so that I can easily change the parameters that I modify from project to project.  On lines 7-9 I set the start and finish diameters, as well as the Z co-ordinate to cut to.  Line 10 is the start point of the cut for the first lead.  I don't start it at Z0 because the steppers need a little distance to accelerate up to full speed.  You may need to increase this number depending on your setup.  Line 11 the cutter is moved to the starting diameter.  Line 12 is where the threading cycle starts.  M08 and M09 tell Mach3 where the threading cycle begins and ends.

On the G76 lines, the following variables have been used: X is the minor diameter, Z is the co-ordinate to cut to, Q is the number of spring passes, P is the pitch, J is the minimum depth of cut, L is the angle to taper the end of the cut, H is the depth of the first cut, I is infeed angle and C is the clearance in X.  J and H in this example are very conservative numbers.  Depending upon the material and machine, you will be able to take deeper cuts on each pass.

Line 14 is important.  This is the line that sets up the second lead.  For the first lead we started the cut at Z0.100.  The second lead starts at Z0.131.  Nothing else changes.  That means it will cut the exact same thread, just offset by 90 degrees from the first one.  Lines 16 and 18 do the same for the third and fourth leads.  Really those three lines are where the magic of multi-lead threads happen.  The last lines are just resetting the cutter and Mach3.

Things don't change much on a manual lathe.  You'll be using a lead screw driven by the spindle, but otherwise the important details are the same.  Setup your lathe to cut an 8 tpi thread.  Set the compound slide to 0 degrees (it should move parallel to the Z axis.)  Engage the lead screw and make sure the spindle speed is set to a low rpm.  Advance the cutter so that it will take a light cut on the first pass.  Start the lathe and keep your finger on the stop button.  You need to be ready to stop the spindle when the cut is finished.  Once you have stopped it, do not disengage the lead screw.  Instead back the cutter out so that it is clear of the piece.  Put the lathe in reverse until you are past the starting point of the cut.  Put the lathe in forward, advance the cutter to take a cut a couple of thou deeper than the last one.  Start the lathe and repeat the procedure from the first cut.

Once you have cut the first lead, reverse the carriage to the start of the cut.  Now you need to move the start point of the cut by 0.031".  Use the compound slide to move the cutter away from the piece by that amount.  Repeat the same cut, stop, reverse pattern of the first lead.  You will now have a second lead cut.  Repeat the same 0.031" movement of the compound slide for the third and fourth leads.

Regardless of if you are using a manual or CNC lathe, please experiment using conservative speeds and depths of cut on a free machining material first.  Once you can reproduce the same part a couple of times in a row, then move on to different materials.

And the final thing I will talk about is the matching nut, or in my case cap.  To make a cap matching the above thread, bore the piece to 0.470" ID, and thread from 0.470" to 0.515"  (if you are using the g-code from above, variable #1 should be 0.470 and #2 should be 0515.)

I hope that all of this rambling has made some sense to someone other than me.  If you have questions, feel free to send me an email.  I can't guarantee that I can answer them, but I would be happy to try.

(Note about the photos below.  Sharp eyed readers will notice that the sequence of the cuts is wrong.  The third photo shows the second lead being cut, except that it is really the third lead.  When I was stopping the machine after the first lead was cut to photograph the piece, it decided to skip the second lead and move onto the third lead instead.  It then cut the fourth lead fine, and I went back and cut the second lead last.  If you follow the instructions above, you will cut them in the correct order, and your piece will look the same as the last photo.)

Engine Turning

Engine Turning, or Guilloché, is a form of decorative engraving that is accomplished with the assistance of an engine. Its origins are rooted in the 18th century when artisan jewellers modified ornamental lathes, or rose engines, to cut metal using a fixed cutter. Rose engines used for ornamental turning typically use a live, or spinning, cutter to cut soft materials such as wood or ivory. I will be discussing rose engines and ornamental turning more in a later post.

The evolution of engine turning eventually led to the invention of the Straight Line Engine. The photo on the left is of the 14" Straight Line Engine that is in my shop. It was produced in the early 20th century by G. Plant & Son in Harborne, England. The 14" refers to the maximum length of cut that the engine can produce. Unfortunately G. Plant & Son closed up shop in the 1950's.

Through the use of different shaped pattern bars, the engine allows the operator to create a series of parallel or radial lines. Adjustments allow precise changes to each cut, creating stunning interference patterns. When combined with transparent enamel, pieces that have been engine turned truly come to life.

Argentium

Argentium Silver is a modern Sterling Silver alloy. It was created by Peter Johns by replacing some of the traditional 7.5% copper with germanium. The addition of germanium to the alloy creates a silver alloy that does not tarnish easily, and is harder than traditional Sterling. These new properties make it ideal for use in making pen and money clips. Unfortunately the layer of germanium oxide that forms on the surface to protect the piece from tarnish, also prevents enamels from fusing to it.

Welcome

Welcome to Silver Hand Studios.  Through this blog I'm hoping to keep everyone updated with what is new and upcoming at SHS, as well as help inform you about the processes and materials I use for making my pieces.  If you have questions concerning any of the pieces, materials or techniques, please contact me.  I am always happy to talk about what I am working on.  There is an RSS feed of this blog available, as well as a mailing list to help keep up to date. I attended the LA International Pen Show for the first time this year.  Besides enjoying the wonderful weather, it was a pleasure to be able to meet everyone who stopped by my table.  I am looking forward to attending again next year.  Throughout the year I will update my show schedule.  If you at a future show, please be sure to stop by and talk pens.