Thursday, November 29, 2012

Designing a Thread Cutting Tool

A good place to start, if one wants to understand design characteristics of bolt threads is the venerable Wikipedia.  There, you will find that most; but not all threads, are based on an equilateral isosceles triangle whose tips have been truncated.  I'll let you explore the referenced article for a deeper understanding that is needed for this article.

Instead, we'll just focus on the 8 simple steps required for the tool that will be used to 'cut' the threads in our making of the bolt for 3D printing.

To create a thread with a 3D design application, like Cubify Invent or Moment of Inspiration, requires creating a "Cutting Tool" shape that is used with a Helix tool ((Coiled Line) to make the threads. 

STEP 1: Create the Primary Reference Triangle at the Thread Pitch size

While it is not absolutely required that the triangle used to form the thread cutter be an equilateral triangle, it is the most common form.  What is required is that the height of one side of the triangle is exactly the length of the desired "Thread Pitch".  In our case, this would be 20 threads per inch, or 0.05".  our starting triangle would look like this sample.

Equalterial Triangle with 0.05" Sides
 STEP 2: Create a "Tip Triangle" to Aid Truncation

Each of the tips of the base triangle will be truncated for reasons explained in the Wikipedia article.  To facilitate the design of the trucation, we copy the primary triangle and then scale the copy to 1/4 the size of the original.  We then place that second triangle right into the tip of the first as shown below.  We'll call this the "Tip Triangle".  It's colored red in this sample.

Create the Tip Triangle at 1/4 Scale of the Base Triangle.

STEP 3: Create a "Corner Triangles" 

We next make two copies of the "Tip Triangle" and scale them to 1/2 the size of the Tip Triangle.  Each of the copies is moved into either the upper left tip or lower left tip of the primary triangle.  All of these triangles are merely reference objects to help us actually draw the cutting tool itself.

Back Triangles 1/2 Scale of the Tip Triangle

STEP 4: Create a Truncation Arc inside the Tip Triangle

The goal of this step is to create two 90 degree reference lines that intersect the side of the Tip Triangle at a point that is 1/4 along the side of the triangle.  We then draw an arc whose center is at the center of the back line of the triangle and that extends from the points where the lines intersect the triangle.

Adding the Trucation Arc

STEP 5:  Add Reference Lines that divide each back triangle.

These lines will aid us in drawing the final cutting tool by giving us snap-to points.  

Add Reference Lines to Facilitate Drawing

Step 6:  Using Continuous Lines Draw the Cutter Outline

Notice that we do not draw to the tips of the primary triangle.  Beginning at the intersection of one end of the arc we draw a series of connected line (Blue Sample) until we get to the intersection of the arc on the opposite side of the triangle.   We then JOIN the arc and lines to form a single object.

Using the Reference Lines, Draw the Cutter Tool Outline

Step 6:  Remove the Reference Lines (Invisible) to Reveal the Cutting Tool

As we make the reference triangles and lines invisible, the outline of the cutting tool is revealed. The front tip is rounded and the back tips have been truncated to result in gaps so that the final thread will not be too pointed.

Remove References to Reveal the Cutting Tool

Step 7:  Add References to Aid Connecting to the Helix

The final step is to add a line and a point, which we have exaggerated for clarity, to aid in positioning the Cutting Tool relative to the Helix path.  Notice that it is NOT attach the cutter to the Helix along the back side of the cutting tool.  We move the attachment point forward to a point along a line anchored at the tips of the small back triangles.  This means that the thread is slightly more shallow then the thread pitch.

Add a Reference to Center the Attachment to the Helix

Thread Result

As the cutter moves around the shaft of the bolt, it removes material to form the threads.  The thing to note is that the tips of the thread ends up being somewhat flat, rather than sharp.  And, the valleys of the thread are also not sharply pointed.  This makes for a stronger, smoother turning thread with less potential for binding up..

It's been very interesting, to me, to explore the more technical aspects of thread design.  Once grasped. it allows us to design any sized threaded bolt or accessory.  But, knowing how to design it is not enough if we are to use a 3D printer to realize those designs.  And, this is going to take some experimentation in scaling, etc.

As usual, it's going to be fun!

Monday, November 26, 2012

General Design Dimensions of Bolts and Threads


It sure has been a while since I've updated this blog.  But, lest you think that is because I have lost interest in the Cube or in creating tutorials for Cube owners, I want to assure you that is NOT the case.  I simply got hung up.

Writer's block?  Fear of leading you astray in what I wanted to convey?  A busier than normal schedule?

Well... a little of all of those things.  But, mostly, I think, a fear that I might lead you down the wrong path as you are learning to use the software package of your choice when it comes to exploring making threaded bolts.  There was also the fact that I try to limit the length of my tutorials to around 10 minutes and this is a reasonably big subject.

So, I have decided to break it into three blog entries.

The first will focus on the general dimensional characteristics of standard bolts and threads.  In this case, we will be looking at a 1/4"x1"x20 standard hex bolt.

The second will focus on design characteristics of the "Cutting Tool" that will be used in conjunction with a Helix to actually create the threads.

The third will be in the form of a video design tutorial, first in Moment of Inspiration and then in Invent.

I am also going to create a companion video tutorial for this blog entry.  But, it will use this blog as the script.  So, it will add little additional information.  It's simply a second available format for those that prefer to watch video tutorials.

General Design Dimensions of Bolts and Threads.

A bolt or screw is generally described by four features.  The first is the HEAD TYPE.   For the purposes of these tutorials, we will be designing a HEX BOLT having a hex shaped head that requires a wrench to tighten it.  Then there is a three element description, such as 1/4"x1"x20.

We will be using the SAE system for describing our bolts.  But, the METRIC system is similar.

The first element, 1/4", tells us the nominal diameter of  the shaft of the bolt.  The next number, 1", tells us the length of the entire bolt shaft, whether it is threaded or not.  And, the last number, 20, describes the THREAD PITCH.  This is the number of thread peaks in 1" in the case of SAE (United States) bolts.

But, this is only the beginning of describing our bolt.  Let's start at the very beginning and work our way to the complete dimensional description.

A)  The Bolt Diameter

The most critical of the dimensions is the diameter of the bolt shaft.  It tells us that this bolt will fit into a 1/4" hole.  It would be a very tight fit, to be sure.  And, usually the hole meant for a 1/4" bolt will be slightly wider than that.  But, that is the diameter of the shaft. 

1/4" Shaft Diameter

B)  Bolt Length

1/4" bolts come in many lengths.  So, it's important to know exactly how long your bolt shaft should be for your particular application.  This is the length from the bottom of the head to the opposite end of the bolt shaft.  In this case, we will be designing a 1/4" that is 1" long.

1" Bolt Length (Shaft Length)

 C)  Thread Length

Descriptions standard thread lengths can be confusing.  Each reference that I explored claimed that the thread length for 1/4" bolts under 6" was 75% of the shaft length and those over 6" were 100% of the shaft length.  BUT, I could not find a single bolt in a hardware store that followed that "rule".  So, I have simply chosen to define the thread length in my design as 90%-95% of the shaft length,  This gives us an opportunity to show how to taper the thread leaving a bit of the shaft without thread.

.90" Thread Length

D)  Thread Pitch (Threads Per Inch)

SAE bolts come in both COARSE and FINE threads.   I'm not even sure that a consumer 3D printer can effectively resolve fine thread pitch.  So, we'll settle on the standard Thread Pitch that we're most familiar with... 20 threads per inch.  This translates to a distance between thread peaks of 0.05".  This is very important to know; because, it is the basis for creating a cutting tool that we will use, in conjunction with a HELIX, to create the threads.

20 Threads Per Inch Thread Pitch

E)  Bolt End Chamfer

While, theoretically, you could create a straight bolt, it's very helpful to taper the very end of the bolt shaft to make it easier to thread the bolt into a nut.  So, we are going to CHAMFER the end of the shaft with a 45 degree slope that begins .025" from the end of the shaft.  Aside from making it easier to begin threading the bolt, it also gives the thread a more finished look.

0.025 Chamfer at the Bolt End

F)  Head Height

1" bolts are actually longer than 1" if you include the height of the head.  While it is true that head height can be thinner or fatter different applications, there is a specified standard for a 1/4" bolt and that is 5/32" or 0.163".  The head shape is actually more complex than we typically think.  The edges slope slightly and the corners are FILLETED or rounded.  There is also a raised area, also called a FILLET that we will discuss later.  The HEAD HEIGHT only includes the head itself and not the FILLET.

5/32" (0.163:) Head Height

G)  Head Width Across Flats

While  we might play a bit with the head height without too many adverse ramifications, that is NOT true of the width of the head from one flat side to the other.  If we are to claim that we have printed a standard 1/4" Hex Bolt, then those who try to use it, expect to be able to use a 7/16" wrench to install it.

This probably presents some interesting hurdles for the home 3D printer.  What we design may or may not print exactly at our expected dimensions.  It could be that reducing the width of the head to an acceptable print width will cause our threads to be too small, etc.  So, we might find that we have to adjust the head width and the shaft diameter separately!

It is possible that this is an individual printer characteristic that can only be guessed in a global tutorial such as this one.  So, we'll simply stick with the standard for now.

7/16" (0.438:) Across Flats

H)  Head Width Across Corners

While the SAE standard calls for a distance of .0.505" from corner to corner on the top of the head of a 1/4" bolt, we really don't have to worry about it too much.  It just happens if we get the distance across the flats right.  And, of course, the converse is true.  If we get the corner distance right, the flat distances will be correct.  It all depends on your preferences as you create your hex shape for extruding into the head.

0.505" Across Corners


The specifications for a hex bolt call for a spacer just under the head that acts like a washer so that the head itself is not absolutely flush with the surface into which it is screwed.  While it might be a bit difficult to see in this illustration, the nominal diameter of this spacer is 13/32" or 0.406".

13/32" (0.406") Head Fillet Diameter


The height of spacer under the head is nominally 0,0125".  But, we are, in fact, limited to the minimal layer height of our printer.  Even so, whatever difference there might be is inconsequential.  But, since we are aiming at a standard bolt size, we need to try to design the bolt as tightly as we can to the specifications.

0,0125" Fillet Height


These are the dimension specifications we will be using to design our Hex Bolt

Summary of Hex Bolt Dimensions

 In our next blog entry, we will explore the dimensions and characteristics of the
Cutting Tool" that we will use to remove material from the shaft to complete our Hex Bolt.

As usual, if you spot an error or think something needs some clarification, please feel free to comment!