Printing Accurate Holes

So in addition to the printed lathe I have been working on a few other designs or ideas, and I thought one of them is worth sharing here on my 3D printing blog.

For the purpose of one new design, I need to control a brushless DC motor, and to read the speed of that motor (RPM).  In order to test out my method for control of the motor and measuring the speed of rotation, I needed to create a test fixture that would allow me to collect data and observe and improve my methods for control and measurement.  I designed a basic fixture shown below, taking advantage of the capabilities of my 3D printer to make the fixture accurate and easy to assemble:

Simple Test Fixture for Brushless Motor Control

The reason I wanted to share this fixture design is the odd shape to the left side of the picture with 10 holes in it.  In the fixture, I need to accurately print holes that are both 1/4" and 8mm in diameter.  Actually, I need to print pieces that have holes the correct size for a press fit of both 1/4" and 8mm steel rod.

Since I am making the parts on a 3D printer, I do not have to take a lot of time trying to get the hole diameters exact--I can just print out a range of hole sizes in a test part and then try out the fit.  Once I've done that, it's easy to adjust the holes in the pieces that make up the fixture accordingly, for a perfect fit in the final assembly.  

The piece with 10 holes in it contains holes that are larger or smaller by about 0.004", allowing me to zero in on the correct size quickly.  After a couple of quick prints and test fits, I can adjust the pieces that make up the test fixture, print, and assemble.

D

Printing A Lathe - Assembly Begins

In the previous post I talked about the 3-Jaw chuck, which I have had assembled for a couple of weeks now.  Beyond the assembly of the chuck I had not been able to work on the printed lathe project any more for some time, but am trying to get back to it this weekend.

A few weeks ago I did finish all of the prints required for the entire lathe, and I also ordered the smooth and threaded rod and bearings that I think I need to complete the rest of the assembly.  I decided to go with all metric hardware, including 8mm smooth rod and 8mm diameter, 1mm pitch threaded rod--to give a nice even 1mm of movement per rotation.

I knew most of my work designing parts will be in metric, and I have used mills and lathes set up for inches with graduation on the dials for metric in the past.  The problem is, that leads to an amount of movement per rotation that is some odd number of metric units, and trying to keep track of positions over several rotations of the dial gets to be very difficult.  1mm pitch means 1 rotation = 1mm of movement (or 2mm off the diameter of the turned piece if the tool advances 1mm towards the center of the piece).

Here's what I ordered from McMaster-Carr as far as smooth and threaded rod, in case anyone is interested (Note: this is not a full list of all components needed for the lathe, and I have not completed the assembly, so some of these items may have to be changed):

• 98861A320   Metric Class 4.6 Plain Steel Threaded Rod, M8 Size, 1 Meter Length, 1.0 mm Pitch, Fine Thread -- $13.52
• 91415A105  Metric Zinc-Plated Steel Fine Thread Hex Nut, Class 8, M8 Screw Size, 1MM Pitch, 13MM W, 6.5MM H, packs of 100 -- $9.27
• 88625K67O1 Tool Steel Tight-Tolerance Rod, 8 mm Diameter, 3' Length -- $5.59 (2 pcs. ordered, $11.18 total)

I went on a tangent this morning and printed a small part for a friend, just to feel cool about having a 3D printer:

Duplo to Brio Converter Brick

It's a piece to convert from Duplo blocks to Brio toy train track pieces.  The design is from Thingiverse here:  http://www.thingiverse.com/thing:14175  Turned out pretty well, but I have to get my hands on some blocks and train track pieces to see if it works.

Now back to very serious things like assembling my own printed lathe.

D

Printing A Lathe - 3-Jaw Chuck Assembled

First, I can't believe it's been about a month since my last post.  Work and life sure can get in the way of trying to take over the world . . .

Although I haven't worked on the assembly of the full lathe, I did manage to assemble the 3-jaw chuck:

3-Jaw Chuck from Sublime on GitHub

Here is an image of the parts that went into the assembly EXCEPT for 3 springs, 3 washers, and 3 pins (shown below):

Most of the 3-Jaw Chuck Components and Hardware

The 3 pins are the items highlighted in red below in this exploded view of the chuck from SketchUp:

3-Jaw Chuck Exploded View with Pins Highlighted

The assembled 3-jaw chuck is working OK, but the three pins that press against the back of the face of the chuck to lock the jaws in place are not holding well.  The instructions call for these three pins to be cut to length from smooth rod.  Since I had to cut the rod with a band saw (I obviously don't have a lathe or I would have used that), I was concerned that I would not get the three pieces to be the same length.  I knew I could print them to a very exact height, so that's what I did:

Printed Pins for 3-Jaw Chuck

At first, these seemed to be working fine, but I think that after some loads and stress have been applied the three small cylinders are binding in the holes and not imparting sufficient load against the back of the jaw face.  I think I need to go back and replace these with smooth rod.

I had to drill out a few of the holes based on the hardware I ended up using, and what drill size you need and how much of this is required will vary from one person to the next depending on the behavior of your printer and your choice of hardware.

Drilling Holes to Size

I also used a drill bit to clean up some strings and stray traces of plastic where the holes are counterbored on one side of the piece shown below.  When the piece prints these counterbores require basically printing a cone, and it came out messy for a few layers inside the hole.  Not a concern for the overall function of the chuck, however.

Cleaning out Counterbores

If you do not have access to a drill press or just want to make the hole sizes exact, I suggest printing out a small test part for a fit check and then updating the models as needed before slicing and printing the full part.  The nut traps in the jaw pieces were too large for what I needed, so I did something similar there and reprinted the jaws.  No sweat!

On to the rest of the assembly . . .

D