Community challenge #10: Milling plastics (closed)

Delrin Bushings and Associated DIY Fixture Vises and DIY Lapping Tool

For this challenge, I would like to submit a seemingly simple project that I’ve been working on for the past 2 weeks: bushings made from 1.250" Delrin. I said seemingly simple because there were plenty of hurdles to overcome throughout this process.

Before I cover the problems I encountered, let me first explain what these bushings are for. I have a blower from an early 1900s metal forge which has worn out bushings.

1

I spent a great deal of time taking this whole assembly apart and realized the original bushings were made of steel. I wanted to make the new bushings out of something softer as to not wear out the shafts of this blower any worse than they already were. At first, I was going to use brass but I realized Delrin was it a lot cheaper, easier to machine, and is also used to make bushings. Let the fun begin😏

Problems

  1. How do I hold a piece of 2 foot long round stock vertically for machining?
  2. My limited selection of cutters is not long enough for 4 out of the 5 bushings I need to make. (longest bushing is 2.200")
  3. One end of the bushing must be machined in a secondary setup.
  4. Holding tight tolerances

To overcome the work holding problem, I created a wooden vise that overhangs the front of the Shapeoko. This vise utilizes a quick zeroing locator hole which corresponds with the machine’s front center rapid travel location. This allows me to slide a 1/4 inch dowel rod from its position in a loose collet and into the hole before tightening the 3 securing bolts. Then I run an indicator across the front of the vise (with the front jaw removed) to verify alignment along the x-axis. This indicator hole allows me to quickly resume work after switching zeros for other programs. Now with this vise, I can quickly hold the piece of Delrin at any height needed for my different length bushings.

4

To overcome my cutter selection problem I purchased a 1/4 inch 3 flute End Mill with an overall length of 4 inches. Before machining the longest bushing of 2.2 in I realized I was significantly close to maxing out the Z travel of my machine. I had already programmed my clearance height to 0.010 inches. However, the G53 code at the beginning of my program commanded the machine to make a Z movement -0.197 inches before moving to the x and y plunge point. If uncorrected this would cause a gouge across the top of my part within the first few seconds of the program. Additionally, the M02 command at the end of the program again caused the Z-axis to move (-0.197 inches I suspect) before traveling to the back of my machine. If uncorrected this would also cause a second gouge across the top of my part. A quick call tech support helped me understand the expected G-Code movements; thanks, Brandon. Next, I did a series of test runs away from my part as I removed the G53 line of code and the M02 line of code. After I was confident in the expected outcome I changed my zeros back to my part and proceeded to cut the bushing.

After operation one finished for each bushing I cut the bushing free of the material with a hacksaw. This brings me to the third major problem; how do I hold a small round object so that I may face the end? My vise only holds the larger round stock. I designed a simple V block vise which I could mount anywhere on the table.Note: a hose clamp can make a quick job of mounting a dial indicator :wink:

And now on to the fourth problem: holding tight tolerances. I started with running my spindle speed at 18000 and a feed rate of around 60 inches per minute. However, I quickly realized things were not rigid enough to hit tight tolerances and go that speed. The major drawback that I faced was cutter run-out. I’m sure the collet was partially to blame, but I noticed that I can wiggle the spindle nut on my Carbide 3D router forward and backward as well as side to side when tightened. This leads me to believe that the spindle bearing in my router is going bad (I’m not sure how tight this was when new). If anyone has any suggestions on fixing this please let me know. As for my project I wanted to continue so I slowed my feeds and speeds extremely.

Contour Operations for both ID and OD
Feed rate: 15 inches per minute
Step Down per Pass: 0.034 inches
Plunge rate: 4 inches per minute
RPM: 10,000.

I was still able to get chips from the Delrin and no melting. The surface finish was okay but not like you would see on the lathe.

I would like to fix the spindle bearing and try this project again to see what improvements can be expected in surface finish and how much faster I could push the feeds and speeds. To compensate for the run-out and spindle wobble I created oversized and undersized geometry for the OD and ID respectively. For each additional bushing (each one varies in size), I copied the previous program and saved it under a different file name so that I could make adjustments as needed without deleting or altering a proven program for the previous bushing (in case I ever need to remake these).

The final problem I’ve had to overcome was the tolerance between each shaft and bushing. I created a makeshift lapping tool from a wooden dowel rod and a piece of a broken belt sander belt. This process slightly expanded the ID of each bushing until it would freely spend on the associated shaft.

And as @Griff would say: where’s the pic, or it didn’t happen (pretty sure I’m quoting the correct person, if not, my apologies). Here is the disassembled puzzle with old bushings, followed by the finished project’s installed Delrin bushings that were fully designed in Carbide Create and machined with my Shapeoko.



Most importantly, thank you to all those who post in these forums, as I have learned a lot from all of you!

Carbide Create CAD/CAM files: 0.880 OD Bushing Vise.c2d (49.6 KB) 1.287 OD Delrin Vise v2.c2d (2.3 MB) Middle shaft - hat bushing oversize.c2d (1.1 MB) Middle shaft - sleeve bushing oversize.c2d (2.2 MB) Outer shaft - sleeve bushing (started with oversize comp).c2d (2.6 MB) Upper shaft - hat bushing (2nd operation_hat OD).c2d (1.9 MB) Upper shaft - hat bushing (started with oversize comp).c2d (2.7 MB) Upper shaft - sleeve bushing (started with oversize comp).c2d (2.7 MB)

CutRocket Link: https://cutrocket.com/p/5efdedf60f0f4/

-James All

21 Likes