A Lesson in Burr Removal

When metal is cut, a sharp edge is often left. Part of making a useable product is removing any sharp edges or (even worse) burrs. Even very small details of parts may have detrimental burrs.
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When I developed my first reel design (this blog 13 Jan 2011), there was the problem of choosing materials for ratchet and pawl. I undertook testing (summary 23 Aug 2011) to find suitable materials for both. I eventually chose Delrin (actually, acetal copolymer, almost the same) for the pawl. I ran many pawl materials against a bronze ratchet.
Once I knew that Delrin would be the pawl, why not Delrin for the ratchet also? I already had made bronze ratchets for my first 5 reels, and the combination had tested well, so I went ahead and built the five with the bronze ratchets.

By now the reels have had some use, and I have been able to see how two have done. Both had the same problem: the pawls were being consumed. They were shorter than when first made, and the tips did not look as if worn in my gearmotor testing. They looked as if they had been machined away. Here is a bronze ratchet, with a pawl that is .020 shorter than when first made.
IMG_4091
The total depth of the 36 DP teeth on the ratchet is .060, so the wear is very signficant.

So why did my testing fail to predict this? I had to think about it for a while, but finally realized the tooth tips of the bronze ratchets were different from those of the ratchet in my tests. The test ratchet had already been through testing with a variety of metal pawls. Some ratchet wear had occurred, effectively burnishing the ratchet tooth tips. By the time I started testing plastic pawls, the ratchet tooth tips were nicely smoothed. On the 5 “production” reels, the ratchets were freshly made and had sharp corners at the tooth tips.

So I am recalling the 5 reels for repair: a new pawl and a substitute ratchet of Delrin.
By the time I made parts for my second reel design (see 10 Aug 2011), I was using Delrin for the ratchet also.

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This entry was posted in Click, Milling, My Reels, Ratchet/Gear. Bookmark the permalink.

2 Responses to A Lesson in Burr Removal

  1. Keenan says:

    How are your going to clean up the edges of the replaced gear or will that even be necessary when using Delrin? Also, excellent choice of material. Delrin has a very high degree of dimensional stability over a wide temperature range and very low moisture absorption, unlike some other plastics. I could keep going, but I’m guessing that most of this you already knew.

  2. dave49 says:

    Keenan,
    Any “burr” left on the Delrin ratchet will not be something that can cut the Delrin pawl. There is clean-up to do after gear cutting, however. There is a “fuzz” down the sides of the teeth on side of the ratchet where the gear cutter exits. This has to be picked off with tweezers.
    If the gear teeth are cut on a blank that is wider than required, then a facing cut (using lathe) will sometimes remove all the fuzz.
    Dave

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