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When selecting a guide bushing from my newly purchased set, how do I determine which one to use for any given router bit when I'm following a template?

  • Do I want the tightest tolerance possible?
    • If so, how tight is too tight?
    • For example, can/should I use a 17/32" ID bushing for a 1/2" (16/32") bit, leaving 1/32" clearance?
    • Or, should I go to the next larger size 21/32" ID, leaving 4/32" clearance?
  • If I'm using a dovetail bit, how do I determine what's the right size? Does the bushing just need to clear the shank of the bit?

NB: I left a very important part of the question out after doing massive editing prior to its original asking. This edit partially invalidates Caleb's answer, but there's enough of his answer to still make it worthwhile.

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  • Isn't the usual requirement that the business end of the bit can fit through the bushing? As for the first q, I don't think it usually matters — there's no zero-clearance effect to try to take advantage of, so you pick a bushing that'll work and size the template accordingly. [And conversely, if the template already exists you pick the bushing based on the bit diameter regardless of what it is.] Obviously this isn't proof of anything but I've certainly seen skinny bits used with large OD bushings, and bits that were pretty tight in the bushing (as pictured in the Q that was just edited).
    – Graphus
    Sep 27 at 7:56
  • Re. the usually above, it seems logical that you need to give slightly more chip clearance for spiral upcut bits than for normal bits. I can't actually find anything that says so (cursory look) but if this is correct, spiral downcut bits may require even lower clearance. But, a seat-of-the-pants judgement, I wouldn't want to try a clearance under 1/32", and would aim for double this normally.
    – Graphus
    Sep 27 at 8:12
  • I don't know, @Graphus, about the business end fitting through the bushing (I presume you're referring to the dovetail bit), that's why I'm asking ;). I could put the bushing into the router base then insert the bit through it, shank only. TBH, When I said "newly purchased", I'd just clicked confirm order, so I don't have it to hand quite yet to even try things out.
    – FreeMan
    Sep 27 at 11:47
  • I think everything is well covered by @Caleb's Answer but to add a bit more to the basic gist of the bushing not having to be a closeish fit to the bit used, there's an old Lee Valley newsletter that had something on this but I couldn't locate it on the current version of their site earlier when I was trying to write my Answer. If you want to see it though the Wayback Machine has a snapshot, here
    – Graphus
    Sep 27 at 16:31
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    Thanks, @Graphus
    – FreeMan
    Sep 27 at 16:39
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how do I determine which one to use for any given router bit?

You don't need a bushing at all unless you're using some sort of template to guide the cut. When you do use one, choose the size of the bushing according to the offset you want between the template and the cutter. Your choice will therefore depend on the template or jig that's guiding the router, and the result you want.

Do I want the tightest tolerance possible?

No, not necessarily. Again, the bushing determines the offset from the template. It would be entirely reasonable to use a 1/4" bit with a 1" OD bushing.

In some cases, the bushing might actually be smaller than the cutter diameter; that's very common with some dovetail jigs, e.g. the Leigh jig. Obviously, you can't use a bit larger than the bushing's inside diameter if you need to make a plunge cut. And if you want zero offset between template and cutter, you're usually better off using a bit with a bearing, like a pattern bit or flush cut bit.

If I'm using a dovetail bit, how do I determine what's the right size? Does the bushing just need to clear the shank of the bit?

If you're using a dovetail bit with a jig to cut dovetails, the manual for the jig should tell you what size bushing to use for a given bit. Different jigs have different capabilities: some only do half-blind dovetails, some do through dovetails with fixed spacing, some can do through dovetails with variable spacing. At least until you really get comfortable with whatever jig you're using, it's best to stick to the combinations recommended in the manual.

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  • "You don't need a bushing at all unless you're using some sort of template to guide the cut." is absolutely 100% correct. And it was a vital part of the question that got edited out after I discovered the answer to the question I was originally going to ask. :(
    – FreeMan
    Sep 27 at 15:56
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    Hmm, "the bushing determines the offset from the template" gives me an interesting thought. I can make a 4x4" hole in my template and use it to make different size cutouts based on the OD of the bush and the OD of the cutter I'm using.
    – FreeMan
    Sep 27 at 16:00
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    @FreeMan Yes, that's a pretty typical use. For example, you could make a template in the form of, say, the letter E. Cut out two E's with the same template but different bushings, paint/stain them different colors, and attach the small one to the large one so that you have an E in one color with an outline in a different color. Templates let you do arbitrary shapes, and changing the bushing lets you vary the size. Inlay is another use, although you typically use a special bit/bushing combo that lets you shift the cut line by exactly the width of the (usually very small) bit.
    – Caleb
    Sep 27 at 16:08
  • Thanks. Obviously, I've not used bushings before, though I've given the router a bit of a workout. Appreciate the ideas.
    – FreeMan
    Sep 27 at 16:10
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    @FreeMan I assumed you were using a template, but stated that assumption because there are also flat inserts without the protruding bushing that screw into a router's base plate like bushings, and figured you might be including those.
    – Caleb
    Sep 27 at 16:14

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