Question about Flush trimming router bits.

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Question about Flush trimming router bits.

Postby Simon Magennis » Sat Jan 13, 2018 8:51 am

The question. I want to get a flush trim cutter the initial purpose of which is to trim the tops and backs of guitars prior to routing out the ledges for binding. I am looking at this http://www.axminster.co.uk/axcaliber-fl ... g-ax803281 UK supplier as I haven't located a local Spanish supplier. (partly a language issue).

That I need a 6.3 mm shank (1/4) is clear (see below). What I don't understand is what is better in terms of diameter of cutter. They offer 1/4, 3/8, 1/2 (6.35, 9.5, 12.7 mm) diameters and a variety of cutter depths (13mm-32mm). Cutter depth is not important to me for this job as the main is a nice clean cut.

What diameter is better in this case? I have close to zero experience with routers.






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Background. I have the UK equivalent of the Bosch Colt. Unfortunately the UK model is "crippled" by a UK unique shaft feature: it can only use 1/4 or 6mm cutters. Despite being delivered with a accessories for 8mm cutters they cannot be used. The apparently identical model sold in the rest of Europe takes the 8mm cutters.
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Re: Question about Flush trimming router bits.

Postby Bob Gramann » Sat Jan 13, 2018 11:04 am

I use a 1/2” diameter bit with an angled-down edge at low speed. I’ve tried smaller diameter cutters and didn’t get any cleaner result and any less perceived risk. Make sure that you do your trimming in 8 segments always cutting from the wide part of the bout to the narrow. Even with sharp bits and that technique, I’ve had an occasional blowout that wrecked a top. #5509 on this page https://www.mlcswoodworking.com/shopsit ... flush+trim is the one I use.
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Re: Question about Flush trimming router bits.

Postby Brian Evans » Sat Jan 13, 2018 4:00 pm

I usually use a 1/2" bit with a bearing on the end, bearing rides against the side for flush trimming and I have a smaller bearing for doing a 1/16" rabbet for binding. It has a 1/4" shaft - all of my router bits have 1/4" shafts and I can't see using a large shaft bit on a guitar. They are usually for quite large diameter bits for cabinet work, making doors and such like that. Almost shaper bits. I just got the Stew-mac router bit set for binding, and it has a flush cut bearing, so I will probably try it next time.

Bob, can you expand on your 8 segment comment? I think I know what you mean, always route away from the grain, but do you also do that for the binding ledges? I try to do binding ledges in one smooth cut as long as possible, I never thought of the 8 step approach for that.

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Re: Question about Flush trimming router bits.

Postby Bob Gramann » Sat Jan 13, 2018 4:39 pm

Yes, I cut the binding ledges (and purfling ledges) in 8 segments as well. All you have to do is grab the grain once, and you will be converted. Many tales of woe have the word “router” some place in the narrative. (In spite of that, I have seven routers and use six on each guitar—the seventh isn’t very good and just hasn’t been discarded yet).

Cutting from wide to narrow minimizes chance that the router bit can grab the grain of the top or back and split the top or back and throw it off the instrument.

I use 1/2” shank bits wherever practical in my big router and with the router table. I know they won’t be slipping in the collet.
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Re: Question about Flush trimming router bits.

Postby Alan Carruth » Sat Jan 13, 2018 6:34 pm

I got a 1/4" spiral cut one (can't remember if it's up-cut or down) a few years ago and have really liked it, as compared with the 1/2" straight cut I'd been using.
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Re: Question about Flush trimming router bits.

Postby Clay Schaeffer » Sun Jan 14, 2018 1:27 pm

Generally you want a larger diameter bit for flush cutting. With 1/4 inch shanks a 1/2 inch diameter with a "shear angle" cutting face will give good service and be reasonably priced . The shear angle bit gives a little bit of the advantage of the spiral bit, but also allows the user to touch up the cutting edge with a diamond hone
I don't do the 8 segment cut, but rather do a "climb cut" all the way around the body. Climb cutting removes the wood in small bites and doesn't allow the bit to "grab" the grain and break out a large section. I think the 8 segment cut accomplishes the same thing if properly done (half of which is climb cutting), but requires more thinking on the part of the operator. Climb cutting requires a firm grip on the tool because it will want to "climb" out of the cut, so isn't recommended for large bits doing large bites, but for flush cutting and binding channels on guitars it isn't too difficult.
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Re: Question about Flush trimming router bits.

Postby Simon Magennis » Sun Jan 14, 2018 1:44 pm

Thanks for the info to date. I will not be using it handheld. It will be in a jib.
Another ever more elementary question. When trimming to prepare for routing the binding channel, does it need to be absolutely flush? I can get a bearing for an existing cutter but it would leave 1mm still standing up. Would that sort of option be OK?
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Re: Question about Flush trimming router bits.

Postby Adam Savage » Sun Jan 14, 2018 5:14 pm

Simon - may I suggest you look at '"Wealden Tools" for router bits. They have a much greater range than Axminster, and are superb. I don't know if they deliver to Germany, but I am sure they will do.
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Re: Question about Flush trimming router bits.

Postby Clay Schaeffer » Sun Jan 14, 2018 6:28 pm

"I can get a bearing for an existing cutter but it would leave 1mm still standing up. Would that sort of option be OK?"

Leaving a mm should be o.k. When you cut the binding channel it will be cutting a little below the top and back in most cases and will easily trim off the "silly little millimeter" (do you remember where that phrase was from?).
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Re: Question about Flush trimming router bits.

Postby Simon Magennis » Mon Jan 15, 2018 5:20 am

Thanks.

I had a look at Wealden. Thanks. I didn't see anything with a 6mm 1/4" shank but it may just be my growers. However, they have a big range of barings so I will check and see if one will fit the cutter I have for bindings. They ship throughout the EU.

I looked up the silly little mm. Never heard it before. :-)
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Re: Question about Flush trimming router bits.

Postby Arnt Rian » Mon Jan 15, 2018 6:12 am

I use a flush cutting 1/4 downcut bit with a bearing on the end. I keep it in a Porter Cable 310, which has a pretty low center of gravity and a big base, so it is quite safe to use hand held for this operation. I use the '8 segment' procedure for cutting from widest points towards waist/end block/neck block, so far so good.
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