Flush/Trim cutter router bit size

[DigitalM]

I needed a flush cut top bearing router bit, don't have much time to leave the house for a few weeks for the 90 min round trip to buy such a thing so grabbed a cheapo one off amazon as it will only be very occasional use.

But hold on. What's the point of different widths/sizes of flush cut bit? Is it just using lighter ones for more delicate work or something or am I missing something huge? Surely, flush is flush...

 

[paulrbarnard]

I don’t think you are missing anything huge, the one on the left is pretty big.

 

[DigitalM]

I don’t think you are missing anything huge, the one on the left is pretty big.

Well yeah of course, especially if you hold it really close to your face. But wait, that's not quite was I was asking.

 

[Cabinetman]

Not an expert routing person, I think it’s so that you can do grooves. Also the wider ones would be more suited to deeper cuts? Ian
Edit, sorry thought I was talking about bottom cut ones, Doh!

 

[Sporky McGuffin]

Different radius on inside corners.

Also you can take off more on each pass with a bigger bit , as a gross over-simplification.

 

[DigitalM]

Ah! Corner radius. Blindingly obvious now you point it out! Isn't it always, after the fact! Thanks

 

[Garno]

Ah! Corner radius. Blindingly obvious now you point it out!

Not to me it isn't

Certainly not a silly question, at least to us two

 

[Spectric]

When you want really clean cuts then I can recomend these,

https://www.infinitytools.co.uk/router-bits/mega-flush-trim-bit/
They are both up & down shear so nice clean edges.

Another reason is when using templates, if you have tight curves you need the smaller ones to get in, but for just edge triming then the bigger the better.

 

[DigitalM]

I'm glad I asked now. I hadn't considered quite a few of these things. Grateful for patient and thoughtful answers. I learn something new every time I come here.

 

[Sporky McGuffin]

I certainly don't think it was a stupid question.

 

[DigitalM]

I should probably change the subject to something more useful for the archive.

[Edit] Done...

 

[Spectric]

I'm glad I asked now. I hadn't considered quite a few of these things. Grateful for patient and thoughtful answers. I learn something new every time I come here.

It is a place where we all can always learn something as no one knows or can remember everything and I find it often ignites a thought that eventually solves something or leads to the purchase of something and if nothing else just keeps the grey mater occupied.

 

[TheUnicorn]

...when using templates, if you have tight curves you need the smaller ones to get in, but for just edge triming then the bigger the better.

so for a template with a mixture of flat edges and tight details would you do a first pass with the larger bit and then a second pass with a small bit? or in that scenario would it just make sense to do it all with the small bit?

 

[Droogs]

depends on how much waste is left outside the template. If you have an 8mm bit to end up with 8mm internal radius corners but have more than a couple of mm outside the template to remove then go over it first with a bigger size and then finish with the smaller one. I know someone who has a 1/32 over size bearing on his largest bit so that he has a little material to remove when using the final pass smaller bit to get a better finish.

 

[Phil Pascoe]

I bit of insulating tape along the edge, removed for the last pass does the same. I've cut the moulding (assuming a straightforward shape like an ovolo, not undercut) before finishing the surface then run the cutter around on the same setting - it's very slightly deeper - that's another way of taking off a very fine final cut.

 

[Spectric]

Not sure if this helps but,

You have a template of a curve and the workpiece roughly cut so when you put the template on the workpiece the amount to be removed to get flush with the template varies by say 4 mm in some places, too much in a single pass for a nice finish so by using a Wealdon T8020 19mm flush trim bit and changing the bearing for one with a larger OD you can remove the material in stages without moving your template. IE fit a TB880 which has an OD of 24mm and would trim to within 2.5 mm of being flush, the next pass with say the TB748 refitted will give the final flush cut that will be more constant and give a better finish.

Bearings for T8020 trimmer

TB748 3/4 (19mm) std
TB760 20 0.5
TB870 22 1.5
TB880 24 2.5
TB890 26 3.5
TB895 28 4.5
TB900 30 5.5