flush trim bit- top or bottom bearing?

[Jbridge337]

Hi, any advantages or disadvantages of one vs the other? Plan on making some radiused shelves. Initially I will be using the router handheld if that makes any difference. Also, is the largest diameter for the task usually the best way to go?

Thanks,

Jim

[LarryG]

Top vs bottom: They work the same way, the difference being in where the pattern (template) you're following must be placed ... on the bottom of the workpiece for a bottom-bearing bit, on top for a top-bearing bit. Bottom-bearing bits are easier to find and are usually cheaper.

Diameter: All else being equal, a larger diameter will usually cut a little better, and will be less prone to bend from side forces imposed as you make the cut.

A couple of unsolicited tips .... if you buy a generic, no-name bit, be sure to test it on some scrap first as budget bits often have bearings that don't exactly match the size of, or align precisely with, the cutters. Name-brand bits, especially those with double bearings, cut MUCH better and IMO are well worth the extra dosh. Also, regardless of how much you spend, always check the bearing's set screw to make sure it is tight and won't vibrate out while making the cut, thereby ruining your workpiece (and yes, that's the Voice Of Experience you're hearing ... )

[LCHIEN]

I think a properly equipped shop should have both. Sometimes its more convenient to use one or the other.

Definately the bottom bearing bit is useful for cleaning up butt joints and matching box tops to the sides of the box.

[cabinetman]

In addition to checking the screw on the bearing for tightness, it might help to put a drop of "loc-tite" on the thread and tighten screw.

As for any advantages of top/bottom, I'll isolate my answer to the router bit question. If you are doing a pattern cut, the pattern can go either on top or the bottom depending on which bit you are using. If you want to see what's happening, it's easier if the pattern is on the bottom. If the visual edge to be seen can be sandwiched by the pattern, there might not be any tearout.

Check the cut of the bit you're using. Some don't cut absolutely flush with the bearing. A tip for trimming to a pattern: If you can size the piece to within an 1/8" or so before trimming, it goes a lot easier.



YOU HAVEN'T FAILED, YOU JUST HAVEN'T FINISHED

[jAngiel]

The next one I want to get is the one from MLCS that has a bearing on the top and bottom. That way it would work for both handheld and table mounted and also whether you wanted the pattern on the top or bottom.

[lago]

The next one I want to get is the one from MLCS that has a bearing on the top and bottom. That way it would work for both handheld and table mounted and also whether you wanted the pattern on the top or bottom.

I have that bit on my Christmas Wish List. With red oak, I have had problems when attempting to rout the end grain. By flipping the work over, I could do it but have to change bits. This bit would make it a bit easier.

[Pappy]

I invested in a pair (1/4" and 1/2") of Whiteside spiral flush trim bits. Well worth the $$$$ spent. For table mounted trimming, I want to get a pair of top bearing bits.

[RayintheUK]

This is what I use - what a brilliant cutter it is too! A = 19mm (3/4") B & D = 50mm (2") C = 1/2"



The downshear gives as clean a cut as a spiral, but you can hone this one easily. Great for working either side of a template, hand-held or table mounted.



Very meaty and heavy, but worth every penny.

Ray.

[Jbridge337]

thanks for all the replies so far...Ray, who makes the bit you mention?

Jim

[RayintheUK]

Ray, who makes the bit you mention?

It's made in the UK by Wealden Tools, but MLCS do a single-bearing shear cut or a double-bearing straight cutter here. HTH.

Ray.