Spiral Flush Bit, Up or Down?

[Mike OMelia]

I am looking for a good bit for trimming the backs and tops of guitars to the sides. Flush bit is obvious. I have several. But there is always a grain direction that I will encounter that can cause catrostrophic tear out. Yes, I watch for it and change my approach. But it still happens. I have been using straight cut flush bits. I want to use a spiral bit. Or an angled cut flush bit. What is the significance of "up-cut" and "down-cut"? And bit suggestions are welcome. The wood being trimmed is either hardwood or spruce. And, it is typically 0.1" thick.

Thanks!

Mike

[Myk Rian]

I would use a down-cut. It pushes the material down to prevent tearout.
This is assuming you position the router on the top and bottom of the body.

[Mike Heidrick]

Up cut spirals twists towards the router - in a typical non-flush trim think smooth dado bottom and fuzzy top surface. Tops are easier to sand in many cases vs the bottom of the dado.

Down cut spirals twists towards the material - bottom of cut is more rough - useful when you want the top cut edge clean or the chips packed in a kerf or in a cut through where the bottom is supported by a sacrifical surface.

In your case I would use a down cut flush bit with bearing on tip. The pattern would be there supporting the fibers of your blank and the shearing action would extend past the edge of your work and alond the pattern.

I like whiteside bits - Spiral Flush 1/2" D, 1/2" SH 3-3/4" OL - Flush Trim Down Cut Spiral Bit, Whiteside # RFTD5125

They are not cheap but one of the best. Brian at Holbren.com gives a woodnet discount on whiteside bits - sales at holbren dot com is his email address.

[Mike Heidrick]

I would use a down-cut. It pushes the material down to prevent tearout.
This is assuming you position the router on the top and bottom of the body.

Even under the work like in a router table, as long as the bearing is at the bottom of the bit the edge of the down cut will be supported by the template that is on top of the work.

[Mike OMelia]

Thank you all. I could not put it in words, but my intuition said downward cut.

Mike

[Kevin Presutti]

Down cut would be my choice as well, up cut are good for mortising, they pull the chips up and out of the hole. Just remember with down cut you are pushing the chips into the material you are cutting, for best results slow feed is required they are not good chip clearing bits but they will leave you a nice clean edge. Just my .02. Best of luck!

[Mike OMelia]

Up cut spirals twists towards the router - in a typical non-flush trim think smooth dado bottom and fuzzy top surface. Tops are easier to sand in many cases vs the bottom of the dado.

Down cut spirals twists towards the material - bottom of cut is more rough - useful when you want the top cut edge clean or the chips packed in a kerf or in a cut through where the bottom is supported by a sacrifical surface.

In your case I would use a down cut flush bit with bearing on tip. The pattern would be there supporting the fibers of your blank and the shearing action would extend past the edge of your work and alond the pattern.

I like whiteside bits - Spiral Flush 1/2" D, 1/2" SH 3-3/4" OL - Flush Trim Down Cut Spiral Bit, Whiteside # RFTD5125

They are not cheap but one of the best. Brian at Holbren.com gives a woodnet discount on whiteside bits - sales at holbren dot com is his email address.

Does something like this come in a 1/4" shank?

[Kevin Presutti]

Mike,

I don't have one with a bearing but I have a down cut from Freud that is 1/4". You could check Whieside, MLCS, Eagle America, there are a bunch out there, you shouldn't have to much trouble finding whay you're looking for.

[Mike OMelia]

My router jig that I use to do this work uses a Bosch Colt Router. 1/4 is the shank size, so I have to find one like that.

[Jerrimy Snook]

Whiteside RFTD2100 is a 1/4" bit with 1/4" shank and bearing on the bottom. About $44 list price.

[pat warner]

"Spiral Flush Bit, Up or Down?"
*******************************
Neither.
Due to the unpredictability of your cuttings and the max diameter of any spiral (=or<1/2") being so small (both parameters negative),
I'd use a larger cutter. The shallow gullets and limited angles of small cutters render them more likely to tear your work spontaneously. Moreover, the up or down cut vectors of theses tools will contribute to the tears, especially upcuts when the overhang is generous.
A modest helix down shear or straight large diameter trimmer may be the better choice.
I'd use something like WMC's 3/4D x 5/8L helix or their 3/8" shanked x 7/8" D bottom bearing trimmers (or somethng bigger).
To be sure, I'd always use a new or near new cutter for the finish cut.
And I'd do some testing on scrap using the same materials.

[Mel Fulks]

With material that thin cant you just climb cut ?

[Stephen Cherry]

I use spiral bits in my cr onsrud inverted pin router. I cut out some pretty intricate doo dads with no real tearout. Upcut or down cut, there are two considerations. First is chip removal. Since you are doing a through cut, the chips are going everywhere, so it does not matter. The second consideration is compression. The upcut pulls the wood up, best when the back side is the show face, and the down cut compresses the material down, which is best for what you are doing.

[Mike OMelia]

I use spiral bits in my cr onsrud inverted pin router. I cut out some pretty intricate doo dads with no real tearout. Upcut or down cut, there are two considerations. First is chip removal. Since you are doing a through cut, the chips are going everywhere, so it does not matter. The second consideration is compression. The upcut pulls the wood up, best when the back side is the show face, and the down cut compresses the material down, which is best for what you are doing.

I do get what you mean by show face. This process is an intermediate process. Eventually, I will rout a channel at the top & side joints and fill with "binding".

[Mike OMelia]

With material that thin cant you just climb cut ?

Would you please explain this a bit more? I try to sand off as much of the edge as I can before apply the router. I really do not want this to be a "through" cut.

Mike