Thread: Routing too rough for neat mortise - what bit.

That bit is a upcut and will tear the surface. Get a downcut bit. Use it in the same manner as the straight fluted bit.
Downcut bits have the flutes twist the opposite way to the bit you have.
Regards, Arie.


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Thanks Breezy, I’ll try that.

Hi Kevin.

If I understood you correctly, after your plunges, you did one pass at full depth. Have you tried with the same technique as with your straight bit? As in progressively increasing your depth over several passes?

Also, a down-cut bit won't resolve this issue. The only thing it will help with is tear-out along the top of your mortice. You should be able to get a perfectly true edge with this bit.

What sort of RPM are you using? The spiral bits have quite an aggressive cut and if running too slow will bite in more and pull, possibly off line. You should be running the router at full speed for this size.

As Lance has advised, don't use a downcut bit. Downcut bits are for shallow mortising where the finished surface will be seen and for mortising across the grain. Upcut is definitely the correct bit for deep mortising, especially in timbers like Blackwood where you can expect significant burning if you're using the wrong bit or your technique leaves something to be desired. Correct procedure is to plunge to full depth at each end of the mortise - this should be a single plunge movement - and then clean out the intermediate material with a series of incremental passes at 5 to 10mm depth.

How is your jig attached to the bench? Is it clamped in your bench vise or is it only clamped with a single clamp that also clamps the stock? The way the whole thing hangs off the side of the bench with seemingly little support for the outer clamping member is possibly making it difficult to maintain alignment of the bit in the mortise. The finish of the mortise makes it obvious that, as you've said, there is significant grabbing and that this grabbing can't be controlled by the jig. Is it possible to re-engineer the jig to provide more substantial support?

Originally Posted by aldav

As Lance has advised, don't use a downcut bit. Downcut bits are for shallow mortising where the finished surface will be seen and for mortising across the grain. Upcut is definitely the correct bit for deep mortising, especially in timbers like Blackwood where you can expect significant burning if you're using the wrong bit or your technique leaves something to be desired. Correct procedure is to plunge to full depth at each end of the mortise - this should be a single plunge movement - and then clean out the intermediate material with a series of incremental passes at 5 to 10mm depth.

DO NOT use a down cut for deep plunging EVER.

This is a link to a post about my jig. It was designed by Dan Phalen and I have used it when cutting all my mortises using the method Aldav posted. It works great.

Dan Phalen mortising jig

I thought I had a down cut bit to try but I don’t. Yes Lance I was also of the understanding that an upcut bit was the preferred bit for this deep mortise. I’m not to concerned about a little tear out at the surface. What I really need is perfectly flat mortise insides to maximise glue strength. I haven’t tried the straight bit method with the spiral bit so I will try that. The grab is happening once, occasionally twice on the plunge at varying depths with the spiral bit & most often when doing the first or 2nd full plunge at either end of the mortise, although I have had it do the same on some middle plunges too. Ie using the first part of Aldav’s process. I’m running the router at speed 2 which the manual says is 11000 rpm. I thought lRger bit were meant to be run at a lower speed. I will however increase that & see if it helps (thanks Treecycle ) The jig is held in my main workbench vice so it is 100% secure Aldav. More photos attached of jig. You did however alert me to the fact that the router is sitting just a fraction off 90 degrees. I wonder if this causing the grab? This slight angle I noticed is caused by a bit of play in the movable jaw of the jig vice which I intended to add additional router support. I’ll see if I can remedy that. Many thanks for everyone’s suggestions.

Originally Posted by Kelvin M

I’m running the router at speed 2 which the manual says is 11000 rpm. I thought larger bit were meant to be run at a lower speed. I will however increase that & see if it helps (thanks Treecycle )

A larger bit in Diameter bit (NOT LENGTH) is meant to be run at a slower speed

Rick

05FEFA63-0924-4668-9BD3-B32037AC16B3.jpg707921A1-7ECA-410F-860D-64D903820868.jpg05FEFA63-0924-4668-9BD3-B32037AC16B3.jpg2758291C-637A-4DDB-AA46-CCDA0EC4420B.jpgOk made 2 changes. 1) swivelled my stop blocks around so they served the double purpose of stopping the the router & stopped the fence lifting slightly out of it’s jig slot. 2) cranked the router up to 21000 rpm. Then did 2 tests a) used the spiral bit with multiple full plunges & a cleanout. The bit didn’t grab but the end result was way better than before even though it still wasn’t particularly smooth. It seemed like a snug fit but there were minute gaps where the 5 plunges were. b) used spiral bit with progressively deeper full length passes. This gave a much cleaner mortise with no plunge gaps but there was an ever so slight taper. Top of mortise was 1/3 to 1/2 mm wider than the bottom, so tenon a bit loose at top but snug down deep. Router must still be moving a fraction in the jig. I also felt it grab once on a mini plunge but couldn’t see any sign of it afterward. Unfortunately I ran out of similar sample blocks to test the 1 method left suggested by Aldav, so will have to wait until the weekend now for that. Hopefully that will be the winning combination.

Originally Posted by Kelvin M

used spiral bit with progressively deeper full length passes. This gave a much cleaner mortise with no plunge gaps but there was an ever so slight taper. Top of mortise was 1/3 to 1/2 mm wider than the bottom, so tenon a bit loose at top but snug down deep. Router must still be moving a fraction in the jig.

Without a proper rigid setup (not hand held), it will be nearly impossible to get perfect repeatability on consecutive passes. I wouldn't be concerned about the strength of the final join, modern glues will bridge a small gap like that.

Just to expand on the downcut bit comments in case you were wondering why the advice against them was so strong; downcut bits do not have cutting edges on the ends, they are not designed to plunge and attempting to do so will likely result in a lot of smoke and a potentially damaged bit.

Some really useful information here, I've never really had much luck with routers so tend to avoid them like the plague, but in the interest of speeding up my work i've been looking into expanding my router bit selection and this thread has been great info in terms of up/down spiral bits.

So whats the benefits of a downcut bit? is the only benefit that you get a cleaner cut on the surface? if so why not just stick with upcut bits as wouldn't the surface but hidden behind the shoulder of the tenon? Would a downcut only be used in through tenons?

Downcut bits are generally used for rebates, grooves and template following, where you want to eliminate breakout on the top, or as finishing bits on CNC machines, where the spoilboard prevents breakout on the bottom to achieve a near perfect finish.

They are also essential for cutting out small parts on a vacuum table CNC as the upward pressure from a normal bit will lift the parts up and throw them across the room.

They have poor chip evacuation and would not be used for mortises at all.

Good advice Elan. I’m curious if router burn in a mortise weakens joint strength when glued up? I’m anticipating some burn in my Tassie Blackwood