Best bits for slot mortiser?

[roger wiegand]

My new-to-me MiniMax jointer/planer came with the slot mortising attachment. I had occasion to use it this week, but am really wondering what the best tooling is. The instructions that came with the machine are silent on that point. What I tried was a four flute flat end milling bit I bought from McMaster. It would take only very shallow cuts (~1mm max), and at that there was vibration and a tendency of the bit to grab and run in the wood.

So, is an end mill the right tool, or am I better off with a router bit, for example a spiral upcut? The clearance for chips is much greater on the router bit than the mill.
Would I be better off with a two flute end mill? Or is there one made specifically for wood?
The two end mills I have have a flat on the shank, it that required to hold them in this machine? (I did manage to knock the bit out of the machine the first time I took too deep a cut, not sure whether it was correctly inserted or not, I couldn't tell by feel if something was engaging the flat or not)

[Brian Holcombe]

A self feeding bit is going to be terrifying with a slot mortiser, so any upcut spiral (end mill or router bit) should not be used (in my experience). A Bird’s Mouth bit is proper, I have some untouched ones if you interested send me a PM. Outside of that I found downshear ‘mortising’ bits to be great. They don’t burn like down spirals, don’t self feed like up spirals and evacuated the chips well.

[Mike Henderson]

I always used 1/2 inch spiral router bits. A slot mortiser is great for making mortises - depending on where the mortise is.

Mike

[Richard Coers]

Best bit depends on method of milling. If you plunge drill a series of holes and then take out the waste, bird's mouth are the best as Brian says. Up cut spirals will work fine on more shallow mortises. If you mill the slot in a see/saw method, ramping deeper as you move the wood, a conventional 2 flute works just fine. I've milled thousands that way. A plunge cut 2 flute will also help, but not necessary. I should say all my experience is with a Multi-router. The higher speed router may skew my preferences.

[Erik Loza]

+1 for Birdsmouths. Try Rangate and Felder's E-shop sells them (Metric) as well.

Erik

[Kevin Jenness]

I have used spiral end mills for many years in my Steton which has dovetailed ways with a fair amount of friction. The only time I had any problems with self-feeding was with an extra long 5/8" end mill which started to pull itself out of the chuck. A shorter 5/8" end mill works fine.

Where I did have a problem was when using a Felder FD 250, which has a very free running bearing system in the x and y axes. Any end mill over 3/8" was impossible to control, so we used birdsmouth bits. The ones I have used are not suitable for boring so they have to be ramped in.

The "best" bit is going to be as short as will do the job. The longer the bit and the deeper the cut, the greater is the tendency for bit whip which can lead to tapered mortises and mismatched mortises in end and side grain. I tend to bore full depth and clean out the waste these days as I find I get the most consistent results in deeper mortises. With ramped cutting I keep the plunge depth pretty low and the feed speed moderate.

A colleague told me that his problems with inconsistent mortises went away when they moved from a manual to an automatic slot mortiser where the infeed and traverse speed are adjustable and consistent. (They have a very cool round end tenoner to make the male part of the joint.)

[roger wiegand]

A bird's mouth bit like these https://www.infinitytools.com/birds-mouth-router-bits-3720?

I'm totally not seeing how that's going to work. You must be referring to something else. --- Ah, found a link to the Rangate page, those look much more promising! The ones I found were seriously expensive. (Set of 5 for $363) I may try one before spring for the whole set.

[Kevin Jenness]

The Rangate bits are pricy, but they do work well and are carbide tipped. I bought a 3/4" one as I couldn't find anything else that diameter and length that would fit my chuck. That said, HSS birdsmouth bits are easy and inexpensive to sharpen.

https://www.icscuttingtools.com/new-products/mortise-bits.htm

It sounds like your machine doesn't play well with spiral end mills, which is unfortunate as they are available in a wide variety of configurations and relatively cheap. I use two flute center cutting bits from vendors like MSC. I think Jeff Duncan has recommended 4 flute roughing mills, but I have not tried them.

If your machine mounts to the front of the jointer, wouldn't it need left hand spiral bits? Or is the motor reversible?

[Jeff Duncan]

Yup, I like the roughing mills in my machine.... birds mouth/specialty bits are just too pricey for a cheap ass Yankee:D Seriously though, I had to try a bunch of bits as some don't feed well at all, the roughing mills cut well they just leave a rougher finish which is fine by me. I take shallow cuts as it makes controlling any type of bit easier..... feed in a bit and cut across, feed a little more, cut across, and so on until you bottom out. I know other guys bore full depth both sides and clean up the middle, but always seemed like asking a lot of the bits. Anyway thats my two cents on it, give them a try and see what you think!

good luck,
JeffD

[roger wiegand]

Mortiser sits on the back of the J/P. so no need to reverse direction. Going to try Brian's birdsmouth bits, as well as some other technique variations I've been reading. The advice to use the shortest bit that will make the cut makes a lot of sense-- I of course bought the longest one I thought I might ever need. As with many things practice may well do wonders.

[Peter Quinn]

I like the onsrud 24-000 series 4 flute birds mouth for shallow to maybe 2" depth slots, Leitz makes some real nice solid carbide 2 flute slot cutters that are more expensive but go much deeper. Neither is a plunge cut, both must be ramped in from left to right. Not sure where to buy the onsrud, haven't bought new ones in a while and the place I used to get them closed, maybe contact Onsrud for a dealer? The Hickory link is the onsrud catalogue, top of page 75 has the slot mortise bits, I like the 4 flute.

http://lexicon.leitz.org/product.php?pgid=978801&itemid=978801&hasleafs=Y&chash=6da98a
http://www.hickorysawandtool.com/Onsrud.pdf

[John TenEyck]

Similar to some others, I have had no trouble using 2 and 3 flute HSS center cutting spiral upcut endmills with my MM FS-35 mortiser. I use mostly 5/8" diameter bits; for anything smaller my horizontal router mortise is much faster and easier to use. With the MM, I plunge 1/8" or less, sweep side to side, repeat until full depth. The comment about using the shortest length possible for the depth you need is spot on, too. If you use end mills that have a flat on the shank make sure the jaws do not land on that flat, otherwise the bit will not run true. Also, do not use reduced shank bits. The Wescott chuck is already challenged enough to hold the bit w/o slipping. That results in a 5/8" max. diameter bit on my mortise.

Stay away from Amana's carbide slot mortising bits in a MM J/P mortiser. They self feed something awful, and whip around leaving a wedge shaped mortise; just pathetic. And they will loosen in the chuck, too, just to add to the excitement.

John