For many routers, including the DW 625, a 3/8″ collet is available. That is the best option.

An alternative [and there is no other choice that I am aware of for 5/16" (8mm) end mills] is to use a collet reducer. This is a sleeve which fits into your 1/2″ collet and fills the gap for the smaller shank bit.

The disadvantage of a collet reducer is that it introduces more stiffness to the collet assembly and therefore the chuck needs to be tightened very snugly to keep a good grip on the bit. The better collet reducers are more flexible.

Infinity Tools sells reducers with six grooves in the wall for flexibility:
http://www.infinitytools.com/3_8-Router-Collet-Reducing-Sleeve/productinfo/SLV-003/

Whiteside also makes reducers, available directly from them or from Woodcraft:
http://www.woodcraft.com/Product/2008167/9245/12-to-38-Router-Bit-Shank-Adapter-Whiteside-6402.aspx

Lee Valley also sells reducers, but note that the 1/2-3/8 reducer has only one split and is therefore rather stiff. It also has no lip, so it inconveniently tends to slide too far into the main router collet.
http://www.leevalley.com/US/wood/page.aspx?p=30126&cat=1,46168,46180

Good luck with your mortising. Take small depth increments, and listen and feel for signs of cutting stress.

Rob

Thank you for the kind words and for reading.

The end mills that I have found work well are classified as:
Single, Standard, Center-cutting, Uncoated, Four-flute, Solid carbide, Plain shank, 30-degree helix.

An example is item #85315240, a 3/8 diameter end mill, from MSC Industrial Supply.

http://www1.mscdirect.com/cgi/nnsrhm

Click on End Mills, etc in the Big Book Index pages on the left. Then Single End Mills in section 3, then Standard.

In the drop-down lists, select Four-flute, Solid Carbide.

THEN put in the size parameters: cut diameter, shank diameter, overall length, and cutting length.

Now you’ve got 1-2 dozen bits to examine. You are looking for center-cutting and uncoated (coating is OK but not necessary for wood and costs more). I would avoid selecting brand, type, and coating from the drop-down lists because you may exclude some bits that you will want to consider. Just look over the list and look for center-cutting and uncoated.

This is what works for me.

Rob

I searched the manufacturers for end mills but the selection is so confusing I have no idea which ones to order. Can you be more specific on which ones you use?

Peter

Fred

I haven’t had problems with burning the wood but I cut the mortise in shallow steps and I clear out the chips regularly. For making a 3/8″ mortise in a dense wood like bubinga, I’ll take probably about 10 passes to cut a 1 1/4″ deep mortise. I listen and feel for signs of strain from the bit or router, but it goes nice and smoothly.

Rob

Thanks for the comment. I am glad you find the posts useful.

I have not used a slot mortiser. The current issue (#206) of Fine Woodworking has a nice review of mortising methods. My go-to method is the plunge router with a template guide that rides in a slot jig. These have all been shop-made jigs, but lately I’m finding the Mortise Pal to be handy. I truly get virtually no vibration or error with this method - unless, of course, I make a mistake (and that happens more than I should admit.)

Rob

As usual, a well written and useful entry. It is interesting to me to read your pieces because I am at the point in my small shop work where focusing on small refinements is the order of the day, and you have mentioned many of them in your blog.

The morticing jig for plunge routers that Tage Frid described in his technique book has served me very well, but quality slot morticers have always been in the back of my mind as machines that might be a subtantial step up in quality, eliminating that last bit of vibration and error introduced by a hand held and controlled tool such as the plunge router. Are end mills used in those machines? Have you ever used one?

Best,

Tico