Thread: Spiral or straight bit for mortices

Hi Jill,

My prefence is for spiral upcuts. They give a smooth base to the cut. Spiral bits certainly remove good amounts of stock. So I think I'd go the upcut. Can you get a larger diameter upcut?

Regards,

Rob

I'd be using a 1/2" bit, but it depends on what you are using to cut them with. If you need to reset the jig to run the next pass then use the bigger jig, as this will require less adjustment. I used both on my last job - a 1/4" spiral cut CMT bit and a generic straight cut 1/2" bit.

My last mortices I cut with a combination of both. I found that cutting the corners in the mortice was easier and more accurate then rounding off the edges with a file. It's easier to see if the mortice fits without the rounded edges, as you are never sure if it is the rounded edge that does not fit or the actual width.

Good luck!!

Tom

Spiral bits are usually solid carbide and as brittle as all getout. I'd say you'd be almost certain to snap a 1/4" bit in this application. DAMHIKT.
But then straight bits are generally not designed for a straight plunge cut. They don't cut in the centre of the end. I'd use a 3/8" or 1/2" spiral bit here.
Lee Valley have HSS spiral bits and my 1/4", and smaller, spiral bits are these. So far I've never broken a one of them.

When cutting mortices i always reach for a spiral bit, hands down. As has been said a 1/2" spiral would be better still but if you only have a 1/4" spiral & a 1/2" straight bit then go for the 1/4" spiral for sure. Just be a little gentle with it & make lighter cuts.

Thanks everyone for your suggestions and support, I really appreciate it.

In the end I went gently with the 1/4" spiral bit, in 4mm deep steps down to 26mm depth. Went well! I actually made a template for this, with a 24mm OD template guide, as there were two morices some 35mm apart. I find there is a lot less chance of damaging the mortice sides or the template itself than when using a top bearing pattern bit.

One last question, how I do I tell from looking at it if it is a spiral upcut or downcut bit?

Hi Jill,
Have a look here. You can see a pic of a spiral upcut and downcut next to each other.

Regards,

Rob

Originally Posted by JillB

One last question, how I do I tell from looking at it if it is a spiral upcut or downcut bit?

Two ways;

1. Hold it up in front of you with the base facing down. Turn it clockwise. If it spirals like a drill bit then it's an up-cut. Drill bits are designed like up-cut spirals to eject the drilled the nasty stuff.

2. Hold it up as before. Turn is clockwise. As the bit rotates watch the flute carefully as it disappears to the back of the bit (from the right side), if it's going up it's an up-cut but if it's going down then it's a down-cut spiral.

Number one is easier to remember.

Up-cuts are great for mortises and table use whilst Down-cuts are great for dados and inlay where you want a clean edge on the surface. In fact to get the cleanest dado cut you would first cut with an undersized up-cut to get most of the dado opened up and then use a down-cut to get that nice clean cut on the edges.

I'm rambling now...

Originally Posted by JillB

I am making a few mortice and tenon joints, and each mortice is 30 x 45 and 25mm deep.

Which is the best router bit to use - a 1/4" spiral bit, or a 1/2" straight bit ?

The advantage of the spiral bit is the very small radius corners which could allow the tenons to be rounded slightly to easily fit, but is it designed to remove relatively large amounts of timber like this? The mortices will be made in timber which cannot be moved to a bench drill to use a Forstner bit to pre-drill the mortice holes.

Jill
Just as a matter of interest I have produced a great number of mortices and have used the standard straight cutter 'Skiing' my way down. The mortice size is the same size as the cutter.