Trouble routing mortises

[Ronak Shah]

I'm routing a large number of mortises for my crib project: 96 in total, into which tenons on the individual slats will fit. They are 1/4" wide, 3/4" long and 3/8" deep.

I did a bunch of them yesterday, but was having significant difficulty on a few things:

1) My mortises aren't straight! Even over their short 3/4" length, they curve somewhat. Is it possible that the bit is catching on wood and walking along the grain or something like that?

2) The bit turned black on my first mortise, burning a lot of wood. It's a straight bit, and seems to burn on anything more than an 1/8" deep cut. I don't see how a 1/4" diameter x 1/8" deep cut should be the limit of the carbide unless I'm doing something wrong. Of course I kept cutting.


It _could_ be caused by a number of factors, and I was wondering what the community thinks is the culprit:

1) I've got the Ryobi 1.5HP router and it's just not powerful enough
2) I'm using one of the cheap 24 bits for $24 bits from Holbren, and it's a 1/4" wide on a 1/4" shank.
3) I'm not holding it against the fence - although this seems unlikely on 90% of the mortises.
4) A 3/8" deep cut in maple is just too much

Thankfully nearly all of the screwups will be invisible since the slats are larger than the tenons - the extra width & depth cover any screwups in the hole. Still, it would be nice to know before I try to do this again!

Ronak

[gary]

Questions:

Is the bit you're using a plunge bit ?
Have you tried a spiral upcut bit which helps clear waste from the mortise?

[dkhoward]

You didn't say the dimensions on the stock that you are working. I suspect that the stock may be moving.

THere are a lot of different grades of carbide. You might search the threads here. I know there have been several different discussions on the quality of carbide bits. My opinion, I never go cheap on router bits or saw blades. Almost all of my router bits are Freud.

MY other observation from xperience is that I almost always take a whole bunch of shallow cuts rather than a few deeper cuts, especially in hard woods. It may take a little longer, but I am almost always happier with the results.

[LCHIEN]

you don't give a lto of specifics about how you are doing the cuts, but...

1. if the cuts aren't striaght and you are using a table mounted router, and fence, then...
- the wood is not moving in a straight line, you need to hold it tightly against the fence which means featherboards, a dust groove along the bottom of the fence
- the router is moving, or
- the fence is moving.

2. if the wood and bit are burning, then
- the bit is not sharp
- the bit is dirty (pine resin is a particularly bad cause)
- the router feed speed is too slow
- the cut is too deep slowing the router and reducing the feed speed
I suspect the latter, 3/8" is a bit much to take at one pass.
It will also help if you use a 1/4" solid carbide spiral upcut bit, approx $15 at places like www.mlcswoodworking.com with free shipping.
it will make good plunge cuts, clean the hole of cuttings as you cut and be more resistant to burning.
Once HSS steel bits are burned they lose their temper, dull quickly and then burn every cut.

[Pappy]

I addition to what has already been said, Maple has a tendency to burn easily. I suspect with the short length of the mortises you are cutting, you are moving the bit too slow. Take 1/8" passes and make sure you are moving the routerin the right direction so the rotation of the bit will pull the stock into the fence.

[Ronak Shah]

Hi everyone, thanks for the responses, especially Pappy who should be asleep at 2:49am.

For my next step I'm going to try shallower and quicker passes. I'm not using a spiral upcut bit, and I do see a lot of burnt sawdust stuck in the mortise when I'm finished. This did happen less often with my shallow cuts.

This is one of the Holbren C2 carbide bits from the 24 piece set; very middle-of-the-road. I am using a router table, but not a featherboard. It is entirely possible that the stock is moving (it's 1" x 1" x 51") since it's pretty hard to manuever.

Thanks for the help, and I'll try these ideas.

[JimD]

Ronak,

I made a bunch of similar mortises in 4 chairs I built (22 mortises per chair, 88 mortises for 4 chairs - lengths varied but all were 1/4 wide). I used a solid carbide upcut spiral bit and a Porter Cable plunge router. I built a jig with 1/2 inch wide cutouts where I wanted the mortises to be. I then put a template guide on the router (1/2 inch) and quickly made a bunch of mortises. If you have as many of the same dimensions as you appear to, you may want to make a similar jig. I also got slightly non-straight mortises but that was because the master jig I made to cut the slots in the jig I used was a little non-straight. Subsequently, I have done better by making my jig-to-make-a-jig using 3/4 stock and cutting strips on the BT3100 then glueing it up. That makes nice straight sides. You may want follow a similar pattern. Make one jig to make a cutout for a template guide to follow that will result in your 3/4 by 1/4 mortise. If you were using a 1/2 template guide and 1/4 router bit, you would need a 1/2 inch wide slot 1 inch long in the template. To make that, I would use a 5/8 template guide and a 1/2 router bit in a slot 5/8 wide by 1 1/8 long. I hope this makes sense. I make one of the "master jigs" - 5/8 by 1 1/8 in the example - and then use it to make a bunch of the smaller openings 1/2 x 1 inch in a piece of 1/4 plywood or hardboard that is then used to route the mortises. Be extra careful about the layout of your template and then you can make several identical pieces and do so pretty quickly.

My slightly non-straight mortises did not seem to affect strength of the resulting joints significantly. I weight over 200 lbs and the chairs are holding up just fine.

With a straight sided bit, I would expect maple to burn. I would think about ordering a spiral bit - carbide - from MLCS (free shipping). It may still burn a little but not enough to affect strength of the joint.

Jim

[Ronak Shah]

So here's what I did:

One of the guys at Woodcraft had a nifty little idea. Use a scrap strip of wood as a pincer fence to ensure that the wood is travelling straight and not twisting in any way. I put my workpiece against the fence, and the scrap strip against the workpiece and clamped the strip down. Even though there's a bit of play in the thin strip, I can easily hold down the workpiece and press on the strip to ensure that it's against both "fences".

This worked pretty well. I don't think it's as good as the plunge guide technique Jim mentioned, but considering that I can hide a lot of slop with the design, worked pretty well for my last 32 mortises. I still cut to full depth.

I think one of the big problems was that it was the wrong bit for the job. It had two cutting surfaces, which the Woodcraft guy thought might result in inconsistent forces on the workpiece, throwing it off-line. As a straight panel bit, it had no way of removing material, and I think this is one of the key causes of the burning.

I'll order the spiral bit soon. It's even more important as I seem to have vacuumed my bit up during cleanup...

Ronak

[jarhead]

Ronak,
I just wanted to chime in and post a link to a post from the past that may be of some use:
http://www.bt3central.com/forum/topi...TOPIC_ID=12712

[crokett]

Ronak,

As an alternative you can do what I did for my crib project:

Run a dado as wide as your slats are thick and as deep as you want the mortise to be in the edges of your rails. Make spacers that fit into the dado to separate the slats. Glue the spacers in, flush sand everytying, then glue up the slats. Instant mortises.