Improving bench top mortise accuracy

[Alex Horvath]

Hi,

I just made my first 6 mortise and tenon joints using the Jet benchtop mortiser and Delta table saw tenon jig. About half of them are near perfect but the rest have noticeable offsets. I turn the work piece to cut from both sides on both mortise and tenon for perfect centering. Also, there was enough variation in the mortise width that I had to sneak up on my tenons to get a tight fit.

I checked the fence and bit on the mortiser for square to the table and they look good.
Also checked the tenon jig for square to the table saw.

Here are my observations-

1. I noticed that if I'm not real careful, chips get between the work piece and mortiser fence which of course really throws things off.

2. I seem to have a hard time getting the mortiser chisel aligned to the work. I can clearly see the uneven sides of the mortise. I suppose this is not a big deal if your spacing of cuts is consistent but going back to clean up with multiple passes will have the effect of enlarging the mortise.

I think #1 is my biggest problem, obviously the mortises were not perfectly centered since the tenons are more likely to be perfectly centered (stock is verified same thickness). I suppose it's just a matter of keeping the work tight to the fence at all times.

Sneaking up on the correct tenon thickness is not too time consuming for say a dozen or less joints but more than that will get tiring.

Also, I nibble the shoulder cuts with table saw but this also can be error prone and very tedious. I suppose if you are doing lots of joints you could set up a dado with a gauge block on the fence.

Thanks for any tips.

P.S. I read once that even production shops generally run frames (mortise and tenon or cope and stick) through a wide belt sander to get the joints in perfect alignment. They have to come back and hand sand the cross grain scratches on the rails. Hand sanding a misaligned joint will show in low angle light unless you feather it across a long distance.

[Neil Brooks]

Alex-

I can't be positive which steps you may or may not have taken, but find this article pretty useful.

Squaring the chisel, to ensure it's straight up and down (90* to the base), and 90* to the fence, seem to be the biggest places to reduce error.

My Grizzly, with its OEM chisels ("tuned up") does a pretty darned good job of cutting consistent mortises, to my layout lines.

Good luck !

[Rod Sheridan]

Hi Alex, here are few suggestions to try

1) obviously square up your chisel to the fence

2) don't flip the piece to "center" the mortise. When you do that one side of the chisel isn't supported by the wood and will deflect, resulting in variable size mortises.

3) taking the same size cut off both sides of the tenon stock only works if the tenon stock is of a consistent thickness, check for that

4) consider buying a tenon jig and cut the cheek cuts with a rip blade.

Regards, Rod.

[Matt Day]

I don't mean to hijack this thread, but my fence is not perpendicular to my table and for the life of me I can't find any way to adjust it. I have a Delta 14-651. I made a temporary fix by cutting a piece of MDF with the proper bevel and clamped it to the fence, resulting in just about a square fence, but I don't think this should be necessary.

[Chip Lindley]

Not to hijack by answering, you will have to shim the fence to 90 degrees. Brass shim stock is sold in hobby stores. Or use an aluminum can. Once you find the right shim thickness, super-glue it where needed.

[Rod Sheridan]

Hi, as I indicated I make the two end cuts, then divide the remaining space into full plunges, leaving less than full pieces in between.

Plunge out the remaining small pieces.............One completed mortise.....Rod.

[Matt Day]

Not to hijack by answering, you will have to shim the fence to 90 degrees. Brass shim stock is sold in hobby stores. Or use an aluminum can. Once you find the right shim thickness, super-glue it where needed.

Chip, the fench is firmly attached to a bar that moves it fore/aft, so the fence isn't clamped down to the table. I don't think shims would do anything, but I'll take a look when I get home.

[John Coloccia]

1) you almost always will introduce an error when you flip the workpiece as any slight inaccuracy in the workpiece thickness will show up in the mortise or tenon

2) is you want to align the mortise accurately, BEFORE you clamp, bring the mortise chisel down onto the workpiece. Yeah, I know....This takes extra effort because of how the mortise drill needs to extend, but if you want it exact you can bring the chisel down, clamp the piece, and then setup your bit depth. With some experience, you'll be able to take some light cuts with everything setup, and then clamp.

M&T joints, if you want them to come out perfectly, take some fiddling sometimes. It helps greatly to run test pieces out of scrap. I don't know if this is helping any other than to let you know that it's not always as simple as setting the jig and cutting if you're looking for precise fits.

[Terry Welty]

I have a super cheapo Harbor Freight's dedicated mortiser. I added a Harbor Freight X-Y Vise and never looked back. Makes cutting mortises super easy uniform and perfectly straight and consistant. I got the 6" vise on sale $49.99 and then used a 20% off coupon. I can only imagine what it would do for a better quality moritser..

I simplified this version of the upgrade...

http://www.woodcentral.com/cgi-bin/r...cles_414.shtml

[John TenEyck]

I got tired of trying to tame a bench top mortiser and built a 2-axis horizontal router mortiser. It is superior in every way except noise. You can also easily make angled and even compound angled mortises. The cheeks of a router cut mortise are straight and smooth compared to a bench top mortiser and the joints fit together precisely with perfect registry, because the stock is held on it's side rather than on edge. I built mine for less than $40 and 8 hours of time. Something to consider if you can't tame that iron beast.