Mortising Bits

I think it is just normal

Mike L

I had a problem like this a bunch of years ago when working with Alder. I was doing through mortises and the tear out was pretty bad. I backed them like you did and some of it got better but still not good.

I was told these type machines are best at doing stop mortises not through. I have the Jet machine and chisels/bits

Maybe you could cut almost all they through with the machine and finish by hand?

Re: Mortising Bits

John in NM

To some extent these kinds of problems are to be expected with hollow chisel mortising.

In general, buying the best bits helps some. I like Star-M's which I've used a lot because I need a 9/16" size for one regular job, and the premium bits are the only way to get above 1/2". Lee Valley and Woodworker.com both sell this brand, they are Japanese pattern bits from Nakahashi Seisakusho Co.

I've read that English pattern bits work better, but the ones I have seem about the same. I've not tuned both up for a proper comparison though. English pattern are dual flute drill bits and sit higher in the chisel because they don't have that brad point/edge burr cutter of the single flute Japanese pattern bits. Supposedly they burn less because two flutes gives you smaller chips and better clearing of them - I've not made a study of that, just read it on another forum.

I've found the most important thing to sharpen is the bit's cutting edge (but then I'm also doing end grain mortises). Keeping that sharp keeps the chips small.

The bit wobble that causes those round cuts outside the chisel is generally solved by moving the bit up in the chisel some. But it is somewhat inevitable too. One thing to watch for is breaking the bit (I've done that pushing them hard) - the wobble becomes extreme before bit breaks. The other nice thing about Star-M is that one of those two vendors sells bits and chisels individually, so you can replace a trashed bit.

Send me an email if you like, we can talk about loaning you an example of each type if you want to tune them up and compare. I want them back though

Re: Mortising Bits

Sgian Dubh

"Chips would get caught up and the drill would frequently heat up to the point of burning the chips."

I suspect there may be a lack of clearance between the the auger and the interior conical section of the chisel. If the gap is too small it usually causes the chips to become trapped as you describe.

I aim for a gap between chisel and auger of about 2.5 - 3 mm, for which I use as a gauge a coin of about that thickness, all depending a bit upon what coins I have handy in my pocket.

To install the chisel and auger, assemble the two parts loosely together, slide the chisel into the machine's bushing or directly into the clamp if the bushing isn't required and let the chisel drop down to rest on a piece of wood resting on the metal table below.

Slip a coin (or similar substitute of the right thickness) on top of the chisel's shoulder, push the chisel up tight to trap the coin against the bushing or clamp and lock the chisel in place.

Next, slide the auger up until it's snug up inside the chisel, and lock it.

Loosen the the clamp holding the chisel, remove the coin, raise the chisel up until its shoulder is tight in the bush or clamp, make adjustments until the chisel is square to the table's fence, and lock it in place.

You should be set to go, having almost certainly eliminated chip trapping and burning, and I apologise if I've just tried to teach you suck eggs regarding correct installation of the chisel and auger.

As to breakout when executing through mortices, probably the easiest or best way to avoid this is to plunge from both edges or faces rather than plunging all the way through from one edge or face. Slainte.

Re: Mortising Bits

Barry Irby

I have not done many through mortises but there were only two ways I managed to get a clean exit hole. Leave the stock about a 1/4" too thick or wide and rip off the breakout. Or, glue on a block of even harder wood and then rip that off.

Re: Mortising Bits

William Duffield

Richard offers good advise.

But wait, there's more!

In the USA, for most hollow mortise chisels, the coin you want to start with is a dime. I can't quote the current exchange rate between dollars and pounds shillings and pence. If you use a coin that is too fat, the bit wobbles inside the chisel, creating the scallops on all four sides of the mortise that you report.

If you are getting chip out on through mortises, are you clamping the work down firmly to the sacrificial board? If not, it is easy for the mortising bit to lift it, and you get chip-out anyway.

If you hone the outside of the chisels, you are effectively creating an almost imperceptible wedge, which increases the amount of force you need to push the chisel into the wood, and extract it.

The hold down forks that are supplied with cheap mortisers and DP mortising attachments are totally worthless. They don't bother to machine them flat, much less parallel to the table, so that attempting to extract the chisel lifts the work and tilts it, sometimes resulting in breaking the chisel. If you tighten them enough to be effective, which is impossible with the supplied wing nuts, they scratch and mar the surface of your work.

If the chisel squeaks, something isn't adjusted properly, and you are creating too much friction, which heats up the chisel and the bit. You've got to watch out for inflation with those thin coins. If you adjust the gap between the chisel and the bit too tight,, it starts heating up as soon as you turn it on. Heat causes the chisel and the bit to expand. This further decreases the gap between them, which causes more heat build-up.

You MUST control the temperature of the HCM bit and chisel. Cheap bit and chisels are not made from High Speed Steel, and high temperatures, squeaking and smoke turn them blue and destroy their hardness and therefore their sharpness. You must pay close attention to chip clearing. If the chips get stuck, they stop carrying the heat away from the cutting edges, but the bit keeps turning and rubbing, generating more heat. As soon as the chips stop coming out of the side of the chisel, back it off quickly, until the chips are cleared. When i used a HCM drill press attachment, I kept it lubricated with candle wax or beeswax and kept it cool and cleared with compressed air, one hand on the drill press plunge control, the other on the air gun, and the third on the wax block.

When I was totally fed up, and it finally came time to sell it, due to the multiple annoyances of the process, I couldn't get one bid for it at our woodworkers' association auction. At that point, I could have gone out and bought a PowerMatic floor standing mortising machine, which I had extensive experience with, and which had given me good results in the past.

https://www.toolnut.com/powermatic-1791264k-719t-mortiser-1hp-1ph-115-230v.html?utm_source=google&utm_medium=shopping

Instead, I bought a Festool Domino DF500, and then the larger DF700XL, and lived happily ever after.

Re: Mortising Bits

Don Stephan

Twenty years ago you said to use a new gold coin each time and then send it to you. Don't you know I have a 5 gallon bucket of those for you the next time you move across the pond? LOL.

Seriously, Sgian's spacing trick ended my tip overheating issues. I keep two nickels, two quarters, and two dimes in a bag with a few other items in the PM mortising machine cabinet.

Re: Mortising Bits

Sgian Dubh

"I can't quote the current exchange rate between dollars and pounds shillings and pence.

Ha, ha. Nor can I. It's been nigh on fifty years since we dropped that crazy currency for the current simpler decimal pounds and pence, 100 pennies to the pound. Pre-decimal was a bit of a nightmare: 12 pence per shilling (aka a 'bob'), 20 shillings per pound, i.e., 240 pence per pound. Then there were, going back seventy or a hundred years, sub-particles of a penny, i.e., half pence and a farthing (1/4 of a penny). To add even more confusion we had a tanner, i.e., half a shilling or six pence; a crown (five shillings); half a crown (two shillings and 6 pence, or more simply two and six, often written 2/6).

Essentially, just a blasted nightmare for those of us (me, for instance) that for some reason have always struggled with simple mental arithmetic. In my case, numbers float around and easily get lost, and I guess it's a bit like a form of dyslexia, but with numbers instead of words. It means I always have to be especially careful to get dimensions right, but somehow I've always coped: and it helped no end when we in the UK largely dropped Imperial measure, e.g., feet, inches and fractions for metric, e.g., metres and millimetres. Slainte.

Re: Mortising Bits

Sgian Dubh

A five gallon bucket's worth of gold sovereigns, Don? Maybe,after the current virus thing eases off, there might be enough there to pay for a luxury cruise over, hire a limo in New York, plus driver, and come and pick them up ... and still perhaps have some change for fripperies, such as a few top notch hotel rooms at suitable junctures to ease the burdens of the journey.

I'm tempted, ha, ha. Slainte.

Through Mortises

Mark Mandell - Gone Round In New Jersey

I solved the chip out issue by not cutting all the way through the stock. I cut until the bit's point just breaks the opposite surface. I then use a 1/4" bearing bit in a trim router from the other side and a chisel to get nice crisp edges and no tearout on the outside surface that will show. Entry chip-out should be covered by the tenon's shoulders

Re: Through Mortises

Lee Ohmart

Thanks for all the insight. On the first mortise, I plunged straight through and saw I had an issue. Thinking it was an issue with the keenness of the blade I tried sharpening all cutting surfaces and only marginally improved the result on a second mortise. On the third attempt, I used hand chisels to start the mortise on both sides. That solved the chipping on the top only. Now the problem is I have three ragged mortises to repair. Would it make sense to try cutting them over size with hand chisels and then gluing some wood strips inside? I suppose I could make it look like a design element.

Re: Through Mortises

Mark Mandell - Gone Round In New Jersey

I clean up the interior walls with a small rasp and a sharp bench chisel to that the trim bit's bearing has a good surface to ride on. All I have to do then is square up the outside corners to get a nice fit on the through tenon.

I would avoid putting in carpenters' helpers. They will show on the outside. That said, if you want to cut your holes 1/2" bigger in both dimensions and then glue in a contrasting wood, that might interest you. I don't think I'd be happy doing all those little mitered corners, however.