Tool Tip #20: Hollow Chisel Mortiser Tune Up Pt3

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Blog entry by robscastle posted 03-21-2016 12:48 AM 1844 reads 1 time favorited 2 comments Add to Favorites Watch
« Part 19: Hollow Chisel Motriser HCM Test run and Results Part 20 of Tool Tip series Part 21: Drilling Dimples or more correctly Pips in wood »

Hollow Chisel Mortiser (HCM) Tune Up Part 3

Well its been a long time coming for part 3 mainly because the HCM does not get much continual use, however because my No 4 son Toby asked for some more clothes dryers for his friends the HCM was brought out to do the job and it prompted me to finish the HCM Tune up blog series, so here we go.

Some Background

The project
The knock apart dryers have pegged mortices on the posts and shouldered tenons on the rails at each end.

The Hollow Chisel Mortiser (HCM)

Is a bench top model and has a capacity up to 5/8 or 16mm chisels


The chisel set consists of five HC 1/4 inch, 5/16 inch, 3/8 inch, 1/2 inch and 5/8 inch
The sharpening Kit comprises of a HSS six flute cone reamer and four guide pins 1/4 inch, 5/16 inch, 3/8 inch, and 1/2 inch only. Notice the absence of a 5/8 guide.

Hollow Chisel Mortiser (HCM) Tune Up Part 3

OK now on with the tune up, first up thanks for all the feedback and suggestions provided regarding the chisel will benefit by sharpening the bevel.

My attempts to buy a diamond cone sharpening set locally were still unsuccessful so I settled for a HSS version. Personally I think this type falls short of the diamond set mainly because of the inability to sharpen a 5/8 chisel where as the diamond set by design covers all the sizes up to 5/8 I am assuming, (some are only 13mm or ½ inch.
And the quality of finish of diamonds as opposed to the chattering HSS fluted type.

HMC Bevel Angles.

An interesting exercise if nothing else was an attempt to find out before I started what the bevel angle for the cutting edge of a HMC at manufacture was.
There seems to be no design specifications provided.

HMC Sharpening kit Bevel Angles.
I can advise that the cones are 60 degrees so I am assuming the Hollow chisels are all the same, however regardless they are going to end up at 60 deg by default after sharpening.

Out of interest I decided to determine the angle differences, so by applying a marker pen on the cutting surface and doing an initial grind it will hopefully show the result, This is what I found.

It appears to be the same angle as the cutter.

After sharpening the Chisel there will be a slight burr left on the four outside edges. Use this burr as a guide as to how accurately you held the cutter on the chisel.
What you should have is a burr of equal proportions on all four sides.

Notice in my photo the left spur has some dye left on it. As small as it was there was a noticeable difference in the burr profile on this corner, so I repeated the process.

Recommended practices for carrying out this procedure vary considerably from: use only a drill press and vice, to to using a wood brace, then by hand only, and finally with a battery drill.

All very confusing if you are doing it for the first time but be guided first up by the suppliers recommendations then modify it to suit the best result.

To remove the burrs from the outside of the chisel draw the chisel across the diamond stone, Job complete!!

Additional sharpening

The cones come in two grits, Coarse 220 and fine 600 the application in sequence is required to achieve a satisfactory result.

There is also thought to filing grooves in the corners of the chisel to assist in chip removal. This was certainly something I was not aware of so I spent some time investigating. There is actually evidence of a patent back in 1929 covering this.

Have a look at the diagram and in particular fig 7.

So was it still done today was the next question, with some more research I found an article in the Popular Woodworking Magazine Blog HCM Tune up and again there was a reference to it.

“The last step is to file a notch on each of the inside corners with a triangular file. I wish I could take credit for thinking of this, but it is a feature found on some chisels. I first saw it on the premium chisel set from Lee Valley. Depending on the hardness of the chisel, this filing may take a while, but I think the results are worth it”.

This provides some more clearance between the outside of the auger bit and the inside of the chisel. I also think it makes it easier to plunge the corners of the chisel into the wood to start the cut. The benefits of doing this aren’t as great as the benefits of smoothing the outside of the chisel, but it doesn’t hurt, and in some species of wood may really help.

Also within the Lee Valley site accompanying a set of HCs there was some more info and reason for doing the activity was also documented.

Relieving Inside Corners

Even with a finely honed mortising bit you may find that the chisel requires a lot of pressure to penetrate into the wood; particularly hardwood. This can be remedied somewhat by relieving the inside corners of the bit to reduce the wedging action of the chips. This is done with a small, very fine-cut square or round file. Clamp the mortise bit in a vise and stroke toward the inside, removing no more material than illustrated.

The next step was to attempt this relief corner work, with my chisels and available tools I was not able to successfully do it so I ceased work on it. I might add trying to do this on chisles smaller than 5/16 would be almost impossibe at home, in a engineering workshop well maybe.
But its there for information.
Dont forget, as an aside you may also be able to use the sharpener to resharpen your wad cutters too, sure beats trying keep the outside surface concentric.

HMC Sharpening the Auger.

To sharpen the HC auger a flat or triangular file is used much the same as a conventional wood auger.

The HCM auger differs slightly from convention as it only has one cutting edge and cutting spur, in most cases, but it appears from research thet you can buy twin cutters at least however I was unable to find any twin cutter and twin spur versions in my investigations.

Cutting surfaces

One cutting edge and spur

Two cutting edges
Legend has it that if there is two cutting edges it’s of an English origin where as if its only one cutter the origin is Taiwanese or Japanese.

I will leave you to form your own opinion here!

No examples two cutting spurs were to be found.

To sharpen the auger simply file the cutting edges on the inside , do not touch the outer edges of auger anywhere.

Polishing the Chisel and Auger.

No doubt you are aware all HCM bits require honing of the business end and sharpening to suit each users requirements before use. This is a one off step which pays dividends later in the life of the HMC.
How I did mine.

I used a fine linisher belt to remove the grinding marks on the chisel body then a Scotch Brite belt to final polish everything I used the Scotch Brite belt only to polish the auger chip extractors up to the cutting surface.

Inspect closely the Point of the Auger
This is an addition point worth mentioning as I had a auger with an off centre filed point. It has a run out of 2mm. this may be of no consequence but reinforces the fact the chisel should always be aligned on the outer edge of the chisel and not the centre point.

Find an HC you want but the shank is too long? They are designed that way simply cut it to length with a hack saw they are soft in this area.

Use of Dry Lubricant.
I did some tests with dry lubricant and it certainly made the extraction easier

Other modifications.

I also added a strip of cloth backed grit to the clamp face. I considered doing the back surface of the vice as well but decided against it due to the fact I use the vertical surface for initial setup alignment of the chisel.

Was it all worth it
This is now up to the user to decide, in my opinion polishing is a must.
If you are not convienced check out the photo of the chisel at the Pt 1 of this series.

Polishing after the square surface on the chisel is purely cosmetic but that was the width of the belt so it got done by default.
A Hollow Chisel Mortiser would have to be the least used tools I have, and is only good for a single use, making square holes. It requires meticulous set up to produce quality work, it’s a heavy brute, but when set up and used correctly an irreplaceable tool if you do a lot of mortices.
Subjectively would I buy another one, the answer would have to be no in my work application.
Dont get me wrong its now a great little machine, and the more I use it the more I like it.
Having a chuck it could suffice for a single speed drill press.

-- Regards Rob

2 comments so far

View Joe Lyddon's profile

Joe Lyddon

10867 posts in 4659 days

#1 posted 03-21-2016 12:52 AM

Very interesting…

Thank you for posting it…

-- Have Fun! Joe Lyddon - Alta Loma, CA USA - Home: ... My Small Gallery:

View JoeinGa's profile


7741 posts in 2614 days

#2 posted 03-21-2016 12:12 PM

Interesting. I have looked at these things before and always though ”Well THAT is something that would certainly spend all of it’s time sitting in a corner” … then I take that money and buy something else. Guess if I did more projects with M & T it might be worth a more serious look.

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