How to sharpen 3/4" mortise chisel

[Jan]

Hi Everyone,

I need to sharpen an very old heavy duty mortise chisel with blade width some 18 mm (3/4") and blade height also some 18 mm (3/4"). Please advice how to sharpen this chisel with the Tormek system.

Jan

[grepper]

Welcome to the forum Jan,

This should help:

http://www.youtube.com/watch?v=6lJHxoQWO8U
http://www.youtube.com/watch?v=kMCwG8xehGE

[Ken S]

Welcome to the forum, Jan.

We had a similar discussion on this forum a couple years ago. Ionut, a longtime member who sadly has not posted recently, was trying to sharpen a 1/2 inch (12mm) mortise chisel.  With the smaller chisel in that discussion, using one of the older square edge jigs provided enough depth to hold the chisel. However, even that jig would not be large enough to handle a 19mm thick mortise chisel.

What would work is one of the Torlock platform jigs. The control is not as precise as the SE-76 jig, however, you should be able to put a very sharp and good looking edge on your mortise chisel.
I would suggest that once you have the platform set for the correct angle that you make a template with either cardboard or plywood. it will fit between the platform and the grinding wheel. Mark it with the bevel angle for your mortise chisel.  That way, setting up for future sharpening will go more quickly.


http://tormek.com/international/en/grinding-jigs/svd-110-tool-rest/

Keep us posted.

Ken

[Jan]

Hi Grepper,
Thank you for your suggestion, unfortunatelly those demos do not solve my problem. My mortise chisel blade thickness is ¾" which is too much for the Square edge jig (SE76).
Regards
Jan

[Herman Trivilino]

The SVS-32 Short Tool Jig should work for you.

http://tormek.com/international/en/grinding-jigs/svs-38-short-tool-jig-svs-32/


Otherwise, free hand.

[Jan]

Hi Ken,
thank you for your response, I already have the Tool rest SVD-110 so I can try your suggestion.
If possible I would like to keep full control over the grinding process. I am wondering if the Multi jig SVS-50 will solve my problem.
I am concerned about the hollow shape of the bevel. If you imagine the bevel length is 1 5/8" than the hollow will be well noticeable.
Best regards
Jan

[Ken S]

Jan,

I understand your concerns. In a perfect world, mortise chisels should have a flat or even slightly convex bevel. In this case, I would not worry about it. The 250 mm diameter grinding wheel of the Tormek produced much less hollow grind than a smaller (150 or 200mm diameter) grinding wheel. If you are concerned about this, add three degrees to the bevel angle. A very sharp mortise chisel with a bevel of thirty five to forty degree angle will be very stout. Keep in mind that the extra width will spread the force over a larger area than a narrower chisel.

Regarding not having your mortise chisel fully  controlled in a jig, if you are reasonably careful holding your chisel with the SVD-110, your bevel and edge will be more precise and probably sharper than the master cabinetmakers of the past could do.                 

Good luck and continue to keep us posted. Good mortise joints will still be solid long after we are gone.

Ken

[Jan]

Hi Herman,
thank you for your suggestion. Yes, the Short tool jig SVS-32 provides enough space to hold my heavy duty mortise chisel. As Ken mentioned in reply #2 the control is not as precise as with the Square edge jig. The Short tool jig SVS-32 works fine provided the lateral blade edges are well preserved and the top face of the blade is perfectly flat and intersects with the plane of the back face of the blade in a line which is parallel to the cutting edge.
Regards
Jan

[Jan]

Ken,
thank for your reply #6. I agree with you fully. You have to guess my mind.
I have just calculated the hollow depth for my mortise chisel with a bevel length 1 5/8" (40 mm). The hollow depth from a 10" stone should be 0.03" (0.8 mm), from a 8" stone it should be 0.04" (1 mm)and finaly from a 6" stone it should be 0.05" (1.35 mm).
Regards
Jan

[Herman Trivilino]

The Short tool jig SVS-32 works fine provided the lateral blade edges are well preserved and the top face of the blade is perfectly flat and intersects with the plane of the back face of the blade in a line which is parallel to the cutting edge.

In my experience the SVS-32 is not precise enough for those issues to be a concern. Regardless of how the chisel is mounted in the jig, the operator must pay attention to how the cutting edge makes contact with the grindstone. This is especially important when trying to get a square edge.

[Jan]

Herman,
thank you for sharing your experience with the Short tool jig SVS-32. This jig offers larger number of degrees of freedom what is however compensated by the fact that some part of the tool control is transferred to the sharpener.

The sentence you have quoted is recollection of my thoughts concerning general requirements on the blade geometry which have to be fulfilled when trying to get a square edge. No jig can help us to get square edge when the geometrical prerequisites are not met. Since I have recognized it, I keep calmer even after several vain attempts to grind a square edge.
Jan

[Herman Trivilino]

You are correct, Jan, in that to get a square edge you do have to make sure all of those geometrical considerations are met. That is, a true grindstone and the tool properly mounted in the jig. The rub is that although these conditions are necessary for a square edge, they are not sufficient. Operator skill is very much a requirement when getting a square edge on any tool using the Tormek.

[Jan]

Herman, your feedback is highly appreciated! I agree with you that an experienced sharpener may be able to grind (possibly even by hand) a square edge on a chisel which does not meet the above formulated geometrical requirements. However, because the geometrical propositions are undoubtedly valid, I came to the conclusion that in this case the "square edge" would de facto be composed of several aligned cylindrical surfaces forming the bevel.
Jan

[Ken S]

Jan,

I believe we are worrying too much about squareness. Mortises are made with some clearance between the end of the tenon and the bottom of the mortise. This allows the shoulder of the tenon to fit snuggly and provides a reservoire for any surplus glue. The end of the tenon is end grain, with no gluing strength, so the clearance does not reduce the strength of the joint. Whether the bottom is square or just near square makes no real difference.

I suggest you try using the platform jig. Blacken the bevel with a felt tip marker. This is discussed in the Tormek handbook. Grind a little and observe how the marking is being removed. This simple method will give you feedback about the squareness of your grinding. Don't worry about removing tii much steel. A chisel with this much bevel will require quite a bit of grinding. Start with a light touch until your confidence builds.

The real test of your sharpening is how well the chisel cuts. Sharpen your chisel using the platform and chop a mortise on a piece of scrap wood.

Ken

[Jan]

Ken,
thank you for your wise advise concerning the perfect squarness. For chopping of mortises it is really not very important. Something else, I thing, is sharpening an old chisel inherited from your grandfather. Such a tool is usually used only occasionally, but all the more shown as a living history artifact. For this occasions the bevel squarness matters. 
In the past I was sharpening my tools on a bench sander equipped with my own roller assembly which was capable to maintain the bevel angle quite well. Now when I have acquired T7 with accessories I am step by step re-sharpening all my tools when I am sure to improve the bevel quality. In the case of my heavy duty mortise chisel I am really not sure whether the bevel will be improved or not.
Jan