Why is skew chisel taking soooo long to sharpen?

[GGee]

Hi everyone I am new to the forum and to Tormek.  Hope you can help.  Have been trying to sharpen an oval, angled skew chisel (HSS).  It was taking ages (about 2 hours) and still nowhere near.  We realised we needed to shape it first, so we got bench grinder with Tormek BGM-100.  Did all that, got it into shape, all looking promising.  Transferred skew over to the Tormek (black silicone) and once again it is barely touching it (we put black pen on both sides and even after 40mins much of it it was still visible, with only tiny areas removed).  As far as I can tell we've been doing everything 100% correct.  But maybe not!  Any suggestions please?  Is the differential between the diameter of the bench grinder and larger Tormek wheels a material factor?
Thanks for your help
G

[Twisted Trees]

Yes the different diameters will be a major factor on a skew as the bevel is so long. Avoid using smaller wheels in future...

but back to the problem in hand, I take it you are using the skew jig, be certain that the wheel is dressed flat and that you have used the course side of the grader more than once to get the best cutting surface, being oval be sure that it is sat flat in the jig.

If it has taken more than 30 minuets then you are doing it wrong, if it has taken more than 2 minutes then you are not doing it often enough

[RichColvin]

Gareth,

I probably have the same one :  a Robert Sorby oval skew. 

This is not uncommon.  The reason is this :  there is a lot of surface area to be ground, especially for shallow included angles like 30° (vs. a bowl gouge).  And this large surface area is what takes so long.

What I have found to accelerate getting it to the next level is holding the gouge against the grindstone (using my right hand) whilst also holding the stone grader (SP-650) against the grindstone (in my left hand).  That keeps the grindstone's surface rough and makes for this process happening faster.  It will sound like you are adding sand to the process.

But, once you get a smooth surface across the whole area, you'll need to set the stone grader down and go from there.


Of course, there is always the option to use the course grit diamond grindstone ... but that is a very different cost option.

Kind regards,
Rich

[Ken S]

I took the liberty of moving this topic to the woodturning section. Gareth, you ask very good questions. If by chance, you do not already have the Tormek Woodturner's Information Box, I recommend it highly. It is, in my opinion, Tormek's best video and well worth the cost. Here is a link:

https://www.tormek.com/usa/en/accessories/other-accessories/tnt-300-woodturners-instruction-box/

In addition to being an expert with the Tormek, Jeff Farris is also a very experienced turner. Jeff worked exclusively with the SG-250. (The other wheels were not available when he left Tormek.) One of Jeff's pearls of wisdom from the DVD is to reshape gradually. By that I mean if your skew left the factory not matching any of the Tormek presets with skew and bevel angles, do not try to regrind the whole change in one session. Just do it partway and leave the skew sharp. Get continually closer during the next sharpenings. You will soon be where you want to be and only need Twisted Tree's fabled less than two minute sharpening.

Twisted Trees and Rich Colvin both have much more extensive turning experience than I do. I would make only two suggestions. The first is the gradual change I just described. I have experienced what you are going through. An interminally long sharpening time is totally frustrating. Even a small amount of reshaping can be frustrating. Do it in small bites. Do not be afraid to use lots of pressure with your SG wheel.
The second suggestion is to keep your TT-50 convenient. It will make your grinding wheel cut faster than just your stone grader. Use the TT-50 and when the stone starts to slow down, switch to the stone grader. Go back and forth as needed.

Once you have the shape duplicated, TT hit the nail on the head.Sharpen often.......

Keep posting.

Ken

PS Jeff's other pearl of wisdom was if the shape you want is radically different than your present tool, buy a tool shaped as you want it. This is especially good advice if you decide you really prefer the original shape. 

[GGee]

Many thanks all for your detailed and prompt responses.  The two most encouraging things are (1) we are not alone and not going mad(!) this is a known issue experienced by others; and (2) there is a solution!  One thing I did forget to mention is that we don't own a DVD player, having got rid of it when we went 'digital' a long time ago and started streaming everything!  Would be good if Tormek could make the DVDs available digitally.  Anyway we'll try the suggestions. Thanks again.
G

[Jan]

GGee, the HSS steel of your chisel contains several hard carbides (tungsten and vanadium e.g.) which are harder than Al2O3 abrasive in the SG stone or the SiC abrasive in the SB stone.
Despite this fact we can use SG or SB stone to sharpen HSS steel. What happens is following: the Al2O3 or SiC grounds away the steel matrix enclosing the hard carbides which are then torn out of the steel. It is understandable that this occurs at the price of low grinding performance and high wear of the grinding stone.

Diamond or CBN wheel will solve the problem.

Jan

[RichColvin]

GGee, the HSS steel of your chisel contains several hard carbides (tungsten and vanadium e.g.) which are harder than Al2O3 abrasive in the SG stone or the SiC abrasive in the SB stone.
Despite this fact we can use SG or SB stone to sharpen HSS steel. What happens is following: the Al2O3 or SiC grounds away the steel matrix enclosing the hard carbides which are then torn out of the steel. It is understandable that this occurs at the price of low grinding performance and high wear of the grinding stone.

Diamond or CBN wheel will solve the problem.

Jan

Jan,

Thank you for the explanation.  That makes sense!  Now I know why (instead of just how).

Kind regards,
Rich

[Twisted Trees]

I sort of remember having the same problem when I switched to the Tormek from a dry grinder with 6" wheels, obviously there is a small curve created by any wheel based system over the length of the bevel on most tools it is barely noticed, but combine that with the low angle long bevel of a skew it makes a difference.

Take Ken's advice, do it in stages over multiple sharpening's use your black marker, but only worry about the first 2mm for today, the bevel behind that is only going to be a tiny bit out so won't affect the use of the tool by much. in a few months it will be fully re-shaped to Tormek diameter 

[Ken S]

Well stated, Twisted Trees. We have a natural desire for instant resolution. I think we need to focus on eventual resolution and present turning. Enjoy the journey.

Jan, you make an excellent point. In ten years of working with the Tormek, I have seen many remarkable innovations. Some areas have advanced quickly; some more slowly. While the Original Tormek grinding wheels still function very well with many tools, I believe some tools with more exotic alloys do better with diamond wheels. The SG is still an excellent and flexible performer with carbon and stainless steels as used in chisels, planes, and kitchen cutlery, as Jan noted, high speed steels respond better with diamonds. One admirable quality about Tormek is that it is continually evolving. When I began, we had only the SG and the stone grader. Today we have several choices in grinding wheels and are starting to see more choices in dressing tools (stone graders).

Enjoy the interesting ride!

Ken

[GGee]

Latest update.  Many thanks again for all your comments.  Desperate to sort the issue quickly, we followed the advice in one of the posts and got the coarse diamond stone, which re-shaped the oval skew very quickly, solving the original problem.   A new issue has arisen: moving to the black silicone (or the original blue) results in a different grind (it is grinding the same tiny place on each bevel, not across the whole bevel).  It's as though the angle has changed, but it can't have; the chisel is in the same jig at the same angle as on diamond wheel and we used the TTS-100 setter . . .  Any reason why simply changing the wheels should result in a different grind, everything else being equal?  Thank you G

[Ken S]

Gareth,

Only one possibility occurs to me. Is your new diamond wheel the same diameter as  your Original SG and SB? Tormek makes two coarse diamond wheels, the DC-250, which is what you should be using (250mm diameter) and the DWC-200, which is designed for the T2 and T4. Are you sure you purchased the correct diamond wheel?

Ken

PS Please don't be offended; such things happen.

[GGee]

Hi Ken - thanks for responding and not offended at all (not least because I didn't realise they did different sized wheels, so I have learned something!)  We do have the correct (same) size wheels. (Very slight difference due to wear on the grindstone wheels, but not such as to explain the extent of the difference. (Plus we've trued up the surface of the grindstone with the TT-50 to remove any difference in wheel surface angle as a possible factor.) G

[Even]

The first thing I think about when I read your problem is:
Are you bushing on the axle worn?
If they are a little worn, the axle will not be parallel to the USB.
So if you true the SG/SB wheel with TT-50 then the wheel will be parallel to the USB, but when you change to the diamond wheel, it will not be parallel to the USB.

Even

[Ken S]

G,
I did not think the wrong wheel size was the problem, however, one thing I learned in 35 years of trouble shooting telephone circuits is always rule out the easy solutions. Now we can cross off the wrong wheel size from the list of trouble candidates.
Even may have a point about worn bushings.
I suggest that you also contact support.
Ken

[RichColvin]

I find that when I change grindstones, I have to adjust the projection, even if slightly.  Using a Sharpie to verify the projection is right is a good way to check this. 

Kind regards,
Rich