SB-250 out of true

[VijayDoshi]

Hello all,

We have 3 T-8s and sharpen thousands of knives a year. About a year ago we switched to SB-250s and generally love them. I'm noticing they constantly get out of true on all of our machines (so I don't think it's a problem with our machines), and I have to think we generally apply equal and random pressure as we sharpen - it's not like we press harder every revolution always in the same place. My only conclusion is that they wear unevenly for some reason.

Has anyone else noticed this problem? Every month or two I have to true them up to the tune on 2-4 mm and it feels extremely wasteful. I wonder if there is a manufacturing defect that is causing inconsistent wear. Would love to hear if other's are experiencing/noticing this problem.

Thanks!

[tgbto]

Hello,

While I don't sharpen enough knives to see this phenomenon with the SB, it is my experience that this kind of issue is inherent to the Tormek principle, said otherwise it is a self-amplified phenomenon.

Sure, you don't intentionally "press harder every revolution in the same place", but think of what happens when a low spot exists for whatever reason : if you are sharpening edge leading, each time your blade will pass this low spot, it will lift ever so slightly at the incoming lip of the low spot, then dig in a bit shortly after. The result is a low spot that gets deeper with each rotation. This kind of oscillatory behavior is of course not limited to Tormek, but it is rooted in a locally favorable combination of resonant frequencies, characteristic times  (those of the tools or the operator), and gains (pressure).

I noticed this phenomenon to be especially obvious when I first toyed around with Wootz' grading technique using diamond plates. It seems I was using the perfect amount of downward pressure for the phenomenon to get noticeable quite quickly. Using less pressure helped, as did using another usb to support the far end of the diamond plate.

I don't think there is anything wrong with your SB's, I would try truing earlier (if you have to true 2-4 mm it is way too late), see if edge trailing gives satisfactory results, and also try using less pressure (I know the SB's glazed feeling makes you want to press real hard). Maybe deglaze often using a diamong plate with little pressure.

Hope this helps.

[Ken S]

I agree with Tgbto.

Sadly, I believe the SB-250 is the least understood and under appreciated grinding wheel in the Tormek lineup.  This is unfortunate, as I believe the SB has a lot of untapped potential. Unfortunately, early on, some users, including me, had glazing problems using it.

What makes the SB unique in the Tormek lineup? Its silicon carbide grits are sharper than the aluminum oxide grits of the SG. One of my mentors switched from using SG wheels to SB and experienced considerably longer wheel life. How is this different than the diamond wheels? While silicon carbide is not quite as hard as diamond, wheel damage and grits which eventually dull can be repaired by using the TT-50 Truing Tool.

The SB-250 was introduced almost fifteen years ago. During that time, it has shared only one paragraph with the SJ in the handbook and received only brief mention in the online classes. It is the only Tormek grinding wheel not available for the T4. Like the SG and unlike the diamond wheels, the coarseness can be modified with the stone grader, although it is generally used only as a coarse stone.

In my opinion, the area where the Tormek really shines is in the initial stage of resharpening, the heavier lifting part of sharpening. Woodturning tools and other HSS and superalloy tools were the reason for the SB. As youe should know, I am a real fan of the Tormek online classes. They are an invaluable learning resource. I am disappointed that there has not been an online class primarily dedicated to the SB, including correct usage, truing, and grading. I also believe that the SB should be included in 200mm diameter for the T4. It is the one missing component keeping the T4 from being a fully capable machine.

Ken

[kwakster]

In contrast to aluminium oxide, SiC carbide grit is harder but also friable: in use the particles break down into smaller ones that will continue to cut & remove steel at a finer and finer level, until they break free from the stone surface which then reveals newer and larger SiC carbides.

For hand sharpening on wet & dry SiC paper (which has only one layer of SiC grit) this effect can be exploited by doing the bulk of the steel removal when the wet & dry paper is still fresh, and then continue using that same paper for finishing the newly established edge, as the grit continuously gets finer and finer.
This way i often do a completely new edge from start to finish using only (a section of) one sheet of 400 grit wet & dry SiC paper.
I also found that i could prolong it's useful life even longer when using a product like WD40, as this helps to protect the SiC grits from premature wear and clogging.

With the SB-250 stone i also use a detergent in the water, named Blue Wonder.
This product is thin like water, has a strong degreasing effect, it keeps the swarf in suspension for longer, and it acts somewhat like a lubricant.
In the US a good alternative might be Windex.

The right detergent does several positive things when used with an SB-250 stone:

- It helps to protects the SiC grit from premature wear.
- It diminishes the grabbing efffect of the stone (which is especially useful when freehanding)
- I helps to avoid glazing by keeping the stone surface clean & free from the build-up of swarf and/or a thin greasy film (which can inhibit the SiC grit from cutting effectively, especially when the grit gets finer)

I found that all of this together helps to keep the stone surface true for longer periods.

[Ken S]

Interesting post, Kwakster.

If you had made the Blue Wonder/Windex suggestion in 2010, I wonder if we would have had the glazing problem with the SB.

A couple questions:
Living in the US, I would use Windex. Should it be used full strength or diluted?
Also, after the tools are sharpened, should they be sprayed or dipped in an anticorrosion solution like ACC?

I hope you will consider passing your suggestion on to support (support@tormek.se).
I suspect there are some SB wheels languishing on shelves which should be put back into service.

I believe garnet sandpaper and PA-70 have the same property of becoming finer grit with use.

Ken 

[Drilon]

Hello kwakster, as I can see there are different Blue Wonder solutions available. Which one are you using?

Regards,
Drilon

[RichColvin]

Vijay,

Are the SB-250s getting out of true whilst still on the machine, or are the grindstones being removed and then replaced onto the machines?

Rich

[kwakster]

This is the version of Blue Wonder i'm using for some time now:



I've tried several other products (among which a few other Blue Wonder products), and so far this seems to work best.

Blue Wonder also makes a degreaser which i haven't tried yet but plan to.

Generally, about one spray dose (~ 1 pull of the trigger) into the full water trough is sufficient.
Less water in the trough also means less Blue Wonder.

The trick is to use just enough so that it doesn't start to foam, which otherwise will obscure your view onto the newly forming edge & burr formation.
The water has to remain as clear as possible during sharpening, which also means thoroughly cleaning the water trough from time to time.

[Drilon]

Thank you for your answer. Will get it and find how it works with my stones.

[VijayDoshi]

I've found that just hitting the SB-250 briefly (2-6 revolutions) with a cheap 120/180 grit diamond lapping plate works to redress the grinding surface. This technique also works really well with the SJ-250 though might take more revolutions on that.

[Ken S]

Interesting conversation.

I wonder if a little bit of the original (not concentrated) Dawn dishwashing detergent work? An old habit I picked up from many years of archival black and white photo processing; I always finished my darkroom sessions by cleaning my trays. I do the same with my water trough.

I give Wootz of Knife Grinders the credit for this idea. He started using three inexpensive diamond plates to grade his wheels. Wootz clamped his diamond plates onto a plane blade. I purchased a set of three credit card diamond plates at my local hardware store. I purchased a 24" x 2" x 1/8 or 1/16" piece of steel and some epoxy cement. Cutting the steel piece in thirds gave me three back plates. (Be sure to soften the edges and corners with a file.) i placed one of the mounted diamond cards in an old square edge jig. This setup is always square to the usb and the grinding stone. This rig gives me one less potential outof square variable.

I think the stone grader predates common use of diamond plates and the SB and SJ wheels.

Ken

[tgbto]

This setup is always square to the usb and the grinding stone. This rig gives me one less potential outof square variable.

Yes, but it might also be a good way of making it out of round by worsening high/low spots. If you make your support long enough that you can rest the far end on a second USB, you also eliminate that risk.

[Ken S]

Good point, T?gbto. I don't remember my source. I believe it was eitherWootz or one of the online classes. T cautioned against leaning in the support bar. The careful workman carries the day.

Ken