Help for decision

[Ken S]

Enjoy!

I had the pleasure of unboxing and discovering a new T4. Savor the experience. Enjoy learning the feel and sounds of your Tormek, and do keep us posted.

Ken

ps I have thought of a situation where the dreaded thirty minute supposed limitation would present a problem. As long as the operator is human, you should have no problem. If you plan to use an android operator, the thirty minute nonsense might prove problematic.

[tkmartinc]

Bonjour Sheang,

I am also trying to convince myself to buy a Tormek and one issue that I couldn't see being considered is the wheel size.

The T-7 wheel is 2" wide with a diameter at start of life of 10" and at end if life is 7".
The T-4 wheel is 1 5/8" wide, starting at 8" diameter and ending at 6".

Using the formula for the volume of a cylinder as 'volume = length * pi * dia * dia / 4' we get

    T-7 starts with 157 cu" and ends at 77 cu" - so volume consumed = 157 - 77 = 80 cu".
    T-4 starts with   82 cu" and ends at 46 cu" - so volume consumed =  82 - 46 = 36 cu".

From this I conclude that there is more than twice as much useable life in the T-7 wheel compared to the T-4.

Ken might like to comment on this observation.

Martin

[Jan]

Welcome to the forum, Martin! 

Your calculations concerning the useable volume of the T7 and T4 grindstones are correct. 

From this point of view the T7 grindstone offers a more economical solution.

To be fair with the T4, when truing the T4 stone you remove slightly less material than when truing T7 stone. This is because the volume of the removed material is proportional to current wheel circumference.

The other thing you should consider is the total price of the unit and its weight.
If these two things are not a problem for you, then buy the T7, otherwise buy the T4.

Jan

[tkmartinc]

Jan,

FYI here is the Aussie pricing from Carbatec in A$:

    T-7 (incl TT-50 & SE76) $989.00
    T-4 (plus tt-50 & SE76) $630 + $124 + $91 = $845
    Premium for T-7 = $144 (+17% compared to the T-4) for:
        Increased grinding capacity (+~100%)
        Larger motor (+66%) with 100% duty cycle (+100%)
        Increased weight (+77%)
        All steel construction + other features.

Martin

[Jan]

Reading between the lines of your reply Martin, I think you've already decided. 

TORMEK is an investment for at least 20 years, and from this point of view, the price difference is negligible.

Owning TORMEK is a pleasure and it's an advantage on your side.

Jan

[jeffs55]

Bonjour Sheang,

I am also trying to convince myself to buy a Tormek and one issue that I couldn't see being considered is the wheel size.

The T-7 wheel is 2" wide with a diameter at start of life of 10" and at end if life is 7".
The T-4 wheel is 1 5/8" wide, starting at 8" diameter and ending at 6".

Using the formula for the volume of a cylinder as 'volume = length * pi * dia * dia / 4' we get

    T-7 starts with 157 cu" and ends at 77 cu" - so volume consumed = 157 - 77 = 80 cu".
    T-4 starts with   82 cu" and ends at 46 cu" - so volume consumed =  82 - 46 = 36 cu".

From this I conclude that there is more than twice as much useable life in the T-7 wheel compared to the T-4.

Ken might like to comment on this observation.

Martin

I yelled this from the mountain tops to no avail. Please read all my comments on this from the beginning of the topic. Here is one quote of mine, "You are starting out with a stone that is 8 inches in size. Which do you think will last longer, the 8 inch stone or the 10 inch stone under the same usage? EVEN if you can use the stone when it is smaller than 7 inches, which one will last longer? The decision yours."

[Ken S]

I believe we have not been looking at the whole picture.

I do not dispute the math showing that the larger 250mm grinding wheel has more useful grinding surface before being replaced. However, those who actually wear out a grinding wheel will discover that the replacement SG-250 costs almost twice as much as a replacement SG-200. (presently $105 USD versus $184, higher on eBay) To be accurate, one should factor in the replacement cost per unit of abraded grinding material. Personally, I do not feel this is a meaningful difference.

If one calculates the surface feet per minute, the T$ and T7 are almost identical due to the slightly higher T4 motor RPM.

One measurement remains relatively constant; the T7 weighs almost twice what the T4 weighs and is physically larger. If you subscribe to the notion that bigger is always better, and are still driving an oversized gas hog car, you will prefer the T7. The T7 may also fit your needs more closely if you are in an industrial work environment. If, like Steve and Doug, your "industrial environment" includes moving your Tormek extensively, you should weigh the difference in grinding wheel life with carrying twice the weight every time you set up.

Bigger is not always better. Lie-Nielsen manufactures updated copies of the old Stanley Chisel plane. These planes have nothing in front of the blade, which allows them to reach into tight places. The original Stanley was a large plane, about ten inches long. Lie-Nielsen makes both a same size model and a new smaller model. The smaller model outsells the larger. In my opinion, it is a more useful tool because of its nimbleness. I believe most of us would be well served with the smaller Tormek unit. Be honest, how many of us really sharpen large planer blades regularly?

My forty year old dry grinder, like most of the dry grinders on this forum, started with a six inch diameter grinding wheel, now reduced somewhat. I think calculating wheel wear is overblown. I also think obsessing over the cost difference between the T4 and the T7 is overdone. Both are capable tools. I would concentrate on the work environment. For use in a factory or fixed location busy sharpening business, the T7 makes sense. I also think that for a more mobile or smaller shop environment, the T4 is the logical choice. I used the T7 for several years before using the T4. I like both. There are no bad choices here, just individual preferences.

Ken

[Jan]

Ken, you're right, the price of the grindstone is only a stone in the mosaic of complex decision-making. 
Yet it seems to me good to clarify which of the stones (T7/T4) is more economical.

Based on the verified results of Martin's calculations, T7 grindstone offers 80 in³ of usable material, while T4 offers 36 in³.

Based on prices reported by Ken, 184 USD for T7 grindstone and 105 USD for T4 grinstone, we will get the following unit volume prices:

T7 184/80 = 2.3 USD/in³
T4 105/36 = 2.9 USD/in³.

Conclusion: The usage of T7 grindstone is more economical.

Jan

P.S.: To be fair with the T4, when truing the T4 stone you remove slightly less material than when truing T7 stone. This is because the volume of the removed material is proportional to current wheel circumference. On the other hand, T4 grindstone will produce larger hollow grinding and therefore wear faster than T7 grindstone.  Maybe these two effects compensate each other. So, the conclusion remains valid.

[jeffs55]

I am once again, vindicated. Based on your math,
T7 184/80 = 2.3 USD/in3
T4 105/36 = 2.9 USD/in3.
            2.9 USD = $2.90
          - 2.3 USD = $2.30
difference is               .60 cents x 44 cu inches = $26.40 saved using the T7 assuming each stone is used to uselessness. I do not see the average hobbyist using either stone to destruction but if that time comes, the end is further away with the T7. The only saving grace of the T4 then becomes the weight factor. If that is paramount to you, then you have only one choice and that is the T4. Good luck. 

[Ken S]

Jeff,

I concede. Based on the math you and Jan have provided, it would appear that for each grinding wheel consumed, the T7 has a $26.40 cost advantage. After seven grinding wheels are consumed, the person who paid the $180 more for the T7 would start to be ahead. Add to this, one would have to calculate the difference in electricity cost between 120 and 200 watt motor usage.

I realize there are people who have consumed more than seven wheels. Steve and Doug are certainly in that category. I also concede that anyone who consumed that many wheels might be better served by the heavier duty machine.

In my opinion, the main factor in which Tormek to choose has always been usage, not cost. As a hobbyist retiree, I appreciate the lighter weight and smaller bulk of the T4. I am not anti T7. In fact, I tend to use both models interchangeably. I am presently revisiting the SB-250. I want to resolve the nagging feeling that my disappointment with it may be because of my own lack of knowledge. Based on Steve's comments, I believe the SB may be useful for more than heavy duty work. I want to find out. As Tormek does not yet manufacture the SB in the smaller size, that would be a plus for the T7. I remain open minded on this.

Whenever I have to move the Tormek, I always prefer to use the T4. It has plenty of power for my work, at half the carrying weight.
The choice is yours; as stated, I do not believe there is a bad choice.

[Elden]

Ken,
   You know you might gain that $26.40 several times because of having to make less trips to your Chiropractor due T7 backache syndrome!

[Ken S]

Point well taken, Elden.

Ken

[jeffs55]

In my opinion it is not a matter of weight so much as it is a matter of awkwardness. The T7 is awkward to carry due to design fault. There is just no good hand hold. I know it has a handle, both of mine do. Still, it is not the easiest thing in the world to move. Forget it if you have water in the trough. EMPTY the trough.

[Jan]

Ken, you are correct, we have to calculate the difference in electricity cost between 120 and 200 watt motor. 

It is not easy to estimate the electricity price for completely consumed grindstone. However, let's try.

Assuming that T7 grindstone can sharpen 2000 knifes before it is completely consumed and assuming we need on average 5 minutes per knife, we get:

2000 knifes x  5 minutes = 10 000 minutes = 167 motor hours.

Assuming further, that auxiliary preparatory work will take up the same time as grinding, we get, that the T7 grindstone may be fully consumed after 2 x 167 hours = 334 motor hours.

Now we can calculate the price of electricity for the consumed T7 stone:

334 motor hours x 0.13 USD per kWh x 0.2 kW = 8.7 USD

where
0.13 USD per kWh is current residential price of electricity in the USA,
0.2 kW is motor input power for T7.

What concerns the T4, with 0.12 kW input power motor, I hope we can assume that the price of electricity will be 60% (0.12 kW/0.20 kW = 0.60) of the amount for T7:

0.6 x 8.7 USD = 5.2 USD

It's really just a rough estimate based on numerous assumptions and simplifications. You can easily modify the figures, based on your experience, and get new results.
Nevertheless, the conclusion that T4 is slightly more economical than T7, what concerns electricity consumption, will remain valid.

Jan

[Jan]

I am once again, vindicated. Based on your math,
T7 184/80 = 2.3 USD/in3
T4 105/36 = 2.9 USD/in3.
            2.9 USD = $2.90
          - 2.3 USD = $2.30
difference is               .60 cents x 44 cu inches = $26.40 saved using the T7 assuming each stone is used to uselessness.

Jeff, I rethought your calculation, and I have to tell you, that the savings using the T7 stone may perhaps be even higher.  We may multiply the unit volume price difference 0.60 $/in³ with the whole usable volume of the T7 stone. So we get:

80 in³ x 0.60 $/in³ = $48.

Explanation: Imagine, that the whole usable volume of the T7 stone is composed of the more expensive T4 stone material. Then the cost of the T7 stone would be 80 in³ x 2.9 $/in³ = $232, which is by $48 more than the real price $184 reported by Ken.

Jan