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Tru Cut Back Lapping

Posted by tomg NC (My Page) on
Mon, May 14, 07 at 16:09

I have a 20" Tru Cut reel mower - about 1 year old. I just bought the back lapping kit - I opened it up and thought to myself "you gotta be kidding". This thing looks really complicated - and I'm all thumbs when it comes to household projects. Has anybody used this kit? Is it possible for an average Joe to sharpen ths blades on a Tru Cut using this kit? Any suggestions?


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RE: Tru Cut Back Lapping

First of all, you are not sharpening the blades and bar, you are lapping them for closer fit. To sharpen, both the reel and cutting bar are ground on a Foley or Neary sharpening machine.

As for backlapping. It's not so much complicated as it is time consuming. The actual lapping is the easiest part. The investment in time comes from adjusting the reel to the bed knife, and doing it several times if necessary to get it right.

I'm not sure which Tru-Cut you have, Home, Pro, or Commercial. The Pro and Commercial adjust the blade by rotating the bearing housings in eccentrics after you loosen three nuts and bolts. You need to scribe match marks, and then very gently tap at the bearing housings with a punch and hammer to rotate them and cause them to cam in or out against the bedknife.

Also, I'm not sure what your lapping kit consists of. At a minimum you need a bell crank for rotating the reel plus medium and coarse grit grinding compound. The compound is also sold at auto supply stores as Clover Compound and was used for years for lapping valves. After rasing the mower on a platform you remove the reel chain and attach the crank. I happen to have a crank for a Trimmer brand, but have been known to use Vise-Grips on the sprocket of other machines.

I recommend against a power back lapping arrangement. These work by re-threading the chain to cause the motor to turn the reel backwards. They spin way too fast, and you can easily gall your reel and bedknife surfaces. At that point you are looking at $150 or more for the reel and $80 or more for the knife, plus you still need a pro to set it up for you. If you haven't ruined both, but only damaged them, a full sharpening grind should be about $100 to $125. The power attachment saves some muscle and maybe reduces the actual lapping time from 15 minutes to 5 minutes. You are still faced with 30 to 60 minutes of clean up (to get rid of all abrasive) and setting of the reel.

I'd suggest you practice setting your reel to the knife, using strips of paper to check the cut every 4 to 6 inches for each of the blades. Once you have mastered that, then you can look at actually lapping the reel.


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RE: Tru Cut Back Lapping

Good stuff there from "ggoyeneche" but i will go against on one matter. Adjusting the eccentrics. Hammer and punch, oh boy. There is a proper tool for this and if you booger up the holes with your hammer and punch and then bring it to me and my tool will no longer work, grrrr. Gotta "Death Wish".

Hopefully that kit you bought has the tool. If not and you use the hammer and punch method, go EZ guy or you may have a very irate shop guy cursing you at some later date.


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RE: Tru Cut Back Lapping

  • Posted by okcdan 7 Oklahoma City (My Page) on
    Tue, May 15, 07 at 11:24

This "tool" being referred to is a spanner wrench. It's what I use to adjust the reel on my Tru-Cut.

You can purchase them online from McMaster-Carr or other industrial supply warehouses.

Good day, Dan


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RE: Tru Cut Back Lapping

  • Posted by tomg NC (My Page) on
    Tue, May 15, 07 at 11:53

Thanks. The kit came with the compound, a spanner wrench (like the one Dan posted), a chain and a gear wheel that you bolt on to the machine. I believe when set up properly it will reverse the direction that the reel turns, and when you adjust the knife you sharpen the baldes and the knife. Sounds like a job for a prefessional - now I just need to find one in the Charlotte, NC area.


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RE: Tru Cut Back Lapping

Mea Culpa. Walrus and OKCDAN are right about the spanner. In my defense I did say gently. I find that with a spanner you tend to move too far one way and then too far the other. By GENTLY tapping with a small drift punch, you can inch the bearing housing to where you need, and shouldn't damage the housing either. The housing are die-cast aluminum and are soft and therefore easily damaged. Trimmer uses the same arrangement on the 20" and 25" Original models.


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RE: Tru Cut Back Lapping

  • Posted by okcdan 7 Oklahoma City (My Page) on
    Wed, May 16, 07 at 13:40

ggoyeneche is right, with the spanner wrench it's really easy to move the reel way too much. But I also agree with nevada_walrus that using a hammer & punch could damage the holes in the eccentric housings. I leave the nuts/bolts kinda snug & tap the spanner wrench with a small ball peen hammer to make adjustments...moving the reel in very small increments while still protecting the aluminum housings.

Good day, Dan


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RE: Tru Cut Back Lapping

Will the spanner wrench work on the chain side??

I amy just learning how to adjust the bedknife (frustrating), and used the punch / tap method. I was thinking about the spanner wrench (which I do not have), but not sure how it fits on the chain side?


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