OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Backlapping

Hi Folks,
I just thought I would provide some feedback about backlapping.
I have a Kermit green Rover - Scotty that hit a screw in the grass and threw the alignment of the reel out. It looked as though it bent the bed knife so I sent it off for spinning grinding and a new bed knife.

On receiving the mower back I noticed a different noise to my last grind and it sounded like the reel was making hard contact with the knife. On inspection it turned out the bed knife hadn't been ground true and to make contact across the full length of the bed knife he had tightened it up significantly. It was so tight that in one spot the reel was so tight that it flexed the bed knife slightly as it passed over. .

In fear of doing damage to the sole plate I quickly lifted the reel up to minimize contact but found then the reel only contacted the bed knife on the outer edges. I was pretty disappointed that I had spent $140 on this considering that he had advised that they grind both together to ensure a true fit.

I found a post regarding having to bed a newly ground reel in via backlapping. I then contacted Cameron in Sydney and he confirmed that if the bed knife is not ground whilst assembled in the frame, it can also lead to it not being perfectly aligned to the reel. So I got myself some backlapping paste and set to work.

The whole procedure didn't take long at all.
Take off the chain cover.
Undo the cutter chain.
Get a 3/4" socket and adapter for my drill.
Adjust the reel just off the bed knife so it flaps a piece of paper when it spins.
Rotate the reel in reverse and apply the paste.

You can hear it grinding away and after a couple of minutes the sound changes and the reel and knife become aligned. Depending on the degree of bow in the bed knife you may need to lower the reel slightly and apply more paste.

When I was happy I then hosed off the compound and re adjusted the reel to make ever so slight contact to the knife. When you rotate the reel there is no resistance and it's now cutting paper. The quality of cut is now great and the reel is no longer making a grinding noise when in use.

I suggest backlapping is a technique that all Scott Bonnar owners should become familiar with to improve the quality of cut.

Cheers

Hello boeing and ODK Members and Guests

Thank you for your observations on backlapping. I could not agree more with your statement:-

"I suggest backlapping is a technique that all Scott Bonnar owners should become familiar with to improve
the quality of cut."

What I really liked was your correct statement on the procedure. The hosing down
of the compound is clearly critical, and there is a lot of nonsense on YouTube.
The SB45 is very much adapted to backlapping because of the RH thread on the reel sprocket.
The drill speed must be slow, though, so lapping paste is not wasted in the process.
I guess you worked that out

I have done a lot of work in unravelling the story of backlapping and reel mower setting;
the so-called 'contact' and 'non-contact' methods. So much damage has been done by the promulgated
existence of a 'controversy' surrounding them. Informed writers seem to spend so much time in
dispelling that 'controversy'; in favour of a commonsense approach based on science.

My reality is that backlapping is a primary method of maintaining blade edge sharpness, and the
method has been with us, used on lawnmowers, for almost two centuries. It should be part of
primary reel mower care for those who choose to use this grass cutting method, and want to do
so with economy.

All very interesting.
-----------------------------------
JACK.

I don't have a reel mower but I found that very interesting, a good thing to be aware of if you have one.

Paul, I suggest you refrain from getting too enthusiastic about back-lapping as a substitute for reel-grinding. It can have its place as a minor supplement to grinding, so the reel can be kept really sharp between grinds, but if carried out too many times it reduces the life of the reel and bedknife. Lapping allows the formation of matching waves in the reel and bedknife, so that grinding has to remove more material from both, to get the cutting surfaces truly straight again.

To clarify for all

Back-lapping is not a substitute for reel grinding and, of course, no one here has said that.
It should, also, not be considered a minor supplement to grinding either.

It is a industry-recognised surface maintenance procedure that should be considered a key part
of keeping reels in top condition, thus increasing the life span of a reel before sharpening
is required; and increasing the life span of the reel in total. Back-lapping will not 'bring back' a
reel that is too far gone, nor will it restore a bottom blade that has been too severely damaged.

This whole farrago - controversy - began because of what I call a Euro-American divide on
these subjects in the 1970s; brought about and driven by the professional sector and professional
turf-care manufacturers in the USA. That, of course, is another story.

In the meantime, boeing's enthusiasm is well grounded.

All very interesting.
-----------------------------------
JACK.

I think we are in agreement on the specifics, Cyberjack. A sharpened reel and bedknife can perform longer before it becomes blunt enough to need regrinding, if mildly backlapped either regularly or when an expert operator believes there is a small diminution of performance. With considerable expertise, an expert can then assess when the time has come for a regrind of the reel and bedknife.

Yes Grumpy,

We are in agreement on the specifics

My interest is in the history - of how ideas originally applied to the professional
turfcare sector could, down the track, undermine long established views about the
correct setting of a reel mower, and the maintenance of machines by back-lapping.

It's a story of engineering marinated in snake oil marketing.
Yet to be told, and shortly showing at a cinema near you.

The rest is history.
-------------------------------
JACK.