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

Hi guys,
I just recently joined this forum, also just recently bought an old second hand Scott Bonnar model 33 cylinder mower, 12 blade cylinder. The blades aren't terribly sharp although they don't cut too badly, but I was just wondering about backlapping it. I used to work on a golf course and we did it to the greens mowers a bit, and they always cut well, but I read somewhere that it's not the greatest idea?? Any thoughts? I'll try and upload a photo of it, it's the only pic I have so far.
Cheers, Kelsey.

I'd be interested to hear more on the back lapping topic as well.

My father-in-law has been using cylinder mowers personally and professionally for over 30yrs and uses the back lapping metthod for maintaining the blades. We just did mine and the neighbours one a couple of weeks ago and the difference in the cut and ease of cut is great.

I'll have to ask him if he ever sends off the bottom blade and cylinder for spin grinding.

Ben.

Spoke to my father-in-law last night. He said they do send of the blades and cylinders for spin grinding once a year and use back lapping for maintaining the edge longer - they do it every couple of weeks given they use them in a commercial application. On his home cylinder, he only back laps 2-3 times a year and hasn't done a spin grind for some years.

I have a significantly bigger yard than him (~700m2 of grass), so I imagine I will probably need to do it yearly or every second year.

Ben.

guys excuse my ignorance but what is backlapping? Have heard the term many times but I dont know what is or how it is done. Assume its some sort of reel sharpening technique..

Ross

It is a two man job for the way I do it. I do it by removing the cog from the drive shaft for the cylinder cutter and use a nut that is welded to a piece of steel rod. The rod goes into the chuck of a powerful drill - a cheapie will not do...need something with some real balls...and the nut goes on the the thead of the shaft.

The drill is then run which spins the cylinder in the reverse direction. While it is spinning, lapping compund it applied along the length of the cylinder with an old paint brush. Because the grit closes the gap between the cylinder and bed knife, this loads up the drill and is why you need a decent drill, otherwise it won't spin and/or will burn out.

It only takes a couple of minutes to do - and more and you are just wasting metal. You should end up with a decent clean endge and face on the cylinder's blades. It is then just rinsed of with water to remove the remaining lapping compound.

The whole process from removing the drive cover to putting it back on takes ~10-15mins.

I am probably going to make up a new bit that uses a socket that just fits over the drive cog nut...saves stuffing around pulling it off.

Ben.

I have a heavy duty Dewalt hammer drill used for concrete drilling. I cut a 16mm SDS bit (which suits the drill) in half and welded a 1/2inch socket extension on.

Put the SDS bit in the drill, fit correct size socket onto the extension to suit the nut on the cutter sprocket, drop the main drive chain to the cutter and I'm backlapping. As Morphias said turning the cutting reel backwards while applying a cutting paste. I use 180 grit paste.

I back lap when the cut is cut is dull. I try to keep my blades cutting paper all year round.

The process is quick if you keep on top of your sharp blades.

My backlapping tool (drill / extension with socket) suits me though if you were doing it frequently on many machines you can buy a backlapping machine. They are simply an electric motor with a couple of pulleys, a shaft where a socket fits to with an adjustable height.

I took my SB65 to a local bowls club with a new bottom blade fitted and they bedded it in for me by backlapping it. I watched the process and am now doing it myself.

A couple of simple points Aussie Blue. First, I take it you run your DeWalt with the impact feature switched off. When I read morphias' post I momentarily visualised following this procedure with my rotary hammer drill, pushing it forward to deliver the standard 3,000 hammer blows of 3.8 Joules each, per minute, and figured the ballraces would last less than a second and the mower frame probably less than a minute. That would be a great way to get plenty of drill-power onto the job, but not a great outcome. Second, the general concern I have with back-lapping is that over time, the bedknife and the reel will both become wavy (the waves in the two parts will match each other, of course), so when you eventually get them ground, you will probably have to remove considerably more metal. So, the secret seems to be to keep a close watch on the flatness of the bedknife (since it will become exactly as wavy as the reel). When the first trace of waviness appears, it is time for a spin-grind and bedknife grind.

Note: 3.8 Joules impact energy is equivalent to dropping a two pound hammer on it from a height of seventeen inches.

Hammer mode is definitely off!

HAHAHA, Def hammer mode off grumpy...

I've never experienced or familiar with any "wavyness" caused by backlapping. If the reel is cylindrical this should not occur. When turning the reel backwards and applying cutting paste to the reel, the grit is worked between the two metal surfaces - one being perfectly cylindrical (the reel) and the other flat (the bedknife). This results in the removal of the dull metal cutting edges (reel and bedknife) leaving the entire top front edge of the bedknife perfectly flat and square, a clean cutting edge on the reel and perfect clearance between the two for the entire width of cut.

If there is any sign of a "wavy" bedknife as a result of backlapping I'd say there would be a serious problem with the cutting reel not being cylindrical. I wouldn't have thought that backlapping could cause a reel to become out of shape. If the reel is out of shape i'd say it would have been caused by a bad spin grind. In this case I would take it to be reground by someone who knows what they are doing.

Backlapping when performed properly should not cause any issues with the reel. The only negative of backlapping is that it removes metal. But if well maintained, only minimal metal is removed with majority of metal wearing from the bedknife. Bedknives are cheap ($40 - $60) compared to cutting reels (upwards of $300). Spin grinds on badly worn reels with rounded or dinged edges remove much more metal than a backlap ever would. If a sharp edge is maintained via backlapping the time between spin grinds should be greater. Backlapping may wear bedknives out quicker though this would be more ecomonical than frequent spin grinds. Removing large amounts of metal from reels with frequent spin grinding results in costly replacement of a reel.

Hi Guys, I have had no experience of backlapping with my Scotty 45, though I have heard about the procedure being carried out by greens keepers with a specialized backlapping machine. I don't possess a heavy duty drill that would be suitable anyway....
I prefer to dress the front edge of the bedknife blade with a gadget that a lot of greens keepers are now using, called the "BMS Bedknife Buddy" shown here;

This dresses the edge and keeps it square and sharp, so as the front edge of the reel can do its job, even if a little dull in itself.
It is quick to do, and if used correctly, removes very little metal.

A better solution is one employed by the professional greens mowers; Scott Bonnar Queen, Winsuttle and others, in that the cylinder reel can be removed, turned end for end, and replaced, the sharp edge on the back of the blade becoming the new sharp front edge....Pity we can't do that with a Scotty 45.

Deejay, I like that gadget because it helps to keep the edge square to the top surface of the bedknife, so less skill is required than with a hand-held oil stone. Don't forget to keep the oil stone in the gadget oiled when you use it, of course, and be careful to remove all of the abrasive and steel grindings from the bedknife when you finish.

It is a bad idea to backlap a reel if you are going to reverse it later, because it has then effectively been forward-lapped, and will have a rounded leading edge on the reel blades. Also, backlapping tends to leave a slight burr on the leading edges of both reel blades and bedknife. This should be removed with a stone or your cute little gadget. Removing all of the grinding compound after backlapping is essential, and note that valve grinding paste is normally emulsified in oil, not water. It will not wash off with water.

Hi grumpy, the bedknife buddy contains a fairly fine file blade that is reversible. Replacements can be bought from any good tool shop. The instructions for use do not mention putting oil on it during use. Naturally, one should clean all filings away after sharpening.

Thanks Deejay, I thought it was a stone, not a file. It isn't usual to oil files because it makes it more difficult to clean the swarf out from between the file teeth, but it does contribute to a better finish if you do it. Whether you oil it or not, getting a good finish requires "carding" the file every few strokes. Unfortunately it is more or less impractical to use a standard file card on the Buddy, but there are substitute techniques for keeping the teeth clear.

Yes grumpy, I agree with you there, and I believe that chalk can help with pinning.
Just for reference, pinning refers to the clogging of the file teeth with pins, which are material shavings. These pins cause the file to lose its cutting ability and can scratch the workpiece. A file card, which is a brush with metal bristles, is used to clean the file.

I'm curious on the Bed Knife Buddy.
It looks like a thick kitchen knife sharpener, just a single balde out of those stay sharp knives, that scrap the edge in a V fashion.
I'd assume that this is done by hand only!!!
Might look for one in AU, or attempt to make one.


My neighbour brought his reel down to me and asked me to 'just throw it on your lathe and skim it'.
My Dad being a machinist - I knew better and tried to explain the results if it were to ever be attempted.
I put it back in his 45 and mildly dressed it with a Oiled Oil Stone and told him to buy a new Bed Knife.
Poor fella doesn't like to maintain things, but that's the thing with Reel Mowers. there's always something that need checking and tending to.

Hi Mersenne, No mate, this is not a V shape kitchen sort of sharpener, it is a flat file set at 90 degrees to the very bottom of the bed knife blade, so that the very edge of the blade is dressed as the "buddy" is pushed and pulled lightly by hand, across the edge surface. It also is designed to push the reel blade out of the way as it goes along.

Thanks Deejay, I see
I really didn't think a file would be that useful of a tempered piece of steel, but obviously it works
I've just been on Youtube the last night watching sharpening of bedknives and the importance of it (duh at myself)
I guess now the Buddy tool is really just for in-between Machine sharpens?
I read in another thread that $200 for a regrind was dear. I went to a local Engineering machine centre and they refused to quote, but told me that it'd have to be in the vacinity of $300
I politely said - Oh really, thanks anyway and bought a brand new 10 blade.
with a bit more experience and an old 6 blade to play with, I now think instead of trying a on machine grind, I'll jig something up with my lathe and a flat pad grinder/linisher

Hi Mersenne, $300.00 is a rip-off for a spin-grind mate. I am charged $70.00 from our engineer here in Wagga. The only thing is I have to wait till he completes the 36" reels for the bowling clubs before he will adjust the lathe for the 17" reels, which is no prob...he usually gives me a call a couple of days before to allow me time to remove the reel and soleplate.

My suggestion, contact your local Bowling Club, ask to speak to the greens-keeper, and find out who spin-grinds their reels. They are usually a friendly bunch of guys who will help you.

Thanks Deejay, I will do
Last when I went to get some parts at the local mower shop, they told me most everyone sends their reels away to Bundy (done by a retired fella) as no one wants to setup their machine for a reel every once and a while. I think I'll go knock on a few greenskeepers doors.