The importance of bed-knives and how much relief do you really need?
A personal view on the debate by Peter Hampton, professional grass machinery supplier and technician since 1963, and Patrick ‘Cal’ Callaby, Workshop Manager at The Celtic Manor Resort
W
e have read several articles about sharpening reel mowers recently, and very few discuss the bed-knife. In fact, some writers thought that the bed-knife was not considered important enough to mention in its relationship to the quality of cut. In our experience, the “bed-knife” is the most important part of the cutting process and needs careful attention when sharpening and setting any reel type cutting unit for a fine finish. While we are on the subject, please allow us to set the story straight on the “relief issue” in relation to the sharpening of reel type grass cutting units, drawing from years of practical experience.
Let’s first clarify a few fundamentals
regarding relief: • Most importantly, what is relief?
• What do we mean when referring to a ‘ground in relief ’?
• How do we actually achieve relief? • What does it do for the mower? Relief is the reduction of contact 110
What a relief...
pressure between reel and bed-knife surfaces, offering a “relief ” to the engine or mechanism that is powering the reel. In the turf machinery industry, making
the profile of each reel blade thinner, so that there is less metal to metal contact between reel and bed-knife, is usually called relief grinding. This relief reduces the mechanical effort required to drive the reel when reel and bed-knife are in contact. It is better to refer to this relief grinding as “blade thinning”, it is a much more appropriate description. The required thickness and angle of blade thinning (or none) varies from one mower manufacturer to another, and you must look at your own supplier’s recommended requirements. It’s worth noting, however, that
excessive relief angles and relieving to a thin edge will weaken the reel blades, and render them more vulnerable to wear and damage from stones or metal objects such as golf shoe studs or coins. Here is a short summary of how blade thinning or relief grinding tends to
operate in practice. When you reduce the profile of each
reel blade to 25% of its original thickness, by blade thinning from the back edge, we could call this a 75% relief, when there is light contact between the reel and bed-knife blades. Most golf course equipment technicians will agree that, after running through sand or back-lapping the cutting units a few times, this area (25% in contact) starts to grow, and it’s not long before over 50% of the reel blade is in contact across the partially worn bed- knife. So now we have only 50% relief remaining. If, however, the same reel blade were to operate without any contact with the bed-knife, then you could say that this is ‘zero contact’ and is, therefore, 100% relief, because the reel is no longer rubbing on the bed-knife. So, surely, it makes more sense to set all of your cutting units with “zero contact” and have 100% relief all of the time? We do at Celtic Manor and it works well for us, however, the real trick
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