Planer knife grinding angles

[Steve Maskery]

I've discovered something I didn't know.

I've changed my planer knives recently and the ones I took out have not been in very long, so they are not so dull that they are unusable. They do, however have a little nick in them from hitting something hard. It's not bad and I'm fairly certain that if I put the knives back the nicks would be offset, so they would cover for each other.

I decided I'd make a little jig to hold them for honing. I thought that they were ground to 30 degrees. Having tried to fit them, I discover that these have been ground at 40 degrees - (fortunately the jig takes all of 15 minutes to make, so I've not lost much).

So a bit of Googling as to what they should be and I discover that planer knives can vary from 30 to 45 degrees. I should not be surprised, given that we use higher angles for hardwoods with hand planes, but I'd never given it any thought for planer knives.

So my q is this - To what angle do you have your planer blades ground and why?

Cheers
Steve

 

[ivan]

Mine are supplied at 40 deg, so I have been repeating that. However lately I have been honing a tiny backbevel of about 15 deg which does reduce tearout in hardwoods. I have been thinking maybe I should have knives ground at a lower angle, and maybe get them to add the backbevel too (if they can do it), but I haven't asked yet!

 

[RobertMP]

Strolling slightly off topic......

My Metabo HC260C came with disposable blades fitted. I turned them round to use the second edge recently so I will need to get some replacements sometime. The disposables are £20+. HSS resharpen-able ones are nearer £40 and need a new holder that could be another £40.

Given that the original first edge lasted ages is there any point to getting HSS ones? Only thing making me wonder is some sellers list the disposables as suitable for softwood not hard... but I've put lots of Oak and Beech through mine with no problem.

 

[OPJ]

I've always had my knives reground to the same 'standard' angle, whatever that is...

Ian John once suggested regrinding them to 60° for use with hardwoods. Sadly, I never got to take advantage of his offer before Dragon's demise.

Steve, what are you honing them on? Wet and dry paper? The only thing that puts me off attempting to sharpen them myself is the fear of unbalancing the cutter block by removing more/less material from one knife against the others.

 

[Steve Maskery]

I've mounted them side-by-side at 42 degrees and then used a bench oil stone on them. Just for the edge, I'm not trying to polish the entire surface. I've not tried them in the machine yet, but visually they look great.
S

 

[xy mosian]

Doesn't the ground angle of the planar knife need to be slightly less than the angle betwen the bed of the knife and the tangent to the circumference of the cutter block at the knife edge?

Is there any point reducing the ground angle if the blade is used bevel down, i.e. to bed?

I have a hand held electric planar with HS blades and also the possibility of using replacement blades. I feel, with pad of thumb, that I can get a finer edge on the HS blades than is standard on replacement blades. Perhaps I should look for better replacement blades.

xy

 

[Racers]

Hi,

Isn't the angle set by the cutterhead? the grinding angle is just for clearance, thats why a back bevel works to reduce tearout be altering the effective pitch.

Pete

 

[Steve Maskery]

Doesn't the ground angle of the planer knife need to be slightly less than the angle between the bed of the knife and the tangent to the circumference of the cutter block at the knife edge?

Well it certainly needs to be less, otherwise it wouldn't cut at all. The q is how much less? And that's what I'm asking. The more acute the angle, the "sharper" it is, but less robust. This is a clearance angle.

One possibility is to change the cutting angle by honing a back bevel. But here be dragons.

Is there any point reducing the ground angle if the blade is used bevel down, i.e. to bed?

I've never seen a knife mounted bevel down, and I can't imagine how it would work.

Cheers
Steve

Edit - Pete beat me to it and put it more succinctly than I did.

 

[xy mosian]

Sorry about the 'bevel down'. I was equating, roughly, the outer surface of the block to the sole of a hand plane. In my mind that would mean the bevel is 'down' when the knife is mounted such that the bevel is nearer a tangent to the block than the larger face of the blade.

xy

 

[Steve Maskery]

Ah, I see, yes, OK.

Having thought about this for most fo the afternoon, I've come to the conclusion that it probably doesn't make much difference, unless you are into back bevels.

Anyway. I'm pleased with the result, I'll just have to wait to see how it performs in practice.

Cheers
Steve

 

[ivan]

Dragon usually knew what they were talking about, but I would have guessd that 60 deg would put the heel of the bevel pretty close to the work. You'd have to measure the bedding angle of your block and do a quick drawing to be sure you weren't polishing with the heel instead of cutting.

Nigel Voysey (Machining Wood) talks about 35 deg for softwood, and a bit more or a backbevel, for hardwood. This looks like bookspeak for suck it and see!

One old book from square block days said to lower the outfeed table a bit and pass a stone over the rotating knife, which sounds a sure way to stop the machine cutting at all: it would ensure all knives at the same height though. I can't quite bring myself to try it

 

[Steve Maskery]

Ouch! Noooooo, I'm not doing that. Actually I've honed them to 42 degrees. They look smashing, but I'm not going to swap them in again until I need to. It will be a pleasant surprise when I need them.

Cheers
Steve

 

[David C]

Steve,

If you have my first book pages 40-41 show an old method of resharpening (honing) planer blades in situ, (if you have an adjustable outfeed table).

I have mine ground at 35 degrees and the honing was close to 40 degrees.

A small 15 degree backbevel is very good for interlocked hardwoods, as Ivan says. Another old machinists trick. Honing is quite sufficient grinding not needed here.

best wishes,
David Charlesworth

 

[Steve Maskery]

Thank you. I do have a blade I keep for my No5 which has a back bevel, but I've never tried it on my P/T. My dad taught me that back bevels were the work of the devil, so it's taken me some time to appreciate their good points.

Thank youall.
Steve

 

[ivan]

Steve, if you spot this coming round again, here's a few more "facts", I came across yesterday.

According to Eric Stephenson, who seems to be a bit of a sawdoctor guru, and author of the HSE reccomended Spindle Moulder Handbook, HSS spindle cutterhead knives can be ground between 35 and 60 deg, (carbide 45 to 60 deg) with the comment "if the angle is too small the edge is prone to damage.....to big and the cutter dulls prematurely".

Looks rather like "as low as you can get away with"; still horses for courses, though! Some spindle blocks appear to have a lower cutting angle than that standard on planers, so 60 deg still gives at least 10 deg clearance. The peripheral cutter speed is higher too, but I guess that the choice of sharpening, or bevel, angle for your p/t is based on similar logic.

* Source of info above. Lots of useful stuff on fences, that apply directly to router tables too

 

[Steve Maskery]

Hi Ivan
Thank you for that.

I have to say I hope that the Stephenson book has been updated. The copy I once had contained working practices that one might consider, how can i put this politely, quaint. State of the art 1930s style. Perhaps I'm doing the gentleman an injustice and perhaps it's a much better volume these days, but I didn't rate it at all.

Thanks for the info, though, very helpful.

Cheers
Steve

 

[MikeJhn]

I'm just thinking of getting a planer blade sharpening system set up and did a search on here for planer angles, rather than starting a new thread, I thought I wold just bump this one.

Interesting thoughts by all on here, my thoughts are the diameter of the block and the angle the blades are set in the block, would alter the angle the blade is presented to the workpiece, so different machines would use different angles, or are all machines set up the same? I would not have thought so.

Mike

 

[Steve Maskery]

The block sets the cutting angle, the grinding only alters the clearance angle, which, given the microscopic amount of work each pass of the blade does, is going to be affected negligibly by the diameter of the cutter block.

 

[MikeJhn]

Agreed, but earlier in the thread the attributes of the difference between a one degree angle was mooted, I just brought up the block angle and blade setting angle to highlight that a difference in honing angle would not make much of a difference.

Mike

 

[RogerS]

Steve, if you spot this coming round again, here's a few more "facts", I came across yesterday.

According to Eric Stephenson, who seems to be a bit of a sawdoctor guru, and author of the HSE reccomended Spindle Moulder Handbook, HSS spindle cutterhead knives can be ground between 35 and 60 deg, (carbide 45 to 60 deg) with the comment "if the angle is too small the edge is prone to damage.....to big and the cutter dulls prematurely".

Looks rather like "as low as you can get away with"; still horses for courses, though! Some spindle blocks appear to have a lower cutting angle than that standard on planers, so 60 deg still gives at least 10 deg clearance. The peripheral cutter speed is higher too, but I guess that the choice of sharpening, or bevel, angle for your p/t is based on similar logic.

* Source of info above. Lots of useful stuff on fences, that apply directly to router tables too

I seem to recall looking into this when I was moulding some tricky hardwood and IIRC went for a smaller diameter block which effectively altered the angle. But then I could be talking twaddle as it's well over a year since I did any woodworking of any merit.