Let's talk cutter blocks

[Deadeye]

I'm toying with buying a replacement planer (wider).
It's an old lump of iron, which always seems a good thing. However the cutter block is 2 knives only (my current is three). Does it matter?
Also anyone have a manual for a multico sl9?

 

[Mike Jordan]

I don't think it matters at all, the finish from my Sedgewick MB has always been good from two knives. It's normal for all faces of the finished work to be hand planed and/or sanded to remove the slight wave effect left by the machine.

 

[MikeJhn]

My Kitty must be a wonderful machine, anything out of it does not need any additional finishing except a light sand, I don't get any wave effect from it when planning or thicknessing, maybe I'm just lucky. (hammer)

 

[Trevanion]

Two knives leave a perfectly good finish, more knives are really only better if you're feeding quicker.

You're in luck! I've got this on my bookshelf

I should really scan it one day and somehow turn it into a PDF.

 

[MikeG.]

Mine has two blades, and with sharp knives it is hard to see how an extra one could improve matters. Three blades means 50% longer to set them up after changing or sharpening.

 

[Trevanion]

I've got an old machining book somewhere that goes over what's the perfect RPM/Amount of Knives/feed speed ratio, which if I recall was something like 60 or so cutter marks per inch for optimum finish and efficiency. If I remember which book it is I'll dig it out and quote it more directly.

Three blades means 50% longer to set them up after changing or sharpening.

I wonder if you get 50% extra life out of a set before needing to resharpen with an extra blade compared with just two blades? I'd never thought of that until now.

 

[memzey]

An interesting topic this. My machine has a four knife Tersa block and leaves a superb finish on almost anything I put through it. I’ve often wondered whether that was due to the Tersa system making it so much easier to swap out dull knives (and therefore always having a sharp set in when needed) or the extra knife giving the benefit of slightly lighter cuts per pass?

 

[Deadeye]

I had thought that a two-cutter block might give a less smooth finish for the same diameter block and same rpm. Indeed, perhaps that is so, but the rpm and block diameter are also varying between machines so maybe the knife speed at contact is similar.
If the latter is true (similar knife speed), presumably a bigger diameter cutter is better as a) it has more greater angular momentum and b) the path of cut is a flatter shave?
Thanks Trevanion for the pictures. Do you also have the exploded parts diagram? I see you did a restoration of a similar machine recently.

 

[Trevanion]

I had thought that a two-cutter block might give a less smooth finish for the same diameter block and same rpm. Indeed, perhaps that is so, but the rpm and block diameter are also varying between machines so maybe the knife speed at contact is similar.
If the latter is true (similar knife speed), presumably a bigger diameter cutter is better as a) it has more greater angular momentum and b) the path of cut is a flatter shave?

Yes, a greater finish is achieved with more cutters in a same-sized block at the same RPM as a two-cutter block. Also as you say, the bigger the block the smoother and flatter the cut too. I had a flick through some of my old machinery books and stumbled upon some information on cutter marks per inch in J.Raymond Foyster's "Modern Woodworking Machine Practice. It seems the golden number is somewhere between 16 and 20 cuts per inch, any less and the finish is quite coarse and will require much more sanding, any more and there will be a tendency for the cutters to scrape rather than cut and the knives will dull far quicker because of more contact with the wood than is necessary. There is quite a bit more technical information and equations on the subject in the book but I doubt anyone wants to hear any of that :lol:

I was way off with my number of 60 cutter marks per inch off the top of my head! :roll:

Thanks Trevanion for the pictures. Do you also have the exploded parts diagram? I see you did a restoration of a similar machine recently.

Sorry, the manual didn't have an exploded diagram of parts, I'm not sure if Multico really did exploded diagrams like that because I've never seen any, all the part numbers were cast or stamped into the machine parts from what I could see. Is there something in particular you're after or are you just wanting it for future reference? I've got a Multico NS which I took to bits and put back together to replace the bearings (As well as the TH restoration I posted on the forum ) so if I can be of any help just say.