powermatic planer knife grinder spindle bearings

[Bill D]

I bought an older grinder for a powermatic 18" planer older. The motor bearings seem to be okay. Hard to say with a universal motor with brushes. The spindle needs new bearings they feel and sound crunchy. I can not read any bearing numbers due to heavy factory green paint. Since it is belted down from 10,000 rpm at the motor I assume I can use rubber sealed bearings. The rpm is probably under 3000.
The arbor is odd. I see nothing holding in the bearings, no snap rings, probably just a tight push fit. I also see nothing holding the spindle shaft in the bearings. It looks like a straight shaft just pushed in. Nothing obvious stopping it from coming off the end. There is no axial load to speak of so they. may have let it float a bit? The belt will prevent it from moving far. It does have a small cross hole in the shaft for a cotter key perhaps.

I know for a tool post grinder they need high quality bearings with very little runout. How critical should these be. I assume better then the average motor quality stiff sold everywhere?
Bill D

looks to be two 9101 bearings on the spindle. I was able to find two exact replacements on the bay.

 

[stephen thomas]

I have built several grinding spindles for sharpeners and for other apps.
You could, of course, opt for the highest quality bearings and perhaps should if the application is to say, re-grind tapers in machine tools.

For sharpening planer knives the "vagaries" in the system will be much greater than the bearing tolerance assuming a good make (brand)

Levels of Precision - Choose Wisely: How Fast Your Bearings Need To Go

For a long time, only bearings with wide tolerances could be mass produced, nowadays, however, technology has advanced dramatically and parts can be manufactured with very tight tolerances, to produce extremely high performing bearings routinely.

blog.emersonbearing.com

IOW, you can probably use "electric motor grade" as i have, even though they are down in the lower levels, because the tolerances are still in tenths (.0001" 's). For grinding a knife the individual striations from the wheel will be in that range anyway. I hone knives (in place) after sharpening, and use that partially as an opportunity to joint them more closely to parallel with planer bed and with each other.

Price them and if grade 5 is little difference in cost and it makes you feel more comfortable, it's your choice. I think -7 and up would be overkill for the system and for the wallet but you can shop surplus if feeling obsessive.

smt

 

[richard newman]

I agree with Stephen, this is not a very precise operation, doesn't require high grade bearings. I think most people joint the knives first, and then grind to thin or just barely eliminate the land.

My experience is with an older SCM 20" planer, knife grinder isn't very heavy or rigid, rides on a couple of round rails rather than a cast iron dovetail beam. That being said, if I'm patient and careful, I skip jointing, and have the knives concentric within .001". My technique is to take a single pass on each knife in order - 1,2,3,4 then next pass is 4,3,2,1. Lower grinder a tiny bit, repeat til burr just barely appears, then hone. The honing is kinda scary, real easy to slice your finger. But I get a great cut - smooth and minimal knife marks. Then again I've never had to plane a lot of material, more about quality than quantity, and I'm careful to avoid dirty wood which might nick the knives.

I'm thinking that my planer probably doesn't even have high grade bearings on the spindle. The import machines are pretty lightly built compared to old American iron which was meant to run 3 shifts a day. But the price was right, and they took over the market.

 

[michiganbuck]

Scary...I had a knife to sharpen that was 31" long and 5/8 or 3/4" thick that needed a razor edge.
it was difficult to handle that with not having fingers in danger.

 

[michiganbuck]

I don't know if low-grade bearings have eccentric marks, but one could roll them with a tenth indicator to find the eccentric.

 

[stephen thomas]

Slip stone for inside (flat side) finishing/de-burring can be put in wooden holder.
Outside/bevel side i use a regular 2" x 8" honing stone (or sometimes 2 grits - a rougher and a finisher) Keep my fingers out of the way, and wear a thick leather glove on that hand.

 

[john.k]

We used to sharpen paper guillo blades,some over 6' long and 1" thick ,and very heavy........the guys refitting the blades always had a cooler packed with ice and some sterile plastic bags .

 

[jmhoying]

I just brought a few blades to be sharpened at a local sharpening shop here in Western Ohio. His main line of work is sharpening large knives. He had many knives there that were either sharp or going to be sharpened. (mostly 1/2" and 3/4" thick x 4" x 2 to 4') His equipment has the ability to sharpen up to a 10' knife with a tolerance of less than 1/2 thousands. My 13" planer blades will look pretty tiny on his machine!
Jack