Cutterhead comparison between 12" SCM and 8" PM

[Joe Jensen]

I am in the process of renovating a 12" wide SCM/Rockwell F3a jointer. Here is the original cutterhead from my PM60 8" jointer side by side with the original cutterhead from the F3a SCM. The SCM cutterhead weights 42lbs. Note how large the bearing for the SCM is compared to the PM.
http://i174.photobucket.com/albums/w99/AZEngineer/_NIK3321.jpg
http://i174.photobucket.com/albums/w99/AZEngineer/_NIK3322.jpg

[Cliff Rohrabacher]

that is small.

[Peter Quinn]

Please educate me. I have seen several discussions here and in archives at another site discussing the 'size' of bearings in a comparison of two machines. One conversation was relative to shaper spindle cartridge bearings. Why would a larger bearing be assumed to be of greater quality or significance than a smaller bearing? Is there some appreciable asset to having very large bearings? I know that there is a wide range of quality difference for any given size of bearing relative to tolerances, rpm capability and life span, but assuming both bearings are of equal quality, what would be the advantage of a bearing being larger?

For instance the largest cars I have ever driven have not been the greatest handling or most precise machines by any means.

[Scott Myers]

Certainly, you have a great point Peter. It comes down to what is the load on the bearing and how much "fat" is left as far as rotational speed and maximum loading. Size is not everything to be sure in a bearing. But larger generally means it will last longer due to spreading the load over a wider surface area, assuming it has the right speed rating.

I did note that the large bearing says "USA" on it and is a SKF. This is second in quality only to a Timken, which is the best in the world as far as tolerances and build quality. Of course, a Timken would have cost A LOT more and probably be overkill in such a lightly loaded application. Obviously, that is a replacement bearing and not the original, as it is brand new. What's the make on the PM bearing?

[William OConnell]

Just to put it out there. I ought a 1943 Powermatic Planer off craigs list. The thing is an unbelievably heavy maneating beast machine. The one thing that really stuck out was the cutterhead. It is massive. There was a grizzly 15" planer in the shop at the time and the diffence between the cutterheads was unbekievable. I was so impressed with it that I too took a picture.
I have this machine tuned nicely now and its a pleasure to use. But not move at 1260 lbs. Even the chip breaker is 1/2" steel plate
I think anything newer is trimmed down to minimum it seems, to bad really
http://woodworkers.us/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=613&g2_serialNumber=1&g2_GALLERYSID=78244b81d7e887af21a01afc8f5c094b

[Joe Jensen]

Certainly, you have a great point Peter. It comes down to what is the load on the bearing and how much "fat" is left as far as rotational speed and maximum loading. Size is not everything to be sure in a bearing. But larger generally means it will last longer due to spreading the load over a wider surface area, assuming it has the right speed rating.

I did note that the large bearing says "USA" on it and is a SKF. This is second in quality only to a Timken, which is the best in the world as far as tolerances and build quality. Of course, a Timken would have cost A LOT more and probably be overkill in such a lightly loaded application. Obviously, that is a replacement bearing and not the original, as it is brand new. What's the make on the PM bearing?

Lots true here. There are differences in bearings within a brand too. The original bearings were a SKF. The distributor had Timken too. He said the Timken he had were made in Brazil or someplace like that. He also said that the SKF were made in the USA, Brazil, and Italy. He could have been full of crap too, who knows :)

I figured that if SCM thought the SKF were good enough, and 33 years later they still spun really really well, that was good enough for me. On the other hand, these could have been replaced a few years ago. I doubt that though, as you have to remove a bed to replace the bearings, and it's a big job.

Can't wait to start reassembly this weekend.

Here is the Byrd head. It has 8 rows of cutters compared to the original 4 knives.

http://i174.photobucket.com/albums/w99/AZEngineer/_NIK3323.jpg

[Scott Myers]

WOW! I have cutterhead envy!

[John Hedges]

I remember going from my original 6" GI jointer to my current 8" Grizzly and thinking how much larger the cutterhead was on the 8". It was a little intimidating the first couple of boards I ran on it. However, in looking at it now the old GI ran soooo much smoother and nicer so I guess size isnt everything, but the point about bearing quality is well taken.

[Ben Martin]

The picture above shows the smaller bearing on the PM60 cutter head, where as the bearing on the opposite side is twice as big as this one. I expect this has to do with it being on the opposite side as the drive pulley and taking the majority of the loading. Which bearing is that from the SCM?

Also, what year is the PM60? I guess it is 1979 or newer, and the "ridges" behind the blade slots would make yours one of the "safe cutterheads" that they speak of.

[Joe Jensen]

The picture above shows the smaller bearing on the PM60 cutter head, where as the bearing on the opposite side is twice as big as this one. I expect this has to do with it being on the opposite side as the drive pulley and taking the majority of the loading. Which bearing is that from the SCM?

Also, what year is the PM60? I guess it is 1979 or newer, and the "ridges" behind the blade slots would make yours one of the "safe cutterheads" that they speak of.

I purchased my PM60 jointer (soon to be for sale) new in 1990. It currently has a Byrd head in it, but I also have a Tersa and the original pictured.

The PM cutterhead uses the same bearing on both sides. The SCM uses a much larger bearing on the pulley side (pictured). The more amazing thing is the weight. I would guess the PM is 1/4th the weight of the SCMI, and the SCMI is only 1/3rd longer...joe

[Ben Martin]

I purchased my PM60 jointer (soon to be for sale) new in 1990. It currently has a Byrd head in it, but I also have a Tersa and the original pictured.

The PM cutterhead uses the same bearing on both sides. The SCM uses a much larger bearing on the pulley side (pictured). The more amazing thing is the weight. I would guess the PM is 1/4th the weight of the SCMI, and the SCMI is only 1/3rd longer...joe

Hmm, my 1979 PM60 uses two different size bearings, a R12V (smaller on the pulley side) and a 6204DU on the opposite side (I just bought new ones two days ago) which is about twice as big (or maybe I have it reversed about which goes where...). I wonder what size are on yours, also where was yours made? I haven't weighed it, but I expect the cutterhead on mine weighs in at 20-25lbs...

[Joe Jensen]

Hmm, my 1979 PM60 uses two different size bearings, a R12V (smaller on the pulley side) and a 6204DU on the opposite side (I just bought new ones two days ago) which is about twice as big (or maybe I have it reversed about which goes where...). I wonder what size are on yours, also where was yours made? I haven't weighed it, but I expect the cutterhead on mine weighs in at 20-25lbs...

You have me thinking. I will weight my PM cutterhead tonight. I was guessing at the weight. I know the weight of the SCMI as I shipped it recently. On the PM bearings I was going from memory, I replaced them a couple of years ago. I'll look at that tonight as well. My jointer was made in the good old USA.

[Ben Martin]

Here is a not-so-good picture of my cutter head:

http://i424.photobucket.com/albums/pp325/martbj/Powermatic%2060%20Restoration/IMG_1831.jpg

[J.R. Rutter]

Please educate me. I have seen several discussions here and in archives at another site discussing the 'size' of bearings in a comparison of two machines. One conversation was relative to shaper spindle cartridge bearings. Why would a larger bearing be assumed to be of greater quality or significance than a smaller bearing? Is there some appreciable asset to having very large bearings? I know that there is a wide range of quality difference for any given size of bearing relative to tolerances, rpm capability and life span, but assuming both bearings are of equal quality, what would be the advantage of a bearing being larger?

For instance the largest cars I have ever driven have not been the greatest handling or most precise machines by any means.

I'm no engineer, but it seems to me that this relates to load requirements and torsional rigidity. A larger shaft is more rigid and requires larger diameter bearings. The bearing mounts on the stationary part of the machine are also subject to this - larger, more massive mounts combined with larger rotating shafts would also benefit from larger bearings. Larger surface area would couple the vibrations into the mass of the base as well. In practice, whenever I have had the opportunity to compare machines with identical specs in terms of function and cutter size, the machine with larger shaft and larger bearings ran noticeably better.

As for the car analogy, the suspension elements would need to be more rigid in a larger car to counteract the forces generated by the increased mass. This is what the larger bearings (and associated support parts) in a machine are doing.

I dunno, does that sound reasonable?

[J.R. Rutter]

Joe - What are the specs on the white paint? Did you match it to the current SCMI factory color?

[Joe Jensen]

Joe - What are the specs on the white paint? Did you match it to the current SCMI factory color?

I couldn't get a paint code and didn't have a sample so I took a cover from my Maggi feeder and I matched that by eye at the paint store :cool:

I figured it was close enough, and at least it would match something :D