Shopsmith Forums

jsburger wrote: Sun May 24, 2026 12:12 pm Still not sure how WELL it would work.

I am pleased by how much interest this topic has sparked. Like some of the other folks I also went back to my old editions of Power Tool Woodworking for Everyone to see if I could find confirmation of my memory of having done some sanding this way many moons ago. And, sure enough, in all the editions prior to the 1984 edition, this technique is detailed and depicted in the chapters on "Jigsaw" and "Abrasive Machines." An important point raised by jsburger is the apparent mis-match between the MT on the cup center and the cylindrical hole in the fence. Not mentioned in PTWFE is that the fence is tapped for a set screw that puts pressure against the center and keeps it from moving. This essential clever feature resolves the MT vs cylinder issue. PTWFE does not, that I could find anyway, talk about using this technique for cutting circles with the tablesaw, but there are many examples on line of such cuts being made using some kind of a pin center. I would have to think that it could be done with the cup center and fence so long as care was taken to keep the workpiece flat on the table.

I think the set screw would interfere with the self centering of the taper into the fence hole. The taper/hole fit will only work with any external force that is in line with the pin axis.

The purpose of the set screw is to secure the mortising holddown shaft. If the holddown shaft is not secured the holddown will not holddown.

As for using this technique with the table saw, I feel it is dicey at best and would only be practical with thin workpieces. Consider the angles(feed direction/saw blade rotation) involved.

JPG wrote: Sun May 24, 2026 6:33 pm I think the set screw would interfere with the self centering of the taper into the fence hole. The taper/hole fit will only work with any external force that is in line with the pin axis.

The purpose of the set screw is to secure the mortising holddown shaft. If the holddown shaft is not secured the holddown will not holddown.

As for using this technique with the table saw, I feel it is dicey at best and would only be practical with thin workpieces. Consider the angles(feed direction/saw blade rotation) involved.

Exactly. The tapered cup center is only touching the fence at the very top of the hole. Tightening the set screw will tilt the center of of perpendicular. Not tightening the set screw will allow the center to wobble around in the hole. Magna had a few dubious ideas about other things in the beginning.

chapmanruss wrote: Sun May 24, 2026 12:15 pm Trying to use a Bandsaw in the place of the Jig Saw would not work due to the orientation of the blade to the Mark 5. Circles can be cut on the Bandsaw with the Aluminum Table, Table Extension and Circle Cutter but there is a much smaller size limitation for that operation.

Maybe... we will see

jsburger wrote: Sun May 24, 2026 7:02 pm Exactly. The tapered cup center is only touching the fence at the very top of the hole. Tightening the set screw will tilt the center of of perpendicular. Not tightening the set screw will allow the center to wobble around in the hole. Magna had a few dubious ideas about other things in the beginning.

I can well imagine something like a dowel center finder, transfer punch, or similar pointed object, set into a bearing and riding in that hole (or even a zero clearance table insert). I think if the rotational pivot point was shorter it would be safter and more stable. Either way, the concept is really cool, especially for a 73 year old idea.

Now.. can you just put a palm router on a stick? Sure. Or mount a table router under the table on the insert and do the same thing but inverted? Probably. But it makes me wonder what other cool contraptions I'm yet to rediscover, or come up with.

Maybe I should use the floor as a fence for the big piece of plywood....... joking! (I wonder if anyones actually ever done that maneuver)

So after thinking about the issues that JPG and the following comments brought up, I set the cup center back into the fence and tried to measure the wobble when the center was loose in the hole. It looks like about 1/8 of an inch or a bit more in all directions. With the set screw tightened, the cup center is indeed pulled off plumb. The set screw bears on the the bottom 1/8 inch or so of the center and when tightened pulls the tip of the pin 1/8 of an inch towards the set screw side of the fence. Without the set screw being tight using this set up would require perfectly vertical pressure along the axis of the cup center during the cut. Probably not attainable. However, with the set screw tight, variations in the direction of the downward holding force do not move the (unplumb) pin. The question then is what difference does it make if the pin is not at a perfect right angle to the table? Seems like as long as the pin is positioned in the desired center of the intended circle and does not move from that spot during the operation that it would work OK.

If using this method to make a cut on the table saw seems unsafe, by all means do not attempt it. A safer alternative might be to mark out the circle and rough cut it, staying outside the line, with a saber saw. Then set up the fence and cup center and using the sanding disk bring the circle right down to the line.

Maurice wrote: Sun May 24, 2026 8:17 pm

chapmanruss wrote: Sun May 24, 2026 12:15 pm Trying to use a Bandsaw in the place of the Jig Saw would not work due to the orientation of the blade to the Mark 5. Circles can be cut on the Bandsaw with the Aluminum Table, Table Extension and Circle Cutter but there is a much smaller size limitation for that operation.

Maybe... we will see

jsburger wrote: Sun May 24, 2026 7:02 pm Exactly. The tapered cup center is only touching the fence at the very top of the hole. Tightening the set screw will tilt the center of of perpendicular. Not tightening the set screw will allow the center to wobble around in the hole. Magna had a few dubious ideas about other things in the beginning.

I can well imagine something like a dowel center finder, transfer punch, or similar pointed object, set into a bearing and riding in that hole (or even a zero clearance table insert). I think if the rotational pivot point was shorter it would be safter and more stable. Either way, the concept is really cool, especially for a 73 year old idea.

Now.. can you just put a palm router on a stick? Sure. Or mount a table router under the table on the insert and do the same thing but inverted? Probably. But it makes me wonder what other cool contraptions I'm yet to rediscover, or come up with.

Maybe I should use the floor as a fence for the big piece of plywood....... joking! (I wonder if anyones actually ever done that maneuver)

Yes, that is one of the dubious ideas Magna had. The table saw in drill press mode and using the floor and adjusting the head stock up and down the set the width of the cut. It is in the early issues of PTWE. Hey Russ, post the pictures.

IdahoEd wrote: Sun May 24, 2026 9:22 pm So after thinking about the issues that JPG and the following comments brought up, I set the cup center back into the fence and tried to measure the wobble when the center was loose in the hole. It looks like about 1/8 of an inch or a bit more in all directions. With the set screw tightened, the cup center is indeed pulled off plumb. The set screw bears on the the bottom 1/8 inch or so of the center and when tightened pulls the tip of the pin 1/8 of an inch towards the set screw side of the fence. Without the set screw being tight using this set up would require perfectly vertical pressure along the axis of the cup center during the cut. Probably not attainable. However, with the set screw tight, variations in the direction of the downward holding force do not move the (unplumb) pin. The question then is what difference does it make if the pin is not at a perfect right angle to the table? Seems like as long as the pin is positioned in the desired center of the intended circle and does not move from that spot during the operation that it would work OK.

If using this method to make a cut on the table saw seems unsafe, by all means do not attempt it. A safer alternative might be to mark out the circle and rough cut it, staying outside the line, with a saber saw. Then set up the fence and cup center and using the sanding disk bring the circle right down to the line.

Well, if the center moves around 1/8" as you say ( I suspect you are correct) then the cup edge is raised up on the side away from the set screw. Does it raise up enough to not let the center pin get a good grip due to the stock bottoming out on the cup rim?

Interesting! My tailstock fits VERY snug. I don't think I'd even need a set screw with vertical pressure, but even with it it only moves ~1mm once I jam it in properly. It all feels way more secure and stable than I had imagined.

Sidenote - has anyone ever opened one of these live centers to clean and add new grease? I can't see how you'd do it without damaging the point. Importing is tricky for me, single pieces on eBay end up being close to $50 USD

John: Good question. My first thought is that the rim of the cup center would not raise up enough to contact the workpiece unless the "dimple" made for the centering pin were needlessly deep. My second thought is that if this sort of interference were a significant issue it would have been mentioned in the chapters of PTWFE that discuss the technique. But, to be fair, I guess the only way to really know is to set it up with an actual workpiece and see if there is interference. I will try to get around to that experiment soon.

Maurice: Thanks for the picture. Very interesting to see that a live center fits so much more snug than the the cup center...I wonder why that is...maybe the MT on the live center is a different diameter?...not sure if that would really make a difference. Another experiment to try tomorrow.

John,

Is this the picture you are referring too from Shop Notes No. 2?

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Keep in mind that in the picture it shows something that appears to be attached to the floor to keep the stock in place. Still a risky operation.

Another "interesting" saw operation can be found on page 1 of Shop Notes No 3.

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