Why are table saw insert plates rounded?

[Stephen Saar]

I was pondering making some more insert plates for my TS last night and I started wondering why do so many TS makers make the inserts rounded on the ends. I couldn't think of any really good reason why you would do that. It makes making your own custom inserts more difficult and I wasn't able to think of any real benefit, outside of maybe they want to sell their own inserts, but I don't see any manufacturers pushing their inserts that hard so it seems unlikely. So can anyone shed any light on this? Am I just missing something very obvious that the rounded ends gives you?

-Stephen

[Mike Goetzke]

Maybe easier to mill a curved surface on the saw (hard to machine a square corner not to mention the stress riser?

[Jon Endres]

My old Craftsman bench saw (ca. 1938) has a rectangular insert plate, but even the corners are rounded slightly (maybe 1/4" radius?).

Maybe for the same reason that manhole covers are round - so they won't fall through the hole if tipped sideways. However, I've dropped an insert into the saw more than once, so that's probably not the reason.

[Myk Rian]

Maybe for ease of getting your hand to some areas.

It's not hard at all making them. Just use the old one, 1/2" BB ply(or whatever), and a pattern bit in the router.

[Mike Circo]

A perfectly square corner would create a stress point in the cast iron. Over time with use and vibrations, that would be a weak point for a crack to form.

Same reason that the windows on pressurized aircraft are rounded, to reduce the incidence of cracking. Talk to a metalurgist or look up the history of the British DeHavilland Comet aircraft, same issue there.

[Prashun Patel]

Reminds me of that old interview question "why are manhole covers round?" They say it's because that way they can't fall in the hole.

Do the curved corners make it harder for the insert to fall thru the throat?

Perhaps it also has something to do with minimizing the lips at the leading and trailing edges of the insert?

[dan sherman]

+1,

It's also impossible to make a square internal corner with an endmill.


A perfectly square corner would create a stress point in the cast iron. Over time with use and vibrations, that would be a weak point for a crack to form.

Same reason that the windows on pressurized aircraft are rounded, to reduce the incidence of cracking. Talk to a metalurgist or look up the history of the British DeHavilland Comet aircraft, same issue there.

[Lee Schierer]

Sharp corners significantly reduce the strength of cast iron. Also most cast iron uses sand molds which are easier to make with rounded profiles and cut outs. Also as you put your throat plate in place the rounded profile won't fall through the opening quite as easily as if it were square.

[glenn bradley]

Mike C. caught my guess on this. In a previous life, if I were fabricating for strength in certain situations I would use round tube vs. square.

[Marty Paulus]

+1,

It's also impossible to make a square internal corner with an endmill.


Stress riser elimination and hard to machine a square hole with a round tool.:(

[Josiah Bartlett]

It also makes for more positive registration of the insert, side-to-side. You can insert a single jam screw on one end and the other end will always register in the same spot, even if you remove the insert and dink with the screw.

[David DeCristoforo]

"...why are manhole covers round?"

Humm. I always thought it was because the holes were round...

[Rob Wright]

My thought is that it is easier for a piece of wood sliding on the table top to overcome any amount of misalignment of the plate and the top surface since you would be hitting just one point - not the flat hard edge of a rectangular insert.

[Van Huskey]

"...why are manhole covers round?"

Humm. I always thought it was because the holes were round...

I thought is was because the men going through them were more round than square.

[Frank Drew]

Not all of them have round ends; my last saw, an Ulmia, had square ends with rounded corners (maybe 1/2" radius), which made making additional inserts particularly easy.

[mark kosse]

I'm going with answer 1, Part A. (ease of machining)

[Peter Quinn]

I use an old SCMI table saw at work, it has a nearly rectangular plate with slightly radiused corners. But that saw is Italian, so maybe its tradition to do rounded plates here?

I think they could make those corners square if they chose to, though it would be quite a bit more work, and I guess people are saying weaker? I wonder, because the intersection between the lip that holds the plate and the slot that forms the outline of the plate is a 90 degree corner (on the scmi, it is continuous, not a few little fingers sticking out), and it desn't seem anxious to break? Come to think of it the lip that holds my shaper inserts forms a 90 degree corner too, and it hasn't broken?

My best guess is "Cause its easy" and thus cheaper. I'd guess ou could pay to have the situation modified should you choose to. On the manhole cover thing, square manhole would weight a fair bit more for the same opening, and it would look funny like those square hamburgers or square fish fillets. Just not natural.

[Larry Frank]

I agree with the idea that the rounded end may cause less of a catching point as wood slides over it and also that it is easier to get it snug int he hole with adjustment to the side and/or end.

The issue of a stress riser seems to be less likely. While there is no doubt that a sharp corner can initiate a crack, it is unlikely that the stresses found in using a table saw would be enough to cause a crack. So, unless your are putting large loads cyclically on the table top or there are huge internal stresses, a crack would not form and run at a square corner. If you beat on it hard enough, no doubt that a crack could run from a sharp corner. Even with a small corner radius, it is difficult to start a crack unless greatly overloaded.

[Cody Colston]

Why are table saw insert plates rounded?

'Cause it ain't hip to be square. :cool:

[Stephen Saar]

Thanks for all the answers. I think the machining makes the most sense, guess it doesn't matter that much one way or another just one of those things that got me curious.

-Stephen

[Josh Bowman]

I was pondering making some more insert plates for my TS last night and I started wondering why do so many TS makers make the inserts rounded on the ends. I couldn't think of any really good reason why you would do that. It makes making your own custom inserts more difficult and I wasn't able to think of any real benefit, outside of maybe they want to sell their own inserts, but I don't see any manufacturers pushing their inserts that hard so it seems unlikely. So can anyone shed any light on this? Am I just missing something very obvious that the rounded ends gives you?

-Stephen
Stephen,
I don't dare ponder you're real question, but have a comment on one of your statements. You said it's "more difficult" for someone to make an insert. Actually using a patten making bit in my router, the roundness makes it much easier.;)

[Terry Welty]

All that and a bag of chips... Also, it probably looks better...

[Jason Jurusz]

I was pondering making some more insert plates for my TS last night and I started wondering why do so many TS makers make the inserts rounded on the ends. I couldn't think of any really good reason why you would do that. It makes making your own custom inserts more difficult and I wasn't able to think of any real benefit, outside of maybe they want to sell their own inserts, but I don't see any manufacturers pushing their inserts that hard so it seems unlikely. So can anyone shed any light on this? Am I just missing something very obvious that the rounded ends gives you?



-Stephen


I'm sorry to weigh in on a topic so long-defunct; I do so not be argumentative, but to offer a contribution I hope might help others. I found myself in a similar position as Stephen last weekend, though in my case I just finished RESTORING a table saw, and had no choice but to fabricate a plate; the saw was missing its when acquired. After pondering, discussing with others, and then coming here, I felt the need to speak up, and then share a solution, to the implied problem, if not the question.



..."why are manhole covers round?" ...make it harder for the insert to fall thru the throat?



... for the same reason that manhole covers are round - so they won't fall through the hole if tipped sideways
...except the reason that works is the shape is a CIRCLE. A "racetrack" shape (a rectangle in which 2 opposite sides are convex arcs instead of straight lines) is EASIER to have fall through, as only the apex of the arcs can catch it, and only in some geometries of the inset slot.



Maybe for ease of getting your handto some areas.
...except said racetrack shape has LESS area than a rectangle of the same dimensions.



Maybe easier to mill a curved...

...hard to machine a square hole with a round tool...

...It's also impossible to make a square internal corner with an endmill.


All fair points. Except it's a CAST iron surface (when it's not STAMPED steel). Why cast something, then turn around and MILL OUT the throat AT ALL? Cast it in place!



Mike C. caught my guess on this. In a previous life, if I were fabricating for strength in certain situations I would use round tube vs. square.


This IS a good point... when compression or expansion are the forces being exerted on the shape. But as Larry Frank points out:



...it is unlikely that the stresses found in using a table saw would be enough to cause a crack. So, unless your are putting large loads cyclically on the table top or there are huge internal stresses, a crack would not form and run at a square corner. If you beat on it hard enough, no doubt that a crack could run from a sharp corner. Even with a small corner radius, it is difficult to start a crack unless greatly overloaded.


And, although, as Lee states...



Sharp corners significantly reduce the strength of cast iron...


...unless you're using your table saw as an anvil (don't do that), it's not really a factor.



My thought is that it is easier for a piece of wood sliding on the table top to overcome any amount of misalignment of the plate and the top surface since you would be hitting just one point - not the flat hard edge of a rectangular insert.


Presupposing the answer is not "because most manufacturer's also sell plates, so if they're more difficult/annoying to make... profit," and it's not being done strictly for aesthetic reasons, Rob's answer does seem the most pragmatic. Finally, while



...It's not hard at all making them. Just use the old one, 1/2" BB ply(or whatever), and a pattern bit in the router.

...Actually using a patten making bit in my router, the roundness makes it much easier.


...no, it's NOT that hard to make one, it's still far MORE of a pain than "zip-zip" with the miter saw, or hand saw, or SCROLL saw, etc. Not least because several brands have oddball insets, slots in the side to accept a corresponding tab, or alignment/retention detents, pegs, or holes.


Anyway, shelving all of that for the moment, the solution I came up with seems to be a fairly solid one, with much to recommend it and I thought I'd share it with the group.


I bought a 1/2" sheet of HMDP plastic from Amazon. I made two inserts with identical outside geometries and laminated them together. the first, the bottom one, has a massive slot cut from the middle, large enough to accomodate my biggest dado stack and most extreme angles the saw can cut within:


https://www.done.cheap/public/throat1.png




The second I cut in half, keeping only the two semicircles, plus about a cm on the straightaways. Since my saw's throat plate is only a half-inch deep, I also drilled out where the single post/setscrew I would have to deal with lived. These I took to the router table and routed a tongue on one side, and a groove on the other.


https://www.done.cheap/public/throat2.png


Finally, I laminated them together using CA glue.


https://www.done.cheap/public/throat3.png




Now, whenever I need an insert, a grab up one of the 3"x12" hardwood floor samples I can grab for either free or 25¢ at any big box store (I have a box of 12 of them I got for free from the blue one under the saw as I type this) slide it sideways into the insert, and drop it right back down into the slot, and crank up the blade. Instant insert. One that's held fast in the "master" plate, that's very low-friction, that sits flush with the surface every time (since the master is already tuned for and the flooring samples are all identical in the two dimensions that count: length and thickness), that costs less than even subflooring (and that's not even factoring the labor savings), per unit, and most importantly? It seems? One that allows my saw to keep it's bizarrely-impractical throat profile.


...for whatever the hell reason it's shaped like that. ¯\_(ツ)_/¯


Hope this helps someone else someday. And if anyone from one of the tool manufacturer's ever reads this? There's been people wondering about it for decades now. Wanna clear this up, maybe?

[dan sherman]

All fair points. Except it's a CAST iron surface (when it's not STAMPED steel). Why cast something, then turn around and MILL OUT the throat AT ALL? Cast it in place!


When you cast metal, you are left with a very rough surface, and with a part that's not dimensionally accurate.

take a look at this video about 1:45 in.
https://www.youtube.com/watch?v=a6sQ8imvcA4

[Bill Dufour]

My Rockwell12/14 which is a copy of a unisaw has a rectangular insert. Of course the corners are rounded a bit. So Rockwell decided not to make it rounded for some reason. It is a different design it all sits to one side of the blade so it is not really possible to make a zero clearance insert. It does have nice fat finger holes at both ends to lift it up. It weighs six pounds.
Bil lD

http://www.limzmachinery.com/Saws-TAS-12quot14quot--Limz-Machinery-265

[Bill Dufour]

I believe the powermatic 20" bandsaw uses a rectangular woods trip as a blade table insert while delta 14" uses. a round disk with. a slot cut half way.