Table Saw Miter Cuts are concave

BKHam

Bradley
User
I was not getting perfect result when tilted my saw to 45. i was getting a bellied or concave cut. I could never figure it out. i did some searching on lumberjocks and redddit and the best advice was slow down your cut or you might have messed up teeth on the blade.

i was curious about the blade part. if you took slow cuts, how would a wildly ground tooth cause a concave surface? i'm just not buying it.

recently, I saw a youtube video on by stumpy nubs on aligning the table saw and started messing around. i have the blade within 2 thousandths front to back, using a dial indicator that rides my miter slots.

Stumpy then takes it an extra step and checks the blade front to back when tilted at 45 degrees. i did that and found out that the back of my blade was slightly lower than the front. Since i can't raise the back, i have to lower the front.

as stumps suggests for my delta contractor type saw, i go in and shim the trunion bolts where they attach to the underside of cast iron table. i just cut up some feeler gauges after bumbling thru some math.

so i have the blade within about .002 at 90 and .004 at 45. seems like i'm ready.

i make some test cuts at 90. dead on.

when i tilt to 45, i get a cut that is CURVED! a slight curve but i can see it nontheless.
my quality checks:
i test my stock. not bowed.
its freshly planed square.
i'm holding it down with a clamp so its not shifting.
tried two blades to put this blade teeth idea to rest.
i use full kerf blades so i believe blade runout is minimal.
arbor runout measures 1 thou

it cuts heavier at the front of the cut and curves to a point.

obviously there is some odd geometry going on with the tilt but my brain is fried. anybody encountered this?
 

Attachments

  • 20200819_163904.jpg
    20200819_163904.jpg
    652.5 KB · Views: 244

nn4jw

New User
Jim
How are you holding the board? With a miter gauge? Sled? What are you clamping the board too?

It sounds (to me) like there may be some kind of movement involving the miter slots, possibly in the vertical direction. Or some kind of flexing with a miter gauge if that's what you are using. Without seeing it happen that's the only wild guess I can make.
 

BKHam

Bradley
User
How are you holding the board? With a miter gauge? Sled? What are you clamping the board too?

It sounds (to me) like there may be some kind of movement involving the miter slots, possibly in the vertical direction. Or some kind of flexing with a miter gauge if that's what you are using. Without seeing it happen that's the only wild guess I can make.
i use a sled that uses the matchfit clamps
 

Jeremy Scuteri

Moderator
Jeremy
This is very interesting, I have not experienced this myself. The first thing that pops into my brain is that cove cuts can be made on a tablesaw. This sounds like the "concave" cut you are seeing. I can't see a curve from the attached picture, it looks like a an 1/8" gap on the left that tapers down to nothing on the right.

I can think of a few things to try:

1. Try making the same cut with a miter gauge instead of your sled. Ensure that the board stays flat against the cast iron and put a stop block on the miter gauge fence to keep the board from shifting.
2. You mentioned that the blade was within 0.004" at 45-deg. Try making this same measurement with the dial indicator mounted to your sled instead of in the miter slot. It can show if the sled is causing something weird to happen.
3. Try ripping a 45-degree bevel on a board using the rip fence and see if that comes out as expected or has the same concave issue.
 

sawman101

Bruce Swanson
Corporate Member
If you're using a blade designed for ripping, this can very well happen. A rip blade will flex in the cut and not cut straight. A good crosscut or combination blade will do thed job you are hoping for.
 

BKHam

Bradley
User
If you're using a blade designed for ripping, this can very well happen. A rip blade will flex in the cut and not cut straight. A good crosscut or combination blade will do thed job you are hoping for.

its an 80 tooth cross cut blade, full kerf.
 

BKHam

Bradley
User
This is very interesting, I have not experienced this myself. The first thing that pops into my brain is that cove cuts can be made on a tablesaw. This sounds like the "concave" cut you are seeing. I can't see a curve from the attached picture, it looks like a an 1/8" gap on the left that tapers down to nothing on the right.

I can think of a few things to try:

1. Try making the same cut with a miter gauge instead of your sled. Ensure that the board stays flat against the cast iron and put a stop block on the miter gauge fence to keep the board from shifting.
2. You mentioned that the blade was within 0.004" at 45-deg. Try making this same measurement with the dial indicator mounted to your sled instead of in the miter slot. It can show if the sled is causing something weird to happen.
3. Try ripping a 45-degree bevel on a board using the rip fence and see if that comes out as expected or has the same concave issue.

the picture i attached was a bit of an afterthought and done with my rip blade so its a bit rough. my earlier tests looked like a wave shape.

the sled is actually screwed to a miter gauge. what i've been doing is using that gauge to cut at 90 and then putting the sled on that.

the rip fence is something i'm going to try.
 

BKHam

Bradley
User
is the table saw top flat?

a simple question but it has me thinking. i could get a curved cut if the piece was being lifted. let me do some more research on this. but i need to check the outfeed side of things.
 

BKHam

Bradley
User
update on all your suggestions: I believe the culprit is a sagging table top. it is out about .004 in the middle, which honestly is quite a lot.

if i'm thinking about this correctly, the workpiece would start out exposed to a small amount of blade at the beginning of the cut, go downhill in the middle (more blade) and then less again as it goes uphill out of the cut. it would be exposed to the most blade when it is directly perpendicular to the blade, then creating a bellied effect in the middle of the workpiece.

when i shimmed the front of the saw, effectively lowering the front of the blade, i think it created an uphill affect which could cause a curve. the workpiece is exposed to a lot of blade at the beginning and middle and then as it exits uphill, progressively less. the blade is probably close to square but can still create this effect.

not sure what i can do about a sagging table. it doesn't affect me much on 90 degree cross cuts and rips. grooves would probably need some manual depth adjustments with a router plane but that is doable. miter cuts are most acutely affected. i'm thinking maybe a overly large sled that contacts the saw table at the high spots throughout the cut.
 

Jeremy Scuteri

Moderator
Jeremy
I don't know Bradley, an error of 0.004" in the flatness of the cast iron top sounds darn good to me.

1. Assume a right tilt blade (left tilt would have the same issue and move the cut line the opposite direction)
2. If the board moves DOWN by 0.004", then the cut line will move LEFT further into the board as you described above by 0.004"
3. The picture showed an error of about 1/8" which is 0.125", that error is about 31-times greater than the 0.004" that would be expected. I think something else is at play here.
 

BKHam

Bradley
User
I don't know Bradley, an error of 0.004" in the flatness of the cast iron top sounds darn good to me.

1. Assume a right tilt blade (left tilt would have the same issue and move the cut line the opposite direction)
2. If the board moves DOWN by 0.004", then the cut line will move LEFT further into the board as you described above by 0.004"
3. The picture showed an error of about 1/8" which is 0.125", that error is about 31-times greater than the 0.004" that would be expected. I think something else is at play here.

you are correct. did a little more investigating.

i'm guessing my original problem, a bellied cut may come from the table sag.

i've obviously exacerbated it somehow, maybe thru my fidgeting with the trunions or....something else. that something else turns out to be that i'm using a zero clearance insert. that insert was contacting the splitter assembly. Thus causing the insert to be raised up at the back and create that uphill affect. i just took a couple test cuts that were dead nuts.

thanks for talking this thru with me folks.
 

Jeff

New User
Jeff
that i'm using a zero clearance insert. that insert was contacting the splitter assembly. Thus causing the insert to be raised up at the back and create that uphill affect. i just took a couple test cuts that were dead nuts.

So how did you fix it to keep the ZCI from contacting the splitter assembly and raising up? What is "dead nuts"?

Good that you got it fixed and working properly now.
 

Jeremy Scuteri

Moderator
Jeremy
Good job getting it diagnosed and sorted out!

I think we have all been there before:
Time to make cut = 10 seconds
Time to figure out why the cut isn't as good as we think it should be = 2 hours
:D
 

BKHam

Bradley
User
So how did you fix it to keep the ZCI from contacting the splitter assembly and raising up? What is "dead nuts"?

Good that you got it fixed and working properly now.

it was actually the bolt and washer on the splitter support so i removed. dead nuts....i cut the miters on my workpiece and they were pretty perfect.
 

Attachments

  • 20200820_123648.jpg
    20200820_123648.jpg
    1.6 MB · Views: 170

Jeff

New User
Jeff
he splitter support so i removed. dead nuts....i cut the miters on my workpiece and they were pretty perfect.

Does dead nuts mean that the miters were dead on perfect? Your first picture sure doesn't look like the last one that you posted today.
 

Premier Sponsor

Our Sponsors

LATEST FOR SALE LISTINGS

Top