Geometry mystery

[Osvaldo Cristo]

Perhaps some of you can cast some light to one fact happened last week and I couldn't explain its reason.

I used my planer sledge to get some tapered legs for a project I am doing - actually I have made tapered cuts in the past but always using my table saw, it was the first time using my planer and its sledge. Please see the attached schematics.

Tappered legs.jpg

I used two supports, one 5 mm height and another one 10 mm. It looked me perfect except the planned surface is around a couple of mm convex!

I cannot figure why as the legs were attached to sledge with hot glue... perhaps the legs flexed at the middle?

Personally I liked the slightly convex legs and I prefer them as a boring straight one but it is disturbing do not understand why I got that. I am used with Geometry but I missed that - your input to explain that will be very welcome!

Thanks in advance for your help!

[Robert Hazelwood]

Looks like there is no support in the middle of the span between the two points where it rests on the sled. So the center portion of the workpiece will flex away from the cutter, and less material will be removed there than at the ends. Result is a convex surface.

[Jacques Gagnon]

Tappered legs.jpg

I used two supports, one 5 mm height and another one 10 mm. It looked me perfect except the planned surface is around a couple of mm convex!

I cannot figure why as the legs were attached to sledge with hot glue... perhaps the legs flexed at the middle?

!

Osvaldo:

I am pretty sure that your geometry is adequate . I suspect that your sled was not sufficiently rigid and that, as you suspect, the legs flexed in the middle section.

The good thing is that you like the end result, even though it was not planned.

Regards,

Jacques

PS: Looks like Robert answered the question while I was typing.

[Al Launier]

Robert is correct. Deflection of the wood while under the cutting forces will result in less material being removed, more at midspan. How long are the leg and what is the cross-sectional size? You could wedge under the legs to provide support. If you do, you could glue sandpaper to the wedges to prevent slippage.

[Bill Dufour]

Agree it is the flex caused by the infeed roller, pressure bar, chip breaker and outfeed rollers all pushing down with a strong force. I do not think the cutter really cases much deflection. I would guess 20-50 KG total force on a smaller planer.
I have found that when I plane a piece like that I get it to the thickness that looks good to my eye then run it one more time removing about 1/16" more. this seems to make the piece a little lighter and better looking when it is all connected together.
Bil lD
Bil lD

[Matt Day]

Flex is my guess as well. Planers put a lot of pressure down.

You can also taper on the jointer.
https://youtu.be/nZTXvsrzbSQ

[Bill Space]

Osvaldo,

I think you may have discovered a new a new way to make cauls ...

Actually I am serious!

[Chris Montoya]

I was just thinking the same thing

[Osvaldo Cristo]

Osvaldo,

I think you may have discovered a new a new way to make cauls ...

Actually I am serious!

You are correct!

I will put that idea at work this weekend!

[johnny means]

I suspect that the the sled is where the deflection occurred. Was it just a peice of plywood or some other sheet good?

[Tom Bender]

Do you have your feed arrow pointing the wrong way?

[Osvaldo Cristo]

Do you have your feed arrow pointing the wrong way?

No. It is that...