achieving flat square bottoms and faces

[Gregg Feldstone]

What is a good method to make sure the bottom of the foot on a bowl or the face of a platter is dead flat and 90 degrees to the sides? I have tried eying it with a scraper but am always a little off. What would I use to reference between the sides of the piece and the face or bottom to ensure 90 degrees? Or is this one that just comes with practice and skill?

[curtis rosche]

for the bottom, the easiest thing is to just make it a little concave, no need to have it perfectly flat. just take a ruler or straight edge and place it against it. for doing the face of platters, the trick is getting your tool rest perpendicular to the bed. probably easiest done with a square or 2 or a laser, then marking somehow so you dont have to do it again. but i think you will find that for a platter, having it perfectly flate, doesnt look or work to good. theres nothing to hold things on it.

[Richard Madison]

Gregg,
A 6" ss rule (the cheap kind that some places used to give away) is very handy for checking flatness on all kinds of things. For vertical sides you could simply place a square (machinists', framing, etc.) across the top. A 45 degree machinists' square works well for pieces that need a 45 degree side slope.

[Scott Conners]

Most gouges are great round straight edges - if it runs against the whole of the bottom, it's automatically square.

[Gregg Feldstone]

Can you please explain that in detail, Scott? Thanks.

[Paul Heely]

If you lay your gouge across the bottom of the turning you can see if its flat. I'm not sure how you can determine that the sides are square to the bottom though using just a gouge.

--
Paul

[Jim Becker]

The only time I've found this necessary is when truing up a wasteblock...and then I use the shank of my gouge as a straight edge. Subtle pull-cuts generally get me flat enough to provide a secure gluing surface.

Tenons don't need to be perfectly flat since they should never bottom out in a chuck. In fact, there should be a small shoulder (doesn't need to be of any significant size) that sits on the top of the chuck jaws to insure the best holding power.

[David Epperson]

If you lay your gouge across the bottom of the turning you can see if its flat. I'm not sure how you can determine that the sides are square to the bottom though using just a gouge.

--
Paul
If it's flat AND turned - it will be perpendicular to the axis of rotation and therefore square.

[Paul Heely]

But it won't necessarily be square to the sides of the piece. Just as the axis of rotation and sides of a piece are not necessarily parallel.

--
Paul

[curtis rosche]

if you get one of those 90 degree toolrests that should help, you just need to make sure that the toolrest is set up so that its parallel or perpendicular depending on how you look at it

[David Epperson]

But it won't necessarily be square to the sides of the piece. Just as the axis of rotation and sides of a piece are not necessarily parallel.

--
Paul
Yes. It will necessarily be square, or at least uniform around all sides. Unless you demount it and remount it out of axis alignment it will have to be. It's just cylindrical geometry. At least until the wood moves due to warpage. If it's flat while it's spinning it HAS to be square with the center axis, and if the sides are symmetrical around the center axis (it doesn't wobble) then the flat base HAS to be "square" with the sides.

[Jim Becker]

David you are theoretically correct but consider a natural edge piece or one where the face is originally not parallel and is left that way. The "phantom" edge will indeed be parallel to the base once turned, but the "physical" rim will not be so.

[Paul Heely]

Here's a picture to illustrate what I am describing. This shape can easily be turned on a lathe. Please explain how the sides are square to the bottom.

--
Paul

[David Epperson]

Just as the axis of rotation and sides of a piece are not necessarily parallel.

--
Paul
I just reread this part. If the sides are not parallel to the axis of rotation then you can get flat (across a diameter) or square, but not both.

[David Epperson]

Here's a picture to illustrate what I am describing. This shape can easily be turned on a lathe. Please explain how the sides are square to the bottom.

--
PaulThat's one of those cases where you can get flat or square, but not both. The angle from the base to the side, at any point around the bowl will be uniform however.

[Paul Heely]

I agree that any point is uniform. But, the original question asked about the foot, or face, being flat AND 90 degrees to the side. Which is what I was commenting about.

--
Paul

[Mike Minto]

give me more of this!

[Scott Conners]

But it won't necessarily be square to the sides of the piece. Just as the axis of rotation and sides of a piece are not necessarily parallel.

--
Paul

You're correct in that the sides will not necessarily be square to the bottom; however, the bottom will be square to the axis of rotation. On a lathe, the axis of rotation is nearly always the reference plane on a piece. If you need the angle at the joint between base and sides to be 90 degrees and want a flat bottom, then you need to make sure that both are properly aligned to the axis of rotation - the base is square, the sides are parallel. A straight edge across the base is an easy way to check for a square-to-the-axis base.
In your picture, because the sides are not parallel to the lathe or each other, the only way to have the angle at the base be "square" (i.e. 90 degrees) would be to have a deeply cupped base. That would defeat the first request of a "dead flat" bottom. If the base is dead flat, the sides are necessarily parallel when the corner angle is 90 degrees "square". With that in mind I assumed Gregg wanted to know how to keep the base flat and square to the axis of the piece, which would ensure it didn't lean to one side. It's also a first step to getting a 90 degree angle and parallel sides, because it gives you a reference plane at 90" to the axis of rotation, so you can easily use a regular square to check the sides.