Cutting a perfectly round pit into a 2" foam sheet

[NevinW]

Here is a problem that I have been giving some thought to. I have 2 inch thick foam in my yards. I want to cut a perfectly round 70 foot diameter HO hole in the foam 1 inch deep with a flat floor to make a turntable pit.

The hard way would be to set an Xacto blade 1 inch long, put a 1 inch deep cut around the edge of the circle and then carefully pull the foam out until you had a 1 inch deep pit and then sand the floor flat. Alternatively one could using a hot wire with the wire bent for a 35 foot radius and a 1 inch deep 90 degree angle and use it around a fixed center point. Obviously more thought needs to go into that one!

Any other thoughts and ideas? The must be some tool with would work for this? - Nevin

[tstage]

Nevin,

Does your turntable have a lip around the edge of it?  I think most of them do.  I would just trace the exterior of the base onto the foam and use a thin serrated knife to cut just on the inside of the line.

Cut all the way though the foam then push the cut out foam piece down 1" for a nice flat bottom.  Your 2" foam base will hold the cut out foam piece in place.

I've used this method on my coaling tower pit and ash pit.  The advantage is, if you should ever decide to "move" your turntable, all you need to do is to push the foam piece back up (so that it is level with the rest of the layout) then cut another 2" OD hole somewhere else.

Does that make sense? 

Tom 

[Don Z]

Nevin,

Here's a messy solution. Use a piece of 1/4" masonite or something similar. Cut your scale 70' diameter hole in the 1/4" material, cutting as close to the line as possible. Use double sided tape to fasten your pattern to the top of your foam. Now use a router with a pattern cutting bit (has the bearing at the top of the bit) and set the bit to cut 1" deep. Rout to your hearts content, enjoy the perfectly flat bottomed hole you cut and oh yeah, enjoy cleaning up all of the mess from the foam!

Don Z.

[SpaceMouse]

Are you sure you are not working too hard here. Why not cut out the 2" circle. Drop it 1 inch and re attatch it. It will be covered anyway so you won't see the cut and it will be perfectly flat.

[twcenterprises]

Are you sure you are not working too hard here. Why not cut out the 2" thick circle. Drop it 1 inch and re attatch it. It will be covered anyway so you won't see the cut and it will be perfectly flat.

That's what I would do. 

Brad 

[Kurt_Laughlin]

First I'd look at Mr. Mouse's suggestion or something similar like cutting all the way through and inserting a 1-inch thick disk in the hole, or cutting a square out of your foam about 6 inches wider than your pit diameter and dropping in two squares from 1-inch foam.  Cut your pit into the top one.

Cutting a perfect circle is very tricky because foam doesn't cut that cleanly, so cheat.  First, fix your axis location in the sub deck and make a hole to firmly but freely mount a 1/2-inch dowel.  Make a tool consisting of a piece of wood with a 1/2-inch dowel at one end and a blade holder at the other.  The tool has to be able to slide in the sub deck hole but not wobble.  Make the pit about 1/4-inch bigger in diameter than it needs to be.  Next line the wall of the hole with spackle or Hydrocal or something that is easily sandable and cutable but not too brittle.  (Perhaps plaster or spackle with some white glue mixed in?)  Use your tool to scrape away (not cut) the spackle as you go around and work your way down.  It should be smoother and rounder than cutting foam, so long as your tool runs true.

I've never tried this but with some experimentation and care it seems it would be easier.

KL

[TheK4Kid]

The router method will work, or also a saber saw with a long fine tooth blade.
Cut the foam out, drop it down an inch and reattach it with latex caulk, then set in your turntable. All done!

TheK4Kid 

[selector]

I'd use a bread knife, a coping saw, a hacksaw with an open end, or simply draw a close-enough circle and then use a jig saw with long blade.

[ndbprr]

First of all a turntable pit is not flat.  It has a slight slope to the center for drainage.  I built mine by mounting a piece of plywood to the bottom after cutting a hole.  I placed a piece of brass tubing in a hole in the center and made a styrene form that had a piece of brass tuibe that went in the hole in the one in the bottom.  The form sloped up gently to the pit rail area where I had mounted small pieces of basswood so it stepped over them and then formed the side wall.  By adding plaster and screting the way they do concrete I made a very presentable pit that filled in all those crooked edges in the top.  The I just sanded the rim flat. The bass wood blocks were for mounting the pit rail.

[Blind Bruce]

Something else to consider. Does your TT require access from the bottom for wiring or service?

[MisterBeasley]

This is what I did for mine:

This is 2-inch foam, with a piece of masonite for the base.  You can see the 4 carriage bolts I used to hold the masonite in place.  Once the scenery and the lead and roundhouse tracks are in place, the only way this turntable will ever come out is from below, so I made the base removeable from underneath.

If you really want only a 1-inch deep hole, you could do something like this and then put a 1-inch piece inside the hole, supported by the masonite base.  Or, you could get a larger sheet of 1-inch foam and replace the 2-inch foam in the area around the turntable, so you'd only be cutting through 1 inch.  For a small area, 1-inch foam should still be stiff enough if you can put some support close by.

I did the cut with a dry-wall saw, by the way.  It made a mess, but it was very easy and worked great.