Will Cut Groove Maintain its Shape After PVC Board is Bent?

No it will not it will tend to compress the walls of the slot together, how much depends on the consistency of the material.

Bending most materials will result in some dimension change.

That is true.When bending tubing,especially thin tubing,wrinkles will form on the inside of the bend.Harley Davidson solved this problem by pressurizing the tubing to extremely high internal pressure while bending.No wrinkles in their bends,no matter how complex the shape.

PVC conduit benders pressurize the inside and apply heat while bending to get the same effect.The material from the outside of the bend and inside of the bend must flow,I think causing a slight increase in the inside wall thickness along the entire length of the bend.

I think lots of times they pack sand inside when bending pipe. The sand prevents a volume change so it will stretch rather than buckle.

I have never seen PVC conduit packed with sand before bending,and I have seen a lot of electrical work.They might do it in some places,but I have never personally seen it.

Electricians use a hot box for bending PVC.They cap both ends and put it in a hot box, the air inside will expand with the heat and prevent kinking.The caps on both ends prevent swelling at the ends so they maintain original joint clearance.

I do remember old bootleggers would pack copper tubing with sand,and wrap the tube around a stump to form their condenser coils.

Yeah, they coulda been moonshiners.

Or a low melting point metal that will be melted out after the bend is completed.

Depending on how sharp the bend and temperature,I would not be surprised if it did not break.If you had a mold in the shape you need,and applied pressure and heat from both sides evenly,you might succeed in not breaking it,but the grooves will not maintain the exact same dimensions.I would cut the grooves after bending,perhaps with a router,or an external groove applied outside of the bend after bending to the same shape,with enough clearance in the bend to allow the rollers or slides to make the turn.

Showing a drawing of the intended purpose will help you get better advice.

I did try to post a drawing using both Firefox and Safari and it seemed to work on Safari but it doesn't show up here when using either, sorry.

I don't know why they can't fix the problem of posting pictures: every time I find a work around they seem to find another way to stop me. But sometimes it still works hopefully

two initial possibilities: not to scale; top or bottom? :-

Secondly: I initially thought you were trying to bend the 4 inches into a C, but now I'm guessing that it's the 12 foot you're bending, so that you end up with a semicircular groove?

Yes it's the top one of your drawings that I wanted to obtain and yes I wanted to bend it along its 12 foot length.

But seeing the comments here it's obviously not going to work as I wanted so I'm going to abandon the idea.

How did you get the drawings on there?

I tried but I couldn't find any way to get them published even after they showed up on my desktop as OK.

"How did you get the drawings on there?"

In this case I cut them from my drawing, and, pasted them direct into the post.

When that doesn't work you can sometimes cut them (from wherever); save them to a folder as a Jpeg, then, insert them using the little camera tool.

When that doesn't work: you can sometimes create a post without any images; preview comment; edit, and, try both methods above.

Often nothing works!

I think the problem is worse when you are replying to a thread you have started yourself. That doesn't make any sense: I must be getting paranoid.

The simplest and easiest way is with a router with a 4mm (.120")bit after bending the piece.It will follow the curve and cut the same dimensions all the way around.Don't know how to use a router?There are plenty of intelligent routers that tell you if you are going too fast or slow.My first use of a router was making molding for my floors and windows.It worked flawlessly after a few practice runs on scrap.Saved a ton of money buying straight planks and routing them myself.

You will find many uses for the router once you use one.

Note:Buy carbide bits.The will outlast dozens of HSS bits.They are worth the extra cost.

K.I .S.S.

Sharp intake of breath 0.12" = 3.048mm

"Note: Buy carbide bits. They will outlast dozens of HSS bits. They are worth the extra cost."

And, you can easily sharpen them with these 22mm diamond cutting discs for Dremel type tools

USA Amazon link, but, you can get them much cheaper: do your local search.

(Those discs are one of the most useful accessories I've come across, you can cut almost anything small with them.)

You are correct,Miscalculation on my part.Got in a hurry(had an urgent nature call from the bilges),I did it like calculating the press-fit bore for a bearing....last 2 digits of bearing number times 5 gives mm for bearing shaft press fit.

(An x204 bearing is 20 mm I.D.)6204,6304,etc.O.D. does not follow this rule of thumb. .

20 mm=.03937x20-.7874"

Getting in a rush will always take longer.

It has been years since I bought any PVC trim, so I am not confident in this. I think the PVC trim has a smooth outside and a dense foam inside. It should all be PVC, but the strength, density, bend characteristics, etc. all, per my best guess, will be different for the surface compared to the interior. I suspect that this increases the problems associated with trying to cut before bending. You probably will need to run a test to see if you can successfully bend, then cut after bending.

The couple of times I bent PVC pipe produced mixed results. If I had a slow heating, I found the PVC had "even characteristics" and my only issues were keeping the round shape and trying to have the desired bend after cooling. If I was in a hurry and didn't heat evenly, the pipe would buckle, bend in undesired directions, and basically remind me that haste makes waste. Flat board should be a little better behaved, but letting the heat soak in will probably produce better results.

It is always a good idea to think about fresh air and having a fire extinguisher handy when heating plastic.

What tolerance do you need on the slot?

How tight is the circumference?

You might be able to devise a cut shape that would give you the desired shape after bending it, I expect the cross profile of the cut would be cut slightly wider at the outer edge. You might be able to cut a slot in a short sample length that is bent to the same radius, then measure the slot dimensions with the board straightened out(?)

From my experience it will deform. Best to bend the board and then cut the groove with a laminate trimmer afterwards. The result will be to your liking as I have done the same myself.

I see that my earlier picture was not just "not to scale" but very misleading:- should have been:-

I wonder if you could make a "former" with several pieces of curved 3mm ply wood: fixed together with bits of scrap wood; then bend the board around it.

Thank you for all your suggestions but I'm abandoning the idea for now and I will let you know if I take it up again.