trimming 3mm off mdf templates

[Paddy Roxburgh]

I run a small dry dock repairing canal boats. A relatively frequent task is to cut out rotten metal sections and replace with new steel (yes this is a woodworking question). What I generally do is make templates for the shapes in 6mm mdf and then cut the steel out (usually 5 or 6mm) using the template with the plasma cutter. Now the plasma cutter cuts a line 3mm from the guide (in this scenario the template). If all the sides are straight it is a simple task to cut off 3mm from the edges with the track saw, infact one side does not even need trimming as I can put it up to a straight edge on the steel sheet.
However some times I need to cut a curve and cutting 3mm from the curved edge is a nuisance. I will generally do it as carefully as possible on the bandsaw and the sand the edge to pencil line. This works but is time consuming. My question is could this be done a better way, perhaps with a single point fence 3mm from the cutter on the router table?
Paddy

 

[Adam9453]

You could route a 3mm rebate into the edge with a bearing guided rebate bit, then use a bearing guided flush trim bit to take off the rest (bearing riding on the rebate edge). If that makes sense

 

[oakmitre]

There was an offer by Scott and Sargeant on here a few weeks ago for a CMT flexible router guide.
They are screwed to the work and enable you to trim route to a curved shape, there are videos on youtube.
They are not cheap, but if you a running a commercial operation, then they may save you time.

 

[RobinBHM]

Could you use a bearing guided router bit that has a bearing to machine a rebate 3mm wide.

Then use a small top bearing flushing bit to trim the remainder.

6mm mdf maybe too thin to work as if you machine off half thickness in each pass which will only be a 3mm land for the bearing to run on. It may work if you are gentle, or could you go to 9mm mdf?

Oops! Posts crossed....

 

[Homers double]

http://www.axminster.co.uk/fisch-flexi-curve-ax783040

Paddy, Axminster sell these flexible curves

 

[Setch]

Depending on how much you want to spend versus how long it takes, you can easily remove 1.5mm using most basic router bit sets.

Remove the bearing from the flush trim cutter, and swap it for the smaller diameter bearing from either the bevel or rounder cutters. This will route a 1.5mm rebate around your pattern, which can be followed up with a pattern bit to remove the step in the sides. Repeat the process a second time and you'll have your 3mm reduction.

 

[MattRoberts]

http://www.axminster.co.uk/fisch-flexi-curve-ax783040

Paddy, Axminster sell these flexible curves

Those look awesome! Shame they're so expensive though

 

[No skills]

Why not buy a spare shroud for the plasma and modify that to give you whatever offset you want.

 

[Droogs]

Having had a quick think could you not use a router insert collar with a straight cutter of known sizes that would allow you then make the template at a reduced size which you could then run the router around. The sticky out bit a the bottom of the router collar will follow the template and the cutter would have it's od cut the corresponding shape at the right size. There's a little bit of maths involved but shouldn[t be too hard

 

[Paddy Roxburgh]

Thanks everyone for the ideas.
I think the rebate followed by the flush trim sounds the most promising, I'm going to give it a go (I'll need to buy a 3mm rebate bit). The curvy thing looks like an interesting bit of kit, useful for some other tasks.
No Skills, I can't modify the plasma so it gives no offset, indeed it is the tip not the shroud that runs against the guide so the offset is as small as possible. but the hole in the tip can't be right at the edge of the tip. It would be fantastic if it were possible and I could put the guide down and cut right next to it (like a track saw does).
Droogs, do you mean making 2 templates with the second one being the one to cut from?
Paddy

 

[Droogs]

hi paddy yeah

 

[Eric The Viking]

How are you making your templates in the first place? I'm guessing you get someone to hold the MDF over a hole and trace round it, or go round the outside of a rotten plate.

There's the old trick when scribing of using a washer that the pencil lead sits in, which keeps the lead a set distance away from an uneven wall. I'd guess that yours are 3D problems to start with, so probably harder, but can you do something similar to get the shape-minus-an-offset? If you have to freehand the shape on a bandsaw first off, does it matter what you mark with, as long as it's clear to see for cutting the MDF?

Would it be easier to make a guide for the torch, that gives an offset from an edge? You could make your templates exact, cut the steel once, then trim 3mm with the guide running on the fresh edge. That way you don't destroy the original template, so can easily check the patch. You'd have to freehand a tiny bit, at sharp corners and the end of the trim cut.

With all of the trimming ideas there's one more question: what accuracy do you need for welding 4mm or 6mm plate? How much wobble in the line is acceptable?

E.

[edit for punkuashun]

 

[SteveF]

how are you making the templates in the first place?
if it is from the scrap you cut out, can you not just use a bearing 6mm smaller than the router cutter
this would give you the 3mm
just seems strange not to start 3mm smaller in the first place

Steve

 

[No skills]

+1 for Steve and Eric, if you can make the template 3mm smaller when making/marking it out then problem solved - but life's not generally that easy.

Eric

Welding clean steel plate that thick is relatively easy even with some bigger than desired gaps - but the more weld you have to put in the more heat your putting in the metal and a bigger risk of distortion (which may or may not be a problem for what Paddy's doing (?)) Plus it's adding time to the job...

 

[Eric The Viking]

+1 for Steve and Eric, if you can make the template 3mm smaller when making/marking it out then problem solved - but life's not generally that easy.

Eric

Welding clean steel plate that thick is relatively easy even with some bigger than desired gaps - but the more weld you have to put in the more heat your putting in the metal and a bigger risk of distortion (which may or may not be a problem for what Paddy's doing (?)) Plus it's adding time to the job...

Yes. I do a bit of MIG but I'm not very good. I'm guessing most of this is stick-welding, which I'm unfamiliar with and I think usually puts more heat in.

To Paddy: do you bevel the edges beforehand, or what? 6mm sounds like "fun" of the worst kind (although I really don't enjoy car bodywork stuff!).

E.

 

[Bright-Spark]

1 min 09 seconds in

https://www.youtube.com/watch?v=VeDnFTmqmI8

Bright-Spark

 

[dickm]

Alternative version of Droogs' suggestion, with no maths. Mark out and cut the template, then cut a second version roughly to size. Pin together and using a bearing guided rebate cutter with appropriate bearing to give a 3mm rebate, cut the second version. Simples.