View Full Version : I want to be straight not bent!

04-11-2006, 02:46 PM
Hi all,

I have a cutting problem and I'm looking for advice.

I've been cutting some 8ft lengths of 9mm 8 x 4ft ply into parts with the intention of butting the pieces back together. But I'm finding that although when I rejoining the edges back together they butt up straight at the ends, there is a 1mm gap in the middle.

I'm running the alpha with a 3hp spindle and the latest shopbot control software. I have noticed something similar in the past but it wasn't an issue then.

My part file is saved as "arcs inches"; however I do all my drawings in metric.

Any thoughts ... cheers robert

04-11-2006, 03:10 PM
Hi Robert, I guess you have to start off by checking if your rails are straight. And then check that the v-rollers seat right down everywhere along the length of the rail.

04-11-2006, 04:17 PM
Hi Robert

You can find out if your rails or V-rollers are out by laying
2 pieces of ply face to face then trim both edges together parallel to the X rail, and then open them like a book … if there is a gap your rails are out by half the gap dimension. The other reason is the stress and tension in the plywood is released when you cut the ply (especially that Chinese stuff) the only way around that is to rough cut them oversize and then trim them. We get the same problem occasionally with 18 and 22mm MDF when we cut rips on the panel saw.

Thanks for the email…I am trying to schedule the Euro camp at the moment.



04-11-2006, 05:05 PM
Many thanks.... I'll make the test first thing in the morning....

Paul...... I'm hoping we can camp together in June!!!

04-12-2006, 01:07 AM
If you cut one piece with the cut away from you, and another with the cut towards you, do they now fit?
If you cut down the centre of a piece, I presume the two bits join back together ok.
If they don't, then it has to be in the material, I think.
If you have a long strip of something more stable, like MDF, try cutting two pieces of that, see what happens.

"Sounds as though it's going to be a gay old time at the camp."


04-12-2006, 02:55 AM
....and he says he wants to be straight?

04-12-2006, 10:21 AM
Thanks Mike,

If you come and camp at mine, I'll make sure you get a Mince Pie...

I've been cutting today and the same problem has not occurred.

I've checked my parts wizard files and I can't find any points that shouldn't be there.

Interestingly I noticed that if you save the toolpath as shopbot inches instead of arch inches the file size 10 to 14 times bigger in memory.

04-12-2006, 11:07 AM
Another bloke working in metric and then converting to inches for the ShopBot. Amazing!

The only other SB I saw in Cape Town, run by Gakiem, was also running in inches.

Paul, how metric are you?

04-12-2006, 12:04 PM
Hi Gerald

I'm totally METRIC....it is SO easy

Robert...Was it the plywood giving you the problem


04-12-2006, 01:17 PM
Paul is pretty much the ONLY reason that the Virtual Tools work correctly in metric...he's the poor guy that does the testing for me and tells me all the things that I've messed up!

04-12-2006, 01:32 PM
Thanks Bill

I have a theory that it would be easier to write a programme in metric and then revert to inches than the other way round...things seem to go wrong when it tries to calculate the sheet sizes.

I lied on the totally metric thing... I use inch collets.


04-12-2006, 01:45 PM
I don't really understand why the 2 different systems of programming inch vs metric? Can't we all just use "units" and apply a scale factor to suit whatever system we are working in? Like Excel, or AutoCAD, or a bookkeeping package, or most other number-crunching packages?

04-12-2006, 02:48 PM
Hi Gerald,

I think its because of my age that I swap all the time backward and forward to metric and imperial when measureing and drawing.

I do want to totally swap over to metric at somepoint, but a previous attempt failed so I didn't bother after that.

Hi paul... I think It could have been the wood... At the camp this year I'm hoping to pick everyones brains. I learnt so much at last years camp.