MDF Expansion or Haunted Jig???

[eaglesplsh]

Why does this type of thing always happen on a Saturday when I'm under a deadline???


About 3 weeks ago I cut a jig for machining some parts - it's a 1'x8' piece of 3/4 MDF that I clamp on my spoilboard. The jig had holes cut into it, spaced evenly along the 8' length, with T-nuts inserted into each of the holes.

The parts I'm making get matching holes drilled through them by the bot before I bolt them to the jig (using the T-nuts in the jig) for additional machining.

I made an initial run of parts at the same time that I made the jig. Everything went fine - all holes/bolts lined up without problems. Today I went to make another run of parts and found that the holes in the jig don't line up with holes drilled by the bot this afternoon.

I measured today's parts/holes and confirmed that they are accurate and match my drawing. The confusing part is that I used the same drawing to create the jig 3 weeks ago. When I measure the jig it appears that it has grown by 0.150" along the 8' length.

Is this possible with humidity/temperature changes? We've all seen our MDF spoilboards grow after being machined flat. I had a scrap of MDF sitting outside the shop for a couple months - it started out 3/4" thick and ended up over 2" thick after soaking up rain water. But, I've never had another situation like this one where I would have noticed expansion in length/width.

I don't remember measuring the jig when it was first created, so I can't confirm this theory, but our machine (PRS Alpha) is only a few months old and from the beginning every time I've measured parts the cuts have been accurate. If there was something going on when I made the jig/initial run of parts, it has fixed itself without my help.

We've had a lot of wet weather in New York this fall. Average temperature has dropped 20 degrees since I made the jig. Perhaps this piece of MDF is just a haunted leftover from Halloween.

[myxpykalix]

Have you upgraded your control software in between the two cutting times?

[harryball]

I always coat my jigs with shellac. I found if I didn't they would change shapes, swell and generally cause cutting problems. The largest jigs I have made at 2' by 4'. I have had parts not fit into a hole the use to fit in. Since using shellac I've not had that problem. Never checked Len and width to see if it changed.

Good idea to still check out your machine but I would not be surprised that is is the mdf.

/RB

[eaglesplsh]

We've been running the same control software since we put the machine together - version 3.6.5

We're using a 1 year old Dell desktop with Vista Home and haven't had any problems or made any changes to the computer/software.

I just remembered one thing... a few weeks ago we experienced a repeating error. I don't remember the error message, but it would stop the program in mid cut at random places within several cut files. A quick call to SB tech support helped us find the problem - the motor cables were longer than I needed to reach the control box and I had several extra ft of cable wrapped into a neat coil before the wires entered the box. Now I have the wires laid out in a big loose loop. The signal/noise problem hasn't happened since.

So that IS one change that I've made between the time that I built the jig and when I went to use it yesterday.

[Gary Campbell]

Russ...
It is very likely that the MDF expanded that much. I have much smaller pieces move more than that. As a rule, MDF is one of the least humidity stable materials out there, but when flat and veneered with equal materials, and then sealed, stays flat fairly well. This is why many cabinetmakers use it.

Gary

[eaglesplsh]

I have to admit that this problem had me stumped for several hours before I came up with the expansion theory.

The holes are laid out about every 6". If you measure the distance between any 2 holes, they're consistently about 0.015" too far apart. That's fine for aligning the holes for the first several feet, but as you go farther down the jig that difference starts to add up.

If I had been missing steps or experiencing a backlash problem then the holes would be closer together, but these are farther apart. I've never heard of a situation where you gained extra steps.

The temperature here has gone down 20 degrees, so that should cause things to shrink instead of expand. And, it wasn't raining yesterday - humidity was probably normal.

The thing that got me thinking about expansion was the 3/4" MDF that soaked up enough water to become 2" thick. That piece never went back to 3/4" when it dried, so I was thinking that it might be plausible that my jig would grow on a rainy day and then keep that extra length forever.

This is all pure speculation on my part... I wouldn't be surprised if I've missed the obvious and have it all wrong.

[navigator7]

Russ,
I worked at a shipyard years ago.
We were under great pressure to cut a hole in a deck and replace a huge hydraulic power pack.
The hole was about 10 x 20' square.
The temp was hot as could be.
We cut the deck, installed the hydie pack and called it a day.
The next morning, things were cooler, we picked up where we left off.
The rigger hooked up the old deck and we were to weld it back in place.
The deck dropped in place and it was nearly 2" shorter in all directions than when we cut it.
2" is a pretty big fillet weld!
I forgot what the rush was...but the boss sent a bunch of us out to look for metal to make up the difference.
Not eager to drag flat bar a long way I processed our dilemma and realized it was the heat. I told my boss and he immediately snapped to, brought all efforts to a stop and we went on to something else.
As the heat of the day returned, we hoisted the deck panel once again and sure enough! Fit like a glove!
This was Seattle where extremes in temps is rare and so is experience with growing or shrinking metal.

I think your jig is not haunted but subject to natural forces.
Why not raise the temperatures 20º???