Not sure I'm ready for this - I've started building!

[arvidb]

Nice work!

What kind of screws did you use in the MDF? Did you pre-drill for the screws in the horisontal "support bars"? If so, any tip on how to get those holes straight? I guess the drill likes to wander when drilling into the edge of an MDF sheet?

Arvid

[Mike F]

Arvid,

The screws are called 'Confirmat screws' and are specifically designed for use in MDF. You need also to purchase the special bit that drills the pilot hole and the clearance hole in both pieces at the same time. The drill is stepped to accommodate this operation. The pieces of MDF are clamped together accurately before drilling through both, once drilled, the screws are inserted before the clamps are removed thus maintaining accuracy.

The screws have a very coarse, thin and sharp thread that anchors well in the MDF 'end grain' and to my knowledge are the only screws that do not allow the MDF to delaminate. Because you drill through both pieces at the same time, the drill bit does not wander in the 'end grain' as it is supported by the hole through the face of the outside piece.

Hope that makes sense,

Mike

[arvidb]

Mike, thanks, that makes perfect sense and will be great to know when my build starts! I've been wondering a bit on how to do this...

Arvid

[Mike F]

I don't know whether anyone else has had similar problems but when I tried to bolt everything together, the gantry did not want to slide at all freely and it seemed as though it was riding on square balls. Individually, the blocks move extremely freely and smoothly - so nothing wrong with the blocks. However, when bolted to the MDF carriers - square balls.

On checking the alignment of the blocks they were found to be slightly out of plane yet the MDF had not been touched. I figured that MDF is nowhere near accurate enough for my linear guides. Has anyone here used THK linear guides on an MDF machine? Have you too found a problem with alignment?

I have now machined some 50mm x 25mm x 4mm box section steel to mount the blocks on which will hopefully be accurate enough for the tolerances of the THK blocks.

MDF is a wonderful material but perhaps I was expecting too much of it.

Mike

[Graham S]

Is it worth doing the final tightening as you would on a cylinder head, just add a little on each screw and go around checking that it is not binding until all is tight and hopefully not sticking.

Graham

[Mike F]

Good point Graham.

I did in fact try adjusting the tightness of each screw and yes, there was a point at which it all rode smoothly but unfortunately, it was not holding the gantry at all well. One of the things I noticed was that after tightening the bolts reasonably well, I could go back a few minutes later and have to re-tighten them. It appears the MDF is compressing under the load and obviously needs some oversized washers on both sides to spread the load better.

I have concluded that if you need the highest accuracy, because of the nature of the running gear, you cannot make do with MDF. Hence the move to steel for gantry attachment.

Thanks for the reply.

[ger21]

MDF is actually pretty soft. Even with big washers it will compress easily. One thing you can do is to drill a counterbored hole, and epoxy in steel washers. But, the opposite side will also compress too. This will happen with any type of wood panels, and even with hardwoods to some extent.

[ger21]

Mike, I'm pretty interesed in the composite stuff you talked about. I've been reading a lot of composite fab info for the last few months, and would like to make a composite gantry machine. Can't wait to see how that goes for you.

[sol]

MDF shrinks and swells a lot over time with temp and humidity.
Recently I noticed how a once smooth MDF work surface had dimpled at every screw head; the wood had become thicker even though it was very well sealed with multiple layers of quality paint.
Yes, and I had the same problem with bearing tensioning...MDF has its place but an application of high precision isn't it.

[Mike F]

Gerry & Sol,

Thanks for your observations, it is surprising just how much MDF does move and expand with moisture. If allowed to, and exposed to high humidity, it can double in thickness quite easily and then becomes quite useless. I don't know whether the waterproof versions fare much better? Attached are photos of the replacement steel holders. I still have to face the edges and bolt on a shoulder piece for the blocks to sit against.

Gerry, composites is one of my main interests and I build competition model gliders with prepreg material. In fact I recently became World Champion in S8E which is rocket propelled, R/C gliders. The whole model only weighs 200g and launches to well over 1000' on a 40Ns motor!

I have to build the MDF version of the router first so I can use it to manufacture the patterns for the composite version. This is going to be a fairly lengthy project so I hope you have the staying power - me also

Mike

[ger21]

Originally Posted by Mike F I have to build the MDF version of the router first so I can use it to manufacture the patterns for the composite version. This is going to be a fairly lengthy project so I hope you have the staying power - me also Mike

I started my first router about 18 months ago. A lot of things have kept me from finishing it. My second router will be the composite one. I already have some THK and IKO rails for it, but it's probably 2 years away.

[Mike F]

Gerry,

You must be an early riser to respond to my post so quickly. It's only 12:40pm over here!!!

Mike