hold down suggestions, please

[i-make-robots]

Hi, CNCZone.

Sorry for the confused nature of this post. I have several issues that need work and I'd really appreciate it if you could lend me the benefit of your experience.

I've got a work surface that's 8'x52"'x3/4" MDF. The surface is the top of a torsion box. The outer 2" on each side are occupied by the rails & chain drive. The torsion box sits atop a wooden frame. Because of the chains & box I can't clamp work around the outside edge and I can't put t-nuts from underneath.

Wood I get from my supplier isn't always dead flat. I'm pretty sure my table is sagging, too - the legs aren't directly under the torsion box because I thought if I made them wider that would give me storage space for large sheets under the machine.

Please help me figure out:

a) how to hold work pieces steady
b) how to stop the sag & properly level the surface

I have a 1/2" bit I can use as a fly cutter to level the surface, once I'm sure it isn't going to move again. I'd like to change the frame to hold the machine better but I'd need a lot of help to move the torsion box (it weighs a hell of a lot)

I've tried putting threaded insert into the table top but they pull out over time, leaving me with a mess to putty shut. I tried weights to hold pieces down. That worked pretty well but fails hard when it gets in the way of the gantry.

Suggestions? My budget is next to nothing, I invested a lot to get this thing and didn't plan for all the delays.

Thank you!

[ger21]

Put T nut's into another sheet of MDF and screw that to your torsion box. That's what I did.
Another First Router

[i-make-robots]

Nice! I'm confused by one thing: why drill 1.2" into a 1/2" sheet?

[ger21]

There's a 3/4" sheet below the 1/2" sheet, and this allows the bolts to go below the t-nuts. This lets me use the same length bolts for different thickness materials, as I have an extra 3/4" to play with.

[i-make-robots]

Oh so you planned it such that when you flipped the board over (with the t-nuts underneath) the holes would all line up? Nice.

I have no room on the edges to put y=0 fences unless I sacrifice some of the envelope. If it were you, would you give up some of the envelope to add a fence? How would you go about it?

I see the fence you use has horizontal slots. Is this for alignment adjustment, or did it serve some other purpose?

Thank you so much for all the help! I just read the entire thread for your CNC build. I'm so dumb I don't even know how to make money off my machine yet, I just knew I had to get one. :T

[jharvey407]

Another option is a T-slot or T-track table. I have tried both.

For a T-slot table simply purchase a T-slot router bit and route T-slots in a spoilboard, typically a high density plastic, that is screwed to your table.

Fot a T-track table, purchase T-tracks, such as;

Buy INCRA T-Track, 48" length at Woodcraft.com

Screw the T-tracks to the table and fill the spaces inbetween with plastic, wood or MDF that is thicker than the T-tracks are tall.

[i-make-robots]

It seems to me the slots would get really annoying when they catch on material being slid on/off the table and adding new t bolts would mean going back and forth a lot.

Having tried both, which do you prefer to use?

[ger21]

I see the fence you use has horizontal slots. Is this for alignment adjustment, or did it serve some other purpose?

I use the machine to route the edge of the fence straight. Every now and then, I loosen the bolts, slide it over a little, and reroute it, to make sure it's exactly zero.


I can't tell you how you should use your machine. Can you give up any usable space? I don't know. But, you can always remove it if you need more room.

[JC50]

Maybe you could use T-nuts secured from the bottom of you spoil board. Seems like they would hold better than wood inserts. I personally don't use them, but it is another alternative.

[Treischl]

If you use Ger21's method of putting in t-nuts from underneath you have to make sure that you control the depth of the c'bore. Otherwise, over time the t-nut can back out and then you have a problem. I used this method for a while, I was not fond of it since the holes tended to fill up with dust and chips.

Another method that works and a lot of people use is t-tracks. You screw them to the top and then fill the space the space between them with either mdf or plywood. I am not a fan of routing slots to put them in. The reason is that if you use 3/4 material to fill in between them, this material acts as your spoil board and it is easy to replace just the sections that need replacing.

[i-make-robots]

ger21, did you level the table before you screwed down the board for the t-nuts?

[frige]

I'm going to have to do something soon myself.

2 things I am considering:

1) 1/4 inch holes, and using cam clamps.
2) keyhole like slots (two big holes, connected by a smaller channel). head of a bolt goes in the hole, and slides under the channel - wing nut on top.

either way, I have to plan to avoid the torsion box underneath.

reading the thread got me thinking, maybe a 1 inch piece of HDPE with T-slot routed would be slick.

-frige