[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.
http://www.cnczone.com/forums/671212-post176.html

[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

[revwarguy]

FWIW, source of less expensive T track:

H0878 36" Aluminum T-Slot Track

Just FYI - its what I used and works well. Haven't had any trouble sliding things on or off using them. Slots vacuum right up.

[ger21]

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

No, I surface the top after it's finished. After I do it a few more times, but before I get too clse to the nuts, I'll add another 1/2" to the top, and have the machine put the holes in over the nuts.

[StuartC]

How 'bout using a biscuit jointer on the edge of workpiece and using z clips (like for holding a table top on) inserted in the slot and screw the clip the table.

[CNC Lurker]

Instead of T-nuts, you ought to try propell nuts (also called hurricane nuts). Instead of having "spikes" on the flat part to bite into the wood, it has "fins" (for lack of a better word) along the barrel that bite into the hole in the wood.

I got some from Amazon, but here is the direct source: 1/4"-20 Hurricane Nuts

Fastenal also carries some 1/4-20 x 1/2 Propell Nut Plain #0141139

Edit:
Forgot to add that if you don't have room for a straight edge/fence to butt up against, you could use some use dowel pins. Simply peck-drill holes along an axis with the CNC stick a pin in there. The dowel pin doesn't have to be the same height as the the wood you are clamping. For example, for 3/4" wood, only 1/2" or whatever needs to stick out to hold the part so that it would be 1/4" from the top of your part.

If one is worried about hitting a dowel pin with the router bit, you could use a wooden dowel rod, but they aren't dimensionally consistent.

1/4" x 1" Bright Finish Alloy Steel Standard Dowel Pin #26753

[RossMosh]

Fences with t-slots are the best way to go. Stick the material against the fence and use the t-slot to secure it the rest of the way. When needed, horizontal toggle clamps to keep the piece from lifting up.

I think it's also necessary to realize spoil boards are just that, spoil boards. When necessary, just screw the darn piece down.

[Treischl]

Here is why I use T-Slots:



Forget the torsion box, that is just what I use because I need one for the way I run my machine.

If you notice, the t-slot track sits well below the surface of the material in between them. You can use whatever you like, MDF, plywood, etc. What is important is that this material can be used as your spoil board since you can easily replace it. Furthermore, you only have to replace the sections that are really chewed up.

Not shown in the picture is a reference edge that is clamped into the t-slots. This can also be replaced easily.

I have tried several other methods, t-nuts, milling my own t-slots, etc. The milling of my own t-slots was probably the worst one. Once I did it, I had to protect that piece of material so I did not have to do it all over again.

Side cam clamps? Uh huh. Tried those too. Worked good until I was cutting one day with an spiral end mill, and you guessed it, the end mill managed to suck the piece right up out of the clamps. I don't view cam clamps as POSITIVE clamping anymore. They do work good with straight flute bits.

[i-make-robots]

I see. My table is very not-flat even before I try to make the hold down (1-2mm z difference over 4'). It's a little difficult to make the holes for the t nuts like that.

Was your surface already nearly perfect?
Should I level it?
What's the best way to find the lowest point on the table?

[ger21]

I see. My table is very not-flat even before I try to make the hold down (1-2mm z difference over 4'). It's a little difficult to make the holes for the t nuts like that.

Was your surface already nearly perfect?
Should I level it?
What's the best way to find the lowest point on the table?

Yes, my torsion box was about as flat as possible without surfacing it.
If yours is not flat, then it's OK to surface it first.
Use a straitedge to find the high and low spots. I'd start at the highest point, and remove .5-1mm at a time, until the whole table is flat.


Everyone has there own methods of working, and using what works best for you is the way to go. I personally don't like the high clamps like Treischl uses, as they can interfere with dust collection, and require a much higher rapid plane for clearance.

For doing repetitive parts, with my CAM setup, I can change parts in about 5 seconds. And with fences, I rarely ever have to find X and Y zero positions.

The perfect table for me would be a combination vacuum table with pop up locating pins, that also had threaded inserts. Vacuum is great, but some parts are better bolted down.