More CNC questions

Carol Reed

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I know Leo made leg extenders for his machine to get the table at a height he likes. I'd like to make my cabinet the right height to begin with. And I know we had a discussion a long time ago about the composition of the table. But I still have questions.

So the base is made up of the 8040 extrusions. I like the idea of t-slots for work hold downs but into what kind of material? I have a sheet of 3/4" HDPE that is big enough, but maybe that stuff is too slippery. Leo said his table has some sort of rubber surface. I remember that fine sawdust made the table even more slippery which can made holding work down interesting.

And should there be a piece of say 1/2"-3/4" MDF (or BB plywood) between the extrusions and the hold down sheet for maybe rigidity's sake?

I need to make this decision now because I have to calculate the height of the lathe table so the lathe is neither too low or too high. That surface fits into a dado on a vertical divider and needs to be milled before construction.

Stuck here until I make a decision.

And now I am thinking about a vacuum table inserted in here someplace.

Thoughts, guys?
 
I have not done anything yet whereas the flatness of the table matters, so the rubbery stuff for now seems to be OK

I have used MDF for several years on my other machine, but is has an Aluminum table under it that I surfaced flat and I bolt the MDF to it.

The MDF is a sacrificial surface. I just resurface it and replace it as needed.

I also polyurethaned the MDF - so slippery would be even worst, but I didn't have problems.
 
Other folks' pictures of their build show a sheet that butts against the side rails which seems like it would add rigidity to the whole table. So my thought was to do that with a piece of 3/4" MDF, surface flat, and seal against moisture. Then add the 3/4" HDPE with the crossed t-slots like Brent did. There would also be a grid of tee-nuts embedded from the back side for hold downs. Likely that surface would have to be surfaced flat as well. Work pieces wold be fastened to a waste board that is held down on the HDPE table. Vacuum pucks could be daisy chained for vacuum hold down when that makes sense.

I think this is the way to go.

Thanks.
 
I gotta say, as much as I like the looks of the crossed t slots, I'm starting to wonder if maybe just having a bunch of them running the long ways might be a little more practical?
 
Wondering the same thing, Brent. Function over aesthetics. Thanks for the insight. Been browsing the web for ideas and as usual have seen I am overthinking this. Did see some some really cool nose clamps to make though!
 
Sorry. But it is so simple I didn't think I needed one. I'll make a SU drawing and you can go from there. An evening project for me.
 
I've been wanting to come up with some lower profile clamps than what I'm currently using.
Actually, I've switched over for the most part to using bolts and the t-track with a wastboard.
 
OK. I got the idea off an Instructables article about tee slot tables. (I think.)

Think low profile and no protruding hardware. Simply a 'bar' (I will use 3/4" thick Delrin because I think I have some.) about 1 1/2" wide, maybe 4-6 inches long. Mill a rabbeted through dado in the top surface for 2/3's of the length offset to one side. Mill a rabbet on the bottom side of the ends with a 1/8" tongue. Mill a slot 1/8" wide slot in your spoilboards. Insert a buttonhead machine screw and washer into the through dado. It grabs the nut in the tee slot track wherever you need for it to be. Insert the tongue into the slot on the spoilboards and fasten. Can even use a biscuit slot cutter for the spoilboards. Just make sure the nose clamp's tongue fits into it.

Still need a picture?

Also made a decision about the table. Because I have a sheet of 3/4" HDPE, it will be the base table directly on top of the extrusions. I will mill dadoes to receive the tee slot tracks (http://www.leevalley.com/US/hardware/page.aspx?p=52400&cat=3,43576,61994,52400) 6" apart from front to back. They will be recessed 1/16".

Now all I need is another Lee Valley free shipping event!
 
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Brent - under what conditions have you found trouble with the X style? I'm genuinely curious because i realize that other folks do things differently and maybe my methods need improvement...

So far, the only time I find myself head scratching a little is when i have a long, narrow workpiece (like say 4x24) -- it really can only go on the table and be reached by the side clamps in a few locations but that hasn't been a problem for me. Since I always home my machine, locating the part's XY zero isn't affected by where things are placed.

I do put down a spoil board when i'm cutting through, which pretty much eliminates the X track so i have to either screw down or double-stick tape things.

Totally not defending my design... truly curious about what I could be doing better :D
 
Brent - under what conditions have you found trouble with the X style?

I think the X design works really well if you put things in the center of the machine.

If I had space all around my machine, it would be no issue, but I've got my machine kind of in the corner of the shop. So I'm finding that I'm working more in a certain quadrant of the machine.

On my machine I have the x axis as the long axis, and the y axis going across the gantry. I'm finding I tend to work more towards the right hand part of the X axis and the 'bottom' of the right axis. So think the right hand near corner.

It's just easier to reach than going all the way to the center of the machine, particularly for the smaller projects I've been working on.

In all honesty, it's working fine though. I'm using a couple of bolts in the t-track to hold down a temp spoil board for most of my projects, as you can see in the picture. This spoil board was a little large, I was using it to cut some interlocking dividers for a spice cabinet.

20160130_092930.jpg
 
I don't like the idea about always working the same area of the machine. All the wear is always located in the same area.

My old machine did not have any sort of hold down. Well is had a grid of 1/4-20 threaded holes but I covered all the holes with the spoilboard.

I screwed directly into the spoil board most of the time. A little bit of a pain, but workable. I made it a point to locate the working area in different places on purpose.

My new machine has many rows of "T" slots, front to back, about 2" or so on center across the width. So far I am really liking that. This is what I think Carol is planning.

I recently put a 1/2 sheet of 1/2" plywood and cut out parts to make a big "V" block. The "T" slots did the trick nicely. I did slide a 3/16 piece of ply under so I could cut through.

There are multitudes of options that can be adopted to "T" slot clamping.
 
Yep, the vacuum stuff makes a whole lot of sense to me too. I can absolutely see the usefulness of the small individual setups.

I plan to someday make something but it will not be big.

A small portable unit can be clamped down to the table.
 
I started out with some t-slots on my table, seems like I could never get things to fit at the distance they were apart. So I've gone to Leo's method of just screwing things down, have yet to have anything pop loose like it did with the t-slots.

We did discuss vacuum hold downs the other day at the CNC SIG meeting. One gentleman was building a vacuum table and plans to use a shop vac for the vacuum source. He mentioned that if anyone was considering this, to make sure their vacuum motor had it's own cooling fan/source as many of the consumer versions use the suction air to also cool the motor. So when using them with vacuum table, they won't get any airflow across the motor and it quickly burns up.

As for using vacuum pumps, it was mentioned that they only work if you don't cut through into the area being held by the suction pod. If you do, they quickly (almost immediately) loose suction and break loose. However, if you're cutting work and know the area won't be cut through, they work great.

If using MDF for the suction table, be sure to seal it as it's very porous and usually won't work if not sealed.
 
Fortunately I have more than a few years of experience with vacuum clamping. Developed the syllabus for and taught a class on making vacuum systems and jigs for a number of years.

Also fortunately, I have a spare vacuum pump and a box full of fittings and gauges.

All the things Darren shared about using a shop vac are true. Also true that a break in the vacuum of a small vacuum pump equals instant release.

Solution is, of course, to plan ahead and use this technology when it is appropriate and something else when it is not. Screwing directly to the table raised havoc with keeping the table flat, in my experience, from former CNC use.

Decision to incorporate a t-slot table came from innumerable YouTube's and researching the big machines and pro applications. A t-slot table does not rule out vacuum, especially the use of vacuum pucks.

We will see where this goes and I will share my reasoning and choices as I go. This stuff is kind of fun!
 
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