Favorite hold downs?

[Carl Beckett]

I am still determining my 'goto' for different clamping configurations on the CNC.

Considering a vacuum table (DIY from a 3 stage central vac motor I happen to have).

Then the usual T bolts. Sometimes these stick higher than would be nice, and always a concern of running the spindle into them on start or start, etc. All manageable as I come up the learning curve.

What other solutions do you all like?

I see these Kreg clamps... might be useful: https://www.rockler.com/kreg-in-line...SABEgJqJPD_BwE

And other traditional dog hole quick clamps I have but havent tried. Not quite suited for the CNC table.

I have an Aluminum T-slot extrusion base and a spoil board sectioned so a T bolt can go through and slide from one end to the other. Spoil board with dog holes on top of that.

I could put some inserts in the spoil board - but it would be fixed location.

What are your favorites?

[Kevin Jenness]

A vacuum table is great for pieces large enough to develop sufficient clamping force. I have a home-made system on my 4'x8' table using three 7 amp central vac motors and three zones. It only pulls about 7"Hg but works pretty well. If not using zones you can concentrate the hold-down force by masking off unused table area.

I bought some t-track but haven't gotten around to installing it. I often drive screws into my ultralite mdf spoilboard, sometimes combined with blocks and wedges. Blue tape and ca glue are quite useful. There are composite gun nails available (Raptor?). Lee Valley has some low profile dogs you might look at, which could perhaps be used with t-track.

[Jim Becker]

Vacuum table is good for big sheets of stuff, but not very effective for small stuff because of the laws of physics. Small stuff and vacuum requires fixtures and a different kind of vacuum pump setup to do a good job. This is why I didn't bother with vacuum on my 4x4 machine. I use a combination of clamps in tee tracks, screws and fixtures to hold materials. I may get one of the pneumatic guns that shoot composite fasteners to at least partially replace using screws as that reduces risk over steel being hit by tooling if a human mistake comes into play. My clamps are a combination of a few commercial aluminum clamps plus a whole bunch of shop made wood and PVC clamps...PVC scraps are a great source of clamping material.

[Keith Outten]

I make these from PVC scraps. No worries about them being hit by the router bit they are so soft it doesn't matter. If you need a custom shape or size cut them on your band saw in just a couple of minutes.

[Grant Wilkinson]

Not to take this off topic, but can you please tell me, Jim, can you shoot composite nails with an "ordinary" gun? The guns sold specifically for these fasteners are very pricey, and I have both a 18 and a 16 gauge ordinary gun.

[John TenEyck]

On large sheets I use a combination of shop-built clamps sliding in T tracks I milled into the MDF spoil board and screws in the waste area. For small parts, however, like the little gears and bushings for clocks, I use blue tape and CA glue or double stick tape for really small parts. It uses quite a bit of consumables, but I haven't found a better way to hold really small parts. Of course you have to go a little slower when milling so the forces don't overcome the holding power of the tape. I have no clue how production shops go about holding these little parts. You can use tabs to keep the parts integral with the sheet, but they are a true pain to deal with on really small parts.

John

[Jim Becker]

Not to take this off topic, but can you please tell me, Jim, can you shoot composite nails with an "ordinary" gun? The guns sold specifically for these fasteners are very pricey, and I have both a 18 and a 16 gauge ordinary gun.

No...it's a special gun which is why I haven't made that move to-date. If I was doing production, no question, I'd have bought it long ago. But for the limited amount of work I do, at least to-date, I'd have to justify the cost of a specialized tool relative to the amount it would get used.

[Kevin Jenness]

Cnc work and turning are similar in that workholding is half the battle. When machining takes place within the outline of a largeish piece, clamps can be used. The best are low profile ones like Keith's, made of material that won't damage the tools. Anything tall like a toggle clamp needs to be avoided when designing the toolpath, and large z-clearances can add time to the operation. Placement of screws and metal nails needs careful attention.

Vacuum is best for parts requiring edge work. High volume through the table works on pieces down to 100 sq ins or so, while smaller parts can be held on a fixture fastened to the spoilboard and connected to a higher level (>20"HG) pump and well sealed to the work.

As John pointed out, when cutting small parts balance must be maintained between holding and machining power. Some parts like shell can be glued to a larger backer for cutting and the glue then dissolved. One method doesn't work in all cases.

The Kreg side clamps you linked to may work well in solid material. I suspect the required holes would wallow out in my soft vacuum spoilboard.

[Cary Falk]

I have a bunch of solutions. I use double sided tape when I can. I sometimes mount a larger board to my table with the standard t-track clamps and then route a pocket the size of my piece then secure with double stick tape. I have a 3D printer, so I print all kinds of hold downs. I have brass toilet bolts and even some nylon bolts in case I hit them with a bit. I have these from Rockler which work in certain situations.
https://www.rockler.com/rockler-stop...t-track-system
https://www.rockler.com/rockler-t-tr...line-cam-clamp
https://www.rockler.com/rockler-auto-lock-t-track-clamp

I also have a mini vacuum puck system that I haven't used yet similar to this.
https://www.rockler.com/axiom-avk500...-hold-down-kit

[Carl Beckett]

Thanks for all the suggestions.

Some home made cams are in order. Those rockler line cam units are interesting. I will start out with something home made to learn from (a number of diy designs out there).

Those mini vacuum pucks are questionable - in that one reviewer noted the piece is always floating on the gasket. I can see that being a no starter for me - if the gasket compressed to where the piece located on the housing maybe, but as noted here it would all need to be sized to accommodate the desired holding force.

For now it will be 'whatever works for the task at hand' until I learn more what gets commonly used for me and go from there.

[Jim Becker]

Gasket material matters on fixtures for CNC use because of what you mention. "Pucks" or similar for hand-work are less concerning if they use a software gasket material, but fixtures for CNC must use a stiffer material for the seal so that workpiece height is maintained.

[Bob Falk]

Hold-downs would be much easier if we didn't have to worry as much about our tools running into them. I have always thought that a big improvement to Vcarve would be the ability to create zones (no fly zones?) around clamps that the cutting tool would avoid. As it is, the tool takes the most efficient route from A to B without the consideration of obstacles (except in the Z). IMHO, the addition of XY avoidance would complement the current Z avoidance.

[Alex Zeller]

I do a combination of what Keith does (but with scraps of wood, not PVC) to hold the workpiece down and blocks of wood on each side so the workpiece can't slide. Other times I make the workpiece larger than needed and screw it down on extra where the bit isn't going to go. Then I let the cnc cut the outside edge of the project with tabs to keep it attached to the outside edge.

[Carl Beckett]

After running some more parts, I am thinking a two step approach:

1) Cam mechanism to hold the part against a reference. This would be X, and Y reference stops set in whatever way the spoil board accommodates. Right now those are sliding guides that tighten in place. So I will try a couple 3D printed cam design to push and hold the piece against this X,Y reference stops

2) Traditional T-Slot type around the perimeter to hold and press in Z. Similar to what many of you are using. The vertical clamping jig does have a camming hold down capability but as you all already know things can stick up in Z and cause crashing. It might work in certain situations, but those scrap made hold downs are likely used by many for a reason - they work.

Will post a couple pics once next iteration in place.

[Jim Becker]

Relative to cams, wedges can also be employed in a similar manner. Do note that both of these methods provide good support to contain lateral movement, but are not as effective with holding the workpiece down to the table. Bit bites with up-cut tooling can sometimes pull the material up off the table, so take care with considering what you are actually cutting and how.