Hold Down solution question?

[GalaticDan]

Hi,

The surface on my CNC router is just 1/2" aluminium plate, No T-Slots. While I've been building it, I was using G-Clamps to hold wood down for test cutting purposes. Side pannels will be mounted at the weekend with luck so I can't use the G-Clamps anymore. I was thinking about drilling multiple holes in a matrix formation (for metric M6, 1mm pitch theads). The alu. used is 6082, a good machining grade.

Now I have to make a decision about the tapped holes, as these will be used repeatedly for mounting work, should I invest in a helicoil kit? I'm guess around 40 holes to give me mounting freedom in the future. I'm asking because I read somewhere?? that Alu. was a poor material for threads, while the machine is built on threaded components, these don't really ever get undone, however the mounting tapped holes is a different story as it's likely each job will require a different bolt down combination. Do you think the theads will be destroyed quickly?

Thanks in advance,

Dan

[Geof]

Aluminum is very poor for threads that will be frequently released and tightened so you should us some type of insert. However, helicoil is not much better. There are solid thread inserts that have a standard thread on the OD and ID; 3/8-16 inside has 9/16-NF outside. These are better because the load is spread over a much larger thread area in the aluminum and they can be replaced if the inner thread wears or strips.

[project5k]

can you get to the back side of this? if so, what about drilling the holes all the waw through, and using tnuts on the back side, you could hot glue them in or something so that they wont fall out, but then they would be easy to replace.. and cheap too...

[--colin--]

Aluminum will hold threads just fine as long as you don't overtighten them. If you happen to strip one out, then install a helicoil insert. My router has a 1/2 inch Al tooling plate base with a matrix of 1/4-20 threaded holes on 3 inch centers. I've been using it for several years and never stripped a thread and don't expect I ever will. Note that many of the components built into the router are also threaded into aluminum - it's very common practice.

--Colin--
www.bdmodeling.com

[jerber]

I did something like project5k suggested.
I bored 49 6.5 mm holes in a 30mm grid (It's a small machine).
Then I used a counterbore for a M6 allen screw.

The diameter of this counterbore is 11mm.
A M6 nut is around 11.3 mm
I just used a M6 bolt to pull the nuts in to the holes.
After that I putted some acetate glue on the nuts.

[project5k]

looks like that should work just fine.. how you have lots of places to hold stuff down