Choice of metal(s) for fixture plates

[dbb-the-bruce]

I've been using 3/8 thick AL for various fixture plates. Both for use in my tiny but capable desktop CNC and my Seig X2.7 type mill.

There are two reasons I'm using aluminum, first I got a reasonable quantity of it in odd sizes for free as scrap. Second, I can use my CNC machine to make the fixture - or at least mark up and spot holes - this makes creating and accurate fixture place very easy.

To be clear, when I say fixture plate, it's something that has a number of holes for locating pins, threaded holes for securing and other stuff for aligning and registration etc.

Using scrap AL and working around whatever holes are already in it has served me pretty well to date. Now I've moved on to making bigger stuff manually on my mill and I'm thinking I need to up my game a bit. The threads in the AL plate wear down / loosen up over time and I often get the depth over cut such that I need to resurface or scrap the fixture. This also happens with using fixture plates on my CNC.

So I'm thinking of using steal plate for the fixtures instead. This would mean that the CNC machine would be used to layout and spot/pilot hole the fixture and I'd need to do more of the machining of it on my manual mill (not a big deal). In the end the fixtures would be more durable.

So first question, what's a good choice for fixture plates?

Just because I've been using Speedy Metals - I look there and they have A-36 and 1045 hot rolled plate. I'd have to fly cut the skin off but would need to flatten pretty much anything I used.

Other options? Pros/cons?
Anything cheap or free is of course a big plus. I haven't checked out the local metal place ends and cutoffs but that also tends to be hot rolled.


Regarding over cutting the depth and messing the fixture up, Any thoughts on how to handle that better? I've used a sacrificial layer ABS sign material in some cases. But that can lead to other issues - uneven thickness and compliance can cause problems.

If I had a bigger pile of scrap aluminum I probably wouldn't be asking!

 

[mmcmdl]

One thing is steel rusts . Thick aluminum jig pate is what we/I use . They are called subplates btw . You need threaded and reamed holes in patterns on your plates . You can easily make overlays of the part being made and use pins in your reamed holes to lift the piece being machined off the subplate . When using indexable drills , you need clearance beneath your part for the slug to break thru .

 

[frankly2]

When I make sub plates I almost always use heli-coils. They hold up under multiple use. Whereas the Al will wear fast when used multiple times. Also I buy cast AL fixture plate, it holds up much longer than 6000 series AL. Cast AL is however more expensive.

 

[dbb-the-bruce]

Great point about rust. And checking AL plate prices, some types are cheaper than steel.
(and I'll start calling them subplates)

Any particular type of aluminum to look for or avoid? I have a good quantity of scrap from an aerospace company it varies between fantastic to machine to so gummy it's almost not worth it.

I'm not quite understanding this:

You can easily make overlays of the part being made and use pins in your reamed holes to lift the piece being machined off the subplate

Do you mean some thickness of material that's the same outline as the finished part?

 

[Nutfarmer]

Check on line with S and S machine they had cast aluminum tooling plate as remnants from a part they make. It made great fixtures for a rotary table. This was the only material they had for sale. It may still be available. If threaded holes start to pull out heli coil inserts work well for repairs. I have just used drill bushings where repeated drilling or locating pins have wore out the aluminum.

 

[mmcmdl]

An overlay of thru holes and features in the part . You then can place your pins in holes to avoid machining into the pins . As mentioned above , heli-coils work great also .

 

[ChazzC]

3/8" is a little thin; my homemade ones are 1/2" or 3/4" 6061. Commercial plates are often made from MIC-6 or ALCA 5, both of which are stronger than 6061 T6 and more dimensionally stable so they don't require as much (or any depending on your needs) metal removal to achieve flat, parallel faces; I have a 6" x 8" x 1" ALCA 5. Unless it's for a specific need (tapped holes not on a grid), I purchase them machined, drilled & tapped since I don't have a CNC and they don't cost significantly more than the material does in smaller pieces. I use SMW 1/4" fixturing pins rather than dowels.

 

[dbb-the-bruce]

So understanding the lingo a little better...

What I am currently doing for anything that I plan to make (small) quantities of on my CNC (small desktop - 8"x8"x3" volume) is to make a custom hunk of 3/8 Al plate with threaded holes and/or holes for alignment pins that work well with whatever the part is. This thing-that-I-would-call (correctly or incorrectly) a fixture is then bolted to what I now understand to be called a sub plate on my CNC that has a regular grid of threaded holes.

Moving to my large manual mill, if a given part is complicated enough (for me) and I want to make a bunch of them I'll do the same sort of thing for larger parts.

So I guess for what I do, I'm on the right track. When I eventually run out of my free scrap aluminum plate, I'll buy some more 6061, possibly thicker.

Based on this discussion, I may drill and ream regularly spaced holes between the threaded holes of my existing sub plate.

Thanks.

 

[rwm]

This is why MIC6 exists. You can sometimes find drops on eBay pretty reasonable.