[akedward]

I'm going to cut some 1/4" mild steel plate on my VF-2 for an engine mount welding jig and I'm looking for a cheap way to fixture the plate above the table. I have use aluminum before so I could go full depth but it gets kinda spendy.

I suppose I could just put spacers at certain locations and rely on the stiffness of the material to support itself seeing how the tolerances on these plates are no more than +/- .010" .

Any thoughts for cnc newb?

-Thanks

[timan]

What are the dim's of the plate? Can you use a couple vises?:rainfro:

[DEAN]

MDF subplate?

[PBMW]

I do jobs like this all the time. I use a sub plate
It cuts into the plate by 10 or 20 thou.

[Donkey Hotey]

+1 for the MDF subplate.

Or use a bunch of carefully placed 123 blocks under the plate clamping points.

I did a piece of 1/2" aluminum plate (22x10) that way. I basically used a 1/4" endmill to 'part off' the finished piece (irregular outline). I clamped the four corners on 123 blocks and put a few more under the plate in the center for support. It chattered a little in the unsupported areas but worked out fine.

To cut it out, I did shallow laps around the periphery. I think the next-to-last cut left 0.025" and I made sure the cutter wasn't 'buried' on its full face (did a clearance pass on every depth so the wall of the 'part' only saw a finish pass).

On the final lap at 'full depth' the part just quietly separated from the edge, the burr broke and the part shifted away from the raw material. It's a little risky but worked in my case.

It left a bit of a knife edge all the way around but that was easy enough to remove with a file.

[PBMW]

I don't mean this to be inflamatory, so please don't take it that way. But you guys must not be doing this for a living.
I cut stuff like this all the time. Stick an endmill in full depth and cut the part. I don't step down as that just wears the corners of the cutter prematurely nad takes 5 or 6 times as long to cut the part. I make one rough pass and one finish pass.
My mini mill won't like full width cuts with a half inch cutter but 1/4 thick material is a no brainer.
I cut 2" thick parts on my box way machine full depth and full width. No issues. I cut everything from 316 to 1018 and alum this way. Every extra rough pass is money out of my pocket.

[timan]

I don't mean this to be inflamatory, so please don't take it that way. But you guys must not be doing this for a living.
I cut stuff like this all the time. Stick an endmill in full depth and cut the part. I don't step down as that just wears the corners of the cutter prematurely nad takes 5 or 6 times as long to cut the part. I make one rough pass and one finish pass.
My mini mill won't like full width cuts with a half inch cutter but 1/4 thick material is a no brainer.
I cut 2" thick parts on my box way machine full depth and full width. No issues. I cut everything from 316 to 1018 and alum this way. Every extra rough pass is money out of my pocket.

You are 100% correct!

[Donkey Hotey]

I don't mean this to be inflamatory, so please don't take it that way. But you guys must not be doing this for a living.

And thank you for sharing it. That's why I'm here: help where I can and learn where I can.

In the case of the part I was making, cosmetic finish was very important on the edge and I only had one piece of material and it had to be done the next morning.

A 2X deep, fully buried 1/4" endmill in 6061 seemed risky. If I broke the endmill, it wouldn't be ready the next morning. If I ruined the material, it wouldn't be ready the next morning. I had all night but only one cutter and one plate.

What about stainless? I am about to start working with a lot of 0.25 and 0.325" stainless plate (alloy TBD). I need to do profile milling, full depth with 0.25" endmills. Any recomendations?

[djr76]

Just like tinman said, get the shortest flute you can get that will cut the job. Carbide rougher with flood coolant at full depth. I work for an oem making food processing equipment. We found it to be cheaper to buy whole sheets and have them pre-lazer cut, then finish machine them.

[Baketech]

Ditto on the sub-plate suggestion. If you do a lot of plate work it's a good idea to have a nice bottom plate with a common grid of holes in order to quickly mount consumable plates to it. You can keep templates in your CAD system in order to quickly fixture jobs and order appropriate sized material.

Low Head Capscrews are your friend here...
The shorter head dimension (compared to SHCS) allows more resurfacing cycles of your fixture plates....

[akedward]

Thanks guys, I like the MDF subplate idea, much cheaper than aluminum and should work fine for the stuff I'm cutting.

[Vern Smith]

Tell me a little about the tolerances that one can expect from MDF used as a sub plate. I can readily see where it would work well for cutting out sections of plate. I have a lot of jobs that require machining features onto the face of the plate as well as a final operation to run an EM around the outside to separate the part from the stock plate. Is the MDF thickness uniform to within say .01"?

Vern

[Baketech]

I can see MDF working for a quickie job, but if you need to run very many parts, the extra tool wear and cutting time (from not running coolant) might stack the costs the other way....

[Donkey Hotey]

Why wouldn't he run coolant? Because it's going to swell? It's a one-time-use spacer. As for thickness: he's using the machine to cut an outline (+/- .010" ). In this case, it won't make any difference.

[Vern Smith]

Let's assume we have .250" alum plate 24" by 5" that we are doing various pocketing operations in and lets say that when finished the plate will be .200" thick in some areas and .125" thick in the pocket bottoms.

We face off the bottom side say .01 then flip it over and do the above operations. If I stick a .5" thick board of MDS under it for the operations described above will my dimensions from the bottom to the top ( .200") vary more then .01". Or put another way, is MDS produced with a tolerance of +-.005 over the 24" length?

Vern

[Donkey Hotey]

Okay, so now we have an aluminum hijack of the OP's thread.

MDF is pretty darned flat but I wouldn't count on it being +/- 0.005. I suppose you could clamp it to the table, let it thermally stabilize to the casting, then face it to ensure flatness before starting. Of course the wood chips and glue are going to make a huge mess out of your machine (BTDT).

In your hypothetical example, you have 0.125" thick pocket floors. In order to hold 0.005" you're going to have to give their backside some very stable support.

I've done 0.125" pocket floors with the stock sitting on four 123 blocks and they weren't consistent (thicker in the center--from flex under the cutter). In my case, MDF would have helped but because of finish concerns, I needed coolant.

Now with all of this discussion, I suppose I could have used MDF and wrapped it carefully in a skin of Saran Wrap or hit it with a coat of paint to seal the surface. I could have used coolant and the MDF would have remained sealed until the final profiling/parting operation.

[Baketech]

Sounds like the aluminum plate is getting cheaper all the time...

[akedward]

My application does not involve blind pockets, they are full depth. I'll give that MDF a try and report back on the damage.

[Geof]

....We face off the bottom side say .01 then flip it over and do the above operations. If I stick a .5" thick board of MDS under it for the operations described above will my dimensions from the bottom to the top ( .200") vary more then .01". Or put another way, is MDS produced with a tolerance of +-.005 over the 24" length?

Vern

Not exactly on topic but possibly of some relevance:

Have you actually taken a 24" length of aluminum plate and face a bit off oneside then flipped it to put pockets in the other side...and then measured how flat it is at the end? My experience is that you can easily get warpage of 0.005" over 24" just from relieving the stresses in the plate so it is almost irrelevant whether your baseplate is level you are not going to get really good precision no matter what the plate is supported on.

[Vern Smith]

I agree, the plate will warp. I've always felt that cutting both sides helps the warping problem. My concern with the levelness of MDF was trying to not make a bad situation worse.