Newbie- Precise Hold Down Fixture? Help?

[acecnc]

Hello,

I'm a little reluctant to post this here because of some of the flaming going on.

This is one of those things that I think experience (which mine is very limited) pays off... I have searched the internet but nothing really meets my needs so I thought I would ask for ideas/advice.

I have a CNC Router and have been doing this for about a year and a half self taught - so cut me a little slack. I had never even seen a CNC Router prior to this.

I am cutting simple plaque molds for injection molding. I have done a couple of these so far with a lot more to come. In my first experiences I have had to go back and do touch-ups on the tool (mold) after it has been test-shot. The material is .5" aluminum plate (which is not really square in any way). Once cut I need a way to be able to take it off of the machine and get the material back in EXACTLY the same spot on the CNC machine for re-machining or touch up.

The way I'm starting to go about this is.... I have piece of plate steel and on the mill, I was going to drill and tap four holes for shoulder bolts. Then drill the same four holes on the aluminum plate. Then permanently mount the steel plate to the machine so I have a X and Y axis starting point that never changes. I would like to be able to do this by using t-track and bolts but I have been told that using shoulder bolts is the better solution. I'm trying to avoid having to drill extra holes which also have to be precise (which takes more time). I was thinking something like a machinist vise but with the material not being perfectly square I doubt that would work.

Any suggestions?

Thanks,
Gabe

[easymike29]

Use three dowel pins to locate the mold plate. Bolt down to your liking. They will repeat.

Gene

[acecnc]

Originally Posted by easymike29 Use three dowel pins to locate the mold plate. Bolt down to your liking. They will repeat. Gene

Gene, thanks for the idea... Will this work even if the plate is not perfectly square? I guess it would as it will repeat not perfectly square. Would it be precise enough to be able to go back over letters that are in the neighborhood of .03-.04"? Originally my plan was to make a raised "L" and slide the part into the far corner of it and then use a couple of those lever clamps that slide out to hold it in place - but because the mold plate isn't square, that ide was shot down.

[denfire]

The first thing i learned in shop class, and something u should put into practice is to always, i repeat always square your workpiece so that opposite side are all parallel and the transverse sides are perpendicular. In this manner u can then set two parallels in a "L" shape or "T" but be sure to indicate them so that they are perpendicular to each other and you will have a simple way to work off of your origin with precision, and accuracy.

[beege]

Two Pins, not three:

http://www.misumiusa.com/LocatingPin...eyareused.aspx

http://www.carrlane.com/Catalog/inde...3C3B2853564055

[easymike29]

Originally Posted by acecnc Gene, thanks for the idea... Will this work even if the plate is not perfectly square? I guess it would as it will repeat not perfectly square. Would it be precise enough to be able to go back over letters that are in the neighborhood of .03-.04"? Originally my plan was to make a raised "L" and slide the part into the far corner of it and then use a couple of those lever clamps that slide out to hold it in place - but because the mold plate isn't square, that ide was shot down.

If you extend the 2 dowel pins through the other side of the mounting plate and use them to align the plate in the t-slots of the mill table the fixture will always be repeatable in terms of the one long axis.
If you then indicate and zero one of the pins from the top you will always have a repeatable starting point for all subsequent plates.
Remember that your method of fastening must not influence the plate moving away from the pins.

Gene

[easymike29]

Originally Posted by beege Two Pins, not three: http://www.misumiusa.com/LocatingPin...eyareused.aspx http://www.carrlane.com/Catalog/inde...3C3B2853564055

2 pins on the long edge, one pin on the short edge. See sketch on my first post.

Gene

[Geof]

Originally Posted by acecnc ....I'm trying to avoid having to drill extra holes which also have to be precise (which takes more time)..... Any suggestions? Thanks, Gabe

In the greater scheme of things sometimes taking the time to put in alignment holes can save a lot of time in the future if/when rework is needed.

Have a standard hole pattern and have alignment pins in your steel base on this pattern.

As mentioned, use other pins. or keys, in the alignment plate so it goes back on the machine in the same location after being removed, but also put a reference hole in the steel base so you can precisely position you work zero on the base by dialing in to this hole.

Always keep in mind the answer to the question; "when I discover I screwed this up how do I quickly, easily and precisely re-align it to fix things?".

[mc_n_g]

Unless you have a good drill bit set and reamers I would hesitate to use drilled holes to located your parts. Maybe for CNC machines with fixtured parts but you don't have that. i would use the pins method or mount yourself a 'L'-shaped bracket to the T-slot to 're-index' the part. Drilling is essentially a roughing operation.

[acecnc]

Thank you all very much! I think I'll try the Pin idea first, then if for some reason that doesn't work, I'll go with the shoulder bolts. I know squaring the plates would be ideal but I'm a one man shop and I think it would take [insert: ME] too long.

Once I get it figured out, I'll post how things turned out.

Thanks Again,
Gabe