Reduce depth of cut, and/or reduce the width of cut on each pass. Holding in a vise will be problematic, as the part will bow up or down as the vise is tightened. A vacuum fixture would probably work well.
Regards,
Ray L.
I have been challenged on how to face aluminum plates.
This is something I have to do often and have had mixed results.
I have tried it with a flycutter and facemill. I was hoping that the facemill would be more 'stable' but in reality it creates a worse pattern since it is more rigid than the flycutter.
I typically have to face 6"x6", 9"x9" and 11"x11" sheets of .25" aluminum down to .1875" thickness.
I typically have the plates either secured between 2 sets of vises/soft jaws or on a fixture plate with mitee bite cam nuts securing the plates from the sides.
Both of these setups allows me to face over the plates without hitting any of the clamping methods.
My problem arises in that these thin plates sometimes span the gaps between the vise jaws and this create an unsupported condition and weird harmonics come into play creating some ugly patterns left behind. Changing the speeds and feeds helps but it never goes away. If you have ever tried this, you know what I mean.
I am trying to come up with a solution to secure these plates via a mechanical method so I can speed up my setup time (as opposed to taping them down, for example).
Any suggestions?
Thank so much,
Nathan
Similar Threads:
Reduce depth of cut, and/or reduce the width of cut on each pass. Holding in a vise will be problematic, as the part will bow up or down as the vise is tightened. A vacuum fixture would probably work well.
Regards,
Ray L.
Is the plate unsupported except the clamped edges? I have trouble with that if it is. I try to support the whole part with about 5/8 to 1" thick plate, the thinner it gets the worse the vibration pattern for me. Something I have been doing is to put strips of electrical tape under a thin part, that I make.
The corner radius of the inserts has a lot to do with it too, I use the smallest corner radius I can and still get a decent finish.
Most of my work was on a lathe but the same principles apply, sharp corner tool rougher finish less chatter. larger corner radius nice finish but prone to chatter.
Just a few things to consider, who knows maybe it will help you out.
mike sr
Thanks Mike,
That was a good education. It turns out that the Tormach Face mill does have rounded corners on its inserts and it made a worse pattern than the Flycutter which has sharper points on the insert.
One problem part is a 5" diameter lid shape I mill from a billet. The top is .1" thick and has a rim on the outside. It gets some serious vibration chatter at the center of this form when I try to face it. No way to put a plate under it since it is a void. I guess I could machine a round piece that sticks up inside the cavity to support the top when it is being faced.
The problem I have with large .25" thick plates that I am facing is that even if I have them on a 3/4" thick solid plate, I have to use MiteBite cam bolt clams and that basically squeezes it ever so slightly putting a very minor bow or tork in the plate so it machines funny. I guess a vacuum clam would eliminate this but I need to find a cheaper alternative. .
Doesn't have to be expensive at all, and you can make a simple one within 1-2 hours. I got some cheap neoprene cord from ebay ($10-15), a $125 vacuum pump, and a few bucks in aluminum invested into it. Just mill some groves into your aluminum plate and attach a air hose fitting, and your are pretty much done. You could mill groves specifically matching your stock.
It's super simple, but quite effective. I actually use it in my vice, so I don't have to align it every single time. (see pic)
The H-F vacuum pump will work just fine. Build yourself a vacuum reservoir, with several gallons volume, using large diameter PVC pipe and PVC caps, then use the pump to keep that evacuated. That will give you instant high clamping force when you open the valve to the vacuum table. PVC ball valves are perfect for switching it on and off, or use electric solenoid valves to control it using a simple switch. The whole thing can be built in hours for about $250-300 in parts, and everything you need should be available from your local hardware store or HomeDepot, except the vacuum pump.
Regards,
Ray L.
I got a quiet two stage pump from ebay (like 2 years ago). I'm using just manual valves that I also got from ebay for cheap. If you want "instant" vacuum (vacuum reservoir), you can also buy a vacuum chamber from ebay as it is used to degas silicone or resin. Sometimes you can buy them bundled with a pump for $200 or so. The nice thing about having a vacuum chamber in the loop is that I can turn off the pump and the vacuum will still hold (be sure to have a valve that closes the connection to the pump as well, or else you might suck the oil out of the pump). My whole set-up cost me around $250. If you don't need a vacuum reservoir, you can probably get it done for $150.
Last edited by cordvision; 09-01-2015 at 11:01 AM.
Cord and Ray,
I see tormach sells a 9cfm pump. Should I shoot for that size of pump?
I see that cnc cookbooks has a page on building one.
The silicone degassing chamber combos look nice and it looks like the pot can be used as a chamber like Ray is referring to, right. I don't need it too fancy since I will be mainly using it for facing. All of my other ops are drilling and I am sure that would ruin the vaccine plate or break the vacuum seal.
Thanks
I'm using a 3.9 cfm two stage pump in combination with a vacuum chamber. Works well. If you do a lot of the same drill operations, you can also put a seal around each hole location so you could use your table to drill as well. Just keep in mind that the vacuum table works best for larger pieces with a lot of surface area. Small pieces might need additional clamps for securing.
Thanks Cordvision.
What about this setup - 2 Gallon Vacuum Chamber and 3 CFM Single Stage Pump to Degassing Silicone Kit | eBay
Do you think 3CFM is enough of a vacuum pull?
Sounds Like I can get the PVC valves for the rest of the job at Home Depot.
Best,
Nathan
Looks good to me. Mine looks exactly the same but is 2 stage (which is probably a little bit more quiet). If you use aluminum stock on your vacuum table, 3cfm should be plenty. You basically just need a pump that can keep up with leaks. Aluminum stock is plenty flat to not cause any major gaps between vacuum table and stock. After turning my pump off, it takes over an hour before I loose the vacuum.
You really don't care about the pumps CFM rating. You do care about it's maximum vacuum pressure rating. Basically, all the pumps you'll find were probably made for evacuating A/C and refrigeration systems, and will pull down to about 25+ in-Hg, which is plenty for this purpose. The setup you linked to on E-Bay with the pump and tank should work just fine. The pump looks like the same one H-F sells.
Regards,
Ray L.
Here is a good article I came across describing what I have been experiencing in more scientific terms. A little overkill on what causes chatter but interesting to look at it from different angles - CNC Milling Chatter and Stable Milling Speeds
I am not sure on the CFM (not at same location at the moment) I regularly face 4" square .125 alum with out a problem. My DIY vacuum plate uses a round slot pattern, to facilitate the use of o-rings for the seals. Next one I make will have a square pattern and use some cord stock. I found the key is the sealing material and slot dimensions need to be sized so the sealing material will squeeze down allowing the part being machined to be flat on the vacuum plate.
As was suggested the CFM shouldn't matter unless you have some type of leak, especially with the added reservoir.