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Topic: A stencil8 PCB tooling block for doing solder paste stencils (Read 26042 times) previous topic - next topic

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #45
Well, I though I could get this thing milled for cheap, but the mill I have access to just can't cut the mustard with aluminum.



So, I've got some Delrin on the way, and I'm going to give it another go.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #46
Too bad it didn't work out (yet).

I thought the main attraction of acetal (delrin) was that it machined about hte same as aluminum (in my experience it tends to flow a little bit when trying to cut though)?

I'm guessing it's one of the home-brew type mills?  What kind of bit are you useing, feed, spindle speed.  Are you trying to use an end-mill and do a hellical path along the inside wall of the hole, or are you doing this with a standard drill bit?

in my limited CNC milling experience - primarily with wood, MDF and FR4 - i've found the solution to most of my issues was solved by using lighter (less agressive cuts).  With Al, most of the manual machining I've done - a bit of cutting fluid goes an awefully long way - helps to reduce heat quite a bit.

After using some HSS bits, I decided that all i wouldn't waste time with steel anymore - I only use carbide cutters now - helps out tremendously - the difference in cutting quality and tool flex is incredible.

Also, if your spindle has limited run-out, you may be able to get a bit that's of the proper diameter (or maybe a fraction under) and just do some drilling operations.  Pretty much no matter what your setup is - i would think that you'd be able to get away with peck drilling and cutting fluid.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #47
In my (also very limited) experience, Delrin is quite a bit easier to cut than aluminum.  This is a homebrew mill setup, designed and built by someone with a lot more experience in machining than I'll ever have.  The bit I used for aluminum was a coated HSS, 2-flute end mill recommended for drilling operations.  Feed speed was 5 IPM, spindle speed was ~10k RPM.  I was using a peck drill cycle, going 1/32" at a time.

The biggest problem is that the mill has a bit of slop in the Z-carriage when placed under high thrust loads. The result is that bit would skip around on the surface when penetrating.  With the delrin, I ordered a bit which claims superiority in drilling plastic.  Hopefully the reduced hardness of delrin (compared to aluminum) will mitigate this problem.  If that doesn't work, I'll order some center-drills, and indent the surface before the drilling operation.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #48
[quote author="Zuph"]The bit I used for aluminum was a coated HSS, 2-flute end mill recommended for drilling operations. Feed speed was 5 IPM, spindle speed was ~10k RPM. I was using a peck drill cycle, going 1/32" at a time.

The biggest problem is that the mill has a bit of slop in the Z-carriage when placed under high thrust loads. The result is that bit would skip around on the surface when penetrating.[/quote]
That certainly sounds reasonable. So was the issue getting the bit centered when starting the hole (which the center drill should help greatly) or actually drilling it? Also, 10k at 5IPM will probably generates quite a bit of heat because of the super light chip load - (not so good for the HSS bit)?  I would say cutting fluid would certainly help this (but it looks like you were using some based on the picture).  Using a lower speed would as well, (you can't really go with a higher feed rate if the z-axis is already having trouble with the load).  The other approach you might consider is to use the mill to accurately place your centers - then a nice sturdy drill press for actually drilling the holes (possibly having more rigidity and lower speeds than what the mill has) - this is assuming you have a nice drill press available.

[quote author="Zuph"]
 With the delrin, I ordered a bit which claims superiority in drilling plastic. Hopefully the reduced hardness of delrin (compared to aluminum) will mitigate this problem. If that doesn't work, I'll order some center-drills, and indent the surface before the drilling operation.[/quote]
This sounds like a great plan - excellent bits can make a world of difference.  Also, like I said before, the Delrin will tend to flow a little when cut, so that will help keep the holes the correct size, even if the spindle does have some run-out.  I haven't tried milling acetal for quite a while - maybe I'll have to give it another shot sometime ( I was thinking about trying to make some better anti-backlash nuts).

The inline picture is pretty small - is sounds like the main issue is that the holes are too large in diameter?

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #49
[quote author="bearmos"]That certainly sounds reasonable. So was the issue getting the bit centered when starting the hole (which the center drill should help greatly) or actually drilling it? Also, 10k at 5IPM will probably generates quite a bit of heat because of the super light chip load - (not so good for the HSS bit)?
[/quote]

Unfortunately, the mill was built for milling wood, so I don't have a reliable way to get the spindle speed below 10k RPM.  I did use cutting fluid.  On all the holes, the drill hit was sloppy, but as it drilled down, it approached the "true" diameter.  Unfortunately, the holes are shallow enough that this still allows some play in the dowel pins.  A bigger problem is that the drill hit would occasionally skip all over the face of the block, resulting in even higher thrust loads, resulting in skipped Z-axis steps.  This happened on 2 holes in the block shown.

The drill press idea is interesting.  I may give that a go if milling delrin goes down-hill.  I do have access to a fairly nice (if ancient) drill press.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #50
[quote author="Zuph"]Unfortunately, the mill was built for milling wood, so I don't have a reliable way to get the spindle speed below 10k RPM. I did use cutting fluid. On all the holes, the drill hit was sloppy, but as it drilled down, it approached the "true" diameter.[/quote]
This sounds a lot like my home setup.  I found that the best way of making accurately sized tooling pins (in MDF) was to use a 1/16" end mill and use a helical pattern to create 1/8" diameter tooling pin holes.  Many attempts at the tooling holes can be seen about half way down the page.  Cambam supports this type of helical drilling operation natively.  There's a free edition at the bottom of this page that I use a lot.  If I start getting more serious about milling, I'll probably spring for the paid version ($150) - it seems to make basic operations really fast.

EDIT: The most important point for the helical drilling operation is that it allows you to tune the "ideal" diameter of the hole iteratively until you get a snug fitting tooling pin.  For example, if you have 2 mils of runout in your spindle you can still get a hole that will snugly fit your 1/8" tooling pin when using a 1/16" end-mill, after making the diameter of the "ideal" hole smaller in CAM.  This method can also be used to compensate for tool wear (as cutters get used their diameter changes ever so slightly) - but there are also other methods of dealing with tool-wear compensation.

[quote author="Zuph"] I do have access to a fairly nice (if ancient) drill press.[/quote]
If taken care of, these are often times better than a lot of the inexpensive newer presses.  The idea is that centering holes (accurately placed with the CNC) will help guide the bit to the precise center once it's on the drill press.  Since the press is better at drilling through metal than the CNC - you get the best of both worlds.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #51
Mine arrived!  Thanks again!

It came so much faster than I expected. I had no idea what this heavy block of a package was :)
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Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #52
That was fast! Please do let me know what you think.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #53
I finally decided to invest in one of these blocks, but they are sold out :-(

Anyone know if there will be another batch?

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #54
The CNC shop who made the first batch gave me a huge amount of trouble and eventually gave up and sent most of the stock back unused. I'm still trying to get a refund out of them, and I had to refund most of the buyers.

I have found someone with much more experience with Aluminium who has offered to do the remainder for a very reasonable price. So I'll be offering them up again on Tindie once they're actually finished this time. If one of the original purchasers doesn't buy again, you might be able to snap one up.

I'm also exploring the possibility of making ones out of polycarbonate (lighter, so cheaper to ship) with a reduced hole count for substantially less than these ones cost.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #55
Sorry it turned out like that, I really appreciate the trouble.

Someone mentioned delrin (?) or some other very hard and stable material too.
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Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #56
Yup, I considered Delrin too, but according to the new CNC guy, Polycarbonate has better dimensional stability. He's going to knock together a sample for me when he does the Aluminium blocks.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #57
Yes, I'd also strongly recommend polycarbonate over delrin for this.

Re: A stencil8 PCB tooling block for doing solder paste sten

Reply #58
[quote author="nickjohnson"]I have found someone with much more experience with Aluminium[/quote]
Wow, sorry to hear this was so miserable for you, Nick.  With all of machinists I deal with Al is the preferred material - I would have never imagined something so straight-forward would become a headache.

Good luck with the future revs, I'll be curious as to how the polycarbonate turns out - I've always seen things machined out of Acetal.  I might have some reading to do ;-)