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I've been playing with my mini-mill. It's very good for isolation routing to produce low cost PCBs but I thought I'd take a shot at something a bit more more mechanical. These should be frames for a Stanier tender. It's double-sided FR4. I'll take a shot using brass and see how that goes.

 

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Very good indeed.... I think many of these things can be extremely useful if you take the time to learn their actual limitations and then think outside the box!

I have Stepcraft, which no use quite a lot for brass and nickel silver, and I now wouldn't be although it. All these things have a significant learning curve, but I think are very well worth it! Do let us know how you get on....

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Now that you have cut the frames in FR4, what would your opinion be to the viability of its use as a structural material for constructing model locomotives. The axle holes would presumably need to be fitted with brass bushes, but apart from that, would the material be strong enough for use as locomotive frames?.

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7 hours ago, DaveyH said:

This is very inspiring....may I ask what spindle speed and feed rate you used to mill the FR4 please?

 

Thanks

 

Davey

 

Hi Davey,

 

Spindle speed at max. I'm not sure what it really is. The controller software says 1000 rpm but I think that's suspect.

 

X/Y and Z feeds are 60 mm/min (1 mm/sec). That's the thin double-sided FR4 (0.75 mm)

 

I used a cheap 1 mm carbide mill. IIRC the feed vertical feed was set to cut the holes in two passes of 0.5 mm.

 

If you are using a Woodpecker (or similar mill) you might see that its a bit different from mine. I added those square plates to hold the angle at 90 degrees and the horizontal beam to reduce droop towards the middle of the gantry. The FR4 is attached to the MDF (a scrap piece of flooring) with hot-melt glue. I use a heat gun to release it.

 

Also, the Woodpecker has a nasty habit of "pulsing" the motor off and on when the controller starts the spindle at full speed. I believe what's happening is the controller goes into overload because of the high starting current, turns off, then turns back on again. I seem to have solved it on mine by putting 1.3 ohms in series with the spindle motor. (It's actually a piece of nichrome wire and that just happened to be the value - 1 or 2 ohms might work just as well.)

 

Cheers,

Andy

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30 minutes ago, rocor said:

Now that you have cut the frames in FR4, what would your opinion be to the viability of its use as a structural material for constructing model locomotives. The axle holes would presumably need to be fitted with brass bushes, but apart from that, would the material be strong enough for use as locomotive frames?.

 

Double-sided FR4 is pretty stable and quite robust. I don't think there should be a problem. You could also go up to the thicker 1.5 mm FR4 which would be stiffer but that might not be necessary.

 

Yes, you'd have to bush the axle holes. I was interested to see if I could mill 2 mm holes with a 1 mm mill and they seem to be OK. One advantage of this method is the axle holes are positioned very accurately. I suspect with far more precision than is possible with a drill, but it's still early days.

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Not a lot of success with milling brass. I've been attacking 16 thou sheet with mixed results. Sometimes it looks like it's going to work then something goes wrong, usually snapping the end off the carbide mill. I've destroyed four 1 mm end-mills so far.

 

Currently pondering my next move, or if there even is one. Based on what I've seen I think it might be practical but it will take a lot of mucking around to get the right recipe. I'm thinking HSS might be better than carbide. Also a larger diameter mill would probably help but that would impact the resolution although it might be OK for some things.

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55 minutes ago, AndyID said:

Not a lot of success with milling brass. I've been attacking 16 thou sheet with mixed results. Sometimes it looks like it's going to work then something goes wrong, usually snapping the end off the carbide mill. I've destroyed four 1 mm end-mills so far.

 

Currently pondering my next move, or if there even is one. Based on what I've seen I think it might be practical but it will take a lot of mucking around to get the right recipe. I'm thinking HSS might be better than carbide. Also a larger diameter mill would probably help but that would impact the resolution although it might be OK for some things.

 

I had similar problems to you with my CNC mill when I started to cut brass.    I had started cutting styrene and there were no great problems,  but carbide cutters were going ping with monotonous regularity when I started on metal.. I dug around the net for speed and feed calculators but still got problems when following the settings they gave.  Until I started questioning the speed of my spindle.  I got myself a cheap electronic tacho and found that my spindle was running at half the speed indicated.  I was able to adjust the speed upwards and matters started to improve greatly  and the life of my cutters improved no end.

 

But my settings are a lot less than yours since I use a 0.1mm depth of cut for my 0.5mm, 1mm and 2mm carbide slotting cutters.  They run at 6000 rpm and the feed is 25mm/min for the 0.5mm,  50mm/min for the 1mm, and 230mm/min for the 2mm.  These cutters should be running at speeds into five figures of RPM according to the manufacturers specs but if you lower feeds and depth of cut you can start to get reasonable performance with a slower running spindle.   Try and nail down your spindle speed if you can.    If it is running at around 1000rpm then you might have problems getting the smallest cutters to work.

 

Jim.

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2 hours ago, flubrush said:

 

I had similar problems to you with my CNC mill when I started to cut brass.    I had started cutting styrene and there were no great problems,  but carbide cutters were going ping with monotonous regularity when I started on metal.. I dug around the net for speed and feed calculators but still got problems when following the settings they gave.  Until I started questioning the speed of my spindle.  I got myself a cheap electronic tacho and found that my spindle was running at half the speed indicated.  I was able to adjust the speed upwards and matters started to improve greatly  and the life of my cutters improved no end.

 

But my settings are a lot less than yours since I use a 0.1mm depth of cut for my 0.5mm, 1mm and 2mm carbide slotting cutters.  They run at 6000 rpm and the feed is 25mm/min for the 0.5mm,  50mm/min for the 1mm, and 230mm/min for the 2mm.  These cutters should be running at speeds into five figures of RPM according to the manufacturers specs but if you lower feeds and depth of cut you can start to get reasonable performance with a slower running spindle.   Try and nail down your spindle speed if you can.    If it is running at around 1000rpm then you might have problems getting the smallest cutters to work.

 

Jim.

 

Thanks Jim,

 

I put my scope on the motor and based on the commutation pattern it look like top speed is about 10,300 RPM. I'll try dialing the speed back and try different speeds and feeds.

 

Andy

 

 

Edited by AndyID
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You may also have a little more success using "D" cutters, rather than traditional milling cutters, as they only have onecutying edge. Your speeds should be down to 0.5mm per sec or less with brass, and yes, cutting 0.25 - 0.33 per pass for reliability. I don't think you will be able to put anything but light side loads (cutting loads) on your spindle, as it doesnt look sufficiently rigid.

I use a low-tack double sided tape to hold the job down, which works well. 

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Runout of the spindle and cutter can also be a problem with small cutters - it can cause some flutes to take bigger cuts than others. Usually measured with a dial indicator. If you don't have one you could measure it indirectly with a vernier caliper by comparing the width of a slot with its nominal width. You would need to make sure that the tool path you use for this eliminates any backlash, and perhaps take spring passes to eliminate cutter deflection as much as possible.

 

I use fusion 360 to create my tool paths on my small mill. One of the most useful features is the option to slow the cutter as it approaches and exits corners. This avoids breakages where the small cutter is flexing somewhat, lagging behind its proper position, and then suddenly turning through 90 degrees. I expect other tool path generators might have a similar option.

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Andy

 

Sorry for the delayed expression of my thanks for your detailed reply...been busy!

 

I have been threatening to create a scratch built chassis for a LNWR Special tank for ages now and have been drafting in Autocad for a while.   I was going to make my first trial milled frames in 1/16 brass sheet, but have now been encouraged by your post to test the design of frames and spacers in FR4 initially.   I ordered some PCB boards this morning so should be ready to go tomorrow or the next day.

 

I have a Proxxon MF70 mill which I have converted to cnc, so I can mill at up to 20,000 rpm !

 

Thanks for the idea......

 

Davey

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Wellll, I'd say my Woodpecker mill is great for PCB isolation routing and it seems to be quite capable of producing structural things like underframes from PCB laminates. It can also mill brass but I think that is pushing the limitations of the machine.

 

I overcame the end-mill destruction routine by sleeving a carbide mill with a "saver" that only leaves 1 mm of the cutter unsupported. But while cutting brass there is enough vibration that the grub-screws holding the end-mill tend to slacken off. A collet would be a better idea.

 

I almost managed to mill one tender frame, but it took a long time. So long in fact that we had a power cut before the operation completed! (We just had a rather violent snow-storm.) But, on the whole, the machine is probably just a bit too flimsy to handle brass effectively.

 

 

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  • 2 weeks later...

The pcb arrived eventually and I had a first stab at milling a frame.   The result was quite good...any imperfections being a result of my lack of cad ability rather than the mill which appeared to cut the frame exactly as designed and, perhaps more surprisingly given the dainty nature of the MF70, exactly to size.

 

Here is the result.

 

3F1A8F03-710A-4BDB-B48B-2039B88636FD.jpeg.1ce5836d313476cc4dc560e61b9de0ce.jpeg

 

The pcb was cut with a 1mm end mill of Chinese origin at a spindle speed of 8,000 rpm and a feed rate of 100 mm per minute.

 

Davey

Edited by DaveyH
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6 hours ago, DaveyH said:

The pcb arrived eventually and I had a first stab at milling a frame.   The result was quite good...any imperfections being a result of my lack of cad ability rather than the mill which appeared to cut the frame exactly as designed and, perhaps more surprisingly given the dainty nature of the MF70, exactly to size.

 

Here is the result.

 

3F1A8F03-710A-4BDB-B48B-2039B88636FD.jpeg.1ce5836d313476cc4dc560e61b9de0ce.jpeg

 

The pcb was cut with a 1mm end mill of Chinese origin at a spindle speed of 8,000 rpm and a feed rate of 100 mms.

 

Davey

 

Very nice Davey!

 

Another advantage of using copper-clad is it can be used to make a split chassis. I have not tried it but it might be possible to make coupling rods and bits of valve gear from FR4 or even FR1/FR2.

 

Andy

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4 hours ago, Giles said:

100mm per second? That seems awfully quick ?...

 

That would be some achievement......................

 

Thanks for pointing the error out, I've corrected the original post.

 

Davey

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