I guess I should have chosen a curvy part instead of this boring one… maybe the next time.
Now some words about my search for a decent linux video editor. I did my previous video using OpenShot; It was a bit crashy and I really don’t like the way it handles titles. So after checking some reviews and doing some tests, I finally settle on Kdenlive, which is really nice.
When I was a CNC girl, I thought things like “Once you make one, you can do thousands, easily!”, “Acetal is the perfect plastic for precision parts and cuts like butter!”. OMG I was so naive. Making these parts were really a pain in the ass. That’s why.
Acetal has internal stresses. This mean that if you remove a significant mass from a block of acetal, the final shape will warp a bit. I knew that. But what I didn’t know was that sometimes this natural “stress relief” can take hours. So one day I was turning a lot of these parts, everyone having a perfect bearing fit. But then, on the next day, the bearing fit was really tight; the parts have had shrank a little bit. May be acetal I got wasn’t annealed? Or may be I should anneal these parts? (yes, plastic annealing is a topic).
BTW, in this specific case, a bearing housing should, ideally, not divert more than 5-7um from the nominal size: 10um less and the bearing will not fit; 10um more and the bearing fit will be too loose.
Now the other issue: long aluminum crosslide = noticeable thermal expansion. So the table temperature will change after stepper warm-up. And, it will change again after some machined parts. From what I remember a machined radius can easily divert 0.03um because of thermal expansion. And re–homming after temperature changes can be a bit misleading.
The good thing is that, once you understand these issues, making something to avoid or reduce them is not too hard. The bad is that now I have a full bag of black plastic decoracion parts (not the ones in the pictures, of course).
Currently I’m doing some modifications to my Sherline lathes in order to add a cover, very similar to the one on my cnc mill. This will result in a really big changes in the lathe look. I don’t feel comfortable doing these brutal changes to these beautiful machines, but it’s something I need to do.
Of course there are always some things that go wrong. If I were a professional machinist working on big machines I will be dead for sure. So be careful when you start cutting on a corner using traditional milling (not climb milling; I’m not saying climb milling is more secure, btw).
I’m doing a lot of work with my cnc mill. I’m working on and with a really useful set of gcode routines that I will like share at some point.
Finally, here’s one of the more simpler yet useful accessories you can make for your Sherline mill/lathe, the “no hammer” (not my idea of course).
Can’t believe didn’t did this before.
My actual cutting parameters for aluminum sheet:
A lot of peoples says 1100 is a nightmare to machine, but as always, you just need to know it. I settle in these parameters by a test-error process, but I don’t now if these are the optimal ones. I just know these values work for me. And of course these small carbide end-mill would enjoy more rpm.
For 5052 I’ve used the same parameters, but you need to lower the feed a bit; a friend broke some end-mills at 800mm-min because of my speed thirst (nevertheless that was fun).
One last thing… the part in the photos is 2.5mm thick, so it was strong enough to withstand all the cutting with just a few screws. If that were a 1mm sheet more screws would be required.
I should say the hardest step in making this part was to press the start button (I’m a chicken).
This 1mm sheet was held to a mdf plate using Carnauba; that seems to work nicely. Badly I made a mistake and the DOC was about 0.5mm (hence two runs where required), so I’m still not sure if 7 IMP is ok for 1mm DOC.
Things I learn:
The next task will be to cut the definitive encoder wheel for the machine spindle.
Up to this point I’ve worked in the lathe and the mill without worrying too much about speed and rpm calculation, trusting in my own experience and “feeling”, as a lot of hobbyists, I guess. Tooling wearing wasn’t ever an issue to me; carbide tools seemed to last almost forever. Until I grasp Machinery’s Handbook and read “tool life for milling… should be approximately 45 minutes” (!!). So clearly in the cnc world choosing the right cutting parameters matters.
The cutting setup of my first cnc project involves:
This LMS table states a speed of 165 FPM for 6061 aluminum (I guess it’s for HSS). So RPM = (165 x 4) / 0.0787 = 8386. Now, according to this, 0.002 IPT (inches per tooth) is suggested for 0.05 DOC, 1/8″ hss end mill over aluminum (closets size); 0.0015 IPT for my 2mm endmill seems reasonable. So using the max rpm’s (2800) gives me a feed of 2800x 0.0015 x 3 = 12.6 IPM or 320 mm/min.
Of course, due to the complex nature of this topic, suggested parameters for material/end-mill can vary a lot. American-Carbide suggest a feed of 16.000 rpm / 11.8 IPM for this cutting setup. And Whitney Tool states a cutting speed of 600 FPM for hss and 1200 FPM for carbide. As always Practical Machinist is a good source of knowledge.
Now some real world experience in the Sherline world. This guy broke his 2mm carbide endmill at 8 IMP, 0.5mm DOC. This other guy broke his 1/8″ 4 fl endmill at 6000 RPM 14 IMP, 1.27mm DOC. In a test in my manual mill three turns per second (7 IPM) doesn’t seem to break the tool.
So i think I will stick to 7 IPM for now and see what happens, and maybe later I get a set of teen end mills to do some testing and push further. Also, It’s clear I need to order the 10.000 RPM pulley set.
I use to do climb milling almost all the time, without any issue. Up to this day. Seems it’s widely known that climb milling can be dangerous if you have backlash in your machine. Well, this was not the case. After I broke the 5mm endmill in the picture I’d found the knobs (both) where pulled out of position by about 0.5mm. Maybe my fault; previously I had disassembled the whole crosslide to clean and install delrin nuts.
A bit later I continue, using 1/4″ three-flute carbide end mill. I don’t remember very well, but I was about to take about 0.3mm on the side, climb milling again. I begin ok, but suddenly, when the endmill was about to exit, the thing grasp into the corner and get stuck into the part. The tip of the endmill was out of position by about 2mm; a rotation in the headstock, purely by flex of all the parts of the headstock and column, was evident (maybe 1º). After unstuck the endmill from the part, the headstock return to his position. I was a bit shoked and forgot to take a picture. Luckily the endmill didn’t break, and there was no damage in the machine (picture doesn’t show original part damage, just a fraction).
Probably as most of my cuts are rather light cuts, there’s no problem with climb milling (max 1mm DOC and 0.5 for side milling). Some things that, perhaps, played against this time, were:
The good thing is that when things like this happens, you can go to Youtube and check videos like this, and feel a bit better.
One last thing…. now I’m convinced that an accelerometer-based automatic stop is a MUST on any cnc.
Here is a very simple but highly useful accessory I made from a 13x13x40 mm block and a broken end mill.
And here’s a 76x140mm flated surface.
There was some vibration due to the unbalanced rotating mass, something that I fix later adding a simple counterweight. And of course I took advantage of a not-perfectly trammed head to get a nice finish (about 15um less in the middle, not bad).
Finally a picture showing how useful is the side plate.
UPDATE (01/09/13)
In the last working round I just note some upset behavior related to, I guess, thermal expansion. The DOC can increase, easily, 0.02mm after a couple of millimiters of feed. You can learn to deal with this, but now I think that a bigger mass cutting bit it’s a better idea (of course this doesn’t matter if you use coolant). Anyway, no matter what flycutter you use, I think a double finishing pass it’s always a good idea (0.03 and then 0.02 mm for example).
Soon I will need to cut some 1″x3″ aluminum bars. I know the right tools to cut thick aluminum bars and plates are things like this:
But of course I can’t have these monsters into my small apartment.
Some guys say you can turn a wood band saw into a metal band saw adding some speed reducer and changing the blade. In fact, up to this point I’ve used a cheap wood band saw as is to cut thick aluminum stock without problems (up to 1″). But I wanted something more appropriate and smaller, so I took a look at the Proxxon band saw.
At first it seemed perfect, but after reading some amazon comments, I discarded this option (easy to understand if you read the specs: 85 watts; my wood saw it’s 250 watts). So I bought a Starret St1010 portable band saw, and start to convert it into a vertical band saw. Here’s the work.
All was going well, but then I realized the damn thing was too noisy, at least to be used into an apartment. Here are the noise measurements at 1 mt of the running saws:
Noise reason was simple… metal gears inside. Tough at low speed noise level it’s similar to my vacuum cleaner, it’s much more annoying. So, I will better off ordering a metal saw blade for my wood saw, and left my converted Starret saw into a corner, at least for now :cry:.
Here is a design of a telescopic cover for the Sherline lathe (please note this isn’t the full drawing).
Why I need that? 1. Because I hate having to clean and lubricate the ways after every use. 2. Because all these debris can increase wearing on sliding parts. 3. Because this is very desired feature on a CNC machine.
I made some delrin parts but drop the design. it I was more committed in doing something cool than functional. Probably it would work, but a simpler design was possible. The simpler, the better, so all these nice parts will go to the bag of sample parts or be reused later.
These parts looks simple, but there was a lot of hours involved in design and building. One good thing I learn about working with delrin was that, due to internal stresses of the material, you should do a rough cut of the shape of the part, and later finish all surfaces.
For example, to cut the straight bars in the picture, starting from a plate, a possible set of steps are: finish the edge of the plate, cut the bar and then finish the other three sides. The first bar below, done this way, has a evident warp.
For the other two, I cut the bars slightly oversize and then did several set of passes over the sides, slowly getting close to the final size. This is a matter with thin parts; for compact and simple parts this is not an issue.
I almost finish the new design for the cover, so hope to begin the building phase soon.
