PDA

View Full Version : Blacktoe Specificiation



kousha
05-11-2012, 01:40 PM
Hi there,

I was wondering if with Blacktoe, I could have a resolution of 50-70 microns? I will be cutting plastic (in terms of hardness, it is the same as the wood).

Is that resolution too much to ask from Blacktoe? I talked to them, and they said that their system is designed for 10microns, and depending on how you assemble the system, you could actually achieve 10microns!

If 70 microns is too much to ask from Blacktoe, what are my alternatives?

Thanks

ahren
05-11-2012, 03:17 PM
Are you really concerned about resolution, or machine accuracy? I'm highly skeptical of an MDF machine claiming 10 micron accuracy -- many high-end vertical machining centers claim about this accuracy, and these are with ground ways and ballscrew drives. However, they may have 10 micron resolution by virtue of their drive reduction -- this isn't that hard to achieve. A 10 microstep drive automatically gives you 2000 steps per revolution with typical stepper motors, and if your gearing is sufficiently low, say like a single start 1/2-10 ACME screw, you'd end up with a resolution of 0.0005" or 1.3 microns! Will you see that sort of accuracy? Not a chance.

Resolution is typically only a small part of machine accuracy, and just sets the increments for movement. Errors in the drive system (I believe the blacktoe is a chain drive), bearings, straightness of the components used, and other factors all come into play in terms of determining how close you get to your actual commanded position. What's your application? If you need local accuracy of around 50 microns (within a 50mm window, say), a lot of systems can provide this fairly easily. If you need 50 microns over a large distance, a lot of machines will struggle, as squaring and other errors tend to compound over larger distances.

Best regards,

Ahren
CNCRouterParts (http://www.cncrouterparts.com)

kousha
05-11-2012, 04:04 PM
Thanks for the reply.

So if this sounds like a retarded question, what is the difference between resolution and machine accuracy? I want to cut a piece within 50microns! What is that called?

The pieces are not that large, they are usually 15cm by 15cm and maybe 4-5cm tall. So it Blacktoe sufficient?

Thanks

PaulRowntree
05-11-2012, 04:40 PM
Accuracy is the ability to place the tool exactly where you specify. Resolution is the distance traveled divided by the number of steps it took to get there, so it is more like an averaged property. I have a chain drive, and even giving the BlackToe a factor of two improvement over my machine (he has been developing and improving his for quite a while), I doubt that 0.002`accuracy will be routinely available with chains. I know that my surface finish can show chatter and cogging effects that are larger than this. It is hard to get steel-on-aluminum tracks to be accurate over large distances as well, and the track will were with use, probably unevenly. So the resolution will stay constant, but the accuracy will suffer.
Cheers!

kousha
05-11-2012, 04:42 PM
Okay in that case, I don't care about accuracy. I want resolution. I don't care where the machine starts on each milling. I just want to be able to get a 15 by 15 cm piece down to 50 microns.

So I am cutting plastic. Are you saying that Blacktoe is not a good choice?

Thanks

PaulRowntree
05-11-2012, 04:52 PM
I am not going to slag off on a machine I do not own. My chains won't work that well, and I would anticipate that MDF+Al rails would create problems for anyone. But really, I think you are looking for a buy/ don't buy recommendation, and I don't think anyone here is going to give it to you. I don't think Patrick would even promise to hit this accuracy spec, since it depends a lot on how you build it and how you use it.

But if you need parts to be precise to within x microns (ie when you measure the piece it is within that specification), then it IS accuracy that you need.

Have you made inquiries on the BlackToe forum? Users and owners are often the best source of real-life performance comments.
Cheers!

ger21
05-11-2012, 06:17 PM
Are you saying that Blacktoe is not a good choice?

You've been here 3 times asking the same question, and you've been told more than once that the Blacktoe is not a good choice. But here you are again asking the same questions??

As Paul said, go read through the Buildyourtools forum. You'll quickly see (and the users will tell you) that those machines have a LOT of flex, and are really only capable of cutting at 1/10 the speeds they claim, and only by taking shallow cuts at that.

louieatienza
05-11-2012, 10:08 PM
Accuracy and precision are two separate things. And you need both to machine to 50 microns. There's a great diagram somewhere on the web; accuracy is hitting 5 shots on the bullseye, though spread around the bullseye, precision is hitting 5 shots may be off center, but in the same hole. Even precision ACME screw can have a lead tolerance of .004"-.009" per FOOT, twice to four times larger than the tolearances you want.

Consider also that precision CNC endmills can have zero to -.002" diameter tolerance. A large VMC can measure this and compensate. I don't know how feasible this is for the DIYer.

Ridgidity plays a big part. Any flex or backlash can introduce chatter and off-tolerance parts. With those cheap v-vbearings (they're not the expensive Bishop-Wisecarver bearings which cost 5-10X more each), soft aluminum rails, one-ply MDF construction, (don't even think about the roller chain option), it seems prone to flex. Fine for MDF.

I'd say it would be tough to find ANY DIY machine that can hold those toleances in your material consistantly over its entire working area. Otherwise you're looking at spending at least 4-6 times what a Blacktoe kit costs.