Cnc Noob, Some Hints And Tips Please!

[British Steel]

Although I'm not too bad as a manual machinist, I'm interested in trying this CNC stuff and have a few questions for Those Who Know...

I'm planning on buying a middle-sized mill (something like an Abene, K&T No.2, Parkson 1N) once I've saved a few more pennies, probably a horizontal with a vertical head, as quills (and quill feeds) are pretty rare, I imagine I'll have to move the knee for Z movement - is this feasible? The main use will be smallish parts in aluminium (for motorbikes, mainly - think rearset footpeg and brake master mounts) and occasional steel parts (gear hobbing? sprockets?) and I realise I may have to mount a second, home-brew high-speed spindle for small work in ally etc..

I plan to mock up the CNC side before getting the mill (unless the workshop at work gets around to "releasing" a nice Deckel universal that they mothballed as a spare very soon...), so:

I've started collecting parts from evilBay, a 48v 20A PSU, two 400 Oz-in steppers (for X and Y?) and a 680 Oz-in (Z?), 5-axis cheapo breakout board, adequate PC for Linux CNC - next step is drivers, the motors are rated as 6A max current, is there a reasonably-priced driver board for this current level? The Chinese ones seem to top out at 4 - 4.5A, which would reduce the available torque, I assume. I have a few smaller, less-torquey (75 and 150 Oz-in) motors for things like a rotary 4th axis (and a 5th?) in future...

Microstepping... all the motors appear to have less problem with half (or finer) steps re resonance at speed, how badly will it degrade the available torque? Would it be best to (at most) half step and gear the motors for improved resolution?

LinuxCNC can do closed-loop, would 1 um linear scales be adequate for feedback? I picture using the "ref" outputs from the scales as home position indicators (and having physical limit switches) - I guess I could also have pulse generators for the three axes for manual machining, with the DRO replacing the handwheel collars - feasible?

What else should I worry about?

Any help very much appreciated!

Dave H. (the other one)

 

[countryguy]

I hate zero replies.... So hello Other Dave.
From the sounds of it you are going into this piecemeal I dare say. That is fine if you are versed on the controller and electronics control basics. Sounds like you are.... There are some proven quick start kits out there. They can get you in the black a bit sooner sometimes. ( just making conversation here ). When you say Linear scales do you mean the magnetic quaturture type outputs? Same q on glass scales? Anyway Jim Dawson is a great resource here. I am also newer to cnc but have the eet and industrial control background so it really helps. For your driver, seen a lot of no-namie chiniglish boards with tons of issues. Guys that bought the $42 board and play with that more than anything else. IMO splurge on a proven board. Search this forum and you will find a few mentioned. One in particular that Jim Dawson likes and uses. Cannot go wrong there as he can probably recite the manual for ya too. Guys a genius. So. Good luck, best on your mill buys. Most of all keep us posted. gc

 

[JimDawson]

I completely missed this thread. I'll study it a bit and reply later.

 

[Boswell]

Is there a specific reason that you are looking for a closed loop system? While I have very limited experience, I have not had any accuracy issues with my open-loop system. I can see that if I was working on expensive parts and was pushing the mill to the maximum possible speed then a closed loop might be critical but for the type of Hobby work that I do, I can keep my machine de-tuned so that there is little risk of missing a step that a closed loop system would catch/correct for. I may be wrong but if you decided to go with Servo's instead of Steppers then it might be more critical to have a closed loop system. Anyway, you can leave out the feedback loop initially to save $ and complexity then add it later if you think it is really needed.

 

[JimDawson]

I imagine I'll have to move the knee for Z movement - is this feasible?

Yes, it can be done. I thought about doing that on my mill, but after some study I rejected the idea as not practical. Too much moving mass, takes a lot of motor power to move it at a reasonable rate. Making a quill feed made more sense to me. Of course it would depend on the mill. http://www.hobby-machinist.com/threads/z-axis-cnc-conversion.21060/#post-199564

adequate PC for Linux CNC - next step is drivers, the motors are rated as 6A max current, is there a reasonably-priced driver board for this current level?

This drive should work fine.

http://www.ebay.co.uk/itm/Leadshine...312c986&pid=100005&rk=2&rkt=5&sd=112057289587

or this one

http://www.ebay.co.uk/itm/Leadshine...220031?hash=item48895d04ff:g:ZcQAAOSwX~dWtX1I

Microstepping... all the motors appear to have less problem with half (or finer) steps re resonance at speed, how badly will it degrade the available torque? Would it be best to (at most) half step and gear the motors for improved resolution?

I run mine at 20,000 steps/rev and have had no problems. I'm running 1280 oz-in NEMA 34 motors on my 48x96 router, and can cut at 150 IPM with it.

LinuxCNC can do closed-loop, would 1 um linear scales be adequate for feedback?

1 um is 0.000039 inches, so yeah, that's close enough for most home shops. I'm using 1 um magnetic scales on both my mill and router, in both cases they will position to 0.0001 in all three axis.

http://www.ebay.co.uk/itm/Renishaw-...hash=item1a1717c6a1:m:m54PF31Bbt3wXtzt3DRQn6w

 

[countryguy]

Hey Jim, Have you ever spec'd out a replacement for the Renishaw LM10? They are just a bit prcey and getting harder to find outside ebay as pulls or nos. wondered if you have looked? Curious is all.
tx
CG.

ps- Must have typed that first reply w/ my thumbs... staring at F360 the past few days- I'm whipped ;-) on the plus side- both half's of the final plasma head shell are 3D printing now.

 

[JimDawson]

Have you ever spec'd out a replacement for the Renishaw LM10?

Not yet, I'm going to be looking pretty quick. There seems to be only 3 pulls left in the world, I may just buy them. I think the new LM10 heads from Renishaw are around $320 each, depending on the Dollar/Pound exchange rate.

 

[den-den]

One thing to watch for; some motors and better drives are rated in amps RMS, some of the Chinese drives are rated in PEAK amps. It reminds me of those "6hp" shop vacuums that run on 120vac.

 

[countryguy]

in digging up some Scales on the 'baba' sites- I did hit a few sources..... More in my range, but who knows if this type of buy ever really works out? I've never purchased via Alibaba? Anyway- Just some Vendors that seem to have something close:

http://www.anaheimautomation.com/marketing/encoder/linear-encoders.php

Delos:
https://delos-tek.en.alibaba.com/product/1632301132-220719326/magnetic_strip_and_magnetic_scale.html
http://www.giden.ru/data/PDF/Optical_Scales/Delos_catalog.pdf


Shenzhen Hengxingxing Precision Instrument Limited Company
produt and mini-co-website.
https://szhxx.en.alibaba.com/
https://www.alibaba.com/product-det..._1204294073.html?spm=a2700.7724857.0.0.twCxrI


Ditron:
https://dcoee.en.alibaba.com/productgrouplist-212994879/Magnetic_scale.html
https://www.alibaba.com/product-det...c_845607666.html?spm=a2700.7724857.0.0.twCxrI

 

[JimDawson]

You've been busy