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richb77
20-10-2008, 08:09 PM
Has anyone converted an axminster turret mill to be CNC?

just wondered if they could be a good starting point. I know they will have pretty shocking backlash but the CNC should take care of that?

If not i guess i am still on the hunt for a suitable or scalable set of plans for a 3/400mm aluminium capable machine.

Cheers

irving2008
21-10-2008, 12:21 AM
He means a machine with approx 300 or 400mm working envelope and capable of cutting aluminium..

Rich wants to build from a plan but there's nothing directly suitable around (I've not found anything either) so he's looking at the option of CNC'ing an existing mill... but TBH Rich, on your proposed budget the sort of mills you could get will be pretty poor.... they'll need a lot of work to get them up to the standard needed...

richb77
21-10-2008, 09:37 AM
Thanks for the info guys.

I did think the backlash would be pretty bad on a sub 800 machine. Just a thought though ;)

I did indeed mean a machine with a bed 300 to 400mm.

I have been doing some scrounging and plan reading. I think i will end up trying to find the time to create my own machine. The frame is the easy part, fixtures and fittings is the tricky bit. I thought initially of using a dremmel for the head....Now beginning to think it wont be man enough.

richb77
21-10-2008, 11:03 AM
Oh for the time :D

Modifying a Harley, Selling an house, starting a new job and now a desktop CNC...I must be mad. Either that or my better half is!

I had come to the conclusion to do it myself. I'll keep asking semi-stupid questions and keep you posted. :beer:

irving2008
21-10-2008, 02:19 PM
Oh for the time :D

Modifying a Harley, Selling an house, starting a new job and now a desktop CNC...I must be mad. Either that or my better half is!

I had come to the conclusion to do it myself. I'll keep asking semi-stupid questions and keep you posted. :beer:
to mill aluminium you need in excess of 150 - 200W cutting power at the spindle.. that means 300+ W input. A Dremel is only 130W max input and the runout is poor. The generally accepted options in reverse order are:
a proper milling spindle... 400 up! main cons: complexity, size, weight
a Kress 1050W (http://cgi.ebay.co.uk/Kress-1050-FME-Milling-Grinding-Spindle-Motor-Router_W0QQitemZ310088271202) ~170 cons: weight (2.4kg), price (there is an 800W one also for 120), small collet (8mm)
a good quality 1/2" router ~100 - 200 cons: size, weight (typically 3 - 4kg), price maybe
a 500W die grinder ~50 cons: runout (but not too bad), small collet (max 6mm dia tools) - this is what I use, it weighs 1.7kg (http://cgi.ebay.co.uk/DeWalt-D28885-500W-Die-Grinder-240V_W0QQitemZ190042122163Q)
Proxxon ~120-150 cons: low power, small collet
Dremel ... just dont bother... just about OK as a low end PCB mill/drill

irving2008
21-10-2008, 03:18 PM
Any chance of some pictures of the machining capabilities of your cnc machine Irving?


I hear you on the lack of time Rich....Are you a farmer?Not yet Kip, the bigger one is still in the planning stages, but stacks of research and planning done so I'm pretty confident of the outcome. in the meantime my little 250 x 250mm MDF unit is nearing completion. It will use the same die grinder spindle as the larger one and the same electronics initially but I'll need new steppers for the big one. As always funding is an issue... and i still have 2 lathes waiting to be refurbed too :) Time? never enough of it...

irving2008
22-10-2008, 12:08 AM
do you use your mill as a drill? I am wondering whether to try and combine things to get better value for money...

richb77
22-10-2008, 05:11 PM
http://s281.photobucket.com/albums/kk208/HOG1450/?action=view&current=cncmill.jpgI've been working on my chassis/platform.

So far any comments? Any glaring issues (bear in mind its not complete)

I am getting confused over the ball screws ends and fixtures. Any suggestions to what goes where?

Cheers

irving2008
22-10-2008, 05:55 PM
rich,

Depending on a number of factors, leadscrews (ball or otherwise) need to be supported in one of the following configurations:

- none: Only suitable for very short screws and low power environments where endfloat/backlash isnt an issue - the stepper motor sets the endfloat as its rigidly coupled to the leadscrew. the stepper experiences reflected axial and radial loading and so its specs determine the maximum work load that can be tolerated.

- far end: use for longer screws (length >20 * dia) where whip at speed is an issue. Usually the bearing just provide radial support. Endfloat still set by the stepper with a rigid coupling, although its possible to use back-to-back angular contact bearings to provide both radial and axial support/control. Stepper will still experience radial loading reflected from the workload though.

- near end: rarely used on its own, but provides a means of decoupling the stepper so the bearing block controls the end float and therefore removes axial and radial loading from the stepper.

- both ends: the best but most expensive option. The far end controls whip, axial and radial loads and uses back to back angular contact bearings to control endfloat. the near end is a sliding fit on the leadscrew and controls radial loading only allowing a sliding coupling to the stepper. Used for maximum loading environments and large screws where temperature-related expansion may be an issue.

Most hobby CNC use none, far end or both; near end rarely seen (bet I'm going to be proved wrong now)

Note: backlash in the workpiece movement is a combination of endfloat in the leadscrew mounting and axial backlash in the nut (movement from one face of thread to other on directional change) and radial backlash in the stepper to leadscrew coupling.

irving2008
22-10-2008, 06:02 PM
http://s281.photobucket.com/albums/kk208/HOG1450/?action=view&current=cncmill.jpgI've been working on my chassis/platform.

So far any comments? Any glaring issues (bear in mind its not complete)

I am getting confused over the ball screws ends and fixtures. Any suggestions to what goes where?

Cheers

Rich, looks good so far... but suggest you start a new thread in the DIY Machine Building forum (http://www.mycncuk.com/forums/forumdisplay.php?f=37)