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proxxon KT70 CNC ?

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john constable24/06/2018 00:31:28
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Does anyone have experience of the proxxon KT70 CNC ready kit?

Basically, I want to make a spiral cutter and one of the options I'm looking at is adding stepper motors to a small milling table for moving the cutter (dremel) along the work and to advance it into the work and mount onto the top table a home made headstock with 3 jaw mini chuck and stepper motor to rotate it slowly.

I'm open to any information or suggestions as this world is new to me - I only heard the name Arduino 3 days ago!

Other options i'm looking at include converting a small lathe but getting the change wheel gear ratios right to get the desired spiral pitches doesn't look easy. The spindle motor would have to be disconnected too and turned by hand which is not necessarily that easy. I wouldnt mind the lathe option because then I'd have a lathe too!

However, the arduino controlled stepper motor approach looks like it'll be enjoyable to learn.

thanks, guys!

richardandtracy24/06/2018 06:31:35
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I have no experience of the Proxxon, but I am in the slow process of doing a 3 axis lathe conversion to do a similar job of engraving the outside of pens.

The Nema 17 motors use in the Proxxon conversion are tiny, with very little torque. I would strongly advise that you check the torque requirements of the machine you are to convert. The torque needed to apply an axial force with a leadscrew is roughly as follows (from 'Ondrives':

Torque (N.m ) = Axial force (N ) x Pitch (mm ) / (2000 x PI( ) x thread efficiency )

Where: thread efficiency = 0.35 for conventional threads and 0.85 for ball leadscrews and PI( ) = 3.1415

The axial loads you will need to consider are:

  1. Table friction, both 'stiction' and the smaller dynamic friction.
  2. Required acceleration of the table
  3. Cutting forces ( these will be tiny compared to the acceleration loads )

It is close on to impossible to guess the friction and stiction loads, so a spring balance pulling the table with the leadscrew removed is needed. You might just be able to calculate the torque by applying a load to a lever attached to the leadscrew drive handle and hanging a weight off.

OK that's one minor technical bit considered. The most important bit is being completely clear in your mind what you want to do. Is it just to create spirals? Or any type of engraving? How fast? How capable. It is an easy field to waste money in, as I have. I wanted to engrave and cut threads for pens, so bought a 4 axis gantry machine, and discovered the zeroing and repeatability of the gantry was such that the thread cutting was insufficiently concentric. For the engraving, it was adequate. But.. The multi start thread cutting on a conventional lathe is close to beyond me as I need up to 5 starts at really wierd pitches, which is one reason why I wanted the CNC. So, one answer is to convert a lathe, which is now what I am slowly doing.

Hope this sets you thinking

Regards

Richard.

JasonB24/06/2018 07:00:44
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Posted by john constable on 24/06/2018 00:31:28:

The spindle motor would have to be disconnected too and turned by hand which is not necessarily that easy. I wouldn't mind the lathe option because then I'd have a lathe too!

No need to disconnect the motor, I did not do anything to mine when I did that video for you, could slip the belt off if you wanted and there is nothing difficult about that. The spindle is driven my the gear train not turned by hand. Leadscrew turned by hand, cordless drill or a stepper if you must.

John Haine24/06/2018 07:10:16
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I bought one of those Proxxon tables to go with a small drill press, also a Proxxon, and it was money wasted. The quality of the table and screws is poor and it's very small. You would be surprised how much space is taken up by your headstock and tailstock and how little is left for for the work.

If you want to the CNC route I suggest either converting a small lathe by adding stepper drive to the headstock, cross-slide and leadscrew; or a small mill such as the X1 from Arc, or a Chinese gantry router - though Richard found its accuracy poor for threading it might be perfectly satisfactory for your purposes..

And depending on whether you want to get lost in Arduino programming as well as all the mechanics, use an established CNC controller such as Mach 3. Actually what you want to do is best done with a mill controller such as Mach 3 Mill as it can drive 4 axes - 3 linear and one rotational.

jann west24/06/2018 08:17:35
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If you converted a lathe you would not bother attempting to get the change wheels correct or turn the headstock by hand ... you would just add a 3rd axis to the headstock.

Either way, you will require 3 axis CNC control ... whether it's a lathe with a headstock control or a mill with a rigid 3rd axis is kinda the same destination, just a different path.

You don't mention the size of the spiral turning project, but a dremmel might well be underpowered ... you could consider something a little more powerful ...

There is more info online on converting mills than lathes to CNC, so that might be the path of least resistance ... CNC mills also seem to be a bit more general purpose.

Brian G24/06/2018 09:20:26
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Can I recommend looking at this thread John **LINK** It is the best step-by-step introduction to using an Arduino to control stepper motors that I have seen, with pictures and descriptions of all the components required, and was the first Arduino project I attempted. The program (sketch in Arduino terminology) includes keypad, display and stepper control, as well as some arithmetic to set the angles, so it is a handy program to tinker with.

The only caveat is that the little blighters are addictive - but with a pack of 3 Elegoo Nanos (The Uno has funny pin spacing whilst the Nano plugs straight into a breadboard or can be socketed or soldered to Veroboard) costing less than £10 on Amazon, it isn't an expensive vice.

Brian

John Haine24/06/2018 09:58:02
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Certainly an interesting challenge to use an Arduino. However, John's requirement is a bit more complex than driving a rotary table, as he has to drive two steppers simultaneously at different speeds which may or may not have a rational relationship. Also to return each axis to a known starting point after each pass. Mach 3 already has this sorted, in fact it can drive 3, maybe even 4, axes in a coordinated way. So it's clearly possible in software but not straightforward. Arduinos are very cheap (in fact Brian, after seeing your post I ordered 3 of those!) but so are old PCs that can run Mach3 happily.

john constable24/06/2018 12:12:42
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Wow, thanks everyone. Loads to consider there. I'll have some foĺlow up questions as I work my wY through it.

May I start by answerving the question of size which was remiss of me to make clear.

Essentially i dont think im likely to want to go above 2" x 8" stock. It wiuld probably be round but of course the convenience of turn down from square would be useful.I have some mobility problems and I'm in a wheelchair with some mobility problems and I had to give up my wood working shop years ago. My solution is to rebuild it in miniature and make small decorative things like boxes and clocks so twists I make will be used in that context.

I do t just want to do bod standard spirals. I'd like to try multistart spirals almost up to straight fluting and higher pitch ones almost approaching a coarse thread. I'll also want to do hollow spirals (with deep cuts rather than drilling the core).

I like the lathe conversion route because it gives me a lathe! Providing the conversions leaves it functioning normally of course. My problems with it have been sourcing a suitable lathe without spending an arm and a leg and also solving the issue of gears. As I understand it, currently change wheels won't give me the pitches I want so I either have to change them - and I'm not sure if that's possible - or I have to drive the lead screw from another stepper motor.

What would your thoughts be on that? Is there a suitable lathe out there?

john constable24/06/2018 12:23:01
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Of course I should have said that if I cen sort the gearing problem then I don't need steppers at all because as you point out, removing drive from the spindle is not a problem.
Frances IoM24/06/2018 12:43:33
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there are several reasons why metal lathes will not give you the gearing required - 1st is the size of the banjo usually used - second is the torque required to be transmitted thro the gear teeth to drive the screw thread driving the cutting tool many times faster than the speed of the chuck

you are into wood lathes where the cutting force is generally considerably less than for metal and the required accuracy is generally lower - the now discontinued Trend router lathe would appear to be the basis of what you want - however with low cost generally comes lower accuracy and it is not clear that you could reset passes on this to give the accuracy you seem to want - the Trend + similar devices would appear to have been aimed at spindles for banisters etc for low production volumes - these days the large professional CNC lathes would turn out such at cost much lower than the skilled labour cost of manual production - however you are looking at serious money here - you may well find it useful to visit Axminster tools who seem to sell into this market but have a deep cheque book.

Edited By Frances IoM on 24/06/2018 12:44:33

john constable24/06/2018 13:04:34
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I've spoken to axminster and they're having a think.

Your right about the trend lathe. Too big and too inaccurate. I need small and accurate.

Research from that sent me to the pen wizard which looks accurate enough but it's a bit too small and offers a lot I don't really need. If they did a larger pen wizard I think I'd get one.

In terms of cutting strength as I'm using a dremel I don't think that's an issue as I'm just positioning the tool and not actually exerting any cutting effort.
John Haine24/06/2018 13:44:21
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The Sieg C2 / Chester Conquest style "7x14" mini-lathe is just about large enough for the size you mention. Often available on eBay at prices 300 - 500 squids.

The book on making the Quorn describes setting up the lathe to make a spiral groove in the main column. IIRC a Potts spindle on the carriage driven from overhead with a slot drill; leadscrew driven by a handwheel and geared down to the headstock. Worth a look perhaps to see how the gears were done.

john constable24/06/2018 14:26:31
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Thanks, I'll look out for that lathe. There are some words there I need to look up.... 'making the Quorn' and 'IIRC a Potts spindle' aren't terms I'm familiar with....

john constable24/06/2018 14:54:42
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someone mentioned mach 3 software for cnc control.Can anyone recommend a primer? I'd like to understand a bit more about what it is and how its used. tia

JasonB24/06/2018 15:15:16
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The Quorn is a grinder that has a spiral cut into one of the parts, much the same as the one I showed in the other thread that is easy to set up and do on a lathe, this pick shows the spital on the vertical column

The Potts spindle would be like a high quality Dremel

As I said in the other thread it just needs a few extra gears which are available as spares, simple handle or cordless drill to drive the leadscrew if you can't physically turn it and a modified banjo & stud to mount the extra gears onto

John Haine24/06/2018 17:02:35
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Mach 3 - you can get everything you can want and more here. It's an application that runs on a PC to control a CNC machine tool. In its standard form it outputs pulses via the parallel port that can drive stepper motor drivers directly. It is programmed either by providing it with G-code or using its internal wizards. M3 Turn however assumes that the spindle is turned in the normal way, if you drive the spindle with a stepper it becomes a "rotational axis" which is best handled with M3 Mill.

IIRC = If I Recall Correctly

john constable24/06/2018 19:44:50
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Posted by John Haine on 24/06/2018 17:02:35:

Mach 3 - you can get everything you can want and more here. It's an application that runs on a PC to control a CNC machine tool. In its standard form it outputs pulses via the parallel port that can drive stepper motor drivers directly. It is programmed either by providing it with G-code or using its internal wizards. M3 Turn however assumes that the spindle is turned in the normal way, if you drive the spindle with a stepper it becomes a "rotational axis" which is best handled with M3 Mill.

IIRC = If I Recall Correctly

Thanks, John. This doc is pretty good. It's talking about windows XP and parallel ports so I'm going to see what the more contemporary setup uses.

john constable24/06/2018 20:38:22
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Ok - I had a massive brainstorm with a friend ealrier and it's helped to clarify my thoughts.

I've dismissed using a lathe and changing the gears. There may be cost and accuracy issues but the main thing is that changing gears may not be easy and won't give me a good range of pitches between the maximum and minimum I need - it'll only give me one discrete pitch per set up. left hand twists will be tricky.

I decided to go the multiple stepper motor route. Not only will it give me a variable choice of pitches, I'll be able to do the opposite thread and even vary the pitch DURING the cut which would be so cool. Imagine how elegant that would look!

Now I don't specifically need the lathe infrastructure, I could either custom make something or use another platform which I guess might be a milling machine that can be converted. Could anyone recommend a suitable milling machine or suggest a better platform?

John Haine24/06/2018 22:37:38
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Many people find the Sieg X1L from Arc a good small mill, and many have been converted to CNC. An alternative if milling metal is not important would be one of the many Chinese gantry routers available on eBay at reasonable prices. I have no experience with those but I'm sure lots of people here have. Just search for CNC milling machine on eBay and stand back...

Mach3 has the benefit of being reasonably priced and versatile. If you want it to run properly using the parallel port then an old 32 bit XP machine is best. It can also run on later Windows machines but may need an external motion controller (for example with a laptop). Mach 4 is probably better, but definitely needs an external motion controller.

Then there is Linux CNC. Many people find it very good, but you need to load Linux on your machine and it is reputed to be hard to get going unless you have Linux experience.

There are several other CNC controller software packages around - UCCNC is one, you'll find others discussed here. Again these need an external motion controller as well as a PC.

You can also buy dedicated controllers that replace the PC. If using a mill platform you will presumably need 4 axes - XYZ & A - that may limit the choice of suitable controllers.

If you are going to start converting a machine then be warned that it is quite involved and you'll need access to machine tools to do it - that may make the gantry router approach attractive if it comes with all the mechanics and steppers already assembled. Another benefit is that they usually have high speed spindles, better for working in wood.

john constable24/06/2018 23:08:37
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If I was going to use something like the proxxon MF70 I would probably still have to mount the dremel to get the spindle speed - however, they do a 3 axis cnc kit that is pre-engineered. Then I have to add the 4th axis.

I've looked for 'gantry routers but nothing is really coming up. Are they called something specific?

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