small mill for machining steel. Max spend £500 including computer

[Andrew Wilding]

Hello all

I thought that I would post my build log here of a small mill. The aims of the project are below:

max spend £500 including computer and cost of base machine - this a real budget as it is the max funds the missis' will release and it will also be an interesting excercise in how to get reasonable results on the cheap.

capable of milling mild and possibly stainless steel with <6mm carbide cutters

Accuracy of +/-0.001" over an area of 4"^3

[Andrew Wilding]

the basis of the mill is a Centec 2 horizontal purchased a few years ago for the sum of £150. This served as my mill until replaced with the AEW show in the pictures below:




This machine is very tired with lots of bed wear and backlash so the first task is to rectify this. It is also a knee mill operated with a rack and pinion system and stops. A rubbish threaded rod lead screw system has been added later to make the knee more controllable. I will have to come up with a better method to raise and lower the mill.

[Andrew Wilding]

Ok I admit that I have already done a lot of the work, but I will go through it like a chronological build log.

Scraping the vertical slideway on the knee (before showing uneven wear):



after showing spots per inch (note rack):



Milling the knee:



The knee was aligned to a ground rod that had already been trammed. Wear was beyond scraping on horizontal part of knee.

[Andrew Wilding]

the x axis was also machined with a dovetail cutter on the mill

[Andrew Wilding]

The ways were built up with PTFE. This was sheet purchased from E bay and had an acid etched back to allow it to be bonded to the ways using epoxy resin.

The PTFE was then machined an scraped to alignment

[Andrew Wilding]

Ball screws are the obvious choice for a cnc mill but alas the budget did not extend this far. I opted to make the best out of conventional screws. I selected 16mm dia by 4mm pitch trap leadscrew stock. This has a max pitch error of 50 microns per 300mm (2 thou per 12") which should be sufficiently accurate to meet my accuracy target. I may experiment with lead screw mapping later on. The 16mm stock is sufficiently stiff over the lengths I am using to only require supporting at one end. At the supported end I am using a preloaded pair of angular contact bearings to reduce support backlash as much as possible.

I am using wear compensating antiback lash nuts from delrin. There are made as a main nut and slave nut biased together using a torsional spring made from piano wire.

Below shows the nuts being made on the small lathe. the thread was roughed in the lathe to ensure parellelsim with the nut. These were finished using a tap made from some of the stock.



The slave nuts were castellated together on the mill. Note the DIY dividing head. This was made when I only had a lathe and uses change gears to provide most of the divisions required in my shop.



This shot shows the three lead screws, bearings and nuts assembled on the bench



Total cost of leadscrew assemblies about £25 as opposed to at least £200 for similar pitch accuracy ball screws.

[Jonathan]

Looking good.
I'm interested in your nut design - do you have a drawing that you could post here?

[M250cnc]

Andrew you have discounted the efficiency of ball screws.

The use of a knee mill is good but the downside is the amount if force needed to move the Z axis.

You could calculate the amount required using a spring balance, have you done this ?

Guys who do Bridgeports use the quill for the Z axis

I do have a Bridgeport myself and did a bit of checking.

The Y axis has an 83mm offset and took 2Kg of force to turn it

The Z axis has an offset of 185mm and took 4Kg to turn it

1000 / 83 = 12.048
1000 / 185 = 5.405

2Kg / 12.048 = .166Nm Required
4Kg / 5.405 = .74Nm Required

.74 / .166 = 4.458 So if the norm was to use a 4Nm motor for the Y axis then amount of force to drive the Z compared to the Y axis would be a 17.8Nm axis motor "4 * 4.458"

Phil

[Andrew Wilding]

Phil,
Thanks for you experiments. I realise that ball screws are more efficient. I have chosen larger steppers than I would have with ball screws. The teflon coated slides also help increase the overall efficiency of the system a bit.

As you will see when I post the detail of how I did the knee, it is counter balanced. This obviously improves the force balance on the system but the knee is still a fairly heavy part to accelerate. I tried to take the easy route out with the design and replace the threaded rod with the screw stock but I was not happy with it and had to go back to the drawing board! more on that later.

Jonathan,
Thanks for the post. I was at Notts 6 or 7 years ago and studied mech eng (ahh the smell of the Coates building!). I now work for a gas turbine manufacturer but still live in Nottinghamshire. I did do an autocad sketch of the nut but it was only to see if it looked right. I will try to post this or possibly just a better picture, although there was not much science behind it. The slave nut is castellated to allow the spring load to be adjusted and so there was a bit of experimentation involved. The two halves mate with a 60 deg chamfer so an increased locking torque is experienced between the two parts when compared with just plain faces for a fixed spring rate. This should increase the thrust required to separate the two halves of the nut with the same spring rate. It seems to work ok. The lash in each axis is about a thou but I an now adjusting everything to see if that can be reduced. Without the slave nuts the lash is 6 or 7 thou (I deliberately made this a loose fit so that the only friction is on the useful faces)

[M250cnc]

Andrew not easy to counterbalance the knee, but good to see you have it worked out.

Good luck with the rest of the build which i will keep an eye on.

Phil