View Full Version : Fourth axis

02-18-2008, 09:49 PM
The last few days I have been busy putting together a fourth axis unit for my mill. It is now finished enough to operate although there is more to do, quite a bit more. It uses a headstock from a mini-mill with a MT3 spindle. I replaced the bearings with smoother ones. The base is a chunk of 7075 aircraft aluminum ( :D ) and it has been milled on all sides for parallelism and flatness. When I built the controller for the mill I included all the circuits etc for a fourth axis as this has been part of the plan all along.


This is however more than just a 4th axis drive. It slots into the mill table with a underneath rail and is fully aligned without further fuss. I will be adding a rotating mount so it can be angled. The drive system uses a 280 oz stepper and a 100 tooth worm wheel driven by a double enveloped worm. It requires 222.222 steps per degree in 1/4 microstep mode giving a resolution of appoximately 16 arc seconds. That can be increased at the expense of speed by using 1/8th step mode. It currently rotates at 3400 degrees per minute.


The drive is a self adjusting zero backlash design. By using a double enveloped worm the gears have a large contact area making it possible to use a spring loaded worm with the worm wheel. Because of the way a double enveloped worm meshes with the wheel it won't skip out of mesh except under very heavy load.

The design is simple. On the left is a needle bearing to locate and control the worm and on the right is a pair of miniature ball bearings face to face and captured by the mount, the worm and the pulley. These two bearings absorb the thrust loads, one for each direction. They are rated for about 50 lbs static axial thrust and near double that when rotating. The worm shaft is hardened 1040 steel and the worm wheel is 660 bronze. I hobbed the wheel with a hob I made a couple of years ago for this sort of purpose. I will build a clear lexan protective housing for the unit to keep chips out of the gears.


Now, about the 'more than just a 4th axis part'. You may have noticed the HTD drive belt pulley. This unit will also serve as the headstock of a CNC mini lathe. By flipping a cam (not yet installed) the worm is disengaged from the wheel allowing a DC servo motor (with encoder, not yet installed) to drive the spindle. A fully functional lathe style tailstock is under construction as well and will be adaptable to the mill as a 4th axis accessory and for use as the CNC lathe tailstock. All that remains to make it a lathe is a bed with carriage ways and a pair of servos, which I have. The headstock and tailstock are simply placed into the waiting bed assembly for use as a lathe. All parts are self aligning to angular tolerances of better than .001" per foot.

02-18-2008, 09:53 PM
How about the software issue of getting all axises to work in concert?

Can it run all 4 at once for complex contours?

02-18-2008, 10:16 PM
Mach supports up to 6 axis.

A lot of folks on CNCZone have used the Mini-Mill headstock as their 4th axis.

02-18-2008, 10:41 PM
Thing looks good Evan! Geez...you get all that other equipment working you'll be able to give the old SB9 a bit of a rest. That ol' girl wouldn't know what was going on if you just turned round stuff on it ;)

02-19-2008, 12:10 AM
Looks real good Evan. Gives me some ideas for for my lathe :D.
Just a thought, maybe consider using a GT2 type belt drive for driving the worm instead of the type you have now. They have less backlash and are more accurate. That's what I use on my cnc mill and cnc lathe.

Michael Moore
02-19-2008, 12:55 AM
Evan, why not just leave it mounted on the mill table and use the mill spindle to hold the lathe turning tool? Your mill with 4th axis is just a lathe with the cross slide rotated 90 degrees if you aren't spinning the mill spindle.


02-19-2008, 01:11 AM
The standard XL belts don't have a lot of backlash, especially if tightened well. The Gates GT series were developed mainly to replace the HTD series since they have terrible backlash. They weren't intended for precision positioning duty but as a High Torque Drive belt instead. I tested an HTD belt recently in a small jig I put together and was disappointed to find that it had over 1 degree of backlash on a medium size pulley with around 30 teeth. I can see the benefit of replacing the belts on the X and Y axes but for this device it won't provide a detectable benefit. For the largest item that can be turned, about 10" diameter, the change in axial position at the circumference per step is only ~0.0004".


Please note that the thumbs down icon was NOT INTENTIONAL.

02-19-2008, 01:14 AM
Evan, why not just leave it mounted on the mill table...

Then I can't do two things at the same time. It has certainly crossed my mind and it wll be operable as a lathe on the mill table.

02-19-2008, 01:27 AM
Very nice, you are a workaholic. You have been real busy in the shop crankin the tooling out. Im impressed!! JRouche

02-19-2008, 06:27 AM
I spent most of two days overhauling that little three jaw chuck. It originally came with my SB lathe and was a complete POS, or so I thought. The back plate was a random disc of metal with threads that looked as though they had been chewed by a squirrel. The body was at an angle and appeared to have been used as a hammer, or maybe an anvil. The jaws were bellmouthed etc.

I made a new back plate using 1/2" 2024 aluminum and a cast iron hub. I threaded the mill spindle at 1 1/4" x 12 tpi and the chuck plate to match with a proper register. The jaws are reground and will hold a rod centered to better than one thou in 12 inches. I remachined the body to clean it up and ensure face perpendicularity to the axis of rotation.

It is now a pretty decent little chuck and repeats a hold nicely. The two pluses that I didn't realize is that the scroll seems quite accurate and the jaws are a nice close fit to the body but without binding.




I don't consider shop time to be work. There isn't much else I would rather do in winter. I spend most of each day in the shop right now because when spring arrives my shop time will be very limited, as usual. I like working with metal and always have since I was a child. It doesn't much matter what sort of metal work either as long as it involves creating something from raw material or restoring functionality. I also like to reuse materials as much as possible. It is cheaper and cuts down the waste stream a bit.

kap pullen
02-19-2008, 07:56 AM
"The base is a chunk of 7075 aircraft aluminum ( ) and it has been milled on all sides for parallelism and flatness. "

That's nice work Evan, but unless you have certs for that plate, it ain't aircraft aluminum........anymore.

That's all I hear, Get the Hardness and Conductivity checked, and material certs on the 6061-t651 projects I get at work.

If that's not done, it could be two weeks work in the dumpster.

They give me jobs so hot, to the drawing is still on a napkin.
I don't always know it's a FLIGHT job till the paperwork catches up.

I always get certified material to cover my butt anymore!


02-19-2008, 08:42 AM
Why the thumbs down?
a 2mm gt2 has about 10 times the positional accuracy of a mxl belt for instance.A 3mm gt2 has around 5 times the HP rating of a xl belt.
SDP/SI has plenty of charts comparing the different belt drives.
I agree I would not use a HTD belt drive for positioning and that the XL belt drive probably works just fine for the rotary axis.
I have used the xl belt drives on my cnc mill and IMHO the GT2 3mm pitch belt drives are quite superior.
They are so cheap and you did such a nice job on the 4th axis I just thought you might want a better belt drive. I wasn't being critical, I just thought maybe you were not aware of how well the GT2 series works.

02-19-2008, 08:57 AM
Sorry, I have no idea how that got there. You may have noticed I don't use reply titles so it's a mystery to me. I tried to edit it but you can't edit the title.

02-19-2008, 09:03 AM
That's nice work Evan, but unless you have certs for that plate, it ain't aircraft aluminum........anymore.

Heh. It's 7075 all right. If you haven't milled 7075 before and you do then you are in for a treat. While it finishes beautifully the swarf sucks. Normally aluminum alloy swarf is nice because it doesn't stick or cut like steel does. 7075 is harder than mild steel though. The swarf from finishing cuts is a lot finer that steel because you can take micro thin cuts. It's almost like fiberglass and you end up with all sorts of little bits sticking you and even itching.

02-19-2008, 10:16 AM
Hell of a nice job Evan, you've got yourself a beautiful one of a kind piece of equipment there. Just think, with this little baby you can make even nicer stuff.

No need for an airworthiness certificate on the 7075...that little unit will fly just fine, CaptainEv.