Harrison Universal Miller Conversion

[will gilmore]

Super. Thanks.

[andypugh]

It's been a long time, largely because I found lots of other interesting projects along the way. But I now have 4 CNC axes on my Harrison Universal and a setup which lets me hob gears too.

This video shows it going through its paces:



Other videos in the same gallery show it hobbing a timing pulley and a gear pinion.

Photos of the build process are in this gallery:
https://plus.google.com/photos/10816...88686374813281

I am more than happy to loan the patterns to anyone else wanting to do the same conversion.

Any questions?

[jsheerin]

Looks nice!

[andypugh]

You might have noticed in that video that there is no end-support for the ballscrew at the left end of the table. As it is a static screw this probably does not matter at all, but it looks wrong so I decided to make an end support.

To make it a little less purely cosmetic I decided to incorporate a tool tightening / height measurement station in to the casting (I am not sure why this isn't common)

The casting was a bit big for my angle plate, making for a rather Heath-Robinson setup:

From Harrison Mill

Tool-holders are generally tapered, and the BT30 on my machine are no different in this matter, with the standard 7/24 taper. So I needed to bore a tapered hole in the casting. Luckily I have a Wohlhaupter boring and facing head, and that can be set up to change diameter by 0.1mm every revolution of the spindle. Then it is a just a matter of synchronising the Z-feed to suit. And there is G33 for that. The flange is 35mm deep and the taper is 7/24, so it's G33 Z-35 K[0.1 * [24/7]]

From Harrison Mill

Trial Fit:

From Harrison Mill

And with a coat of paint, ready to start populating that tool table:

From Harrison Mill

[andypugh]

I have always been rather conscious that the 12mm Y-axis screw and standard-clearance ballnut probably wasn't really ideal, but I din't think I could fit anything more in. Then I found that GTEN list an RSH nut that is only 1" diameter in the 16mm ballscrew size. The existing RSW 1205 nut is 25.5 diameter, so I could have a bigger screw and a fractionally smaller nut. I also found that the factory were able to supply it special-order as a preloaded variant. £100 for the ball and the screw, but I decided to go for it.

When it arrived it looked a lot beefier than the 12mm, a serious improvement:

From Harrison Mill

However, when I unwrapped it, there was a nasty surprise.

From Harrison Mill

The new nut has an external ball transfer tube not shown on the drawing in the GTEN catalogue! This seems like something of an oversight to me.

The nut and screw sat in their box all summer while I tried to decide what to do. Eventually I got the CAD out and started to play, and I worked out that I could _just_ squeeze it in. The tube meant that it couldn't screw into the housing and lock with a nut like the RSW nut did, it would have to be a split housing with clearance for the transfer tube.

From Harrison Mill

One advantage of this is that I do at least know the orientation of the nut in the housing. The Harrison has an oil nipple for the leadscrew (with a complicated drilling to get the oil to the bore where the nut is attached). The transfer tube isn't oil-tight, so works nicely to get oil into the nut. That is what the secondary little hole is for.

This is it all bolted up around the nut:

From Harrison Mill

You might be wondering what the taper on the end of the screw is for. There is a bit of a problem with assembly:

From Harrison Mill

The original leadscrew simply went in from the front. Not an option when you want to put the nut and the screw in as an assembly (trying to assemble the nut onto the screw inside the knee of the machine seems not so much to be tempting fate, as poking fate with a stick and calling it rude names). So I borrowed a brilliant idea from Wohlhaupter. (I did the same with the 12mm screw). I machined a taper on the end of the screw (Ballscrews are really fairly easy to machine with CBN at 120m/min. It's moderately spectacular too.) There is a matching taper in the socket, and the socket has a M10 x 1.5 thread in it.

From Harrison Mill

The screw has an M8 x 1.25 thread in the end, and I made a special 2-thread socket screw. The screw goes in the end of the ballscrew, the socket is screwed on until snug, then the special differential screw is backed out using an allen key up the end of the shaft.

From Harrison Mill

Because the M10 thread is coarser than the M8 thread the effect of this is to pull the two parts very snugly together, as if by using a 0.25mm pitch screw. It also means that I just need to screw the screw back in again to push the taper apart.

From Harrison Mill

On with the swarf-shield:

From Harrison Mill

And the servo-mount casting and drive system:

From Harrison Mill

The handle is just used to move the table by hand for jobs like checking the Gib resistance. It pulls out, and drives the screw by means of a really quick and easy sort of dog-clutch that I found in Machinerys Handbook:

From Harrison Mill

By this time it was getting late, so no test cuts yet. I did find one issue, but a very minor one. The saddle now runs to the limit-switch, because the new screw is 15.08 mm pitch, and I forgot to change the scale setting...

[bogus105]

Great project and fantastic work! Would you share details of your steady screw-driven nut assembly? any dimensions for reference? I wonder how you made it so compact. What bearings did u use - a pair of AC bearings? Mated/preloaded in the factory or preload is provided by you by means of some sort of shims? Have you got more photos of your conversion? I bet many wants to see as much as possible What size ball screws did u use for XYZ? XY are 12mm dia?? I thought they are way bigger by the photos. And please reveal some details regard that ATC gripper - what steel, how did you machine split grippers, what hardness after tempering, what belleville springs, how many and what pulling force you have? So many questions but you've made SOOO interesting project!
cheers
Bogi

[andypugh]

Would you share details of your steady screw-driven nut assembly? any dimensions for reference? I wonder how you made it so compact. What bearings did u use - a pair of AC bearings?

The 3D model of the parts can be found (and downloade din various formats) from Unsupported Browser ~ A360.
The bearings are double-row angular contacts, but used and preloaded as single-row. (basically with a wasted row) because that was the only way to get a small-enough section bearing. I probably made it a bit too compact, and would have been better letting the ballnut protrude into the space under he table and have a bit more material around the bearings.

Mated/preloaded in the factory or preload is provided by you by means of some sort of shims?

The bearings are preloaded by adjusting a threaded collar. There are slots in the inner bearing sleeve and one more slot in the collar to give a vernier-style adjustment. A wire clip fits into one pair of slots to lock it. I didn't bother making 3D models for the threaded collars that clamp the bearing outer-races into the main casting and that clamp and preload the inner races.

Have you got more photos of your conversion? I bet many wants to see as much as possible

If you click the album link in the small thumbnails above you should get (pretty much) all the photos I took.

What size ball screws did u use for XYZ? XY are 12mm dia?? I thought they are way bigger by the photos.

No, only the Y was ever 12mm, and that was later expanded to 16mm. The X and Z axes both use 25mm ballnuts.

And please reveal some details regard that ATC gripper - what steel, how did you machine split grippers, what hardness after tempering, what belleville springs, how many and what pulling force you have?

The ATC gripper is made from BS1407 tool steel (also known as Silver Steel). The split collet was made by clamping together 4 pieces of square material in a 4-jaw chuck. I then turned a short section and pressed on a collar and centre-drilled the end (actuallly into the 4-way intersection, the collar was needed to stop the lathe centre spreading the bars.) It was then moved further out of the chuck to machine the outside profile. I then cut off the excess and held it in a collet while I machined-off the collar and machined the inner profile. when it was removed from the collet I was left with 4 separate petals.

[bogus105]

Thank you Andy for reply. Now i get the concepts behind the driven nut. I came to the same conclusion (4 squate pieces) when thinking about ATC grippers. Did you use Silver Steel for rod and other elements to be hardened? Ball screws are from GTEN (taiwaneese?) or some chineese sort or what?
cheers

[andypugh]

I used silver-steel for the round parts of the ATC gripper and the central rod. The outer tube was some thick-wall hydraulic pipe drilled-out and machined on the OD. The end where he gripper lives was hardened silver-steel, screwed in to the tube with (IIRC) a 14.75 x 0.9 mm thread. With a CNC lathe I wasn't limited by standard threads, so chose one that gave me a 1:1:1 split through the threaded region.
I suspect that I made the actual gripper petals out of key-steel, having proved that it could be hardened. Or I might have later-on re-made them with square silver-steel. It's been quite a long time since I did that part.
(Key-steel is .55% carbon, silver steel is 1%.)
GTEN is a Chinese brand of ballscrew. But one with at least a name, and a reasonable reputation.

[bogus105]

.... But one with at least a name...

I wonder how did you drill-out so long hydraulic pipe accurately?

... so chose one that gave me a 1:1:1 split through the threaded region. .

do you mean outer tube thickness is the same as thread thickness and inner element thickness in cross sectional view?

What is outer tube O.D? Do you recall the X-axis driven nut bearings type?
Thank you for link to your CAD project - now i get the overall arrangement. By the way what do you think about fusion360? I've read i can use it for free as hobby amateur

[andypugh]

I wonder how did you drill-out so long hydraulic pipe accurately?

I used a long drlll and trusted the existing bore to guide it.

do you mean outer tube thickness is the same as thread thickness and inner element thickness in cross sectional view?

Yes. Mainly because it amused me

What is outer tube O.D?

16mm I think. YOu would need to choose something to suit your machine. And might well not be able to fit the stack into the spindle drawbar bore like I could.

Do you recall the X-axis driven nut bearings type?

7209

By the way what do you think about fusion360? I've read i can use it for free as hobby amateur

It's very good, especially for the price. And you can even use it commercially as long as you make less than $100k a year.