Homegrown articulated arm hollowers - Steel or Aluminum?

[Tom Giacomo]

I used one inch square steel for my homemade unit.

[Adam Petersen]

I used one inch square steel for my homemade unit.

Tom, how is it attached to your tailstock? That's what I want to do with mine.

Adam

[Robert Willing]

Ed your entire system is very unique "love it."

[Dane Riley]

Victor sells Morse taper to bolt thread adapters for cheap. Tap a block of metal and thread it in. A chain around your tailstock will lock it in.

6061 T6 aluminum is almost as hard and stiff and scratch resistant as steel.

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PM me with an email adress for a writeup.

[Tom Giacomo]

Adam here are two pictures of the clip I made. The clip consists of a 1 1/4 inch square tube 1/8 inch thick 2 1/2 inch long with one side cut off. Then a piece of metal as you can see in the picture was fabricated and welded to go from the tube to the tailstock depth locking handle. The things you cannot see in the pictures is a 3/8 inch bolt going through the center of the clip all the way through the tailstock barrel and you can see in the second picture it is connected with a turn locking handle. Also just behind the clip are three 2 inch washers with 1 1/4 inch center holes to take up the space left by the barrel not retracting all the way into the tailstock. I have been using this setup for about 4 years now with no problem. There are several advantages to this setup. 1. It is relatively inexpensive to make but requires one weld joint 2. It leaves the hollowing arm dead center no need for a lot of adjustments 3. You can move your tailstock with it attached 4. And the best of all I can install my hollowing arm in less the 30 seconds.

[Adam Petersen]

Okay, plans are set. Here's what I have. I'm going to take a bit of several people's designs and not reinvent the wheel here. I've attached a drawn out plan and pics of a wooden prototype I threw together to make sure everything articulated well, lined up, etc. This should be fun.

The arms will be 1" sq cold roll. It willl be attached to the tailstock with an MT2 that I ordered that will be welded to the 3" block at the tailstock. That MT2 has a 1/4-20 drawbore that will secure it in place to the tailstock with a handknob. The joints will be 1/2" grade 8 bolts with locknuts. There will be bronze washers between the arms at the joints. I will attach the laser to a 3/8" vertical bar that will have a square union of some aluminum I have laying around. It will be locked with set screws. The horizontal arm will also be 3/8" round and it will travel through the block and also have set screws so I can adjust it in and out and swing it left and right. The laser has been ordered from digikey (thank you Dane Riley for the email with the part numbers). The end bore will be 3/4" with set screws. I've ordered a John Jordan 3/4" straight hollower and also the bushing reducers to 5/8 and 1/2". That straight hollower, coupled with my Sorby bent arm, Harrison Simple Hollower and Crown multi-tip should give me a good array of tools to use. I can also work on making some of my own as needed.

I have one last question. I've read all types of info on bearings, bronze Oil-lite, etc. Do I really need anything other than a well drilled 1/2" hole and the Grade 8 bolts? I figured I'd Teflon coat them and put them in place and I can't imagine the wear would be too significant as to create slop in the joint. Any thoughts on this?

Again, thanks for everyone's input and I'll post the final product when I'm done. I'm going to have my dad use his auto-painting skills and make it red to match the lathe. Hopefully I'll have a sharp final product to show you.

AdamPrototype2.jpgPrototype1.jpgPlans.jpg

[Richard Casey]

Adam, it will work as you described, but it will work much freer with brass or bronze sleeves. If you want it perfect you can also add toroidal end thrust bearings between each part as well.
Rgds,
Richard

[John K Jordan]

I have one last question. I've read all types of info on bearings, bronze Oil-lite, etc. Do I really need anything other than a well drilled 1/2" hole and the Grade 8 bolts? I figured I'd Teflon coat them and put them in place and I can't imagine the wear would be too significant as to create slop in the joint. Any thoughts on this?

I've never used one of these hollowing arms so all my experience with pivots is on other things. Seems like a large hollowing arm could be subject to significant twisting force.

With four pivots on the arm I would certainly want zero play in each joint. In my experience bolts make sloppy bushings but of course that would depend on the precision of the hole and length of the contact area. Note that teflon is pretty soft and might distort and make a sloppy joint, but again, a longer contact would help. Looking at things intended to pivot without play I usually see two bearings/bushings on each joint.

BTW, precision holes can be machined by drilling undersized and reaming on a milling machine or metal lathe.

At my local fab shop I found cold rolled rod and heavy wall tubing that fit well together made smooth and strong pivots. The rod, about 1/2" if I remember, fit the tubing very well. I made the pivot about 1.5" long since the longer the contact the less inherent play. I welded all of these in place but the rod could be threaded or grooves cut for retaining rings. With a little dry lube

JKJ

[Dane Riley]

The needle thrust beatings reduce the static friction. This gives a much better feel for the inside surface, which is all you have. And they are not very expensive.

[Brice Rogers]

Hot roll might be less expensive.
For your bolts, I'd suggest getting the kind that have a section (like 1" long) that is not threaded. That'll keep the threads from chewing into the bar or the bar from reforming the threads and getting sloppy.
The thrust bearings seem like a good idea but is something that you can add later.
I considered using Teflon washers between the sliding surfaces - - like a low-tech thrust bearing. But I ended up using HDPE (poly from the bottom of a bucket) and they are low friction.
Many drills will drill oversize. Some worse than others. It has to do with unequal sized cutting surfaces.. John's idea of drilling undersize and then reaming is good. Another alternative is to drill undersize (by perhaps 1/32nd) and then drill the final hole. Drill a test hole and then check it for slop or measure it.
Good luck. Keep us posted.

[Marvin Hasenak]

I used nylon lock nuts, so that I could tighten them and still have movement and they will not come off. For bearings, I drilled my holes larger then the bolts I used, 1/2" bolt and drilled 7/8" holes. Then back plugged the holes with sleeves I made from 7/8" diameter Delrin with 1/2" holes center drilled. The 1/2" bolts are a little less than 1/2" in diameter, so I had smooth movement.

[Tom Giacomo]

You have some good advise here may I also add as to fitting bolts I used 3/8" bolts with smooth shaft on them not have threads in the bore I used a letter drill for the hole making a tight fit, I agree with the nylon locking nuts. If you try to use bearings you'll find a need to drill a very large hole in a 1" square bar to fit a bearing or brass sleeve.

[Dane Riley]

Needle thrust bearings do not require drilling, although you certainly could. Nylon lock nuts are a must.

Why weld the Morse taper? http://www.victornet.com/detail/DCA-2M-1220.html and a tap set has to be cheaper and more likely to be straight.

[Adam Petersen]

Pfft. Because.....


I actually don't have a good answer other than I wanted one with a drawbore on the end to secure it in place. I couldn't find one with the threaded end with a drawbore on the other end. But of course looking at your link to victornet I see they have one and for about $6 CHEAPER than the one I ordered from littlemachineshop.com. Hmm.....maybe I'll ordered one of those and send the other one back. I'll look at it when it comes and decide. Thanks for that link!

Needle thrust bearings do not require drilling, although you certainly could. Nylon lock nuts are a must.

Why weld the Morse taper? http://www.victornet.com/detail/DCA-2M-1220.html and a tap set has to be cheaper and more likely to be straight.

[Mark Greenbaum]

I am completing one that I made from 1.5" square aluminum tubes (square corners inside and out), 1/8" wall, and used about $50 worth of Oilite Bronze bushings and shoulder bolts, and nylon lock nuts. All that I have remaining to do is make the actual cutting end. It should be fine because all of the rotational loads are going to be carried through the 1/2" diameter shoulder screws and bronze bearings. I'll take pictures and report on how it works after I get the end effector completed.

Here's some crude pictures:
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