View Full Version : Welding Machined Parts

06-22-2009, 02:14 PM
My brother, being just about the Chief Hodaka Nut in the world, has me building a rectangular steel swing arm for one of his early bikes. The original arm is known for being a bit of a wimp and since there's nothing better than overkill ...:D

Here's a pic of the general idea.


In addition to being a flexi-flyer the original arm also pivoted by flexing rubber bushings. Kinda like the eye of a shock asborber. That's OK for small amounts of movement, but if you're gonna stretch the frame to run taller shocks and a longer arm to get more suspension the rubber bushings need to go.

Later model bikes went to a steel on steel pivot with a zerk to keep it greased up, and I've been arguing for bronze bushings, but he's stuck on Delrin. He read that was the way to go in a motorcycle mag somewhere and that's all there is to that! So .... realizing the Delrin would get fried when I welded the arms on I made up some temporary steel inserts to take their place and keep everything straight. I'm running the Delrin .005 over for a tight press fit but I made the steel inserts .001 under to keep everything lined up and still be able to remove them.

And that's the problem.


You can see the steel insert in this pic and apparently due to distortion from welding they are now fixed in place. I'm talking stuck.

This particular arm is prototype #2 so I'm not concerned about losing it. Ron thinks I can sell these things to the other Hodaka Nuts so I've built a jig and intend on making more.

I do need to figure out how to weld them up without destroying my machined dimensions tho. It ain't like I can throw a swing arm up on the lathe ...

Anybody have any suggestions?


06-22-2009, 02:31 PM
Welding distorts, not much you can do about that. You can minimize by careful process, and your steel bushing limits somewhat, but it's going to move. Hydraulic press might remove without too much trouble, and like clamping a weldment to a jig, the movement has been limited, so it might still work.

Alternatively, you could line bore on the lathe, or (with difficult setup) use a boring head on a mill (don't recall if you have one).

06-22-2009, 02:38 PM
An expandable bushing, then after shrinking it to remove,
go in with a bar and "line hone" or "line lap".

Make the line up bar like an expanding reamer, drill and
tap the end for a setscrew (or tapered pipe plug),
saw slit the end in 4-6 places.

The reaming or honing bar would be sort of the same,
add some grit somewhere. Spin with a hand drill.

Ian B
06-22-2009, 02:41 PM
If you're pressing delrin bushes in with a 5 thou interference, they'll distort anyway, and end up with a smaller hole than they started with. Why not ignore the distortion that happens when you weld, press the bushes in, and then ream out the hole to the right size?

If a customer ever asks about the hole in the steel bush being out of round, tell him it's a specially machined ovoid which maximises bush retention - and charge him more for it :-). Mutter something about CNC - he'll like that.

If a round hole in the steel's really important, use an expanding reamer to open them out.


06-22-2009, 02:49 PM
I doubt you will press that bushing out of the sleeve. Once you welded half the diameter of the OD you pretty well locked it in place. That is a trick used to lock pins, etc. in a bored sleeve.

You may be able to hacksaw a slot in the bushing and then push it out or you will have to bore it out.

You can make a sleeve slightly over size as you did and then saw a slot lengthwise then push it in the bore and use it to align everything and then drive it out and use it on the next swing arm. You may have to drive a wedge in the slot to expand it from time to time but it should last a long time.

Peter N
06-22-2009, 03:10 PM
Try and persuade him to go for Phosphor Bronze bushes rather than Delrin.
I should mention that this advice is coming from someone (me) who has spent his life in the plastics industry from apprentice to university to current business owner.

The Yamaha RD series used Nylon bushes carrying steel inner bushes in all their swingarm pivots, and lots and lots of experience has shown that these wear in a very short time, and all in a very limited arc of course, and this seriously affects the handling. Delrin will be better than Nylon, but for this application the harder phosphor bronze will last much longer.
One of the first mods RD owners do is to fit Ph. Bronze bushes.

Incidentally, I just made a pair of these the other week for an RD400. Got loads of Delrin and it's lovely stuff to machine, but I wouldn't use it here.



06-22-2009, 03:24 PM
Try and persuade him to go for Phosphor Bronze bushes rather than Delrin.

In industrial applications, I have good success with ampco 18
which is better yet than standard bronze (and more expensive
as well)

06-22-2009, 03:39 PM
Thank You for all the responses gentlemen. It occurred to me I should show you guys the jig so you'll have a better idea of what I'm working with, so I slipped the arm back in and snapped one.


I'm not quite sure how I evolved to it, but there's a 16mm tube inside of the "used to be temporary" steel bushings. The tube is 10mm on the ID which accomodates the 200mm long 10mm bolt that accurately clamps the whole mess up between the angle iron plates.

The ID of the stuck bushings still seems to be fairly accurate BTW. The 16mm tube is a light tap fit. I do believe I could run this if I can figure a way to feed it grease with a zerk ......

What if I used this as an excuse to lose the Delrin and substitute bronze? Would welding with the bronze already pressed in be detrimental in anyway?


06-22-2009, 03:41 PM
Try and persuade him to go for Phosphor Bronze bushes rather than Delrin.Peter

Well you've convinced me! He's next!


06-22-2009, 03:47 PM
Easy solution is to coat your steel bushings with a couple layers of teflon tape. They should be easy to remove after welding.

06-22-2009, 03:50 PM
"What if I used this as an excuse to lose the Delrin and substitute bronze? Would welding with the bronze already pressed in be detrimental in anyway?"

Well, maybe, I don't know how hot it's getting right around the bushing.
But it's a bad idea anyway, as the alignment is getting wiggled
out of of true by the heat. This means there will be binding in
the finished product.

As it's going to near impossible to weld perfect alignment,
an after welding machining is best. Honing (or "sanding")
would be quick and dirty, no need for an expensive sunnen
hone on this job. But it's got to be done inline (both at once).

Jim Shaper
06-22-2009, 04:19 PM
Are you talking about sintered bronze, or plain bronze?

Sintered bronze has the oil impregnated in it and they don't like being heated up any more than pvc does.

As was mentioned; your real task here is to do the welding in a fixture and then clean up the weldments via some form of machining. Bicycle builders don't true any mounting surfaces until the frame is completed. The geometry is the important part, and you can't do that when you're expanding and then shrinking the components.

When you weld, the material grows as it's heated, but then it also shrinks when it cools down. It's precisely why you can straighten steel with a torch. You simply get the long side (outside of the bend) to contract by heating it up first.

06-22-2009, 08:40 PM
I'm just curious why you wouldn't build those swingarms out of aluminum?
Steel will give you more unsprung weight.
Geez...I just took over 100 alu swing arms to the scrapper...
BUT...nice job you did there.
You could heat the pivot and the bushing up...then run an icicle in the bushing...have it layin so the melted icewater runs straight down the hole instead of around the pivot....sometimes works if you are quick enough with the hammer and punch.

06-22-2009, 09:49 PM
I would love to be able to make aluminum swing arms Russ. But I not only got no tig, I've got no experience using one either. I've heard about spool guns to mig Alu but I'm under the impression that ain't good enough for structural stuff. Dirt bike guys learn how to land from falling off in the all usual ways, but having the swing arm come out from underneath 'em ain't one of 'em! Better stick with what I know.

Surprisingly it's not as heavy as I thought it was gonna be. Using a bathroom scale the early stocker's 5-6 lbs. The 2nd gen is much beefier and is roughly another pound. Mine's about a pound over the 2nd gen. You can feel the difference with 2 hands, but not by much.

For my next attempt I'm gonna press some hard bronze in and see what happens when I weld 'er up. Might work.:rolleyes:

Thanx again guys. Whenever I get stuck ......


Jim Shaper
06-22-2009, 10:13 PM
What's the reluctance to machining the finished assembly?

You've got a jig to weld them, why not make a fixture that attaches to the jig to line bore them after the tube is welded in? Nothing more complicated than a cutter (like a fly cutter) mounted on a shaft would fix your issues and could be powered by a hand held drill.

Consistency, and accuracy is the goal here and you'd get both.

06-23-2009, 12:57 AM
I still say if you make a split bushing to do the alignment and welding and tap it out you would be better off than welding with a bushing in place.

06-23-2009, 12:57 AM
The only reluctance to secondary machining is I don't have a clue how! LOL!

When line boring the part is typically traversed across the path of a spinning cutter, right? Alternatively, could the cutter be accurately traversed across the part?

Anybody got any pics of a line boring set up?

Meanwhile I'll google ....


Jim Shaper
06-23-2009, 01:06 AM
You'd just need to make pillow block mounts to hold the ends of the arbor holding the cutter. Supporting it from both sides would minimize the fuss and bother of a rigid device while retaining the alignment of the cutter.

I think you're making it much harder than you need to. ;)

I also think the multi piece or even an expanding mandrel for the welding might be helpful.

06-23-2009, 01:26 AM
I think you're making it much harder than you need to. ;)

Could be ... LOL! It's happened before.


06-23-2009, 01:35 AM
I still say if you make a split bushing to do the alignment and welding and tap it out you would be better off than welding with a bushing in place.

This concept finally dawned on me Carld. I could make the temporary bushing say .010 under and then splay it out with a slot? The slot would collapse as I tapped it in and hold the OD alignment. Maybe not perfectly but probably good enough. During the welding process the slot can go wherever the surrounding metal wants and still not get all bound up. Press the bronze after. I'm gonna try that.

But don't tell my brother! He'll get back on the Delrin kick ....


Ian B
06-23-2009, 01:41 AM
Bronze bushes, line bored in situ would certainly give you the very best of results. Have a look at this for some ideas:


(just don't pick the swarf out of the way with your fingers while it's running!)

Same principles would apply, just with smaller kit.


06-23-2009, 04:44 AM
Why not use the heat-and-bend (no wet rags) and heat-and-shrink (wet rag) techniques to "correct" it so that the spindle slides through the frame bores and aligns to the jig?

Panel-beaters and Boiler-Makers do it as a matter of course.

Go down to your local panel-shop and watch them stretch and twist a frame and panels to fit a jig!!

All of the twist may not necessarily be in the frame members. At least some may be "out of round" as well as "out of line" in the member that houses the bearings for that shaft/spindle.

Does that welded frame need to be normalised? Or stress-relieved?

Will it be affected by welding/heat to the extent that it work-hardens such that machining is not practical?

If I had to bore it I'd do it on the mill with the frame in a milling jig. Whether the bearings were pressed in without boring the steel tube first or not is for the builder to decide. Even if boring later after the bearings are in, I'd at least align it by boring before I reamed it although my choice would be for boring in preference to reaming at any time.

06-23-2009, 06:30 AM
Your title would better read "Machining welded parts".
I did R&D work in an industry that relies on welding steel bushings to mechanical tube, in a variety of joint types, many being full encirclement.
There is no way to avoid weld distortion. Even machined flat surfaces in compression would shrink unequally from a fillet weld. Your swing arm weld makes for ovoid distortion, and there is no straight line through the bushing on the weld side. If you mic that bore you'll see what I mean.
The long standing work around was to press in oilites or solid bronze, and line ream with an adjustable, spiral fluted reamer that had tapered pilot. This would ensure subsequent steel pin alignment over perhaps a two foot span. It did not assure anything having true centers, and with an assembly having thirty or so pins, innaccuracy really added up.
If you are assuming longevity of the arm with occasional bronze, Delrin, or other bearing replacement, boring the steel true to a press in size that gives desired final ID of the bearing is best.
There are a number of suggestions through the thread on how to do that. If you re-configured your assembly jig to register OD's it could possibly act as the boring line up in a lathe or even with a hand reamer (though I wouldn't pursue the latter).
The above mentioned adjustable, spiral fluted reamers were supposedly going out of production around 2000-2001. They would work, with some difficulty, if you could locate one in your size.