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View Full Version : Quick Fix For Broken Cast Crank Handle ? Probably NOT



randyc
11-30-2010, 11:18 PM
(This post describes a repair procedure performed on a broken knee-mill crank some time ago. For once, I remembered to photograph the "fix". I doubt that I'd do this the same way again, LOL.)

I bought a Taiwanese 8 x 30 vertical mill a dozen years ago and the machine has been useful for more than the small non-ferrous work for which it was purchased. The knee crank on the machine was broken within days of delivery - clumsiness during a move. The price for the cast-iron replacement part that the distributor (Enco) quoted was unacceptable so I didn't buy a new crank. I cleaned and brazed the crank together. All went well for about five years until the crank separated at the brazed joint. I repeated the fix which separated again seven years later.

I ran out of oxygen while attempting to braze it again and of course this occurred late Friday afternoon - the oxygen supplier is fifteen miles away. Options were limited - the brazing "fix" obviously wasn't a permanent one (I'm blaming the unknown casting material rather than lack of skill, ha-ha). Even if WAS a good fix, I'd have to wait until Monday morning to refill my tanks and I wanted a quicker repair here's what I came up with. Note that this CAD drawing was made AFTER the repair. The sketch that I actually worked from was not fit to post here or anywhere else. (When something is modified, I try to document the modification and place a copy in the machinery manual. I don't always remember but I did this time.)

http://i97.photobucket.com/albums/l219/randy9944/crank.jpg

An obvious problem is that the cross-section doesn't allow large fasteners to secure the broken crank to the "castle" coupling. Also, the constant raising/lowering of the estimated 300 pound knee would be sure to loosen the fasteners if they are required to withstand rotational torque. My thinking was that a vee-joint would be helpful. There was enough room for a pair of 1/4-20 socket head cap screws to secure the two parts and, if located along the axis that locks-up the vee, then the vee mostly transmits the torque - not the screws. The two cap screws are loaded only axially and the joint should be solid.

Oh yeah, before I forget - I needed to use the mill to modify the parts but had no crank to raise/lower the knee. Happily that was a simple fix - split a brass bushing and clamp over the pin with a large pair of vise grips to rotate. Awkward, takes a long time to move the knee but acceptable until the crank is repaired.

http://i97.photobucket.com/albums/l219/randy9944/P1010399.jpg

The vee in the cast-iron castle needs to be cut with reasonable accuracy and the setup needs to be stable. Can't just put the rounded, rough casting into the milling vise, prop it up at a 30 degree angle and assume that the angle plate will stay put during the milling operation. After clamping the castle in the milling vise, resting on a 30 degree angle plate, the angle plate was prevented from slipping out from under the castle with a C-clamp (securing the plate to the castle).

A brass bushing, turned to fit and inserted into the crank pin bore, provided a solid surface on which a step clamp could apply vertical pressure to the castle. The setup for this part could be implemented in many different ways so long as every axis of movement is constrained. (It's not easy to see the brass bushing in the photo of the set-up - cramped quarters, poor lighting and hard to focus so close to the work.)

As we know, coolant isn't a good idea for cast-iron so I used a shop vacuum to suck up the chips while cutting (cleaning the machine thoroughly after machining cast iron is always a good idea). After the setup was secure, a four-flute end mill produced the required vee configuration.

http://i97.photobucket.com/albums/l219/randy9944/P1010403.jpg

randyc
11-30-2010, 11:18 PM
Before drilling the mounting holes, the castle needs to be aligned in the mill so that the vee is perpendicular to the mill spindle. Remember that the castle is round - there is no reference surface. There are many setup possibilities but the spirit level in the protractor head of a machinists square is a quick and handy method. The spirit level was calibrated by adjusting so that it reads true when held in the milling vise on parallels. Then the castle was lightly clamped in the vise with the level resting in the vee groove. The castle was lightly tapped with a small brass hammer until level (and therefore perpendicular to the spindle). The photo leaves a bit to be desired but shows the level resting in the vee of the castle, clamped in the milling vise.

http://i97.photobucket.com/albums/l219/randy9944/P1010409.jpg

The centerline of the vee needs to be determined because the hole locations will be referenced from the centerline. A conical edge finder is a useful tool for this purpose:

http://i97.photobucket.com/albums/l219/randy9944/P1010410.jpg

After cranking to the desired offset, tapped holes to secure the clamp arm can be produced. As can be seen in the above sketch, the holes aren't perpendicular to the pocket, they are inclined at 30 degrees. Can't tap drill at this angle without the drill skidding off the inclined surface so I first spotfaced the hole locations with a 1/4 diameter end mill, just deep enough so that mounting holes can be centered, drilled and tapped.

http://i97.photobucket.com/albums/l219/randy9944/P1010411.jpg

I was a little worried about the casting material (especially considering the poor brazing qualities), I've heard a lot of horror stories about hard spots and foreign objects in imported castings. Trying to make the tap drill run true, I used a 2-flute 3/16 solid-carbide end mill to bore a starter hole about 3/4 deep followed by the tap drill before tapping the mounting holes. (The tap wrench is steadied by using a lathe dead center in an R-8 to Morse taper adaptor. Any number of similar guides will serve the same purpose.)

http://i97.photobucket.com/albums/l219/randy9944/P1010413.jpg

randyc
11-30-2010, 11:19 PM
The crank arm surfaces mating with the castle need to be milled true (eliminating the casting draft angle and the rough surface finish). Similar clamping problems exist with this rough, tapered cast part as in the setup described above. The two perpendicular surfaces can be milled in one setup and their angular relationship to the remainder of the cast surfaces isn't critical. In this situation it's convenient to clamp the part between scraps of wood. The wood, under heavy clamping pressure from the mill vise, will conform closely to the irregular surfaces of the workpiece providing a sturdy and secure platform (if only a temporary one).

About five minutes were required to produce this cradle. After tracing the outline of the crank on a scrap of 4 x 4, the outlines were cut on a bandsaw, using a tapered scrap of wood to hold the cradle at the proper angle.

http://i97.photobucket.com/albums/l219/randy9944/P1010404.jpg

The cradle was cut in half and the bandsaw used to cut a ledge for the crank to rest on.

http://i97.photobucket.com/albums/l219/randy9944/P1010406.jpg

Here's the crank, held tightly in the cradle after two surfaces have been roughed perpendicular to one another. One more pass should do it:

http://i97.photobucket.com/albums/l219/randy9944/P1010408.jpg

After truing, the crank arm was spotfaced with a 7/16 end mill at the location of the mounting screws to provide a perpendicular surface for drilling the clearance holes and to provide a flat seating surface for the heads of the mounting screws. As when performing a previous operation, the crank arm must be oriented 30 degrees from the spindle axis.

Hardwood scraps were also used in this operation (for their ability to conform with the rough-cast draft angles of the handle) and the angle was again established with the spirit level. Since through holes were required, I didn't want to leave an angle plate permanently in the setup under the workpiece risking collision with a drill. After completing all operations and de-burring, the parts can be securely assembled with the socket head cap screws and here's the finished crank (I painted it a week or two after the fix):

http://i97.photobucket.com/albums/l219/randy9944/P1010415.jpg

Repairing the crank twice (by brazing) took about five minutes and lasted about five years each time while the machining fix required several hours. I'd have brazed the thing again had I not exhausted the oxygen tank with no refill available until Monday, LOL !

Hope that the description has been useful - or at least AMUSING.

Cheers,
Randy C

ADGO_Racing
11-30-2010, 11:33 PM
Coincidentally, I too recently had to run my knee with a pair of vise grips as you did....Long story....Ghost stole my knee handle. Ghost thinks he is being helpful...I guess.:confused:
I had a new handle shipped to me, (Family member works for a used machinery company). Wrong handle, it needed to be bushed, and "drive keys" added it came with a "drive slot".

You are right, running your mill, using a pair of vise grips on the handle is a bit awkward! Got the job done though.:D And I am soooo happy to have a knee crank again!

Carld
11-30-2010, 11:35 PM
Interesting repair. You could still braze it and it would be even stronger.

randyc
11-30-2010, 11:46 PM
Interesting repair. You could still braze it and it would be even stronger.

You bet ! And the first time those little screws show any sign of loosening, OUT comes the torch -

Toolguy
12-01-2010, 10:57 AM
If the screws get loose, put red Loctite (271) on them. They will be there for life.