Model Train Journal

I have several locomotive drive projects on my bench for the next few months that will involve various amounts of machine work, so I thought I would start a thread to capture things of interest that may arise. We'll start with a problem I had today machining a piece of brass bar stock. Here is a close-up:


The stock is 3/4" wide and 1/16" thick brass bar stock from K&S. I need to make some L-shaped pieces. If I wasn't adverse to waste I would saw out rectangles as scrap -- fast and simple. However, I can get near 100% yield by nesting the L-pieces, which requires using my mill. I am using a brand-new 1/16" diameter end mill, and to my surprise the material rolled up, as can be seen in the photo. I have no experience with end mills that small and I was afraid to take a deep cut, so I took several passes at the cut, going deeper each time. I used cutting oil.

Any tips from the Council of Collective Wisdom?

Jim

Jim,

It's late and I'm tired but a few questions. What did you use to hold the end mill? Please tell me it wasn't a Jacobs three jaw chuck! How long is the cutting portion of said end mill (the longer it is the more deflection and ragged finish)? Better to start with a larger end mill and finish with a smaller one to make the final cuts. Better to waste some brass than break or otherwise ruin a small, and expensive, end mill.

Oil isn't required for machining soft brass like K&S uses.

I'm sure before I awake others, more knowledgeable than I, will offer better solutions.

Jay

RK.

It looks to me like your milling cutter is not running true. If you have the mill chucked in a Jacobs chuck and not a precision collet this may be the problem. Also, if the workpiece is not rigidly mounted you can also have problems but looking at the cut, it looks like a wobbly cutting tool to me.

I think Jay 's comments above are correct.

Joe

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So here's my setup:


No Jacobs chuck there, but I think Jay and Joe were onto something when they said it might be a wobble problem. I found that I had only hand-tightened the collet chuck . I will tighten properly and try again. Having said that, Jay's advice to not use a very fine cutter for a rough-cut seems right to me on reflection -- something similar to being penny wise and pound foolish. I will rethink my approach.

Many thanks to Jay and Joe. I will report back

Jim

I was dropping in to read and saw this, so I'll add a couple thoughts that hopefully will help, or at least inspire some debate. I agree with the above, and would add these thoughts; admittedly I might be being a bit neurotic. I don't know your feedrate, cutting speeds, or number of flutes, but I cut soft brass dry with a slow feed and a fast cutting speed. Nice square chips means you have the feeds and speeds right.

1) A 1/16th dia end-mill is a small sucker. I think you might need to pick up the tool speed/spindle RPM from the looks of it. Remember a smaller diameter end-mill has a slower cutting speed for the same RPM as a larger diameter end-mill.

2) If you are pushing material, chances are you aren't cutting it or clearing it, so I'd also think your feedrate might be too high.

3) Last one is arguable as I see many people cut with a buried end-mill, but I was taught that, for a nice clean finish, the tool should only be cutting material on one side with the front and opposing side clear to allow chips to throw clear. So, don't expect anything remotely pretty when hogging out material with a buried end-mill rather than using a saw for your initial shape.

Jim,

I'm sure all of us could continue to elaborate on all the possibilities for your dilemma. I'll add only a few more.

K&S stock isn't particularity flat it's punched out and, if you're not careful, your clamping technique may be causing a problem. One side tends to be convex and the other concave. Place the concave side downwards. Not sure what kind of wood you're using as the base. Hopefully, not balsa.

I also much prefer a four flute end mill.....yours looks to be two. Either will work but I just have better luck (finish) with four.

Yeah, tightening the collet holder is probably a good idea!!

Anyway, by the time you see this you'll probably have it worked out.

Jay

I dunno - it looks like you are holding the work with wood screws. That is rarely rigid enough. Good mills are cast iron.

I'm sorry; not feeling very frisky today so forgot one important consideration that could also contribute to what you see, now that I see your cutter. I agree with Butch regarding four-flute vs. two, but perhaps as important is the depth of the flutes.

Especially cutting buried, you really shouldn't be any deeper than 1/2 the length of flute to properly clear the chips or swarf. Those tiny tapered-shank cutters are notoriously un-stiff for anything remotely heavy in cut, so I'm really guessing the job is beyond the ability of the cutter to clear if speeds, feeds, and depth aren't pretty strictly adhered to.

Looking at the cut and the cutter from several states away, I think a larger diameter (hence stiffer) four-flute cutter with an eye on the basic parametres of tool-speed, feed, and depth of cut, would do a nicer job.

sarge wrote:I'm sorry; not feeling very frisky today so forgot one important consideration that could also contribute to what you see, now that I see your cutter. I agree with Butch regarding four-flute vs. two, but perhaps as important is the depth of the flutes.

Especially cutting buried, you really shouldn't be any deeper than 1/2 the length of flute to properly clear the chips or swarf. Those tiny tapered-shank cutters are notoriously un-stiff for anything remotely heavy in cut, so I'm really guessing the job is beyond the ability of the cutter to clear if speeds, feeds, and depth aren't pretty strictly adhered to.

Looking at the cut and the cutter from several states away, I think a larger diameter (hence stiffer) four-flute cutter with an eye on the basic parametres of tool-speed, feed, and depth of cut, would do a nicer job.

Good tips!
Also, it's delightful to hear from you.

One of these years I will give the machining a try myself but as long as I had to look for these blanks. I will send them to the pros today.
Ok Engines here they come.

I know I owe a reply on the brass machining issue that was the start of this thread, but I am waiting for some new material to arrive before I do so. In the meantime, I have distracted myself with a quick project to fix a drive shaft issue with a U.S. Hobbies SD-45 that is passing through my shop for a tune-up. The unit arrived with motor-to-truck drive shafts that had a universal joint at the motor and a defective (wobbly) home-made horny-ball and socket setup at the truck. I decided to give Bob T's Toyota hose setup a try. Bob informed me that it was best to put the hose at the motor as the bend angle on curves is less at the motor then at the truck (which is obvious once you see it -- the truck swing angles both the gearbox shaft and the drive shaft). That required that I move the universal to the truck. It turned out the gearbox shaft was boogered and would not accept the original universal joint, so I wound up sleeving the motor and gearbox shafts, using a larger diameter drive shaft, and replacing the original U-joint with a CLW U-joint that was sized for the new shafts. Works fine on the bench:


I will rework the other truck drive shaft the same way and then give it a test run on track. By the way, I do not know why there is a gearbox missing from the first axle, but I would speculate that if there was a gearbox there originally it might have split a gear (common on these), so the modeler simply removed it. The other side is setup the same way. I just wish he had put the original axle back in so the wheels would match.

Jim

Jim,

Should of said something. I would have sent you two gearboxes with geared wheels.

I mostly throw them away but keep a few just for these types of emergencies.

Jay

Jim,

By the way, I've seen this done before because the original drives don't like tight radius curves. Very short drive shafts. Sometimes you can help mitigate it by moving the kingpin further outboard on the frame and the trucks. Changes the geometry just enough.

Jay

Jay Criswell wrote:Should of said something. I would have sent you two gearboxes with geared wheels.

Jay, I did consider contacting you, as I would have bet a dollar to a donut you would have some of these gearboxes. This unit is destined to run MU'ed with other units so maximizing drawbar pull was not necessary. I was also a bit concerned about the possible limitation on curves that you also mentioned, so I decided to just leave it as it is. However, should you find two blown-out gearboxes in your hand someday please consider setting them in some dark corner for me. The unit would look better with matching wheelsets.

Now that I think about it, maybe we could set up an on-going deal where I pay you by the pound for all the motors and gearboxes that you would otherwise trash….

Jim

Here is the drive with the other side converted:



These trucks have "hard stop" pins in them at that limit how far they can swing. The drive shafts operate fine out to the limit of travel, so I think this is ready to go. I am going to hold on to this until I can get it on a test track and then send it on its way.

I realized after the fact that once I had all the shaft adapters installed then this setup would have supported a standard double U-joint installation. I thought about converting to that configuration but stayed with the hose on one end to get some experience with it. Technically, it does not provide the same "constant velocity" kinematics that a properly installed U-joint setup does, but the hose does provide some compliance, which should absorb some or most of the speed variations of the single U-joint. One would have to do a carefully controlled side-by-side comparison of drive life to know for sure, but that's not going to happen. My guess is this will run fine for a long time and if it doesn't it will be something else, not the drive-shaft setup, that is the cause.

Jim