Machining wider than the travel of your mill?

[TroyO]

I have a 12" long hunk of cast iron I need to rough machine in preperation for making a precision straight edge and dovetail master for scraping.
This means shaving one side flat, then the other side parallel. Then, the two opposing sides that remain will get a 60^ and 55^ angle

I have a mill with 8" of travel and 12" of stock to remove.

I figure my options are to set up and mill ~6" and then set up and mill the remaining 6" and count on being able to repeat the setup with fair accuracy OR to mount it to the table once and use a fairly wide fly cutter and start the cut in the middle. The two ends would only get a single pass but the hand scraping would cover minor surface differences pretty easy.

Any great tricks out there for machining stuff that is over length?


Pic of the basic shape I am going for...

 

[Florian]

Hey Troy

I would look for someone who can do it in one pass! (At least one side for reference!)
It is very ridiculous to remove a lot of material by scraping.
Also filing needs lots of time if you have to remove something like 1/10 mm.

Cheers Florian

 

[Chazz]

If this is going to be a 'True' master reference bar, I'd send it out for machining, hardening and grinding. There is no way (at least with my equipment) I'd try to make a precision master reference tool that is beyond the scope (reach) of my table.

My 2 pesos,
Chazz

 

[Lew Hartswick]

Another case of "no location" so anyone with a Bridgeport can't offer to do it for the
OP. :-(
...lew...

 

[TroyO]

It's cast iron, being hand scraped in line using a reference surface plate and hand scrapers.

So, the rough machining really only needs to get me within a "couple thou" but the closer it is, the fewer hours I spend refining it, LOL. Surface grinding would be a nice start, though.

I thought I had my location in my profile, but it wasn't (It is now... Denver CO.)

I do have potential access to a larger mill, we have a great place called "Club workshop" here.. picture an athletic gym but with machine tools instead. It would be a while before I can afford another membership though. I love the place... and it's fairly priced, but more than I can spend at the moment.

What the heck... plenty of material and the dimensions aren't critical.... I can always take a pass and see what happens. If it doesn't go I can regroup and figure out other options.

 

[steamer]

Your last comment is a fair one Troy. Give it a shot with the tools you have and see how far you get.

If your table is accurate, and you spend the time to blend the cut, and tram the part back in using the cut surface as a
reference, you should be successful.

NOW....cut the back first flat. that will help get the part set back up.

Then cut the angles rough...like within 2-3 thou.

Now the trick, flatten the back . This is not easy to do as the part tends to want to walk around from stress relief and from the warmth of your hands. Scrape a little and then leave it for an hour on the plate to get back to temperture.

You will need to cobble up a "nest" to hold the part while you scrape the bottom flat.

Once it's flat to gage tolerance...you need to set the part up on a sine table and take stock of the angles.

Scrape the angles until you zero out, and then alternate cycles using the indicator and the plate for bearing.

Do the same for the far side at 55 degrees.

Here's the most important part....strap on a whole lot of patience.....and pay attention when things start to go pear shaped and figuire out what's not working....and adjust. this will not be quick

I would make the part a bit wider if I were you.....and put some handles on the ends....

I wouldnt harden this part as you can correct it if it's worn with a scraper...otherwise you need a grinder and a fixture.

Dave

 

[MachineTom]

Troy, I did a 26" lathe bed with 20 " travel on the BP, a pita it was. For your project rough out the piece angles and all then mail to me and I'll run it through the mill and grinder, that should give you a nice start to scrape down.

 

[my65pan]

Can you swing the ram around like on a Bridgeport to make up the 4 inches?

 

[TroyO]

Thanks for the input and offers of help, everyone!

I keep reviewing the idea of an overhanging fly cutter and I just can't seem to find the flaw. As long as I have the mill trammed as best I can (Which would need to be done in any method) and I have a nice sharp and well radiused cutter in it I think it will be OK. I certainly don't think it would be dangerous in any way that I can see. This of course is the method I intend for the final passes.... for roughing it out I can go a bit more "Gorrilla" on it.

I do plan on adding handles... current plan is pretty basic. I'm going to drill/tap each end for 3/4" all thread and just bolt the all thread handles in with some Loc-tite. (Before any machining). I will then use the handles to secure stuff to the table.

As to width... I can maybe eek it out to 3.25x1.5 but the length is pretty much it... 12" is how long a bar I ordered, LOL.

It's pretty much benchtop size stuff I will be working on, so I picked something not TOO long as to be cumbersome. I almost decided to cut the bar to 8" so it would be less of a machining challenge but decided that was going to be too short to properly overlap for spotting longer runs.

Ohh, and the 55/60 layout is pretty arbitrary. I need 55 for the Mini Mill and the standard dovetail is 60. I'm open to other suggestions.

So... for now I will wing it and see how far I get. The really funny thing is I waffled on buying the "Mini-Mill" becase I was worried I would run in to "lots of things" that would be too big to mill. This is the very first time it's come up, LOL.

If I get close, but not close enough I will PM you MachineTom, thanks for the kind offer!

 

[steamer]

If the 55 and 60 are not critical, leave them a little shallow...like five degrees. You can still mark up but you won't need to use the sine plate to make the angle. Beware long narrow parts during scraping. You will find that they are easily influenced during mark up. When I made the angle master for my Waltham, I made the angle shallower, but because of the size, it was easy to roll it during mark up. I brought to a scraping mentor of mine who suggested that I make a small flat master first to mark the angle master as you will be less likely to roll that.

http://www.homemodelenginemachinist.com/index.php?topic=7692.0

Hope that helps Troy.

Dave

 

[TroyO]

Correct... my understanding is that the 55 and 60 aren't critical to an actual value, but that whatever they are is the same all the way across the bar is more important.

That gets covered by both being flat planes (I think) but if it's 55.2^ or 52.1^ it won't really matter too much. The whole machine gets scraped to that standard, whatever it is.

It could, to a slight extent mean more material removal.... but chances are I will be closer using my ground angle blocks than the factory was anyway.

Hmmm, I can probably come up with some scrap for smaller bars too. Good idea!