O/T Broaching Machines

[chris m]

Good afternoon,

 

We are looking at tightening the tolerance on the slots we currently mill in one of our 'core' components and I am thinking that broaching the parts would be the best way to make a repeatable feature. The part is, essentially a thick washer that, when laid flat, has a slot across the face about 1/2 way through the thickness. The parts range in size from .720ID x 1.250OD x .270 thick to 1.280ID x 2.880OD x .460 thick and the slot widths range from .197 to .472. Tolerance on the slot width is +/-.0002 and symmetry is critical.

 

Since I know absolutely nothing about broaching I am looking for some input on horiz / vertical, manufacturer, method, etc.

 

 

C

[crazy^millman]

+/-.0002 for broaching IMHO is asking a lot. I had a .0005 to hold on thru slot and that was a PIA. Had to keep that broach very sharp. Might need to look to specialty house and talk to them and see what they say.

 

HTH

[Guest]

I agree, that is a tall order for broaching.

 

EDM a possibility?

[chris m]

EDM is probably too slow; I need to bang these out. Milling currently runs about 2 minutes or so but I have +/-.0005; not real confident about +/-.0002 day after day. One of the parent companies [theoretically] broaches this feature now, so it is [theoretically] attainable. Of course I don't really know what they do because I don't have the parts to inspect and we have uncovered some....discrepancies in the past between the paper and the parts.

 

C

[Kevangel]

I have seen some BIG broaching machines sell for almost scrap prices at toolshop auctions. I am only guessing, but if the broach were guided on both ends AND close to part with guides and there is no galling going on and broach is correct width "seems" doable especially on a cheaper washer type of part, where some could get tossed.. don't know if there is guided tooling made by broach companies.

[Tim Johnson]

Don't know the material but would you be able to grind them on your machine after rough milling?

[Guest]

wire edm it.

[Surface]

If you are using an endmill, I'm surprised and impressed you can hold that tolerence (.0005) currently. .0002 isn;t much to play with, but have you tried using a plain milling cutter i.e a slitting saw, one pass.

Your not telling us the material, but if that dont work, I'd say grind it.

[Kevangel]

quote:

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has a slot across the face about 1/2 way through the thickness.

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Wonder how many of us read over that in a hurry? Looks like a bunch of special fixtures and tooling and $$$ to do ...that maybe tough to sort out.

[chris m]

Material is 1144 Stressproof steel.

 

We have used a slitting saw with mixed success on very small slots in the past but the location on center can be a PITA.

 

I pulled apart one of my mother ship's units yesterday and it looks like they are rough milling and then pulling a broach thru it to get the walls but not the floor. Mother ship manufacturing guy says the part I looked at was made by an outside vendor so he is not completely sure of the process. This particular part has a slot that's .2366 +.0004 and it measures right on the low limit.

 

I am going to look at the parameters of my machine and see if I can use comp on the diameter instead of the radius of the tool; if I can split that tenth I'll give it a go with some test parts and see how well I can do.

 

We aren't afraid of spending money on machines, tools, or fixturing so I don't sweat that stuff too much but I need to be sure I can make these damned things right day in and day out

[Surface]

quote:

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if I can split that tenth

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.0001 is the smallest resolution in inches, but .001mm (.00004) is the smallest resolution in metric.

[chris m]

Yeah, I know, but I can't run one machine metric and 29 English. If I can get the radius comp over to diameter it essentially does the same thing; there are other ways to fake it, too, but I don't want to make a contraption out of it.

[Gorn_TY]

Sure sounds like grinding to me. Get a diamond wheel dresser line up 10-15 of them and cut them in a single pass,,,, OK our equipment is a bit heavy you might have to take 2 passes and dress before each final pass.

[elbro]

How critical is the slot to center? If it has a little room .0005 or so. Why not a horizontal with a slotting saw.

[chris m]

I'm not much for grinding; I may have to check that out

 

Scott, we've tried the slitting saw before with mixed success; I might head back that way though

 

C

[elbro]

Slitting saws on a vertical mill or a horizontal? I would use a manual horizonal, it's supports both side of the arbor. If the machine it tight you should be able to hold size.

[chris m]

Nothing manual here, baby; push the button and duck

[Charlie Pierson]

How about an undersize slitting saw + 3 passes.

1 up the middle and 2 to widen to size w/ seperate offsets in the Z for each of the 2 finish passes (or , if available 1 or 2 custom macro variables to control the size)

 

cp

[chris m]

That gives me the same issue that endmilling does; if I make an offset it will be .0001 up and .0001 down in order to maintain the required symmetry to the bore. This uses up 1/2 of the tolerance in one whack.

[Matt Berube at Ferron Mold]

Creep Feed Grinder

 

If you are looking for the ultimate repeatable solution, these machines can do it.

 

A place I used to work at had one of these and were holding ridiculous tolerances with it on a slotting operation.

[Tim Simmermon]

I was reading all of this, had a solution for you, got to the end and Matt beat me. It's the perfect solution for your application. I'd outsource a batch to a creep feed grinder to check results. If they came back good, then buy one if you have the quantity to justify it.

[Charlie Pierson]

Chris,

No matter what you do you are still stuck w/ the least input increment of the machine (presumably .0001) If you have a little leeway on the centrality-symmetry tolerances you could make .0001 adjustments and get .0001 using two passes w/ 2 seperate offsets.

 

then again, perhaps the creep feed machine or a machine w/ more precision than usual (.00001).

 

cp

[Kevangel]

Personally before I would spend a billion dollars and multiple fixtures, set ups etc. I would buy a bunch of end mills around the size I need and use it (maybe special order, extra good quality and new holder) rough end mill , then one finish pass with perfect end mill, replace as needed and buy some slot gages, red end no go, green end go..or atleast try the cheapo way first...glad I don't have to do this job especially when your customer, etc say the old "we do this all the time" ......well,we weren't born yesterday. I have read some amazing things about creepfeed grinding.

[Gorn_TY]

We have creep feed griders here. Up to 80 hp creepfeed grinders. One job will not pay for a machine like that. Thats why I said get a diamond roll made to the size of the slot. Dress the wheel thru diamond roll so the grind wheel has the slot width in it. You could use any mid size surface grider and hold that tolerance. With the light cuts you are taking you might even skip dress and dress the wheel every other cut. You would have to test that.

 

CNC's are just not designed for 2 tenths. Most machines New hold .0001. the best tools are +/-.0002 and anyone with a vision type presetter knows how much runout new tool holders have. Most coolant tanks are to close to the floor and the collant can be 10 degree differnt from the air temp. Your part can changes size more then .0002 while the coolant is on.

 

The most common place .0002 is help repeatable is a bearing manufacture. Bearing races are gound with a dressed grinding wheel then honed to mirror finish.

 

Does your measuring system pass a gage RR? I would think any less then air gaging or a lazer form analizer would have a tuff time with .0002 across multi operators.

[Bruce Caulley]

http://www.hornusa.com/eng/products/new_product/index.htm

 

Have a look at cross broaching with something like this while still in the machine. It will obviously need some custom precision ground inserts, but once you are out of tolerance you can just change the insert and carry on.

 

HTH

 

Bruce