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View Full Version : How to Flatten Bar Stock on a Mill? Not O/T!



ckelloug
07-16-2008, 11:42 PM
Hi,

I'd like to mill a 2 inch wide 1/2 inch thick by 18 inch long piece of 4140 prehard barstock flat. I tried clamping it to the mill table with t-bolt clamps but this bends the metal elastically to conform to the mill table (until it's unclamped when it becomes as crooked as a dog's hind leg).

I am assuming that one either needs two mill vices or a set of custom clamps that clamp on the sides of the metal instead of in the dimension I am trying to mill.

Any suggestions?

--Cameron

lazlo
07-17-2008, 12:06 AM
Hi Cameron,

For that I use Mitee Bite clamps, which are hexagonal clamps with an eccentric button-head screw. As you twist the screw into the T-Slot nut, it pushes the hex side-ways, and it locks the workpiece. Great for milling thin stuff:

http://www.miteebite.com/images/product_pages/t-slot_kit_2.jpghttp://www.miteebite.com/images/product_pages/t-slot_kit_1.jpg

You can also use a toe-clamp:

http://www.jergensinc.com/JergensCatalog/Photos/toclamp.gif

Cheers,

Robert

Ian B
07-17-2008, 01:32 AM
Cameron,

I think you have 2 separate problems here;

1. Clamping a piece of bent bar to a flat table. As you say, when you unclamp, it springs back. Can you cut strips of shim stock and slip them under the bar before clamping to support it? Machine the top face flat, and if problem 2. isn't an issue, it should be ok.

2. Stresses in the outer skin of the bar. Cold rolled steel will go banana shaped if you mill one side off. The other side is still in tension and the bar bends.

Ian

JRouche
07-17-2008, 01:32 AM
I tried clamping it to the mill table with t-bolt clamps but this bends the metal elastically to conform to the mill table (until it's unclamped when it becomes as crooked as a dog's hind leg).

--Cameron


Ok.. If the bar stock is way outta being semi flat and it springs then a toe clamping is in order if you dont have two vises. JR

darryl
07-17-2008, 03:37 AM
I'd be tempted to try a shimming routine- here's one which I have used. This may be a bit tedious, but it works. I got some epoxy putty and rolled a few 'worms' of it, then laid these worms on paper strips laying on the mill table. Put more paper strips on top of the worms, then laid the crooked piece across them. I sort of 'helped' the piece to indent into the putty strips in fairly level fashion, then left it to bed down as it wanted to. I went for coffee. Once the putty was hardened, I added some clamps to hold the material down only at these points. I had to remove one clamp at a time as the flycutter passed that point, then reclamp behind the cutter to carry on.

Once I had a flat side, I turned the piece over and clamped that flat to the table. This clamping took out the slight warp that came about from the first milling. After the second side was milled, the piece stayed fairly straight. I found out how far off my mill table was end to end on the x axis :)

I sometimes take a piece I've milled this way and even out the materials surface by dragging it across sandpaper laying on a flat surface. To control the workpiece while sanding it, sometimes I'll take a piece of wood that's longer than the workpiece and put two screws into it so the workpiece is held between these screws, a crude form of toe clamping. The wood serves as a handle to push the piece back and forth with across the sandpaper.

Forrest Addy
07-17-2008, 03:52 AM
One miserable problem you'll have to contend with is material warp from a variety of sources. Removing stressed metal from the material results in the stresses that opposed them to warp the material. The cutter will wipe compressive stress in the metal unleass it's dead sharp. Then there is the fact the material is long and skinny.

I suggest a thermal cycle stress relief to allow the locked in stresses to equalize. Soak the material in a temperature controlled furnace for a day at 1000F then allow it to cool in the oven to no more than 400 F. You will lose a little hardness. The important thing after stress relief is to get a light cut off both sides of the material without regard for straightening. The purpose is to condition the material surface for further machining. Then shim and go for straightness. I've never tried the epoxy putty trick but it seems very sensible if you can afford the curing time.

Use relatively heavy feeds, light cuts, HSS cutters, and lower than usual surface feet per minute. Also mist on soluable oil coolant to reduce heat input to the work.

The thinner you cut it the harder it will be to make flat. You will never get a skinny piece like that straighter than 0.003" of flat overall without heroic care. Try for parallel, as straight as you can get it, and call it good.

A good surface grinder hand could get it to 0.0015" of flat on a good day. If the work can tolerate a little peening in the concave sides you can get it straighter than that. Making long skinny stuff flat and straight can be a real art, especially alloy steel. Good luck.

oldtiffie
07-17-2008, 06:38 AM
I agree with Forrest and Darryl.

It was stresses that got the material into that shape and there will be residual stresses remaining. As soon as the "skin" is broken, those residual stresses will manifest themselves and so a new warped shape will occur. It is a slow, gradual process.

I would be inclined to use brass shims on a magnetic chuck or chucks on the mill table. The face of one of the magnetic chucks could be packed and shimmed on the table to bring both chuck faces into line.

I would pack/shim the material as it was on the chuck face/s with steel "stops" on the ends and sides of the work-piece to stop it moving longitudinally and laterally and turn the magnetic chucks "on". Stops placed in the mill table "Tee" slots will be OK as well.

I would then use a very sharp fine pointed HSS fly-cutter with a very small nose radius and large clearance and rake angles honed to very fine edges.

I would not use end or toe clamps if the section height to width to length ratios were too small as they can make light section "bow" (usually) "up".They are OK for the more "solid/stiff" sections.

I'd use "suds" or cutting oil with a relatively slow sped and feed. I'd "listen" to the work to "tell" me when it was getting too high. I would leave "witness".

I would reverse the work piece and repeat the process on the other side.

I would taken it off the magnetic chuck and allow the stresses to at least partially relieve themselves.

I would keep repeating it until I had a relatively stable work-piece and then finish it off.

I agree with Forrest as regards thermal heat relieving. I'd heat it in a brick ("fire"/refractory brick preferred but slowly heated non-extruded ie "pressed" house bricks will do if care is taken). I'd heat it until it was red and "sweating", "soak" it for say 1/2 an hour and then remove it and cover it with common builders/Brick-Layers lime for 24 hours.

If you only have one magnetic table the ends can be supported with Machinists Jacks.

Two vices are OK provided that the vices don't put any more stresses into the work-piece.

Frankly, I'd be surprised if that work-piece would stay "still" enough to get an accurate flat un-warped job out of it.

If I were to surface grind it, I would use a coarse well-dressed aluminium oxide wheel with lots of coolant on it as there is localised heat in the "moving" grinding zones which can not only cause thermal distortion but surface micro-cracking as well.

If only one face was required I'd think very seriously about seeing a good Boiler-maker Welder who is experienced in using heat, welding and cooling to "move" structural parts. Its a work of art the way they do it - just like a good panel-beater but on a much larger scale.

Ask torker (Russ) for advice on this aspect.

torker
07-17-2008, 07:48 AM
Whoa tiffie..LOL! I've straightened lots of steel with a torch...everything from big heavy beams used in sawmill chain runs to shortened Ford 9" housings. But I've never been asked to bring it to a "thou".
It could be done with patience but the scale produced would drive you nuts. and no... you can't wrap it in foil cuz you can't see the color then.
My "rule of thumb" for something like this... warp it back the opposite way the same amount that it's out of whack...hold that opposite warp mechanically with something that will allow the piece to be free (as in no heatsoak)then heat up and let cool very slow. Usually works. Take off the restraint and it'll pop back flat.
"Free" stress relieving with a torch... now that is the black art. We've had 6"x10"x1/2" wall rectangular HSS 30 feet long with a 6" bow in it caused by overzealous weldors pouring too much weld on one side. You can take the bow out almost perfect with a rosebud and a lot of time.
But that "perfect" is sawmill perfect :D
Russ

oldtiffie
07-17-2008, 08:41 AM
Whoa tiffie..LOL! I've straightened lots of steel with a torch...everything from big heavy beams used in sawmill chain runs to shortened Ford 9" housings. But I've never been asked to bring it to a "thou".
.................................................. .....
.................................................. .......

. You can take the bow out almost perfect with a rosebud and a lot of time.
But that "perfect" is sawmill perfect :D
Russ

Thanks Russ.

What a shame - I had you "up there" with the "God Squad"!!!

I only meant to bring it back so that the OP had enough material to get his job out of it!!

My biggest concern on the OP's job would be getting under the skin and staying under it as it is usually quite hard/tough and any "rubbing" prior to (trying to) "break-though" will cause more spot work-hardening.

I forgot to mention too, to reduce "rubbing" I'd climb-mill it but would be very careful how and where I placed the cut(ter).

I've seen Boiler-Makers - and Blacksmiths - in Naval Ship-Yards do some marvelous stuff with moving and "rescuing" all sorts off stuff. The whole loft floor is one big swaging plate. Watching them mark out and bend/form rib/frame sections on the "loft" is incredible.

Some of the best is done "on site" (on the ship) with heat, wedges, "come-alongs", "handy billies" and what-ever. Seeing them replace a removed whole section out of the side of the ship or decking is awesome. Seeing them join two pre-fabricated section of the hull on a new ship is very impressive too.

Sheet-metal Workers there were not far behind - and Plumbers and Pipe-Fitters too.

mayfieldtm
07-17-2008, 09:05 AM
Skim one side and...
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.
.
.
.
.

Tom M.

Evan
07-17-2008, 09:20 AM
You need some cam clamps. Easy to make even without a CNC machine.

http://vts.bc.ca/pics3/engclamps.jpg

http://vts.bc.ca/pics3/camlock.jpg

Note the dark one is 1/4" steel and is tightened with an angle grinder wrench as it is locked down.

lazlo
07-17-2008, 09:29 AM
I suggest a thermal cycle stress relief to allow the locked in stresses to equalize. Soak the material in a temperature controlled furnace for a day at 1000F then allow it to cool in the oven to no more than 400 F. You will lose a little hardness.

Forrest, I'd think that you'd lose all the Pre Hard hardness. Sub-critical annealing is 900 to 1100F for an hour per inch of thickness.

http://www.key-to-steel.com/articles/art15.htm

Seems a shame to pay for 4140 Pre Hard and then anneal it...

I think Tom has it right:


Skim one side and...
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.

oldtiffie
07-17-2008, 09:54 AM
Skim one side and...
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.
Flip her over and do-it again.

Seems a shame to pay for 4140 Pre Hard and then anneal it...

I think Tom has it right:

I don't necessarily agree.

The part will have to be clamped/secured and shimmed/packed as I said previously. Any movement due to clamping will be repeated as soon as the clamping force is released and so partially if not entirely negate the packing and the reason for it.

The cutter has to get under the skin for each cut on each side. That will take off a lot of material. If the piece that the OP (Cameron K) has has not got enough "meat" in these circumstances, he is not going to get that job out of it unless he pre-straightens and anneals it.

It will get progressively more difficult as the work-piece gets "thinner".

He either straightens and anneals it or perhaps gets another/better/alternative bit of material.

I'd be interested to know how that piece that he has got distorted the way that it has been. I'd be surprised if the original piece was that distorted when it came from the supplier/merchant.

I'd be interested to know if that "pre-hard" is a "must".

lazlo
07-17-2008, 10:16 AM
It will get progressively more difficult as the work-piece gets "thinner".

It depends whether it's 4140 PH Hot Rolled or Cold Rolled. If it's cold rolled, which I'm guessing, once you get past the extrusion skin, there's very little internal stress.



He either straightens and anneals it or perhaps gets another/better/alternative bit of material.

I'd be interested to know if that "pre-hard" is a "must".

Agree with the first part 'Tiff, but Cameron's a smart guy, so I'm guessing that he had a really good reason to use Pre Hard Chromoly :)

Mcgyver
07-17-2008, 10:31 AM
two parts to the puzzle, 1) how to get rid of/minimize the stress so it stops bouncing around and 2) how to get it flat.

for 1) if the piece wasn't heat treated, sending it out for normalizing is something I've done and it works. Forrest, you're recommending a thermal cycle - this is prehardened 4140 - would you not have heat treat to get back the same hardness hence recreating all the stresses again?

I'm not steeped in experience in getting rid of the stresses in hardened material but have followed the bouncing ball in enough chats with T&D friends to make some suggestions. A couple of ideas that will reduce the stress - hot to cold cycling and time - one friend when making tooling would keep it (before finish grind) in the freezer and then into oven whenever a roast was going in and did this over and over. Or you could get more sophisticated and use cryogenic treatment which is supposed to be effective and removing stresses with changing the hardness. if it matter, i'd look into that route.

I've also used Tom's idea, based on the stresses being concentrated in the outer layers - that's so when the stress are mostly from rolling, I wonder if with something that has been heat treated if it would be as effective...ie are the stress more through out rather than at the surface

Once the stress has been dealt with, To machine flat, you've got to be able to hold it without clamping force distorting it. The shim method on a mag chuck is one way - 'map' out the surface with an indicator and then shim until you get the same indicator reading with the clamp on. Tedious eh?

How I like doing it, if the material is soft enough, is a quick and dirty scrape and file job. I use the basic concept of scraping; blue on a flat surface, to get one side with bearing points distributed over it enough so that it won't distort when clamped. This is faster than it sounds because we don't about get the whole surface flat, just a good distribution of spots. Wholesale removal with a power scraper or coarse file (yes a file will work for roughing in) can make this approach less tedious as the shim and indicate process.

if you are limited to a mill, I'd scrape/file one side flat, clamp it with mighty bites, flycut, flip & check. repeat as necessary.

As said, the cutting action will also add stresses. Coolant stops heat build up but doesn't do much about the temperature at the point where the cutting is happening - this can be 1500 F in grinding so the challenge never completely disappears

Jpfalt
07-17-2008, 11:01 AM
The method you settle on is going to depend a lot on the outcome you are looking for.

So far most of the posts deal with reducing stress to get a flat part. The answer may be to go the opposite direction.

A technique used by saw filers for large circular and band saws is "leveling" the saw plate, and may take to form of hammering down, "picking up" or pressing.

Levelling by hammering down works for steels up to and including Rc60 hardened tool steel in thicknesses up to about 3/16". The steel is worked on a leather pad on a large, thick, steel block called appropriately a "levelling block".

4140 prehard is somewhat hard, but also tough. 1/2" material is a little too thick to hammer down, but pressing or "picking up" may be quite suitable. A good technique for pressing is to set up two round rods under the workpiece and place a dial indicator against the upper surface. You can then press down between the rods and be able to see the deflection, springback and resulting set. I once worked for an old tool and die maker who used to straighten files that way.

Picking up involves the use of a small round pein on the upper surface of the bar to put in compressive stress, something like shot blasting. I saw this used on 48" diameter, 3/8" thick hardened steel circular saws for cutting aluminum ingots. Quite literally, the saw body arches up as it is peined. The saw body will move over time as the stress levels change, but is quite maintainable.

Evan
07-17-2008, 11:25 AM
Peining the part will certainly work for straightening, I use the method all the time. It leaves marks in the surface but more important the stresses that it produces are mainly very near the surface. As soon as the surface is machined the part will assume some other shape.

lazlo
07-17-2008, 11:46 AM
4140 prehard is somewhat hard, but also tough. 1/2" material is a little too thick to hammer down, but pressing or "picking up" may be quite suitable. A good technique for pressing is to set up two round rods under the workpiece and place a dial indicator against the upper surface. You can then press down between the rods and be able to see the deflection, springback and resulting set. I once worked for an old tool and die maker who used to straighten files that way.

That's intriguing, if you're saying that you're actually stress relieving the 4140 by pressing it, and not just plastically deforming it? If you're just bending it straight without reducing the skin or internal stress, it'll warp again when you mill it.

Forrest Addy
07-17-2008, 11:56 AM
Lazlo. Pressing both sides of a crooked piece results in a balanced yielding of the surface layer and a more stable straightening job. The stresses are still there but there are more of 'em and they oppose one another. Sounds counter-intuitive but it works. If you want a fun job spend a few months on a straightening press mashing anything and everything that comes your way. There's some practical rules to the specialty but sooner or later instinct - and superstition - takes over from a rule based system. I swear some things (a periscope tube come to mind - imagine a 40 ft long steel tube with a full length 6 1/2" dia monel sleeve shrunk over it) straighten easier in the quarters of the moon than the full or the new moon.

There's been questions raised about my suggestion of a thermal cycle to reduce locked in stresses. I suggested at temperature of 1000F for 4140. It took a while to find a chart. Anyway 1000F corresponds to a hardness of 32 Rc or there abouts - very close to the pre-hard spec. I don't think there will be much loss of hardness but there may be some given a long soak and the drawing temperature. We used to stress relieve Nitralloy (essentially 4140 with a specified aluminum content) to 1050F over several cycles to relax it enough to bore and hone it for steam chest gland bores holding 0.0005" diametral clearance and better than that straightness for 15" length. The stems when inserted clean and dry in the bore and spun with the fingers would spin for several minutes lubricated by the film of air.

It's the chrome content that gives 4140 its heat tolerance. 1000 degrees in a plain carbon steel is enough to draw the material back to nearly the annealed state. Mil-S 1222 is a spec for high temperature steel used for superheated steam fasteners. The material was chrome moly very close to 4130 heat treaded to 25 Rc. It wias rated to 1100 degrees as I recall.

If a close flatness tolerance is called for one could always scrape it. You will chase it around for a while but in the end you could get it pretty flat.

What is the end use of the workpiece anyway? It's fine to discuss flatness in the abstract but in the end we need to actually produce something practical. The end use determines the tolerances to be held.

ckelloug
07-17-2008, 03:06 PM
Wow, My first On Topic Post that generated interest.

Specifically, the piece is intended to be turn into a blades for a special manual hedge trimmer I'm prototyping. I made the first one with intentionally bent blades to preload them but due to a notched tooth design, the testing of which is the purpose of the item, this was very tricky.

I'm using brand new fresh 4140 prehard cold rolled from Crucible Metals (yay local office) since I don't have a furnace yet and my machine tool budget has been exceeded for a number of lifetimes at the moment precluding quick procurement of a furnace.

The piece is out by about a 16th in 24 inches which is touch too far for a straight blade design but not heinous.
I've tried the skimming approach but it's too springy to have cutting it with ordinary clamping on the table due anything but make things worse.

Ideally, this would be a forged part if it were going to be manufactured but proof of concept demands a working prototype that isn't as clumsy and kludgy as the first one.

I'll definitely take the advice from Carld and Forrest on the flycutter thread. I got some Rex95 HSS over at crucible the other day because the 4140 was eating my standard grade HSS alive.


Thanks you very much for all of the input. I'll make some cam clamps now and perhaps acquire some mightee bites and eventually a decent sized toolroom furnace.

Regards all,

Cameron