making square stock round on lathe

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thomas harris
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making square stock round on lathe

Post by thomas harris » Sat Jul 28, 2007 5:41 pm

what are the limits, if any. Do they differ greatly from mahcine to machine. I have done this with plastics, but wonder if it's wise to do with metals. I always find drops in the square form. They seldom exist in round bar. I tried soem A-2 tool steel, (very small piece), and it was knocking the machine pretty hard on each corner so I aborted the mission. I have a chunk of Al. that is also square and about 3.5x3.5x10 and am considering trying this. Will the softer Al be a touch easier on the machine and my nerves? Thanks for any suggestions or answers. I'll try to never ask the same question twice...unless I forget the previous answer.

Harold_V
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Post by Harold_V » Sat Jul 28, 2007 7:20 pm

Your assessment is correct, and you got baptism by fire. The tougher a material is, the harder it becomes to machine when the cut is interrupted. Avoiding such an operation if possible is in your best interest, particularly on small machines.

When such an operation can't be avoided, it's important to shear the material instead of force it with negative rake, so a HSS with chip breaker is often the solution to the problem. Avoid carbide. It doesn't do well in an interrupted cut, especially on light machines.

Harold

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mcostello
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Post by mcostello » Sat Jul 28, 2007 9:19 pm

Done my share of knocking off square corners. One trick that helps is to put carriage break on slightly(IF AVAILABLE) cuts down on hogging cut.

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Post by Harold_V » Sat Jul 28, 2007 11:22 pm

Another good trick is to take repetitive facing cuts instead of turning the length. Lots of positive rake and light feed.

Harold

RET
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Post by RET » Sun Jul 29, 2007 10:48 am

What you want to do is quite possible but to be successful, you need to use the power feed combined with a slow headstock speed and HSS tooling as Harold says. Make sure the tool bit is firmly supported. Sometimes an interrupted cut sounds worse than it is, but whenever you are in doubt, it pays to stop & think again.

What is also important is to make sure the work is very firmly supported. The last thing you want in this situation is to have the work move. Use a 4 jaw chuck & a tailstock center, ball bearing if you have one. Slowing the machine down helps a lot (back gears) and the power feed is necessary to give a uniform cut. I wouldn't really recommend trying to do something like this by hand on a light machine.

For something like this, a belt drive machine can be an advantage because the belt may slip if the machine jams, but if you are using back gears, don't count on it; the mechanical advantage with back gears is pretty high.

Just my experience.

Richard Trounce.

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boaterri
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Post by boaterri » Sun Jul 29, 2007 8:48 pm

With aluminum try knocking the corners off with a woodworking table saw with a carbide tipped blade set at a 45* angle. Just be sure to wear long sleeves and safety glasses. The table saw will slice it with no problem.

Rick

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Post by gzig5 » Mon Jul 30, 2007 11:11 am

Use a belt sander or grinder to knock the corners off as much as possible first.

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BadDog
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Post by BadDog » Mon Jul 30, 2007 2:39 pm

Strangely, that last suggestion seems to be overlooked by many machinists. I guess it's the "everything looks like a nail" syndrome? I watched a guy turning a piece of square stock on a light lathe. "BAM, BAM, BAM, ...", shuddering the whole time. He needed a square end about 1/2" long, the rest turned down to a considerably smaller diameter. Asked him why he didn't just mark off the 1/2 "leave it alone" end, and free hand grind away most of the corners. Even grinding it to sort of an octagon would dramatically ease the transitions and reduce the number of hammering cuts. His response, "Hmmm, didn't think of that..." "Me machinist, me need round thing, so me put in lathe..." ;)

One of the first books I read when this sickness first began had a section on process selection (or whatever they called it). Basically boiled down to "You'll save a lot of time and tooling if you do bulk removal with the band saw (generally fasted material remover in the shop short of plasma/carbon-arc), rough grind if it will aid in fixturing or process (i.e. reduce hammering on the lathe), and only then move to the final processes (which it went into in more depth regarding rough/finish/etc.)
Russ
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thomas harris
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Post by thomas harris » Mon Jul 30, 2007 9:08 pm

BadDog wrote:Strangely, that last suggestion seems to be overlooked by many machinists. I guess it's the "everything looks like a nail" syndrome? I watched a guy turning a piece of square stock on a light lathe. "BAM, BAM, BAM, ...", shuddering the whole time. He needed a square end about 1/2" long, the rest turned down to a considerably smaller diameter. Asked him why he didn't just mark off the 1/2 "leave it alone" end, and free hand grind away most of the corners. Even grinding it to sort of an octagon would dramatically ease the transitions and reduce the number of hammering cuts. His response, "Hmmm, didn't think of that..." "Me machinist, me need round thing, so me put in lathe..." ;)

One of the first books I read when this sickness first began had a section on process selection (or whatever they called it). Basically boiled down to "You'll save a lot of time and tooling if you do bulk removal with the band saw (generally fasted material remover in the shop short of plasma/carbon-arc), rough grind if it will aid in fixturing or process (i.e. reduce hammering on the lathe), and only then move to the final processes (which it went into in more depth regarding rough/finish/etc.)
It was the first attempt at such a procedure. Yes it will not be attempted in such a way again. Even the plastic is probably worth saw cutting the corners off. One thing about the grinding method is it creates two messes instead of one, and the mess with the grinder needs to be isolated some from the lathe. I also wonder if the tool steel could become work hardened with the agressive grinding needed to remove that much material. I hoped that the lathe would do the whole job, as that keeps all the mess in one area, and lathe chips are easier to clean up than grinder dust. Especially that much grinder dust. I may just go outside next time I need to turn square steel round, that way the mess will add minerals to the soil. Thanks to all who offered advice.

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BadDog
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Post by BadDog » Mon Jul 30, 2007 9:41 pm

If you have a clean, aggressive (coarse) wheel, it shouldn't work harden. If it gets hot enough to discolor significantly, you should probably back off and use more dipping sauce.

On the matter of mess, I've tried to address that by making a "grinding cart/table". It's a ~500+ lb industrial roller cabinet on 8" double tire casters, something like 7' x 3' or so, with 1/4" and 1/2" plate/angle construction. On it are mounted my 4 angle grinders and 3 belt sanders. In the cabinet are grinding consumables and dressing stones along with hand stones. Basically, anything grit related. When I need to use it, I roll it out onto the apron to keep the mess out of my shop.

I'm thinking of adding another 2" vertical receiver (as in hitch) to mount my bench grinder as well. Currently the grinder is on a separate pedestal that I just drag outside. I've also got a makeshift quick mount plasma cutting rack that goes on the same heavy cabinet (and needs a better mount). Another of those "round tuit" projects...
Russ
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Harold_V
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Post by Harold_V » Mon Jul 30, 2007 9:51 pm

BadDog wrote:If you have a clean, aggressive (coarse) wheel, it shouldn't work harden. If it gets hot enough to discolor significantly, you should probably back off and use more dipping sauce.
Nope! Not a problem. Unless you reach a temperature that hovers in the 1,500°F range, then quench, you will not harden steel-----although, depending on the type of material involved, you can soften it with even lower temps. Getting it hot, under red heat, isn't normally a problem. If you're dealing with low carbon steel, even heating and quenching won't make any changes in hardness.

Harold

Jose Rivera
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Square material

Post by Jose Rivera » Mon Jul 30, 2007 11:54 pm

As Harold knows, I used to run a Grazziano just like the one he owns. This lathes have a tracing unit on them.

This lathe at Jorgensen Steel Forge Division in L.A. was more or less exclusively used to machine test bars that came from the ends of large forgings that we machined into large ship shafts.

The test bars covered the full spectrum of materials, from aluminum to 300 series stainless to tool steels like 4130 and 4140.
Trying this on a less sturdy lathe may be dangerous though I did the same at U.C.B. on a Monarch EE10 and blew everyone away. They could not figure out how to machine a square into a round without all the banging.

These bars came to our shop un six inch long by 1' square pieces, rough sawn.

In one night the record to beat was 39 test bars completely machined.
(For the ones that don't know how test bars look, they have two .750 dia. with fine threads bosses at each end. Two radiused corners slopping down to .375 diameter for about 70% of the whole length looking sort of like a bone or a dumbbell).

39 test bars in eight hours is about 5.5 bars per hour with threads, polished and all excluding brakes (seven hours work time).

The fact that they where square did not slowed down the process no matter how hard or thought the material.

Here is how we did it:
One need a constant variable speed lathe to make good time.
Turning the piece between centers I would cut with the tool (5/8 carbide indexing holder with triangular insert, .015 radii) just bellow the area where I could get a full clean diameter by at least .015.

The spindle will be at almost full speed but using a fine feed of about .008 per rev., and with the constant variable speed one will slow down the machine if the insert was not taking it because of the type of material, or speed up farther to make better time.

The amazing thing about doing it this way is that the banging of the corners will almost disappear and the chips will come out as strings of little triangles attached by at each end by the amount of material that the tools is cutting bellow the maximum circle possible on the square.

I have never tried this with squares larger than 3/4 though. The larger the square the deeper the tool need to cut, but it may be possible if one uses high speed bits on aluminum or mild steel.

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