The problem is with machining flat bar along its length ; There is not usually a problem with tuning round bar but there probably would be if you try to mill a flat along its length. Colin

I suspect that "precision" simply refers to the OD being consistent, rather than precision in terms of tenths, which is important when using collets on a repetition lathe. Hot rolled steel is generally oversize with a wide tolerance.

Many cold drawn materials warp if machined asymmetrically; I've had the problem with brass rectangular section. As a matter of course I anneal cold drawn sections before milling; and use hot rolled when I can. Anecdotally hot rolled seems less prone to rusting, although it's more difficult to get a good finish.

I've certainly bought poor quality cold drawn sections, and from so called professional suppliers. Hot rolled seems more consistent.

Andrew

Does that prove metric stock is superior?

Neil

When I buy metric plastic stock it often comes a good deal oversize to the nominal, for e.g I buy 22mm Nylon 6.6, and it actually measures 22.7mm, However, the roundness of the extruded bar is often left to be desired, so I presume this is the reason why it seems a fair deal over 22.

Michael W

this is fairly standard practice for flat bar too, if you mill one side at full depth, you will often find you have trouble holding the size due to the inherent stresses.

But if you take both sides, half as deep, you'll get a more accurate cut. They will also be more parallel to each other.

Michael W

You can use Bright Drawn Mild Steel for machining, but it helps if you stick it in the fireplace first to get nice and hot and let it cool slowly over night.

I too have experienced flat section BDMS moving quite dramatically when milling one side of it. The heating and slow cooling relieves some of the stresses from the cold rolling process.

Phil

In a past life I used to machine large lumps of steel up to 60" diameter. We used to rough them out and leave them for a couple of days to let the stress relieve it self, then final machine to size. Some of the brake drums we mad would go egg shaped over night!

I had a problem last night that the forum wasn't responding after I posted, so I refreshed the page several times before it would load.

It obviously accepted the 'new posts' although it wouldn't send them back to me!

I also have 4,000 posts as 'Stub Mandrel' so my real total is over 14,000, of which several are quite useful

Neil

Despite the length of the bar it's only the length in the region of the slot that is affected. And the deeper you cut the less the distortion, as the stresses tend to be concentrated near the surface. Try milling a millimetre or two off the whole length.

Andrew

Size does not come into it much. Like Andrew I machined some 5/16" sq section brass down to 1/4". Took 1/16" off one side and it bent really badly maybe 10mm of light shining through on a 300mm length.

I also leave things like those crank shafts for a few days after stitch drilling so that they settle and again after rough turning.

I suppose to some extent it will depend on how much the bar has cooled down as it goes through the last set of rollers.

Edited By JasonB on 06/07/2017 14:35:21