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How much difference can tooling make on a small, low power lathe?

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  • Evan
    The vice grips are clamping the insert to a carbide tool as a test to see if I should bother to make a tool holder.

    The large cone shaped object in the tailstock is a live pipe center for large hollow work.

    Rake angles:

    The insert is set to cut at the centre line.

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  • gfphoto

    Very informative post for me as I've just found a bunch of inserts of that shape and had been wondering how to use them and If I could make a holder. I think mine are plain steel or maybe carbide and without holes.

    I want to make sure I understand "negative rake": is that when the cutting corner of the insert is lower than the opposite end?

    Is the tool height set so that the insert contacts the work at the centerline?

    Also, in the fourth photo, what it the device opposite the tool holder? Is that a kind of follower rest? And what are the Vise-Grips for?



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  • How much difference can tooling make on a small, low power lathe?

    It can make a great deal of difference. Here is an example.

    It is frequently said that negative rake carbide inserts will produce the best finish. In the same breath it is also said that they require a lot more power and rigidity to use. Both statements are more true than false but not entirely accurate. Negative rake is not the cure-all for finish problems and negative rake cutters do not necessarily require a large, rigid and powerful machine.

    I was turning some schedule 40 welded seam pipe of dubious metallic content which was imported from India, that bastion of quality control and unmatched consistency of product.

    I normally use hand sharpened solid carbide cutters for this, especially the first pass to remove scale. I was not seeing a good enough finish although it was passable for the purpose it wasn't quite good enough for my eye.

    Recently I traded an item to Rockrat in exchange for some old stock cermet (CERamic-METallic) coated carbide inserts. They are as plain as they come, simple radiused squares with no chip breaker, no rake and symmetrical top and bottom.

    They are intended to run at a negative angle in respect of the stock surface which provides the requires clearance under the cutting edge.

    I made up a tool holder from a square bar of cast iron by milling a 45 degree angled step on one end to bring the centre height on line. It is also canted to provide the correct rake angle and exposes two of the corners for use.

    I applied this to the job running the lathe at 800 rpm and taking light cuts of no more than .020" and less for the finish pass. The insert cut a near mirror finish with none of the usual tearing, skipping and other usual grief seen when turning mild steel. No lubricant or coolant was used.

    As can be seen, even if you have a light import machine there is much you can do to improve the quality of your work by simply spending the money to buy quality tooling. These inserts have 8 usuable corners and the one I used turned at least three passes of nearly a foot by 1.5" diameter. If used for only the finishing pass on 1018 steel that equates to turning at the very least a total of 24 feet of pipe with a realistic expectation of much longer life than that.