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[How-To] Hitting a measurement
- Thread starter DavidR8
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- Jul 8, 2018
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That's pretty much what I've been doing with carbide. I tried to describe it early on yesterday but it just sounded flakey so I didn't post it.
To begin with, establish a DOC that works well with the tool, the lathe, and the material. This is easy enough to do during roughing, and as it's mostly a rule of thumb, you don't need to be too scientific -- all that matters is that when you take that cut, the results are accurate to .001. For my 14" lathe, with trigon (WNMG) inserts, on mild steel, this is around .020.
When making a part, take it down to double this measurement, then take two cuts of equal depth, measuring between the first and second as a sanity check if the measurement is critical. Generally, you will find that the cut is spot-on, and if it isn't, well, break out some HSS tools and sneak up on it.
Basically, what you are doing here with carbide is the opposite of sneaking up on the final measurerment: you know the final measurement and how to get it, so all you are doing is preparing the part so that you can make those last two cuts. There is a lot of leeway around the magic number; yesterday I was making some 10-32 bolts, and with the first one I roughed down to 0.030 and then did two 0.015 cuts, but for subsequent parts I just did two .027 cuts with no preliminary roughing cut. All came out to the same diameter with sub-0.001 variation.
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Except for a few 60 degree carbide tool bits that I believe are for threading all I have is HSS. Once I got the belt sander grinding tool bits became no big deal. By no means am I anywhere close to being an expert at it. Yet I can now grind any shape that I want to. For example the other day I was turning up to a shoulder and decide I needed a little more front clearance (wrong term probably). So I changed the angle from 80 degrees as recommended by Mikey in his absolutely awesome thread on grinding tool bits to 70 degrees. Probably didn't make any difference in the cut but I was more comfortable making the cut. I still need lots more practice turning up to a shoulder.
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- Mar 17, 2018
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Nowadays, to be honest, I mostly use HSS tools on the lathe and am happy to leave this carbide business to you guys!
This is where I find myself, over and over. My son bought a qctp and a set of indexing lathe tools (he is an actual machinist) for my very capable lathe (Clausing 6329). I have used the carbide tooling with success, but I feel more comfortable and find it much easier to hit my numbers with hss. I will probably buy some tool holders and put hss in them. I'm sure that I will get a wtf look when he sees them.
Btw, thank you mikey, for all of the advice!
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- Dec 20, 2012
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Chuck, this is exactly the kind of thing that makes all the work I did on that Model Tools thread worthwhile ... good for you! The ability to grind a tool to do what you need it to do is a bigger deal than you might think. You ground the tool for better access. Now you need to think about how to make your tool work better for your lathe.
Not all lathes are the same; therefore, no one tool geometry is optimal for all lathes. If your lathe loads up with a 0.050" deep cut with a square tool, grind a bit more side rake and see if that helps you go deeper. If you are work hardening that stainless rod, increase side rake a bit to lower cutting temperatures and see how that works. Mess with it until you find a tool geometry that allows your lathe to work to its full potential. THAT is the beauty of HSS but it takes someone like you to see it.
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You're welcome, Sir!
Yeah, I'm with you. Carbide is good for some stuff but a good HSS tool it is just so much easier to use. My brain is happier when I don't have to think too hard to come in on size!
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- Dec 20, 2012
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If you dial into the next cut without backing off the handwheel then the threads of the leadscrew and nut should be in intimate contact and cannot drift. This is true even when there is back lash present. However, loose gibs can be an issue and allow the cross slide itself to lift under load. This is commonly dealt with by locking the cross slide before a cut is made. Since most gibs are locked by a locking screw that presses into the gib, most slack is taken up and chatter or movement is reduced.
EDIT: I should amend this by saying that if the cross slide screw is not anchored solidly then yes, it can shift even if you set your depth of cut properly.
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On my 12×36 lathe I hog down in 0.020 (like above) until I am 3×DoCs away from the desired diameter (+/- a couple of thou) and take an accurate measurement (micrometer)
I then dial in 1/3rd of the difference between where I am and where I want to finish and take that cut.
I then measure to see how much actually came off and divide that by 1/2
I then dial in the 1/2 remaining and take the cut.
I then measure to see how much actually came off and calculate how much is left. Based on the previous 2 cuts and subsequent measurements I adjust the final cut.
So, if I had 59.5 thou to go, I would dial in DoC=20 and take a cut, this might come out still needing 48.5 though (1 thou more than I though I dialed in)
Half of the 48.5 is 19 thou so I dial this in take the cut and then measure.
So, now I find I have 19.7 thou to go (0.2 less than dialed in). So, I dial in 19.7-1.0+0.2 = 18.9
Notice that you only dial in fractions of a thou on the last cut.
The change in tool pressure from 20 thou to 19 thou to 18.9 thou is essentially negligible, and what makes this procedure self correcting.
When I used the 2 tries method I would end up +/- 0.001,5" and when I use the 3 tries method I generally hit +/- 0.000,7" or better.
But watch out !!:: when you hit a number this precisely, the number will change as the part cools down (overnight, for example.)
I once hit a target of 2.000,0" on the lathe and came back the next morning to find it was 1.999,7" ?!?