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Boring With 7x10 Mini Lathe

mzvarner

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Apr 7, 2016
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#1
I recently ordered a 9 piece set of variable length boring bars from LMS that can be used with my QCTP. So far the set seems nice but what i have bored does not seem to come out paralell. I do not really have a good way to measure the internal diameters right now.

I say "it seems" because this is by feel sinc i cant measure. As i advance the boring bar i notice more resistance the deeper i go. Maybe im making mkre than it needs.to be though because so far the parts made (2 total) have fit together whell enough for my tolerances.

Any thoughts? Also whats the best way to get the tool on center for boring?
 

compsurge

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#2
Check your travel on the bed and see if the tightness is observed. You may need to adjust the gibs or work on bringing the bedways or saddle more true. Did you indicate the travel of the boring bar?
 

Groundhog

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#3
An easy way to set lathe tool height is to trap a 6" scale (or any flat thing) between the tool tip and the side of the work piece. If the top slants away from you the tool is too high, if it slants towards you it is too low. Better to be too low than too high. Pic is just on center.
 

higgite

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#4
Mini lathes aren’t all that rigid, even when all gibs are snugged up. Nor are mini lathe sized boring bars, tool holders, tool posts, etc. Depending on the depth that you are boring to and the overhang of the boring bar and the overhang of the work piece, it is quite possible that you are boring a tapered hole. It’s the nature of the beast. To overcome that you can make spring passes, that is, repeated "clean up" passes when near your target dimension without changing the depth of cut. I had a 7x14 and now have an 8.5x20. With both machines, I have found it necessary to sneak up on final dimensions. If I don’t, more often than not I will overshoot the mark when I finish making spring passes. Hope this helps.

Tom
 

mikey

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#5
I recently ordered a 9 piece set of variable length boring bars from LMS that can be used with my QCTP. So far the set seems nice but what i have bored does not seem to come out paralell. I do not really have a good way to measure the internal diameters right now.

I say "it seems" because this is by feel sinc i cant measure. As i advance the boring bar i notice more resistance the deeper i go. Maybe im making mkre than it needs.to be though because so far the parts made (2 total) have fit together whell enough for my tolerances.

Any thoughts? Also whats the best way to get the tool on center for boring?
Are you referring to the Chinese brazed carbide boring bars? These typically come in a set of 9 for like $10.00 or so. If so, then they should work okay for rough work. I have a set and while I don't use them on the lathe, I used to. I don't recall them boring tapered bores, though. You might want to buy a set of telescoping gauges and check what your diameters are at the top and bottom of the bore to be sure. A boring bar should not encounter more resistance as you go in; the cutting action is the same. Is it possible the hole is packing with chips?

As for setting them on center, don't. Boring bars need to be about 0.005 - 0.010" above center to offset tangential cutting forces. Best way to do that is to make a tool height gauge or use a ruler as groundhog suggests, then raise the tool holder 1/4-1/2 turn on the knob before locking it down.