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Reverse threading blind hole

rabtrfld

Aluminum
Joined
Oct 11, 2016
Location
WI USA
Threading question...
I need one piece of a threaded blind hole, with as little wasted space as possible at the bottom of the hole. Don't want to turn forwards, doing multiple passes I will surely crash on the bottom of the hole. If I turn the threading bar over and cut the back side in reverse I can work towards the open end. But I'm not sure if the toolpost etc. will take upward force without deflection.
No risk to try it, the workpiece is just a chunk of round bar. On the other hand, I could still crash it while resetting for the next pass, but I'm usually not that clumsy if I don't overdo the coffee.:crazy:
I can afford maybe 1/4in. gap at the blind end, so maybe should just do it forwards with care. Its about 1-1/4 ID, 12TPI, on a Clausing C1440S.
 
Why not put a groove in the bottom of the hole to allow the thread tool some clearance, then the threaded component can go to the bottom of the hole
 
Just by turning the threading bar upside down and threading away from the chuck will produce left hand threads. Don’t ask how I know. You will need a left hand threading bar, spindle turning in reverse and cutting on far side of part.
 
I do most all ID threading on the back side of the chuck. It is easier to see what is happening. Your tool needs to be a left hand tool, but the cutting edge will be on the top. No need to worry on uplift of the compound or toolholder.
 
Just by turning the threading bar upside down and threading away from the chuck will produce left hand threads. Don’t ask how I know. You will need a left hand threading bar, spindle turning in reverse and cutting on far side of part.

I have seen rules with all the possible threading configurations as to which will produce left or right hand threads. I like the simple rule: If the spindle and lead screw are turning the same direction you get right hand threads, if they are turning in the opposite direction you get left hand.

If left hand thread leads crews exist it would be the opposite.

For the OP's problem if he could thread mill it would be real easy. A lot of times on the mill I start on the bottom of the hole and mill out so as not to deal with the chips.
 
All the configuration options are just choices of what you gain or lose. I've got a 1-1/2 x 8 internal thread I do frequently and usually cut on the far side. It's a right hand thread so the threading tool is upside down. That's just the same as right side up on the near side, but rotated half a revolution along the thread lead. The lathe rotation is also forward and not reversed. Since it's going into a blind hole I've got about 3/16" of thread relief in the bottom of the hole.

To signal when to release the half nuts I've got a machinist clamp on the front way as a mark when I know the tool is safely in the relief. (Tape, carriage stop or anything else convenient would work as well) I choose this arrangements because I'm accustomed to backing off the crosslide at the same time I flip the half-nuts and the muscle memory is fast-turn-reverse. If I momentarily lose my concentration I don't want to zip that crank instantly in the wrong direction.

For something like threading that's most often OD threads and that I do frequently I really get used to a consistent sequence. On OD threads I sometimes don't even create a relief but just get on those controls in that fraction of a second at the end. If, for some reason, it has to be done differently I spend a little time before starting psyching myself for the difference -crank IN not OUT. Bad shit happens in that tiny moment of going the wrong way on a machine handle or control and I hate it when I do that.
 
You can have a thread with no runout and flat flush bottom. How?

Thread insert, press fit, not always the worst solution
 
Thread chart
http://www.steelmasters.co.nz/webfi...read_Dimensions_for_UNF_Screw_Thread_2016.pdf

SHARS Internal Threading Boring Bar 3/8" RH Shank for 1/4" Insert | eBay
Not a bad idea to mount a dial indicator some place to tell when at depth. Not a test indicator and a wider range like .001 or so https://www.amazon.com/Fowler-Nsk-F...EZKS92QSPK5&psc=1&refRID=VVWTW1FXREZKS92QSPK5

To avoid confusion put a large right-hand nut in the chuck and look to see that your spindle/chuck direction, the cutting tool set up or down, is correct and the direction of travel will make the thread you need. Yes, have a left-hand nut handy in your toolbox also. You don’t want to begin cutting the wrong way to mess up a part. looking at a thread will make it sure that your set-up is correct. That is much better than looking at a hole and trying to think it out.

You should know how you will check it for size or fit. It is not a bad idea to make a male thread gauge checking that with wires/ or a hand feel into the thread where it needs to go.

Yes, it can be tough to internal notch/undercut, you may need to take very fine in feeds /perhaps with a little controlled /careful bed travel back and forth (left and right)with watching your indicator so to not break an expensive boring bar. A 12 thread is about .083 deep so when deep into the notch/undercut that is a lot of stock cutting on both sides of the bit/insert.

If new to internal threading it might draw the bore at 5 or 10 x scale to decide what clearance is needed below the cutting edge if grinding your own bit. Clearance may need primary, secondary and a third angle or rolling off the heal to miss rubbing in the part. Clearance is often the reason for failure. Boring bars need more clearance than OD cutters, and often a number of secondaries to be sure the heal will clear the ID of the bore.

Bought boring bars with inserts are good/better but expensive to crash. Many bought boring bars have back rake on the cutting edge and so can travel in or out effectively, and a flat on the shank to ensure they are set at the right square angle. Oil is often good for boring.
Home-made boring bars with having a round shank are set to the desired rake angle so to be used for different marerials with care to look see it might have enough clearance.
 








 
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