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Thread: Threading problem
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12th March 2012, 08:13 PM #1
Threading problem
Ok
I have been trying to thread a 1 1/2" 8 tpi internal thread. I am making a ER32 chuck for the Sheraton
Problem is: As the sharp internal HSS threading cutter hits the bore , the front end ( closest to headstock ) of the carriage rises and the tool runs along the bore , rather than cutting .This occurs when the heavier cutting is needed. It's OK when doing the initial shallow cuts, but when the cutter gets deeper, it jumps up and doesn't want to cut deeply .
As you can imagine , this ruins the thread , it ends up too narrow and malformed .
The toolpost is very rigid . No problem there .
The carriage has a lock at the rear end . I have this tightened up , but not too tight . the arriage still moves freely .
External threading is no problem
MIKE
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12th March 2012 08:13 PM # ADSGoogle Adsense Advertisement
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12th March 2012, 08:19 PM #2Member
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Hi Mike
Are you feeding with the cross slide or top slide? If the former, then the cut will get progressively harder as the length of the cutting front increases. If you set the top slide to the flank angle of the thread then you can feed along one flank only, reducing the cutting forces (and generally allowing finer feed).
Cheers
Si
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12th March 2012, 08:26 PM #3
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12th March 2012, 08:49 PM #4GOLD MEMBER
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Hi Mike,
Are you sure you have enough clearance (side, front or both?) for the helix angle and that its a boring bar?
Stuart
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12th March 2012, 09:06 PM #5
boring
Hi Stuart
Yes , I did think of the same thing. The clearance angles are fine .
The problem is the lathe itself. The SB 9 family has a weakness in the design, you can lift up the carriage from the front side . The back side of the carriage has a sort of gib underneath , this holds the carriage well at that side . But, the front side has no gib , it just floats on the bed
A force of a certain type, will lift the carriage , not good
MIKE
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12th March 2012, 09:11 PM #6Distracted Member
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The cutting forces should be downward.
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12th March 2012, 09:16 PM #7GOLD MEMBER
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All the cutting force should be pushing down,there should be no indication that the carriage is lifting.
Are you running the lathe in reverse with the tool upside down?
I think that AMWH sells blank threaded pieces to suit collet chucks.
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12th March 2012, 09:17 PM #8
picture
Pic
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12th March 2012, 09:19 PM #9
Yes
YES
I agree , the cutting force is downwards , but the carriage lifts up ... I cannot understand it
The cutter has side rake on it .
MIKE
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12th March 2012, 09:24 PM #10
another pic
another pic
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12th March 2012, 09:25 PM #11GOLD MEMBER
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Is this the first internal thread of this size that you have cut?
Not a good photo but is there clearance on the back edge of the tool?
From the photo the rear edge of the tool looks to be almost vertical,there dosent appear to be any clearance,it should be sharpened virtually to mirror the leading edge.
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12th March 2012, 09:28 PM #12Philomath in training
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This isn't one of those things where you run the lathe in reverse so that you are feeding towards the tail stock is it?
My thoughts would be that 1.5" is a substantial size so that you should be able to get a nice (big) rigid boring bar to hold the tool to reduce/ eliminate tool deflection. Investigate cutting the thread with the compound parallel to the bed - the 30 degree stuff can be used but especially internally adds an extra degree of complication.
Feeding towards the chuck on an internal thread is not difficult. Space the item out a little from the chuck so that you can see the tool come out from the back. Alternately work out your maximum depth and put a piece of tape on the boring bar at the correct depth (so that when you are at full feed into the 'nut', the tape is level with the start of the nut or similar). Try doing this on a scrap piece of something soft (plastic, Al, machining wax) until you are comfortable with it. Doing something new on a 'good' piece becomes nerve racking in itself.
Michael
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12th March 2012, 09:30 PM #13
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12th March 2012, 09:33 PM #14GOLD MEMBER
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Obviously you are doing something different,you will have to work that one out with trial and error.
Bit hard to give suggestions or guess when it worked for you before.
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12th March 2012, 09:34 PM #15
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