Thread: Threading problem

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

Originally Posted by SiJ

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

Hi

I am feeding using the top slide , set over 30 degrees .

MIKE

Hi Mike,
Are you sure you have enough clearance (side, front or both?) for the helix angle and that its a boring bar?

Stuart

Originally Posted by Stustoys

Hi Mike,
Are you sure you have enough clearance (side, front or both?) for the helix angle and that its a boring bar?

Stuart

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

The cutting forces should be downward.

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.

Pic

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

another pic

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.

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

Originally Posted by pipeclay

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.

Yes there is clearance on the back edge .

I did do a very good internal 1 3/4" 7 tpi thread for the PREMO lathe spindle protector . It worked beautifully on that occasion using the same cutter . But now, it doesn't work

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.

Originally Posted by Michael G

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

Michael

The tool isn't deflecting , it's a solid piece of 3/8 x 1/2 HSS ground down .

The carriage itself is deflecting upwards under load .

No not reverse . Normal dorection

MIKE