Clarification please

[DuncParticipant @dunc]

[DuncParticipant @dunc]

Have been looking through the past issues for some tips on threading. I noticed, in Dave Fenner's Making a Simple Internal threading Tool (MEW 165, page 39) the following: "… Harold Hall's trick of using a tap as an internal thread cutting tool. [last paragraph]"

Surely, by definition, a tap is an internal thread cutting tool.

I am missing something here. Please enlighten me.

[JasonBModerator @jasonb]

You can use a tap that is smaller than the thread required mounted in the toolpost and feed this in using the lathe, Did it myself only this weekend, I did't have a 1/2"x40ME tap so used a 5/16" one to cut the 1/2" internal thread.

 

Sorry I did not take a pic as I was doing it but this is the Lukenheimer carb that I was working on and you can see the 1/2" thread which turned out well. Sorry its a bit black as I have just silver soldered some plugs into the convoluted fuel passages.

 

 

J

Edited By JasonB on 14/05/2012 19:20:00

[Chris TriceParticipant @christrice43267]

Posted by Dunc on 14/05/2012 18:53:25:

Have been looking through the past issues for some tips on threading. I noticed, in Dave Fenner's Making a Simple Internal threading Tool (MEW 165, page 39) the following: "… Harold Hall's trick of using a tap as an internal thread cutting tool. [last paragraph]"

Surely, by definition, a tap is an internal thread cutting tool.

I am missing something here. Please enlighten me.

 

 

 You can use an M6 x 1mm tap to thread a 30mm hole with a 1mm thread. The tap is held in the toolpost in the same manner as an internal threading tool. The only difference is the tap gives you (in theory) the perfect thread form. I've been using taps this way for a while now and is my preferred method.

Edited By Chris Trice on 15/05/2012 00:05:19

[DuncParticipant @dunc]

Thanks for the replies. Are the change gears are set up for the correct threads/inch and the feed engaged?

[Quote] "The tap is held in the toolpost in the same manner as an internal threading tool."

Does this mean the 29.5 degree set-over (assuming 60 degree threads) of the top slide? Also, how is the tap aligned to the thread axis; what takes the place of the fish?

[JasonBModerator @jasonb]

Yes the change gears are set to suit the pitch of the thread.

I've only done it without angleing the topslide, tool parallel to lathe axis, set the cut with the cross slide.

I can mock it up tonight and take a photo.

Whats the "fish"

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

When using a tap to screwcut internal threads of size larger than the tap a problem can arise especially with larger and coarser threads .

For a given pitch the helix angle varies with thread diameter so if correct on tap it won't match what the helix angle should be on the larger thread . The tooth shape is only actually correct on the cutting edge (assuming set to centre height) below that it progressively becomes more and more of a mismatch .

On smaller and finer threads the tap flutes usually provide clearance before the mismatch becomes acute but on larger and coarser threads large parts of the teeth under the cutting edge may need to be ground away

[Clive HartlandParticipant @clivehartland94829]

Thats what 'Chasers' are for, only trouble is I never have the one I need to cut the thread in large diameter components.

I can see the problem with large pitch threads abd clearance would be needed, possibly a three flute tap would be OK.

Clive

[DuncParticipant @dunc]

"fish" is the tool used to align a single point threading bit parallel to the axis of t5he thread.

[Tony Pratt 1Participant @tonypratt1]

When you disengage the feed at the end of the thread do you not then get a series of radial grooves corresponding to the taps pitch as the tap will have teeth in contact all the way along the job/

best wishes,

Tony

[david lockwoodParticipant @davidlockwood10028]

i have tryd this method of cutting a thread but i simply could not get the tap to cut at all so i gave it up for a bad job

would any one have any tips for using this method

regards David

[JasonBModerator @jasonb]

Yes, its best used for through holes or if blind machine a small undercut at the end of the thread and just stop the machine when the tool runs out, back it out of the cut and then withdraw the carrage.

If you can't get an undercut in and the part is not tool large then the spindle can be turned by hand stopping as the tap touches the bottom of the hole.

On the one in my photo because I had a valve seating face right at the end of the thread that I did not want to damage I managed to get a very fine undercut and then rotated the spindle by hand and left the feed engaged for the whole job, just disengaging the tap from the thread with half a turn of the cross slide before with drawing under power with the machine in reverse.

J

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