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Firebag
09-02-2004, 12:13 AM
Hi guys; I have a problem which one of you may be able to help me with. I am somewhat of a novice at machine work. I have a lathe
(Grizzley 11 x 26 ) and have been attempting to cut a 22 x 1.5 mm metric thread. I have made sure that the change gears are correct for the thread pitch. I have set the compound at the required 29 degrees (even using a protractor to check). I have insured that the cutting tool is at the correct height, and perpendictular to the work. When I attempt to cut the thread, I get grooves cut in the work piece, but no real thread form. I have checked the cutting tool for the correct angle and that is correct. Before I tear my hair out, what could I be doing wrong. Please help.

Thanks

WJHartson
09-02-2004, 03:04 AM
I assume what you mean is that you are getting several different grooves when you engage the halfnut. The tool is starting in a different place each time you engage the halfnut at the same point.

When cutting metric threads on a lathe that is set up with and inch feed screw you have to back the tool out and reverse the feed screw without disengaging the halfnut. Changing the gears will give you the correct pitch but it won't change the relationship of the leadscrew and the thread gage.

Hope this helps

Joe

zl1byz
09-02-2004, 04:56 AM
You really need to explain what you are getting or you will get a lot of suggestions that arn't relenvant.
I don't know the lathe you are using, there are others here that will. They should be able to comment on any special procedures relating to what numbers you can engage the half nuts on.
One thing I have found most usefull for thread cutting on manual lathes and that is use some thread cutting lubricant. I think this becomes most important as we are usually using less than optimum speeds when manually cutting threads, so using one of the products specially made for thread cutting certinaly helps the cutting action at the tool.

zl1byz
09-02-2004, 05:11 AM
Some things that will help others help you. You mentioned the make and size of lathe, this is good. Many will know the specs of that machine, and any special procedures to do the thread you want. What material are you threading is helpfull. What you think is wrong with the result ie poor finish, bad form, multiple threads.

Cheers, John.

Ian B
09-02-2004, 06:14 AM
Do you mean that you're left with a ripped up surface, ie. something that resembles a screw thread of the correct pitch, but the surface finish is terrible?

If so, it could be down to the material you're trying to cut (try a piece of brass, see if it looks any better), the sharpness / rake angle / clearance angles of the tool (wrong clearances can wipe the thread out), or lubricant.

Maybe this will help others to give the answers you're after.

Ian

Carl
09-02-2004, 11:55 AM
Since it's a metric thread, do not disengage the half nuts once you start the thread, the spindle must be reversed with the tool retracted and the half nuts engaged to reposition the tool for the next pass. Make sure you set the compound to 29 degrees from perpendicular and not parallel to the lathe axis. Set the tool on center vertically and perpendicular to the work AFTER you set the compound to 29 degrees NOT before. Advance the compound and cross slide screws before you zero them to take up backlash in the screws. When you retract the tool with the cross slide to reposition for the next cut, retract it at least a full turn so that when you advance it back to zero all backlash is taken up. Get Martin Cleeves "Screw Cutting in the Lathe" book and study it, it is a great text on threading in the lathe.

zl1byz
09-02-2004, 03:30 PM
<font face="Verdana, Arial" size="2">Originally posted by Carl:
Make sure you set the compound to 29 degrees from perpendicular and not parallel to the lathe axis.</font>

Yep this is a good point, I have seen this done before. Doesn't make good looking threads, http://bbs.homeshopmachinist.net//wink.gif

lynnl
09-02-2004, 05:31 PM
A similar problem I had soon after I got my lathe was because the thread dial gage, which is on a pivot to swing it out of engagement or 'into', was not quite rotated enough for good engagement with the leadscrew. I can't really tell from the comments if that might be a player here, but it's worth considering.

..disregard. I was kinda mentally mangling the essential elements of two ongoing thread threads. Got'em confused. Sorry.

[This message has been edited by lynnl (edited 09-02-2004).]

Firebag
09-02-2004, 11:38 PM
<font face="Verdana, Arial" size="2">Originally posted by WJHartson:
I assume what you mean is that you are getting several different grooves when you engage the halfnut. The tool is starting in a different place each time you engage the halfnut at the same point.

When cutting metric threads on a lathe that is set up with and inch feed screw you have to back the tool out and reverse the feed screw without disengaging the halfnut. Changing the gears will give you the correct pitch but it won't change the relationship of the leadscrew and the thread gage.

Hope this helps

Joe</font>Thanks for the help. I cut the threads by making a pass and then backing
out without disengaging the lead screw, and
the threads were perfect. I knew it had to be simple.

Thanks again