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Thread: Thread cutting on a lathe
06-13-2006, 02:09 PM #1
I've been taught to set the compuond at 29.5 deg when cutting threads. I understand this is so the back side of the tool dosen't rub and cause chatter. I wanted to get a visualization of this so I drew up the angles on cad. Now I'm only more confused as setting it to 29.5 deg would actually cause the back off the tool to have more contact than if it were set to 30 or 30.5 deg. I've tried it set at all angles with no noticeable difference. Maybe you guys can clear up the reasoning as why you set it to 29.5 deg.
06-13-2006, 02:15 PM #2
Hmm, something not right.
Point is a perfect 60 degrees. Set to 30 deg and you should have full contact on all faces...no good as the "trailing" edge will rub.
Goto 30.5 and the "trailing" edge will start to cut into the threadform already produced, also no good.
However, back off 0.5 degree the other way to 29.5 and there should be clear air right down to the point on the "trailing" edge.
Note that the angle is with zero in front of your belly button and increasing counter-clockwise to the right as viewed from above.
06-13-2006, 02:27 PM #3
Picture this. If you go straight in (0 deg), both sides of the tool will cut equally. If you start angling compound to the right (and resetting the tool straight, of course),then when feeding with the compound, you will cut more and more on the left (leading) tool side and less and less with the right (trailing) tool side the more you increase the compound angle. Once you get to the half angle of the tool (30 deg) the tool will advance along the line of the right edge, and that side will just rub. If you increase beyond 30 deg, a gap will open on the right side the further you feed in with the compound. The extreme case is with the compound aligned with the lathe bed. In that case, it's obvious you would never cut on the trailing side. 29.5 deg is just shy of 30 deg, so the right side will feed into the metal just slightly.
06-13-2006, 02:34 PM #4
The point of the 29.5 is NOT the for setting the tool but for Feeding the tool. You still need to have the tool perpendicular to the centerline, i.e. 30 degrees left and right. You would set the compound to feed in at 29.5 degrees. The point of this is to minimize any cutting on the backside of the tool. This works well with roughing cuts, however, on the final pass or two, I do not feed in with the compound but feed straight in with the crosslide to generate the final profile. Cutting threads is just like cutting a shaft. No one cares what the finish looks like when your roughing, it's the finish pass that counts. If you have a 62 rms on a roughing cut, you won't last long at your job.
06-13-2006, 02:45 PM #5
The reason to set at 29 1/2 is to make the chip behave. If you feed straight in, you are cutting a v-shaped chip. The cutting forces are much higher, and both flanks will often be rough. At 29 1/2, almost all the cutting is on the leading flank. This will also allow you to grind rake on the tool for improved cutting action.
At 29 1/2 degrees, the trailing side of the tool still scrapes the flank of the thread. This means the entire thread geometry is formed by the shape of the cutting tool.
06-13-2006, 03:02 PM #6
Remember that .1" clocked on the compound only cuts to a depth of .1 x cos 29.5 = .08703" .So do your calculations first to get the correct depth of thread.
06-13-2006, 05:46 PM #7
Ok I think gbent has got it right. Obviously you don't want to go straight in and have both sides cut that would be a mess. The instructors were telling me the compuond was set at 29.5 so the back side wouldn't do any cutting. As gbent said the back side does some cutting but very minimal just a half a degree so the forces are much less. If it was set to 30deg it wouldn't cut at all and 30.5 would leave a gap. Thanks gbent I get it now.
06-13-2006, 06:55 PM #8
I'm not totally sure about how important the 29.5* angle is.A couple of weeks ago had set up to cut Acme threads,1"X4TPI.After finishing I left the compound set for the Acme.A few days later I had the occasion to cut regular threads(I don't remember the size and tpi)but I forgot to set the compound at 29.5* and just started cutting away.I did set the cutting tool as one would normally do.The threads turned out perfect.In fact they were better than any I had previously cut in all the years I can remember.So I wonder about the 29.5* setting.
06-13-2006, 07:17 PM #9
If you are saying that the the trailing edge of the tool cut deeper at 29.5 than at 30.5, it makes me wonder if you set the angle from the correct axis... Remember, when you set the compound, you have to swing it to 90* (parallel to the cross slide) then back it up 29.5* Maybe you missed that when you did it on CAD.
06-14-2006, 12:52 AM #10
Yup thats right jim 29.5 the back side of the tool will cut material 30 is neutral and 30.5 makes a gap. There is only one axis that will get you even close to a 60 deg thread when drawing it no way could it be drawn different and be even close to 60 deg. South would be 0 north 180 and east 90. Draw up a picture and see for yourself.
Thats interesting Ray I can see how it would make a good thread though. Going straight in would obviously cause too much cutting force but when you came in at 14 deg it took enough pressure off the back side to not chatter. Yet sence it was cutting more than it would at 29.5 it made both sides cleaner.
06-14-2006, 12:56 AM #11
I think it's a big mistake to keep talking about
this " 29 1/2 degree " bit. Almost all of the lathes
I've seen lately the compound is marked the other
way. ie. it would be 59 1/2 degrees. The Clausing/
Metosas at school and the Jet I just saw Monday.
I would specify it as: Advance the compound feed
in the direction so as to just (skim) the trailing
side of the cutter.
06-14-2006, 02:23 AM #12
I like to cut threads. I have tried a lot of ways to do it. I get the best results by setting the compound at just short of 30deg. By that I mean that the mark on the compound just touches the 30deg mark. When cutting a right hand thread for example, the tool will cut on the left side and will just chase the right side of the V form. It cuts a perfect V form without the rough looking right side and I very seldom have to feed the tool in with the compound on the last cut to clean up the V. If you draw it out on paper you will find that using a 29 1/2deg setting will feed the tool in so that it will cut on both sides of the V. However it is a very light cut on the right side. If you set the compound at 30deg you will find that it will feed the tool in with little or no contact on the right side of the V and may leave a rough finish on the right side of the V causing you to have to feed the compound in .ooo5 to clean the V on both sides. Feeding in at 30deg with the compound causes the tool to feed parallel to the right side. Feeding in at the 29 1/2deg setting causes the tool to feed in on a line 1/2deg off of parallel line to the right side with the result that the right side will sustain a slight shaving action. Perhaps it was necessary when they did not have high grade tool steel and carbide. The early machinists only had carbon steel to cut with.
06-14-2006, 03:07 AM #13
I believe you're right! I just re-read rklopp's response. He nailed it. Like you, I always thought the 29.5 was to provide clearance at the trailing edge. live and learn