Grizzly 9729 half nut

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kennycrawford
Posts: 2
Joined: Wed Feb 08, 2017 3:11 pm

Grizzly 9729 half nut

Postby kennycrawford » Wed Feb 15, 2017 3:31 pm

Good afternoon,

First off I would like to admit that I am far from a machinist. Secondly I have had my lathe since 2011. The first machine tool I purchased was my Grizzly 9729. I didn't do a lot of research before I bought it but knew the Grizzly name and thought I would not be that seriously dedicated to this hobby and that the shortcomings of this unit I could deal with. In a way I was right. I now have Grizzly g4000, taig micro lathe 2, Logan 400, Precision Mathews pm2o mill, Jet and Craftsman drill presses band saw, measuring equipment granite plate .........

I like 3 in 1 as it is relatively large (I can turn a rifle barrel on it), easy to understand how the controls work and mine is not really modified. It is the only lathe I have not done a slew of "improvements" on. I think as a lathe it is a really good machine, except for the half nut. My original brass one is completely worn through and makes it useless for threading and leaves patterns in longer work due to the "wrapping effect" that the leadscrew exhibits as the slot comes around to take another "cut" from the half nut. Being new to this I looked everywhere as to why this was happening only to discover it after the warranty expired and I am staring at a 80 dollar parts that weighs in at a few ounces. By reading a few posts here I was able to diagnose the problem and a cure was put into place. I spent a huge amount of time deburring the slot in my leadscrew so it would behave less like a one flute mill. I thought I would make my own half nut only to be set back again by not being able to purchase a tap for the incredibly odd .767 (if I remember the number right) by 6 tpi left hand thread. Lots of ideas went through my head from an electronic leadscrew with ballscrews to all out cnc (another area I have no knowledge in).

Last week I had just finished the Logan lathe set up clean up and realized this lathe could turn left hand threads. This machine is missing a few gears but I had everything except a 72 tooth gear to do it. Ebay solved the problem for about 50 bucks. So what I did was turn a left hand 6 tpi tap blank that matched the diameter of my leadscrew (again .767?) I used a form tool I ground myself and used the leadscrew as a gauge to get it right. The material was water hardening drill rod. I did not photograph any of it (disappointed) because at first I did not think I would get very far and later because I was superstitious. :). It actually came out very good! Before I cut the slots to make it a tap I remembered someone saying something about Acetal being a wonderful material for half nuts on another forum for a different lathe. I had a foot of this in 1 1/2 inch diameter so went about measuring the old part and turning a blank to try and mould a half nut. Using the tap blank, propane torch, bench vise and water I carefully heated the metal and tightened the vise until I forced the blank into the plastic. I could not figure an accurate way of measuring this so I "eyeballed" it until the blank tooth just bottomed out and I stopped cranking and poured water over everything to stop the process. After a little cleanup on the bandsaw I installed it and to my amazement it worked. I played with it a little bit last night but was wondering if anyone else has any experience with this material. More importantly will this material hold up or should I complete the tap and make one out of brass.

Sorry for the length of this essay but I wanted to explain my thought process so as to make it easier to point out any errors in my logic.

Thank you
Kenny

Harold_V
Posts: 16147
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Grizzly 9729 half nut

Postby Harold_V » Wed Feb 15, 2017 4:21 pm

I've not had any experience with making a Delrin half nut, although I do have extensive experience in machining Delrin, in general.
It is made in various combinations, some better suited to acting as a bearing than others. Delrin AF comes to mind, as it has Teflon included in its makeup, but I don't know if it would lend itself to heat forming. Never been down that road.

One thing troubles me a little. I get the impression that you've been using the half nuts for power feed. As I'm not familiar with your machine, perhaps that's the only option you have, but the fact that the lead screw has a slot indicates to me that is not the case. The slot generally drives the power feed mechanism, located in the carriage, and takes the load off the half nuts and screw, so they aren't worn unnecessarily. Armed with that thought, I would expect the apron of the carriage to have either knobs or levers, one or more, to activate power feed.

Welcome to the board.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

Torch
Posts: 1531
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Grizzly 9729 half nut

Postby Torch » Wed Feb 15, 2017 7:06 pm

kennycrawford wrote: I played with it a little bit last night but was wondering if anyone else has any experience with this material. More importantly will this material hold up or should I complete the tap and make one out of brass.


I've read the same method of making a delrin half-nut, and considered doing that myself one of these days, just to have a spare. That said, since I deburred the slot in my lead screw, I haven't damaged another brass half-nut so I haven't had need of it. Yet.

BTW: I believe the unique thread diameter is the Chinese solution for turning a machine dimensioned with a metric lead screw into a machine with an inch lead screw.

Harold_V wrote:One thing troubles me a little. I get the impression that you've been using the half nuts for power feed. As I'm not familiar with your machine, perhaps that's the only option you have, but the fact that the lead screw has a slot indicates to me that is not the case. The slot generally drives the power feed mechanism, located in the carriage, and takes the load off the half nuts and screw, so they aren't worn unnecessarily.


In this case, the slot is used to drive a gear that translates lead screw rotation into cross-feed. There is no separate feed screw on these machines. There is a really fine gear combination for feeding.

kennycrawford
Posts: 2
Joined: Wed Feb 08, 2017 3:11 pm

Re: Grizzly 9729 half nut

Postby kennycrawford » Thu Feb 16, 2017 12:31 am

Thank you Harold and Torch. I hope that deburring the lead screw will go a long way to solve the problem. I will keep the delrin (I am guessing acetal is another name) half nut and set my lathe up for cutting a thread when I get some time here in the near future so I can see how it well it works. I did cut some 1018 steel tonight with the fine feed gears (forget exactly how many inches per revolution) and everything worked better than new. I am ordering some more drill rod tomorrow or the next so I may do some research and order another chunk of acetal with it. Does anybody have any recommendations for what to order for my application? I had no idea that more than one type existed.

Thank you,

Kenny

Torch
Posts: 1531
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Grizzly 9729 half nut

Postby Torch » Thu Feb 16, 2017 5:47 am

Acetal and Delrin are very, very similar. Same chemical structure, different manufacturing process (IIRC). I don't think it makes a big difference in this application, but I might give acetal the nod here over Delrin.

Here's a short summary of the differences: http://www.lionep.com/uploads/files/Ace ... Delrin.pdf


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