Lathe tool geometry for threading aluminium

I'm making a couple of exhaust adapters for a 20cc four stroke engine using aluminium block but I'm having problems forming the threads. As the inlet and outlet of the adapter both have m14 X 0.75 threads I really have no alternative but to form the threads on the lathe as dies in this size are ruddy expensive. Although I have a Myford ML7 I'm not a experienced machinist and don't use it regularly. When I tried cutting the thread the aluminium would not cut cleanly, and the thread was very rough on the surface. I used the slowest possible speed, the tool was set at centre height, freshly sharpened, and multiple shallow cuts made. Any advice on where im going wrong? Any advice on the best setup to thread aluminium would be welcome!

Could it be rough because you are cutting multiple threads ?

This will happen if you disengage the half nuts and re-engage again. You have to leave them closed and wind back either under power or by hand with the tool retracted.

Sorry if I'm telling you something you know and are doing but it's often a valid point

As mentioned above, crank the headstock by hand and don't disengage the lead nut

(Aluminium is a doddle)

GL

Edited By Ady1 on 02/01/2016 00:24:37

I am only a beginner myself but have threaded 6061 and 6082 aluminium with good results.

Did you grind a flat into the front of your HSS 60 degree threading tool? It needs to be the exact width of that in the fish tail tool you use for checking angle and getting the tool squared up to the work. If the point is knife sharp then your tool may be digging in creating a ripping/digging in affect in your work?

I found this channel on Youtube (Toms Techniques) both educational and informative.  Worth watching Parts 1, 2 & 3 of this series as well where Tom explains grinding a 60 degree threading tool:

https://youtu.be/f2y36EGHpts

Also worth checking that your topslide has no play in it and you don't have excessive amounts of backlash in your leadscrews.

Like others, I use paraffin for lube.

Some more experienced members will be along shortly to offer further advice.

Good luck.

G

Edited By Grant Nicholas on 02/01/2016 00:41:18

You don't say anything about cutting fluid, it always pays with aluminium to use a cutting agent such as parrafin, WD40 or something similar.

You haven't said which grade aluminium you are using, some of the cast variety are hopeless to form threads on or even get a decent finish when turning. I thread 6082T6 regularly and the finish is good, best if you can use a full form insert type tool.

Emgee

On a fine thread like this i would check the tool angles with a magnifying glass as its easy to get the side clearances a bit vertical ie no or not enough clearance and have 10 to 15 degrees of top rake for aluminium , as said above 6061 (HE15) and 6082 (HE30) type alloys should cut nicely but some of the softer stuff can be a pig to get a finish on at times , especially if you are limited to using low speeds.

You didn't mention whether you have the topslide set over or are plunge cutting. The reason for set over is so you are only cutting on one face of the tool so can apply the high top rake as mentioned above.

You don't say what grade of Aluminium Alloy you're using. I've had alloy that was so gummy almost no amount of lubrication or tool care would yield a decent finish.

With an ML7 you can't rotate the topslide far enough to cut down the flank. What you have to do is leave the topslide parallel to the spindle axis, but apply half the feed you've applied to the cross slide to the topslide, so if you advance the cross slide 10 thou, advance the topslide 5 thou. If you don't do this the tool is cutting on both sides, and the swarf from each side collides and just makes a raggy mess. You can also put some rake on the tool as you're only cutting on one face. Then use WD40 as a lubricant.

Having given some thought to what I was doing wrong I realized that John Stevenson was at least partly right, I was disengaging the half nuts and winding back, but I was using the thread dial indicator on the carriage as I had done when I last cut some imperial threads. Being a raw beginner I had assumed this would work with metric threads, which it clearly doesn't. Winding back by hand was very hard work as the back gears were engaged and I didn't have a spindle driving handle. So, I've spent the weekend making a handle from my own design and it works a treat. The design is my own with ideas from photos I found trawling the net and it gave me the opportunity to use the knurling tool and tailstock die holder I was given for Christmas. This should make life a good deal easier!

As yet I haven't attempted to redo the exhaust adapter as I'm still considering the best angle to make this as I want to keep the silencer completely within the cowl of the aircraft and as I now have my lathe sorted I may even make a complete new silencer. For anyone who is interested the aircraft is a 71 inch span Mitsubishi Zero powered by an ASP 120 four stroke, it's a big b***er, should weigh around 12 pounds ready to fly

As I've said, I'm a complete beginner having bought the lathe some time ago to make parts for rc aircraft and it's taken ages to get to this point and I'm learning from that very hard teacher, trial and error, mixed in with advice from forums and books. Great fun though! The lathe was an EBay purchase, but I had the opportunity to inspect before buying as it was just a few miles away, and £300 for a decent ML7 with loads of accessories and tools struck me as a good deal. My biggest problem I sorted by myself; after I'd had the lathe a couple of months it started to chatter when cutting and this got progressively worse until I was despairing of ever being able to use it. I checked the bearings (plain ones), negligible play, carriage, cross and top slides were as rock solid as could be, lathe well bolted down and level, no significant vibration from motor, etc, etc. Having left it alone for a couple of months I went back to it last week and whilst checking the spindle I noticed that the locking collar on the end of the spindle was loose, giving about 20 thou of end float. Nipped it up to give about 0.5 thou float and lo and behold the chattering has gone completely. Not something I'm likely to miss again!

Many thanks for all the replies, if I still have problems when I redo the adapter I'll be sure to use some of the suggestions.

Post a picture of the tool you are using. It is much easier for people like myself to be able to give you constructive feedback on the changes that will be required to it geometry or the way it is made. Sometimes very small geometry changes in a tool will have a big impact on it's effectiveness which is not always obvious especially to beginners or people who seldom ever make that style of tooling.

Neil

You can disengage the back gear and wind the mandrel back by hand. The back gear is not involved in the gear train from the end of the mandrel to the leadscrew. Should make life a lot easier, even with your very nicely made handle.

Edited By Hopper on 04/01/2016 08:57:41

No but it is involved with the position of the chuck.

With back gear in neutral you could rotate the chuck say 1/2 a turn and then engage back gear which would stuff your threads up

Edited By John Stevenson on 04/01/2016 09:04:06

I'm using the back gear to keep the speed low, would it be better just to cut the thread by turning the spindle using the handle? It's only a short length of thread, about 3/4 inch long.

.

Yes

MichaelG.

How about getting the threads nearly there and cleaning up with a chaser they are a bit cheaper than a die. You could use a M4.5 tap at a pinch. WD40 is not bad as a lubricant for something like this.

regards Martin

What kind of lathe are we talking about here? Surely most are like the Myford where the lathe mandrel(aka spindle) is one solid piece with the chuck firmly attached on one end and the first gear in the screwcutting chain firmly attached to the other?

Thus rotating the chuck half a turn also rotates the mandrel gear on the other end by the same amount, and the screw cutting gear train is intact, independent of the back gear, and so the leadscrew moves the correct amount and all is still in alignment?

Turning the chuck with with back gear disengaged affects only the relationship between the drive pulley and the bullgear, which has no bearing on the screwcutting gear train.

Or are other lathes different to the Myford set up?

I agree with Hopper, the backgear is only going to drive the motor train if it is engaged for hand powered threading on my lathe. I would disengage the backgear and leave the drive pulley on the spindle unlocked so the motor was not able to drive the spindle, much safer when a crank handle is sticking out of the back of the spindle and no effort going into manually turning the motor.

Martin

I agree with Martin and Hopper,the chuck is either screwed to the spindle,or keyed and so is the gear at the other end of the spindle driving the screwcutting train whatever method is used to power the spindle (gears,pulleys,back gear) it is entirely independent from the gear cutting train. I have intended a number of times to make a mandrel handle for the myford but usually get over the problem though my thoughts on the subject would be a complete circular disc with a folding handle to save getting an accidental clout. Turning a lathe over by hand was a lot easier in the days when lathes were driven by flat belts from overhead pulleys,it was a case of grab the belt (when it was not under power ) and pull the lathe round by hand particularly when chasing threads up to a shoulder.Metric screwcutting on an imperial lathe can be a right pain,if one needs to do a lot of metric/imperial screwcutting either buy two lathes or get a lathe with a forward and reverse clutch,and remember to check the leadscrew dial on an all metric lathe as some of the dials have a range of pick off off gears to mesh with leadscrew.

Grant Nicholas:

I have just been reading your post of 2 January.

"Did you grind a flat into the front of your HSS 60 degree threading tool? It needs to be the exact width of that in the fish tail tool you use for checking angle and getting the tool squared up to the work"

I don't think anyone else commented on this advice which I feel needs some qualification.

I assume that by "fish tail tool" you mean a screw-cutting angle gauge eg the M&W No 200. (I have a Chesterman gauge of this kind which does indeed look a bit like the head and tail of a fish). To begin with, these tools don't claim to give the width of the flat (which varies with the pitch where the root is flat) and what you call the flat is, I think, a relieving slot designed to facilitate the manufacture of the gauge; in a good one the gauging surfaces will be ground.

Secondly, whilst I agree that a sharp point is undesirable, it would be impractical to have a separate tool for every pitch and, in practice, I think most single-point tools will have an arbitrary flat or radius which needs to be less than or equal to the specified width or radius.

The OP appears to be cutting an external ISO thread which theoretically should have a small radius at the root and a flat at the tip; details are given eg in the well-known Zeus data booklet. However, most amateurs will not have the equipment to produce a precisely formed tool and will simply cut their bolts to fit their nuts or vice versa.

As an entirely separate point and as one who knows nothing about motor bikes, I did wonder about aluminium as the choice of material for an exhaust adapter.