Fine feed carriage stop starts!!

Hello fellow members.

My Myford Super 7 (3/4" lead screw) has started to stop/start and hesitate as I go about fine travel machining leaving the job with grooving marks every 3/4mm along the process. Threading speeds I have no issues with?

I have had the apron off to inspect the half nut which after cleaning looks to be in a reasonable condition leaving me with no concerns. Likewise, the lead screw.

I am inclined to suspect that the half nut is the problem therefore I have serched the ususal outlets for preferably a new or pre-owned one without success.

1) Have any readers encountered this phenomenon themselves and have a satisfactory outcome?

2) Should I require a new half nut please point me in the right direction!!

Thanking you in anticipation.

Rgds.

Paul.

I’m assuming you can’t actually see the carriage stopping but only a regular banding along the length of your workpiece. I think the is not uncommon - from time to time I’ve seen it on work I have done. I think most likely reason is some harmonic frequency. I just try and reduce the visual effect by using a tool with a reasonable radius on the tip rather than a real point. Of course after making sure gibs are adjusted. I think it only shows on the finish rather than leaving ridges. Also worse with ‘finishing’ cuts.

Steve

Has the shear pin / woodruff key broken between the lead screw and the gearbox?

Banding usually as Steve says, but if the carriage is visibly hesitating I suggest looking for:

• Half-nut or lead-screw worn or very dirty (which Paul's check makes unlikely)
• Half-nut not engaging properly - an adjustment
• Slipping clutch (if fitted!), either worn out or needs adjustment
• Missing gear teeth (check all the gear wheels)
• Slipping belt (worn-out, age hardened, oil-contaminated, or maladjusted)

+1 to speedy's broken sheer pin. It happened to me, most confusing!

Dave

Edited By SillyOldDuffer on 09/10/2022 08:39:35

Hi to everyone.

Thanks for your thoughts so far! To clarify, the situation is this. There isn't a gearbox on the machine, the tips are tungsten carbide the gibs are as near as dammit spot on. The carriage actually stops momentarily/hesitates
to produce the ridged finish.
The carriage will perform this activity without any work in the chuck, idling down the bed is just as bad.
If I hold onto the apron hand wheel as it is turning creating back pressure, the carriage acts normally although I sense at times a feeling of a hint of the malady is present.
To reiterate. The lead screw is in good condition and is in constant motion whilst all this is happening, yet the carriage is not following orders!
As mentioned in my original post, after spending quite a considerable amount of time scrolling through the net, Super 7 half nuts seem to be sharing the same stable alongside rocking horses' doo doo!
Does anyone have a comment on these elusive items please?
Rgds.
Paul.

Try cleaning the carriage rack and the pinion on the end of the hand wheel.

Additionally when trying to diagnose a problem on an imperial lathe this are more obvious when measuring in the same system of units. You say 3 to 4 mm. Is it actually 1/8 inch which is the pitch of the leadscrew, so you have an immidiate clue.

regards Martin

If the carriage is actually stopping as you say if only momentarily then there must be a lapse in the drive momentarily.

You haven't mentioned it but have you checked the gear train for free movement - might be something as simple as a piece of swarf stuck in a tooth gap or a gear sticking on a drive bush giving a slight lag as the key moves in it's slot

I've seen banding before, usually a result as described by Steve and when taking very fine cuts however I don't recall it ever happening on my Super 7 which I've had since new

Just a thought that may help

Tug

Many thanks for your thoughts, folks.

I think that Martin has taken the clue vis the 3/4mm banding spaces as a source for deeper investigation hence my search for half nuts.

RDG/Myford are my first port for a phone call albeit not with much confidence of a positive immediate outcome as they aren't showing Super 7 half nuts on thier site. Considering half nuts are a fundamental must have component on any lathe, surely they should be a constant item in thier catalogue?

We'll see in the morning!

In the mean time I'll check out all of your advice found on this post and keep you informed as to how things progress.

Best rgds.

Paul.

I think you should think a little deeper into this. There are 2 ways of altering the travel the first and obvious is intermittent drivewhich led you to look at the half nuts. However if there is an intermittent frictional force this can generate the same symptoms as you are seeing. You say that when you apply a retarding force to the saddle handwheel the problem dissapears. If something else is causing a periodic friction then swamping that friction by pressure on the handwheel would similarly swamp the variation in the drive. There is always a little slop in the leadscrew arrangement (oil, excessive clearence on the half nuts, slop in the bearing at either end of the leadscrew etc) so a periodic tight spot caused by muck in the rack and pinion on the saddle travel which causes the lead screw to hunt a little and create bands. A milder form is exibited when the leadscrew and half nuts are not dead concentric so engagement causes a slight bending of the leadscrew.

As I say, whip the saddle apron off and take a look at the handwheel pinion. What have you got to loose.

regards Martin

I don't have a modern Myford so not sure of the drive - is it by the rack or a very slow screw thread? Is the leadscrew imperial and the marking actually every 1/8 inch ? Or does it correspond to the pitch of the rack?
If the latter then it firstly points to a bit of swarf in the drive gear.
If the former and it uses a slot in the leadscrew to drive the rack gear (as on Boxfords) the sliding key inside the worm may have a burr or swarf catching on the edge of each pitch of the leadscrew.

On my Boxford I once found the weight of the sticking out handle part of the carriage handwheel as it went over top dead centre caused it to take up the gear slack and make a mark if everything was very loose.

This is a non gearbox Myford and as such I'm assuming non power cross feed. The leadscrew slot only appears on the power cross feed version. Auto feed is by the leadscrew and hand feed is by the handwheel rack and pinion. When auto feeding the leadscrew drives the handwheel pinion along the rack (permenant engagement). Hence my notion that swarf build up on the pinion can cause a periodic tight spot. I have had one occasion where my lathe picked up some swaff between the rack and the pinion sufficient to lock the travel. The swarf was stuck to the rack and therefor only caused a problem at one point down the bed. Build up in the bottom of the pinion teeth would cause a periodic issue.

regards Martin

Seems like everything has been suggested, but I would check if the leadscrew rotates smoothly by itself without any end float and shows no sign of being bent. You could try different leadscrew gears if you have any and see if the pitch of the banding changes. Suspicions of the pitch of the leadscrew matching the banding have already been posted.

Keep us up to date with results as you go, the information is valuble.

Eureka! How's my Greek? I have found the cause of the Stop go, carriage issue.

Of course, I had nothing to lose in following Martins sound advice and whipping my apron off and revealing my inner parts as they say.

After washing off the rack and its mating parts reassembling and oiling nothing changed! Stop go had not been eliminated. Oh Dear!!!

There is more!

Whilst my rubber gloves where mucky and close to my wits end, I took another look at the gib setting on the half nut dovetails, adjusted the two side screws hardish up, to prevent movement then backed them off until the half nuts moved with a tad bit of friction, then locked up the two Allen cap screws again plus the two side screw lock nuts.

Guess what?

No more stop starts.

Just goes to show how promptings on the forum can nudge one into reassessing what one has already "sorted" previously to then get it sorted correctly by revisiting with "nothing to lose"

I can now get back to making some more nuts and bolts for my '72 Commando, otherwise known as the "Black Hole".

Keep up the good advice my fellow twiddlers and many thanks to you all.

Best rgds.

Paul.

Whilst I agree with Martins assessment of dirt/swarf in the handwheel pinion causing a tight spot (that does happen on occasion) it would be unlikely to create such a 'banding' every 3-4mm. Swarf there would, and is, felt every time the saddle is moved by the handwheel too.

Paul,

your initial post states that your lathe has 'started ' to "stop, start and hesitate" - are you saying this is new compared to previous use?

You also sate that the leadscrew nut is clean and appears to be okay. If this is the case then the issue is elsewhere. A new nut will not necessarily cure the problem.

As I said previously if the carriage stops moving then there has to be delay in drive transmission somewhere - when in drive any backlash is constantly taken up so with several leadscrew pitches in contact with the clasp nut it is unlikely to be between the two. It is more likely to be something that is impeding the leadscrew drive allowing the tool to dwell in as the saddlle stops.

If you can't feel the banding nor measure it the it's probably the oft seen 'banding' as a result of fine cut and feed that sometimes occurs. It's usually a direct copy of the leadscrew pitch though.

EDIT - Well done, new one on me but one well worth remembering. Keep on turning

Edited By Ramon Wilson on 09/10/2022 16:33:36

Well done. Just goes to show how most things can be sorted by cleaning and adjusting.

Electronics/Computing Press buttons until something happens.

Mechanical devices Clean lubricate and adjust until something happens.

regards Martin

Many thanks to you both with this one. Although the stopping etc has ceased I am at a loss as why considering the fineness of the adjustments made.

It is just as well this issue showed up during the weekend otherwise, I most probably would have shelled out for a new half nut if the suppliers were open!

Most if not all my spare spondulix have been set asside for the "Black Hole" mentioned on here before.

Now I can concentrate on producing swarf of many colours and shapes from various metals until the next hiccup happens! Watch this space!

Most times a thing of beauty emerges from my doings much to my amazement and utter joy.

Best regards to all.

Paul.

Good result, it's nice to hear of success.