I have the Warco version of the minimill and looked at all the various methods of overcoming this problem – gas struts, wall bracing (I think) and eventually went for the Phil Dawes idea in ME251 of using a counterweight.This works very well, but I discovered that whilst there does appear to be more travel than enough, it can't all be used because the rack isn't long enough, so much so that I have had to add stops to both top and bottom to limit the head travel.

In respect of upwards weight, my counterweight is indeed slightly heavier than the head. The main problem I have is that I've yet to find a way of limiting drop when the cutter takes charge and starts to pull down. I think the main problem I now have is that there is a large amount of backlash introduced into the system by the rack & pinion not being tight enough, plus the slop between the various parts on the slow up/down feed mechanism. I have tried packing out the rack to try and eliminate some of the slack, but in reality, I think it needs a better, or a different mechanism. I suppose using a much heavier counterbalance would go some way towards reducing the slack.

Peter G. Shaw

Weak struts avoid backlash.

In the worst case, a strut powerful enough to lift the head until the lead-screw engaged in the reverse direction would leave all the backlash free to be applied to the job. More bouncing and vibration and maybe a cutter will disengage entirely, requiring more operator effort to control it.

When a head doesn't have a strut at all, it's full weight has to be overcome before any looseness between screw and nut reveals a backlash problem. Good rigidity, but winding the head up and down is hard work. So there's advantage on counter-balancing the head up to a point, but overdoing it is likely to cause trouble.

How much trouble a big strut causes depends on how much slack exists in the lead-screw. A recently adjusted or new machine might have so little slack that an over-powerful strut seems harmless. Actually the problem emerges over-time as the nut and screw wear; poor results gradually getting worse until the cause becomes bleeding obvious. Beware internet videos where naive chaps report novel improvements to their machines based on 'it was great when I tested it that day' rather than 'still good after two years hard use'. Constantly tightening up an ordinary nut & leadscrew to remove backlash isn't a good idea because it causes excessive wear.

CNC machines can take advantage of full balancing because their design includes anti-backlash features such as ball screws. Expensive. In practice manual machines don't suffer too much from backlash because the cutting forces are normally supported by the lead-screw and nut. Consequently the cheaper lead-screw and nut arrangement is almost universal on manual machines.

Climb-milling is perhaps the most common operation where backlash bites, but generally it's best to minimise backlash and certainly not to encourage it.

That weedy strut is your friend!

Dave

Edited By SillyOldDuffer on 05/12/2020 11:58:36

Ed,

Yes, mine does. The problem for me is the actual setting of the cut to be taken which can be difficult if using the fine feed mechanism to set it. Another problem is that I'm not too sure just how secure the z-axis lock actually is. I suspect that vibration can cause it to move.

I have to say though, that age and illness have combined to reduce my workshop efforts so I don't do much milling at all. And when I have done it, I do sometimes resort to using the lathe & vertical slide as being easier to use and see.

Suspect the answer to the first paragraph might be some sort of separate measuring device for the z-axis.

Peter G. Shaw

recent thread on here where the Doc fitted too strong a strut and found that if a drill or cutter being plunged snatched then it would pull the head down into the work as the strut was holding it at the far end of any backlash so too strong is as bad as too weak. Maybe that's why the makers don't fit one?

Not had any problems winding my X3 head up and down for the last 13yrs without a strut, maybe get a bit more experience with the machine before making modifications

One of the problems on the SX2P is that there is no quill, the whole head has to move. I packed out the rack to eliminate as much backlash as I could without making it too tight. Also the gib has to be tightened to give as smooth a movement as possible to give some feedback to the feed hand wheel. It is still better to plunge a slot drill into a drilled hole to lessen the the snatch.

Was about to post here. I fitted the specification of strut recommended by a few folks online who’d done the conversion: 150N. This over-compensates for head weight, I assume allowing the head to be pulled into the job, (as you said). This may have been exacerbated by me not modifying cutting tools for spot facing or plunge milling, but who knows.

Only yesterday I ordered an adjustable strut. By trial and error I’ll bleed the strut until it bottoms on the rack, then continue bleeding pressure until it gives a hand lever load that’s acceptable. I suppose I’ll have to lock the head or leave fine feed engaged to stop it dropping under gravity. Then again, it doesn’t move up under the excess strut load, so maybe stiction will be on my side there.

So if anyone wants a pretty much brand new 150N strut for an SX2P, let me know, and you too can experience the horror of watching your work being randomly mangled.

having fitted a gas strutt a few years ago to my SX2P (published in MEW), I can not understand the problems people seem to be having. My strutt just ballances the head rather than trying to take all the weight, a lot of others have done the same so we can't all be wrong. from my experience I have had no problem with cutter being pulled into the work or excessive backlash.and find the mill a delight to use.

perhaps it is methodology rather than the gas strutt that is causing the problems

You're going through exactly the same issues and process I was going through. There can be a lot of misunderstanding when explaining what you're thinking, which can make the responses confusing:

https://www.model-engineer.co.uk/forums/postings.asp?th=168240

Just to be clear, here is the video taken from that thread (should work if you click on the image):

It's pretty obvious what's happening, and what will eventually happen when drilling or plunge milling. There is no issue when machining with the head locked. What some people don't appreciate is that you can't lock the head on an SX2P, and feed using the quill; it doesn't have a quill feed.

There are a lot of subtleties when thinking this through. I was going to fit the adjustabe strut, de-rate it until the head fell under gravity (to eliminate the pull-down issue), and adjust the z-gib so the head wouldn't drop on its own by default. Of course, that then defeats the whole object of de-rating the strut: Friction of the gib would then hold the head up against the rack, giving exactly the same issue you were trying to avoid…

Fitted the adjustable strut today, and on compressing it slightly it immediately began hissing and leaking fluid from around the ram. This stopped when returned to fully extended, so it seems like the piston itself is plugging a leak at the seal end. Clearly not fit for purpose, so I’m awaiting advice from the supplier. To be honest with the amount of excess pressure It’s got, I’m now a bit reluctant to go anywhere near the thing.

If it’s not one thing it’s another.