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SX2P Gas Strut Modification Issue?

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Dr_GMJN03/10/2020 20:07:33
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1208 forum posts

All,

Today I was trying to spot-face part of my Stuart 10V cylinder casting with a 6.5mm slot drill. I was using the fine feed wheel. The slot drill grabbed, and damaged the casting. Luckily it's not an issue this time, since it's invisible and I was eventaually able to complete the process successfully. This is not the first time it's happened when drilling or using a mill which requires z-feed, and one day I will end up scrapping a part becasue of it.

I have a theory that the gas strut conversion I made to the mill is causing this issue: I think the strut is slightly over-compensating for the weight of the head. This means that when I'm using the z-feed, all the (considerable) backlash in the rack is pretty much uncostrained, and allows the haed to be pulled down by any slight grab of whatever tool I'm using (drill, slot drill etc).

I think the head is almost balanced, giving no significant upward or downward residual load on the rack. Effectively I've ended up with a free-floating z-axis. I did try an experiment by moving the head downwards, letting go of the wheel (or lever) and then manually pushing down on the head. Every time, it moved down by what seemed to be the backlash amount with a thump.

Is it likely that this is what is causing the issues I'm having whenever I'm doing a process where I can't lock the z-axis?

If so, can anyone suggest the remedy (without buying another strut!).

Thanks!

HOWARDT03/10/2020 20:39:37
776 forum posts
28 photos

I have a gas strut assembly I built into the column, it over compensates and I have no trouble but I nip up the gib to stop the head from moving. Photo in my album. If the head moves too freely then it is likely to snatch.

Dr_GMJN04/10/2020 00:15:35
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1208 forum posts
Posted by HOWARDT on 03/10/2020 20:39:37:

I have a gas strut assembly I built into the column, it over compensates and I have no trouble but I nip up the gib to stop the head from moving. Photo in my album. If the head moves too freely then it is likely to snatch.

Thanks. If I had some residual weight on the rack and pinion, the worm drive fine feed should pretty much eliminate any downward free play. Trouble is I’ve currently got the opposite, which I think is making life very difficult for any work involving z-feeds.

Dr_GMJN04/10/2020 00:47:32
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1208 forum posts

I wonder if I could temporarily load the feed handles downwards to eliminate the lash, then use the fine feed?

oldvelo04/10/2020 05:16:31
274 forum posts
54 photos

Hi

The gas strut needs enough power to push the head up to the top with with the fine feed disengaged and the lock slackened off.

Fit "Air Spring" to counter weight of mill head 24 kg rated is the one I have fitted to a much modified X2 mill.

z-axis 15.jpg

If the castings you are spot facing is Çast Iron, Bronze, or Brass you need an end mill with the cutting edges backed off to 90deg to prevent the end mill from diving into the castings. The piddly little hand wheel is a pain to use the photo shows the mods done on X2 Mill Drill

Eric

Ron Laden04/10/2020 08:12:53
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2236 forum posts
443 photos

What rating is your gas strut, mine is 120N and I have never had the problem you are seeing and I use the fine feed all the time.

I assume your head gib is well adjusted, my SX2 has the usual fine feed backlash which I keep meaning to modify but once the backlash is taken up its never been an issue.

Cabinet Enforcer04/10/2020 09:20:29
106 forum posts
3 photos

Test your theory by strapping some extra weight to the head?

Clive Foster04/10/2020 10:30:40
2815 forum posts
101 photos

Could the rack be shimmed out a bit to give nominal zero backlash against the drive pinion? Needs the pinion and rack to have been cut deep enough to prevent tooth tip interference tho'.

An (expensive) alternative would be to find a skewed rack and pinion set as used in microscope focus mechanisms which can operate with very close fits and always have proper pinion tooth to rack engagement as engagement / disengagement points for adjacent pinion teeth have significant overlap. Maybe its practical to make a set as a project.

Clive

SillyOldDuffer04/10/2020 10:58:44
Moderator
7476 forum posts
1648 photos

Posted by Dr_GMJN on 03/10/2020 20:07:33:

...

If so, can anyone suggest the remedy (without buying another strut!).

Thanks!

Not easily.

I think you've correctly identified the problem. Due to the strut removing most of the force needed to lift the head against gravity, backlash emerges as an issue. A weaker strut would make backlash less likely whilst making it more work to wind the head up. Fitting one is probably the easiest fix.

More complicated would be to fix the backlash problem. It may be the mill has a crude anti-backlash adjuster like that fitted on my lathe's cross-slide, or even something smarter. I suspect it doesn't have anything, in which case fitting a compensator or replacing the lead-screw with a CNC-style ball-screw would do the job. Or remove the strut and motorise the head.

All harder to do than fitting a weaker strut!

Dave

Ron Laden04/10/2020 12:34:05
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2236 forum posts
443 photos

Just a thought and probably unlikely but could be worth checking that the 3 allen csk screws which fix the rack to the column are tight, at least worth a check.

Roger Best04/10/2020 13:03:19
289 forum posts
31 photos

frown Put a DTI on the head and touch against the bed or visa versa and pull down on the head hard - what happens.?

Its worth noting that you can get adjustable gas struts, if you don't know the weight of the head for sure you could fit one, and bleed it down until the backlash disappears.

All of this is all very well but it does rely on all your operations having the same direction of force and clearly drilling is the issue here.

Jeff Dayman04/10/2020 15:06:38
2165 forum posts
45 photos

Could a second rack be bought, and both reduced in width to half, then mounted one ridgidly the other free to move a tooth width with a heavy die spring pushing on it to take up backlash at the pinion?

Was the slot drill that grabbed and walked out of the OP's machine mounted in a tight collet or was it in a drill chuck?

Dr_GMJN04/10/2020 20:46:55
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1208 forum posts

Thanks all.

To answer a few questions:

* The gas strut is rated at 150N, this is the same one used by Andrew Whale on his SX2P in his “Learning Turning” series on YouTube. Obviously the vertical component of that changes slightly with the position of the head.

* There’s no point lowering the head and using a DTI to see if there’s any movement - as I mentioned: I can see and hear the movement - it’s basically all of the backlash.

* On the last occasion the slot drill was in a drill chuck, but it’s happened plenty of times before when using a collet chuck.

* I’m not using modified tools, they are standard mills, drills or slot drills. I wouldn’t know how to modify them anyway.

Having considered all the options, a lower rated, or adjustable strut would be the best way forward. Adding weight to the head seems like the wrong way to solve the issue.

As it stands, it’s just wrong to have the head forced up by default. It must be detrimental to accurate work - if you have a hand feed lever, you intuitively assume it’s limiting the tool position downwards. Currently, it’s basically moving the tool to its highest limit - all bets are off in terms of where it could end up in the workpiece. I’m surprised nobody else has picked up on this potential issue. Perhaps I’ve just got the wrong strut? I wonder if I could use some scales and a bottle jack to get the actual vertical force required to balance the head, resolve it for the angle and knock 5kg off?

not done it yet04/10/2020 22:01:37
6273 forum posts
20 photos

Adding weight to the head seems like the wrong way to solve the issue.

Cabinet Enforcer was not offering that option as a solution to your problem - only as a means of testing your theory. Little point in changing the strut if it is not the solution.

Michael Gilligan04/10/2020 23:07:54
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18710 forum posts
915 photos
Posted by not done it yet on 04/10/2020 22:01:37:

Adding weight to the head seems like the wrong way to solve the issue.

Cabinet Enforcer was not offering that option as a solution to your problem - only as a means of testing your theory. Little point in changing the strut if it is not the solution.

.

Agreed yes

MichaelG.

Emgee04/10/2020 23:59:09
2147 forum posts
265 photos

Dr-GMJN

Ron has stated the gas strut he uses is rated at 120N but yours is rated 150N, could that be the answer to your problem ?

Emgee

oldvelo05/10/2020 00:42:34
274 forum posts
54 photos

Hi

A lot of solutions quoted until you address this the problem will continue

( I’m not using modified tools, they are standard mills, drills or slot drills. I wouldn’t know how to modify them anyway)

Higher rated Air spring will give higher resistance to the cutter being dragged into the softer metals by the cutter edge angle and maintain pressure taking out any backlash.

The cutter can be modified with a Dremel grinder and back off the cutting edge to parallel to the Centre Line.

Eric

Roy Vaughn05/10/2020 00:59:30
53 forum posts
4 photos

This is the latest manifestation of the Z-axis backlash problem with the old X2. An Internet search will bring up an extensive literature. The main problem is plunge cutting when cutter pull-in is a perpetual threat.

Roy

oldvelo05/10/2020 02:50:29
274 forum posts
54 photos

Hi

"As it stands, it’s just wrong to have the head forced up by default. It must be detrimental to accurate work - if you have a hand feed lever, you intuitively assume it’s limiting the tool position downwards. Currently, it’s basically moving the tool to its highest limit - all bets are off in terms of where it could end up in the workpiece".

"intuitively assume" are not terms consistent with precission engineering. To hold the head from being pulled into the job and elliminate the effect of the backlash is with a higher spring pressure to have the head held firmly UP at all times.

1 ) Backlash in an X2 is the clearance in the Rack and Pinion

2 ) The clearance in the Worm and Worm Wheel

3 ) End float on the worm to housing

4 ) Clearance on the Teeth on the Dog Clutch

5 ) Clearance on the Key and Keyway on the Pinion Shaft in The Dog Clutch

6 ) Two Universal Joints on the Worm Shaft Drive

All adds up to an excessive ammount of backlash. Around 3/4 of a full turn on my reasonably adjusted machine.

Eric

Roy Vaughn05/10/2020 09:09:02
53 forum posts
4 photos

After shimming the rack on my X2 I found that most of the remaining backlash was in the end float in the worm shaft. I fitted an adjusting screw at the back of the worm shaft housing to take up the free play and it has made a considerable difference, the free movement on the fine feed wheel is down to 10 degrees or so. It is still necessary to tighten or lock the head to eliminate the the possibility of a pull-in. I also have the head biased up.

Roy

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