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Lathe Rigidity Issues - Modification Opinions

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Ed Page18/03/2021 05:03:39
13 forum posts
32 photos

Hi,

I'm no stranger to the forum, although not logged in for such a long time it seems I needed a new account.

I live in Canada which seems to have no market for used small lathes like the UK did, and I didn't have the room for a large lathe, so I had to purchase one new and convert it to CNC. I am a machinist by trade so the lathe is a little under-whelming. I also have a 4 axis CNC mill to make some bits on if I have to. I no longer have access to any other machining tools apart from this lathe and mill.

This lathe is the Grizzly 0602 equivalent, a pretty generic lathe design with lots of different names to it. I first removed the compound and replaced it with a new holder, made from 4140 to hold 1 inch standard lathe tools, also to make it rigid as possible. I converted it to CNC, it is running some high torque servos, runs at up to 10m/m, so it's fast. However it is not very capable in terms of rigidity. I have been making some pistons out of 1018 steel which I managed to turn at 0.4mm/rev and 10 thou DOC, that was the limit of the motor, but also shallower cuts caused vibrations. I could only hit 3 thou before the CNC conversion, it now maintains half a thou. It finally got to cutting the grooves in the pistons to find all it would do is chatter, in the end I used my 4th axis on the mill to do the grooves, but that was quite time consuming. The piston is 5 inch in diameter.

I have tightened the gib on the X-axis as tight as it should be, there is no adjustment on the saddle, the weight of the servo actually lifts it off the back ever so slightly. There is a small strip of steel bolted at the back underneath the saddle to keep it from lifting, without that the saddle would literally fall of the front. I know there are lots of solutions but I would like to know your guys opinions. Here are a few of my ideas in order of complexity.

1. Bolt a 20lb slab of lead on top of the X slide, that should dampen resonance and the weight should take out any slack. However not sure that will take slack out of the X axis, and it may put a lot more wear on the bed.

2. Mill the saddle flat, hopefully its not hardened. Bolt some linear rails on, make a new X- slide out of a chunky piece of steel. I never need to turn any more that 6 inches, and I do have a 2 x 6 x 12 piece of flat bar left over.

3. Make a completely new saddle and x-slide. Line the saddle with either plastic or bronze against the bed surface, make it very heavy, 100lbs or more total.

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Ady118/03/2021 07:59:38
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6137 forum posts
893 photos

The piston is 5 inch in diameter

lol

Its never going to be easy to do a huge lump like that on a hobby lathe

No real stiffness so high speed and sharp carbide

The real solution if you're doing a lot of work that size is a bigger lathe

Dave S18/03/2021 08:19:44
433 forum posts
95 photos

My first thought looking at the photos is that your grooving tool has a lot of stickout.

If your saddle isn’t in contact with the bed properly I suspect there is a

potential for “bounce” for want of a better description - the saddle dips as the cut comes on, then as the chip peels it lifts, changing the tool angle, which causes it to dip again....

Can you add ball bearing gibs like on a 10ee to the back of the saddle?
Dave

JasonB18/03/2021 08:26:43
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25215 forum posts
3105 photos
1 articles

You could also try a narrower tool. You say you could only manage 10thou DOC when turning so as you are now using a grooving tool that is maybe 100thou width of cut that is putting a lot more load on the light hobby machine. Something like a 1mm wide carbide insert of the "Minithin" style would be how I would do it on a manual machine at those diameters.

Servo at the back may balance out the apron at the front.

Some people also change the plate that held the topslide to a rectangular one with a fixing in each corner and say it helps with rigidity, looks like you have retained the original plate.

Edited By JasonB on 18/03/2021 08:29:09

Nick Wheeler18/03/2021 08:55:52
1227 forum posts
101 photos

Posted by JasonB on 18/03/2021 08:26:43:

Some people also change the plate that held the topslide to a rectangular one with a fixing in each corner and say it helps with rigidity, looks like you have retained the original plate.

I could see the original plate on WM250 bending when parting off or making heavy cuts. I replaced it with a 15mm thick plate, using all of the original topslide fittings and four M6 t-nuts. The original plate needed to be done up really tightly for any clamping effect. The new one just needs the four bolts to be nipped up with a 10mm spanner for much better clamping.

The new plate was hacked out a larger chunk with an angle grinder, the cut edges filed smooth, and the mounting/clamping faces machined in the lathe. I keep promising myself I'll paint it, but that's unlikely to happen until I paint something yellow....

I can't imagine using 1" tools on such a small machine, and definitely wouldn't want to pay for them. Most of mine are 8mm shanks, used in a Myford size Dickson QCTP, and they're not the limiting factor.

Speedy Builder518/03/2021 09:02:35
2878 forum posts
248 photos

A bit of a job now, but put the stepper motor on the rear of the saddle ??

Oops - just saw Jasons comment.

Edited By Speedy Builder5 on 18/03/2021 09:03:05

Andrew Johnston18/03/2021 09:02:43
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7061 forum posts
719 photos

Insert blades have a propensity to chatter unless the feed per rev is above a minimum value. I use inserts for parting off with a minimum feedrate of 4 thou per rev. But for grooving I grind a HSS tool bit which will give a better finish and no chatter even when sneaking up on a dimension.

Andrew

Hopper18/03/2021 09:08:36
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7881 forum posts
397 photos

If I was doing that job in my Myford -- which looks to be a similar sized lathe -- I would be using back gear to spin that 5" piston at less than 100rpm somewhere. Will your Chinese lathe run that slow without losing torque?

And I'd reduce the stick-out on that grooving tool to the absolute minimum, like the groove depth plus 1mm. The round toolpost has the disadvantage of curving back the wrong way right where you need max support on the cutting edge side of the tool unfortunately. A grooving tool with the cutting edge on the right-hand side of the tool would be better supported.

Fort the wider grooves in the final pic, I might use a regular left-hand and right-hand turning tools to make that. Cuts nicer than a grooving tool when moving it from left to right to widen the groove. Just have to use the two different turning tools to get the square shoulder at each end.

And I find that in rigidity-challenged situations such as grooving on these small lathes, a razor sharp HSS tool can chatter less than carbide.

Tony Pratt 118/03/2021 09:11:25
2319 forum posts
13 photos

Worth trying a shorter grooving tool, easy & cheap. I found on my Myford super 7 the revs had to be way down low to stop chatter like this.

Tony

Niels Abildgaard18/03/2021 10:46:52
470 forum posts
177 photos

The lathe is a 26mm bore 250 size like the much loved one I had before my son liberated it.He will get a surprice when estate is parted.

First thing is that it shal have a solid foundation.

wp_20190418_003[1].jpg

The old Emco toolholder system is idiotic and a momemade came onTool System

Standard EMCO copy

Niels Abildgaard18/03/2021 10:50:28
470 forum posts
177 photos

For parting on my Boxford I made a very stiff system and for my 250 with solid flange another.

The ME photo insert system is a disater waiting to happen for us unpaid writers

wp_20200607_001[1].jpg

 

stepstik 1.jpg

Edited By Niels Abildgaard on 18/03/2021 10:52:13

Dave Halford18/03/2021 11:33:28
2536 forum posts
24 photos

Firstly skim the plate seating (not the saddle) on the back of the saddle so you have minimal clearance, that will stop the saddle lifting and dropping and lifting ------- in time with you chatter. This is due to the front being forced down by the cut which can't move and so the back is lifted instead, which raises your tool height, and stops cutting.

If you throw excess weight at the saddle the lathe bed will begin to bend.

I would make the tool tower square. If you look at the tool / tower contact patch that you have it's also quite small.

 

Edited By Dave Halford on 18/03/2021 11:34:16

Tony Pratt 118/03/2021 11:49:26
2319 forum posts
13 photos

If people are looking for 'stiffness' in their tooling systems why do some many have a tendency to use round tool towers etc when square would be so much better?

Tony

JasonB18/03/2021 13:09:26
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25215 forum posts
3105 photos
1 articles

This guy has a nice solid replacement for the top slide

mgnbuk18/03/2021 13:45:29
1394 forum posts
103 photos

The undersides of the bed front and rear are bearing sufaces for the keep strips that constrain the saddle vertically. These faces should be machined parallel to the top bed formation to a similar standard of finish. The keep strips can be solid blocks, ground or scraped to be in running contact with the bed when bolted up tight, or a bolted on piece designed to hold a tapered gib strip for easier adjustability. The saddle should not be able to lift regardless of how the load is applied - adding weight to the saddle to keep it in contact with the bed is not the right way to address the lift issue. IMO correctly adjusted saddle keep strips are an essential part of the machine.

Changing to linear bearings would probably make the situation worse rather than better - they make for very "lively" axes where what you want is better damping.

The bed design on these doesn't seem too good if it is similar to the model Nils shows - in one of his other posts Nils showed one of these machines stripped down & I was struck by how poorly the headstock is supported due to the amount of metal removed underneath it to make space for the motor. Most Western lathes continue the bed at it's full form (or beef it up) under the headstock to properly support it.

If I was to add weight it would be in the form of a substantial flywheel mounted at the rear of the spindle.

Tool overhang, cut width, toolpost footprint, direct drive variable speed spindle drive shortcomings & the requirement for a solid foundation have already been mentioned above. I would also look at adding non-drive end support bearings to the ballscrews & really the Z axis arrangement is "the wrong way round" - the motor & end support bearing bracket would normally be positioned where the feed gearbox is, so that the ballscrew is in tension when under normal cutting loads & so not subject to buckling, with a support bearing at the tailstock end. On a screw of that length, I would have a bearing arrangement that allowed the screw to be tensioned rather than just a single deep groove ball race support. Hiwin Ballscrews application notes give details of how ballscrew performance is affected by support bearing arrangements - a copy can be found as a .pdf on line.

If it is any consolation, even large industrial lathes can struggle with grooving. At my last employment we sold a straight bed Boehringer CNC lathe to an aerospace company & I was sent over to investigate when they complained about chatter when grooving. The machine was (IIRC) 800mm swing x 4 meters between centres, 22Kw drive with 4 gear ranges & probably weighed in at around 16 tonnes, but it chattered when the customer attempted to use an 8mm wide grooving tool. Tried all sorts of mechanical adjustments, control and drive settings but to no avail. Cutting the groove with a 4mm wide tool was chatter free, but the customer was adamant that the part had to be made with the 8mm item & the machine, in the end, just could not do it.

Nigel B.

Michael Gilligan18/03/2021 14:36:21
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23121 forum posts
1360 photos
Posted by JasonB on 18/03/2021 13:09:26:

This guy has a nice solid replacement for the top slide

[…]

.

yes __ I would probably even be tempted to tap the LIKE Button if we had one !

MichaelG.

JasonB18/03/2021 14:47:14
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25215 forum posts
3105 photos
1 articles
Posted by Michael Gilligan on 18/03/2021 14:36:21:
Posted by JasonB on 18/03/2021 13:09:26:

This guy has a nice solid replacement for the top slide

I would probably even be tempted to tap the LIKE Button if we had one !

He also has a very nice home made epoxy granite CNC mill, have a look at his "Elke" videos. Youtube's subtitles are not too bad.

J

PS Should not be long until you can like the new software!

Edited By JasonB on 18/03/2021 14:47:55

Niels Abildgaard18/03/2021 15:38:16
470 forum posts
177 photos
Posted by mgnbuk on 18/03/2021 13:45:29:

The bed design on these doesn't seem too good if it is similar to the model Nils shows - in one of his other posts Nils showed one of these machines stripped down & I was struck by how poorly the headstock is supported due to the amount of metal removed underneath it to make space for the motor. Most Western lathes continue the bed at it's full form (or beef it up) under the headstock to properly support it.

.

 

The bed of the 180/210 lathes is Lousy but the Canadian gentleman lathe is a 250 and frame is miles better than Myfords and on par with Southbend/Boxfords

wp_20200923_002[1].jpg

 

 

WM250 plus Huanyang

wp_20190418_004[1].jpg

 

 

Edited By Niels Abildgaard on 18/03/2021 15:44:35

mgnbuk18/03/2021 15:48:10
1394 forum posts
103 photos

Thanks for the clarification Nils.

The lowest photo is the one I was thinking of - light use only there, I think !

Nigel B.

Niels Abildgaard18/03/2021 16:14:24
470 forum posts
177 photos
Posted by mgnbuk on 18/03/2021 15:48:10:

Thanks for the clarification Nils.

The lowest photo is the one I was thinking of - light use only there, I think !

Nigel B.

 

 

It depends on what You define as ligth

Modified 180 Lathe

 

it was done with a sewing machine motor

Stiffened 180 lathe bed

 

Edited By Niels Abildgaard on 18/03/2021 16:34:12

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