# SB milling machine finished and modified



## Mike23 (Sep 5, 2021)

I can't find any SB specific forum pages. But I thought Ide share with you all. I just went through my old SB knee mill. It was all gumed up and stuck from 20 years of siting in my back shed. It took a couple of weeks but being retired now I have the time.  So Ive been following some of these youtube guys. they are really a lot smarter than me. And you all probably are up on this stuff. I'm pretty new to it.  One guy (Mrpete222) is like an encyclopedia and has the answer for everything.  he has a ton of followers. Then theres another guy (Abom79) who is mostly big stuff but very interesting.  Another guy who has almost no followers (Don's engine) but he has really brilliant ideas. When I did my mill I saw Don put some struts on his knee to help save the knee gear.  My SB was cranking up kind of hard, but I'm 69 now and don't have the energy I once had so maybe its just me.  So after I put the struts on, the knee now comes up so easy like butter, even with really heavy parts on the bed.  I thought it may be a good tip to share for anyone else that has a SB knee mill.  His other ideas are really great too especially on the lathe.   Mike


----------



## markba633csi (Sep 6, 2021)

Shoot us a picture of that bad boy- not too many of those SB mills around
-M


----------



## jwmay (Sep 6, 2021)

What rating for the struts? That sounds like a heck of an idea to me.  I may just try it. Actually I guess I can search for the Youtube video. Thanks!


----------



## jwmay (Sep 6, 2021)




----------



## Mike23 (Sep 6, 2021)

jwmay said:


> What rating for the struts? That sounds like a heck of an idea to me.  I may just try it. Actually I guess I can search for the Youtube video. Thanks!


The struts he used are 200 pounds each and 32 inch long. I found some 150 pound struts here locally but they only they are only 29 inches so I lost a couple inches on the Z axis but that's ok.  I'll try and get a picture or two for you.   Mike


----------



## B2 (Sep 6, 2021)

So Guys,

 I have a couple of comments/questions and would welcome comments back.  I have a bench mill PM940M CNC and have trouble with the head actually falling as it weight in at close to 300#s and the steppers cannot hold it when the power goes off.  The gib was also cut wrong at the factory, but I have shimmed it to get it tighter.  Nevertheless falling is still a problem as there is more going on.  I actually think the head is so heavy that it tilts forward a bit at times.  This falling, as well as the head tilting maybe due to the weight causing the column ways to bend together when the head is vertically centered and away from the column ends.  I will eventually measure the way separation to determine if it is so.   So I have been trying to figure out a way to counter balance some of the weight.   I am currently thinking of a counter balance weight with cables over the top to weights hanging down the back.  But this is kind of spatially messy, but so would be struts.    

1.  Should the springs in the knee mill application not be at the center of gravity of the table weight so as to not create an additional torque on the table vertical ways? Maybe it could be far from the column to do so.  At least it does not seem that putting the gas springs near the ways is the best place.  

2.  In the Don's Engine video he did not mention what the length of the strut rods vs the gas cylinder, nor what the table height was when the springs were installed (but I assume that it was fully up when installed).  So at this up point the springs (gas) do not supply as much lift as when the table is fully down where they supply the maximum lift.  So the lift  would vary  as the table moves from top to bottom, and so it provides less support when the table is high.  The pressure in the gas spring follows the ideal gas law, PV=nRT.  Of course, since the diameter is constant the V is proportional to the length, L.  So P =k/V= m/L.  where k and m are constants.  Or, since the cylinder of the spring is prefilled it has pressure (force) when fully extended, but is against the end stop.  Po = k/Vo= m/Lo.  When the piston is compressed this pressure changes to P1=m/L1.  So the Delta P=Po-P1= m(1/Lo -1/L1)= m(L1-Lo)/LoL1, where L1<Lo.    Assuming for a second that Lo is initially something like 15 (inches?) and that L1 is Lo/3 (inches as it cannot go to zero before it hits some stop).  Then the delta P =m(L0/3-Lo)/(Lo*Lo/3)= -2m/Lo=-2Po.  So the force is twice as large when the knee is down than when it is up.  Did I get this right?  It would seem that you would want to choose the strut rating (400#s) to be a corrected value to be equal to the knee weight at the point where the knee was most likely to be operating!  This way the struts would actually be stronger than gravity force (weight) when compressed and less than the gravity force when fully extended.  

3) Some of these struts are also shocks, which might slow up the response time of vertical motion on a CNC machine.

Dave


----------



## Mike23 (Sep 7, 2021)

Hi B2.  I can't keep up with all your fancy math. I barely made it out of HS having to cheat through Algebra to do it. And that was back in the 60's. I do know that these gas struts are a constant pressure. I think Don said it on his video.  You are probably thinking that the gas compresses like in a closed cylinder. They are actually valved through the piston to maintain a constant force.  I'm no engineer, but I know there is enough info out there to explain what I can't.  Don could probably do it better than me.  I know he floats around on practical machinist and the Burke mills site. maybe you could catch up with him here. or ask him on his youtube site.  Mike


----------



## B2 (Sep 7, 2021)

Hi Mike,
It seems we are about the same age!  Graduated HS in 65!  There was no Algebra in my little HS in Missouri, but I did get it and others in college. 

Thanks for your response and comment.  However, I do not see how a valve in the cylinder could keep the pressure constant and still allow the spring to work in both directions. To keep the pressure constant it would have to bleed gas into the non-pressurized side during compression, but there is no way to put it back on the opposing stoke.  Bleeding gas (actually oil) from one side to the other, via small port, would be how a shock would work.  So I googled the concept of a valve to see if such a thing exists.  I did not find it but did find several sites talking about how the spring works.  A link that indicated that they do work the way I thought is:  https://industrialgassprings.com/how-gas-springs-work/ .   It seems that their two tricks in the design of all of this.  One it to start with a very high pressure when the piston rod is completely extended.  The other is to never let the piston go completely to all the way to compressing the gas to zero volume (infinite pressure would destroy things) nor to letting it become decompressed completely when extended. They show a cross-section of what is inside as well as simple graph of the pressure versus piston position.  Note they plot both pressure and volume and the are inverse of each other.   For their simple example, the pressure when the rod is in the center would be the value I was suggesting for the Mill application.... if the point of typical mill use was at the center of travel of the mill knee.  In their plot  For example, I went out and looked at the springs in my car trunk.  The rod moves about 3 inches, but the cylinder is about 6 inches long.  So its over all length is about 9 inches when extended.  So in this spring the volume (length) to contain the gas would be 6 inches when extended and and about 3 inches (6-3=3) when compressed.  So the volume decreases to about 1/2 of what it is when extended. 

I went an looked at my bench mill again.  It would be pretty hard to support the moving head with these springs as they would be in the way of the moving table!

There is another interesting thing about Don's placement of the spring anchor points.  He made them in a straight line, but on my car trunk the end move in some what of an arc.  An arc would mean that the spring arm compression length would follow an arc and so be adjusting the force some what with motion.  It might be that if Don had put the bottoms out farther then the upward force would be more of a constant as a function of the mill knee height position.   i.e. When the knee is up the spring is pushing mostly in the upward z direction, but when the mill knee is down the force is divided between the z and the y direction.  But because the spring is compressed when the knee is down it is exerting more over all force  so the the portion in the z direction is more like it was when the knee was up!  This math is computed via trig., in fact the cosine function.  I bet we could figure out the optimum location (angle) for the bottom  strut anchor if you want to.  

Dave


----------



## Mike23 (Sep 7, 2021)

I graduated in 69' and year later was up to my knees in rice paddys in the Mekong Delta...  Not much fun to be had there for a WV mining town boy.  Saigon was OK though... 

My struts are rated 150 pounds but they say, support weight 125, lifting 170. So I guess the 165 is the average.  I was going to get the ones Don was using but didn't want to wait.  They are just listed as 200 lbs constant and with a 14.5" stroke.  I know I set mine up the same way and they do make a huge difference all the way from bottom of the stroke to the top.  I think when you close a trunk lid the fulcrum point changes through the arc in relation to the position of the mounting points making it easier to close at it moves downward.  Putting the struts directly underneath the load seems to work really well.    Mike


----------



## B2 (Sep 7, 2021)

The first draft was in 65 and I lucked out.  I had already taken my physical and then they stopped the draft until the lottery!  I'm sorry you had to go. 

Sounds like you are all set with your install.  I am glad you posted it as it has made me think my mill problems over again.  If you really need the extra stroke you can probably easily change the struts for the longer ones.  However, if it is working why fix it?!!!  My mill has a vertical range of about 18-19 inches without tooling or vice... so it would take a long stroke and probably need a extended length of greater than 2x that to be the best.  Not room for that much length in any place I can see so far.     However, I have yet to ever work over anywhere near that range and so your struts would probably be long enough for me.  The best I can tell my head weight about 250-300#.   Anyway it is a cheap China Mill and so is not built all that well.  I did not see any place on Don's video where he said anything about the specs on his struts other than the 200#.  But you seem to know the stroke.  Do know what the total length is or better still do you have a model number?

I am a little puzzled about how some of the struts seem to have a throw of L and the total length is only slightly larger than 2L.  Maybe they have a double walled cylinder and the piston is only in the inner cylinder and the outer wall is just for the extra compressed gas.  If that is the case, it is amasing that they are so cheap!  Large volume production maybe?

 I have a SB lathe, SB10-Heavy, which has a lot of wear, backlash, and some gearing problems. I think it was built about the time we were in HS.   I may try to rebuilt it one of these days, but even without doing so I can tell it was made much better made than the Mill.   I recently purchased a new lathe which is Taiwan made and it is hand over foot better than what is made in China.  The lathe came 3phase, so I have spend quite a bit of time installing new electronics and a VFD to make it run on 220 1phase power.  I have been writing up my conversion in great detail and hope to post it to H-M one of these days.  

Only in the last few years did I get into metal working much (with machines other than a drill press) .  No formal training.   I was suppose to take metal working in HS, and the order equipment, but it did not arrive until after I graduated.  So my metal working in HS was pounding stuff with a hammer!  So I am still learning! 

PS.  I miss stated in the last posting.  My HS did have an algebra course.  It just was not much good.  However it did offer Trig.  which is really handy in the shop. 

Dave


----------



## Mike23 (Sep 26, 2021)

Hey guys.. I had to take a break... had a heart attack the morning after my last post here. It was number two. They're supposed to get easier, but don't believe everything you hear. Luckily I knew it was coming on so got to the H pretty quick.  Wife has forbidden me from the shop til I get the OK from the Doctor.  He'll probably drag his feet on that. Headed to Florida for my Mother in laws 95th and some R&R sometime next week.  Hoping to stop in and see Don from don's engine, the youtube guy.  SO I'll get some one on one with his plasma welder and hopefully can make a decision on buy in. Hope to spend more time here when I get back.. Mike


----------



## B2 (Sep 26, 2021)

Hi Mike,
Sorry to hear.    My brother-in-law had more than one heart attacks, but in the end the smoking gave him cancer and that took him.  Wishing you well and the best and wish I could visit Don's with you.  That should be interesting!


----------



## Mike23 (Sep 27, 2021)

Thanks B2.  Luckily I gave up smoking years ago.. wouldn't do it again.  Its just that damn Bacon... I can't get enough of it


----------

