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Steve M
Unregistered
| | Posted on Sunday, December 11, 2005 - 06:09 am: | 
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Gerald_D
Registered
Username: Gerald_d
Post Number: 17
Registered: 11-2005
| | Posted on Thursday, December 15, 2005 - 09:54 pm: |
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On the other side of the y-car we now have the facility to mount a second y-motor. (More pics) But, while we think if a second motor is really necessary, there is a dummy plate held in there with 4 screws. The plate has a spigot with a plain ball bearing on it. This runs under the angle iron rail and duplicates the hold-down force of the pinion running under a rack.
A very much simpler version of this principle can be knocked together as a dedicated hold-down.
It works like a dream. The 4 rollers of the y-car are always firmly seated and there is no fiddly adjustment to be done. |
rhfurniture
Unregistered
| | Posted on Wednesday, February 08, 2006 - 04:49 pm: |
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Gerald,
I like your holddown. Are you using the sprung motor as a hold-down on the other side?
I am asking because I have just fitted a second motor to the gantry carriage which has brought about a remarkable improvement in quality (I think - but know not how to measure it). I will at some time be fitting hardened rails and am wondering whether to abandon the shopbot holddowns (which are working ok at present - #iiabdfi#). Remembering that I work mainly in thick heavy hardwood:
1.Do you think it is fair on the motors/r&p to act as holdowns - and would you advise against tightening the spring a little?
2.With such a setup how much force would be needed to tilt the carriage ie would I be safe pushing my 2hp bosch to the limit with a 50mm cutter?
I am asking you because of your experience and mechanical background. Any help most appreciated.
Ralph. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 33
Registered: 11-2005
| | Posted on Wednesday, February 08, 2006 - 08:54 pm: |
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Ralph, a normal motor/spring arrangement does provide a good hold-down force at the motor, whether it likes it or not. From the geometry of the "levers" involved, a 10kg force on the turnbuckle/spring gives about 15kg of vertical holddown force at the pinion gear. The motor spec allows about 25kg load on its bearings/shaft and we see that our motors are happy with this load (at least, no one is complaining of worn stepper motor bearings)
So, my logic is that we already have 15kg of hold down on the one side, why not balance it with another 15kg on the other side? A total of 30kg is quite a lot.
Who knows if the ShopBot "hold-downs" are really holding down? Some guys find that the rail edges are not that parallel to each other and that the y-car gets confused as to which of the 8 ground surfaces it is supposed to follow. Sometimes the car is lifted off the primary rails by the supposed "hold-downs" depending on what y-position the car is sitting. Others find the car motion tightens up depending on y-position. The SB "hold-downs" are a waste of v-roller bearings.
Is 30kg of hold-down enough? Well, if we grab the router/spindle by the nose and push/pull as hard as what we think the cutter will do, we cannot get the y-car to lift a primary wheel off the primary rail edges at the top. That says our holddown is enough.
It is just so much more logical that if you want to hold something down you need an anchor point under the rail or gantry - not next to it. |
Ralph
Unregistered
| | Posted on Thursday, February 09, 2006 - 01:39 am: |
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Thanks Gerald, I will give it a go.
Does the standard instruction "take up the slack in the (standard shopbot)spring and turn it 3 more" result in a 10Kg force or do I have to do something different? - I don't really understand springs other than that they pull (or push).
I did manage to get the present ones working ok after an awful lot of faff and faddle - and there is still a short run where one of them doesn't turn.
My main quarrel with the shopbot holddowns is that the forces are in the wrong direction (45 deg at best) - see my solution (benchtop table cariage)at: http://www.mechmate.com/Forum/messages/11/157.html?1134285925
Thanks,
R. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 34
Registered: 11-2005
| | Posted on Thursday, February 09, 2006 - 07:18 am: |
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Ralph
I took the 4 springs that came with our SB, hooked each one to a load sensor, and applied the take up the slack .... and turn it 3 more rule. The average force was 11kg (25lb), but the spread of results was too wide for this to be a reliable method of setting tension. (See also this thread)
As you see, if there is a short run where a "hold-down" wheel does not turn, then:
1. The rails are different in that run, ie. they are not ground parallel, or they are curved. And,
2. The wheel experiences zero force from the rail, therefore it certainly is not holding down anything - removing the wheel will make no difference.
Your solution is a vast improvement on the standard. You can effectively "spring-load" your hold-downs because you have long slender screws holding them. |
Scott Worden
Registered
Username: Scott
Post Number: 2
Registered: 06-2006
| | Posted on Friday, August 18, 2006 - 06:31 am: |
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I thought I'd add a little chrome to my ShopBot and finally get rid of those hold downs that we love to hate so much. Actually, it was just a piece of flat bar 1/4" thick and when I started sanding off the black scale with my random orbit sander so I could paint it, I decided to take it through the grits up to 1500. It looked great so I put some "Wizards Power Seal" on it to help keep it *shiny*.
The rail is part of one of the utility struts that came with my machine that I didn't use. I cut off one edge, belt sanded the edge clean and true, and then bolted it onto the Y rail with the rounded "J" part tight to the underside of the rail. The V-bearing rides nice and smooth along the underside and the Y car is held down firmly.
The Y car now rides much smoother and there's no fiddle fartin' with the four horizontal V-bearings!
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Gerald_D
Registered
Username: Gerald_d
Post Number: 178
Registered: 11-2005
| | Posted on Saturday, August 19, 2006 - 05:46 pm: |
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Good job Scott! |
Scott Worden
Registered
Username: Scott
Post Number: 3
Registered: 06-2006
| | Posted on Sunday, August 20, 2006 - 05:32 am: |
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Thanks, I've been wanting to do something about those things for years but just haven't had or taken the time. I finally decided to just do it and started looking around the shop to see what I might have for a rail. I remembered the two spare struts that have been sitting in the corner for six years and said "That's it" and started cutting. ;-) |
Gerald_D
Registered
Username: Gerald_d
Post Number: 287
Registered: 11-2005
| | Posted on Wednesday, October 11, 2006 - 10:32 am: |
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On the MechMate there is no special rail surface for the hold-down wheel - it runs directly under the angle iron's "raw" side. (This tiny bit of roughness is still far better than the pinion running on a rack)
Here is a crude something you guys can knock together out of your scrap boxes:
The bearing is a sealed/shielded 6202 - (similar top bearing of PC router)
Bearing "shaft" is 3/8" countersunk head bolt and inverted Nylock nut, etc.
Flat bar is about 1.5" x 0.25" at thinnest. 5.0" long with 3.5" between hole centers
Spring & hinge point same as motor.
Not pretty, but it makes a fine holddown....exactly the same as what a second y-motor would have done. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 299
Registered: 11-2005
| | Posted on Friday, October 13, 2006 - 08:42 pm: |
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And the best part of this mod is that you get a couple of spare V-rollers that can be put to better use at the bottom of the z-slide:
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Mike John
Registered
Username: Mikejohn
Post Number: 89
Registered: 12-2005
| | Posted on Saturday, October 14, 2006 - 07:52 am: |
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Your Z slide is very different to mine.
My springs go up the tube, and the rack is enclosed.
I am beginning to see why having a PRT with an Alpha 'framework' doesn't suffer some of the problems requiring strengthening.
Maybe I should repeat all your photographs on mine and we can compare them side by side.
For what purpose I don't know 
..................Mike |
Gerald_D
Registered
Username: Gerald_d
Post Number: 300
Registered: 11-2005
| | Posted on Saturday, October 14, 2006 - 09:00 am: |
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Mike, I dumped a lot of pics in the Test Area yesterday. That's a circa late 2000 z tower with some mods - if the dark-blue paint is disturbed then it is modified. I would be very interested to know how the Alpha Z-tower differs from that one.....dump some pics down there too. I'll tell you later how to make those pics appear anywhere on this Forum, without uploading them again.
Will probably start a separate thread for a z-tower discussion, but don't let that stop you now  |
30. Y-Car