Newb question.

[gundog]

I am looking into buying a new Shopbot. Is it feasable to cut a profile in one side of a sheet in this case the final parts are round and then turn the sheet over and profile the other side and match the parts up? I make this one part on a lathe now but I make the part in 6 steps useing round stock. I am wondering if it could be cut from flat sheet. The part is made from 1.25" UHMW and it is 4" in dia. I am not sure if what I wrote makes sense or not.
Mike

[scottcox]

Michael,

I cut two sided parts often. The key is to place registration holes on the top side of the blank, then matching holes on the table. Flip it and use small dowels or drill bits to match the holes.

[gerryv]

I'm in precisely the same position. Looking at buying a Shopbot but needing to make some plastic spool valves (in this case PVC or Delrin). Scott (or anyone): Q. Doing a two sided part this way and assuming great care is taken on the setup, do you have any indication what sort of accuracy on roundness e,g. out-out-round would be realistic and 2nd, with a good bit and not rushing, what sort of finish might one expect? Hope this helps your question as well Michael. Thanks much - Gerry

[scottcox]

Gerald,

I've only done edge rabbeting and profile cutouts of PVC, so never machined a 3D contour into the "face" of PVC. (I think that's what you were asking.) My edge cuts went fine with a downcut spiral from Onsrud.

Concerning the 2-sided cutting, I do this with full sheets of MDF most often and would guess an error of no more than a few thousands. My customers have been happy with it, so I was also.

It's important that your machine is very square. Any error will mean the registration holes don't line up.

[oddcoach]

If the part has a rounded surface on the top you could machine a cavity in a block. then use vacuum to hold the part while you machine the other side.
If the part only has rounded edges there are tools available to do that machining from the top side
john

[gerryv]

Good info thanks. The easiest way to explain this "spool" is in terms of what I do on a lathe. I begin with a length of solid round rod, I 1st, machine the diameter down to size for the entire length say 2 inches, I next move down an inch and machine it down to 1.5 inch diameter for an inch, leave the next inch at full diameter, do another step e.g. larger dia, smaller dia, larger dia, smaller dia, all being round. Without a rotating fourth axis, I'm wondering if I can come fairly close to true round and reasonably smooth, assuming my setup (and alignment, thanks much John, I'd not thought of that :-) are good. Better no doubt with a spindle but, The spool will have O'rings, (o-ring slots routed in larger ID's) making contact with the chamber walls it's sliding back and forth in so there is room for tolerance but not being able to get close to true round by two sided machining (assuming I do my part right) would be my bigger concern, being inexperienced enough to not know what to expect. - Gerry

[myxpykalix]

Gerald, A picture is worth a thousand words so if you have a pic of what you are trying to accomplish it might be easier for someone else to envision what you are trying to do and how to make it work on a bot.

Many times i have tried to describe something and because i see it my head it makes sense to me but my description might have not been clear enough. This could be just me but i'm not real clear on what you are trying to do. But if it can be done someone here can tell you how to do it.

[bcammack]

Didn't Brady Watson or someone else here machine the guts of a water pump off an ancient Buick out of aluminum a while back?

[beacon14]

If I understand correctly you want to make something that would typically be turned on a lathe, and machine it flat by working one side at a time on the ShopBot. While it may be possible, and if done correctly the registration would let you match the two sides up quite precisely, you could do this more easily with an indexer or even a small lathe setup on the ShopBot.

I've made turned legs by rigging up a "lathe" on my machine table consisting of a plywood jig which supports the workpiece using a lag screw at each end. I turn the exposed hex head end of one of the lag screws with a power drill while the 'Bot shapes the part. It's a little crude but it works. I may have another one like it coming up soon in which case I'll try to get some pictures.

[wmcghee]

Sounds like a good job for an indexer (4th axis). Machine your OD on the lathe and then do the steps and o-ring grooves with the shopbot indexer. Or maybe skip the lathe altogether and mill the OD to desired size on the 'bot.

[oddcoach]

AS DAVID said you could do this with just a lathe and not an indexer. I started with a $200 jet mini lathe from woodcraft supply. I mounted it off the end of the shopbot table in the y direction. you can then use partswizard to draw a cross section of your part. the indexer tool in the shopbot software will take this file and translate it for the indexer( thanks Bill). you can then chuck a square part in the lathe and start is spinning. then run the partfile.
i have since added a stepper motor to this setup.
John

[bill.young]

Gerald,

I may not be understanding the shape correctly, but it sounds like a series of alternating 2" and 1.5" discs that are 1" thick. If that's right, can you cut them flat as discs out of sheet material and then assemble them into the spool? Maybe add a hole in the center of each to align them.

Bill

[gerryv]

Thanks all,
You've confirmed that there is a way to do this and, in fact, "more ways than one to skin a cat." My decision is based on whether I can accomplish this, even slowly and carefully, until such time as I could afford the CNC indexer. That said, I feel I may have hijacked Michaels thread so hopefully you can get back to him.
Again, thanks much.
- Gerry

[gundog]

Not a problem Gerry I actually have another part similar to what you are talking about. My part is a 4" roller shaped like an hour glass large diameter is 4" it is 4" long and the small diameter is 2" it has a 5/8" wide rope grove in the center at that smallest diameter or 2".

Right now I outsource them to a guy with a cnc lathe. I can turn them on my manual lathe but it is real time consuming the material is UHMW and it is hard to turn because it does not make a chip it makes strings and then they ball up on the part until it binds and stalls the machine. To turn it manually I turn until it balls up then stop the machine and cut off the strings and start over. The cnc lathe pecks at it and it doesn't ball up that way.

My thought was to rough it out on the shop bot. I was thinking I could cut the 4" stock to length and bore on the lathe then make an arbor mounted solid to a couple pieces of angle mounted to the table of the Shopbot then machine half of it then loosen the arbor and rotate 180* and machine the other side take it too the lathe and finish it. I would make the arbor so it could hold mulitiple parts maybe even several arbors each holding a number of parts. The arbor would be low to the table the round stock actually touching the table so it would not deflect down when cutting. This is what I have envisioned anyway until I could afford the indexer but the small lathe might work also.
Mike

Here is a picture of that roller. Copy the link and paste it in your browser I do not have a copy of this picture on my computer.

http://img.photobucket.com/albums/v292/millnut/DSC03339.jpg

[myxpykalix]

See i told you a picture is worth a thousand words! The shape in the pic is simple to do with an indexer. It essentially is just a short tapered cylinder, reverse the deminsions for the other side with a ballnose bit cut to your depth in the center.