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Help With sizing the DRO For PM-25

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HellawellCustoms

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Hello I own a PM-25 and I am looking at buying a DRO for it. I found a thread on here by searching that brought me to this kit on ebay (http://www.ebay.com/itm/NEW-DRO-3-A...087225?hash=item25cd5b1079:g:wuIAAOSw4GVYUPuF) He has install pics etc... But does not show what size scales he bought. I figured I would just find the pm25 datasheet and hand the seller the sizes. I did and they said they do not have those sizes.

"

hi...1inch=25.4mm. 7inch=177mm. but we have 170 or 220mm travel. 19inch=482mm. we have 470 or 520mm travel. 13inch=330mm. we have 320 or 370mm travel. so which scale you need ?"



This is the message from the seller, I needed 177mm but they only have 170 or 220, I needed 482 and they only got 470 or 520, i also needed 330 but they only got 320 or 370. My question is do I go with the slightly undersized? As far as I can understand (wich isnt much) I'd have to go with the undersized as the larger ones would stick out? I asked the seller for the undersized and this is what they said ok...we will send 150*300*450 but it can read to 170*320*470mm.

I guess my question is do I go slightly undersized or do I go with the oversize.

 

darkzero

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#2
Always go with the next size up rather than undersize.... unless the next size up is extremely oversize but in that case I would look elsewhere.

Also never trust the machine's spec for travel, measure for yourself. For each axis, move it all the way to one side, make marks, then move it all the way to the other side, then measure your marks. Add 1" to your measurement then choose the next size scale (scale travel, not OAL) over your measurement.
 

mksj

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Per what Will mentioned above, if you are between sizes you go to longer scale. There are a few exceptions where it is very close, say withing 10mm. On a smaller mill like yours I would go with slimline scales GS30/31 if available that are much smaller than the standard size GS10/11, usually the cost difference is nominal but they may not have them for the longer travels. The 5 micon scales are all you need on a mill.
http://www.dropros.com/DRO_PROS_Digital_Readout_Scale.htm
The PM-25 travel and scale size:
X axis travel 19" so this works out to 623 mm per their calculation so you needs a 650 mm scale
Y axis travel 7" so this works out to 318 mm per their calculation so you needs a 350 mm scale
Z axis travel 13" so this works out to 471 mm per their calculation so you needs a 500 mm scale
So in each case your scale is about 1" longer which will not be an issue. There scale sizes are different than those indicated on the DROPros site, so I would go based on the sellers information.

You might look at this post of the DRO/scale install on a PM25. You could PM the poster as to scale size and any recommendations.
https://www.hobby-machinist.com/threads/pm-25-dro-install.55624/
 
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HellawellCustoms

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Per what Will mentioned above, if you are between sizes you go to longer scale. There are a few exceptions where it is very close, say withing 10mm. On a smaller mill like yours I would go with slimline scales GS30/31 if available that are much smaller than the standard size GS10/11, usually the cost difference is nominal but they may not have them for the longer travels. The 5 micon scales are all you need on a mill.
http://www.dropros.com/DRO_PROS_Digital_Readout_Scale.htm
The PM-25 travel and scale size:
X axis travel 19" so this works out to 623 mm per their calculation so you needs a 650 mm scale
Y axis travel 7" so this works out to 318 mm per their calculation so you needs a 350 mm scale
Z axis travel 13" so this works out to 471 mm per their calculation so you needs a 500 mm scale
So in each case your scale is about 1" longer which will not be an issue. There scale sizes are different than those indicated on the DROPros site, so I would go based on the sellers information.

You might look at this post of the DRO/scale install on a PM25. You could PM the poster as to scale size and any recommendations.
https://www.hobby-machinist.com/threads/pm-25-dro-install.55624/
GAH! I just emailed them and they said they already shipped them! :( Are these smaller ones just not going to work? Or can I manage with them.
 

darkzero

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GAH! I just emailed them and they said they already shipped them! :( Are these smaller ones just not going to work? Or can I manage with them.
They'll function of course but with short scales, if you ever work close to you end capacities, you risk damaging the scales if you over travel. Looks like you got glass scales with are very easy to damage the read head if you over travel. With the X axis you can always just use the stops if you still have them in place to prevent over travel but for the Y axis you'd have to make your own stops to prevent over travel if your x axis scale is too short also.

So it comes down to what type of things you work on. I currently have a PM45 mill & while I don't normally don't work on large parts I have reached my tables end limits many times. All it takes is one bump of overtravel to ruin your day. Luckily the reader heads are replacable but they may not always be easy to get or are expensive.
 

mksj

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#6
I was a bit in error as the scales I noted above was overall length, I see both overall scale sizes and travel. I should have read more closely.
X axis travel 19" so this would be a 500 mm travel scale
Y axis travel 7" so this would be a 200 mm travel scale
Z axis travel 13" so this would be a 350 mm travel scale

I do not follow their scales as far as what you need to order per their instructions and the actual travel of the scales. If you go strictly by the scale travel, the X, Y and Z scales will most likely be an issue. Per their information the respective travel would be 18.5", 6.7" and 12.6" and you do not want to hit the absolute limits be it glass or magnetic. So you may want to add some stops on these axis so you do not damage the scales. I would have gone with the next size up on both which would have added about 2". Bad information on the sellers part or the appropriate travel information was not provided.
 

HellawellCustoms

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I was a bit in error as the scales I noted above was overall length, I see both overall scale sizes and travel. I should have read more closely.
X axis travel 19" so this would be a 500 mm travel scale
Y axis travel 7" so this would be a 200 mm travel scale
Z axis travel 13" so this would be a 350 mm travel scale

I do not follow their scales as far as what you need to order per their instructions and the actual travel of the scales. If you go strictly by the scale travel, the X, Y and Z scales will most likely be an issue. Per their information the respective travel would be 18.5", 6.7" and 12.6" and you do not want to hit the absolute limits be it glass or magnetic. So you may want to add some stops on these axis so you do not damage the scales. I would have gone with the next size up on both which would have added about 2". Bad information on the sellers part or the appropriate travel information was not provided.
So you think 200mm,500mm and 350mm travel scales will be ok for the PM25, the seller said that Actual physical length is 141mm longer than travel so the one step bigger should still fit?
 

mksj

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#8
Yes, the 200, 350 and 500mm scale travel would work, the 150, 300 and 450 mm would not work based on the information provided. As mentioned and outlined in the link provided: "Recommended Digital Scale Travel refers to the maximum travel we recommend is utilized for a particular scale installation. It allows 10mm or ~1/2" clearance on either end of the read head so that it does not physically run into the end of the digital readout scale." So if you look at the table in the link provided a 350 mm glass scale (assuming this is a GS10/11) has a maximum travel of 14.7" but a recommended scale travel of 13.7". A 300 mm scale on the Z axis scale would be too short.
 

HellawellCustoms

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Yes, the 200, 350 and 500mm scale travel would work, the 150, 300 and 450 mm would not work based on the information provided. As mentioned and outlined in the link provided: "Recommended Digital Scale Travel refers to the maximum travel we recommend is utilized for a particular scale installation. It allows 10mm or ~1/2" clearance on either end of the read head so that it does not physically run into the end of the digital readout scale." So if you look at the table in the link provided a 350 mm glass scale (assuming this is a GS10/11) has a maximum travel of 14.7" but a recommended scale travel of 13.7". A 300 mm scale on the Z axis scale would be too short.
Thank you for this information! I'm gonna email them and see if I can have those ones sent to me.
 
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